Voltage Transients

What are Voltage Transients?

Although it is rarely noticed, power systems are constantly subject to voltage transient behaviour.  Voltage transients can come from many sources but the most severe generally tend to originate from local load switching events, grid switching events or lightning strikes.

Figure 1 shows a typical voltage transient resulting from an indirect (induced) voltage strike.  It is obvious that there is a severe, albeit brief, overvoltage imposed on the system voltage. These transients produce no visible effect in the way that voltage dips / swells can in lighting that alert users to the fact that a voltage event has occurred.

Sub-cycle transients such as that shown below do not produce any discernible increase in the general voltage level but can have far more damaging effects on any load exposed to it. These effects can range from erroneous operation of digital electronic equipment to actual physical damage resulting from exposure to such fast rising, high voltage transients.

Figure 1 – typical ground induced voltage transient resulting from lightning strike

Protection from voltage transients

Protection from the damaging effects of voltage transients is generally provided by transient voltage surge suppression (TVSS).   These devices are connected in parallel with the load and conduct virtually no current until a voltage threshold is crossed, at which time the TVSS exhibits very low impedance and limits any further increase in voltage across it.

Figure 2 shows the voltage / current characteristic for two types of surge arrester, namely a Zinc Oxide (ZnO) and a Silicon Carbide (SiC) arrester.  You can see that both arresters draw very low current current up until the threshold voltage of approximately 300 Volts.  Once this voltage has been exceeded, both types of arrester start to exhibit a much reduced impedance to voltage.

Clearly, the Zinc Oxide arrester has a much better characteristic for a surge arrester as the voltage developed across it is limited due to its much lower impedance.

The installation of surge arresters need careful thought as it is important to match the characteristic of the arrester to the load being protected.  Often it will require a series of coordinated TVSS devices to protect a sensitive load.  This may require a high energy arrester at the main incoming supply which has a reasonably high let through voltage, and a more sensitive lower energy, lower let through voltage nearer to the load being protected.

Figure 2 – Voltage / current characteristic for typical surge arresters

How we can help

Using advanced power quality analysis equipment, PureSine can capture and diagnose transient events lasting microseconds and provide mitigation recommendations and arrange installation of any proposed solutions.

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