POWER QUALITY Q&A

Our Expert Offers Answers to Frequently Asked Questions About Power Quality

Question:
How can I tell if I have transients that may affect sensitive electronic equipment that I plan to purchase?

Answer:
If you are contemplating installing sensitive electronic equipment in your facility, especially if it is a factory, you may wish to know ahead of time if you have the potential for power quality problems.

The best way to determine this is to attach some higher quality monitoring equipment to look at the waveform, harmonic content and of course, transients in your system. This equipment must be capable of recording very fast transients as they may occur in microseconds. An example of a damaging, very fast transient that I have seen on an industrial power system is shown in Figure 1. This is almost impossible to detect without good equipment.

However, when its comes to transients, there are many possibilities. Some transients are long, some short, some very large, and some moderate. The various transients may each have completely different effects on the equipment that you are going to install.

So, if I go to the trouble of finding out what transients I have on my system, how do I know if they will adversely affect my sensitive equipment?

The Computer Business Equipment Manufacturers' Association (CBEMA) has developed a diagram that helps to define the types of transient that will affect our electronic equipment, and those types that will likely not affect our electronic equipment. The diagram (shown in Figure 2) has time as one coordinate and amplitude (in percent) as the other. If we place the characteristic time and amplitude of the transient on the diagram, it will fall into one of the three regions. If the transient falls into region I or III, we can reasonably expect to have a problem. If the transient falls into region II, it is not likely to cause a problem.

So how does this work? The manufacturers have agreed that equipment they manufacture will be designed to withstand transients in region II. So, if you are buying equipment from a reputable, quality-oriented manufacturer, you will likely not have any worries.

Of course there are no guarantees; the table is only a guideline. However, if you buy electronic equipment on price alone, you will likely guarantee yourself problems.

Question:
I have heard of something called a harmonic mitigating transformer. What is this and how does it work?

Answer:
In a distribution system where there are a significant number of single-phase loads, there is the potential for a cumulative effect of third order harmonics. These are also called zero sequence currents. Manufacturers of harmonic mitigating transformers have stated that this can reach levels of 80 per cent of the fundamental current. Figure 3 shows a typical waveform for a personal computer.

Typically, the zero sequence currents are additive in the common neutral conductor of single phase loads such as computers and other electronic equipment using switching mode power supplies. This creates higher than expected currents in the circuit and neutral conductors.

Basically, the harmonic mitigating transformer works by presenting a high impedance to ground for normal positive and negative sequence currents and an ultra low impedance for the zero sequence or ground currents. This effectively cancels the third order harmonics and returns the currents to more reasonable levels.

The claim for this type of transformer is that it will significantly reduce the level of third order harmonics on the secondary side. This, of course, will benefit the distribution system by saving energy, releasing system capacity, improving power factor and by reducing harmonic levels.

However, the harmonic mitigating transformer does not cancel or block the harmonics associated with a three phase non-linear load. These are typically the 5th, 7th, 11th and 13th harmonics and higher.

This type of transformer may be a better solution than a K-rated or de-rated transformer because it can be smaller and more energy efficient. The theory is that the harmonics are cancelled in the secondary winding with the harmonic mitigating transformer. The K-rated or de-rated transformer simply tolerates the excess harmonic loading rather than reducing it.

This solution is appropriate for distribution systems with a significant amount of computer and electronic non-linear load. Other techniques that can be used to mitigate harmonic loading problems are correct grounding practices, dedicated circuits (with separate neutrals), and oversized circuit conductors.

Systems with primarily three phase loads do not exhibit the same characteristics and are not likely to benefit from this type of equipment.