Due to the proliferation of TVSS (transient voltage surge suppressors), while voltage transients are still here, they may not be captured by most power-quality monitors much longer.
Just about everyone with a computer at work or home has the line cord plugged into an outlet strip that is sold to prevent power-quality problems that would “wipe out your data and may even destroy your computer.” These outlet strips usually have several TVSS devices within them to clamp voltage transients at “safe” levels. Manufacturers of this safety equipment have quite effective campains. And while it is true that the potentially damaging voltage transients (also referred to in the past as spikes or impulses) are suppressed, this doesn’t mean that the transients should be ignored, nor are all the power quality problems eliminated.
There are different types of components used within the outlet strips to clamp the voltage. Some are very fast-acting components that may not be able to absorb lots of energy, and then there are slower-reacting components that can divert a significant amount of energy. Some of the more effective outlet strips employ both types, so that even the very fast transients are clamped, and the high-energy transients can also be suppressed through combinations of semiconductors, capacitors and even gas-discharge tubes. These components can be connected between the line-to-ground and neutral-to-ground, as well as line-to-neutral clamps.
When the line-to-ground transient does occur from a nearby lightning strike the voltage transient is absorbed and/or diverted into the ground conductor by the TVSS. The result is that there will be an accompanying current transient, which can be quite significant in magnitude. This requires that there be a low-impedance path in the ground conductor circuit to allow the current to flow into earth without causing a voltage rise on the ground conductor. If it were a line-to-neutral transient, there would also be large line-current transient, as the TVSS device acts like a short circuit across line-to-neutral to prevent the transient from being seen downstream at the load.
If your power quality monitor only triggers on peak-positive voltage transients, you will no longer see such events being recorded. If the TVSS is doing its job, the peak-positive voltage will most likely not exceed the trigger point of the monitor. However, these devices cannot absorb this energy forever. It is wearing down the capacity of the device each time it happens, just like the shock absorbers in your vehicle will age faster on bumpy roads than on smooth highways. Some devices may eventually degrade; others may eventually fail catastrophically. You need to be sure that your monitoring equipment can trigger on the accompanying current transient.
A lot of transients are also negative transients; that is, they subtract energy from the voltage sine wave, rather than adding to it. TVSS devices wouldn’t do much to help this situation, as they are designed to clamp the peak values that can cause destruction by exceeding the voltage-breakdown ratings of components within equipment. Negative transients can occur when two conductors are either shorted due to a fault, or briefly shorted during the commutation period of the SCRs in three-phase AC-to-DC converters used in equipment such as adjustable-speed drives. This is another phenomenon that your power-quality monitor may not be able to capture, if it only captures peak voltages. To see these transients, the monitor must be able to capture the transient relative to the normal sine wave voltage; that is, strip out the AC waveform and just look at what is left.
An additional note of caution is that TVSS devices in outlet strips do not provide any protection against the most common type of power-quality phenomenon, the voltage sag. Mitigation of this requires equipment that can provide energy to fill in the missing voltage, not clamp it. This doesn’t diminish the value of the TVSS for doing its job for positive voltage transients. And it doesn’t diminish the need to keep monitoring, to find out what “unseen” things are happening to your equipment, that may one day, become “seen” in a most unfortunate way. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.