The Triple-Harmonic Equivalent Circuit in Three-Phase Power Transformer Banks

Abstract

The importance of triple-harmonic voltages and currents in electrical power systems is generally recognized by engineers associated with the power and telephone industries. It is desirable to have some convenient method of representation and calculation of the triple-frequency effects in three-phase power systems. The path of the third-harmonic current is different fromn that of the fundamental current and for this reason the circuit can be considered as separate and, distinct. The characteristics of the triplefrequency circuit as determined by laboratory experiment are given in this paper. Methods of measurement of the required quantities for accurate solution of the triple-frequency circuit are described. The results of an experimental investigation of the equivalent triple-frequency circuit as proposed by H. S. Osborne in 1915 are set forth. A typical simple three-phase power network has been set up and the measured currents compared with computed values. Oscillograms are presented of voltages and currents with typical transformer connections demonstrating the principles of operation of the circuit both for normal and abnormal excitation conditions. The effects of various types of connected loads are discussed, with particular attention given to the condition of triple-frequency resonance. It is shown that triple-frequency resonance occurs when the external capacitive reactance of the circuit is equal to the triple-frequency no-load impedance of the transformer. Resonance produces large triple-frequency currents and serious distortion of the line-to-neutral voltage of the system.