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Transient Overvoltages

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General Explanation

Transient overvoltages are brief, high-frequency increases in voltage on AC mains.


Broadly, 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”


Protecting susceptible electronic equipments from AC power line disturbances is a major concern in commercial or industrial business environments. Computers, programmable logic controllers (PLCs), solid-state motor controllers, variable-speed drives (VSDs), and communications equipment all can be damaged by voltage transients.


High Frequency Transients

High frequency transients are caused by lighting, and by inductive loads turning off. Typical rise times are on the order of a microsecond; typical decay times are on the order of tens to hundreds of microseconds. Often, the decay will be an exponential damped ringing waveform, with a frequency of approximately 100 Hz, 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 equipments, remove EFT’s.


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

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 circumstances there can be “multiplication” of this transient by resonance with other power factor correction capacitors.


Capacitor switching - While this is one of the most common switching events on utility systems, it is one of the main causes of oscillatory transients. Figure 1 (below) shows a typical utility- capacitor switching one-line diagram and resulting oscillatory transient waveform from a capacitor switching operation.


CapacitorSwitchingTransients.png
















Figure - 1: One-line diagram (top) depicts capacitor-switching operation, resulting in the wave-form below (the graph below)

[Transient Overvoltages, Power Standards Lab, (http://powerstandards.com/tutorials/TransientOvervoltages.htm)]


This transient can propagate into the utility's local power system, pass through its distribution transformers, and enter into the end-user's load facilities. The extent of the transient's energy is a function of the turns ratio of the transformer feeding the end- user facility.


A common symptom that directly relates to utility capacitor switching overvoltages is that the resulting oscillatory transients appear at nearly identical times each day. This is because electric utilities, in anticipation of an increase in load, frequently switch their capacitors by time clock.


The resulting conditions, as a result of this switching include VSD tripping as well as disoperation of other electronically controlled equipment — all without any blinking of lights or other noticeable effects on more conventional loads. There's a potential side effect to this scenario: magnification of the utility capacitor-switching transients. This can occur when the end-user adds power factor correction capacitors at its facility. These capacitors can, in fact, magnify the transient overvoltages for certain low-voltage capacitor and step-down transformer sizes.


One way to counter this potential problem is for the electric utility to use synchronous closing breakers or switches with pre-insertion resistors. Another way is to use high-energy surge arresters at the end-user location to limit the magnitude of the transient voltage at the end-user bus. Still another way is to convert end-user power factor capacitor banks to harmonic filters. Appearing as an inductance in series with capacitors, these filters decrease transient voltages at the end-user bus to acceptable levels. This solution has multiple benefits:

• Correction for displacement power factor,

• Control of harmonic distortion levels, and

• Limitation of magnified capacitor-switching transients.


Important Aspects

• It is very important to identify the source or sources of these transient overvoltages in order to correct power system problems that affect susceptible electronic equipments

• Symptoms and timing play an important part.

• In the case of transient overvoltages, nature may be the culprit, as in the case of lightning.

• Applying equipment designed to improve system performance and save money, as in the case of switched capacitors, motors, and contactors, may also cause problems.


References

1. Transient Overvoltages, Power Standards Lab, Available online at http://powerstandards.com/tutorials/TransientOvervoltages.htm

2. Sources of Transient Overvoltages, Industry News, ecmweb.com, Available online at http://ecmweb.com/news/electric_looking_sources_transient/