EFFECTS OF HARMONICS

In electrical power system, harmonics are not desirable in most of the applications and operations. Harmonics have adverse effect on power system equipment as well as on its operation. Harmonics can create resonance in power system network. Damping property may change due to the presence of harmonics. Also it has some adverse effects on performance of rotating machines, transformers and transmission networks.

Accuracy and operating characteristics of measuring instruments and protective devices may change due to the presence of undesirable harmonics. Performance of reactive power compensation devices may change. Moreover presence of harmonics has some adverse effects on different consumer equipment. Effects of harmonics are classified in the following way.

1) RESONANCE

Capacitors used for power factor correction cause system resonances due to harmonic frequencies. This results in excessive high current, which can produce damage to the capacitors. Resonance occurs when the frequency at which the capacitive and inductive reactance of the circuit impedance are equal. At the resonant frequency, a parallel resonance has high impedance and series resonance low impedance. Harmonic resonances create problems in operation of power factor correction capacitors.

There are three types of resonance occurred in power system

1. PARALLEL RESONANCE: which offers high impedance at the resonance frequency and increases harmonic voltages and harmonic currents.

2. SERIES RESONANCE: which results in high capacitor current at relatively small harmonic voltages. Magnitude of this current depends upon the quality factor of the circuit.

3. COMPLEMENTARY AND COMPOSITE RESONANCE: Normally, composite resonant frequency is of non-integer type frequencies generated from conversion from the fundamental frequency and DC components of the converter control circuit.

2) POOR DAMPING

In presence of harmonics, variable speed drive motors or a switched mode power supply introduces small negative impedance or resistance. It decreases the current with the rise of voltage, which reduces the damping or broadband energy absorption capability of the system. Undesirable variation of the degree of damping changes the operating performance of different electrical devices, especially different measuring and controlling instruments.

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