1. Role of HVDC Configurations in Modelling of NZ National Reserve Market Vladimir Krichtal SO Development, Transpower NZ

2. Contents Existing Real HVDC Operation Proposed Real HVDC Operation SPD Reserve Transfer Modelling Options 2

3. 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. 3

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5. 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. 5

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8. SPD Reserve Transfer Modelling Options (1) 8

9. SPD Reserve Transfer Modelling Options (2) 9

10. 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. 10

11. 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. 11 SPD Reserve Transfer Modelling Options (4) HVDC flow(MW)30165 Bipole losses (MW) 1.29 Monopole losses (MW)0.082.59 Round Power losses (MW)0.43 Table1

12. SPD Reserve Transfer Modelling Options (5) 12

13. 13 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.

14. 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). 14 SPD Reserve Transfer Modelling Options (7)

15. 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. 15

16. Thank you