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Transient overvoltages are brief, high-frequency increases in voltage on AC mains.
Broadly speaking, there are two different types of transient overvoltages: low frequency transients with frequency components in the few-hundred-hertz region typically caused by capacitor switching, and high-frequency transients with frequency components in the few-hundred-kilohertz region typically caused by lighting and inductive loads. Low frequency transients are often called "capacitor switching transients". High frequency transients are often called "impulses", "spikes", or "surges". Surge suppressors are devices that conduct across the power line when some voltage threshold is exceeded. Typically, they are used to absorb the energy in high frequency transients. However, the resulting high frequency current pulses (often in the hundreds of amps) can still create problems for sensitive electronic systems, especially delicate instrumentation. Low frequency transients are caused when a discharged power-factor-correction capacitor is switched on across the line. The capacitor then resonates with the inductance of the distribution system, typically at 400 - 600 Hz, and produce and exponentially damped decaying waveform. The peak of this waveform, in theory, cannot exceed twice the peak voltage of the sine wave, and is more typically 120% - 140% of the sine peak. However, in some specific cicumstances, there can be "multiplication" of this transient by resonance with other power factor correction capacitors. High frequency transients are caused by lightning, and by inductive loads turning off. Typical rise times are on the order of a microsecond; typical decay times are on the order of a tens to hundreds of microseconds. Often, the decay will be an exponential damped ringing waveform, with a frequency of approximately 100 kHz, which corresponds to the frequency of equivalent inductor/capacitor model of low voltage power lines. Typical peak voltages for end-use applications are hundreds of volts to a few thousand volts; several thousand amps of current may be available. (Extremely fast transients, or EFT's, have rise and fall times in the nanosecond region. They are caused by arcing faults, such as bad brushes in motors, and are rapidly damped out by even a few meters of distribution wiring. Standard line filters, included on almost all electronic equipment, remove EFT's.) Contact Alex McEachern for more information about:
Alex McEachern, 1/2004, based in part on work done with Mark McGranaghan © 2004 Power Standards Lab. All rights reserved. |
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