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)

The grid owner will implement a control change during the planned HVDC maintenance outage (from the 23rd to the 28th November 2017).  This control change:

  1. Will block a pole in the event of a loss of a second HVDC filter.
  2. HVDC transfer capability will then reduce to only 700 MW not to 140 MW which occurs with current bipole operation.
  3. Means there will be no change in the HVDC risk subtractor for an outage of one HVDC filter.
  4. Will come into effect from the 28th November 2017

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