PURPOSE

The purpose of this Design Handbook is to supplement the existing Technical Specifications and Protection Application & Setting Policy for LVAC supplies. This includes supplies from DNO, Tertiary, Auxiliary/Earthing Transformers, Power Stations and Diesel Generators.

Where a Tertiary winding is supplying Reactive Compensation equipment then the requirements for the HV protection application and settings are adequately covered in PS(T) 10. In this case there may also be a voltage regulator to maintain the LVAC voltage to within limits defined in TS 2.12. See Appendix B.

Where the Tertiary is supplying an earthing/auxiliary transformer exclusively for LVAC supplies then this Design Handbook should be referenced for correct protection application and settings.

The need or otherwise for an off-load tap changer or voltage regulator supplied with the auxiliary transformer should be considered at the design/development stage taking into account the historic normal running voltage of the substation busbars. National Grid have experience of a substation historically running at 406kV, more recently running at 396kV due to local wind farm connections.

The complete LVAC system design will consist of a number of Circuit Breakers (ACBs or MCCBs with integral protection modules), and Switch/Fuse units. Calculation reports should be produced to demonstrate discrimination as far as reasonably practicable with all these devices.