scope:

PURPOSE AND SCOPE

The normal operational security of a transmission substation is dependent on the availability of its auxiliary LVAC supplies to drive site assets such as transformer tap changers and cooling, air compressors, switchgear drive motor circuits and site battery chargers.

The ideal is to provide LVAC diversity to site, that is to have site supplies which are independent and of high security. In achieving this, normal site operations will be maintained in the event of the loss of the permanent LVAC supply under fault conditions.

It is recognised by National Grid that following a major system disturbance, when large parts of the transmission system may become islanded, substation auxiliary supplies reliability and availability plays a major role in the recovery of the system back to normality.

Substation LVAC supplies are derived from combinations of the following feeder arrangements:

• On-site aux/ earthing transformers - permanent supply.
• An adjacent Generating Substation - permanent supply.
• A local Distribution Network Operator (DNO) - permanent supply.
• By on-site diesel generating set(s) - standby supply.

This policy details the LVAC connection strategy at both new substations and legacy substations where retrospective enhancement or refurbishment work on the auxiliary supplies is planned.