Extended Reserve

How extended reserve works

The electricity system has tools in place to ensure its resilience during a significant event. Extended reserve deals with sudden loss of supply by reducing demand in a managed way.

AUFLS in action

The automatic under-frequency load shedding (AUFLS) scheme is the extended reserve mechanism currently used in New Zealand. It sheds large blocks of load to prevent the electricity system collapsing if there is a significant loss of supply.

AUFLS is a ‘last resort’ mechanism to restore the balance between electricity demand and supply. If a power station suddenly stops generating electricity or there is a major interruption in transmission, and instantaneous reserve cannot restore grid frequency, AUFLS relays automatically trip to disconnect consumers on segments of the network. This shedding of demand is AUFLS in action.

Disconnecting a portion of load for one to three hours during a large under-frequency event prevents a complete electricity system collapse, after which it could take more than a day to restart and fully restore the system.

Obligations under the Code

The Electricity Industry Participation Code 2010 places obligations on distributors and direct connect consumers to automatically disconnect blocks of load at specific frequency and time settings during large under-frequency events.

This obligation to provide AUFLS rests with distributors and direct connects in the North Island, and with Transpower in the South Island. Each block of AUFLS represents a fixed proportion of the participant’s load.