Harmonics Electrical - Power Quality
The quality of your electrical power supply matters!
Understand basic Power Quality issues your facility may suffer... and how to address them by reading a series of blog posts on Power Quality.
If you own or run a business, it’s very unlikely you think about the quality of your electricity supply... but you are likely to think about the cost of energy!
For a start, what is power quality and how can it vary? Surely this falls under the energy supplier's remit!? Does power quality really matter or is it just something specialist engineers need to worry about?
Chauvin Arnoux have written a white paper to address all of these questions and we've utilised this knowledge to produce a blog post! Let’s start off by saying that power quality is most definitely a concern for all of us. Have you ever had a piece of equipment... most likely a computer or other electronic item that misbehaved for no apparent reason?
Maybe you have lights that flicker, or if you operate a factory, motors that run hot and fail sooner than expected. All of these could be the results of poor power quality and if you don’t realise this, time and money you spend trying to fix the symptoms is likely a waste time and energy. And that’s not all: power quality issues can also increase your energy bills, eating further into your hard-earned profits. For these reasons, everyone who owns or runs a business needs to be aware of the basics of power quality, to understand how to assess it, and to know what to do if and when problems are identified. Or to put it another way, a few minutes spent reading these blog posts could save you a lot of time, trouble and money!
First, let’s make it clear that we’re only going to talk about AC supplies. This is because although DC supplies are possible, and were actually quite common until the middle of the 20th century, today's network operators only offer AC supplies. DC supplies may make a comeback in the not-too-distant future because in the modern world they have certain benefits, but that’s a topic for a different blog post/white paper!
In a perfect world, you would expect your network operator to provide you with an AC supply at a constant voltage, a fixed frequency, and with perfectly sinusoidal voltage and current waveforms that have no nasty 'spikes' on them. Also, if it’s a three-phase supply, you would expect the voltages of the three phases to be exactly the same. This would be perfect power quality. However, as we don’t live in a perfect world, your supply may not actually meet these requirements and, even if it does meet them at the point it enters your premises, it may well become degraded as it passes through the electrical installations on your site.
As this suggests, if you have power quality issues, in many cases it’s not the fault of your network operator. The operators do, in fact, go to great lengths to ensure that they deliver ‘clean’ supplies, but some of the factors that affect supply quality, such as thunderstorms and the equipment you’ve got installed in your own premises, are beyond their control.
That said, what can possibly go wrong with power quality?
In practice, almost all power quality issues can be divided into 6 main areas:
- Harmonics
- Dips and swells
- Transients (spikes)
- Interference
- Voltage imbalance
- Poor power factor
What is Harmonics in Electrical Power?
As was mentioned in the introduction, the voltage and current waveforms in an ideal power system would be perfectly sinusoidal and this would not be too difficult to achieve if all the loads connected to the power system were linear – that is, the loads where the current drawn from the supply is always proportional to the applied voltage. Simple heaters and incandescent lamps are examples of linear loads and, until the last few decades of the 20th century, loads were predominantly of this type.
Within the last 30 years, however, there has been a big increase in the number of non-linear loads connected to the electrical network. These include computers and associated IT equipment, uninterruptable power supplies, variable speed motor drives, electronic lighting ballasts and LED lighting, to name just a few. The growing use of such equipment, and the use of electronics to control nearly all types of electrical load, are beginning to have some worrying effects on the electricity supply and, in particular, on individual site installations. It is estimated that today over 95% of the harmonics present on a given site are generated by equipment installed on that site.
As we have stated, when a linear load is connected to the supply it draws a sinusoidal current at the same frequency as the voltage. Non-linear loads, however, draw currents that are not necessarily sinusoidal. In fact, the current waveform can become quite complex, depending on the type of load and its interaction with other components in the installation. Non-linear loads produce distorted current waveforms in the supply system, and in severe cases this can result in noticeably distorted voltage waveforms. The consequences can include significant energy losses, shortened equipment life and reduced operating efficiency of devices.
The distortion of the waveform produced by non-linear loads can be mathematically analysed to show that it is equivalent to adding components at integer multiples of the supply frequency to the 'pure' supply frequency waveform. That is, for a 50 Hz supply, the distortion takes the form of additional components at 100, 150, 200, 250, 300 Hz and so on - an example is shown in Figure 1.
Figure 1: A distorted waveform can be analysed as multiple sine waves added together.
These additional components are the harmonics, and in theory, they can go all the way up to infinity. In practice, however, it is rarely necessary to consider harmonics above say, the 50th, which has a frequency of 50 x 50 Hz = 2.5 kHz and, in most cases, only the lower order harmonics, up to the 25th, will be of importance. Unfortunately, unless they are prevented from doing so, harmonics from a non-linear load will propagate through the supply system causing problems elsewhere.
Knowing that a distorted current waveform can always be represented as a series of superimposed sine waves (using a mathematical procedure known as Fourier analysis) makes it possible to devise a measure of the amount of harmonic distortion present in the current in a supply system. This is known as the total harmonic current distortion or THDi, and is calculated with this formula:
Where 'I' is the current at the supply frequency, 'I' is the current at twice the supply frequency, I is the current at three times the supply frequency, and so on. Fortunately, it’s unlikely that you will ever have to use this formula in practice, as modern instruments for analysing harmonics carry out all of the necessary calculations automatically and simply present you with the THDi figure.
Harmonic currents have negative effects on almost all items connected to an electrical system; they upset sensitive electronic devices, they increase heating and they produce mechanical stresses. Among the most common effects of harmonics are computers crashing, IT equipment locking up, lights flickering, electronic components failing in process control equipment, problems when switching large loads, overheating of neutral conductors, unnecessary circuit breaker tripping and inaccurate metering.
While some of these effects, such as flickering lights and IT equipment crashes, could be dismissed as no more than irritants, others such as process equipment failures, can lead to costly downtime. Worst of all are failures of power factor capacitors and electrical distribution equipment like cables, transformers, motors and standby generators. Here the replacement equipment is likely to be expensive and may only be available on a long lead time. In these cases, both the repair costs and the consequential costs can be enormous. And, even if there are no outright failures, the presence of harmonics will cause reduced electrical efficiency within the installation leading to excessive power consumption which you will be paying for.
The risk to power factor correction capacitors relates to the fundamental property of any capacitor: its impedance decreases as the frequency rises. At high frequencies, a capacitor can behave almost as a short circuit. Power factor correction capacitors are generally designed with operation at the fundamental supply frequency in mind, and the reduced impedance they present to higher frequency harmonic currents leads to increased current flow and increased heating, which may result in premature failure. Capacitors can also be permanently damaged if the parallel circuit they form with an associated transformer happens to be resonant at one of the harmonic frequencies. Eddy current heating in motors and transformers is proportional to the square of the harmonic frequency, so it follows that as the presence of higher order harmonics in the supply system increases, the heating effect will increase even more dramatically. Not only does the generation of heat waste energy – which you are paying for - it also increases the risk of failures of or even fires in wiring, motors, transformers and other distribution equipment.
In addition to the losses that result from heating effects, harmonics in motors can give rise to the problematic phenomenon of torsional oscillation of the motor shaft. Torque in AC motors is produced by the interaction between the air gap magnetic field and induced currents in the rotor. When a motor is supplied non-sinusoidal voltages and currents, the air gap magnetic fields and the rotor currents will unavoidably contain harmonic frequency components.
These are grouped into positive, negative and zero sequence components. Positive sequence harmonics (1, 4, 7, 10, 13, etc.) produce magnetic fields, and hence torque, rotating in the same direction as the field and torque produced by the fundamental frequency of the supply. Negative sequence harmonics (2, 5, 8, 11, 14, etc.) produce magnetic fields and torque that rotate in the opposite direction. Zero sequence harmonics (3, 9, 15, 21, etc.) do not develop torque, but produce additional losses in the machine.
The interaction between the positive and negative sequence magnetic fields and currents produces the torsional oscillations of the motor shaft, which appears as shaft vibrations. If the frequency of these vibrations coincides with the natural mechanical frequency of the shaft, they become amplified and severe damage to the motor shaft may occur. It is sometimes possible to literally hear a transformer or motor 'sing' or 'growl' due to these vibrations and this is often one of the first observed indications of a harmonic problem. Some of the most troublesome harmonics are the 3rd, and odd multiples of the 3rd, i.e. the 9th, 15th etc. These harmonics are called "triplens". The triplen harmonics on each of the supply phases are in phase with each other so they add rather than cancel in the neutral conductor of a three-phase four-wire system. This can overload the neutral conductor if it has not been sized to allow for the potential presence of harmonics.
Fortunately, the identification and measurement of harmonics is easily achieved using a power quality analyser or power and energy logger (PEL) with harmonic capabilities. While the harmonics usually cannot be eliminated, since they are generated in the course of the normal operation of many types of load, they can be prevented from spreading throughout the distribution system and the wider power network.
This is usually done by installing passive or active filtering close to the source of the harmonics, and in some cases, by the use of tuned power factor correction equipment. Bringing harmonics under control will eliminate, or at least mitigate, all of the problems we have discussed in this section of the white paper, leading to benefits that include improved efficiency and longer life of equipment, and reduced energy costs.
A note of caution is, however, needed. Adopting measures to mitigate harmonics is unlikely to be a once-and-for-all-time solution. In today’s dynamic business environment, it’s likely that new loads will be connected to your electrical installation quite frequently. And, without measuring, how can you know how these are affecting overall harmonic performance? In other words, regular monitoring of harmonics is strongly recommended if the benefits of harmonic reduction are to be maintained.
The Power Quality Blog Post Series by Chauvin Arnoux
In practice, almost all power quality issues can be divided into 6 main areas and our Power Quality Series ('Understanding The Basic Issues of Power Quality and How to Address Them!') covers these 6 main areas:
- Harmonics
- Dips and swells
- Transients (spikes)
- Electrical interference
- Voltage imbalance
- Poor power factor
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