Harmonic resonance in power systems - a documented case

Harmonic resonance occurs in a power system when the power system natural frequency corresponds to the frequency of a source of harmonic current. This paper studies a real case of power system harmonic resonance that resulted in failure of 20-kV capacitor bank fuses. The load current harmonics are measured and analyzed to investigate the individual harmonics of the current and then by using the system impedance frequency characteristic, the voltage harmonic distortion and the corresponding capacitor current are determined. Calculation of the power system natural frequencies are given for various configurations of system shunt capacitance and results of the power system harmonic current and voltage on the capacitor fuse blown is studied. The paper proposes a criterion for the proper size of capacitor to put in service according to system frequency characteristic and current harmonics.

I. Introduction

Shunt capacitors have been utilized for voltage support and power factor correction as a common practice in the electric power grids. Every power system has a natural frequency that is a function of the system reactance and the amount of power factor correction capacitors connected to the system [1]. Therefore, the installation of shunt capacitors can result in harmonic resonance and cause an increase in the system impedance at the corresponding frequency. In the case that a source of excitation; such as harmonic currents of nonlinear loads exists with a frequency near or equal to the system natural frequency, large harmonic voltage distortions can result due to the occurrence of resonant conditions. With the increasing spread of harmonic-producing loads such as motor drives, power electronic converters, electric arc furnaces, … the possibility of parallel resonance due to shunt capacitors has become a routine concern for capacitor applications [2] [5]. The problems associated with the parallel resonance are typical of harmonic related heating, including blown fuses, circuit breaker heating, and unexplained general equipment failures.