Harmonics generated by variable speed drives are a form of pollution of the electrical plant that can cause severe problems. From flickering lights to exploding capacitors, the range of symptoms is diverse and not to be ignored. Recognizing you have a harmonics problem is the first big step to its cure. This article examines how variable speed drives cause harmonics and how they can be prevented.
Air pollution, pollution of the sea, fresh water pollution. All are highly visible and the cause can be easily recognized. But there is another form of pollution that is invisible, can be very damaging and yet, even when the symptoms are clearly seen, can be hard to diagnose. This pollution is harmonic distortion of electrical supplies and it is a phenomenon that must be guarded against carefully.
The symptoms of harmonics can be severe and serious problems can result. Transformers may overheat, causing damage to insulation, even though they may be correctly sized or even oversized for the expected load. Cables may get too hot, resulting in insulation breakdown. Motors may overheat or become noisy, and torque oscillations in the rotor can lead to mechanical resonance and damaging vibration. Capacitors overheat with–in the most severe cases–the risk of explosion as the dielectric breaks down. Electronic displays and lighting may flicker, circuit breakers can trip, computers fail and metering give false readings.
Often, the cause of these problems is not recognized and measures taken, such as installing extra cooling and higher rated transformers, cables and capacitors, may alleviate the problems but will not eliminate them. Such measures are also costly and disruptive.
If you suffer any of these symptoms and can see no obvious cause, then you probably have a harmonics problem.
Non-linear loads connected to the electrical supply cause harmonic distortion. Common non-linear loads include motor starters, variable speed drives, computers and other electronic devices, electronic lighting, welding supplies and uninterrupted power supplies. Of these, variable speed drives probably need the most attention.
All power electronic converters used in the many different types of variable speed drives can increase power line disturbances, distorting the supply waveform by injecting harmonic currents directly into the grid. Figure 2 shows how the current harmonics (ih) in the input current (is) of a power electronic converter affect the supply voltage. The grid in the primary transformer is assumed to have zero internal impedance, but due to the feeding transformer’s impedance (represented by Rs and Ls), the voltage waveform (u) at the point of common coupling to other loads will be distorted.
The theoretical amplitudes of the harmonic currents caused by an ideal six-pulse static switching circuit are inversely proportional to the order of the harmonics. So, for example, the fifth harmonic is one fifth or 20% of the fundamental current amplitude (see Figure 3). However, such circuits are not ideal and the amplitudes in practical circuits are different and usually higher.
Filtering is an effective cure
Utilities are required to provide a voltage that is sufficiently sinusoidal for equipment to perform correctly. In the face of non-linear loads, the utility has two choices for maintaining adequate supply voltage quality. First, the utility can filter out current distortions to prevent them flowing via the supply transformer to other loads, or second, the utility can place limits on the harmonic levels that its supply users are allowed to generate.
Filtering can be an extremely effective cure, but it involves a cost on the utility’s part. There are two basic approaches to filtering–passive and active.
Passive filters comprise a series of LC circuits tuned to the harmonic that is to be filtered. By connecting several in parallel, a bank of filters can be constructed to filter out all the troublesome harmonics.
Active filtering introduces an additional power electronic converter to the non-linear loads. The converter’s input current is controlled to produce a level of harmonics that is equal to what the load is producing, but in the opposite phase. These two levels of harmonics cancel each other out at the point of common coupling.
To allow drive users to meet the relevant standards, manufacturers should produce drives and offer expert advice that minimize any distortion produced.
Although it would be possible to filter out all harmonics at their point of generation, or even better, not produce them at all, this can add considerable and even unacceptable cost to the drive itself.
A convenient measure defined in standards that allows users to be categorized according to the level of harmonics they produce is given by the short circuit ratio (Rsc). The short circuit ratio is defined as the ratio of the short circuit power provided by the supply at the point of common coupling to the nominal apparent power of the drives. Users with small short circuit ratios will generally be subject to more stringent limits on the permissible level of harmonic current emissions than those with higher ratios.
The short circuit ratio gives a quick rule of thumb for assessing the vulnerability of a plant to harmonics. Simply measure your network’s short circuit power (fault level) at the point at which the drives are connected and divide the result by the sum of the installed power of your variable speed drives. If the end figure is above 200, the risk of harmonics is low, between 200 and 100 the risk is moderate, and below 100 the risk is high.
For example, a 30MVA HV/MV transformer has a 10% short circuit voltage, resulting in an approximate short circuit power of 300 MVA on the medium voltage terminals. If the installed variable speed drives total 1MVA, then this equates to more than 200, which is at a low risk level.
Choosing the right type of drive
The magnitude of supply harmonics generated is largely unaffected by the type of output inverter circuit used in the drive. Instead, the size of the motor load and the configuration of the drive’s input converter and DC link circuits mainly affect the level of harmonics.
If the motor load is relatively small compared to the supply short circuit capacity, harmonics problems can generally be avoided by choosing a Pulse Width Modulated (PWM) frequency converter with effective DC link filtering.
The PWM converter with six-pulse rectifier
A voltage source PWM frequency converter comprises a rectifier circuit, a DC link and an inverter. The rectifier circuit converts the incoming constant frequency AC voltage into DC, which in turn is converted into a variable frequency AC voltage by the inverter. The function of the DC link is to smooth the DC voltage to allow the inverter to function properly.
The most common rectifier circuit in a three-phase PWM frequency converter is a six-pulse diode bridge as shown in Figure 4. This comprises six uncontrolled diodes and an inductor, which together with a DC capacitor form a low-pass filter to smooth the line current. The inductor may reside on the DC or AC side or it can be left out completely, leaving only the source inductance of the mains for smoothing. This type of rectifier is robust and low in cost, but the input current contains considerable low-order harmonics.
Consequently, if the majority of the transformer load is made up of this type of converter, the feed transformer must be over-dimensioned and there may be difficulties in meeting the requirements. Some harmonic filtering may be required. Despite this, the six-pulse PWM frequency converter is likely to remain dominant in industrial applications.
The 12-pulse solution
For larger installations, say above 500kW, a PWM drive with a 12-pulse rather than a six-pulse rectifier can eliminate certain harmonic frequencies. The drive’s input transformer has two secondary windings, each supplying a six-pulse rectifier. By phase shifting the secondary windings by 30 degrees, the sum of the secondary currents in the primary eliminates, for example,
the 5th, 7th, 17th and 19th harmonics. This can result in Total Harmonic Distortion (THD) of less than a quarter that of a six-pulse installation. In addition, adding inductance on either the AC or DC sides will smooth the current waveform even more if required.
The disadvantage of this type of drive is that a special transformer is required. But in higher power applications a transformer for each drive would be used anyway. For smaller drives the cost is relatively high but even so some customers will specify a 12-pulse solution at lower powers where low harmonic distortion is critical.
The role of the drive’s DC link inductance
Drives with large DC link inductance will produce less harmonic line currents. Therefore, one effective way to limit harmonics is to use drives with a large DC or AC inductor.
The importance of the inductance is demonstrated by Figure 5, which shows how different harmonic currents decline as DC inductance increases. The diagram is based on a 415V 50Hz installation with the maximum x-axis value of 1150 representing the rated load impedance. At a tenth of that value, THD is still at about 30% but at about 1% of rated load impedance (10 on the x-axis, which is a logarithmic scale) the THD sharply increases. Choosing an inductor larger than about 20mH per motor kW, less than half of the total harmonic current can be achieved. If no DC inductor is used, the current distortion can be up to 130% of the fundamental current.
To many engineers, harmonics can be something of a mystery. But by choosing the right drives and using the expert help of drive manufacturers, the problems of electrical disruption can be overcome before they even begin.
Mauri Peltola is marketing manager, ABB Industry Oy Drives, Helsinki, Finland. Author’s Note: All these assumptions are reasonable to make and will give a rough estimation in most cases. However, if these assumptions do not correspond to the actual values in an installation, software such as ABB’s DriveWare can be used for an exact calculation.
Table 1. A comparison of the THD values produced by different types of input bridges with a practical short circuit ratio of 500, is shown in this table:
|Six-pulse diode rectifier without inductor
|Six-pulse diode rectifier with small inductor
|Six-pulse diode rectifier with large inductor
|Twelve-pulse diode rectifier with large inductor
|Drive with IGBT inverter front end (active front end)
||Less than 70%
Assessing harmonic levels
A quick assessment of the level of THD in your system can also be made using the Nomogram shown in figure 6. A number of assumptions have been made for this diagram
the maximum rated motor has been connected to the drive
the drive’s efficiency is 97%
standard efficiency motors are used
transformer impedances are those of a typical 20/0.4kV distribution transformer
the supply impedance is 10% of the transformer’s short circuit impedance.