ACTIVE FILTER DESIGN AND SPECIFICATION FOR CONTROL OF HARMONICS IN INDUSTRIAL AND COMMERCIAL FACILITIES Mark McGranaghan Electrotek Concepts, Inc. Knoxville TN, USA can be less than a conquerable passive filter for Abstract the same nonlinear load and the active filter will not introduce system resonances that can move a Active filters have become a viable alternative harmonic problem from one frequency to for controlling harmonic levels in industrial and another. commercial facilities. However, there are many different filter configurations that can be The active filter concept uses power electronics employed and there is no standard method for to produce harmonic components which cancel rating the active filters. This paper describes the harmonic components from the nonlinear the active filter operation characteristics and loads. These active filters are relatively new and develops standard ratings that can be used for a number of different topologies are being filtering different types of nonlinear loads. proposed [6-10]. Within each topology, there Limitations of the active filters are also are issues of required component ratings and described. methods of rating the overall filter for the loads to be compensated. Development of a detailed 1.0 Introduction model for the active filter using the Electro- Applications of active filters have been Magnetic Transients Program (EMTP) described in a number of previous publications facilitates the evaluation of these design and [1-5]. The increasing use of power electronics- application considerations without extensive based loads (adjustable speed drives, switch field tests. This paper describes the results of an mode power supplies, etc.) to improve system extensive investigation to evaluate specific efficiency and controllability is increasing the design and application concerns. concern for harmonic distortion levels in end use facilities and on the overall power system. 2.0 Active Filter Configuration The application of passive tuned filters creates The active filter uses power electronic switching new system resonances which are dependent on to generate harmonic currents that cancel the specific system conditions. Also, passive filters harmonic currents from a nonlinear load. The often need to be significantly overrated to active filter configuration investigated in this account for possible harmonic absorption from paper is based on a pulse-width modulated the power system. (PWM) voltage source inverter that interfaces to the system through a system interface filter as Passive filter ratings must be coordinated with shown in Figure 1. In this configuration, the reactive power requirements of the loads and it filter is connected in parallel with the load being is often difficult to design the filters to avoid compensated. Therefore, the configuration is leading power factor operation for some load often referred to as an active parallel filter. conditions. Active filters have the advantage of Figure 1 illustrates the concept of the harmonic being able to compensate for harmonics without current cancellation so that the current being fundamental frequency reactive power concerns. supplied from the source is sinusoidal. This means that the rating of the active power Electrotek Concepts, Inc. 1 Active Filter Design and Specification I I s L M I f NONLINEAR LOAD Main Customer Bus Interface Filter IGBT Controls PWM and Inverter Gating Signal Generators Figure 1. Diagram illustrating components of the shunt connected active filter with waveforms showing cancellation of harmonics from an ASD load. Therefore, the dc capacitors and the filter The voltage source inverter used in the active filter components must be rated based on the reactive makes the harmonic control possible. This inverter power associated with the harmonics to be canceled uses dc capacitors as the supply and can switch at a and on the actual current waveform (rms and peak high frequency to generate a signal which will current magnitude) that must be generated to achieve cancel the harmonics from the nonlinear load. One the cancellation. leg of the inverter is shown in Figure 2 to illustrate the configuration. The current waveform for canceling harmonics is achieved with the voltage source inverter and an interfacing filter. The filter consists of a relatively large isolation inductance to convert the voltage signal created by the inverter to a current signal for canceling harmonics. The rest of the filter provides smoothing and isolation for high frequency components. The desired current waveform is obtained by accurately controlling the switching of Figure 2. One line diagram for one leg of the active the insulated gate bipolar transistors (IGBTs) in the filter. inverter. Control of the current waveshape is limited by the switching frequency of the inverter and by the The active filter does not need to provide any real available driving voltage across the interfacing power to cancel harmonic currents from the load. inductance. The harmonic currents to be canceled show up as reactive power. Reduction in the harmonic voltage The driving voltage across the interfacing inductance distortion occurs because the harmonic currents determines the maximum di/dt that can be achieved flowing through the source impedance are reduced. Electrotek Concepts, Inc. 2 Active Filter Design and Specification by the filter. This is important because relatively rolling FFT on the sampled load current waveform high values of di/dt may be needed to cancel higher and then reproducing a current waveform that has order harmonic components. Therefore, there is a the same harmonic components with the opposite tradeoff involved in sizing the interface inductor. A phase angle. This calculation is performed each larger inductor is better for isolation from the power cycle and the desired compensation is implemented system and protection from transient disturbances. in the successive cycle. This one cycle of delay could However, the larger inductor limits the ability of the be a problem for nonlinear loads with rapidly active filter to cancel higher order harmonics. varying characteristics. 3.3 PWM Firing Pulse Generation 3.0 EMTP Model of the Active As in most PWM applications, the interval between Filter two consecutive switching actions varies constantly The Electro-Magnetic Transients Program (EMTP) within a power frequency cycle. A rigid definition of is an ideal tool for studying the effectiveness of the the switching frequency is not applicable. Thus, the active filter and evaluating control system concept of an average frequency is commonly used. requirements. The model permits evaluation of the In principle, increasing the inverter operating possible configurations without expensive field tests frequency helps to get a better compensating current and prototype development. Important elements of waveform. However, the actual performance of the the model are described briefly here. active filter becomes limited by the isolating inductance once a high enough switching frequency 3.1 IGBT Voltage Source Inverter is achieved. Control of the average frequency is The voltage source inverter is the heart of the active realized by introducing a hysteresis characteristic filter. This three-phase, full-wave inversion bridge into the PWM firing pulse generation logic as shown is built using three identical IGBT inverter legs. A in Figure 3. dc link neutral is established by equally dividing dc capacitance between the positive and negative poles. I comp - This design combined with separate controls of the I ref individual legs allows the filter to compensate for + I unbalanced loads or even single phase loads. + PWM Modulation - Hysteresis Signal Specification Hyst. -1 3.2 Sampling and Control Reference Figure 3. PWM firing single generation with In this parallel active filter configuration, control is hysteresis characteristic. accomplished by monitoring the current to the nonlinear load and then generating gating signals for In Figure 4, Iref is the desired compensation current the inverter to create a current waveform that will reference signal. Icomp is the actual inverter leg cancel the harmonics in the load current. Sampling output current. An unmatched quantity ∆I is the of the load current must be at a high enough rate to current shaping error which is sent to the positive accurately characterize all the harmonics to be terminal of the comparing unit. The negative canceled - 256 samples per cycle is used in this case. terminal of the comparing unit is connected to the Then the sampled load current is further processed to output of a hysteresis characteristic generator. obtain a harmonic power compensation reference. When Iref is greater than Icomp, the resultant ∆I is There are many different control methods that can be positive. If the magnitude of the ∆I exceeds the used to generate the compensating current that upper boundary of a specified hysteresis band, the cancels the harmonics in the load current. They are comparing unit output goes high, firing the upper distinguished by how the current reference signal for bridge device of the leg and making the leg current the harmonic compensation is derived from the increase. When Icomp becomes greater than Iref, ∆I measured quantities. This paper focuses on one becomes negative. If the magnitude of the ∆I particular method, known as the FFT method. exceeds the lower boundary of the hysteresis band, the comparing unit goes low, firing the lower bridge This method compensates for individual harmonic device of the leg and making the leg current components in the load current by performing a Electrotek Concepts, Inc. 3 Active Filter Design and Specification decrease. By increasing or decreasing the allowable inductor (L2f) to prevent overloading due to current shaping error, which is determined by the transients and high frequency harmonic components bandwidth of the hysteresis characteristic, the from the power system. average switching frequency can be controlled. This firing algorithm is sometimes called a delta- Sizing of the inductor and capacitor values must take modulation. In the simplest delta modulation into account control of the inverter switching control, an equal bandwidth over the whole power frequencies, power system switching transients, and frequency cycle is used for the hysteresis. In this the characteristics of the nonlinear load to be case, the modulation frequency is a constant. A compensated. One of the main objectives of the nonlinear modulation can be used to help reduce the application study was to evaluate the possible variations in the switching interval associated with a impacts of system switching events on these filter constant bandwidth. components. 3.4 System Interface Module 4.0 Example System for Active The system interface module provides the isolation Filter Performance Evaluation and filtering between the output of the voltage source A simple example system was modeled to evaluate inverter and the power system where the active filter the active filter performance for different types of is connected. The isolation consists of two loads and to evaluate the impact of system switching inductors, L1f and L2f (refer back to Figure 2) and a events on the design requirements for the active high frequency attenuation filter formed by a small filter. A typical distribution circuit as shown in capacitor with these inductors. Figure 4 was selected for this evaluation. Important parameters are as follows: The isolation inductance allows the output of the active filter to look like a current source to the power Source strength at transmission supply point = 200 system. The inductance makes it possible to charge MVA the dc capacitor to a voltage greater than the ac 138/13.8 kV Transformer: 10 MVA, 7% impedance phase-to-phase peak voltage. The isolation Substation capacitor bank size = 3.0 Mvar (switched) inductance also functions like a commutation Equivalent load for parallel feeders = 3.0 MW impedance. It limits the magnitude of a current Modeled feeder circuit: 3.0 miles to example spike during commutation and prevents the customer switching device from seeing an excessive rate of Feeder capacitor bank on 13.8 kV side at example current change. customer: different sizes evaluated Customer low voltage capacitor bank: varied The PWM switching of the filter inverter generates Customer service transformer: 1500 kVA, 6% high frequency components. To prevent these impedance components from being injected into the source, a Customer load = 1.0 MW capacitor is included with the isolation inductance to Active Filter size = 400 Vrms, 30 Arms form a second order passive filter. The capacitor Nonlinear load: different loads evaluated must also be isolated from the power system by an 3 MW 138 kV Bus Other Feeders and Loads 200 MVA, 138 kV Feeder Equivalent Source Cap. Zs 3-mile Overhead Distribution Feeder Bank 1500 kVA 13.8 kV Bus Z=6% Customer Bus Substation Cap. Bank Customer Nonlinear AF Cap. Bank Load Figure 4. Example distribution circuit for active filter application evaluations. Electrotek Concepts, Inc. 4 Active Filter Design and Specification would be to provide only enough compensation so that the load/filter compensation was within some 5.0 Determining Active Filter specified guidelines for harmonic generation (e.g. Ratings for Nonlinear Load IEEE 519-1992). Types 5.1 Effect of Load Waveform on Filtering One of the confusing aspects of applying active Effectiveness filters is trying to figure out the active filter rating The effectiveness of the active filter in compensating that is required to compensate for the harmonics for harmonic components of the load current from a particular load. A parallel-connected active depends on the specific load current waveform filter should be rated in terms of the rms current it involved. Two different waveforms may have the can provide. Then the task is to figure out the rms same rms harmonic content but the active filter may current required to compensate for the harmonics do a better job of compensating for one of the from different types of loads. Simulations were waveforms because of the waveshapes involved. performed for a number of typical nonlinear loads to develop some guidelines for active filter ratings. An ac voltage regulator is used for illustration. Two cases are compared in Figure 5. The only difference One advantage of the parallel-connected active filter, between the two cases is the load of the ac regulator. as compared to passive filters, is that it is self- In the waveforms on the left side of the figure, the limiting in terms of the harmonic cancellation load is a pure resistance. The waveforms on the provided. there is no concern for overloading the right side are for the case where the load is a series filter due to harmonics from the utility supply system combination of resistance and reactance. The or under-rating the filter for the loads involved. The performance is much better for the smoother load worst case scenario if the filter is under-rated is that current waveform (RL load). It is worthwhile to note it just won’t completely compensate for all the that the majority of applications for the active filter nonlinear load current harmonics. In fact, it may not will involve waveforms like those on the right hand be necessary to compensate for all the harmonics side of Figure 6 (e.g. adjustable speed drives with from a nonlinear load. With the active filter, the diode bridge rectifiers or single phase electronic size can be selected to achieve any desired level of loads), rather than the left side. cancellation. One good way to use this concept Regu lator w ith Regu l ator w ith resi sti ve l oad RL l oad REGR-INJ>ILOADA-ACREGA(Type 9) REGL-INJ>ILOADA-ACREGA(Type 9) 150 150 Max: 190.929 Max: 91.7568 Min: -103.18 Min: -67.4758 Avg: 30.623 Avg: 32.5594 100 100 Abs: 190.929 Abs: 91.7568 RMS: 44.7151 RMS: 40.9831 CF : 4.26991 CF : 2.2389 50 FF : 1.46018 50 FF : 1.25872 0 0 Load Current s -50 -50 -100 -100 -150 -150 360 370 380 390 400 350 360 370 380 390 400 Time (mS) Time (mS) REGR-INJ>TACS -ICREFA(Type 9) REGL-INJ>TACS -ICREFA(Type 9) 0.100 0.10 0.075 0.050 0.05 Compensat or 0.025 ref erence and 0.000 0.00 -0.025 act ual out put -0.050 -0.05 -0.075 -0.100 -0.10 360 370 380 390 400 360 370 380 390 400 Time (mS) Time (mS) REGR-INJ>MAINSA-PTBUSA(Type 9) REGL-INJ>MAINSA-PTBUSA(Type 9) 150 150 Max: 417.374 Max: 399.957 Min: -237.178 Min: -220.969 Avg: 50.3947 Avg: 49.7815 100 100 Abs: 417.374 Abs: 399.957 RMS: 71.1235 RMS: 66.5263 CF : 5.8683 CF : 6.012 50 FF : 1.41133 50 FF : 1.33637 Net current 0 0 fro m syst em -50 -50 -100 -100 -150 -150 360 370 380 390 400 360 370 380 390 400 Time (mS) Time (mS) Figure 5. Comparison of active filter performance for an ac voltage regulator with resistive load and with RL load. Electrotek Concepts, Inc. 5 Active Filter Design and Specification In general, the current waveform of an ac regulator It is important to note that these simulations were for with resistive load is an example of the waveshape steady state conditions (load was not changing). that poses the severest challenge for an active filter. Therefore, the effect of the response time associated The problem is the high di/dt that is required of the with the FFT control was not a factor. filter to compensate for the high di/dt at turn on of the regulator. The problem is most severe when the A number of important observations can be made regulator is turned on with a firing angle close to 90 based on the results summarized in Table 1: degrees because this is when the available driving voltage stored on the dc capacitor is at a minimum. • The overall filtering effectiveness depends The output di/dt capability can be raised either by significantly on the types of loads being increasing the dc voltage setting or by reducing the compensated. There is no simple size of the interfacing inductance. The limiting relationship between the load current THD factor for increasing the dc voltage is the voltage and the filter effectiveness. withstand capability of the IGBT devices. The limiting factors for reducing the interfacing • The active filter is most effective when the inductance include the IGBT di/dt withstand load current waveform does not have abrupt capability, control requirements, the interface changes. As a result, it is very effective for passive filter requirement, and overall system most voltage source inverter-type loads, stability. If the interfacing inductance becomes too even when the distortion is high. small, the dc voltage cannot be kept constant for normal operation. • The active filter effectiveness was not as good for 12 pulse loads. This is caused by 5.2 Steady-State Rating Requirements and the fact that the higher frequency Active Filter Effectiveness components are more dominant in these loads. The best way to provide a rating for an active filter is • The rating requirement for the passive filter in terms of the rms current that it must provide to capacitor is also dependent on the load compensate for harmonics from nonlinear loads. current characteristics. Load current Table 1 provides a convenient summary of different waveforms with more high frequency nonlinear load types with example waveforms and content (e.g. ac regulator with resistive load typical levels of harmonic current distortion or 12 pulse converters) result in higher associated with each load. Using these typical duties on the filter capacitor. waveforms, it is possible to calculate a theoretical value for the required harmonic compensation from the active filter. The summary includes the THD for each nonlinear load waveform and the required active filter rating in rms amps per kVA of load rating. These ratings assume that the active filter rating should be based on the total rms harmonic current content of the load. A simulation waveform illustrating the active filter effectiveness for each of these waveforms is also provided. The ratings in Table 1 assume ideal active filter characteristics. That is, they assume that the active filter can compensate for every amp of harmonic current created by the nonlinear load. It is clear from the simulation result waveforms also included in the table that the harmonic cancellation is not perfect. The distortion in the supply current is also provided in the table to illustrate the effectiveness of the active filter. Electrotek Concepts, Inc. 6 Active Filter Design and Specification Table 1. Required active filter ratings for different types of nonlinear loads. Active Filter Rating (rms Amps/kVA of Load) Nonlinear Supply Type of Load Nonlinear Load Supply Current Load Current Current 120/208 volt 480 volt Current Waveform with Active Filter THD (%) THD (%) applications applications 200 200 Single Phase 100 100 84% 8.2% 1.79 0.77 Power Supply 0 0 -100 -100 -200 -200 360 370 380 390 400 360 370 380 390 400 100 100 75 Semiconverter 50 50 87% 9.6% 1.82 0.79 25 0 0 -25 -50 -50 -75 -100 -100 360 370 380 390 400 360 370 380 390 400 100 100 ac Voltage Regulator, 50 50 23% 4.3% 0.62 0.27 RL Load 0 0 -50 -50 -100 -100 360 370 380 390 400 360 370 380 390 400 150 150 ac Voltage 100 100 Regulator, 51% 16.7% 1.26 0.54 50 50 Resistive Load 0 0 -50 -50 -100 -100 -150 -150 360 370 380 390 400 360 370 380 390 400 100 150 100 6 Pulse Drive, 50 31% 10.3% 0.82 0.36 50 Current Source Inverter 0 0 -50 -50 -100 -100 -150 360 370 380 390 400 360 370 380 390 400 200 150 6 Pulse Drive, 150 100 Voltage Source Inverter 100 109% 13.2% 2.05 0.88 50 50 no series inductance 0 0 -50 -50 -100 -100 -150 -200 -150 360 370 380 390 400 360 370 380 390 400 150 150 6 Pulse Drive, 100 100 Voltage Source Inverter 45% 5.0% 1.14 0.49 50 50 3% ac input choke 0 0 -50 -50 -100 -100 -150 370 380 390 400 -150 370 380 390 400 360 360 150 150 100 100 12 Pulse Converter, 13% 5.6% 0.36 0.15 50 50 Current Source Inverter 0 0 -50 -50 -100 -100 -150 370 380 390 400 -150 370 380 390 400 360 360 150 150 100 100 12 Pulse Converter, 13% 6.3% 0.36 0.15 50 50 Voltage Source Inverter 0 0 -50 -50 -100 -100 -150 360 370 380 390 400 -150 360 370 380 390 400 350 350 Electrotek Concepts, Inc. 7 Active Filter Design and Specification Simulations were performed to evaluate the effect of 6.0 Effect of Power System switching the 3.6 Mvar capacitor at the substation in Transients on the Active Filter Figure 4. Substation capacitor switching usually The active filter is not designed to control transient causes the most severe transients and has the most currents flowing to the load. It is a steady state potential to cause magnified transient voltages. The controller. However, it is important to understand characteristics of the transient seen at the active the impacts of transient conditions on the filter were varied by changing the size of the feeder components of the active filter. The inverter controls capacitor bank located close to the customer facility include hard limits to prevent excessive and by evaluating the effect of a low voltage compensating current generation during either capacitor connected at the customer bus. The steady state or transient conditions. The main waveforms in Figure 6 illustrate the magnification concern during transients is for the passive filter within the customer facility for a case with both a components in the interface module between the feeder capacitor and a customer capacitor in service. inverter and the power system (see Figure 1). Note that the transient voltage across the interface filter capacitor is approximately the same as the The interface module includes inductors and transient voltage at the customer bus due to the capacitors. Surge suppressors can limit high relatively low frequency of this transient (no frequency voltage spikes that may be associated with additional magnification across the inductor L1f). It load switching events within a facility or coupled is important to evaluate conditions such as this as transients during lightning strokes. The concern part of the active filter design process. If the evaluated in this study involves transient voltages isolation inductor size was increased, the concern for caused by capacitor switching on the utility supply transient magnification on the interface capacitor system. Capacitor switching on the primary would be even more severe. Also, a damping distribution system has the potential of causing an resistor is added in the interface circuit to help oscillation frequency that could be magnified by the prevent magnification problems. inductor/capacitor series combination in the interface module. 1.57 pu SRC1INJ>BS13KA(Type 1) 2 1 1.94 pu 0 SRC1INJ>TX13KA(Type 1) 2 - 1 13.8 kV Bus 1 - 2 380 400 420 440 0 Time (mS) -1 -2 380 400 420 440 Time (mS) 2.12.pu SRC1INJ>MAINSA(Type 1) 3 2 1 0 -1 -2 3 80 400 420 440 Time (mS) Nonlinear Substation 2.26 pu Load DERIVED>MAINSA-VC(Type 1) 3 Cap. Bank 2 1 0 - 1 - 2 38 0 400 42 0 440 Time (mS) IGBT PWM Inverter SRC1INJ>DCPLUS-DCMNUS(Type 8) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 380 400 420 440 Time (mS) Figure 6. Transient voltage waveforms caused by substation capacitor energizing. Electrotek Concepts, Inc. 8 Active Filter Design and Specification It is also interesting to evaluate the effect of this transients and many utilities are making efforts to transient on the active filter currents. Since the control substation capacitor switching transients. transient voltage exceeds the voltage on the dc capacitors for this case, all control is lost during the 8.0 References transient (no driving voltage for the IGBTs). 1. Richard M. Duke and Simon D. Round “The Instead, there will be a significant transient current steady-state performance of a controlled current that flows through the converter to charge up the dc active filter,” IEEE Trans. Power Electronics, capacitors. The waveforms in Figure 6 shows that Vol. 8, No. 3, April 1993, pp 140. the additional charge on the dc capacitors is not 2. V.B. Bhavaraju and Prasad N. Enjeti, “Analysis significant but the current waveform in Figure 7 and design of an active power filter for below illustrates that the transient current during the balancing unbalanced loads,” IEEE Trans. capacitor switching can be quite high. This current Power Electronics, Vol. 8, No. 4 October 1993, does not flow through the IGBTs since they are not pp 640. being gated. Instead it all flows through the anti- parallel diodes, resulting in possible overloading and 3. Janko Nastran, Rafael Cajhen, Matija Seliger, failure of the diodes. and Peter Jereb “Active power filter for nonlinear ac loads,” IEEE Trans. PE, Vol. 9, SRC1INJ>ACFLTA-JOINTA(Type 9) 200 No. 1, January 1994, pp 92. 100 4. Mukul Rastogi, Ned Mohan and Abdel-Aty Edris “Hybrid-active filtering of harmonic 0 currents in power systems,” IEEE 95 WM 258-4 -100 PWRD. 5. W.K. Chang, W.M.Grady and M.J.Samotyj -200 “Controlling harmonic voltage and voltage -300 380 400 420 440 distortion in a power system with multiple active Time (mS) power line conditioners,” IEEE 95 WM 257-6 Figure 7. Active filter current output during PWRD. capacitor switching transient. 6. Fang-Zhang Peng, Hirofumi Akagi, and Akira Nabae”A study of active power filters using These transients must be considered in selecting the quad-series voltage source PWM converters for capacitors to be used for the interface filter. All of harmonic compensation,” IEEE Trans. Power the capacitor switching transients can cause high Electronics, Vol. 5. No. 1. January 1990, pp. 9. currents in the anti-parallel diodes. 7. Joe F. Chicharo, Damrong Dejsakulrit, and B. Sarath P. Perera “A centroid based switching 7.0 Conclusions strategy for active power filters,” IEEE Trans. Active filters could have wide application for Power Electronics, Vol. 8, No. 4, October 1993, controlling harmonic currents from nonlinear loads. pp 648. The best performance is obtained for loads such as 8. M. B. Brennen and B. Banerjee “Low cost, high PWM type ASDs and switch mode power supplies, performance active power line conditioners,” where the current waveform does not have abrupt Proc. Conf. PQA’94, Part 2, Amsterdam, The changes that are hard for the active filter to follow. Netherlands, October 24-27, 1994. Guidelines for rating the active filters are presented. 9. C. Lott and H.Pouliquen “High power voltage Capacitor switching transients should not be a major source PWM active filter,” Proc. Conf. PQA’94, problem for the active filter inverter and controls. Part 2, Amsterdam, The Netherlands, October However, the interface filter capacitor could 24-27, 1994. experience high transient voltages that may exceed 10. G. C. Damstra and A. H. Eenink “An active the capabilities of the capacitor and surge harmonic compensator for four-wire LV suppressors. The capacitor switching transients networks,” Proc. Conf. PQA’94, Part 2, could also cause overload of the anti-parallel diodes Amsterdam, The Netherlands, October 24-27, in the inverter bridge. Other devices in customer 1994. facilities can also have problems with these Electrotek Concepts, Inc. 9