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Electricity is transmitted at high voltages in order to minimize the loss of power during transmission. The power loss during transmission is proportional to the square of the current (P : I2), so there is a strong incentive to reduce the current by as much as possible.

For a given amount of energy, the current is inversly proportional to the voltage (I : 1/V), so the higher the voltage, the lower the current. And the lower the current, the lower the power loss squared.For example, if the transmission power loss was 9 watts at 100 volts, then at 200 volts (double), the current would halve and the power loss would quarter. Hence the transmission power loss would only be 2.25 watts (1/4).

Owned by

Types

State(in Circuit

Kms)

Central (Power Grid)

(in Circuit Kms)

Total(in Circuit

Kms)

HVDC 1,504 3,532 4,836

800 kV 400 550 950

400kV 13,000 32,500 45,500

220/132kV 2,06,000 9,000 2,15,000

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Why to prefer High Voltage DC over ACThe AC resistance of a conductor is higher than its DC resistance because of skin effect, and eventually loss is higher for AC transmission. The switching surges are the serious transient over voltages for the high voltage transmission line. In case of AC transmission the peak values are two or three times normal crest voltage but for DC transmission it is 1.7 times normal voltage. HVDC transmission has less corona and radio interference than that of HVAC transmission line . The long HVAC overhead lines produce and consume the reactive power, which is a serious problem.

Power system stabilityStability in AC power system depend on the sending, receiving end voltages, phase angle difference between them and line reactance. Increase in line length will increase the line reactance and thereby reduce the stability of the system. This requires shunt reactors and series capacitors to compensate. But DC line is not affected because it is governed by the DC resistance of the line, thermal conditions and current carrying capability only.

Tower Size :DC insulation is lower than AC insulation for a given power transmission. Therefore the size of the tower s and the corresponding right-of-way are also less.

Insulation :In steady state operation there is no charging current or reactive kVA taken by the cable as in AC systems. Hence no dielectric loss in DC cables. Reduced voltage stress also minimizes the insulation level. Hence Underground and Under sea transmission is cheaper.

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Increase Power transfer capability

Voltage profile :AC line has surge impedance or natural loading. Voltage profile varies when with the load on the system.

When the load and surge impedance are same, voltage profile will be flat.When load is less than the surged impedance , voltage in the middle of line risesWhen the load is more than the surge impedance voltage in the middle drops.

Although the DC converter stationsrequire reactive power depending onthe line loadings, the line itself doesn't require reactive power.

AC Interconnection Problems :AC interconnected system has the following problems :

• presence of larger power oscillations that lead to frequent tripping• Increase in fault level• Transmission of disturbance from one system to the other.• frequency variation among the interconnected system

DC line eliminates these problems.

Generation harmonic during converter operation will flow in the converter transformers on the AC side, causing audio frequency telephonic interference. This needs huge filters to suppress.

Since DC system generate reactive power needed by the load, we need additional devices to supply it on both the AC sides of HVDC system.

Complexity in control

High cost of conversion equipment

Inability to use the transformers to change voltage levels

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Comparison of AC and DC Transmission could be grouped into Economics of transmissionTechnical performanceReliability

In order to compare the cost it is necessary to consider the cost of all main component of the system. In case of the AC system capital cost of the step up/ step-down transformer, transmission line, reactive power compensation, light load compensation and circuit breaker must be accounted. In case of the DC system, the capital cost of converter, transmission line, AC input output equipment and filters used to remove the harmonics must be accounted.

The cost of control system needs to be accounted for in both cases.

AC Transmission Line Cost: -The cost of the AC transmission line depends upon many factors including the power capacity to be transmitted, safety and environmental conditions. A three phase AC transmission line has three conductors where as in DC transmission line has only two conductors due to this fact the cost of the AC transmission line is more in comparison to the DC transmission line.

DC Transmission line Cost: -In case of the AC system the cost of the transmission line predominates and the cost of the station is less. The cost of the converter station makes the total cost higher than that of the AC system

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Variation of break-even distance with power transmitted

–Controlled Power on either direction

–Asynchronous operation possible between regions having

different electrical parameters

– No restriction on line length as no reactance in dc lines

–The ability to enhance transient and small signal stability in

associated AC networks.

–Fast control to limit fault currents in DC lines.

Reliability of HVDC is measured with two metrics :

where equivalent outage time is the product of the actual outage time and the fraction of system capacity lost due to outage.

Recordable AC system faults are those faults which cause one or more AC bus phase voltages to drop below 90% of voltage prior to fault.

The Energy availability and Transient reliability for existing DC systems with thyristorvalves is 95% or more.

%1001 xtotaltime

outagetimeequivalenttyavailabiliEnergy

%100xfaultsACrecordableofNumber

designedasperformedsystemsHVDCtimesNumberyreliabilitTransient

A power system planner must consider the already briefed factors likeCostTechnical performanceReliabilitywhile considering HVDC as an alternative.

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For long distance bulk power transmission, the voltage level is chosen to minimize the total costs for a given power level(P). The total costs included investment (C1) and cost of losses (C2). The investment costs per unit length are modeled as :

Where V is the voltage level with respect to groundn is the number of conductorsq is the total cross-section of each conductorA0, A1 and A2 are constants

nqAnVAAC 2101

The cost of losses per unit length is given by :

Where is the conductor resistivityT is total operation time in a yearL is the loss load factorp is the cost per unit energy

C2 can be simplified as

Where A3=TLp

q

TLpnVPn

C

ρ2

2

nq

VPA

C

ρ2

3

2

By minimizing the sum of C2 and the third term in C1, we have

Where J is the current density. The total costs can be written as

The above equation ignores the variation of terminal costs with the voltage.

VP

AAnq .

2

3

ρ

ρ3

2A

AnqV

PJ

VPAAnVAACCC .2 321021 ρ

The voltage level is chosen to minimize C. The following figure shows the selection of optimum system voltage to minimize the sum of converter and line costs.

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Modern trends in DC transmission: Recent trends or developments in the technological aspects of power electronics, power semiconductor devices, digital electronics, DC protection equipment have been increased in the applications of DC transmission.

The main objective of these developments is to reduce the cost of converter stations to improve the reliability and performance.

Power Semiconductor and Valves:The cost of converter depends upon no. of devices used in it. If no of devices connected in series or in parallel decreases then total cost of converters also decreased.Overload capacity of device at reasonable cost will be high if its current rating increases, moreover it leads to the reduction of transformer leakage impedance, there by improving the power factor. The cost of valves is reduced by using zincoxide gapless arresters and protective firing methods.Most of the power semiconductors devices uses silicon the cost of silicon may be decreased by using magnetic CZ(czochralsli) method, rather than the conventional FZ(float zone) method.

Usually, it is uneconomical to use forced commutated converters operating at high voltages, which leads to the development of devices which can be turned off by the application of gate signal.

Gate turn off devices (GTO) operating at 2500V and 2000A have a drawback of large gate current to turned off.

But technology was developed for a metal oxide semiconductor (MOS) controlled thyristor for which a very small gate current is sufficient to interrupt a very large line current.

Converter Control: The development in converter control equipment is micro-computer based converter control.

With the use of such converter control it is easier to design systems with automatic transfer between systems during false operating.

This micro computer based control has the flexibility to use adaptive control algorithms for fault identification and protection.

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DC Circuit Breaker: Recent developments in DC circuit breakers are useful in tapping of existing DC link parallel operation of converters is allowed rather than series operation shows some flexibility in the system growth.

In order to limit the fault current the dc breaker current should not to exceed the full load ratings.

Conversion of Existing AC Lines:o In certain cases such as to increase the power transfer limits it is necessary to convert existing AC circuits to DC.

oIn India there is an experimental project of converting single circuit (or) double circuit 220kV line is currently under commissioning stage.

Operation with weak AC systems: The strength of AC systems connected to the terminals of DC link is measured in terms of short circuit ratio (SCR) SCR = short circuit level at the converter bus / Rated DC power If SCR<3, then it is weak AC system. For a weak AC system, conventional constant extinction angle control may not be satisfactory.

In order to overcome the problems of weak AC systems constant reactive current control (or) AC voltage control have been suggested. By using static VAR systems at the converter bus fast reactive power control can be achieved.It is necessary to limit the dynamic over voltages during load rejections through converter control.The dynamic stability of power systems can be improved by power modulation techniques in the presence of weak AC systems.Co-ordinated real and reactive power control must be necessary inorder to overcome the problems of voltage variations which can limit the power modulation.

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Thyristor valve stacks for Pole 2 of the HVDC Inter-Island between the North and South Islands of New Zealand. The man at the bottom gives scale to the size of the valves. Gotland – The worlds first

HVDC link connected the island of Gotland to the Swedish mainland by a 100km HVDC cable in 1954

Originally rated 20MW at 10kV is then increased to 30MW at 150kV.

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Important HVDC links in India

NameConverterstation 1

Converterstation 2

Overhead line (km) Volt (kV)

Power (MW) Year Type

VindhyachalB2B Vindhyachal24°05′38″N82°40′44″E

Vindhyachal24°05′38″N82°40′44″E ---- 176 500 1989 Thyr

Silru-Barsoor Sileru17°52′01″N81°39′21″E

Barsoor19°8′20″N81°23′47″E 196 200 400 1989 Thyr

Rihand-Delhi Rihand24°01′13″N82°47′21″E

Dadri28°35′36″N77°36′16″E 814 500 1500 1992 Thyr

Chandrapur-Padghe Chandrapur20°0′36″N79°17′06″E

Padghe (near Mumbai)19°21′26″N73°11′18″E 900 500 1500 1999 Thyr

Chandrapur B2B Chandrapur20°5′21″N79°8′36″E

Chandrapur20°5′21″N 79°8′36″E 0 205 2x500 1998 Thyr

Vizag 1Visakhapatnam

Gazuwaka17°38′33″N83°7′57″E

Gazuwaka17°38′33″N83°7′57″E ---- 205 500 1999 Thyr

Talcher-Kolar Talcher,Orissa21°06′01″N85°03′49″E

Kolar, Karnataka 13°10′39″N 78°7′0″E 1450 500 2000 2003 Thyr

Sasaram B2B Sasaram25°07′42″N83°42′29″E

Sasaram25°07′42″N83°42′29″E 0 205 500 2003 Thyr

Vizag 2 Visakhapatnam17°38′26″N83°8′10″E

Visakhapatnam17°38′26″N83°8′10″E ---- 176 500 2005 Thyr

Ballia-Bhiwadi Ballia Bhiwadi28°11′0″N76°48′58″E 780 500 2500 2009* Thyr

Biswanath- Agra Biswanath Agra 1825 800 6000 2012* Thyr

Mundra - Haryana Mundra22°49′46″N69°33′22″E Mohindergarh 960 500 2500 2012 Thyr

A back-to-back HVDC converter can be used when two asynchronous AC systems need to be interconnected for bulk power transmission or for AC system stabilization reasons. In an HVDC back-to-back station there are no overhead lines or cables separating the rectifier and the inverter, hence the DC current can be kept high and the DC voltage low. The low DC voltage means that the air clearance requirement is low, which favours a compact design of the valve hall. The back-to-back HVDC converter station at Vizag is of the conventional type with indoor thyristor valves and smoothing reactors located outdoor. The HVDC Back-to-back utilizes a special mid-point (6 pulse bridge) grounding design for the thyristor valves on the southern side with two smoothing reactors installed in series on the neutral side.

The HVDC back-to-projects at Vizag are part of Powergrid’sEast-South Interconnections. The first 1 x 500 MW HVDC project was commissioned in 1999 thus enabling power transfer from the Eastern region to the Southern region.

The second 1 x 500 MW HVDC project at Vizag forms the East-South Interconnector.

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The next step in the interconnection was the construction of the 2000 MW HVDC bipolebetween Talcher in the Eastern region and Kolar in the Southern region. This bipolar HVDC long distance transmission with a line length of around 1370 kms. was completed and commissioned in 2002 and forms the East-South Interconnector-II.

Part of the power from the 3000 MW coal-based thermal power Rihand complex in Uttar Pradesh is carried by the Rihand-Delhi HVDC bipolar transmission link, which has a rated capacity of 1500 MW at 500 kV DC. Some of the power is transmitted via the existing parallel 400 kV AC lines.

The Rihand-Delhi HVDC transmission is the first commercial long-distance HVDC link in India.

The two converter stations in Rihand and Dadri, outside Delhi, were supplied jointly by ABB and Bharat Heavy Electricals Limited, a government of India undertaking. The transmission was, which was commissioned in 1990, is owned by Power Grid Corporation of India

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Rihand-Dadri , 500kV, 1500MW

Station control room at Dadri

Rihand-Dadri , 500kV, 1500MW

Dadri converter station

Valve hall interior

Maharashtra State Electricity Board (MSEB) has constructed a 1,500 MW HVDC link between Chandrapur and Padghe near Mumbai (Bombay). The converter terminals have being constructed by ABB (Sweden and India) and Bharat Heavy Electricals Limited (BHEL) of India. The HVDC transmission was commissioned in 1999.

The 500 kV Chandrapur - Padghe HVDC Bipole feeds Mumbai with 1,500 MW from the thermal generation plant located in the Chandrapur area, in central India, 752 km away. The HVDC link stabilizes the Maharashtra grid, increases the power flow on the existing East-West 400 kV AC-lines and minimizes the total line losses. The power to be evacuated from the Chandrapur 400 kV Bus is around 2,700 MW. The AC transmission network, comprising three 400 kV circuits between Chandrapur and Mumbai, can safely transmit around 1,200 MW of power without taking into account any contingency outage. It was therefore necessary to provide an additional transmission capacity of around 1,500 MW.

Chandrapur- Padghe, 2x500MW

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Chandrapur- Padghe, 2x500MW

Padghe Valve room interior Suspended converter valves of a complete pole at Talcher Figure shows the completely assembled

converters, arranged as three quadruple valves, for one pole of the East-South system. The 12 modules in total needed to make a quadruple valve are best arranged as a twin tower as can be seen in Figure.The converter twin towers are suspended from a special ceiling construction of the valve hall, and all connecting components between the modules like suspension insulators, bus work and pipings are of flexible design to ensure maximum seismic stress withstand capability.To reduce the risk of any fire to a minimum, exclusively flame-retardant materials for insulation and barriers within the converter valves are used. The modular structure of the valves has not only simplified replacement of any faulty component, but also transportation and installation as a whole.

Converter transformers of one pole at Talcher, in front of the valve hall

Between the converter valves and the ac grids, on both sides of the ±500 kV dc transmission line, are the Converter Transformers (see Figure), another key component of an HVDC system.

• In the rectifier station Talcher, they transform the eastern ac grid voltage of 400 kV down to a value, as is optimal for the converter valves, based on design calculations. • In Kolar, where dc is converted back to ac, the converter transformers do the reverse, i.e. step up the voltage from the valve side to the level of the southern ac grid (also 400 kV), thus completing the interconnection.• Converter transformers experience combined ac & dc stresses in the winding insulations, high harmonic content in the currentand need special competence and skill in design, construction and testing, compared to conventional ac power transformers.• The HVDC system basic design calculated and defined important parameters of the transformers like short circuit impedance, on load tap changer range etc., taking consideration of all special factors such as the permissible s.c. current of the thyristors, operation requirements at reduced dc voltage as also at high firing angles.

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DC harmonic filters of pole 1, TalcherAnother key component is the Smoothing Reactor which limits the dc fault current as also suppresses the dc harmonics to a permissibly low level. In the design phase, calculations considering different dc circuit configurations were carried out for an adequate dimensioning of the smoothing reactor, which is installed outside the valve hall and connected to the 500 kV dc valve hall bushing.

The harmonics mentioned above, which the smoothing reactor is supposed to limit, are a necessary evil of the current conversion process in the converter valves. The converters are sources of harmonics, which if allowed to infiltrate unhindered into the ac or dc systems, would distort the system voltage. The dc harmonics can be kept within specified levels by an adequately designed smoothing reactor in combination with DC Harmonic Filters. For absorption of the ac harmonics, AC Harmonic Filters are needed. They are tuned to the specific frequencies of the harmonics aimed for elimination. The ac harmonic filters are installed in the outdoor ac switchyard and connected to the 400 kV ac bus. The dc filters, located behind the smoothing reactor, are connected to the outgoing or incoming 500 kV dc line at the rectifier or the inverter station respectively(in Figure).Not only that the converters generate harmonics. Depending on the art of converter control as well as the commutation process, an amount of phase shift between the fundamentals of ac current and voltage occur, causing a demand in reactive power which has to be met.

• AC harmonic filter area at Talcher

Unless a proper balance of the reactivepower demand in the system isachieved, inadmissible fluctuations inthe ac grid voltage may occur. Thesame ac filters that absorb the acharmonics, offer here a dual function.They provide this reactive power, andin a detailed reactive powermanagement study, out of acombination of all reactive powerelements: ac filters, shunt capacitors,on need also shunt reactors, theoptimal choice is made to establish thisbalance.

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EXISTIN G/ U ND ER CO NST.

APPR OVED/PRO POSED

SASARAM

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M ERAM U ND ALI

HVD C BA CK T O B AC K

HVD C BIPOLE

PAR LI

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BH U SAWAL

PEN C H ST PS

PAT RAT U

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PU RN EA

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NAR EN DR A

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40 0K V L IN ES

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The remote Vindhyachal region hosts three super-thermal projects of NTPC within a radius of 40 km: Singrauli, Rihand(supplying power to the Nortern grid) and Vindhyachal. Vindhyachal is the largest project of NTPC with a total capacity of 2,260 MW supplying power to the Western grid.

Vindhyachal Back to Back HVDC, 70kV,2x250MW

Inside one of the valve hallsThe 500 MW Vindhyachal back-to-back

HVDC station interconnects the Northern and Western Regions.

Vindhyachal exterior with the world's first HVDC transformers with extended delta windings.

Chandrapur Back to Back HVDC, 205kV,1000MW

Connects Chandrapur Thermal Power Stations (Western Region ) to Ramagundum (Southern Region).Bidirectional power flow is possibleSecond commercial Back to Back HVDC station in India.

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Gazuwaka Back to Back HVDC, 1000MW

Connects Jeypore(Eastern Region) to Gazuwaka (Southern Region)Bidirectional power flow is possible

Sasaram Back to Back HVDC, 205kV 1000MW

Balli- Bhiwadi HVDC, 500kV, 2500MW

Balli- Bhiwadi HVDC, 500kV, 2500MW

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Balli- Bhiwadi HVDC, 500kV, 2500MW

The world's first HVDC system, exceeding 500kV, was set up during the 1970s in Cahora Bassa, Mozambique, South Africa. Photo: Siemens