Comparative Study ofac Side Passive and Active Filters for Medium Voltage PWM Current Source Rectifiers

Transcription

1 Comparative Study ofac Side Passive and Active Filters for Medium Voltage PWM Current Source Rectifiers Ahmed K. Abdelsalam, Mahmoud I. Masoud, Stephen J. Finney and Barry W. Williams Kadry_2012(?geee.strath.ac.uk, m.masoud(?geee.strath.ac.uk, s.finney(?'yeee.strath.ac.uk, and barry.williarns(??~eee.strath.ac.uk Electronic & Electrical Engineering Department, Strathclyde University, Glasgow, Scotland, UK Keywords - Active front end, current source rectifier, passive filter, and active power filter. Abstract In this paper, a comparative study for both AC side passive and active filters for medium voltage PWM current source rectifiers which utilize selective harmonic elimination technique is introduced. Modeling, design and performance analysis of both power filters are given to improve the total harmonic distortion (THD) of the input current and ensure near unity power factor operation for the whole loading range. The topologies used are second order passive LC filter with damping resistor and shunt active power filter based on instantaneous active-reactive power theory (p-q method). 1 Introduction Current Source Rectifiers are widely used in medium voltage high power AC drives. The basic types for this front end are either a thyristor based rectifiers or a current source rectifiers (PWM-CSR) [1]. Thyristor based rectifiers are difficult to meet IEEE-519 standards. Those types of rectifiers usually use a group of tuned filters to improve their performance. On the contrary, the PWM-CSR harmonic requirements are often accomplished by a simple LC low pass filter, and near unity power factor operation is possible without additional equipment [1, 2]. As the switching frequency is very limited in high power applications, the preferred modulation technique is the SHE. This technique enables the mitigation of low order harmonics with low switching frequency [2]. A general model of SHE technique is presented in details in [3]. The design of filters for input stage converters is described in many literatures [4, 5, 6]. In [7], a systematic approach to design a passive LC filter for PWM current source rectifiers with considerations related to cost, power factor and parameters variation. The filter transient response could be improved by splitting the filter into two parallel sections with the same capacitance value. The purpose ofthe second branch is to provide a low impedance path for spikes discharge due to the insertion of the damping resistors [8]. To meet the updates in the harmonic standards [9], an approach presented in [10], to use amplitude and phase variation of the modulation index of PWM-CSR to reach near unity power factor. Their practical results show acceptable performance from 30% to 100% loading conditions but the performance degrades rapidly in lower loading conditions. On brief, the usage of the passive filter gives acceptable results but with some limitations specially; 1. Near unity power factor operation for the overall loading range is not feasible. 2. Increase and distortion ofthe rectifier input voltage. Consequently, the usage of active power filter is investigated as an alternative for the passive filter. Theory of reactive power compensation is described in details in [II]. There are many techniques regarding how the harmonic components are extracted from measured voltages and currents. Those techniques differ from complexity, applicability on non ideal supply and robustness [12]. Among the various types ofthose techniques, the active-reactive power component method (p-q method) is used in this paper with modifications on the harmonic filter order and type [13]. Review on active filters is illustrated in many publications [14-16]. In this paper, the design and analysis ofpassive LC filter with damping resistor to achieve near unity power factor are introduced. Also, active power filter are used to have a comparative study with the passive filter, where, modeling, equations and performance of each topology are given. Losses are not considered in this study as it focuses on filter performance. 2 PWM-CSR Topology with Passive Filter The complete circuit diagram of a three-phase PWM CSR with passive filter is shown in Figure 1. It consists of three phase AC mains, an input LC filter with a damping resistor, a PWM rectifier, a DC link inductor and a passive load. The PWM-CSR is modulated using SHE technique to mitigate the 5 th, 7 th and 11 th harmonics from the input line current. The system under study is 3.3 KV supply system with closed loop controlled DC current rated at 150 A. the load is a 200 resistor while the DC link inductor is 300 mho where, Loe Load Figure 1: PWM-CSR with passive filter topology. A. Passive Filter Mathematical Model The filter equivalent circuit is considered in its simplest LC form. The passive LC filter transfer function (i/i r ) is a second order system and is given by; loe z = l/cfr f, ~ =R f l2m r L f (1) 578

2 The resonant angular frequency w,. depends mainly on C J and L J values, while the attenuation and the damping factor ; depend on the damping resistor R J as well as C J and values L J. B. Passive Filter Design Steps (1) Select the comer frequency of the filter according to the first unwanted harmonic frequency fi.the gain of the filter at this frequency (a) is usually taken 1.5 [7]. fr == fh~ (2) (2) Substitute with the selected comer frequency in the following equation f = 1/(2Jr~LfCf) (3) (3) For unity power factor, assign the displacement angle with zero in Equation 4 for certain DC link current and corresponding modulation index M. f} = tan- 1 [V c I(M.ldc.XJ]- tan-l[(m.ldc,xi)/~.(l-(xl/xc))] (4) (4) Solve Equations (3) and (4) to find the values ofljand C f (5) With the obtained values of L J and Cfi find the value of damping resistor R J by substituting in Equation 5 with a gain usually taken 0.01 [7]. I:: 1= Ik[ S2 +2;;,: +m/ ]I (5) C. Passive Filter Performance Analysis The system investigated is 3.3 kv supplying a PWM-CSR. The rectifier load is a 200 resistor. A large DC link inductor (300 mh) is used for DC side smoothing. The performance of the passive filter is investigated with different comer frequencies to show the advantages and disadvantages of each comer frequency selection. The results are for two different loading conditions, full load (M == 1) and light load (M = 0.1). Figure 1 shows the supply current without any filtration at M==1. The rms value is 142A with 44.4% THD. ~w otq 0"" 0,M !> 015:' el~ Ot5& G'5e o 1ft Tlme(s) Figure 1: Supply current Tables 1 to 3 summarize the results at different comer frequencies for the full load and light load conditions, respectively. For each filter, the tables illustrate the filter parameters (Rfi L J and C J ), the supply current and its corresponding THD, the peak rectifier input voltage and its corresponding THD. Filter Parameter 350Hz 300Hz 200Hz 100Hz Filter resistance(o') Filter capacitance (J.lF) Filter inductance (mh) Table 1: Filter parameters Full Load (M=1) 350Hz 300Hz 200Hz 100Hz Supply current (A) THD ofis (%) Rectifier input voltage V r volt THD ofv r (%) Power factor Table 2: Full load results. Light Load (M= 0.1) 350Hz 300Hz 200Hz 100Hz Supply current (A) THD ofis (0/0) Rectifier input voltage V r volt THD of V r (0/0) Power factor Table 3: Light load results. Figure 2 shows the performance of PWM-CSR with passive filter tuned at comer frequency of 200 Hz as an example. 1 f'/""- I, 1 I ),- +-..it I I Ij 1 1 I \1 I 1 I I g -ti -1- t- -j ~\ -t -I - r -j - ~.Ii 1 1 I 1\ I ~ 7i - 1- ri -r\<t-,-ri- ~ ~ _ 1_ ~ -' _ 1_ ::i _' _ ~ ~ -J ::J I 1\ I 1 If. ~ _:_~ ~ _:_ +1\:_~ }(L 'I F ",," (a) Line current..., IlM llotl...,.t Cl.ltl D'" I'et Iltl III Time(s) i 100r ==-----< --t ----i (b) Rectifier input current (c) Filter current (d) Rectifier input line voltage Figure 2: PWM-CSR performance with passive filter tuned at corner frequency 200Hz at full load The behavior of main parameters, (Line current and its THD, filter current, power factor and peak input line voltage of the rectifier and its THD), is investigated at different comer frequencies. Line Current Figure 3 shows that all filters tuned at different comer frequencies almost have the same line current, but the filter tuned at 100Hz has increase in the supply current. This is because the filter impedance is greatly decreased, (C J is approximately doubled and L J is halved), compared to the one tuned at 350Hz. I 80 I FuR~ I a 60 ~ ,..--1_ < ~ ~ ~ 100 '.111"....r comer...ncy 1*).Li!t'~ Figure 3: Line current at different comer frequencies Line Current TUD As the comer frequency of the filter decreases, more harmonics are mitigated and hence improve the line current 579

3 THD as shown in figure 4. Although, the difference in impedance between the filter tuned at 300 Hz and that tuned at 350 Hz is small, there is a big improvement in the THD. This is because at 350 Hz, a small value of the 7 th harmonic component (mitigated by SHE technique). The filter at this frequency amplifies that small value of the 7 th harmonic component while at 300Hz there is no existing harmonic. Therefore, the filter at 300 Hz does not amplify any extra harmonic and produces better THD than that tuned at 350 Hz P..II"....r comer hquency (Hz) Figure 5: Filter current at different comer frequencies Power Factor _ < ~ 25~ < ~ '.Fuliloed i 15.Ugrtload ij ::i l! 10 5 ~ ---< Figure 4: Line current THD at different corner frequencies Filter Current The filter impedance decreases with the decrease of the comer frequency. Therefore, the filter tuned at 100 Hz draws the largest current compared to the filters tuned at higher comer frequencies as illustrated in Figure 5. 90, : ' C : !iO J i&:2o+--_-~- P...w..., comer...ncy (Hz) The design steps ensures near unity power factor operation at full load for different comer frequencies but, no passive filter can achieve near unity power factor for the whole loading range. The power factor is very poor especially at light loads. Figure 6 shows the power factor at different comer frequencies for both full load and light load. J0.8 I 0.6~ I-- J-- f----: : I- I- I--- f------< P..II"...., comer hquency (Hz) Figure 6: Power factor at different comer frequencies Rectifier Input Voltage Passive filter affects the rectifier input voltage because of the presence of the filter inductance. The rectifier input voltage is the phasor summation of the supply voltage and the drop on the filter inductance. As the filter inductance increases, the rectifier input voltage increases and becomes load dependant. The peak line voltage should be 4.66 kv, while in all the investigated filters, the peak line voltage input to the rectifier exceeds this limit as shown in Figure 7. Also, the existence of the filter distorts the voltage input to the rectifier. Figure 8 shows that the rectifier input voltage is distorted and has relatively high THD. As the capacitance ofthe filter increases (filter tuned at lower comer frequencies), the THD of the rectifier input voltage is improved. 3!iO P..II"....r comer hquency (Hz) Figure 7: Peak Rectifier input voltage ~ 40 t : J I ~ 20 ~ ! i- :~ +--.I----I----I----~ ~ I : ~--~~ P..."...., come,...ncy 1Hz) Figure 8: Peak Rectifier input voltage THD 3 PWM-CSR Topology with Active Filter Figure 9 shows the circuit diagram of a three-phase PWM CSR. It consists of three-phase ac mains, an active filter, a coupling transformer, a PWM rectifier, a DC link inductor and a passive load. Figure 9: PWM_CSR with active filter topology A. Active Filter Mathematical Model There are many techniques used for active filters [ ]. In this paper, the instantaneous active-reactive power theory is used due to its simplicity and robustness [ ]. The transformation of the rectifier currents and supply phase voltages to the stationary a-pframe are given by, [:::]=H[~ AA][~:] 2 2 Ire [vp]=h[o ~/2 ~/2]:: [ Va 2 1-1/2-1/2 a V ] The instantaneous load real and imaginary determined by, [ Pload] [Va Vp][ira ] qload = - VP Va irfj (6) (7) power can be (8) 580

4 There are two instantaneous power components represent the load, (rectifier and passive load). Each component can be divided into ac and dc components as follows, Pload = P Load + PLoad qload = q Load+qLoad The negative of the ac components of the real and imaginary load power components must be supplied by the active filter to mitigate the harmonics of the load. In this case, the supply recognizes only the dc components of the real and imaginary load power [ ]. To improve the power factor, the negative value of the dc component of the load imaginary power must be supplied by the active filter as given in equations 11 and 12. The filter must draw a small amount of active power component to fulfill the losses ofthe semiconductors P jilter = - P load + Ploss q jilter B. Active Filter Performance Analysis (9) (10) The performance of the PWM-CSR is investigated with the presence of the active filter instead of the passive under the same loading conditions. The active filter operates at 3 khz switching frequency and uses a 1:4 step down transformer to use available low voltage high current IGBT modules. Figure 10 shows the active filter performance at 100% loading. (a) Line current = -(q load + q load) (11) (12) (b) Rectifier input current excellent results with acceptable filter current. The comparison covers the performance at high and low loading conditions. The main parameters concerned are the line current, the filter current, the power factor, the rectifier input peak line voltage and its THD. Line Current The line current in case of the active filter is less than that in case of the passive filter at both high and low loading conditions as illustrated in figure 11..Fuflload '.ligtliloed Unec:urrentlAj passi..eilter Figure 11: Line current for active and passive filters Filter Current The active filter adapts itself with the loading conditions while the passive filter draws approximately the same current over the whole loading range. This is cleared in figure l I ~ _ j i 5 30 I.li~1oad! ii: pessiloe tiller I F~~ I Figure 12: Filter current for active and passive filters Power Factor Although the passive filter is designed to make the system operates at near unity power factor at full load, yet this can not be fulfilled at low loading conditions. On the contrary, the active filter can achieve near unity power factor in the high loading conditions as well as the low ones. This is illustrated in Figure ,., ; I FuH~I.1.ifI1l1oad (c) Filter current 'Time (5)" (d) Dc link current actiloelilter passm! filter Figure 13: System power factor Time(s) (e) Capacitor voltage (t)rectifier input line voltage Figure 10: PWM-CSR performance with active filter at full load 4 Comparison between Passive and Active Filters The comparison is held between the active filter and the passive filter tuned at 200Hz. This passive filter gives Rectifier Input Voltage The usage ofthe active filter does not affect the rectifier input voltage because the active filter is connected to the system as shunt source. The passive filter increases the rectifier input voltage because the system operates at leading power factor due to the filter impedance (specially the shunt capacitors). Moreover, the introduction of the filter series inductance makes the rectifier input voltage be the phasor summation of the supply voltage and the filter inductance voltage. This contributes to the increase of the rectifier input voltage and 581

5 the rectifier becomes load dependant. Figure 14 shows the rectifier input voltage for the passive and the active filter. In this analysis the supply voltage is assumed to be harmonic free. Hence, The THD of the rectifier input voltage in case of active filter is almost zero. Figure 14: Peak line rectifier input voltage for active and passive filters Conclusion ~ 7000, "-' t ;;;;=--: l5ooo ~ 4000 :; ~3000 i2000 i 1000 I 0 actiwilter passileilter The passive power filter shows good results from the point of harmonic cancellation as well as power factor improvement at high loading. The performance of the passive filter is poor at low loading. The use of damping resistors in series with the filter capacitors is mandatory (for damping any harmonics at the filter corner frequency) but contribute additional losses in the system. The active power filter shows excellent results from the point of harmonic cancellation as well as power factor improvement. The use of an active filter does not increase the voltage stresses on the input converter switches, as the passive case does. The THD in case of active filter is within the IEEE-519 range for the input utility current and voltage. The supply current is lower (for the same loading conditions) in the case of the active filter than that in the passive case. The power factor is approximately constant over the loading range if an active filter is used. As a result, the performance of the active filter is superior to that of the passive one in the full range of loading. Small capacitors should be added in the case of active filter to decouple the supply impedance to the rectifier. System cost, complexity and reliability mitigate against active filters. References [1] B. Wu. "High Power Converter Systems", IEEE Press and John Wiley, (March 2006). [2] N. R. Zargari, S. C. Rizzo, Y. Xiao, H. Iwamoto, K. Satoh and J. F. Donlon. "A New Current Source Converter Using a Symmetric Gate-Commutated Thyristor (SGCT)", IEEE Trans. On Industry Applications, vol. 37, pp , (June 2001). [3] H. R. Karshenas, H. A. Kojori and S. B. Dewan. "Generalized Techniques of Selective Harmonic Elimination and Current Control in Current Source Inverters/Converters", IEEE Trans. On Power Electronics, vol. 10, pp , September [4] S. B. Dewan, R. S. Segworth, and P. P. Biringer. "Input Filter Design with Static Power converters", IEEE Trans. on Industry Applications, vol. IGA-6, pp , (July/August 1970). [5] D. A. Gonzales and J. C. McCall. "Design of Filters to Reduce Harmonic Distortion in Industrial Power Systems", IEEE Industry Applications Soc. Conf. Rec., pp , (October 1985). [6] S. B. Dewan and E. B. Shahrodi. "Design of an Input Filter for the Six-Pulse Bridge Rectifier", IEEE Trans. on Industry Applications, vol. IA-21, pp , (September/October 1985). [7] N. R. Zargari, G. Jooz and P. D. Ziogas. "Input Filter Design for PWM Current- Source Rectifiers", IEEE Trans. on Industry Applications, vol. 30, pp , (November/December 1994). [8] J. I. Guzman, J. R. Espinoza. "Improvement Issues on the Input Filter Design for PWM-CSR that are SHE Modulated," PESC'05, IEEE, 36 th, pp , [9] C. K. Duffey and R. P. Stratford. "Update of Harmonic Standard IEEE-519: IEEE Recommended Practices and Requirements for Harmonic Control in Power Systems", IEEE Trans. on Industry Applications, vol. 25, pp , (November/December 1989). [lo]n. R. Zargari, Y. Xiao and B. Wu. "Near Unity Input Displacement Factor for Current Source PWM Drives", IEEE Industry Applications Magazine, pp , (July/August 1999). [11]H. Akagi, Y. Kanazawa, A. Nabae. "Instantaneous Reactive Power Compensators Comprising Switching Devices Without Energy Storage Components", IEEE Trans. on Industry Applications, vol. IA-20, pp , (May/June 1984). [12]A. M. Masoud, S. J. Finney and B. W. Williams. "Review of Harmonic Current Extraction Techniques for an Active Power Filter", Harmonics and Qualitv of Power 11 th International Conference.. pp i59, (September 2004). [13]V. Scares, P. Verdelho and G. D. Marques. "An Instantaneous Active and Reactive Current Method for Active Filters", IEEE Trans. on Power Electronics, vol. 15, pp , (July 2000) [14]B. Singh, K. AI-Haddad, A. Chandra. "A Review of Active Filters for Power Quality Improvement", IEEE Trans. on Industrial Electronics, vol. 46, pp , (October 1999 [15] H. Akagi. "Trends in Active Power Line Conditioners", IEEE Trans. on Power Electronics, vol. 9, pp , (1994) [16]M. EIHahrouk, M. K. Darwish, P. Mehta. "Active power filters: a review", lee Proceedings Electric Power Applications, vol. 147, pp , (2000) 582