30 Jun Common issues with power supply
There are a number of problems which may occur with the incoming power supply to a site. These problems cause voltage and current instability which can have a significant impact on equipment operation and power usage.
Voltage stability refers to the ability of a power system to maintain steady voltage levels after a disturbance. The main forms of voltage instability can be categorised as:
- sustained under or overvoltage
- transient events and
- waveform distortion.
Undervoltage describes a sag in voltage for a short period or a sustained reduction in the system voltage level for a period of time (sometimes described as a brownout). Common causes of undervoltage conditions include the start-up of large loads or other long-term system faults.
Conversely, overvoltage refers to a swell in voltage levels supplied which may occur over a short or sustained period and is the reverse of an undervoltage condition. Common causes of overvoltage include sudden or persistent load adjustments which are not compensated for, or phase fault events.
A transient is a sharp change in voltage or current away from normal operating levels for a short period of time (usually no more than 1ms in duration). It normally occurs due to an event which causes a change in the circuit conditions, internally or externally to the circuit. As a result, an oscillating (both positive and negative change) or ‘spike’ (high positive change) of voltage occurs, which can be transmitted throughout the system.
Examples of events which may cause a transient to occur in an electrical system include lightning, inductive electrical equipment, electrostatic discharge, and circuit faults such as short circuits or circuit breaker operation.
Waveform distortion involves the changing of the fundamental supply AC waveform due to influences such as:
- Harmonics – Harmonic voltages and currents are caused by non-linear loads such as variable speed drives (VSD), uninterruptible power supplies (UPS), low energy lighting and switched mode power supplies in electronic devices such as computers. Non-linear loads generate harmonics by drawing current in abrupt short pulses, rather than in a smooth sinusoidal manner, introducing currents of additional frequencies which are reflected back into the system, distorting the AC waveform.
- DC Offset –DC voltages introduced into a system are referred to as DC offsets. They are superimposed onto the fundamental supply waveform, causing distortion and are often caused by conversion devices such as rectifiers which convert between AC and DC.
- Noise – Noise refers generally to any unwanted voltages or currents which are superimposed on the fundamental supply waveform, causing distortion. There are many sources of noise in a system but one particular example may be electronic devices (specifically power electronics and switch mode power supplies or control circuits within such devices).