Interaction between AC and DC Power Systems: the … · Interaction between AC and DC Power Systems: the Need for Controllability | | § Aging power system ... § Comparison of AC+FACTS

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Spyros Chatzivasileiadis, Thilo Krause, Göran Andersson

Power Systems Laboratory, ETH Zurich

09.09.2013Spyros Chatzivasileiadis 1

Interaction between AC and DC PowerSystems: the Need for Controllability

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§ Aging power systeminfrastructure

§ Increased shares of RES*integration

§ Massive cross-border flows dueto electricity trade


Challenges

*RES: Renewable Energy Sources

Need for investments in powersystem infrastructure

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§ Infrastructure Roadmap for the Energy Networks inEurope for the next 40 years

www.irene-40.eu

§ Partners:

§ 5 European Universities:RWTH Aachen, Imperial College, Delft, NTUA, ETH Zurich

§ ABB, Siemens, Alstom, Energy Center of the Netherlands

3

IRENE-40 European Project

09.09.2013Spyros Chatzivasileiadis

http://www.irene-40.eu/

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IRENE-40

§ Identify expansion measureswith respect to:

§ Sustainability à more RES

§ Security

§ Competitiveness à efficientmarket operation

§ Possible Options:§ AC Lines, FACTS devices, HVDC lines, regulatory measures, etc.


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Which technology improves security and reduces costs?


Objective

§ Run Simulations and Compare Results w.r.t.:

§ Network Utilisation

§ Cost of Operation (Generation Dispatch)

§ Cost of Security

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Some findings from theory and practice:

§ Practice: Application Survey§ Which technology tackles better which problem?§ Findings based on state-of-the-art current practice/expert

knowledge

§ Theory: AC expansion§ Take advantage of the power system properties§ Use the Kirchhoff laws§ Extract “universal truths”


But first…

|| 09.09.2013Spyros Chatzivasileiadis 7

§ Practice: Application Survey§ Theory: AC Expansion

§ Case Study

§ Expansion Options for the European System

§ Cost of Operation

§ Cost of Security

§ Comparison of AC+FACTS and DC Options

§ RES curtailment

§ Concepts for the Future

Outline

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§ Ranking is not straightforward. Each technology performsbetter for certain problems, while it tackles not so well, oreven aggravates, other problems.


“Technology Ranking” w.r.t. Security

In general, addition of newlines is the most effectivemeasure for increased powersystem security

§ FACTS contribute significantly inspecific problems

HVDC

AC-400kV

AC-750kV


§ Practice: Application Survey

§ Theory: AC Expansion§ Case Study



§ Cost of Security


§ RES curtailment


Outline


Supergrid or Local Network Reinforcements?

© IFHT© IFHT

10°W

0°10°E 20°E

30° E

40°N

50°N

60°N

70°N

source: irene-40.eu (RWTH Aachen)

LocalReinforcementsSupergrid


AC or HVDC ?

HVDCAC-400kV

AC-750kV



Assumptions

NTC (INTC) is determineddue to congestions inthe internal network

#2The bottleneck is not inthe interconnection

Internal Networkmore meshed thanthe Interconnection

per unit length#1

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§ Adding the “same” line as: internal reinforcement, overlayline, and interconnecting line


Zint

Znew,int

Zext Zext

Znew,ext

ZintZext

Znew,ext

Zint

Zint

,≤

+, + ,

≤,

≤

Flow through theoverlay line

Flow through the line parallelto the interconnection

Zint Zext

Internal Network Interconnection

A BC

Flow through theinternal reinforcement

A C B A C B A C B

≤

= ∙

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Assumptions:§ Weakly interconnected meshed networks§ Bottlenecks are inside the networks (not on interconnections)

Findings:§ Overlay networks are preferable: they carry more power

for the same line-kilometers compared to localreinforcements

§ Upper bounds for AC overlay networks:§ One additional cross-border AC-400 kV:

Total NTC cannot increase more than twice§ One additional cross-border AC-750 kV:

Upper bound for total transfer capacity is 2.6*NTC09.09.2013Spyros Chatzivasileiadis 15

Main Findings



§ Theory: AC Expansion§ Case Study



§ Cost of Security


§ RES curtailment


Outline


Case Study: Maximum Power Transfer from Bus #222

1. Set bus #222 as slack bus

2. Increase active powerconsumption at Bus #123 insteps of 50 MW

3. Run DC Power Flow

4. Calculate the sum of the power flows over theinterconnections

5. If no congestion in the internal network of Area 2, go toStep 1


IEEE RTS-96 Two Area System Bus 222: Graduallyincreasing thepower injection

Bus 123: Graduallyincreasing the

power consumption

Maximum PowerTransfer


IEEE RTS-96 Two Area System Bus 222: Graduallyincreasing thepower injection

Calculatingthe sum of the powertransferred to Area 1

Bus 123: Graduallyincreasing the

power consumption

Congestionappears


IEEE RTS-96 Two Area System Bus 222:Increasing thepower injection

Calculatingthe sum of the powertransferred to Area 1

Bus 123: Increasingthe power

consumption


Overlay vs. Local ReinforcementsOne existing interconnection (230 kV)


Overlay vs. Local ReinforcementsTwo existing interconnections (230 kV & 138 kV)


Overlay vs. Local ReinforcementsThree existing interconnections (2 x 230 kV & 138 kV)


Maximum Power Transfer from bus #222All additional lines on the same voltage level

0

200

400

600

800

1000

1200

1400

1600

1800

1 line 2 lines 3 lines

Max

imum

Pow

erTr

ansf

er(M

W)

Number of Existing Interconnections

No expansionLocal Reinforc.Overlay


Maximum Power Transfer from bus #222Overlay network on higher voltage level

0

500

1000

1500

2000

2500

3000

3500

1 line 2 lines 3 linesMax

imum

Pow

erTr

ansf

er(M

W)


No expansionLocal Reinforc.OverlayOverlay higher volt.


Upper bound on utilization of AC overlay lines

≤ =

Compareline loadings

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§ Additional Remark: with increasing level of interconnections,the utilization of the AC overlay lines decreases

need for controllable power flows09.09.2013Spyros Chatzivasileiadis 27

Upper bound on utilization of AC overlay lines

0

100

200

300

400

500

600

700

1 line 2 lines 3 lines

Line

Load

ing

(MW

)


Overlay

InterconnectingLine

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European System


§ Single-node per country;real network data(ENTSO-E)

§ Generation Scenario:80% RES in 2050

§ Addition of 19 new AC orHVDC lines

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0%

20%

40%

60%

80%

100%

120%

AT-DE AT-HU FR-ES FR-CH DE-NL DE-PL DE-CH IT-SI RO-RS

Line

Load

ing

Maximum Utilization of New Lines for DifferentExpansion Options AC-400 kV

AC-750 kVHVDC

Highly Meshed Underlying Grids do not allow highutilization of Overlay AC Networks without FACTS


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§ Analytically shown:For weakly interconnected meshed power systems,overlay networks are preferable.

§ Derived an upper bound on utilization of overlay AC lines(over weakly interconnected meshed power systems)

§ Controllable power flows are necessary over meshedpower systems§ Higher utilization of the installed lines


Wrap-up



§ Theory: AC Expansion

§ Case Study

§ Expansion Options for the European System§ Cost of Operation§ Cost of Security§ Comparison of AC+FACTS and DC Options§ RES curtailment


Outline

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§ Model based on:Optimal Power Flow (OPF)

andSecurity-Constrained Optimal Power Flow

(SC-OPF)

§ We emulate network and market operations

The SC-OPF takes into account thepost-contingency control capabilitiesof the HVDC lines(Chatzivasileiadis et al., SEPOPE 2012)

Modeling Framework (in brief)


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Generation Scenarios 2050compiled from ECN

EFF-iciencyBAU

CCS DES-ertec RES

“Robust Choices”: Scenarios cover a broad spectrum offuture possible G&D developments

Fina

lEle

ctric

ityD

eman

d(T

wh/

year

)

RES Share (%)


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Three expansion options for 2050 (all for overlay grids):

Long AC-400 kV/3000 MVA with fixed seriescompensation

Long AC-750 kV/3900 MVA with fixed seriescompensation

VSC-HVDC lines 3000 MVA

Note: For all expansion options, the submarine expansionswere always VSC-HVDC cables (9-10 lines in eachgeneration scenario)


Expansion Methodology

HVAC

UHVAC

HVDC

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Cost of Operation (RES 2050)HVAC-400kV

10 AC lines + 9 submarine HVDCs

Generation Cost= 95.3 billion Euros

UHVAC-750kV10 AC lines + 9 submarine HVDCs


HVDC19 HVDC lines


Reduction= 10.3% Reduction= 11.5% Reduction= 16.5%

Minimum Expansion: Generation Cost = 106.31 billion Euros09.09.2013Spyros Chatzivasileiadis 35


Operating Cost Reduction of Expansion Options

§ HVDC expansion leads to the lowest Cost of Operation§ Controllability (= FACTS) is necessary in the AC Expansion in

order to be as cost-effective as HVDC

0%

5%

10%

15%

20%

25%

BAU 2050 CCS 2050 EFF 2050 RES 2050 DES 2050

AC 400 kVAC 750 kVHVDC



§ Case Study

§ Expansion Options for the European System§ Cost of Operation

§ Cost of Security§ Comparison of AC+FACTS and DC Options

§ RES curtailment


Outline

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§ The “Cost of Security” represents the additionalgeneration dispatch costs, in order to satisfy the N-1criterion.

§ Equivalent to “Redispatch Costs”

§ Goal: find the lowest “Cost of Security”


Cost of Security

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§ HVDC expansion leads to the lowest Cost of Security formost scenarios (~1-2 billion Euros/year less than AC)


Cost of Security

§ CCS is anexception

CoS: from 1 to 7 billion Euros/year

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

BAU 2050 CCS 2050 EFF 2050 RES 2050 DES 2050

Billi

ons

ofEu

ros

Cost of SecurityAC 400 kVAC 750 kVHVDC

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0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

CSS 2050 - 85% Loading CCS 2050 - same #contg.

Billi

onso

fEur

os

AC 400 kV

AC 750 kV

HVDC


Cost of Security

CCS-2050: All expansions with the samecontingencies in the SC-OPF


HVDC with and without Post-Contingency Control (PCC)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

AC-400 kV AC-750 kV HVDC - no PCC HVDC

Billi

ons

ofEu

ros

Cost of Security (RES 2050)

HVDCno

PCC

HVDCwithPCC

AC750kV

AC400kV

Post-Contingency Control from HVDC lines results inadditional savings of 1 billion Euros per year

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% of SnapshotsAC-400 kV 0.0 %AC-750 kV 0.2 %HVDC - no PCC 2.1 %HVDC – with PCC 3.7 %


Snapshots with Zero Cost of Security



§ Case Study


§ Cost of Security

§ Comparison of AC+FACTS and DC Options§ RES curtailment


Outline

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0%

1%

2%

3%

4%

5%

6%

7%

UHVAC (750 kV)

WithPSTsand

SVCs;

andFixedSeriesComp.

FixedSeriesComp.

0%

1%

2%

3%

4%

5%

6%

7%

HVDC

MeshedDC grid

DC gridpoint-

to-point

FACTS lead tosignificant cost savings

Radial DC Networksslightly more effective

(but higher installation costs)

Cost Savings: AC+FACTS and DC Options




§ Case Study


§ Cost of Security


§ RES curtailment§ Concepts for the Future

Outline

|| 46

Ecological Impact –Reduction of Network Congestions (RWTH Aachen)

FACTSHVAC++FACTS

HVDC

UHVACFACTS

HVAC+RES curtailment

HVAC+


|| 47


FACTSHVAC++FACTS

HVDC

UHVACFACTS


HVAC+


Network development is highly necessary to reach CO2reduction targets, especially in case of a high penetration of

renewables

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§ Upgrade with HVDC is the preferable infrastructure scenariocharacterized byØ reduced RES curtailmentØ lowest cost for security (SC-OPF) AND operation (OPF)

§ Overlay networks are preferable

§ Need for increased controllability is identified in all scenarios i.e.AC+FACTS or HVDC flows

§ Post-contingency control capabilities of HVDC can result insignificant cost savings of redispatching costsNovel operation schemes and control algorithms are necessary.


Conclusions



§ Case Study



§ Cost of Security


§ RES curtailment


Outline

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§ Decouple “completely”the Market Operationsfrom the SecurityConsiderations

§ Proof for maximumnumber of controllablelines in a system:


Towards a Fully Controllable Power System

SCH, TK, GA, IEEE PES GM 2011

Cost of Security

14 Lines10 Buses

= 5 Max Ctrl Lines

|| 51

The Global Grid

Long transmission lines toharvest remote RES

and connect regional Supergridsin one “Global Grid”

LowLoad

HighWind

HighLoad

Time difference: Transferexcess energy to where it ismostly needed


• Emerging Opportunitiese.g., less storage,

less peak power plants

S.Chatzivasileiadis, D. Ernst, G. Andersson,The Global Grid,

Renewable Energy, vol.57, p. 372-383, 2013


Thank you!

www.irene-40.eu

http://www.irene-40.eu/

Appendix

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§ Several studies for a 100% Renewable Energy Future

All these studies suggest the need formore transmission lines:

§ Interconnecting RES increases reliability in supply

§ Long transmission lines can harvest abundant renewablepotential in remote areas



Cheap RES production over long transmissionlines and Supergrids

Desertec

North-Sea Grid

Gobitec«Google» Project


The Global Grid



Telegraph 1866-1901

1866: First successful submarine cable

1901: Global Telegraphy Network


Wind Farm in Greenland

Quebec City

New York City

London

North UK

Faroe IslandsIceland

§ Sell wind power always at peak prices§ Trade electricity with the remaining line capacity

387 km (OHL)

550 km


Smoothing out electricitysupply and demand

Load

Load

Wind

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§ Oportoà New York : 5334 km

§ 5’500 km , 3 GW submarine cable§ Low Cost: $0.023 per delivered kWh§ High Cost: $0.035 per delivered kWh

§ RES Cost in 2020*§ below $0.04 up to $0.13 per delivered KWh

§ Conventional plant cost in 2020 in the US*§ $0.08/kWh, with the social costs: $0.14/kWh

§ Except for the most expensive RES generators, it is moreeconomical for the US to import RES power from Europethat operate its own conventional power plants


§ Oportoà Halifax : 4338 km

*Delucchi and Jacobson, 2010

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§ Alleviate the need for bulk storage

§ Reduce volatility of electricity prices

§ Minimize power reserves and defer the construction ofnew peaking power plants

§ Enhance power systems security

§ and others...


Additional Benefits

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§ Investments§ Investment Mechanisms§ Case of NorNed

§ Operation§ Operation Schemes§ “Global Regulator”§ “Global System Operator”

§ Alternatives to the Global Grid§ Challenges


Additional Topics covered within “The Global Grid”

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§ The Global Grid can be technologically feasible andeconomically competitive for a 100% RES future

§ Several new opportunities emerge

§ Working groups can be established to substantiate thebenefits and identify the risks

S.Chatzivasileiadis, D. Ernst, G. Andersson, The Global Grid.Renewable Energy, vol.57, p. 372-383, 2013. [online]http://arxiv.org/abs/1207.4096v4


Conclusions

http://arxiv.org/abs/1207.4096v4

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Duration of NTC Line Loadings

Hours per Year

4000 hours

Examples ofLines loadedat 100% formore than4000 hours

0% loading

100% loading



Reinforcements: Robust Choices

*Reinforcementsidentified based onDuration of LineLoadings

Additional Linesidentified asnecessary* inalmost all 2050scenarios(in red colour)

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§ Degree of compensation: 40-50%

Total Compensation

AC-400 kV (Gvar) AC-750 kV (Gvar)BAU 2050 14 16CCS 2050 10 11DES 2050 16 18EFF 2050 14 16RES 2050 16 18

Series Compensation

High Costs!


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§ Controllability Index(a vector based on PTDFs)


Controllability and Placement

= max. controllability

§ Only dependent on the networkcharacteristics (off-line)

§ Find the vectors that are mostindependent from each other(orthogonality)

||

§ Adding the same line as internal reinforcement or parallelto the interconnection


=,

=,

′′ ,

≤′

′ ,

≤

Zint Zext

Zext

Znew,ext

ZintZint

Znew,int

Zext


Flow through the internalreinforcement


A BC

A C B A C B

||

§ Adding the same line as internal reinforcement or parallelto the interconnection


Zint Zext

Zext

Znew,ext

ZintZint

Znew,int

Zext


A BC

A C B A C B

||

§ Add same line as internal reinforcement or overlay line§ Assume that the existing network has transfer capacity

limited to


Supergrid vs. Internal Reinforcements

Zext

Znew,ext

Zint

Zint

≤

Zint

Znew,int

ZextA BCA BC

Overlay networks lead to higher utilizationcompared with internal reinforcements

Flow through theinternal reinforcement


||

§ Assume that the existing network has transfer capacitylimited to


Overlay Networks: Upper bound on utilization

Zext

Znew,ext

Zint

Zint

≤

Zext

Znew,ext

Zint

Upper bound on utilization for overlay networks:Line with equal length and parallel to the interconnection

A BC A BC



||

§ Existing interconnection: at least one AC-400 kV line

§ Adding one cross-border AC-400 kV: ≤§ Adding one cross-border AC-750 kV: ≤ 1.6 ∙


Overlay Networks: Upper bound on utilization

Zext

Znew,ext

Zint

Zint

≤

Zext

Znew,ext

ZintA BC A BC

|| 74


FACTSHVAC++FACTS

HVDC

UHVACFACTS


HVAC+

Network development is highly necessary to reach CO2reduction targets, especially in case of a high penetration of

renewables


Documents

Interaction between AC and DC Power Systems: the … · Interaction between AC and DC Power Systems: the Need for Controllability | | § Aging power system ... § Comparison of AC+FACTS