Chapter 14 - Power Transformers

Abstract

This chapter concentrates on highlighting certain important aspects of voltage selection—calculation of transformer voltage ratio, specification of insulation levels, examples of voltage regulation, rating, tap ranges, and impedance calculations, and thermal aspects—specification of temperature rise and ambient conditions. The transformer equivalent circuit is a fundamental basis for transformer calculations involving voltage drop or regulation under various load conditions (short circuit currents, tap settings, power factor, and load currents). Vector representation of voltage and currents in transformer windings allows the practicing engineer to visualize the relationships involved. The length of the vector is made equal to the maximum or rms value of voltage or current. The convention is for the arrow heads to point away from the source of generation towards the load. The systems engineer, as opposed to the transformer designer, is chiefly concerned with the representation and characteristics of a given power transformer in the transmission and distribution network. In fault and load flow studies transformers are represented in the network diagrams by their equivalent impedances. The positive and negative sequence impedances of two winding power transformers are equal and equivalent to the ordinary leakage impedance used in three phase calculations. Transformer impedance is usually expressed as a percentage reactance (a transformer is highly inductive) on the base of the transformer rating.