INTRODUCTION

The electrical systems in a power station includes those parts associated with connecting the station to the grid and the ones that distribute power to the auxiliary systems inside the station. The reliability of the power supply depends on the importance of the equipment. For example, if the impairment of a piece of equipment will result in a unit trip, its power supply should be more reliable than a sump pump used occasionally. It is therefore necessary to determine the importance of each piece of equipment to the power plant when the degree of reliability of the power supplies is required. The sources of power supplies in the plant vary from gridderived ac supplies to battery-backed ac and dc supplies.

The electrical supply to the auxiliary loads in the power plant can be taken from the following sources:

• Grid: derived ac supplies through a station system transformer (SST)

• Unit: derived ac supplies through a unit service transformer (UST)

• A combination of grid- and unit-derived ac supplies

Most modern power plants are choosing the second option while relying on the grid-derived ac supplies as backup power. The complete details of the electrical loading, rating, and duty information for all station loads including reactor, boiler, and turbine-generator should be established at the onset to enable accurate design of the electrical systems. The design ratings of the transformers, switchgear, cables, etc., can be determined when all the electrical system loadings are assembled. The electrical loading, rating, and duty schedules of all plant...