There’s a tenaciously prevailing opinion that mains-related problems can essentially be traced back to the power utility. This assertion is only partially correct. The truth is far more complex. Sources of power disturbances can be found at the power utilities as well as at the facilities of the power consumers. However, 85% of all power quality disturbances are indeed “homemade”, i.e. caused by the equipment operators themselves.
Under certain circumstances, connecting and disconnecting large loads on the part of the power utility may result in voltage fluctuations (i.e. overvoltage or undervoltage) or frequency fluctuations in the dynamic network. This mains pollution spreads throughout the entire grid and influences the functioning of sensitive devices such as rectifiers, networks and PLC systems.
Harmonics are caused by non-linear consumers such as thyristors, IGBTs and varistors. These components are used in, amongst other devices, DC power supplies (power packs), computers, electronic ballasts and dimmers, as well as in power converters and frequency converters for drive units using motors with adjustable speed. Modern electronics works with extra-low voltage. Incoming alternating current is first of all rectified by means of a bridge rectifier and smoothed by a smoothing capacitor.
Current consumed by these devices is pulsed. This results from intermittent charging of the smoothing capacitor downstream from the rectifier. The current’s waveform is changed by this intermittent charging: it’s no longer sinusoidal but rather superimposed with harmonic current. The consequence for the consumer is that sinusoidal current is no longer available from the mains.
Due to the fact that voltage and current frequencies are linked via line impedance, they cause current and voltage distortion at the terminals and vice versa. If current supplied to linear consumers is not distorted, i.e. if it’s sinusoidal, and if the voltage has a different waveform, i.e. not sinusoidal, the current at the mains terminals is also non-sinusoidal. The following table describes the various phenomena and their causes.
| Phenomenon | Primary Cause | Can be Limited by | |
|---|---|---|---|
| The Power Utility | The Power Consumer | ||
| Frequency fluctuation | Load changes, generation failure |
Yes | No |
| Gradual voltage changes |
Load changes | Yes | No |
| Rapid voltage changes / flicker |
Switching, special loads |
No | Yes |
| Voltage asymmetry | Asymmetric loading of the phases |
To some extent | Yes |
| Harmonics and sub-harmonics |
Special devices | To some extent | Yes |
| Signal voltages | Data transmission | Yes | Yes |
| Direct current or voltage |
Special devices (half-wave rectifiers) |
No | Yes |
| Voltage dips and interruptions |
Faults in the power utility’s or the power consumer’s network (short-circuits, interruptions) |
No | No |
| Temporary overvoltage | Faults in the power consumer’s network, power resonance |
No / to some extent |
To some extent / yes |
| Transient overvoltage | Lightning, switching operations | No | No |