Role of HVDC Configurations in Modelling of NZ National

Reserve Market

Vladimir KrichtalSO Development, Transpower NZ

Contents

• Existing Real HVDC Operation• Proposed Real HVDC Operation• SPD Reserve Transfer Modelling Options

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Existing Real HVDC Operation

• Real HVDC configuration depends on total HVDC power flow and change via steps:

• Bipole (both poles forward)• Monopole (one pole forward, second pole shutdown) • Monopole (one pole backward, second pole shutdown) • Bipole (both poles backward). • Each pole in real operation HVDC has dead bands

35MW from both directions.• HVDC is modelled in the SPD as bipole without dead

bands.

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4

HVDC bipole operation (No round power)

Bipole Power Pole2 Power Pole3 Power PoleMin=35MWW

Dead Band

Monopolar Operation P2

Monopolar Operation P3

Bipolar Operation

Bipolar Operation

PHVDC

PRef

Proposed Real HVDC Operation

• Real HVDC configuration depends on total HVDC power flow and change via steps:

• Bipole (both poles forward)• Monopole (one pole forward, second pole shutdown) • Round Power (one pole forward, second pole backward)• Monopole (one pole backward, second pole shutdown) • Bipole (both poles backward). • If HVDC is offered in monopole only mode in real

operation HVDC has dead bands 30MW from both directions.

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HVDC bipole operation (Round power)

Bipole Power Pole2 Power Pole3 Power PoleMin=35MWW

Round Power Operation

Monopolar Operation P2

Monopolar Operation P3

Bipolar Operation

Bipolar Operation

PHVDC

PRef

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HVDC Risk in Bipole operation with Round Power (MW)

1000 Southward Risk

1400

700 Bipole Power (MW)

700 (-overload capability)

400

400

Risk exists for duration of Block sequences- for

reducing power

-334 (+overload capability)

Northward Risk

-270

Possible Lost Power (MW)

-666

-1000

Transition at 400MW

Transition at -270MW

1000

1000

500

500

SPD Reserve Transfer Modelling Options (1)

• Cap Export Equation. • , (1)• Can Only Export What You Got. • (2)• Energy and Reserve Transfer Limit.• , (3)• Constraint (3) effectively limits transfer of Forward Reserves. Reserve transfer

in opposite HVDC flow direction (Backward) is still limited by constraint (3). In real operation at some transitional modes like Bipole-Monopole, it impossible to reverse pole quickly, so in the operational RTD schedule we have to address this and set more tight Backward Reserve constraint.

• Deadband issue will not exist if round power mode is used.

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SPD Reserve Transfer Modelling Options (2)

• In operational RTD schedule we can introduce new constraint• , (4)

where reserve exported is limited by HVDC flow.

• Reserves exported are bigger than reserves imported when reserves and HVDC power flow are in the same direction and reserve exported is smaller than reserves imported when reserve and HVDC power flow are in different directions in bipole and monopole configuration. We use lossless power flow for reserves transfer.

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SPD Reserve Transfer Modelling Options (3)

• HVDC losses. During real operation a total HVDC losses becomes non-smooth, non-convex function, see solid curve in Figure 1. Estimated HVDC losses for above HVDC configurations are in the Table 1.

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Figure 1. Losses for different HVDC modes (pole configurations).

Losses(MW)

HVDC Flow (MW) 165 30

Bipole HVDC

losses Round power HVDC

losses

Monopole HVDC

losses

Convex approximation of HVDC losses

• HVDC losses. Loss difference is very small at HVDC flow level of 165 MW.• Inconsistencies between modelling HVDC as Bipole and real configurations

can be resolved with the following options:• Approximate total HVDC losses at Figure 1 by piece-wise convex loss

function, see dashed curve in Figure 1. This will allow LP modelling.• Use Bipole losses (existing model). Inconsistencies are resolved via

constrained on/off payments. LP model is used. • Create a detailed HVDC model which reflects operational consequence and

configuration. It can be used in operational RTD schedule to model precisely transition bipole-monopole, monopole – round power modes.

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SPD Reserve Transfer Modelling Options (4)

HVDC flow(MW) 30 165Bipole losses (MW)   1.29Monopole losses (MW) 0.08 2.59Round Power losses (MW) 0.43  

Table1

SPD Reserve Transfer Modelling Options (5)

• HVDC CE risk. In the SPD model HVDCCE risk is calculated as max(0, ) in Bipole mode and should be in Monopole mode. In real operation Monopole mode, the second blocked pole can be available to be de-blocked within 3 seconds.

• CE Risk in Round Power mode can be estimated at 35+25*5=160MW as maximum of poles flow at Round Power to Monopole transition point because 5 minute time waiting for the second pole reversal with 25(MW/min) ramping. In the North Island it always smaller than the minimal generation risk. In the South Island it can be larger than 120MW manual risk. This situation lasts only 5 min, so real SI HVDC risk should be modelled in this transitional RTD run. Reserve HVDC transfer capacity in Forward direction is not affected.

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SPD Reserve Transfer Modelling Options (6)

HVDC Instantaneous Reserves Transfer• Bipole, Monopole modes. Reserve transfer into Forward direction is limited by

constraints (1,3). Real operation in Monopole mode does not affect HVDC ability to ramp-up in Forward direction because second pole can be de-blocked in less than 3 seconds. Reserve transfer in Forward direction in Monopole mode can be modelled like it is in Bipole mode.

• Backward Reserve transfer is limited by constraint (1). In real operation Backward reserve transfer is possible by ramping down monopole or Bipole poles. In this case backward reserve transfer will be restricted by constraint (4). This is a conservative level of Backward reserves transfer. To make Backward reserve transfer effectively unrestricted in case of island risk event we can change the pole direction. The pole has to be blocked and then wait 5 minutes to be de-blocked in the opposite direction in the bipole-monopole transition.

• The 5 minute waiting time is six times less than most market schedules 30 minutes trading period. So we do not apply constraint (4) in 30 min. schedules. We will apply constraint (4) in RTD run during transition Bipole-Monopole.

• Round Power mode. Reserves transfer into forward or backward directions is limited by constraints (1). Constraint (3) does not affect a solution.

• HVDC National Frequency Keeping Market. • The same HVDC control system is used for

moving power from one island to another when there is a difference in frequency between the islands. In the future FK National Market we need to limit the sum of FK reserve transfer and Instantaneous reserve transferred by constraints (1,2,3).

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SPD Reserve Transfer Modelling Options (7)

Summary and Recommendation

• Use Option 1 to model aggregated HVDC losses and use reserve transfer constraints (1,2,3) for pre-dispatch and final pricing (30 minutes) schedules.

• Use Option 3 with more detailed HVDC losses can for all RTD (5 minutes) schedules. – Use reserve transfer constraints (1,2,3) for all RTD runs except

transitional runs between bipole-monopole and monopole – round power.

– In RTD schedule add constraint (4) and a new developed constraint for round power – monopole transition with a separate algorithm to address the Pole reversal 5 minutes waiting time issue.

– Reserve exported can only be used to cover AC type risks: Manual, ACCE and ACECE. It cannot cover any DC risks: DCCE, DCECE.

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Thank you