Historically, harmonics were mainly caused by magnetisation non-linearity, for example, in transformers. Nowadays, power electronics based equipment is the main source of harmonic pollution in the low-voltage network.
Examples of such equipment include Variable Speed Drives, UPS’s, Welders, PC’s, Printers etc. In general, the semiconductor switches in this equipment conduct only during a fraction of the fundamental period. This is how such equipment can obtain their main properties regarding energy saving, dynamic performance and flexibility of control. However, as a result a discontinuous current containing a considerable amount of distortion is drawn from the supply.
Many types of equipment found in modern electrical installations can be considered as harmonic current sources. The currents are injected into the supply system distorting the supply voltage. This then gives rise to harmonic currents at other locations, even when at these locations, no harmonics generating equipment is present.
Also, a resonance phenomena is introduced by the interaction between the inductive supply system and power factor correction capacitor banks. This amplifies the harmonic distortion when the resonance occurs around harmonic frequencies.
Harmonic pollution is costly
Harmonic pollution causes a number of problems. A first effect is the increase of the RMS value and the peak value of the distorted waveform. This increase in the RMS value leads to increased heating of the electrical equipment.
Furthermore, circuit breakers may trip due to higher thermal or instantaneous levels. Also, fuses may blow and capacitors may be damaged. KWh meters may give false readings. The winding and iron losses of motors increase and they may experience perturbing torque’s on the shaft. Sensitive electronic equipment may be damaged. Equipment which uses the supply voltage as a reference may not be able to synchronise properly and either apply wrong firing pulses to switching elements or switch off. Interference with electronic communications equipment may occur. Also, in installations with a neutral, zero phase sequence harmonics may give rise to excessive neutral currents.
Excessive harmonics leads to premature ageing
Overall, it may be concluded that an excessive amount of harmonics leads to a premature ageing of the electrical installation. This is an important motivation for taking action against harmonics.
The harmonic pollution may only affect equipment in the polluting plant but may also disturb equipment in other plants. In order to limit this disturbance, maximum allowable distortion limits have been defined in standards and recommendations (see Electricity Authority UK recommendation G5/4 below). Also, the International Electro-technical Commission (IEC) has issued a technical report (IEC 1000-3-6) which outlines assessment procedures to determine whether distorting loads may be connected to the supply system.
In many cases the regulations impose a limit for the total harmonic distortion (THD) of the voltage or current present at the point of common coupling (PCC). The PCC is the location at which the plant is connected to the public power system (generally at the primary of the main transformer). The utilities may impose penalties on users, which introduce too much harmonic distortion on the supply. This is another motivation for taking action against harmonics.
Inverter Drive Systems delivers a harmonic distortion survey to determine any problems with the quality of the incoming supply or the extent of the harmonic distortion being created within your site. We have a state-of-the-art software package on a laptop that connects via a power meter directly to the mains supply. This software is able to measure simultaneously harmonic orders up to the 23rd harmonic.
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