Variable speed drives. A Guide to. Supply harmonics and other low-frequency disturbances - PDF

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Variable speed drives. A Guide to Supply harmonics and other low-frequency disturbances This guide is one of a series covering subjects such as harmonics, safety features, EMC, feedback devices, industrial
Variable speed drives. A Guide to Supply harmonics and other low-frequency disturbances This guide is one of a series covering subjects such as harmonics, safety features, EMC, feedback devices, industrial communications and motion control. These can be accessed via Contents Page. 1 Overview 4 2 Regulations Regulations for installations Regulations and standards for equipment 6 3 Harmonic generation within variable speed drives A.c. drives D.c. drives 10 4 The effects of harmonics 11 5 Calculation of harmonics Individual drives D.c Individual drives A.c Systems Isolated generators 15 6 Remedial techniques Connect the equipment to a point with a high fault level (low impedance) Use three phase drives where possible Use additional inductance Use a lower value of d.c. smoothing capacitance Use a higher pulse number (12 pulse or higher) Use a drive with an active input converter Use a harmonic filter 23 7 Typical harmonic current levels for a.c. drive arrangements 24 8 Additional notes on the application of harmonic standards The effect of load Choice of reference current, application of IEEE Std Interharmonics and emissions up to 9kHz Voltage notching Voltage dips and flicker References 30 3 1 Overview Supply harmonics are caused when the a.c. input current to the load departs from the ideal sinusoidal waveshape. They are produced by any non-linear circuit, but most commonly by rectifiers. The supply current waveform is generally measured in terms of the harmonics of the supply frequency which it contains. The harmonic current flowing through the impedance of the supply, causes harmonic voltage to be experienced by other equipment connected to the same supply. Since harmonic voltages can cause disturbance or stress to other electrical equipment, there are regulations applying to public supply systems. If installations contain a high proportion of power electronic equipment such as Uninterruptible Power Supplies or variable speed drives then they may have to be shown to satisfy the supply authorities harmonic guidelines before permission to connect is granted. As well as obeying regulations, users of drives need to ensure that the harmonic levels within their own plant are not excessive. In the general realm of electronic equipment design and regulation, harmonics are considered to be just one of the many aspects of the discipline of Electromagnetic Compatibility (EMC). For variable-speed drives, because of the high power levels involved and the intimate connection between the basic design principles and the harmonic behaviour, the subject of harmonics is normally considered independently from other EMC aspects, which are predominantly concerned with high-frequency effects. Some of the practical problems which may arise from excessive harmonic levels are: Poor power factor, i.e. high current for a given power Interference to equipment which is sensitive to voltage waveform Excessive heating of neutral conductors (single-phase loads only) Excessive heating of induction motors High acoustic noise from transformers, busbars, switchgear etc. Excessive heating of transformers and associated equipment Damage to power factor correction capacitors An important property of harmonics is that they tend to be cumulative on a power system. That is to say, the contributions of the various harmonic sources add up to some degree because they are synchronised, and only their phase angles differ. It is worth emphasising this difference from high-frequency electromagnetic compatibility (EMC) effects, which may cause interference in sensitive data and measuring circuits through unintended coupling paths. Highfrequency effects tend to be localised and not significantly cumulative, because the various sources are usually uncorrelated. It is important to be clear that with few exceptions, 4 if harmonics cause disturbance it is through direct electrical connection and not through stray paths. Shielding is rarely a remedial measure for harmonic problems. It is necessary to consider the effect of supply harmonics both from the point of view of the possible effect of harmonic emissions from drives on other equipment, and also their possible effect on the drive. However since most a.c. drives use a simple rectifier at their input, their immunity to harmonics is inherently good and requires no special attention here. In addition to harmonics, consideration is also given to other possible low-frequency effects on the mains supply, i.e. interharmonics, voltage notching and lighting flicker. 2 Regulations Regulations may exist to protect the public power network from excessive harmonics, or as part of wider EMC regulations. Although the category of low frequency for EMC standards extends officially up to 9kHz, in most cases only harmonics up to order 50 are considered which is 2.5kHz on a 50Hz supply and 3kHz on a 60Hz supply. There are currently no limits to emission in the range from 2.5kHz/3kHz to 9kHz. Measurements should be made using equipment which conforms to the current IEC standard for harmonic measuring instruments, which at the time of writing is IEC :2002. The use of a correctly specified instrument is particularly important in the presence of fluctuating quantities. There are two kinds of regulations which may be relevant: 2.1 Regulations for installations These are imposed by the electricity supply authority to protect other electricity consumers from the effects of excessive harmonics. They are usually based on an agreed level of voltage distortion which can be tolerated by correctly designed equipment. This is specified in terms of a total harmonic distortion (THD), which is the ratio of the harmonic voltage to the fundamental expressed as a percentage. (Where there are a number of harmonic voltages present it is usual to calculate the total harmonic voltage as the square root of the sum of the squares. Alternatively if the r.m.s and the fundamental voltages are known then the harmonic voltage is calculated as the square root of the difference between the squares of these values). The internationally accepted maximum THD compatibility level in a low voltage system is 8%, and to achieve this with a high degree of confidence it is usual to aim for a rather lower level as the planning level, typically 5%. Individual harmonics are also subject to limits. Some relevant standards and regulations are given in the References and source of information section. 5 From the point of view of the supply authority, the relevant harmonic voltage is at the point of common coupling (PCC) with other power consumers. The harmonic voltage levels within the consumer s premises may be higher because of the impedance of cables and transformers. In large installations measures may be necessary to prevent harmonic problems within a site. Since there are no statutory requirements, a relaxed version of the authority limits can be applied internally. It is not advisable to allow the 8% THD compatibility level to be exceeded, because the majority of equipment will have been designed to be immune only up to this level. Predicting the voltage distortion for a proposed installation by a calculation can be an expensive undertaking, because it requires existing harmonics to be measured over a period of time, the system parameters such as source impedances to be derived, and the effect of the planned new load to be estimated. For a large installation with a high proportion of the load comprising electronic equipment, it may be cost-effective to complete this exercise in order to avoid either initial overdesign, or the application of unnecessary remedial measures. For simpler cases a full analysis would be burdensome. Regulations such as the UK Energy Networks Association recommendation G5/4-1 provide simplified staged procedures to permit connection based only on harmonic current data, which can be obtained quite readily from the manufacturers technical data. This involves making some simplifying assumptions biased in a cautious direction. If the simplified stage does not permit connection, the full calculation procedure has to be applied. 2.2 Regulations and standards for equipment A further simplification of the guidelines can be made if a product conforms to a relevant harmonic standard, when it can be connected without reference to the supply authority. The international standard for equipment rated at less than 16A is IEC , the corresponding CENELEC standard being EN These are applied to consumer products and similar equipment used in very large numbers, where individual permission to connect would not be practical. In the EU, EN is mandatory for equipment within its scope. Small variable speed drives rated at less than about 650W shaft power fall within the scope of this standard, and can be made to conform to it by the application of suitable measures. However where they are used in large quantities in a single installation it may be more cost effective to assess their total current and obtain permission to connect from the supply authority. A further more recent standard IEC (EN ) covers equipment rated up to 75A and is mandatory in the EU for equipment within its scope. 6 3 Harmonic generation within variable speed drives 3.1 A.c. drives Harmonic current is generated by the input rectifier of an a.c. drive. The only exception is for an active input stage ( Active Front End, AFE), where PWM is used to create a sinusoidal back e.m.f., and there is, in principle, no harmonic current. The only unwanted current is at the PWM carrier frequency, which is high enough to be relatively easy to filter. This arrangement is discussed later. The essential circuit for a typical a.c. variable speed drive is shown in Figure 1. The input is rectified by the diode bridge, and the resulting d.c. voltage is smoothed by the capacitor and, for drives rated typically at over 2.2kW, the supply current is smoothed by an inductor. It is then chopped up in the inverter stage which uses PWM to create a sinusoidal output voltage of adjustable voltage and frequency. Supply harmonics do not however originate in the inverter stage or its controller, but in the input rectifier. Figure 1 Essential features of a.c. variable speed drive L 1 or 3 Ф supply C The input can be single or three phase. For simplicity the single phase case is covered first. Current flows into the rectifier in pulses at the peaks of the supply voltage as shown in Figure 2. 7 Figure 2 Typical input current waveform for a 1.5kW single phase drive (with supply voltage) 400V 80A 200V 40A 0V 0A Supply voltage Current -200V -40A -400V -80A 0 10ms 20ms 30ms 40ms Time Figure 3 Corresponding harmonic spectrum for Figure 2 12A 8A 4A 0A 0kHz 0.4kHz 0.8kHz 1.2kHz 1.6kHz 2.0kHz Frequency Figure 3 shows the Fourier analysis of the waveform in Figure 2. Note that all currents shown in spectra are peak values, i.e. 2 times their r.m.s. values. It comprises lines at multiples of 50Hz. Because the waveform is symmetrical in the positive and negative half-cycles, apart from imperfections, even order harmonics are present only at a very low level. The odd order harmonics are quite high, but they diminish with increasing harmonic number. By the 25th harmonic the level is negligible. The frequency of this harmonic for a 50Hz supply is 1250Hz which is in the audio frequency part of the electromagnetic spectrum and well below the radio frequency part which is generally considered to begin at 150kHz. This is important, because it shows that supply harmonics are low frequency effects, which are quite different from radio frequency EMC effects. They are not sensitive to fine details of layout and 8 shielding of circuits, and any remedial measures which are required use conventional electrical power techniques such as tuned power factor capacitors and phase-shifting transformers. This should not be confused with the various techniques used to control electrical interference from fast switching devices, sparking electrical contacts etc. Three phase drives cause less harmonic current for a given power than single phase drives. Figure 4 shows the input current waveform for a 1.5kW three phase drive. The line current is less in any case, and there are two peaks in each mains cycle each of about 20% of the peaks in the single phase drive. Figure 4 Typical input current waveform for a 1.5kW three-phase drive 20A 10A 0A -10A -20A 0 10ms 20ms 30ms 40ms Time Figure 5 Corresponding harmonic spectrum to Figure 4 12A 8A 4A 0A 0kHz 0.4kHz 0.8kHz 1.2kHz 1.6kHz 2.0kHz Frequency 9 Figure 5 shows the corresponding harmonic spectrum for the current waveform in Figure 4. Compared with the single phase case the levels are generally lower, and the triplen harmonics (multiples of three times the supply frequency) are absent. The actual magnitudes of the current harmonics depend on the detailed design of the drive, specifically the values of d.c. bus capacitance and inductance. Therefore the supplier must be relied upon to provide harmonic data. 3.2 D.c. drives There is no difference in principle between the harmonic behaviour of a.c. and d.c. drives, but the following aspects of d.c. drives are relevant: The current waveform is not affected by the choice of design parameters (inductance and capacitance) in the drive. It does not therefore vary between drive manufacturers. It can be calculated from knowledge of the motor armature inductance, source inductance and pulse number. The phase angles of all harmonics change with the rectifier-firing angle which during the constant torque region of operation up to the base speed of the motor is approximately proportional to the inverse cosine of speed. For multiple drives, unless their speeds are co-ordinated, the phase angles are effectively random and the harmonic amplitudes do not add up arithmetically. Today d.c. drives tend to be most often used at relatively high power levels, and often a dedicated transformer is required, so 12-pulse and higher pulse numbers are more readily provided. Effect of loading In the case of an a.c. drive, the input current is proportional to the load power, i.e. the product of motor shaft torque and speed. As the load power falls, all of the main harmonics also fall, but not as rapidly as the fundamental. In other words, the THD deteriorates as the load falls. This applies whether the power reduction is through reduced speed or torque or both. In the case of a d.c. drive, the above applies for variations in output current, and hence motor shaft torque. However for a given torque the current does not fall significantly as the speed falls. At light load the waveform may improve somewhat at low speed if the d.c. current becomes discontinuous, but at full torque, low speed the harmonic structure is much the same as at maximum speed. The highest harmonic current for a given drive invariably occurs at maximum load, but in a system with multiple drives it may be necessary to look in detail at the effect of various possible load combinations. 10 4 The effects of harmonics Some of the effects of harmonics were summarised in section 1. Figure 6 shows a voltage waveform where a distribution transformer is loaded to 50% of its capacity with single phase rectifiers. It shows the characteristic flat top effect. Figure 6 Supply voltage waveform with single phase load of 50% supply capacity 400V 200V 0V -200V -400V 60ms 70ms 80ms 90ms 100ms Time Although this waveform looks alarming, most modern electronic equipment is undisturbed by it. However the harmonic content can cause excessive stress in components, especially capacitors, connected directly to the supply. The diode bridge input circuit in a single phase a.c. drive is the same as used in a very wide range of electronic equipment such as personal computers and domestic appliances. All of these cause similar current harmonics. Their effect is cumulative if they are all connected at the same low voltage (e.g. 400V) supply system. This means that to estimate the total harmonic current in an installation of single phase units, the harmonics have to be added arithmetically. Phase-controlled equipment such as lamp dimmers and regulated battery chargers causes phaseshifted harmonics which can be added by root-sum-squares to allow for their diverse phase angles. In a mixture of single and three phase loads, some of the important harmonics such as the fifth and seventh are 180 out of phase and actually mutually cancel. Sometimes this information can be very helpful even if there is no certainty that the loads will be operated simultaneously - for example, in an office building which is near to its limit for fifth and seventh harmonic because of the large number of single phase computer loads, the installation of three phase variable speed drives will certainly not worsen the fifth and seventh harmonics and may well reduce them. 11 Over-loading of neutral conductors is a serious concern in buildings containing a high density of personal computers and similar IT equipment. It is caused by the summation of triplen harmonics in the neutral conductor the neutral current can equal or even exceed the individual phase currents, whereas it is common for the conductor to be of reduced cross-section. Single phase a.c. drives would have a similar effect, but it is unusual for them to be used at such a high density. 5 Calculation of harmonics 5.1 Individual drives D.c. The calculation of the input current for a controlled rectifier is covered in most of the standard textbooks of power electronics. A particularly clear and comprehensive account is given in IEEE Std The basic analysis for the controlled rectifier assumes an infinite inductance load. Then for a p-pulse rectifier the input current has a stepped waveshape with p regularly spaced steps in each cycle. This can readily be shown to contain no even harmonics, and only odd harmonics of the order n = kp±1 where k is any integer. The amplitudes of the harmonics follow the simple rule for a rectangular wave being inversely proportional to the harmonic number: For some purposes this simple calculation is sufficient, but the influence of finite inductance on the d.c. side should be taken into account, and the a.c. side inductance may also have a significant effect. Figure 7 Six pulse converter Variation of line current harmonic content with ripple current % Harmonic current Typical practical values Peak to peak current as % of Id.c. (Av.) % 5th 7th 11th 13th Figure 7 shows the effect of d.c. current ripple on the four dominant harmonics of a 6 pulse drive. The fifth harmonic increases steadily with increasing ripple, whereas for moderate ripple levels the other harmonics fall. Figure 8 Variation of line harmonic content with line impedance - ɑ = % Harmonic current 5th th % Line impedance Figure 8 shows the effect of supply inductance on these harmonics for a firing angle of zero and a ripple of 25%. All of them fall with increasing inductance, particularly the 11th and 13th. However the benefit is reduced at large firing angles because of the more rapid commutation. So for operation at high torque and low speed (low voltage) the benefit of supply inductance may be minimal. Further information is given in IEEE Individual drives A.c. Whereas for a d.c. drive the harmonics are determined largely by external circuit parameters, for an a.c. drive they are determined mainly by the internal inductance and capacitance. It is therefore not usually practical for the user to calculate the harmonic current for an a.c. drive, and it is the responsibility of the drive manufacturer to provide harmonic current data. This should be provided at least for full load, and preferably at part load also. Linear interpolation of the harmonic currents as a proportion of the fundamental can then be used to estimate other loadings. For small a.c. drives with no internal inductance, the supply impedance has a considerable influence. IEC recommen
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