Harmonics prevention is better than cure

Transcription

1 ABB Feature Article Harmonics 1-5 Harmonics prevention is better than cure Harmonics generated by variable speed drives is a form of pollution of electrical plant that can cause severe problems. From flickering lights to exploding capacitors the range of symptoms is diverse, but should not be ignored. Recognising you have a harmonics problem is the first big step to its cure. Here, Geoff Brown, Principal Applications Engineer of ABB Automation Ltd, 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 recognised. 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 carefully guarded against. The symptoms of harmonics can be severe and serious problems can result. Transformers may overheat, even though they may be correctly sized or even oversized for the expected load, causing damage to insulation. Cables may get too hot and their insulation break down. Motors may also 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 recognised 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.

2 2 T he Principle of H armonic Distortion Funda m enta l 5th Ha rm onic The Sum Figure 1. The distorted current or voltage waveform is the sum of the fundamental (e.g. 5 Hz) wave and harmonic (25 Hz) wave. If you suffer any of these symptoms and can see no obvious cause, then you probably have a harmonics problem. What causes harmonics? 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 uninterruptible power supplies. Of these, variable speed drives probably need the most attention partly because of stringent regulations recently introduced by authorities. Further regulations are likely to be imposed for industrial environments in the near future. All power electronic converters used in the many different types of variable speed drive 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.

3 3 Transfomer is(t) = i1(t) + ih(t) u(t) Power electronic converter Rs Ls Point of common coupling, PCC Other loads Figure 2. A line diagram showing a typical power electronic converter generating harmonic currents The theoretical amplitudes of the harmonic currents caused by an ideal 6- pulse static switching circuit are inversely proportional to the order of the harmonics. So, for example, the fifth harmonic is one fifth or 2% 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. Harmonic current (%) Figure 3. Harmonic content in a rectangular current from a six-pulse rectifier

4 4 Filtering is an effective cure Utilities are required to provide a voltage that is sufficiently sinusoidal for equipment to perform correctly. The quality of the supply is set by national and international standards, some of which may be legally binding. 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 so preventing them flowing via the supply transformer to other loads or second, the utility can place limits on the harmonic levels that supply users are allowed to generate. Filtering can be an extremely effective cure, but 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 nonlinear loads. The converter s input current is controlled to produce an equal level of harmonics that the load is producing, but in the opposite phase. These two levels of harmonics cancel each other out at the point of common coupling. Minimising harmonics To allow drives users to meet the relevant standards, manufacturers should produce drives and offer expert advice that minimise 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 the standards that allows users to be categorised 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 your plant to harmonics. Simply take your network s short circuit power (fault level) at the point at which the drives are connected and divide by the sum of the installed power of your variable speed drives. If the end figure you obtain is above 2, the risk of harmonics is low, between 2 and 1 and the risk is moderate, while below 1 and the risk is high.

5 5 For example, a 3MVA HV/MV transformer has a 1% short circuit voltage, resulting in an approximate short circuit power of 3 MVA on the medium voltage terminals. If the installed variable speed drives total 1MVA, then this equates to more than 2, 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. Figure 4. Drive with six-pulse rectifier front end and DC link with an inductance and a capacitor 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 forms 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 cost, but the input current contains considerable low order harmonics.

6 6 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 twelve pulse solution For larger installations, say above 5kW, 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 sixpulse rectifier. By phase shifting the secondaries by 3 degrees, the sum of the secondary currents in the primary eliminates, for example, the 5 th, 7 th, 17 th and 19 th 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 inductances will produce less harmonic line currents. Therefore, one effective way to limit harmonics is to use drives with a large DC or AC inductor. 1.4 Harmonic Current (pu) V, 5 Hz 5th 7th 11th 13th 17th 19th 23rd 25th THD D C Inductance/mH = This Figure/Motor kw Figure 5. The size of inductance has clear influence to the harmonic currents

7 7 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 5Hz installation with the maximum x-axis value of 115 representing the rated load impedance. At a tenth of that value, THD is still at about 3% but at about 1% of rated load impedance (1 on the x- axis, which is a logarithmic scale) the THD sharply increases. Choosing an inductor larger than about 2mH 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 13% of the fundamental current. Table 1. A comparison of the THD values produced by different types of input bridges with a practical short circuit ratio of 5, is shown in this table: Drive THD current Thyristor rectifier 5% - 15% Six-pulse diode rectifier without inductor 13% Six-pulse diode rectifier with small inductor 7% Six-pulse diode rectifier with large inductor 45% Twelve-pulse diode rectifier with large inductor 1% Drive with IGBT inverter front end (active front end) Less than 1% To many engineers harmonics can be something of a mystery. But by choosing the right drives and utilising the expert help of drive manufacturers the problems of electrical disruption can be overcome before they even begin. 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 2/.4kV distribution transformer the supply impedance is 1% of the transformer s short circuit impedance.

8 STOP TURN LEFT No DC-Inductor, 6- Pulse Small DC-Inductor, 6- Pulse Large DC-Inductor, 6- Pulse Large DC-inductor, 12-Pulse Supply Transformer (kva) 1 TURN LEFT TURN UP START 2 Motor kw Example: 45 kw Motor is connected to a 2 kva Transformer. THD = ca. 3% with a Large Inductor Drive and ca. 11% with a No Inductor Drive Figure 6. Total Harmonics Distortion diagram 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.