Network Enhancements by HVDC Transmission

McAllen, TexasOctober 11-12, 2007

Michael Bahrman, PEABB Grid Systems

Topics

Historical PerspectiveComparison of HVDC & EHV TransmissionLine Loading CapabilityEconomic ExampleTransmission Expansion Model

Historical PerspectiveJanuary 12, 1883

2nd transcontinental railroad completed with the driving of a golden spike atop the Pecos River bridge near Del Rio, TXConnected gulf coast ports with California and the greater southwestProvided broader access to more diverse competitive markets

October 10, 20071st HVDC interconnection between ERCOT and CFEEnables economic energy tradeProvides mutual assistance, reserve sharing for increased reliabilityLeast cost, lowest losses

Asynchonous borders

Mine-mouth

Remote renewable

DiversityCongestion

CongestionEconomy

DiversityEconomyReliability

HVDC in North America

Asynchronous InterconnectionGeneration OutletInterconnection

Area 1

Area 3

Area 2

Thermal path limit

Stability path limit

Network path limits:Thermally constrainedStability constrained (voltage, angle)Parallel flow issuesConstraints result in sub-optimal dispatch

Transmission Constraints

Net 1

Net 3

Net 2

HVDC Asynchronous Ties:Limited by converter capacity and local network characteristics Provide mutual assistance, decouples gridsPrevents cascading outages, ‘firewall’Allow incremental interconnectionsNo inadvertent flow, active or reactiveImprove reliabilityLeast cost, lowest losses, modular, fast control

ERCOT SPP

CFE

Congestion

Area 1

Area 3

Area 2

Increased short-circuit levelThermal path

limit

Stability path limit

Raise path limits by new AC line:No direct flow control (generation dispatch)Raise thermal limitRaise stability limit (voltage, angle)Parallel flow issuesIncreased short circuit levelsDistributed reactive power demandSingle circuit or double circuit configurationCorona & audible noise issues with higher voltages at altitude

Transmission Expansion – EHV v HVDC

Area 1

Area 3

Area 2

Thermal path limit

Stability path limit

Raise path limits by new DC line:Flow control adds operational flexibilityRaise thermal limitRaise stability limit (voltage, angle)No parallel flow issues due to controlNo increase in short circuit levelsLumped reactive power demand at terminalsDouble circuit (bipolar configuration)

Distance Effects

Area 1

Area 3

Area 2

Increased short-circuit levelThermal path

limit

Stability path limit

New AC line:Need for intermediate switching stationsLower stability limits (voltage, angle)Higher reactive power demand with loadHigher charging at light loadParallel flow issues more prevalent and widespreadIncrease stability limits & mitigate parallel flow with series compensation (FACTS)Thermal limit remains the same

Area 1

Area 3

Area 2

Thermal path limit

Stability path limit

New DC line:No distance effect on stabilityRaise stability limit (voltage, angle)No need for intermediate stationNo parallel flow issues due to controlNo increase in short circuit levelsNo increase in reactive power demand

Area 1

Area 3

Area 2

Increased short-circuit levelThermal path

limit

Stability path limit

Add second AC line:Increases thermal limitIncreases stability limits (voltage, angle)Increase stability limits & mitigate parallel flow with series compensation (FACTS)Higher short circuit levelsImproves reliabilityTwo circuits

Staged Transmission Expansion

Area 1

Area 3

Area 2

Thermal path limit

Stability path limit

Add second DC line:Increases thermal limitIncreases stability limits (voltage, angle)Improves reliabilityFour circuits (bipolar configuration)Add converter capacity as complement or alternative to new line (higher current or voltage)

2

2

+ 400 kV, ≤1600 MW

± 400 kV, ≤ 3200 MW

± 800 kV, ≤ 6400 MW

Staged Transmission Expansion HVDCStage 1:

Build bipolar transmission lineInsulate one pole to 400 kV, second pole as neutralAdd up to 1600 MW converter at each end

Stage 2:Raise insulation on second pole to 400 kVAdd up to 1600 MW converter at each end on second pole

Stage 3:Raise insulation on both poles to 800 kVAdd up to 1600 MW series-connected converter at each end on each polePower doubled, no increase in losses

Area 1

Area 3

Area 2

Thermal path limit

Area 1

Area 3

Area 2

Thermal path limit

Stability path limit

Stability path limit

Minimum short-circuit level

Minimum short-circuit level

Dynamic Voltage Support

Dynamic Voltage Support

HVDC

HVDC Light

Conventional HVDC:Minimum short circuit level restriction (S > 2 x Pd)Reactive power demand at terminals (Q = 0.5 x Pd)Reactive compensation at terminalsHigher ratings possibleGreater economies of scale

HVDC Light:No minimum short circuit levelsNo reactive power demandDynamic reactive voltage support (virtual generator)Leverage ac capacity by voltage supportConducive for but not limited to underground cable transmissionIsolated wind farm, black start

Transmission Expansion – HVDC v HVDC Light

Area 1

Area 3

Area 2

Area 1

Area 3

Area 2Gen

Gen

AC Transmission:Power flow from generation distributes per line characteristics (impedance) & phase angle (generation dispatch)Variable generation gives variable flow on all pathsMay be limited due to congestionNew resources add cumulatively clogging existing pathsFlow controlled indirectly by generation dispatch

HVDC Transmission:Controlled power flow adds flexibilityPd = P schedule or by Σ generationPd = Pg or,Pd = Pg + P schedule or,Pd = k * PgPermits optimum power flowBypasses congestion

Indirect v Direct Control – AC v DC

Pg

Pg

Pd

Area 1

Area 3

Area 2

Area 1

Area 3

Area 2

Tapping – AC v DC

HVDC TapElectronic clearing of dc line faultsFast isolation of faulty convertersReactive power compensationMomentary interruption due to ac fault at tapLimitations on tap rating, location and recovery rate due to voltage stability

HVDC Light TapNo momentary interruption to main power transfer due to ac fault at tapLess limitations on tap rating and locationNo reactive power constraintsImproved voltage stability

AC TapAdd transformer & substation equipmentExacerbate parallel flow issues

Area 1

Area 3

Area 2

Off-ramps

HVDC Light off-ramps:Delivers bulk power allocation to selected distribution substations in congested areaProvides dynamic voltage support (virtual generator)Doesn’t increase fault current dutiesAllows shared use of narrow rights-of-wayStealthy and healthy

Area 1

Area 3

Area 2

AC off-ramps:No control of power injectionPotential for unequal utilization and local congestionReactive power compensation required for light & heavy load conditionsNo voltage supportIncreases fault current dutiesIncreased right-way-requirements

Area 1

Area 3

Area 2

Area 1

Area 3

Area 2Gen

Gen

AC Transmission:Capacity of new line v reserve margin (stability, thermal) in parallel pathsReserve margin v remedial actionSeverity v probability – single circuit, double circuit or corridor outage, circuit reliabilityCapacity factor, spinning reserve (amount & location), restoration speedNo control

Contingency Response – AC v DC

HVDC Transmission:Capacity of new path v reserve margin (stability, thermal) in parallel pathsReserve margin v remedial actionSeverity v probability - monopole, bipoleor corridor outageReliability of line & terminals (outage probability - monopole or bipole)Capacity factor, spinning reserve (amount & location), restoration speedControl – preposition or post-contingency

Generator tripping

Generator tripping

Perm outage

Perm outage

HVDC Bipole – Contingency Operation

0

400

800

1200

1600

POLE POWERMW

0 2 64 8MINUTES

-60 MW/MIN1200 MW/MIN

Overload

Pole loss compensation

DC Transmission:Firm capacityHigh utilization possibleCan operate with reserve capacitySimilar to double circuit ac lineExpandableMore power on fewer lines with lower losses

Line Loading Capability Line Type Voltage Conductor Thermal Limit

of LineThermal Limit

of Terminal Equipment

SIL 1.5 x SIL Charging per 100 mi

kV kcmil GW GW GW GW 8 GW 12 GW MVAr345 2 x 954 1.2 2.4 0.4 0.6 14 21 80500 2 x 2312 2.9 3.5 0.8 1.3 7 10 180765 5 x 795 6.0 4.0 2.3 3.5 3 4 4601000 8 x 795 12.5 5.2 3.9 5.9 2 3 800±500 3 x 2515 5.3 4.0 n.a. n.a. 2 3 n.a±600 3 x 2515 6.3 4.8 n.a. n.a. 2 3 n.a±800 5 x 795 7.6 6.4 n.a. n.a. 2 2 n.a

Req'd no. of lines - EHVAC

at 1.5 x SIL

EHVAC - UHVAC

HVDC

More lines are needed with lower transmission voltage. Loadability increases and number of ac lines can be reduced by series compensation.Fewer lines are needed with higher transmission voltage. More shunt compensation is needed at light load to absorb excess charging.Larger contingency impact with fewer lines or circuitsConventional HVDC converters are subject to minimum short circuit capacities relative to transmitted power – ac voltage stability issueReactive power compensation issues: AC – distributed, DC - localInterconnection issues are similar – the collector and receiving networks must be able to handle the power

CREZ Transmission Challenges

Asynchronous Boundary

High Load Density

Congestion

6 GW

6 GW

Sparse network

Panhandle Outlet HVDC, Central Outlet UHV AC

Asynchronous Boundary

High Load Density

Congestion

6 GW collector system synchronous with 230 kV network or isolated with ‘anchor tenants’

6 GW collector system integrated with 345 kV

network

3000 MW HVDC bipoles3000 MW UHVAC lines

Sparse network

Preserves asynchronous intrastate environment for the ERCOT grid

Cost Comparison for 6000 MW Transmission

0.00

2.00

4.00

6.00

8.00

10.00

12.00

Cost 250 mi(B$)

Cost 500 mi(B$)

Cost 750 mi(B$)

Cos

t (B

$)

345 kV AC 8 single circuits345 kV AC 4 double circuits500 kV AC 4 single circuits500 kV AC 2 double circuits765 kV AC 2 single circuits± 500 kV 2 HVDC bipoles± 600 kV 2 HVDC bipoles± 800 kV 1 HVDC bipole

Series Comp

Cost of 6000 MW Transmission Alternatives

Note: Transmission line and substation costs based on Frontier Line transmission subcommittee and NTAC unit cost data.

HVDC Light – On-Off Ramps, Offshore CREZ

Asynchronous Boundary

High Load Density

Congestion

6 GW collector system radial or synchronous with 230 kV network

6 GW collector system integrated with 345 kV

network

1000 MW HVDC Light circuits 3000 MW UHVAC lines

Sparse network

Offshore CREZ

Preserves asynchronous intrastate environment for the ERCOT grid

Cost Benefit Screening Analysis Comparison Scenario T1 T2 T3 T4Source Coal - PC

Coal - CCS Wind 3,500 3,500 3,500 3,500

Sink Gas CC 900 900 900 900Gas CT 1,000 1,000 1,000 1,000Coal - PC 500 500 500 500

Net Line Capacity MW 3,000 3,000 3,000 3,000

Line Segments2 x 345 KV (AC)

dbl ckt1 x 500 KV (AC)

dbl ckt1 x 765 KV (AC)

single ckt1 x 500 KV (DC)

bipoleLine + S/S Costs $ Million $2,081 $1,717 $1,802 $1,302Line + S/S Losses 10.2% 7.4% 5.6% 4.8%Financing Utility Utility Utility UtilityGHG Adder $40 $40 $40 $40Dependable Capacity Yes Yes Yes YesGWH 10,731 10,731 10,731 10,731

T1 T2 T3 T4ResultsB/C Ratio 1.58 1.98 1.95 2.66Benefits ($MM) $334 $350 $360 $365Costs ($MM) $212 $177 $185 $137Savings ($MM) $122 $173 $175 $228Value ($/MWh) $11.37 $16.12 $16.31 $21.25

Texas Panhandle to Dallas-Ft Worth 3000 MW Transmission Alternative

Export transmission costs only – does not include collector system costs or receiving system reinforcements.Transmission distance of 400 miles to account for practical line routeTransmission capacity deemed firm, i.e., no congestionScreening tool only – not production cost simulation

PDCI

PACI

Transmission Expansion - Pacific Intertie ModelRegional resource & load diversityHybrid AC & DC transmissionMix of investor-owned and public powerShared costs and benefitsMultiple jurisdictionsControllability for optimal power flowServes intermediate load & resources while bypassing congestionPACI terminal upgrades - 1500 to 2300 MWPDCI upgrades - 1600, 2000 to 3100 MWWould same concept work today for integration of diverse resources?Where are the champions?