Q&A: High-voltage DC insulation

Harmonics in power systems are a major concern as they are becoming increasingly prevalent and they can give rise to serious problems that include excessive heating and premature equipment failure. While there is good general awareness of these issues, many questions still arise, not least in relation to terminology. Here are answers to a few of the most common of these questions.

Q: Why is there a tendency for harmonic levels in supply systems to be greater now than in the past?

A: Harmonics are, for the most part, produced by non-linear loads. Linear loads, such as incandescent light bulbs and electrical heating elements, draw current throughout the whole cycle of the supply waveform. Non-linear loads, such as the power supplies used in computers and televisions and the variable speed drives widely much used in industry, draw current for only part of the supply cycle. Because of this, they tend to distort the supply waveform, which is the same thing as saying that they generate harmonics. Since non-linear loads are becoming increasingly common, the level of harmonics in many supply systems is also increasing.

Q: What is meant by the order and sequence of harmonics?

A: Harmonics are voltages and currents at frequencies that are integer multiples of the supply frequency. The order of the harmonic is simply a way of stating which multiple applies to a particular harmonic. For example, with a 50 Hz supply system, the frequency of a second-order harmonic is 100 Hz, that of a third order harmonic is 150 Hz and so on. The sequence of harmonics in three-phase systems - and, in principle, in any other type of multiphase system - defines whether the particular harmonic tends to produce a rotating magnetic field that adds to or opposes the rotating field produced by the fundamental. Positive-sequence harmonics add to the field of the fundamental, while negative-sequence harmonics oppose it. Zero-sequence harmonics create a stationary field that does not rotate, but they do increase overall current demand and generate heat. Harmonics with orders divisible by three always generate a zero-sequence field and are often referred to as triplens.

Q: Why are harmonics so troublesome?

A: The biggest problem is that they cause heating. Triplens in particular produce currents in the neutral conductor that do not cancel out, even if the load is nominally balanced across the three phases. This leads to heating of the neutral, which can be a significant issue as in many three-phase installations the neutral conductor is not fully rated - that is, it has a lower current carrying capacity than the phase conductors. Also, in equipment like a transformer or a motor that has a magnetic core, even relatively small harmonic currents can produce a lot of heat, because the heating effect increases with frequency. Other problems that can be caused by harmonics include, but are by no means limited to, buzzing in transformers and motors, and flickering of lights.

Q: What can be done to reduce the impact of harmonics?

A: The best solutions involve tackling harmonics at the source, typically by fitting chokes or filters to the equipment that is producing the harmonics. As always, however, to control something effectively, it is necessary to be able to measure it and for this a power quality analyser is needed. A good instrument will detect and measure harmonics up to at least the 50th multiple of the power frequency, and will provide information about the amplitude of individual harmonics as well as the total harmonic distortion of the supply. This means that the sources of harmonics can be readily determined and the effectiveness of harmonic mitigation measures quickly assessed.