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 (Pre 30 November 2016)

When the HVDC is in bipole operation it is able to cover some or all of the unexpected loss of one of the HVDC poles by increasing the transfer on the HVDC pole which remains in service. Transfer can be increased up to the maximum overload on the pole remaining in service.


Maximum overload (MW) received value

Time overload is sustained for (minutes)

Pole 3 (both directions)



North Transfer Pole 2

528 to 650


South Transfer Pole 2

515 to 619


The limiting component which causes P2 to be variable is the initial temperature of the subsea cable, pre-event.  The initial temperature of the cable before overload is a function of the power transfer through Pole 2.

The variability in overload quantity means the minimum value is assumed by the market system when modelling under frequency reserves.

HVDC overloads (Post 30 November 2016)

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

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

To maintain the current HVDC bipole capability of 1200 MW, Pole 3 maximum transfer will increase from 700 MW to 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 (Pre 28 November 2017)

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 (Post 28 November 2017)

A HVDC control change came into effect from the 28th November 2017.

  1. From the 28th November the control system will block a pole in the event of a filter trip that previously caused a bipole runback to 490 MW (sent) or less. This change will often result in increased HVDC risk subtractors during filter outages.
  2. Appendix G of the updated Bipole Operating Policy (Dated the 1st March 2018) shows the new worst case n-1 limits as sent values) for each filter outage. These values converted into a received value, will represent the new 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.

A full explanation of HVDC risk subtractors including appendix G can be found by logging into your Transpower customer account on the website and clicking this link Bipole Operating Policy 1st March 2018. For further questions please contact Market Operations on 04 590 7470 or 021 684398, or Market.operations@transpower.co.nz.

Subscribe for updates

Subscribe to this page to receive email notifications when this page has been updated.