HVDC bi-pole trip and change to under frequency reserve risk: technical commentary

On 11 December Transpower’s HVDC equipment suffered a bi-pole trip while transferring 90 MW to the North Island. The cause was unexpected and, while an explanation was quickly found, it was deemed necessary to change the risk treatment of bi-pole trip risk from ECE to CE until a fix could be determined. This change in risk status, with consequent market impacts, lasted 8 days until an interim fix to the faulted assets could be implemented.  A permanent fix will be developed and implemented in 2016. 
This report on the event is a technical commentary on the equipment, the event and the management of impacts from both Transpower’s grid owner and system operator perspectives. It is intended to provide a background to the nature of the event and describe how Transpower reacted to and managed the event outcomes. Readers are assumed to have reasonable knowledge of relevant grid, power system and market arrangements.