Project no.: 219123 development of pan-European key GRID ...realisegrid.rse-web.it/content/files/File/Publications and results... · development of pan-European key GRID infrastructures

Project no.: 219123

Project acronym

REALISEGRID

Project title: REseArch, methodoLogIes and technologieS for the effective

development of pan-European key GRID infrastructures to support the achievement of a reliable, competitive and sustainable electricity

supply

Instrument: Collaborative project Thematic priority: ENERGY.2007.7.3.4

Analysis and scenarios of energy infrastructure evolution Start date of project: 01 September 2008

Duration: 30 months

D1.3.3

Comparison of AC and DC technologies for long-distance interconnections

Revision: Final

Actual submission date: 2010-03-19

Organisation name of lead contractor for this deliverable: Technical University of Dortmund

Dissemination Level

PU Public PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission Services) X CO Confidential , only for members of the consortium (including the Commission Services)

D1.3.3: Comparison of AC and DC technologies for long-distance interconnections Page 3

Deliverable number: D1.3.3 Deliverable title: Comparison of AC and DC technologies for long-distance interconnections Work package: WP1: Performances and costs of innovative grid technologies Lead contractor: Technical University of Dortmund (TUDo)

Quality Assurance

Status of deliverable Action By Date Verified (WP-leader) Athanase Vaféas, TECHNOFI 2010-03-19 Approved (Coordinator) Gianluigi Migliavacca, ERSE (former CESI RICERCA) 2010-03-19

Submitted

Author(s) Name Organisation E-mail Sven Rüberg TU Dortmund [email protected] Arturs Purvins JRC - Institute for Energy [email protected]

Abstract The present report gives a brief technological introduction to long-distance power transmission based on both HVAC (High Voltage Alternating Current) and HVDC (High Voltage Direct Current) transmission technologies. Furthermore, experiences in the operation of long-distance power transmission lines from other large scale networks, like those ones in North America, South America, India, China, Russia and Africa, are evaluated to derive proposals for the usage of HVAC and HVDC technologies for long distances within Europe. A special consideration is given to the parallel operation of HVAC and HVDC systems and their mutual impact on system security. In addition, the impact of an increased power infeed from Renewable Energy Sources (RES), in particular wind plants, on power transmission in Europe and the potential of long-distance power transmission to help solving related system issues are also described.

mailto:[email protected]

mailto:[email protected]


TABLE OF CONTENTS Page

ACRONYMS AND DEFINITIONS .............................................................................................. 7

1 EXECUTIVE SUMMARY .................................................................................................... 9

2 INTRODUCTION ................................................................................................................ 13

2.1 Objectives of this deliverable..................................................................................... 13

2.2 Expected outcome ...................................................................................................... 13

2.3 Approach.................................................................................................................... 14

3 TECHNOLOGICAL ASPECTS OF LONG-DISTANCE POWER TRANSMISSION...... 17

3.1 Traditional HVAC transmission ................................................................................ 17

3.2 HVDC transmission ................................................................................................... 21

3.3 Practical solutions for long-distance power transmission.......................................... 23

3.4 Parallel operation of HVAC and HVDC.................................................................... 24

3.4.1 HVAC and HVDC links in the same right-of-way ........................................ 24

3.4.2 HVDC link within a synchronized HVAC power grid .................................. 24

4 ENVIRONMENTAL AND ECONOMIC CONSIDERATIONS ON LONG-DISTANCE POWER TRANSMISSION .................................................................................................. 27

5 LONG-DISTANCE TRANSMISSION PROJECTS IN OPERATION TO-DATE............. 31

5.1 North America: United States of America and Canada ............................................. 32

5.1.1 Pacific Intertie ................................................................................................ 34

5.1.2 Square Butte ................................................................................................... 34

5.1.3 CU .................................................................................................................. 35

5.1.4 Intermountain Power Project.......................................................................... 36

5.1.5 Quebec-New England .................................................................................... 37

5.2 South America............................................................................................................ 37


5.3 India ........................................................................................................................... 41

5.3.1 Rihand-Delhi .................................................................................................. 43

5.3.2 Chandrapur-Padghe........................................................................................ 44

5.3.3 East-South Interconnector II .......................................................................... 44

5.3.4 Ballia-Bhiwadi ............................................................................................... 45

5.4 China .......................................................................................................................... 45

5.4.1 Gezhouba-Nanqiao......................................................................................... 48

5.4.2 Tianshengqiao-Guangzhou............................................................................. 48

5.4.3 Three Gorges-Changzhou .............................................................................. 49

5.4.4 Three Gorges-Guangdong .............................................................................. 50

5.4.5 Guizhou-Guangdong ...................................................................................... 51

5.4.6 Three Gorges-Shanghai.................................................................................. 52

5.4.7 Guizhou-Guangdong II .................................................................................. 53

5.5 Russia ......................................................................................................................... 53

5.6 Africa ......................................................................................................................... 55

5.6.1 Cahora-Bassa.................................................................................................. 56

5.6.2 Inga-Shaba...................................................................................................... 57

5.6.3 Caprivi Link ................................................................................................... 57

5.7 Lessons learnt from the operational experience described ........................................ 58

6 FUTURE TRENDS OF LONG-DISTANCE TRANSMISSION WITHIN EUROPE ........ 59

6.1 Green energy power balance...................................................................................... 59

6.2 Green energy power import ....................................................................................... 61

6.2.1 Desertec Foundation....................................................................................... 61

6.2.2 OffshoreGrid initiative ................................................................................... 63

6.3 Proposals for long-distance transmission in Europe .................................................. 65


7 CONCLUSIONS................................................................................................................... 69

8 REFERENCES...................................................................................................................... 71


ACRONYMS AND DEFINITIONS

AC Alternating Current

CO2 Carbon Dioxide

CSC Current Source Converter

CSP Concentrating Solar thermal Power

DC Direct Current

EHVAC Extra High Voltage Alternating Current

EHVDC Extra High Voltage Direct Current

ENTSO-E European Network of Transmission System Operators for Electricity

EU European Union

EUMENA European Union, Middle East, North Africa

HVAC High Voltage Alternating Current

HVDC High Voltage Direct Current

LCC Line-Commutated Converter

RES Renewable Energy Sources

TREC Trans-Mediterranean Renewable Energy Cooperation network

TSO Transmission System Operator

UK United Kingdom

USA United States of America

VSC Voltage Source Converter

XLPE Cross-Linked Polyethylene


1 EXECUTIVE SUMMARY

The present report aims at describing the two key technologies of power transmission, i.e. High Voltage Direct Current (HVDC) and conventional High Voltage Alternating Current (HVAC) transmission, as well as their applicability and limitations in power transmission over long distances, where in this context the term “long distance” refers to overhead lines with a length longer than approx. 500 km.

Between 1962 and 2009, a total transmission capacity of 47.6 GW for long-distance HVDC power transmission has been installed worldwide, mainly to connect remote hydro and coal-fired power plants to the load centers. 50% of this transmission capacity has been constructed after the year 2000, while in 2009 additional 26.5 GW of new long-distance HVDC transmission capacity were under construction. These numbers underline the high growth rate of installed HVDC transmission systems which has been observed in the last years. This is obviously due to the technical and economical benefits that come along with HVDC in long-distance power transmission compared to conventional HVAC transmission technologies. In addition, since HVDC is a relatively new technology, the growth rate of HVDC systems in power transmission is expected to remain steady or even rise in the next years due to the results of research activities in this field. In particular, the development of higher current and voltage ratings of self-commutating switching devices will open up new fields of application for Voltage Source Converter (VSC)-based HVDC (VSC-HVDC) in bulk-power transmission.

Today, existing long-distance HVDC transmission systems are primarily in use for the point-to-point interconnection between remote power generation (mainly hydro and offshore wind power) and highly urbanized areas (e.g. Los Angeles, Mumbai, and Shanghai). Almost all long-distance HVDC transmission systems in operation to-date are overhead lines, for the most part carrying between 1500 and 3000 MW of active power over distances between 500 and 1500 km. Most of the lines are bipolar with two conductors and with DC voltages in the range of ±500 kV (+500 kV in one conductor and -500 kV in the other, with respect to ground). Two converter stations are needed at both ends of the line. The generated power at the sending end is converted from HVAC to HVDC for lower transmission losses on the line and converted back from HVDC to HVAC at the receiving end, where the transmitted power is supplied to distribution grid. The main reasons of bipolar transmission are zero earth-current flows, when both poles are symmetrically loaded, and a higher reliability: if an outage appears in one pole, the healthy pole can continue to deliver approximately half of the rated power using the earth return path or a dedicated metallic low-voltage return in case the earth return path is not licensable for environmental concerns.

The main purpose of the existing HVDC long-distance transmission systems described is bulk-power transmission from the sending end to the receiving end of the line, hence, from areas with high generation but low load to areas where additional power support is needed. The main choice reasons for HVDC instead of conventional HVAC transmission are lower transmission losses on the line, environmental advantages (higher power density per ground unit), and the positive impact on the stability of the existing network. In the case of the interconnection of two asynchronously operated AC networks and the interconnection between two AC networks with different frequencies, HVDC constitutes the only feasible solution.


Main operational experience is described for the CU link1, Pacific Intertie and Square Butte (North America), Itaipu (Brazil), Tianshengqiao-Guangzhou, Guizhou-Guangdong, Three Gorges-Changzhou and the Three Gorges-Guangdong (China) HVDC transmission projects. Additionally, flashover reliability records are presented for the Rihand-Delhi (India) and Gezhouba-Nanqiao (China) projects. Some forced outages, which appeared in the first years of operation, were prevented mainly by modifying the apparatus. The described problems in the operation of HVDC systems in China were mainly caused by apparatus malfunction due to deficient design and manufacturing. The quantity of forced bipolar line outages is relatively small, if compared with monopolar faults.

The average energy availability of the existing HVDC lines as a function of time tends to rise after the first 10 years of operation and remains relatively constant afterwards. The overall average energy availability between 1993 and 2004 is approximately 94.3%.2 Scheduled energy unavailability increases for the facilities older than 20 years, perhaps indicating that a higher level of maintenance is required for older HVDC systems. Approximately two thirds of the forced energy unavailability of HVDC transmission systems around the world in the time period from 1983 to 2006 are caused by failure of AC and/or auxiliary equipment of the HVDC system in service.

Due to the geographical location and other causes, each of the lines in the following has its own specific features. Quebec-New England is a multi-terminal HVDC transmission system with the possibility to supply power from the middle of the line. Itaipu has the highest voltage level of ±600 kV, but long-distance overhead HVDC lines with higher voltages up to 800 kV are under construction in China at the time of writing. The Zhengping converter station of the Three Gorges-Changzhou project is exposed to very heavy industrial pollution. Therefore, the electrical insulation has to be higher than conventional, all high-potential DC equipment is installed indoors, and all the DC neutral equipment is installed outdoors. Because of the extreme line length and the difficult logistics along the route, the Inga-Shaba project is composed of two monopolar lines (in contrast to one bipolar line) in electrically parallel connection. The Caprivi HVDC system is the first system worldwide that uses VSC-HVDC for long-distance power transmission in combination with overhead transmission lines.

From the operational experience of the existing HVDC transmission systems described, it can be seen that HVDC transmission technologies are essential in efficient long-distance electrical bulk-power transmission: its low losses and its smaller environmental impact made it the preferred choice over conventional HVAC transmission. In addition, the experiences gained from the operation of HVDC transmission projects for years and the reliability records presented underline the feasibility and reliability of HVDC for long-distance transmission. HVDC transmission can therefore be a feasible option for the necessary upgrade and expansion of the European transmission grid in order to ensure a stable integration of RES in the future.

1 CU stands for the names of the first owners – Cooperative Power Authority and United Power Authority

2 Analysis of 51 HVDC transmission systems. Not only long-distance systems are included.


An alternative to the long-distance HVDC transmission system for decreasing energy losses on a long-distance AC line is the EHVAC technology (Extra High Voltage Alternating Current), featuring a higher voltage level than traditional AC transmission systems. The first link of the 1150 kV EHVAC transmission system in the world was commissioned in 1985 in the Republic of Kazakhstan between power substations in Ekibastuz and Kokshetaus. In total, the 1150 kV AC line connects the following cities: Itat, Barnaul, Ekibastuz, Kokshetau, Kostanay and Chelyabinsk. actually, step-down transformer substations were installed only on Kazakhstan territory. Therefore, the energy transmission at voltage level of 1150 kV is only possible between Ekibastuz, Kokshetau and Kostanay, while the remaining part of the system operates at 500 kV.

Due to environmental and political drivers, the future generation mix in Europe will contain a considerable share of renewable energy resources, such as offshore and onshore wind energy from the north-west Europe and solar energy import from North Africa and the Middle East. Solar power plants with the possibility of radiation accumulation using molten salt technologies ensure a stable power supply all 24 hours of day. Using HVDC technologies, an efficient energy transmission of this green power to South-Eastern and Central Europe is possible. HVDC transmission can also be used for green-power balance within Europe by transmitting the wind power from places with high wind and low load to places with low wind and high load.

Ongoing feasibility studies, such as Desertec and OffshoreGrid, have shown that an HVDC overlay grid can help TSOs to cope with the issue of long-distance bulk-power transmission as it is foreseen to arise with the increased integration of renewable energy sources (RES). In addition, this report provides some elements related to ongoing pilot initiatives, such as Kriegers Flak, the North Sea Offshore Grid project, and the Mediterranean Energy Ring on power transmission in Europe.


2 INTRODUCTION

Today, the field of electrical power generation and transmission is in a deep transforming phase. Driven by the public and political will to accomplish the nuclear-power phase-out, to responsibly utilize conventional sources of generation (such as nuclear energy, lignite, and coal), to reduce obnoxious gas emission, in particular carbon dioxide (CO2), and therefore to promote the exploitation of Renewable Energy Sources (RES), the need of electrical power transmission over long distances from remote RES to the load centers all over Europe increases. Furthermore, the deregulation of the European electricity market puts additional pressure on the power grid: extra transmission capacities have to be allocated by Transmission System Operators (TSOs) to allow for a pan-European power trade. While the power grid was originally designed for the exchange of reserve power in emergency situations only, the above stated changes result into an operation of the power grid that is closer to its thermal and stability limits than ever before.

2.1 Objectives of this deliverable

The present report deals with the comparison of Alternating Current (AC) and Direct Current (DC) technologies for long-distance interconnections. Experiences from other large scale networks, like those ones in North America, South America, India, China, Russia, and Africa are evaluated to derive suggestions and benchmark solutions for the usage of the AC and DC technologies for long distances within Europe. A special consideration is given to the parallel operation of AC and DC systems and their mutual impact on system security. In addition, this report gives an outlook towards the impact of feasibility studies, such as OffshoreGrid and Desertec projects on the one hand, and elements related to ongoing pilot initiatives, such as Kriegers Flak, the North Sea Offshore Grid project and the Mediterranean Energy Ring on the other hand, on power transmission in Europe.

2.2 Expected outcome

This report provides and compares the technical key facts of HVAC and HVDC power transmission as well as the operational experience that has been gained from existing long-distance transmission projects in operation to-date. Hence, this report provides assistance in finding feasible technical solutions to long-distance power transmission problems that are foreseen to arise within the European continent. On this background, possible future installations of AC or DC long-distance transmission lines are suggested.

In order to lay a profound theoretical basis, Chapter 3 provides the fundamentals of power transmission over long distances. In particular, the key factors that limit the active power transfer and the transmission distance are presented for both HVAC and HVDC power transmission. Furthermore, economic, environmental and political considerations on long-distance power transmission are given in Chapter 4 to enable for a widespread analysis of possible transmission solutions. In Chapter 5, experience that has been gained from the operation of long-distance


transmission projects throughout the world complements the theoretical basis laid in Chapters 3 and 4. Finally, the impact of the increased use of RES and of feasibility studies, such as OffshoreGrid and Desertec projects on the one hand, and elements related to ongoing pilot initiatives, such as Kriegers Flak, the North Sea Offshore Grid project, and the Mediterranean Energy Ring on the other hand, on the future role of long-distance transmission within Europe are presented in Chapter 6. Also, the future trend in the construction of a long-distance transmission infrastructure is described. Finally, Chapter 7 recaps the main points addressed and provides elements for the way forward.

2.3 Approach

The information and the data presented in this deliverable are based on internal knowledge and experience of the REALISEGRID authors as well as on the technical and scientific literature available on long-distance transmission. In particular, the reliability survey of HVDC transmission systems throughout the world, which is published every two years by the Study Committee B4 of the International Council on Large Electric Systems (CIGRE), has been a key source for the reliability assessment that is conducted within the present report. In addition, public documents, sources, and links to ongoing feasibility studies (e.g. OffshoreGrid), initiatives (e.g. Desertec) and projects that are under realization (e.g. North Sea Offshore Grid and Kriegers Flak) in Europe have been consulted and compared in order to have a broad and consistent picture on the need for long-distance power transmission to promote the consequent use of RES.

The close interrelation of this report with the REALISEGRID Deliverables:

• D1.1.1 (“Synthetic description of performances and benefits of undergrounding transmission”),

• D1.2.1 (“Improving network controllability by FACTS and by HVDC transmission

systems”),

• D1.2.2 (“Improving network controllability by coordinated control of HVDC and FACTS devices”) and

• D1.4.1 (“List of promising innovative grid technologies”)

is stressed.



3 TECHNOLOGICAL ASPECTS OF LONG-DISTANCE POWER TRANSMISSION

The transmission of electrical power through a transmission channel is subject to physical phenomena. This Chapter describes the physical models and principles behind electrical power transfer through an electrical transmission line using the examples of conventional HVAC and modern HVDC technologies. The corresponding restrictions and limitations that electrical transmission system operators are confronted with during operation will be presented. In this context the term “long distance” refers to lines with a length longer than approx. 500 km.

3.1 Traditional HVAC transmission

SE REXj

I

(a) Simplified line diagram

(b) Phasor diagram

Figure 1(a) shows a diagram of electrical power transfer through a transmission channel in which the complex power SS is transmitted from the sending end to the receiving end in order to balance

RE

SE

IXj

I

δ

ϕ

SS RS

Figure 1: Model of transferring electrical power through an electrical transmis-sion line in AC systems [1]


the complex load RS . The line reactance X corresponds to the series impedance of the simplified representation of an electrical over-head transmission line in which the shunt capacities (and also the line resistance) have been neglected for the sake of simplicity. SE and represent the complex busbar voltages at the sending and the receiving end, respectively, where is assumed to be real-valued, i.e.

RE

RE°∠= 0RR EE (see Figure 1(b)). I is the complex line current.

With

RRRR j: QPIES +== ∗ (3.1)

and

XEEI

jRS −= , (3.2)

it can be easily obtained that the complex power at the receiving end of the line equals RS

XEEEES ])cos([j)sin( RSS

RR−+

=δδ , (3.3)

more precisely

)sin(SRR δ

XEEP = (3.4)

and

XEEEQ

2RSR

R)cos( −

=δ . (3.5)

The corresponding phasor diagram is shown in Figure 1(b).

Similarly, it can be found that at the sending end of the line

)sin(RSS δ

XEEP = (3.6)

and

XEEEQ )cos(RS

2S

Sδ−

= . (3.7)

Equations (3.4) to (3.7) indicate that with 0=δ no active power can be transferred through the line. In case , and are positive and reactive power is transmitted from the sending to the receiving end of the line. If , then and become negative, i.e. reactive power is flowing from the receiving end to the sending end of the line

RS EE > SQ RQ

RS EE < SQ RQ[1]. For both cases, Figure 2 shows the

corresponding phasor diagram for 0=δ . The reactive power loss on the transmission line is in this case

lossQ


22

RSRSloss

)( XIX

EEQQQ =−

=−= . (3.8)

SERE

XIj

I

I

RESE

XIj-

(a) RS EE > (b) RS EE <

Figure 2: Phasor diagrams for 0=δ (after [1])

From the above results, it can be derived that transmission of lagging current, i.e., the transmission of positive reactive power from the sending to the receiving end, causes a voltage drop in receiving end voltage. Similarly, the transmission of leading current, i.e. the transmission of negative reactive power from the sending to the receiving end, causes a voltage raise in receiving end voltage. In another way of interpreting, this means that the transmission of reactive power (for 0=δ ) mainly depends on the difference between the voltage magnitudes at the ends of the line. Furthermore, it can be noted that a transmission line consumes reactive power, which is equivalent to reactive losses on the line.

In any case, the transmission of reactive power through a transmission line causes a variation of voltage at the opposite end of the transmission line. This voltage variation is proportional to the transmitted current, i.e. the transmitted reactive power, and the inductive reactance X of the line which is proportional to the transmission distance. Since for a stable system operation this voltage variation needs to be kept within specified limits, the amount of reactive power that can be transmitted decreases with increasing transmission distance.

0≠δ and , it is possible to derive from equations Considering now EEE == RS (3.4) to (3.7) the expressions that now are

)sin(2

RS δXEPP == (3.9)

and

22

RS 21)]cos(1[ XI

XEQQ =

−=−=

δ . (3.10)

Since , no reactive power is transferred through the line0RS =− EE 3. With 0>δ , active power flows from the sending to the receiving end. With 0<δ , the direction of active power flow

3 This is true only for E=0 or δ=0.


reverses. The reactive power loss on the line is equally balanced by each end of the transmission line.

lossQ

From these observations, the following conclusion can be drawn that with positive δ , active power is transferred from the sending to the receiving end. Similarly, with negative δ , active power is transferred from the receiving to the sending end. This leads to the finding that the transfer of active

power mainly depends on the load angle

SE

δ .

Taking a more detailed representation of the transmission line that now also considers the ohmic series resistance R (see Figure 3) and allowing any possible values of 0≠δ , and , it can be found that the active and reactive losses on the transmission line and Q , respectively, can be expressed as a function of the power transfer and by

SE REP

P Qloss loss

R R

2R

2R

2R2

loss EQPRRIP +

== (3.11)

and

2R

2R

2R2

loss EQPXXIQ +

== , (3.12)

while again taking advantage of equation (3.1).

From equations (3.11) and (3.12), it can be deduced that an increasing active power transfer P results into increasing both reactive and active losses on the line, and with increasing reactive power transfer Q , both active and reactive losses on the line increase. Furthermore, equations

R

R (3.11) and (3.12) show that for a given transmission power, the losses on the line can be reduced by increasing the operating voltage and hence decreasing the line current RE I .

The above stated deductions provide the basic framework for the engineering practice of long-distance power transmission. Since the line reactance X and the ohmic series resistance R are proportional to the transmission line length, for long lines X and R will take high values which in turn lead to the following constraints in long-distance power transmission: With increasing transmission line length

ISS

REXj

RS

R

Figure 3: Simplified line diagram considering ohmic resistance


(1) the reactive power demand of the line increases, see equation (3.8);

(2) the voltage drop on the line (due to the increasing reactive power demand) may lead to unacceptably low voltages at the receiving end of the line, see equation (3.8);

(3) both reactive and active losses on the line increase, see equations (3.11) and (3.12);

(4) for a given active power transfer , the load angle RP δ may exceed the stability limit of maximum °= 30δ [2].

Reactive compensation of the line can solve problems (1) and (2) but may exhibit the line to resonance phenomena which put the stability of the line at risk. Therefore, transmission lines can only be partially compensated in practice. Problem (3) could be solved by increasing the operating voltage of the line, but this operating voltage step-up results in increased insulation requirements that make the transmission line and the downstream equipment uneconomically expensive. Problem (4) cannot be solved and constitutes a strong restriction in long-distance power transmission that limits the line length depending on the active power to be transferred through the line.

Taking a slightly compensated 3-phase HVAC line as an example with an operating voltage , a line length , a typical reactance per unit length kV 400S =E km 1000=l km15.0 Ω=′X and a

typical series resistance per unit length km027.0 Ω=′R , a more detailed simulation shows that a maximum of can be transferred through this line without exceeding the operational limits of and

MW 300R =P10%kV400R ±=E °= 30maxδ .

3.2 HVDC transmission

LX 0ω= is ineffective in direct current systems with =For the reason that the line reactance ω , only the ohmic series resistance R of the transmission line needs to be considered, see Figure 4.

SE

ISP

RE

RP

R

Figure 4: Simplified line diagram of a transmission line in DC systems


With and being the bus voltages at the sending and the receiving end, respectively, the system can be described according to the following equations

SE ER

IEP SS = , (3.13)

IEP RR = , (3.14)

and

REEI RS −= . (3.15)

From equations (3.13) and (3.14), the active power losses on the transmission line can be derived as follows:

lossP

2RSRSloss )( RIIEEPPP =−=−= , (3.16)

while also taking advantage of equation (3.15).

In the above case, the transport of active power through the transmission line does not require any reactive power or any specific load angle δ . In another way of interpreting, this means that reactive power cannot be transmitted through a direct current operated transmission line.

From equation (3.15) it can be deduced that the voltage drop RS EE − on the line is proportional to the line current I and the ohmic series resistance R . It can be concluded that with increasing transmission line length,

(1) the ohmic series resistance R of the line increases,

(2) the voltage drop on the line increases for a given line current RS EE − I .

The above conclusions constitute the main restrictions in long-distance power transmission by direct current since for a stable system operation the bus voltages and need to be kept within specific limits, i.e. the voltage drop

SE RE

RS EE − may not exceed a specified value. While the line current I is interrelated to the amount of active power to be transferred through the line, problem (2) can be solved by taking into account the maximum allowable voltage drop RS EE − during the design stage of the transmission project: The ohmic series resistance R can be easily controlled by increasing the cross-sectional area of the conductor without putting the economic efficiency of the line at risk. Therefore, HVDC technology is in many cases the most suitable solution for the transmission of bulk power over long distances.

Taking again the above stated transmission line as a monopolar HVDC transmission example with an operating voltage kV 400S =E , a line length km 1000=l and a typical series resistance per unit


length km009.0 Ω=′R , a more detailed simulation shows that a maximum of can be transferred through the same line without exceeding the operational limit of .

MW 1500R =P10%kV400R ±=E 4

3.3 Practical solutions for long-distance power transmission

From sections 0 and 3.2 it has been deduced that with increasing line length and active power to be transmitted (see also [60]), the most feasible application shifts from HVAC to HVDC. As a guideline, Figure 5 shows possible transmission technologies for selected transmission projects up to 1000 MW and 300 km [6]. It clearly indicates that HVAC up to 345 kV represents a feasible solution for the transmission of small amounts of active power over short and medium distances only while the fields of application then shift with increasing power to be transmitted and increasing line length over Voltage Source Converter (VSC)-based HVDC to LCC (Line Commutated Converter)-based HVDC, also known in literature as line-commutated Current Source Converter (CSC)-HVDC. In addition, it can be seen that for some transmission tasks the fields of application of HVAC and HVDC transmission may overlap, and both HVAC and HVDC technologies constitute a technically feasible solution.

Figure 5: Solutions for power transmission (power against distance, reproduced from [6])

4 In this simulation it is assumed that all 3 conductors of the HVAC line are used in parallel operation in order to form one HVDC pole.


For transmission projects of more than 1000 MW over distances longer than 300 km, the use of HVDC transmission technologies (i.e. VSC-based or LCC-based HVDC) constitutes the only technically, environmentally and economically feasible solution.

3.4 Parallel operation of HVAC and HVDC

3.4.1 HVAC and HVDC links in the same right-of-way

Today, transmission system planners are confronted with the problem of transmission capacity increase under strict environmental constraints. The regulatory grant of right-of-way for the construction of new overhead transmission lines is subject to strict regulations. This leads to a denser utilization of existing transmission routes, often resulting in less interspace between conductors than technically recommended.

If an HVDC-operated transmission line runs in parallel with an HVAC-operated one in the same right-of-way, currents of the fundamental frequency will be induced in the DC system. The same particularly applies when only one out of two or more HVAC-operated transmission circuits on a tower is converted to HVDC. The fundamental frequency component on the DC side will generate AC-side harmonics of 0th and 2nd order and higher when transferred through the converter. Especially the 0th order harmonic, i.e. the DC component of the transferred current, will saturate the converter transformer and will in turn reinforce already generated 2nd order harmonics. This phenomenon may put the stability of the HVDC transmission control at risk [3].

It is therefore suggested to evaluate this mutual influence during the planning stage of the construction or conversion process. Countermeasures such as the provision of dedicated filters on the main circuit side are advised if needed [3].

3.4.2 HVDC link within a synchronized HVAC power grid

In addition to the positive contribution of HVDC to the transmission capacity increase of a power grid, which has been described in detail in REALISEGRID Deliverable D1.2.1 ([4]), the wide literature on research in the field of transmission system stability as well as the experience in operation of HVDC transmission lines within a synchronized HVAC power grid have shown that HVDC transmission can be used to influence the dynamic performance of a power system and thereby augment its transient stability.

In particular, with its ability to provide instantaneous and continuous control of both active and reactive power independently of each other, embedded self-commutating5 VSC-HVDC allows for flexible power flow control in real-time, bottle neck mitigations and optimized power sharing between HVDC and HVAC transmission lines. In combination with WAMS, VSC-HVDC enables 5 Also known in literature as self-commutated VSC-HVDC.


effective damping control, dynamic voltage support and prevention of undesired power flow loops [6]. For further information, see REALISEGRID Deliverable D1.2.2 ([5]).

In terms of long-distance power transmission, no negative impacts from the parallel operation of HVDC systems within a synchronized HVAC power grid is known to date (but the number of such installations still stays very limited)..


4 ENVIRONMENTAL AND ECONOMIC CONSIDERATIONS ON LONG-DISTANCE POWER TRANSMISSION

Long-distance transmission naturally comes along with an appreciable environmental impact since a complete undergrounding of long transmission lines is economically unfeasible, although in principle possible in case of HVDC. Long-distance transmission lines are therefore generally overhead lines that need towers and possess a specific visual profile. However, in relation to conventional HVAC transmission, the environmental impact can be reduced to the minimum by the use of HVDC transmission technology due to the lower visual profile of its overhead lines, its reduced non-pulsating EMF emissions, and its feasibility of cabling not only the entire but also parts of the transmission line where necessary for environmental, public, or political reasons [4]. The land use can be reduced by 33-50% when choosing HVDC-operated overhead lines instead of HVAC-operated ones [4]. Especially for these above stated environmental advantages, it is expected that HVDC transmission is more acceptable to the public than HVAC transmission. Table 4.1 recalls the environmental key facts of both HVAC and HVDC transmission as they have been presented in detail in REALISEGRID Deliverable D1.2.1 [4].

Land use System component Voltage level Power rating min Max Unit

HVAC OHL, single circuit 400 kV 1500 MVA 40000 60000 m2/km HVAC underground XLPE cable, single circuit 400 kV 1000 MVA 5000 15000 m2/km

Reactive power compensation unit for HVAC cable line 400 kV 1000 MVA 2000 3000 m2

HVDC OHL, bipolar ±150..±500 kV 350..3000 MW 20000 40000 m2/km HVDC underground cable ±350 kV 1100 MW 5000 10000 m2/km HVDC undersea cable ±350 kV 1100 MW 0 m2/km HVDC VSC terminal, bipolar ±150..±350 kV 350..1000 MW 5000 10000 m2

HVDC CSC terminal, bipolar ±350..±500 kV 1000..3000 MW 30000 40000 m2

Table 4.1: Typical surface occupation for selected HVAC and HVDC transmission system components [4]

Furthermore, the construction of new long-distance transmission lines increases the transmission capacity of the power grid, allows for green power balance, and enables for long-distance power trade. In case remote RES are directly connected to the load centers by dedicated long-distance transmission lines, this power does not need to be transmitted through the main power grid. This relieves the congested situation that predominates the European transmission grid to-date and frees up transmission capacity that now can be used for the local power trade and the exchange of primary reserve in emergency situations. In addition, with long-distance transmission lines, green power can be transmitted from areas with high RES generation and low load to areas with low generation and high load. Thus, the green power trade within entire Europe is promoted.

In terms of investment costs, Table 4.2 recalls the typical cost ranges of HVAC and HVDC transmission equipment as they have been compiled in REALISEGRID Deliverable D1.2.1 [4]. It can be seen that the installation costs for HVDC overhead lines can also be by approximately 40% lower than the costs for the HVAC equivalent. Particularly in long-distance transmission, these cost


savings in the transmission line and the absent need for reactive compensation make up for the higher HVDC station costs. Furthermore, the lower transmission losses of HVDC decrease the annual operational costs [4].

Cost range System component Voltage level Power rating min max Unit

HVAC OHL, single circuit(1) 400 kV 1500 MVA 400 700 kEUR/km HVAC OHL, double circuit(1) 400 kV 2×1500 MVA 500 1000 kEUR/km HVAC underground XLPE cable, single circuit 400 kV 1000 MVA 1000 3000 kEUR/km

HVAC underground XLPE cable, double circuit 400 kV 2×1000 MVA 2000 5000 kEUR/km

Reactive power compensation for HVAC cable line, single circuit 400 kV - 15 15 kEUR/MVAR

HVDC OHL, bipolar(1) ±150..±500 kV 350..3000 MW 300 700 kEUR/km HVDC underground cable pair ±350 kV 1100 MW 1000 2500 kEUR/km HVDC undersea cable pair ±350 kV 1100 MW 1000 2000 kEUR/km HVDC VSC terminal, bipolar ±150..±350 kV 350..1000 MW 60 125 kEUR/MW HVDC CSC terminal, bipolar ±350..±500 kV 1000..3000 MW 75 110 kEUR/MW

(1) cost ranges correspond to the base case, i.e. installation over flat land. For installations over hilly landscape +20% and +50% for installations over mountains or urban areas have to be factored in. Table 4.2: Typical cost ranges for selected HVAC and HVDC transmission system components [4]

From the political point of view, long-distance power transmission provides additional transmission capacity for the power grid which is absolutely necessary to fulfill the political target of increasing the share of RES in the European power generation pattern. The power grid as of today is not able to cope with the large amounts of power which are locally injected into a grid node and which need to be transmitted to the load centers. Furthermore, long-distance transmission lines can only be planned and constructed when supported at political level since usually many European TSOs and countries are involved6. In addition, long-distance lines may cross countries which are not tapped to the line. Local compensations and incentives need to be prepared at the political level. However, long-distance transmission will also be necessary when large amounts of solar power shall be imported from North Africa (as proposed by the Desertec foundation) in order to fulfill the political will to promote energy efficiency, to increase the use of RES, and to decrease CO2 emissions, more precisely, to fulfill the 20/20/20 target7 set by the European Commission on January 23rd, 2008. The very significant costs for the network expansion necessary for importing RES energy from very remote areas could be allocated to the responsible generators. In the latter case, even if extra costs allocated to the generators would in any case be traduced into price bid-ups and, therefore be ultimately paid by the final customer, this would reveal the true costs associated to RES, making

6 A comprehensive discussion on the subject of TSO incentivization and regulation is included in the REALISEGRID Deliverable D3.6.2.

7 20% cut in emissions (respect to 1990 levels), 20% improvement in energy efficiency and 20% energy consumption covered by renewables by the year 2020.


them less competitive with respect to conventional generation. In this framework, this policy could not be convenient in sight of achieving the 20-20-20 targets in terms of RES generation.



5 LONG-DISTANCE TRANSMISSION PROJECTS IN OPERATION TO-DATE

The total installed electrical generation power in the world was 4034 GW in 2006, in which 2752 GW were fueled by fossil resources, 798 GW by hydro power and 377 GW by nuclear power. The average annual growth rate of installed generation capacity in the world was in average 2.8% from 1980 to 2006 [56].

Mainly due to the increased deployment of renewable energy generation, more and more electrical power is generated far away from the energy consumer, which in turn leads to the need of bulk-power transmission over distances of several hundred kilometers or more. For this reason HVDC transmission can be more economically and technically efficient if compared with conventional HVAC transmission. In this context long-distance links refer to overhead lines with a length longer than approx. 500 km.

This Chapter briefly describes existing long-distance HVDC transmission lines8 in North America (mainly United States of America, and also Canada), China, South America, Russia, India, and Africa in order to see the world’s operational experience for a further analysis of the electrical energy transmission sector. Main operational experience is described for the Pacific Intertie, Square Butte, CU, Itaipu, Tianshengqiao-Guangzhou, Guizhou-Guangdong, Three Gorges-Changzhou and Three Gorges-Guangdong HVDC transmission projects. Also, flashover operational experience for the Rihand-Delhi and Gezhouba-Nanqiao projects is presented. As an alternative to HVDC, some EHVAC installations have been carried out over long-distances. One of them connects Russia with Kazakhstan .

8 Even if a transmission line length of more than 500 km is considered, a 473 km line in Russia is also included.


5.1 North America: United States of America and Canada

The total installed power of the electrical energy generators in the United States of America (USA) in 2007 was 995 GW; fossil fueled generators comprised the majority of 764 GW, 100 GW were fueled by nuclear energy and 77.9 GW were generated from hydro power [56]. From 1995 to 1999 only a few new electrical energy generation stations with a total power of some 10 GW were built in the USA. In the period from 2000 to 2002 there was a higher growth with about 144 GW of new power plants installed [22]. From 2003 to 2007 the construction of new generators slowed down [56]. The total generation capacity gain in the USA from 1949 to 2007 was in average 4.9% per year [56].

A significant amount of hydro energy is transferred from hydro-electro generators in the north-western regions of the USA to the south-west consumers; thereby, less power is generated in south-west area from fossil fuel. In this hydro power transmission the first long-distance HVDC project in the USA called Pacific Intertie was included and realized by ASEA and GE for the Los Angeles Department of Water and Power and the Bonneville Power Administration [9].

From coal-powered power plants located in North Dakota the energy is transferred to Minnesota through two HVDC lines – Square Butte and CU9 – built by General Electric for Minnesota Power Cooperative and Minnesota Power and by ASEA for Cooperative Power Association and United Power Association (later Great River Energy) respectively [9].

The second HVDC transmission to Los Angeles is part of Intermountain Power Project. The transmission system from coal-fired electrical generators in Utah was realized by ASEA and is owned by the Intermountain Power Agency and Los Angeles Department of Water and Power [9].

Some amount of hydro power from Quebec region in Canada is transmitted to the USA through HVDC technologies built by ABB and owned by Hydro Québec (Canada) and National Grid USA (formerly New England Electric Systems) [9].

Main technical data of existing long-distance HVDC transmission systems in the USA and between USA and Canada are listed in Table 5.1 (as at commissioning year) and their approximate geographical locations are indicated in Figure 6.

9 The first letters of the tag CU are taken from the names of the first owners: Cooperative Power Authority and United Power Authority


Table 5.1: Long-distance transmission projects in operation to-date in the USA [7] [9]

Project Length1

(in km) Power

(in MW) No. of poles

DC voltage(in kV)

Converter locations2

HVDC Technology

AC voltage3 (in kV/Hz) Year4

Pacific Intertie 1362 1440 2 ±400 Celilo/ Sylmar Mercury 230/230,

60/60 1970

Square Butte 749 500 2 ±250 Young/ Arrowhead CSC - 1977

CU 701 1000 2 ±400 Underwood/ Dickinson CSC 235/350,

60/60 1979

Pacific Intertie 1362 1600 2 ±400 Celilo/ Sylmar Mercury 230/230,

60/60 1982

Pacific Intertie upgrade 1362 2000 2 ±500 Celilo/

Sylmar CSC 230/230, 60/60 1985

Intermountain Power Project 785 1920 2 ±500 Delta/

Adelanto CSC 345/500, 60/60 1986

Pacific Intertie expansion 1362 3100 2 ±500 Celilo/

Sylmar East CSC 230/230, 60/60 1989

Quebec- New England 1500 2250 2 ±450

Radisson/ Nicolet,

Sandy Pond CSC

315/ 230, 345,

60/ 60, 6051990

1Length of DC cable or line; 2Converter station sending end / receiving end; 3AC voltage sending end / receiving end, frequency sending end / receiving end; 4year of commissioning; 5Connections – Radisson to 315 kV 60 Hz, Nicolet to 230 kV 60 Hz, Sandy Pond to 345 kV 60 Hz.

Celilo

Sylmar

Young Arrowhead

Underwood

Dickinson

Adelanto

Delta Sandy Pond

Radisson

Nicolet

Figure 6 Long-distance transmission projects in operation to-date in the USA; geographical map from [59]


5.1.1 Pacific Intertie

The Pacific Intertie connection ensures electrical energy flow from the hydro power generators on the Columbia River in the north-west of the USA to the area around Los Angeles in the south-west coast of the USA. The electrical connection includes three AC lines (500 kV 60 Hz) and one HVDC line with transmission power 4800 and 3100 GW respectively [9].

The northern end (sending end) of the 1362 km HVDC transmission line is connected to the Celilo Converter Station, which is just south of the Dalles Dam little more than 100 km east of Portland, and the southern end (receiving end) is joined to the Sylmar station located on the northern outskirts of Los Angeles [9].

The overhead long-distance bipolar HVDC transmission system was firstly energized in 1970 with rated power 1440 MW and voltage level ±400 kV. In 1971, after one year of operation, the San Fernando earthquake damaged the Sylmar converter station, which was rebuilt in 1973. In 1982 the transmission system was upgraded to 1600 MW and in 1985 the DC voltage level was stepped up to ±500 kV by putting a 100 kV thyristor converter in electrically series connection with the mercury arc converters. This led to a transmission power rise up to 2000 MW. At last in 1989 the rated transmission power of the ±500 kV line was extended to 3100 MW in a way that new 1100 MW converter stations were installed in electrically parallel with the existing stations. At Sylmar it was necessary to build a new terminal, Sylmar East, located few kilometers from the first Sylmar site. In 1994, the mercury arc valves in the Sylmar converter station sustained damage during the Northridge earthquake. In 2004 the Sylmar East station was upgraded and modified to full transmission power of 3100 MW. Because of cost saving, some part of the existing equipment in the Sylmar station that survived former earthquakes was used for Sylmar East construction. In the same year mercury arc power valves in the Celilo converter station were replaced with thyristor valves. After 2004 each power valve in the converter stations at both ends consists of 84 water cooled thyristors [8] [9].

• Main choice reasons: Relatively low energy transmission losses, environmental concerns, network stability [9].

• Operational experience: The reliability statistics from 1999 to 2006 are given in Table 5.2.

Table 5.2 Reliability statistics of the Pacific Intertie HVDC link [42][43][44][45]

1999 2000 2001 Energy availability in % 88.0 88.9 79.5 Forced energy unavailability in %1 2.87 1.70 5.20 Planned energy unavailability in % 8.90 9.40 10.9 1Converter station outages only

5.1.2 Square Butte

The Square Butte overhead HVDC transmission system ensures 500 MW power supply from the Milton R. Young coal-fired power plant situated near Center in North Dakota to Duluth in


Minnesota in the USA. A ±250 kV bipolar line with a total length of 749 km connects sending end converter station Young with receiving end station Arrowhead (near Adolph) and was commissioned in 1977 [9].

• Main choice reasons: Relatively low energy transmission losses, environmental concerns, network stability [9].

• Operational experience: The reliability statistics from 1999 to 2006 is given in Table 5.3; in the mentioned period the major part of equivalent forced outage hours (607.4 of total 846.2) was caused by 24 forced outages in transmission line equipment; in AC and/or auxiliary equipment there were 29 forced outages with 113.6 equivalent forced outage hours, and in DC Equipment 11 outages with 68.6 equivalent forced outage hours; approximately 35% from total outages in period from 1999 to 2006 occurred in other equipment parts and caused little below 7% from total equivalent forced outage hours [42][43][44][45].

Table 5.3 Reliability statistics of Square Butte HVDC link [42][43][44][45]

1999 2000 2001 2002 2003 2004 2005 2006 Energy availability in % 95.9 94.8 80.8 92.8 94.9 85.0 95.0 95.4Forced energy unavailability in %1 0.10 0.38 0.45 0.61 0.09 0.57 0.42 0.12Planned energy unavailability in % 2.37 4.44 18.64 4.19 3.55 14.0 4.09 4.49Quantity of forced outage 17 10 13 10 11 23 11 5Equivalent forced outage hours2 152.2 64.0 52.1 266.8 133.2 90.6 76.7 10.61Converter station outages only; 2the sum of the forced outage hours after the outage duration has been adjusted for the percentage of reduction in capacity due to the outage (at monopolar outage 50 % loss of capacity and at bipolar – 100 %), for example, for an outage of one pole of a bipole system which lasted two hours, the equivalent outage hours would be one hour)

5.1.3 CU

CU is a HVDC overhead long-distance transmission system between Underwood in North Dakota and Dickinson (near Minneapolis) in Minnesota with commissioning in 1979. The project's name "CU" originates from the name of the first owners: Cooperative Power Authority (CPA) and United Power Authority (UPA). The 701 km bipolar ±400 kV DC line ensures 1000 MW power transmission from coal-fired electrical generators of Coal Creek generating station. The sending end is connected to the 235 kV 60 Hz grid, while the receiving end converts DC back into AC at 350 kV, 60 Hz. The power valves in both terminals are built from air-cooled thyristors. CU was one of the first project to employ metallic return during monopolar operation. Transmission system overload capacity: 1375 A continuously, 1500 A for 1 hour or continuously if the temperature is below 8°C [9].

• Main choice reasons: Relatively low energy transmission losses, environmental concerns, network stability [9]


5.1.4 Intermountain Power Project

One part of the Intermountain Power Project is a long-distance 785 km ±500 kV bipolar DC line that ensures the transmission of 1920 MW from the coal-powered Intermountain Power Agency power plant in Utah (sending end in Delta) to the Adelanto converter station near Adelanto in California, which is located in a seismically active area. Suspended thyristor valves are therefore used to achieve maximum security. Extremely stringent requirements were imposed on reliability. Each pole has a 1200 MW continuous and 1600 MW short term overload capacity in order to minimize the impact on the power system in the event of a pole outage. The Delta and Adelanto converter stations are connected to the 345 kV 60 Hz and the 500 kV 60 Hz grid, respectively. An additional upgrade of the transmission system to 2400 MW is in process at time of writing [9].

• Main choice reasons: Relatively low energy transmission losses, environmental concerns;

• Specific project features: Relatively high monopolar overload characteristics [9].


5.1.5 Quebec-New England

Quebec-New England is the first long-distance multi-terminal HVDC transmission in the world. The power is generated in Canada at the La Grande II hydro power station in the James Bay area, converted into DC at the sending end converter station Radisson (Quebec in Canada), and transmitted over the multi-terminal system to load centers in Montreal (Canada) and Boston (USA). The receiving end terminal is Sandy Pond in Ayer (Massachusetts, USA). The total length of the ±450 kV bipolar DC line is 1480 km [9].

In the first construction phase of the project the HVDC transmission system was built and commissioned in 1986. The DC link connects two converter stations – Des Cantons (near Sherbrooke in Quebec) and Comerford (near Monroe, New Hampshire) – each with rated power 690 MW [9].

The second phase includes three additional converter terminals as well as modifications to the existing ones. The line was extended to the North from Des Cantons to the 2250 MW Radisson terminal, located within the La Grande hydroelectric generating complex. Furthermore, the line was also extended to the South from Comerford to a new 1800 MW converter terminal at Sandy Pond in Massachusetts (USA). These extensions were taken into full commercial operation in 1990 [9].

In 1992 another terminal – Nicolet – was put into service on the multi-terminal HVDC system. This terminal has a rated power 2138 MW and is located at Nicolet in the Montreal area in Canada [9].

The Radisson converter station receives the power from the 315 kV 60 Hz grid. At Nicolet station power is delivered to Montreal area via the 230 kV 60 Hz grid and Sandy Pond is connected to 345 kV 60 Hz grid in Boston area [9]

• Main choice reasons: Relatively low energy transmission losses, environmental concerns, connection between two asynchronous networks;

• Specific project features: Multi-terminal HVDC transmission [9].

5.2 South America

Brazil is the biggest electrical power producer among the South American countries. The total electricity generation in 2006 in Brazil was 93 MW, in which 71 GW were produced from hydro power, 14 GW from fossil fuel and 2 GW from nuclear power. The total generated capacity gain in Brazil from 1980 to 2006 was in average 4.0% per year [56].

The significant part of Brazil’s energy is provided by one of the world’s biggest hydroelectric power plant, Itaipu on the Parana River (15 km north of the Friendship Bridge), located on the border between Brazil and Paraguay. Itaipu is a binational project with a power rating of 14 GW


(with 20 generating units providing 700 MW each) at commissioning year 2004. With such an amount of power Itaipu provides approximately 30% of Brazil’s electrical power needs and more than 90% of the needs of neighboring Paraguay. 10 of the 20 Itaipu’s electrical power generators operate at 50 Hz (the standard frequency in Paraguay) and the other 10 generators at 60 Hz (the standard frequency of Brazil). The 60 Hz generators of the Itaipu power plant supply only the Brazilian Interconnected Power System through the Foz do Iguassu substation, where the voltage is stepped-up to 765 kV, 60 Hz. Subsequently, the electrical power is transmitted from there by three HVAC transmission lines to the Sao Paulo area, located 900 km far away from Foz do Iguassu. The South-Southeastern Brazilian electrical power generation is 95% hydraulic [10][12].

Itaipu supplies ranges of 100 to 500 MW from its 50 Hz generators to Paraguay, where the rest of the 50 Hz power is supplied to Brazil through the only one long-distance HVDC transmission line in South America [7][9]. The main technical data of the line are listed in Table 5.4 (as at commissioning year) and the approximate geographical locations of the power converters are indicated in Figure 7.

Table 5.4: Long-distance transmission projects in operation to-date in South America [7][9]

Project Length1 (in km)

Power (in MW)

No. of poles

DC voltage(in kV)


HVDC Technology

AC voltage3 (in kV/Hz) Year4

Itaipu 1 785 1575 1 300 Foz do Iguacu/Sao Roque CSC - 1984

Itaipu 1 785 2383 1 ±300 Foz do Iguacu/Sao Roque CSC - 1985

Itaipu 1 785 3150 2 ±600 Foz do Iguacu/Sao Roque CSC 500/345,

50/60 1986

Itaipu 2 805 3150 2 ±600 Foz do Iguacu/Sao Roque CSC 500/345,

50/60 1987 1Length of DC cable or line; 2Converter station sending end / receiving end; 3AC voltage sending end / receiving end, frequency sending end / receiving end; 4year of commissioning.

Two bipolar ±600 kV DC lines (Itaipu 1 and Itaipu 2) with an overall power rating of 6300 MW were realized by ASEA for Furnas Centrais Eletricas in Rio de Janeiro (an Eletrobras company). Still, at time of writing, it operates at the world’s highest long-distance transmission DC voltage magnitude [10][12], but HVDC transmission systems operating at voltages of ±800 kV are currently under construction by ABB in China and are planned to enter service in 2010.


Foz do Iguacu Sao Roque

Figure 7 Long-distance transmission projects in operation to-date in South America; geographical map from [59]

Both long-distance overhead transmission systems are designed to operate independently of one another under normal conditions. They connect the rectifier station (with input voltage 500 kV, 50 Hz) at Foz do Iguacu in Parana and the inverter station (with output voltage 345 kV, 60 Hz) in Sao Roque near Sao Paulo, in the industrial centre of Brazil. Each line has a rated power of 3150 MW. HVDC electrical power transmission started on a monopolar line in 1984 with a rated voltage of 300 kV DC and in 1985 on a bipolar line with a rated voltage of ±300 kV DC. In 1986 the first and in 1987 the second DC bipolar line ±600 kV (transmission length 785 and 805 km, respectively) were commissioned. The converter stations were commissioned stepwise in order to match the generating capacity built-up at the Itaipu hydropower plant [7][9].

The HVDC converter stations have a current overload capability of 12% of rated power for two hours, temporarily at rated ambient temperature of 40°C or continuously if below 30°C. There is also a five second overload capability of 30%, which ramps down to 12% according to thyristor junction temperature [20].

Four 300 MVAr synchronous compensators are available for fine control of the AC voltage in Sao Roque converter station. In Foz do Iguacu station there is a limit to the total capacity of filters which may be connected, as a function of number of generators, in order to avoid a potential self-excitation situation [10].

• Main choice reasons: relatively low energy transmission losses, environmental concerns, interchange of power from the 50 Hz generators to the 60 Hz system [9];


• Operational experience: In 1985 after about three months of operation, a flashover occurred on the DC pole bushing at Foz do Iguacu due to heavy rain; at the end of the year, also because of the heavy rain, five flashovers occurred at Foz do Iguacu and two flashovers at Sao Roque on both the positive and negative poles for five different bushings; by that time it had become clear that this was a general problem for the 600 kV wall bushings; in 1986 silicone grease was applied to all 600 kV wall bushings as a temporary measure and the experiences from the temporary silicon grease application was excellent, preventing further flashovers in that area; from 1985 several failures occurred in both inverter and rectifier power converter transformers – 6 failures in the first operation year of the transformers, 3 in the second, 2 in the third, 1 in the fourth – and no more failures occurred in the transformers in the following 5 years of the operation; the main fault reason was gassing in oil due to physical properties of the transformers, which were prevented by modification of the transformers; in 1989 a fire broke out in power converter at the Foz do Iguacu substation mainly due to the cooling water leakage.

Twelve-year (1988-2000) outage statistics reported a total of 54 events (either monopolar or bipolar) with a total duration time of 25 hours, or around two outages/year and 1.9 hours/year as average. One should have in mind that these lines crossed a region of high values of extreme wind speed, which has been responsible for several tower breakdowns over the years [10][26].

The reliability statistics from 1999 to 2006 for the bipole 1 and 2 are given in Table 5.5 and Table 5.6; the major part of the equivalent forced outage hours in the time from 1999 to 2000 in Itaipu bipole 1 and bipole 2 are caused by forced outages in AC and/or auxiliary equipment and are 10.4 (by one outage) and 42.5 (by two outages) hours respectively, what is more than half from the total equivalent forced outage hours in mentioned period; in time period from 2005 to 2006 the great part – 116.8 equivalent forced outage hours – was caused by 2 forced outages in transmission line and/or cable equipment in 2005 and 33.8 equivalent forced outage hours was caused by 4 forced outages in AC and/or auxiliary equipment part in 2006 [42][43][44][45].


Table 5.5 Reliability statistics of Itaipu bipole 1 HVDC link [42][43][44][45]

1999 2000 2001 2002 2003 2004 2005 2006 Energy availability in % 97.2 97.7 - - - - 95.4 96.6Forced energy unavailability in %1 0.22 0.05 - - - - 0.18 0.41Planned energy unavailability in % 2.55 2.24 - - - - 3.06 3.00Quantity of forced outage 7 9 - - - - 17 7Equivalent forced outage hours2 19.3 4.2 - - - - 132.2 35.51Converter station outages only; 2the sum of the forced outage hours after the outage duration has been adjusted for the percentage of reduction in capacity due to the outage (at monopolar outage 50 % loss of capacity and at bipolar – 100 %), for example, for an outage of one pole of a bipole system which lasted two hours, the equivalent outage hours would be one hour)

Table 5.6 Reliability statistics of Itaipu bipole 2 HVDC link [42][43][44][45]

1999 2000 2001 2002 2003 2004 2005 2006 Energy availability in % 98.0 97.3 - - - - 97.6 96.5Forced energy unavailability in %1 0.71 0.05 - - - - 0.10 0.14Planned energy unavailability in % 1.28 2.64 - - - - 2.32 3.21Quantity of forced outage 19 6 - - - - 8 8Equivalent forced outage hours2 62.4 4.1 - - - - 8.8 13.11Converter station outages only; 2the sum of the forced outage hours after the outage duration has been adjusted for the percentage of reduction in capacity due to the outage (at monopolar outage 50 % loss of capacity and at bipolar – 100 %, for example, for an outage of one pole of a bipole system which lasted two hours, the equivalent outage hours would be one hour)

• Specific project features: highest voltage level (± 600 kV) among the world’s long-distance HVDC transmission systems in operation at time of writing [7][9].

There is also an ongoing project Rio Madeira whose scheduled commissioning year is 2012. It can become the longest HVDC transmission line in the world with an overhead line length of 2500 km. It is planned to connect two hydro power plants in the Amazonas down (on Madeira River in Porto Velho in the north-west of Brazil) to the Sao Paulo state (Araraquara in the south-east coast of Brazil). 3150 MW HVDC converter stations are to be placed at each end of a new ±600 kV line. An additional HVDC station will be 800 MW back-to-back station that will transmit power to the surrounding AC network in the north-west of Brazil [9].

5.3 India

In the Republic of India in 2006 the installed electrical generation power reached 144 GW, in which 32 GW of generators used hydropower, 102 GW fossil fuel, and nuclear powered generators share 3 GW. The generation power rose with average 5.8% per year between 1980 and 2006 [56].

The electrical power grid in India has developed to regional power systems which are operating asynchronously. The interconnections between regions are made by AC or back-to-back DC links. The traditional HVAC transmission technologies in such interconnections between asynchronous networks can reach their system stability limits and are marked by relatively high energy transmission losses because of bulk power long-distance transmission.

http://www.abb.com/industries/db0003db004333/4d5c52413d94ad11c12574810046e529.aspx


To increase the electrical bulk-power transmission in parallel to the existing AC transmission lines from the thermoelectric generators (mainly coal-based) located in the eastern part of India to the energy consumers in highly urbanized areas the HVDC transmission systems described in the following were constructed.

The first long-distance HVDC transmission project in India – Rihand-Delhi – was built by ABB and Bharat Heavy Electricals Limited and is owned by the National Thermal Power Corporation (after the reorganization of the Indian power sector the transmission system belongs to Power Grid Corporation of India). The same companies realized the second project – Chandrapur-Padghe – for the Maharashtra State Electricity Board. Further Siemens AG developed the East-South Interconnector II project for the Power Grid Corporation of India Ltd. The fourth long-distance HVDC transmission link in India – the Ballia-Bhiwadi project – was built together by Siemens AG and Bharat Heavy Electricals Ltd for the Power Grid Corporation of India Ltd [8][9].

Main technical data of the HVDC long-distance lines in India are listed in Table 5.7 (as at commissioning year) and their approximate geographical locations are indicated in Figure 8.


Table 5.7: Long-distance transmission projects in operation to-date in India [7][8][9]

Project Length1

(in km) Power

(in MW) No. of poles

DC voltage(in kV)


HVDC Technology

AC voltage3

(in kV/Hz) Year4

Rihand-Delhi 814 750 1 500 Rihand/ Dadri CSC 400/400,

50/50 1991

Rihand-Delhi 814 1500 2 ±500 Rihand/ Dadri CSC 400/400,

50/50 1992

Chandrapur-Padghe 752 1500 2 ±500 Chandrapur/

Padghe CSC 400/400, 50/50 1999

East-South Interconnector II 1450 2000 2 ±500 Talcher/

Kolar CSC 400/400, 50/50 2003

East-South Interconnector II upgrade

1450 2500 2 ±500 Talcher/ Kolar CSC 400/400,

50/50 2007

Ballia-Bhiwadi 800 2500 2 ±500 Ballia/ Bhiwadi CSC 400/400,

50/50 2009 1Length of DC cable or line; 2Converter station in sending end / Converter station in receiving end; 3AC voltage in sending end / AC voltage in receiving end, frequency in sending end / frequency in receiving end; 4year of commissioning.

Padghe Chandrapur

Dadri

Rihand Talcher

Kolar

Ballia

Bhiwadi

Figure 8 Long-distance transmission projects in operation to-date in India; geographical map from [59]

5.3.1 Rihand-Delhi

The first HVDC transmission to Delhi – Rihand-Delhi – is also the first commercial HVDC long-distance overhead transmission system in India with a rated power of 1500 MW (bipolar ±500 kV). Energy is transferred from the coal-based thermal power Rihand complex with generation power


3000 MW in Uttar Pradesh in the eastern part of India. The monopolar transmission was commissioned in 1991 (750 MW) and in 1992 the bipolar line was opened for operation. The sending and receiving end converter stations – Rihand and Dadri (outside Delhi) respectively – are connected to the AC grid at 400 kV, 50 Hz and by a 814 km long DC line between them. In electrical parallel with Rihand-Delhi DC link, the energy is also transmitted through the 400 kV, 50 Hz AC transmission systems [7][9].

• Main choice reasons: relatively low energy transmission losses, environmental concerns, network stability and control, and also better overall economy, interconnection between two asynchronous AC regional power networks [9][13];

• Operational experience: since beginning of operation until 1997 there have been flashovers in Dadri power converter station during fog and/or light rain; the countermeasure adopted was to apply silicone grease coatings on practically all DC yard insulators and bushings, after than in interval of one year no more flashovers have occurred in Rihand-Delhi transmission system [32].

5.3.2 Chandrapur-Padghe

The first HVDC transmission system in the area around Mumbai is the Chandrapur-Padghe long-distance overhead bipolar DC transmission system, which was commissioned in 1999. It transmits 1500 MW, ±500 kV DC power from Chandrapur in the eastern part of Maharashtra to Padghe in the western coast area of India. Chandrapur is located in the middle part of India, where thermal power generation is concentrated due to the abundance of coal in that area. On the AC sides the sending and receiving end converter stations – Chandrapur and Padghe respectively – are connected to the 400 kV, 50 Hz grid and have a 752 km DC line between them. In electrical parallel with the HVDC transmission system, there exists a AC transmission tie between Chandrapur and Mumbai, which comprises three 400 kV, 50 Hz lines with a total power around 1200 MW. At Chandrapur as well as at Padghe converter station an 800 MVAr AC filter bank is provided for each converter [7][9][14].

• Main choice reasons: relatively low energy transmission losses, environmental concerns, network stability in Maharashtra region, interconnection between two asynchronous AC regional power networks;

• Specific project features: Active DC filters for an overhead long-distance line transmission has been provided for the first time to further minimize possible disturbances to nearby telecommunication lines [14].

5.3.3 East-South Interconnector II

The East-South Interconnector II is one of the longest existing HVDC transmission systems in the world with a DC line length of 1450 km. The overhead bipolar HVDC line was opened for commercial operation in 2003 with a rated power 2000 MW, ±500 kV for electrical energy


transmission from the Eastern region of India (Orissa province) to West-South India (Karnataka province). The AC voltage levels on both sides of the transmission system in sending end converter station Talcher and in receiving end station Kolar is 400 kV, 50 Hz. Because of the growing demand for energy in the region around Bangalore (Karnataka province) the line was upgraded to a rated power of 2500 MW and was commissioned to operate with increased power in 2007 [7][8] [9].

• Main choice reasons: relatively low energy transmission losses, environmental concerns, interconnection between two asynchronous AC regional power networks [8].

5.3.4 Ballia-Bhiwadi

The HVDC long-distance bipolar overhead transmission system project Ballia-Bhiwadi was commissioned in 2009 to transmit 2500 MW electrical power from the Ballia Power Pool in the East of Uttar Pradesh province (eastern part of India) to the Bhiwadi substation in the province of Rajasthan, about 80 km from Delhi (north region of India). The total length of the ±500 kV DC line is 800 km. Both ends of the DC transmission system (Ballia and Bhiwadi converter stations) are connected to the 400 kV, 50 Hz AC power grid [7][8].

In order to satisfy the maximum reactive power demand of the converters at two hour-overloading and at minimum AC voltages and frequencies, a total of 1904 MVAr and 2054 MVAr AC filters (at 400 kV, 50 Hz) are installed in Ballia and Bhiwadi respectively [33].

• Main choice reasons: relatively low energy transmission losses, environmental concerns, interconnection between two asynchronous AC regional power networks [8].

5.4 China

In the People's Republic of China the total installed power of the electrical energy generators in 2006 was 518 GW, in which more than a half (391 GW) was due to generators using fossil fuel, 117 GW from hydro power and 7 GW from nuclear power generators [56]. The expanded rate of the installed electrical generators in average was about 8.2% per year from 1980s to 2006 [56]. At such rising rate the total generation power in 2030 will be around 1000 GW.

A part of the electrical power generators (mainly hydro power) is located in the South-West region of China. From there the energy is delivered to the East-South coast, a highly industrialized urban area, over a distance close to thousand kilometers. For this bulk-power transmission several HVDC systems were constructed.

The first HVDC long-distance overhead transmission system project in China – Gezhouba-Nanqiao link – was realized by BBC and Siemens PTD and is owned by China National Technical Import & Export Corporation. Several further HVDC transmission projects were accomplished by Siemens


AG such as Tianshengqiao-Guangzhou and Guizhou-Guangdong for State Power South Company and Guizhou-Guangdong II for China Southern Power Grid [7].

For the energy flow from Three Gorges region, ABB developed the following HVDC transmission systems: Three Gorges-Changzhou, Three Gorges-Guangdong, and Three Gorges-Shanghai, which are owned by the China’s State Grid Corporation [8][9]. This is a part of the Three Gorges project planned to be the largest hydropower plant in the world located on the Yangtze River with a generating capacity of 18200 MW from 26 generating units of 700 MW each in 2009 [11]. In 2007 there was a 14.7 GW generation power capacity [17].

Xiangjiaba-Shanghai is an ongoing ±800 kV EHVDC transmission project in China with scheduled commissioning year in 2010. With 2071 km DC line the 6400 MW transmission is planned from the Xiangjiaba hydro power plant, located in the southwest of China, to Shanghai area. It includes two main breakthroughs in the technology of electric power transmission: the system voltage ±800 kV will be the highest DC voltage used (the highest being so far in Itaipu, Brazil: ±600 kV) and the power rating 6400 MW is approximately twice higher than the nominal power of the HVDC transmission in operation to date [9].

Main technical data of the HVDC long-distance lines in China are listed in Table 5.8 (as at commissioning year) and their approximate geographical locations are indicated in Figure 9.

http://www.abb.com/cawp/gad02181/c1256d71001e0037c1256833006cb3a4.aspx


Table 5.8 Long-distance transmission projects in operation to-date in China [7][8][9]


Power (in MW)

No. of poles

DC voltage(in kV)


HVDC Technology

AC voltage3

(in kV/Hz) Year4

Gezhouba-Nanqiao 1000 600 1 500 Gezhouba/ Nanqiao CSC 525/230,

50/50 1989

Gezhouba-Nanqiao 1000 1200 2 ±500 Gezhouba/ Nanqiao CSC 525/230,

50/50 1990

Tianshengqiao-Guangzhou 960 1800 2 ±500 Tianshengqiao/

Beijiao CSC 230/230, 50/50 2001

Three Gorges-Changzhou 860 1500 1 500 Longquan/

Zhengping CSC 500/500, 50/50 2002

Three Gorges-Changzhou 860 3000 2 ±500 Longquan/

Zhengping CSC 500/500, 50/50 2003

Three Gorges - Guangdong 940 3000 2 ±500 Jingzhou/

Huizhou CSC 500/500, 50/50 2004

Guizhou-Guangdong 980 3000 2 ±500 Anshun/

Zhaoqing CSC 525/525, 50/50 2004

Three Gorges - Shanghai 900 3000 2 ±500 Yidu/

Huaxin CSC 500/500, 50/50 2006

Guizhou-Guangdong II 1200 3000 2 ±500 Xingren/

Shenzhen CSC 525/525, 50/50 2007

1Length of DC cable or line; 2Converter station in sending end / Converter station in receiving end; 3AC voltage in sending end / AC voltage in receiving end, frequency in sending end / frequency in receiving end; 4 year of commissioning.

Anshun Xingren

ZhaoqingShenzhen

Gezhouba Nanqiao

Tianshengqiao

Beijiao

ZhengpingLongquan

Huizhou

Jingzhou

Huaxin

Yidu

Figure 9 Long-distance transmission projects in operation to-date in China; geographical map from [59]


5.4.1 Gezhouba-Nanqiao

The Ge-Nan (Gezhouba-Nanqiao) HVDC long-distance bipolar overhead transmission system ensures 1200 MW, ±500 kV electrical power flow from the hydroelectric plant on the Yangtze River in central China to Nanqiao (about 40 km from Shanghai) on the East coast of China for the Shanghai conurbation. Rectifier station Gezhouba in the Hubei Province receives generated power from the 525 kV, 50 Hz grid and the inverter station Nanqiao converts delivered power to AC 230 kV, 50 Hz. The length of the DC line between two converter stations is 1000 km. Commercial operation of the line started in 1989 as monopolar (600 MW, 500 kV) and in 1990 as bipolar transmission. The power switches in the converter stations are thyristors with water cooling system [7][8][9].

• Main choice reasons: relatively low energy transmission losses, network stability and environmental concerns [9];

• Operational experience: from 1989 to 1997 in total 26 flashovers occur in the Ge-Nan transmission system, where the PLC (power line carrier) capacitors contribute with almost 50% of the flashovers; the flashovers have happened in fog and/or rain conditions; the RTV (room-temperature vulcanization) coating was done in 1991; it was very effective for preventing the uneven wetting which caused flashover on the wall bushings; however, it had no effect on the PLC capacitors flashovers; that led to conclusions to change the hardware design sufficient at environment conditions similar to those in Gezhouba and Nanquiao [32].

5.4.2 Tianshengqiao-Guangzhou

The HVDC long-distance bipolar overhead transmission system Tian-Guang (Tianshengqiao-Guangzhou) carries 1800 MW, ±500 kV of electrical power from the hydropower plant Tianshengqiao in South-West China Yunnan Province to the load centre of Guangzhou in the Guangdong Province at the South coast of China. Both converter stations are connected to the respective AC networks through 230 kV, 50 Hz buses. The 960 km DC line connects both power converter stations, where water-cooled thyristors are used. Active DC filters are implemented in this system for absorption of DC harmonics in order to avoid interference on neighboring communication lines. The transmission line has been put into operation by the middle of 2001 and then China Southern Power Grid (CSG) became the first AC & DC hybrid grid in China, where the HVDC system operates in parallel with three AC transmission lines. The DC link helps damping low-frequency oscillations (typically 0.1 to 0.7 Hz for AC power transmission systems) and stabilizing the system [7] [8] [29] [31].

• Main choice reasons: relatively low energy transmission losses, environmental concerns, network stability [8][31].

• Operational experience: The reliability statistics from 2001 to 2007 is given in Table 5.9. After two years of operation in the converter station transformer severe defect in design and manufacture emerged what caused gassing in transformer oil.


In 2003 one of the tuning devices for coupling capacitor of AC PLC filter at Beijiao station was burnt out because of overheating of a resistor; in the same station in 2007 a lightening on DC line caused an arrester firing in a line trap.

Shunt capacitors and filter capacitors units on shunt banks failed frequently usually in summer when environment temperature was high – until 2007 there were totally 71 unit failures at Tianshengqiao and 97 at Beijiao converter station; the main reason was the rated voltage of capacitor had been designed a little lower; to improve voltage stress, one unit was added in series into a string of capacitor units, but the effect was not satisfactory; inrush current was another reason causing capacitor failures; new HVDC project installed a switching synchronization device on every breaker for filter/shunt bank to avoid inrush current; the dielectric film of the capacitor was found not enough to bear the electrical stress and capacitors for new projects are suggested to use three layers of film instead of two layers. In 2007 stainless pipe in power valve cooling system in Tianshengqiao station broke repeatedly at welding joints for several times; one of the reasons was the vibration caused by pumps and motors.

During initial operation stage of the DC line, two highways were built across by Beijiao station; also the local developing industry forced the pollution increasing; in 2005 all post insulators and bushings on DC yard at Beijiao station were painted with RTV. Overheating on DC disconnector took place sometimes if mechanism operation had not been done properly; the problem was the pressed spring in apparatus losing its elasticity. In spring of 2007 the DC voltage measurement link fluctuated frequently, several sensors were found abnormal and replaced [41].

Table 5.9 Reliability statistics of Tian-Guang HVDC link [41]

2001 2002 2003 2004 2005 2006 2007 Energy availability in % 91.99 89.98 93.66 89.23 91.78 93.84 93.78 Forced energy unavailability in % 0.56 0.16 0.17 4.68 2.97 0.47 0.16 Planned energy unavailability in % 7.46 9.86 6.18 6.09 5.25 5.68 6.06 Quantity of monopole forced outage 21 10 8 11 9 13 13 Quantity of bipolar forced outage 1 1 0 0 1 1 3 Quantity of monopole scheduled outage 35 30 32 31 22 16 21 Quantity of bipolar scheduled outage 1 5 3 2 5 1 3

• Specific project features: Active DC filtration installed [8].

5.4.3 Three Gorges-Changzhou

The Three Gorges-Changzhou ±500 kV HVDC overhead long-distance bipolar electrical power transmission system is the first of two links to connect Three Gorges and Eastern China at the time of writing. Converter stations at both ends of the 860 km-long DC line ensure AC/DC bidirectional conversion of the electrical power of 3000 MW from ±500 kV DC to AC 500 kV, 50 Hz and vice versa. The electrical energy is transmitted from the Three Gorges hydropower plant in central China


to the eastern coastal area of Changzhou city. The line came into commercial operation in 2002 as monopolar line with 1500 MW transmission power and in 2003 as bipolar with full power 3000 MW. The sending end converter station is located at Longquan, approximately 50 km from the Three Gorges power plant, and the receiving end station Zhengping is in the city of Changzhou, approximately 80 km northwest of Shanghai. The transmission system has a continuous overload capability of 3480 MW and a 5 second overload capability of 4500 MW. There are three possible connection modes: bipolar, monopolar ground return, and monopolar metallic return mode. In total 90 thyristors per single valve are installed at Longquan and 84 thyristors per single valve at Zhengping converter station. Power switches are air insulated and water cooled [7][9][15][19].

• Main choice reasons: relatively low energy transmission losses, network stability, environmental concerns [9];

• Operational experience: during 2004 several monopolar outages occurred – three times in DC control and protection part because of software bug and false trip from transformer saturation protection, twice in AC and auxiliary equipment part because of transformer guard malfunction and manual fire call point miss-operation, once discharge fault in one of the DC line tower; there were also several monopolar outages in 2005 – twice in DC control and protection part because of optic circuit fault and malfunction of station service power control system, once because of human factor during DC disconnector maintenance work, once in AC and auxiliary part malfunction of converter transformer tap changer pressure release function, four times in DC line because of line icing, all the restart attempts failed [18];

• Specific project features: A part of the line is crossing the Yangtze River near Wuhu in Anhui Province using two 229 m tall pylons with two conductors on each and the span width is 1910 m; the Zhengping converter station is exposed to very heavy industrial pollution, therefore the pole insulators had to be longer than conventional and all high-potential DC equipment and all the DC neutral equipment are installed outdoors [9][27].

5.4.4 Three Gorges-Guangdong

The 3000 MW, ±500 kV HVDC overhead long-distance bipolar transmission system Three Gorges-Guangdong transmits electricity from the Three Gorges hydropower plant in central China to the Guangdong province in the South coastal area of China. The sending and the receiving end of the system are both connected to the 500 kV, 50 Hz grid. The transmission system with a 940 km long DC power line went into commercial operation in 2004. The sending end converter station is located 16 km from Jingzhou city near the Three Gorges power plant, and the receiving converter station in Huizhou near Guangzhou. The overload characteristics and power switch performance and also possible connection modes are practically equal to the Three Gorges-Changzhou project [7][9][18].

• Main choice reasons: relatively low energy transmission losses, connection between two asynchronous AC networks, network stability, and environmental concerns [9][31];


• Operational experience: in 2004 one bipolar outage occurred in DC control and protection part first in one pole and later in both because of LAN network disturbance, in the same year several monopolar outages occurred – once in AC and auxiliary equipment part caused by malfunction of on-load tap changer gas relay, flashover fault due to icing in DC line, other flashover over DC voltage divider in primary DC equipment, one human error and DC line fault due to hill fire; in 2005 one bipolar outage occurred in AC and auxiliary equipment first in one pole and later in both because of malfunction leading to loss of station power supply; in the same year four faults occurred in DC line because of line icing all the restart attempts failed, malfunction caused by inconsistent status in DC control and protection part [18].

5.4.5 Guizhou-Guangdong

The Gui-Guang (Guizhou-Guangdong) project is HVDC long-distance bipolar overhead transmission system developed in 2004 and allows the transmission of 3000 MW of electrical power from the Anshun converter station in Guizhou Province in South-West China, where the hydro and thermo-electro plants are located, to the Zhaoqing converter station in Guangdong Province near the load center of Guangzhou. It is a bipolar ±500 kV DC transmission system (1500 MW per pole) with a total DC line length of 980 km. The Gui-Guang system is electrically parallel to the other HVDC transmission tie Tian-Guang to integrate large AC transmission systems. The thyristors in both converter stations are water-cooled and AC voltage connection at both ends is 525 kV, 50 Hz [7][8].

Due to the long transmission distance (about 1000 km) the AC system in the mentioned transmission area experiences severe power oscillations after faults, close to stability limits. Due to its ability to damp these power system oscillations, the DC transmission system helps to increase the overall system reliability [28][30].

• Main choice reasons: relatively low energy transmission losses, environmental concerns, network stability [8][28][30].

• Operational experience: The reliability statistics from 2001 to 2007 is given in Table 5.10.

The very low energy availability in 2005 was mainly due to several explosion of the coupling capacitors for DC PLC filter, all took place in rainy or fog weather; research showed that the uneven voltage distribution on capacitor sections was the main reason; inside the porcelain capacitor column, there are no resistors to even the voltage distributions among elements; finally, external resistors have been added in parallel with porcelain capacitor sections; this method ensures voltage distribution evenly among sections and has avoided further explosions.

Oil leakage happened on some of units of shunt and filter capacitors banks mainly caused by mechanical damage in transportation from factory to converter station (explanation from supplier).


Light triggered thyristor used for Gui-Guang link was first introduced into China; in first half year of operation totally 32 of thyristors failed.

As the industry developed somewhere near the line path, the pollution becomes heavy so that flashover of insulator takes place in spring; to improve the insulation level, additional porcelain or glass insulators were added into the string; composite insulators replaced porcelain insulators.

From 2005 to 2006 there were 4 times of signal transmission faults caused by the problem in data verification. In humidity season of 2005 it was found that DC voltage measurement fluctuated frequently, several sensors were found abnormal and replaced [41].

Table 5.10 Reliability statistics of Gui-Guang HVDC link [41]

2004 2005 2006 2007 Energy availability in % 96.07 92.53 96.32 96.67 Forced energy unavailability in % - 3.22 0.10 0.81 Planned energy unavailability in % - 4.25 3.58 2.52 Quantity of monopole forced outage 6 11 10 9 Quantity of bipolar forced outage 0 0 1 1 Quantity of monopole scheduled outage 12 9 4 3 Quantity of bipolar scheduled outage - 2 3 3

5.4.6 Three Gorges-Shanghai

The Three Gorges – Shanghai is the second of two 3000 MW HVDC power links to connect Three Gorges in central China and Eastern China, by a long-distance bipolar overhead HVDC power transmission system, and delivers electrical energy to the coastal city of Shanghai. Both converter stations are connected to the 500 kV, 50 Hz grid, where sending end station is located at Yidu, approximately 58 km from the Three Gorges power plant, and the receiving end station, Huaxin, at the western outskirts of Shanghai. The receiving station will feed power directly into Shanghai. The ±500 kV DC transmission system with a 900 km DC line length was commissioned in 2006 and from that time the amount of electrical power delivered from central China to the Shanghai area raised from 4200 MW to 7200 MW. The transmission is designed to transmit full rated power up to specified maximum ambient temperature and without any redundant cooling in service. With redundant cooling in service, a continuous overload of 105% and two hour overload of 113% of rated power is achievable. With lower ambient temperature, the overload can be even higher i.e. for an ambient of 20°C, a continuous overload of 115% and 2 hour overload of 131% of rated power is achievable. The power switch performance and also possible connection modes are practically equal to the Three Gorges-Changzhou project [7][9][16].

• Main choice reasons: relatively low energy transmission losses, network stability, environmental concerns [9].


5.4.7 Guizhou-Guangdong II

The HVDC long-distance bipolar overhead transmission system developed under Gui-Guang II (Guizhou-Guangdong II) project is located in the South-East part of the China. The DC line distance is 1200 km and connects the converter station Xingren in the region of the hydro and thermo-electro plants in Guizhou Province in the South-West China with the converter station in Shenzen – city in the Guangdong Province on the South coast of China. The bipolar ±500 kV DC overhead line ensures 3000 MW electrical power transmission and has a long-term overload capability of up to 115%. Power transmission in the reverse direction is also possible. The AC voltage at both ends of the system is 525 kV, 50 Hz and AC filters in sending end have 140 MVAr reactive power and in receiving end 155 MVAr. Power switches are thyristors with water cooling. The line was inaugurated for operation in 2007, what is the latest realized long distance HVDC project in China so far [7][8][31].

• Main choice reasons: relatively low energy transmission losses, environmental concerns, network stability [8][31].

5.5 Russia

In 2006 the total installed power of electrical generators in the Russian Federation was 218 GW, in which 149 GW is ensured using fossil fuel and 46 GW hydro power, and 23 GW from nuclear power [56].

Part of the hydro power is generated in the largest hydroelectric station in Europe – Volga Hydroelectric Station or Volga GES – which is located on the last of the Volga-Kama Cascade dams before the Volga River flows into the Caspian Sea. The system was commissioned in 1955. At time of writing the Volga GES has 2551 GW generation power [25].

The power generated by the hydroelectric station is transmitted primarily via 220 kV AC transmission to the city of Volgograd (formerly called Tsaritsyn and Stalingrad), which is located on the western bank of the Volga River. The other part of Volga GES energy is transmitted to Moscow (via AC 500 kV transmission) and to Donbass (Donets Basin in Ukraine) via the only long-distance HVDC line in Russia in operation today called Volgograd-Donbass [24].

The main technical data of the line are listed in Table 5.11 and the approximate geographical locations of the power converters are indicated in Figure 10.


Table 5.11 Long-distance transmission projects in operation to-date in Russia [7]


Power (in MW)

No. of poles

DC voltage(in kV)


HVDC Technology

AC voltage3

(in kV/Hz) Year4

Volgograd-Donbass 475 750 2 ±400 Volzhskaya/

Mikhailovskaya CSC 220/220, 50/50

1962/ 1965

1Length of DC cable or line; 2Converter station in sending end / Converter station in receiving end; 3AC voltage in sending end / AC voltage in receiving end, frequency in sending end / frequency in receiving end; 4 year of commissioning.

Volzhskaya

Mikhailovskaya

Figure 10 Long-distance transmission projects in operation to-date in Russia; geographical map from [59]

The Volgograd-Donbass project was realized by the Ministry for Electrotechnical Industry of USSR for power companies Volgogradenergo and Donbassenergo with commissioning in 1962 and 1965. Both converter stations – Volzhskaya (at Volga Hydroelectric Station) and in Donbass called Mikhailovskaya – ensure the 475 km DC line connection between the two 220 kV, 50 Hz power grids. The ±400 kV DC bipolar overhead power transmission system with a rated power of 750 MW has an overload capacity of 15% continuously. Earth return is possible with only one pole in service. Depending on hydro power available in the Volga River and power demand in each end of the transmission system the energy flow can be reversed. In the first operation period, mercury power valves were used. Later, power switches in the converter stations were replaced by thyristor type valves. At present, the transmission system operates at only 25% of its design capacity with a DC voltage level of 100 kV [7][24].

• Main choice reasons: relatively low energy transmission losses, environmental concerns, connection between two asynchronous AC networks [24].

An alternative to the long-distance HVDC transmission system is the EHVAC technology with an extra high AC voltage level, which is higher than in traditional AC transmission systems, to decrease the energy losses on a long-distance AC line.

The first 1150 kV EHVAC transmission system in the world was commissioned in 1985 in the Republic of Kazakhstan. The length of the AC line is 494 km ensuring connection between power substations in Ekibastuz and Kokshetaus. Other links were the one in 1988 between Kokshetau and Kostanay with a 1150 kV AC line length of 396 km and a 697 km line between Ekibastuz and Barnaul. The further expansion of the transmission system was built in 1995 between the power


substations in Barnaul and Itat with a line length of 448 km, where the AC line, autotransformer and reactor equipment are installed for a nominal voltage of 1150 kV, while the other part of the electrical equipment is designed for a nominal voltage level of 500 kV. In the 1990s, the power substation in Chelyabinsk was included in the transmission system, linked with a 1150 kV AC line to Kostanay. In total the 1150 kV AC line connects the cities in the following sequence: Itat, Barnaul, Ekibastuz, Kokshetau, Kostanay and Chelyabinsk, but step-down transformer substations were installed only on Kazakhstan territory. Therefore, the energy transmission at voltage level 1150 kV is possible between Ekibastuz, Kokshetau and Kostanay, but the other part of the system operates at 500 kV. From 1996 until 2005, the whole 1150 kV line operated with a voltage level of 500 kV, mainly because of a significant lowering of required power capacities [54].

The main purpose of the line was to strengthen the electrical transmission grid and to ensure the 5 GW power transfer from the thermo-electro plants in Ekibastuz area and hydro-electro stations in the Siberian Federal District to the Urals Federal District [54].

Without specific extra high voltage level equipment, there are many different specific operation properties of the EHVAC line, like relatively high corona discharge losses, radio disturbances and the like. It stands to reason that the EHVAC transmission technologies are relatively expensive, like many of new technologies (also the HVDC) in research process.

5.6 Africa

The total electricity generation in Africa in 2006 was 110 GW, in which 22 GW of generators used hydro-power, 2 GW were powered by nuclear energy and the majority of 86 GW was powered by fossil fuels. The total gain of the generation capacity from 1980 to 2006 in Africa composes in average 3.6% per year [56].

For bulk-power transmission there are three existing long-distance overhead HVDC transmission systems in Africa and all of them are located in the Central, Eastern and Southern regions of Africa. The first long-distance HVDC transmission project in Africa, Cahora-Bassa, was developed by BBC, AEG-Telefunken and Siemens AG for the Eskom South African electricity public utility and Hidroelectrica de Cahora Bassa (owned by Mozambique and Portugal). The next HVDC long-distance transmission project, Inga-Shaba, was realized by Morrison-Knudson Co., as main contractor with ASEA and GE and is owned by the SNEL, the national electricity company of the Democratic Republic of the Congo. Developed by ABB and owned by the Namibian transmission system operator, NamPower, the newest project that will start operation in 2010 is called the Caprivi Link [9].

Main technical data of the HVDC long-distance lines in Africa are listed in Table 5.12 (as at commissioning year) and their approximate geographical locations are indicated in the Figure 11.


Table 5.12 Long-distance transmission projects in operation to-date in Africa [7][8][9]


Power (in MW)

No. of poles

DC voltage(in kV)


HVDC AC voltage3 Technology (in kV/Hz) Year4

Songo/ 220/275, 1975, 1998 1456 1920 2 ±533 CSC Cahora-Bassa Apollo 50/50

Kolwezi/ 220/220, 1700 560 2 ±500 CSC 1982 Inga-Shaba Inga 50/50 Zambezi/ 330/400, 970 300 2 350 VSC 2010 Caprivi Link Gerus5 650/50

1Length of DC cable or line; 2Converter station in sending end / Converter station in receiving end; 3AC voltage in sending end / AC voltage in receiving end, frequency in sending end / frequency in receiving end; 4 year of commissioning; 5Not determined sending and receiving end; Zambezi converter station / Gerus converter station. 6

Songo

Apollo

Gerus

Zambezi

Inga

Kolwezi

Figure 11 Long-distance transmission projects in operation to-date in Africa; geographical map from [59]

5.6.1 Cahora-Bassa

The Cahora-Bassa long-distance overhead HVDC transmission system is used to transmit 1920 MW electrical power from a hydroelectric plant on the Zambezi River in the northern part of the Republic of Mozambique to the Republic of South Africa. The 1456 km long ±533 kV DC line project was commissioned in 1975. The system includes two converter stations: Songo connected to the 220 kV, 50 Hz grid in Mozambique and Apollo connected to the 275 kV, 50 Hz grid in South Africa. Cahora Bassa is the first HVDC project that used thyristor valves, which were outdoor located, oil-cooled and oil-insulated designed [7][8].


During the civil war in 1980s the transmission line was heavily damaged and the system was down. Besides the restoration of the main equipment, the complete DC control was exchanged by a fully digital, computerized system including a modern Human-Machine Interface. The new system increases the availability and reliability of the HVDC transmission system and the most powerful DC line in Africa has been back in operation since 1998. The upgraded Apollo substation contributes to an expansion in transmission capacity of the HVDC transmission system in the future from 1920 MW to a planned 2500 MW and was completed in 2008. The upgrade significantly increases the availability and reliability of the system and reduces the maintenance of the Apollo station. Air-insulated outdoor thyristor valves have replaced the old oil-insulated valves and are mounted on the existing support insulators for the old valves. The total reactive power generation per AC filter has been increased from 167 MVAr (old AC filters) to 300 MVAr [9].

• Main choice reasons: relatively low energy transmission losses, environmental concerns [7][8][9].

5.6.2 Inga-Shaba

The second longest HVDC overhead bipolar electric power transmission system in the world – Inga-Shaba – with a DC line length of 1700 km is located in the Democratic Republic of Congo (DR Congo). It allows to transmit 560 MW electrical power from the Inga hydroelectric complex at the mouth of the Congo River (western part of DR Congo) to mineral fields in Katanga province – prior Shaba (eastern part of DR Congo). The DC voltage level is ±500 kV and the AC voltage at both ends of the transmission system – at Inga and Kolwezi converter stations – has a value of 220 kV, 50 Hz. The line was commissioned in 1982 [7][9].

The power valves in both converter substations are equipped with six double-valves of air-cooled design, where each single valve has 258 series connected thyristors [9].

• Main choice reasons: relatively low energy transmission losses, environmental concerns;

• Specific project features: because of the extreme line length and the difficult logistics along the route, it was decided to build two monopolar lines; the converter stations were built so that the two converter poles can be operated in parallel with ground return, in case of a monopolar line outage [9].

5.6.3 Caprivi Link

The Caprivi project – HVDC overhead long-distance transmission system – is constructed in Namibia and will start operation in 2010. At present there are 300 MW power connection between the Zambezi converter station in the Caprivi strip in eastern part of Namibia, close to the border of Zambia, and the Gerus converter station, about 300 km North of Windhoek in the middle of Namibia. The AC voltages are 330 kV and 400 kV at Zambezi and Gerus converter stations respectively, which are interconnected by a 970 km long 350 kV DC line. The electrical power


transmission is possible in both directions [7][9]. The Caprivi Link will be the longest overhead line based on the modern VSC-HVDC technology.

• Main choice reasons: relatively low energy transmission losses, environmental concerns, two relatively weak AC networks stability, enable power trading in the expansive region of southern Africa.

5.7 Lessons learnt from the operational experience described

From the above description of existing HVDC transmission systems, the following deductions that are of special interest for TSOs can be derived: HVDC transmission technologies are essential in efficient long-distance electrical bulk-power transmission. Its low losses and its smaller environmental impact made it the preferred choice over conventional HVAC transmission. In addition, the experiences gained from the operation of HVDC transmission projects for years and the reliability records presented underline the feasibility and reliability of HVDC for long-distance transmission.

HVDC transmission can therefore be a feasible option for the necessary upgrade and expansion of the European transmission grid in order to ensure a stable integration of RES in the future.


6 FUTURE TRENDS OF LONG-DISTANCE TRANSMISSION WITHIN EUROPE

The combination of increasing demand for electrical power and strict environmental regulations entail that more and more energy generation from remote hydro, solar and wind plants is under consideration. Renewable generators are naturally built in areas, where the renewable resource, such as wind and solar, has a high potential and is easily available. Generally, this is far away from densely urbanized areas and the main energy consumers. A considerable number of these resources is located several hundred kilometers from the load areas. Considering the future renewable energy supply in Europe, there is a notable potential of solar radiation from North Africa and the Middle East as well as an important potential of offshore and onshore wind power from Northern Europe. In order to increase the share of renewable energy sources within the European generation pattern, efficient electrical power transmission systems are needed.

6.1 Green energy power balance

Europe has a relatively high amount of wind power resources, especially in the northern part of the continent, which can cover a significant share of the electrical power demand. That has been already successfully realized in many European countries, for example, Germany with 23.9 GW, Spain with 16.7 GW, and Denmark with 3.2 GW of installed wind power in 2008 [46]. The highest onshore and offshore wind potential is in the northern and north-western part of Europe and has an average value from 350 to more than 1500 W/m2 in offshore regions (mainly depending on the height above sea level) and from 150 to more than 1800 W/m2 in onshore regions, where the highest energy density is found in the hill areas [48]. The EU market for onshore wind grew by an average of 32% per year in the 12-year period from 1992 to 2004 [47]. Therefore, wind power experienced the highest growth in the European electricity generation, more than any other technology in 2008 [46]. In Europe, the total installed wind power was 57.1 GW in 2007 and reached 65.9 GW in 2008.

The main characteristic of wind power is its variable nature: the power output of wind generators depends on the weather and is therefore fluctuating. This makes transmission system operation difficult, especially in times when the generation of electrical power from RES is high while the local power consumption is low. In this case, the RES power surplus shall be transmitted to areas where it is needed and where it can replace power generation from conventional resources. Also, the maximum share of wind power within a certain area depends on the power balance in this area, taking also into account the minimum power demand and the exchange possibilities with neighboring zones (e.g. between countries). Furthermore, the activation of reserve power to compensate for wind power fluctuations and its efficient allocation throughout Europe need to be considered: since power generation and consumption have to be balanced in order to guarantee a stable grid operation, energy storages and/or conventional power plants need to provide reserve power and therefore will still have a role to play within a RES-dominated generation pattern. The accumulation of wind power in battery electric vehicles and/or in hydro reservoirs must also be considered as leading factors to provide reserve power, to maximize the electricity generation from wind power and to increase the wind power share in the total energy system.


In [50], the wide-area interconnection of wind power plants through a transmission grid is estimated as a simple and effective way of reducing swings in wind power delivery caused by wind fluctuations. As more wind farms are mutually interconnected in cluster, the probability that all sites experience the same wind regime at the same time decreases. The cluster consequently behaves more and more similarly to a single farm with steady wind speed and thus steady deliverable wind power. It was found that an average of 33% and a maximum of 47% of yearly averaged wind power from interconnected farms can be used as a reliable base-load power generator. Equally significant, interconnecting multiple wind farms to a common point and then connecting that point to a far-away city can allow the long-distance portion of transmission capacity to be reduced, for example, by 20% with only a 1.6% loss of energy.

Efficient energy transmission over long distances is possible using HVDC technologies (described in section 3.2). HVDC transmission can lead to increase the wind powered electrical generation share in Europe by supplying the wind power from the place where wind is blowing to the consumers around Europe, even between asynchronous AC networks with or without different frequencies.

There are many plans of new wind parks, mainly focused on the high wind energy potential in offshore sites. Over 100 GW of offshore wind projects in European waters are already in various stages of planning [48].

One of the ongoing EU funded project at the time of writing is OffshoreGrid, which is a techno-economic study within the Intelligent Energy Europe programme. It will develop a scientifically-based view on an offshore grid in Northern Europe along with a suited regulatory framework considering technical, economic, policy and regulatory aspects. The project is targeted for European policy makers, industry, transmission system operators and regulators. The geographical scope is, first, the regions around the Baltic and North Sea, the English Channel and the Irish Sea. In a second phase, the results will be applied to the Mediterranean region in qualitative terms [51].

High political support is devoted to a European supergrid idea which includes long term develop-ment of an international electricity grid including new wind (mainly offshore), and also solar power plants in and around Europe. In developing the European supergrid, it has been suggested that the approach should focus on modular development, with particular attention being paid to the regional projects like Kriegers Flak, the North Sea Offshore Grid and the Mediterranean Energy Ring pro-ject [58]. Such an increase in scale in presence of large amounts of variable RES generation will require a clear and harmonized regulatory framework and innovations in system planning, opera-tions and also market issues. It will also require reinforcements of existing high voltage networks onshore. Transmission assets will hence continue to play an ever increasing role to connect remote RES to customers and to manage the load-supply balance, along with a more efficient and smarter use of energy by end consumers [62].

A large quantity of offshore wind power generators are being considered in the Kriegers Flak pro-ject, concerning the Baltic Sea area. Spread out over the German, Swedish and Danish parts of Kriegers Flak a total of 1600 MW future wind power generation capacity has been assumed in the literature [57] where the classical solution with the offshore power plants connected nationally is compared to solutions where the grid connection of the offshore wind power plants would also function as an interconnector between Germany, Sweden and Denmark. In this combined solution


at Kriegers Flak it is planned to deliver wind power to the European consumers, strengthen the en-ergy markets and increase the security of supply by providing international transmission capac-ity [57]. It must be mentioned that at the beginning of 2010 the Swedish TSO has informed that it will withdraw from the project, mainly due to uncertainties concerning the construction of wind turbines in the Swedish zone of Kriegers Flak during the near future [62].

The North Sea Offshore Grid is another project included in the European supergrid. The relatively high wind energy density area – North Sea – is chosen for the offshore wind power plants. Up to 70 GW of new offshore wind generation is planned to supply the Europe’s wind resources to the northwest and middle part of Europe by international networks in Ireland, the UK, Norway, Germany, Denmark, and the Benelux countries [58]. During high wind speed in the North Sea area and at low energy demand, the wind power could be stored in hydro reservoirs located in the North Europe countries mainly in order to balance wind power fluctuations.

Significant increase of the renewable energy generation and share in Europe could be achieved by the Mediterranean Energy Ring project with the idea of the electrical connection around the Mediterranean Sea ensuring energy import in Europe from the Middle East and Sub-Saharan Africa [58]. Developing the ring and interconnection across the sea would enable mutual support, a common Europe and Mediterranean energy market, and exploitation of the regions vast solar and wind energy potential [62].

From the offshore wind experience in literature [52], the future generation of offshore wind farms, being mostly of larger capacity, will be connected into and regulated by the transmission rather than distribution systems.

6.2 Green energy power import

6.2.1 Desertec Foundation

The Desertec Foundation was created in 2009 by the German association Club of Rome and the Trans-Mediterranean Renewable Energy Cooperation (TREC) network. Aim of the foundation is to serve as a hub for realizing the Desertec concept, which describes the perspective of forming a sustainable supply with electrical power from renewable energy resources located in the Mediterranean EUMENA (Southern Europe, Middle East, and North Africa) regions until 2050. The main RES is decided to be solar radiation from the deserts in North Africa and the Middle East, where the majority of electrical power will be generated by concentrating solar thermal power (CSP) technologies [34].

In the CSP plant, the solar radiation is concentrated by a receiver in order to heat up the working fluid. This heated working fluid is used in a thermoelectric generator to produce electrical power. Solar heat can be stored in molten salt tanks and used for the thermoelectric generation of electrical power during the nighttime. However, the thermoelectric generators can also be powered by conventional fuel, such as fossil fuels, biomass, and others [37].


The planned transmission system provides the connection of the electrical energy generators from 20 to 40 different locations in the Middle East and North Africa to the main consumers in Europe (see Figure 12), where the energy transmission distance can be around 3000 km (or even higher). The power rating of this network is planned to be 100 GW until 2050 [36].

Figure 12: Power transmission network as proposed by Desertec [35]

[36]In , there are three electrical energy transfer options described: via hydrogen, through HVAC, and through HVDC. Using hydrogen as electrical energy carrier requires two energy conversions between two types of energy: at the sending end of the transmission line electrical energy is transformed to chemical energy (hydrogen) and vice versa at the receiving end. This conversion together with the hydrogen transport and storage processes would lead to about 75% of losses of energy in transmission system over a distance of 3000 km. The energy transfer with conventional HVAC over the mentioned distance also has relatively high losses: about 45%. By contrast, only some 10% of the generated electricity will be lost using HVDC transmission from Middle East and North Africa to Europe over 3000 km distance.

Therefore, three possible future HVDC power transmission lines for the main energy transfer are described in [36]. Additional interconnections between the western part of Algeria and Germany,


the southern part of Libya and Italy, and the central region of Egypt and Poland (see in the following for further details) are proposed for the purpose of exporting solar power from CSP in the desert areas of North Africa to Europe. The areas for electricity generation from solar radiation were simply found by looking at the solar irradiance map of the Middle East and North Africa regions and selecting three sites with major solar irradiance potentials of over 2800 kWh/m²/y mentioned above. The distribution ends of the power transmission lines are placed in the major centers of electricity demand in central part of Europe, in order to effectively backing and taking off some load from the main existing mainly HVAC transmission grids and effectively using local transmission and distribution grids for the further transfer of imported electrical power. The major centers of electricity demand in Europe were found looking at the density of the electricity grid, the people population density and the nightly light emission concentration as major indicators for electricity demand. The Ruhr area in Germany, London in the UK, Milan in Italy and Warszawa in Poland were selected as possible headers of the HVDC links coming from North Africa [Ref.].

The first HVDC power transmission line path begins in sending end converter station in Algeria and leads through Morocco, Spain, France, Belgium until receiving end substation in Germany. The total overhead line length is calculated 3099 km plus 18 km submarine cable in the Strait of Gibraltar [36]. The second HVDC power transmission line with sending end converter station in Libya transfers energy through Tunisia, Sardinia, Corsica, Italy (continent) with the total overhead line length of 2735 km and 373 km submarine cable [36]. The third HVDC power transmission line is planned to be installed through east coast continental part of the Mediterranean Sea. With the sending end station in Egypt the energy flows as follows through Israel, Jordan, Syria, Turkey, Bulgaria, Romania, Hungary, Austria, Slovakia, Czech Republic, Poland with the total overhead line length 5113 km and 30 km submarine cable [36].

The benefits of such a large grid interconnection described in this subchapter are the gain of additional reserve capacity and the compensation of local power and plant outages respectively. Within a large grid there is a higher utilization of the single power plants, and the electricity exchange over the national borders is easier [36].

There are indications in the literature [38] that 90% of the present world people population could be supplied by necessary electrical power only by electrical power generated by CSP in potential desert areas in a distance not longer than 3000 km through HVDC transmission technologies. Less than 0.4 and 2.8% of the electricity potential of worldwide potential CSP areas would be required for electric and non-electric energy needs, respectively, on the today’s European energy consumption level. CSP potential areas were assumed by a direct normal irradiance greater than 2000 kWh/m²/y with territory larger than 9000 km².

6.2.2 OffshoreGrid initiative

OffshoreGrid is a techno-economic study initiated by the European Commission within the Intelligent Energy Europe programme [51]. The target of this study is to develop the fundamentals for an offshore power grid that comprises the regions around the Baltic and North Sea, the English Channel and the Irish Sea. For this purpose, OffshoreGrid builds on information initially gathered by 3E for Greenpeace and for the Belgian Federal Public Service for Economy, which has been complemented with inputs from the German Energy Agency (dena). Already proposed power grid


configurations, such as e.g. Czisch’s SuperGrid [Ref.], the Eumena / Desertec backbone grid, and the Power Cluster are considered, see Figure 13. The findings were updated and extended by the research and the inputs of offshore grid experts [53].

(a) (b)

(c)

Figure 13: Selected power grid configurations under consideration by OffshoreGrid [53]

While a number of the proposed network configurations under consideration by the OffshoreGrid initiative already form a complete overlay power grid and therefore do not rely on transmission capacities provided by the European ENTSO-E transmission network, offshore network configurations such as presented in Figure 13 (c) have a significant impact on the European ENTSO-E transmission system since they rely on the ENTSO-E transmission network to distribute


the power that has been generated offshore and fed into the onshore connection points. In particular, electrical power fed into the connections points CS15, CS16, CS17, and CS18 needs to be transmitted from the coast areas to the load centers. This requires the construction of additional transmission lines since the transmission capacity of the existing power grid within this area has already reached the upper limit.

6.3 Proposals for long-distance transmission in Europe

Especially grid configurations as shown in Figure 13(c) in section 6.2.2 result into an increased need for transmission capacity within the ENTSO-E transmission network. Therefore, the construction of new transmission lines becomes necessary. A proposed power grid extension [55] is shown in Figure 14: three HVDC lines pick up the power fed into the connections points located in coast of Germany and transmit it to the load centers in Central and Southern Germany, i.e. the Ruhr-Basin, the Rhine-Main-Basin, and the city of Stuttgart [55].


Figure 14: HVDC power grid extension with three direct connections [55]

Furthermore, on a north European scale, HVDC interconnections between Norway with its large capacities of hydro-pumped storages and wind farms located in Northern Europe could help to exploit hydro-pumped storages as a compensation for wind power fluctuations and therefore – from the economic point of view – to exploit and promote arbitrage transactions between complementary markets.

However, while the above presented example constitutes only a local solution for Germany and the North Sea Region respectively, and while similar solutions need to be developed for the Netherlands, Belgium, and France, the construction of multinational long-distance transmission lines or a multinational HVDC overlay grid seems to be a more viable and more comprehensive solution. On this background, the HVDC overlay grid presented by the Desertec Foundation already constitutes the most promising transmission infrastructure. Also, the future development of the OffshoreGrid initiative, the Kriegers Flak project and the North Sea Offshore Grid should be followed.


However, the opportunity of big investments in supergrids and transmission highways in Europe should be always carefully evaluated on the basis of costs and benefits, so as to achieve a clear view and to co-evaluate these projects in perspective with less costly initiatives (like reinforcing the existing network punctually tackling the most limiting bottlenecks). From this point of view, the cost-benefit analysis proposed in the REALISEGRID Deliverable D3.3.1 ([61]) appears as an important decision tool in the hands of the European Commission and of the European TSOs.


7 CONCLUSIONS

From the technical point of view, HVDC transmission has proven its advantages over conventional HVAC transmission when it comes to the transmission of bulk power over long distances:

• no need for reactive compensation along the transmission line and therefore practically no limitation in line length,

• lower losses as well as less environmental impact compared to conventional HVAC transmission with same ratings,

• no stability problems.

Today, existing long-distance HVDC transmission systems are primarily in use for the point-to-point interconnection between highly urbanized areas (e.g. Los Angeles, Mumbai, and Shanghai) and remote power generation (mainly hydro and offshore wind power). Almost all long-distance HVDC transmission systems in operation to-date are overhead lines, for the most part carrying between 1500 and 3000 MW of active power over distances between 500 and 1500 km. Most of the lines are bipolar with two conductors and with DC voltages in the range of ±500 kV (+500 kV in one conductor and -500 kV in the other, with respect to ground). The main purpose of the existing long-distance transmission systems described is the bulk-power transmission from the sending end to the receiving end of the line, hence, from areas with high generation but low load to areas where additional power support is needed.

From the description of existing HVDC transmission systems in the Chapter 5, the following conclusions that are of special interest for TSOs can be deduced: HVDC technologies are essential in efficient long-distance electrical bulk-power transmission. Low losses and less harm to the environment are the main choice reasons with respect to conventional HVAC transmission. In addition, the experiences as well as the reliability records presented underline the feasibility of HVDC for long-distance transmission.

These advantages of HVDC technologies can be the key solution of the many international electrical energy transmission lines worldwide. The future trends in the European energy supply show an increased need for the transmission of large amounts of power over long distances from remotely (partly offshore) located RES to the load centers all over continental Europe. In the European supergrid project the HVDC technologies offer efficient energy transmission from the north-west areas with relatively high wind potential to the whole Europe wherever the supply is needed, and also efficient energy import from the remote North Africa and the Middle East regions is possible. Besides, the international long-distance transmission lines can lead to increase the wind power share in the Europe’s electrical energy generation system due to efficient supply of remote load and/or remote energy accumulator (e.g. hydro reservoir, battery electric vehicle). Ongoing initiatives, such as Desertec, OffshoreGrid, Kriegers Flak or the North Sea Offshore Grid projects, show that a comprehensive solution for the power grid of the future cannot be found on a national level: multinational transmission projects in the form of long-distance point-to-point HVDC transmission lines comprising more than just one country or in the form of a multinational HVDC overlay grid are necessary. Furthermore, the idea exists to create an extra high voltage backbone to connect wind generation in the North Sea (with a possible future interconnection between the wind farms of the United Kingdom and Ireland) to the load centers in south-continental Europe. This


requires the cooperation among all European TSOs in order to coordinate the construction of the proposed infrastructure.

Of course, the opportunity to put in place in Europe infrastructural reinforcements requiring very significant investments, like supergrids and transmission highways, should be always carefully evaluated on the basis of costs and benefits, so as to achieve a clear view and to co-evaluate these projects in perspective with less costly initiatives (like reinforcing the existing network punctually tackling the most limiting bottlenecks). From this point of view, the cost-benefit analysis proposed within REALISEGRID ([61]) appears as an important decision tool in the hands of the European Commission and of the European TSOs.


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