Extended Reserve

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AUFLS is the acronym for Automatic Under-Frequency Load Shedding and describes the set of relays in New Zealand which automatically trip blocks of load, following a severe under-frequency event, to restore the system frequency.

The System Operator relies on these relays to prevent system collapse following under-frequency events which have the potential to cause a system black-out.

New Zealand's current AUFLS scheme is made up of two blocks of at least 16% of demand in each island. This means that 32% of customers' load can automatically disconnect to restore stability to the power system following the loss of net injection which drops the system frequency to below 48 Hz.

A review of the current AUFLS arrangements identified that the scheme could be amended to provide a better level of cover for both large defined risks and also a broader range of undefined larger "other" risks.

To improve overall performance and therefore reliability of the scheme, the System Operator identified dividing the AUFLS load into more blocks of smaller sizes in the North Island, and amending the trip settings of the second AUFLS block in the South Island. In addition for the North Island, the investigations also indicated a further performance improvement in the AUFLS scheme through using the rate of frequency fall to trigger AUFLS for at least one block.

AUFLS Animation

We have prepared an animation that summarises the findings of the Scheme Design Report and in available in our Learning Centre.

Additional Information

See Extended Reserve Project page for more information on the Extended Reserves Project.