Harmonics

Definition

A sinusoidal component of a periodic wave having a frequency that is an integral multiple of the fundamental frequency.

Effects of harmonics

•          Increased losses in the machines

•          Increased skin effect losses in conductors and hence higher I²R losses

•          Peak value of the waveform increases due to this increase in electric stress, hence reduction of life of equipment

•          Higher frequencies cause mal-operation of equipment (energy meter of induction type read more when harmonic load connected)

•          Protective relays can have unpredictable behavior (Some relay operate slower when energized with distorted inputs, while some tends to be more sensitive  and operate faster).

•          In three phase four wire network, if harmonics are present, large harmonic currents will flow through the neutral conductor. These harmonic current will be larger than the fundamental current. Hence the size of the neutral conductor will have to be increased

•          Fan motors when used with the electronic type regulators produce humming sound

•          Current harmonics produce interference in the communication networks when the communication lines run parallel to the power transmission lines.

•          Harmonic in the system affect the nature of fundamental wave

•          When the harmonic is in the phase with fundamental, the resultant wave is distorted with a squashed top

•          If the phase of harmonic wave relative to the fundamental is 90⁰ leading, the composite waveform changes to peaked form

Effect of harmonic on capacitor

The impedance of capacitor is inversely proportional to the frequency.   Hence a capacitor offers a low impedance path for the harmonic currents.

The effect of harmonics on capacitors are:

•          Increased capacitor current

•          Increased kVAR losses

•          Higher temperature rise

•          Blowing of fuses

•          Bursting of capacitors

•          Resonance between capacitors and the system inductance

Sources of harmonics

•          Transformer under no-loads and light loads

•          Saturated reactors

•          Rotating machines

•          Arc furnaces

•          Induction furnaces

•          Gas discharge lighting

•          Rectifiers

•          Industrial drives

•          Electrolysis plants

•          Energy conservation devices : soft starters, electronic chokes for tubular fluorescent lamps, electronic fan regulators

•          Equipment with switched mode power supplies : TV receivers and personal computers

Total Harmonic Distortion

Where    I_sc = Maximum short circuit current at point of common coupling.

And        I_L = Maximum demand load current (fundamental frequency) at point of common coupling.

TDD        =     Total demand distortion in % of maximum demand

Harmonic Distortion Solution

Þ           Specify the type of controller or drive to minimize harmonic production: If two 6-pulse thyristor DC drives are connected in series or parallel with 30⁰ phase shift between them a 12-pulse circuit results and the harmonic production is greatly reduced (5^th & 7^th harmonic are generally reduced).

Þ           Pulse multiplication and PWM circulating have been used to create the “perfect harmony” line of medium voltage variable frequency drives: No filtering required

Þ            If there is a choice to be made in laying out the distortion system one should isolate the harmonic producing equipment as much as possible.

Þ           Shunt Networks: Because the harmonics are current sources, something to divert these currents from electrical utility at the point of common connection is necessary:

                - If a shunt network, which has an impedance at a harmonic frequency much lower than the utility source impedance at that frequency, the most of the harmonic current can be diverted.

Harmonic Filter Circuits