Harmonic resonance at a medium-sized industrial plant

Harmonic resonance is a concern for industrial plants because approximately 50% of the normal electrical load consists of computer equipment, switch mode power supplies and AC induction motor adjustable speed controllers. Due to the nonlinear characteristics of these loads, significant harmonic voltages and currents are produced on the electrical distribution system. Measurements were made with a harmonic analyzer at several locations in an electrical distribution system to quantify harmonic currents and voltages. The system impedance diagram, prepared from the system one line diagram, was used to model the electrical system and calculate power system natural frequencies. Calculations indicated harmonic resonance occurs near the seventh harmonic frequency depending on configuration of the electrical power system. The plant equivalent circuit was prepared for several harmonic frequencies to show the amplification of currents and voltages due to the effects of harmonic resonance. Calculations to determine network harmonics are presented using a simplified system representation in combination with traditional circuit reduction methods. The results of these calculations are shown to be valid by actual system measurement