HVDC reserve modelling

The “HVDC risk subtractor” is the value used in the market system to model the under frequency Instantaneous Reserves (IR) required for a HVDC Contingent Event (the loss of a HVDC pole).  

It represents the overload capability of the HVDC pole which remains in service.

IR is purchased using the formula below:

IR = HVDC received – “HVDC risk subtractor – Net Free Reserve (NFR)

Increasing the HVDC risk subtractor will therefore decrease the amount of IR required to cover for HVDC Contingent Events.

HVDC overloads

To increase the overload to a guaranteed minimum value of 650 MW for north transfer and 619 MW for south transfer the grid owner operates the pole in two ways;

  • The Pole 2 overload time has been reduced to 15 minutes
  • Pole 2 will be operated to a maximum pre-event dispatch value of 420 MW.

To maintain the HVDC bipole capability of 1200 MW, Pole 3 maximum is 780 MW, provided ambient temperatures do not exceed 28C.  The maximum transfer on Pole 3 will be 770 MW when (occasionally) ambient temperatures exceed than 28°C limit cooling capacity.


Maximum overload (MW) received value

Time overload is sustained for (minutes)

North transfer Pole 2



South transfer Pole 2



HVDC filter outages

In normal operation the loss of one HVDC filter does not reduce HVDC transfer capability.  

If one HVDC filter is on outage and another filter trips there can be a reduction in HVDC transfer capability, to a value of around 140 MW. The reduction ensures filter rating limits are maintained.  If HVDC transfer is above the current transfer capability the HVDC controls quickly reduce transfer to the rated capability. This may cause an under frequency event in the island receiving the HVDC transfer. IR is purchased to maintain power system frequency limits for such an event.

To purchase the correct amount of IR the risk subtractor is set to the worst HVDC transfer capability for any one of the HVDC filters tripping. This results in an increase in the quantity of IR required to cover HVDC operations.

HVDC control change for the loss of a second HVDC filter

The control system  blocks a pole in the event of a filter trip. This will often result in increased HVDC risk subtractors during filter outages.

Appendix G of the updated Bipole Operating Policy (Dated the 1st Nov 2021) shows the worst case n-1 limits as sent values) for each filter outage. These values converted into a received value, will represent the HVDC risk subtractor.  Please note that the risk subtractor may vary from that listed in Appendix G due to other DC or AC grid conditions at the time of the outage.

For further questions please contact Market Operations on 04 590 7470 or 021 684398, or [email protected].