HVDC Reserve Modelling

The “HVDC risk subtractor” is the value used by 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 that remains in service.   

IR is purchased on the basis of 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.

Existing HVDC overloads

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 that remains in service. Transfer can be increased up to the maximum overload on the pole which remains in service.


Maximum overload (MW) received value

Time overload is sustained for (minutes)

Pole 3



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.  

Proposed HVDC overloads

To increase the overload to a guaranteed value of 650 MW the grid owner is investigating reducing the Pole 2 overload time to 15 minutes and holding Pole 2 at a maximum pre-event dispatch value of 420 MW.

To maintain a HVDC bipole capability of 1200 MW, Pole 3 maximum transfer would increase from 700 MW to 780 MW. (The maximum transfer on Pole 3 will very occasionally be 770 MW when ambient temperatures of greater than 28 C limit cooling capacity.)

The proposed new overload is given in the table below for comparison;


Maximum overload (MW) received value

Time overload is sustained for (minutes)

North transfer Pole 2



South transfer Pole 2



Existing 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 new capability the HVDC controls quickly reduce the transfer to the new capability. This causes an under frequency event in the island receiving the HVDC transfer. IR is purchased to maintain power system frequency limits for the 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 the HVDC.

Proposed HVDC control for the loss of a second HVDC filter

The grid owner is investigating a control change which would trip Pole 2 (leaving Pole 3 in service) in the event of a loss of a second HVDC filter. The HVDC transfer capability would then reduce to only 700 MW not the 140 MW with bipole operation.

This change means there would be no change in the HVDC risk subtractor for the outage of one HVDC filter.

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