POWER QUALITY Q&A

Our Expert Offers Answers to Frequently Asked Questions About Power Quality

Question:
Some transformers used for harmonic loads are rated with a 'K' factor. What does this mean?

Answer:
When a non-linear load is supplied from a transformer, it is sometimes necessary to derate the transformer capacity to avoid overheating and subsequent insulation failure.

The reason for this is that the increased eddy currents caused by the harmonics increase transformer losses and thus generate additional heat. Also, the rms load current could be much higher than the kVA rating of the load would indicate. Hence, a transformer rated for the expected load will have insufficient capacity.

The K factor is a number derived from a numerical calculation based on the summation of harmonic currents generated by the non-linear load. The higher the K factor, the more significant the harmonic current content

The calculation goes like this:
where I_h = current at harmonic h
      h = harmonic number
Details of the calculation method can be found in IEEE Standard 1100-1992.

To help get around the problem of successfully applying derating factors to conventional transformers, the K factor is used by transformer designers to develop transformers made especially for non-linear loads and the extra heating caused by the harmonic currents. Transformers come in basic K factors such as 4, 9,13, 20, 30, 40, and 50.

The strategy is to calculate the K factor for your load and then specify a transformer with a K factor of an equal or higher value. In this way, the transformer can be sized to the load without derating.

The advantage of using a K factor transformer is that it is usually more economical than using a derated, oversized transformer.

Question:
When grounding a VFD system, do I have to take any special precautions?

Answer:
When one is designing a system for signal or communication purposes, we generally think in terms of normal 60 hertz grounding methods. In this case, a copper conductor of sufficient size to carry the expected fault current will usually meet all of our needs.

However, there are applications, such as VFD applications, where higher frequencies are involved. Here carrier or signal frequencies get into the kilohertz or higher ranges. In these cases, we have to think about how a copper wire will react to increased frequencies. One of the more commonly known effects is called the skin effect.

As the frequency increases, the current flow in a solid conductor will tend to track more closely to the outer surface, leaving the interior part of the conductor unused. Hence, there is less cross-section of copper available for the current flow and thus the impedance of the conductor increases.

How does this affect grounding? Well, if we have a high frequency component to the power, although we have lots of copper, the impedance may be high enough to disrupt our bonding and equipotential strategies and goals.

The increased impedance may cause potential differences between equipment, unwanted ground current loops, and perhaps even some safety issues.

How do we get around this? The way to reduce the impedance is to use smaller, multiple conductors to do the same job. In effect, multi-stranded wire is used. Braided straps or welding cable have many fine conductors and make excellent high frequency ground conductors.

In this way, the skin effect is minimised by keeping the conductors small but increasing the number of conductors. There is more usable copper and thus frequency will not have a significant effect.