Elimination of Harmonics and Dc Voltage Fluctuations Due to Non Linear Loads using Hysteresis Controlled Active Power Filter

Transcription

1 Elimination of Harmonics an Dc Voltage Fluctuations Due to Non Linear Loas using Hysteresis Controlle Active Power Filter J Venkatesh M Tech stuent Department of EEE AVANTHI S St. Theresa College of Engineering Abstract: This project proposes a new control metho of three phase active power filters for reucing the c capacitor voltage fluctuations an source current harmonics for non linear loas. The propose control metho introuces a new k-step compensator, which maintains the mean active power flowing into the c capacitor at zero every 1/(k 1) ac line perio. Therefore, the compensator can suppress the current ue to non linearity. The control technique inclues time elay reference currents which are compare with c bus voltage generate currents. A pwm technique is use to generate pulses for the filter. A hysteresis technique can be replace for faster operation of the converter an to further improvement of the performance. Key wors: Active power filters, c capacitor voltage fluctuations, harmonic etection, loa change, transient response. I.INTRODUCTION: Active power filters (APFs) have been evelope an put into practical use for reucing the harmonic current prouce by harmonic-proucing loas in power systems [1], [2]. An APF reuces the source harmonic current by absorbing harmonic current which is anti-phase from the harmonic currents rawn by the harmonic-proucing loa. The APF requests a high-accuracy an a smallphase elay in the harmonic-etection an the currentcontrol circuit an/or metho to improve the compensation performance [3], [4]. Various control methos have been propose mainly to improve harmonic compensation characteristics [5] [12]. Nakata etal.[6]have propose the application of moving average filters to a comprehensive harmonic etection metho for reucing the steay-state error. The current control B Sankar Prasa Associate Professor&HoD Department of EEE AVANTHI S St. Theresa College of Engineering performance of the APF is also an important factor for the improvement of harmonic compensation charact eristics. A eabeat control metho [7] an a quaruple sampling technique for single-phase APFs[11] have been reporte to expan the current control banwih. Any control metho causes an amount of instantaneous active power flowing into/out of APFs, which is forme by the source voltage an the compensating current. Akagietal.[17]have reporte that compensating only instantaneous reactive power causes no c-capacitor voltage fluctuations. However, harmonic compensation performance ecreases. On the other han, c-capacitor voltage control methos have also been iscusse to improve the control stability an to reuce the voltage fluctuations[16],[18].a control metho using the Lyapunov function has been propose to improve the stability of the feeback control of the c-capacitor voltage against the quick reference change [18]. The authors have propose a harmonic etection metho for a single-phase APF, which can greatly reuce the capacitance value [19]. The harmonic etection metho in [19] can control the energy flowing into the c capacitor to be zero for one source cycle after the suen loa change. However, the harmonic etection metho is not enough to ecrease the voltage fluctuations in a threephase APF because the transient voltage fluctuations is much greater than the voltage ripple in steay states. This paper proposes a new control metho capable of reucing the capacitance value of the c capacitor for three-phase APFs. The propose metho employs a newly-evelope k-step compensator to reuce unesire energy flowing into or out of the c capacitor. As a result, the propose metho makes it possible to restrain the voltage fluctuations across the c capacitor even when a suen loa change occurs. Thus, the APF using the propose metho can continuously operate without overvoltage in Page 52

2 transient states even when a small c capacitor is employe. This paper theoretically analyzes characteristics of the newly evelope k-step compensator paying attention to the reucing performance of the voltage fluctuations. In this paper, the harmonic etection metho in [19] is referre to a two-step compensator. Since the rapi evelopment of the semiconuctor inustry, power electronics evices have gaine popularity in our aily use electrical house-hol appliances. Although these power electronics evices have benefite the electrical an electronics inustry, these evices are also the main source of power harmonics in the power system. These power harmonics are calle electrical pollution which will egrae the quality of the power supply. As a result, filtering process for these harmonics is neee in orer to improve the quality of the power supply. Thus, active power filter seems to be a viable alternative for power conitioning to control the harmonics level in the power system nowaays. Power system normally operates at 50 or 60 Hz. However, saturate evices such as transformers, arching loas such as florescent lamp an power electronic evices will prouce current an voltage components with higher frequencies into the power line. These higher frequencies of current an voltage components are known as the power harmonics. The harmonics isturbances in the power supply are cause by the nonlinearity characteristic of the loas. Due to the avantages in efficiency an controllability of power electronic evices, their applications can be foun in almost all power levels. II.CONTROL OF HARMONICS: Passive filters: Traitional solutions for these problems are powerfactor-correction (PFC) techniques, passive filters ue to their easy esign, simple structure, low cost an high efficiency. These usually consist of a bank of tune LC filters to suppress current harmonics generate by nonlinear loas. Passive filters have many isavantages, such as Resonance, Large size, Fixe compensation character, Possible overloa, With the PFC approach, a PFC unit is usually in cascae in the energy pass, which processes all the power an corrects the current to unity power factor. Those kins of approaches are usually suitable for low-power (less than 5kVA) applications. Active filters: To overcome the isavantages ue to Passive Filters, Active Power Filters (APFs) have been presente as a current-harmonic compensator for reucing the total harmonic istortion of the current an correcting the power factor of the input source. The Active Power Filter is connecte in parallel with a nonlinear loa. The approach is base on the principle of injecting harmonic current into the ac system, of the same amplitue an reverse phase to that of the loa current harmonics.. This will thus result in sinusoial line currents an unity power factor in the input power system. In this case, only a small portion of the energy is processe, which may result in overall higher energy efficiency an higher power processing capability. These kins of approaches are applicable for low-power (less than 5kVA) to highpower applications (aroun 100kVA). Figure 1: Block iagram of propose APF A three-phase shunt APF is typically compose of a three-phase brige converter an control circuitry. Most of the previous control approaches nee to sense the loa current an calculate its harmonics an reactive components in orer to generate the reference for controlling the current of a brige converter. Those control methos require fast an real-time calculation; therefore, a high-spee igital microprocessor an highperformance A/D converters are necessary, which yiels high cost, complexity, an low stability. Page 53

3 the DC-link voltage an reucing the high frequency current harmonic components of the power system. Figure 2: Basic principle of harmonic currents compensations The harmonic current compensations by the active power filter are controlle in a close loop manner. The active power filter will raw an inject the compensating current, I f to the line base on the changes of the loa in the power supply system. The supply line current, Is is escribe by the following equation, I s = I f + I l The line current, I S is shape to be sinusoial by aing the compensating current, I f into the istorte loa current, I L. III.CONTROL SCHEME OF ACTIVE POWER FILTERS: The main aim of an active power filter (APF) is to generate compensating currents into the power system for canceling the current harmonics containe in the nonlinear loa current. This will thus result in sinusoial line currents an unity power factor in the input power system.the exclusive features of this propose PWM controlle APF are conclue as follows: (a) The reference frame transformation an a igital low pass filter are use to compute the harmonics of the nonlinear loa current. (b) The voltage ecouplers an pole-zero cancellation metho are use in the current controllers of the active power filter to provie fast current harmonic compensation an simplify the control scheme. (c) The elay times of both current response of an active power filter an DC-link voltage feeback are consiere. This results in ecreasing the settling time of Figure 3: control block iagram of PWM controlle APF e a,e b,e c an v af,v bf,v cf represent the phase voltages of a power system an the input voltages of a power converter, i af,i bf,i cf an vc2 enote the input currents of the active power filter an the DC-link voltage, respectively. Neglecting the reactors Ls of the input power system, the ifferential equations of the threephase active Power filter can be escribe as follows. L2 iaf ea - R2iaf - vaf L2 ibf eb - R2ibf - vbf L2 icf ec - R2icf - vcf C v f i f i f i 2 c2 a af b bf c cf Page 54

4 Where C2 is the capacitance of the DC-link capacitor, R 2 an L 2 are the resistance an inuctance of the active power filter line reactors, respectively, f a, f b, f c are Switching functions, an the possible values are 1 2 0, an. For moel analysis an controller 3 3 esign, the three-phase voltages, currents an switching functions can be transforme to a -q-o rotating frame. This yiels, 2π 2π sin θe sin θe- sin θ e+ 3 3 x xa 2 2π 2π x = cos θ cos θ - cos θ + x q e e e b x o x c Finally L i e - R i L i - v L i e - R i - L i - v 3 C2 vc2 ( fif fqiqf ) 2 where v v 2 f 2 f e 2 qf f 2 qf q 2 qf e 2 f qf f v f c2 f v qf q c2 Figure 4: Block iagram of - an q-axis reference current of active power filter Figure 5: Control block iagram of - an q- axis current controllers of active power filter. IV TEST SYSTEM MODEL: In the existing literature, the analytical moel of an active power filter is complicate an is ifficult for the esign of current regulators an DC-link voltage regulators. To improve these isavantages, a current controller with voltage ecoupler is presente here to simplify the analytical moel. This Section first iscusses the timeelay concepts incluing reference current elay, current response elay an the DC-link voltage feeback elay. Consiering these elay times, the analytical moel an close-loop transfer functions for the active power filter are then erive. Finally, the bounary conitions between the stable an unstable operations for the active power filter system are iscusse in etail. e is the frequency of the power system an the subscripts an q are use to enote the components of the - an q-axis in the rotating frame, respectively. Equations will be use to erive the block iagram of the active power fitter an calculate the input voltage commans of power converter. Figure 6: Equivalent control block iagram -an q- axis current control loops of active power filter. Assume that the -q voltage e couplers for current control loops are fully ecouple an the -q voltage commans are not saturate for linear operation of PWM Page 55

5 moulation. The -q current control loops can be simplifie for the -axis current control loop, the closeloop current transfer function can be erive as. KP f KI f s If () s L2 L2 * () s K 2 P f R2 K I f s s L L I f 2 2 The turn on an turn off instants of inverter switches shoul be such that the loa an the connecte RES coul appear as balance loa to the system. The c link voltage, Vc is sense at a regular interval an is compare with its reference counterpart Vc*. The error signal is processe in a PI-controller. The output of the pi controller is enote as Im. The block iagram of a DC-link voltage regulator is shown in Figure. The proportional-integra1 controller (s) for the DC-link voltage control loop in G c equation.(3.18) has the Characteristic Gc( s) K P c K s I c To obtain a smooth current comman i c for the Dc-link voltage regulator, the voltage etection using a first orer low pass filter is ae to the DC-link voltage feeback Control loop as shown in figure where Tc is the elay time constant between the real DC-link voltage Vc2 an the measure DC-link voltagev, an c2 c represents the Cut off frequency of the low pass filter of the DClink voltage feeback. ' Figure 8: control scheme of the moel Total system is shown as five sub systems; those are Three-phase ac source, which is supplying for Non-linear loa. Active Power Filter (APF),that is connecte parallel to the loa an also PWM controller subsystem which is giving pulses to the APF such that it will inject compensation currents into the power line which are opposite in phase to the harmonic currents introuce by the nonlinear loas. Total controller subsystem will provie reference voltages to the PWM controller subsystem. Here the reference voltages are compare with the triangular wave which is having frequency of 10KHZ an prouce the switching gate pulses for the power converter. Figure 7: 8 Analytical moel for active power filter Figure 9: Simulink Moel of PWM Controlle APF for the Three-Phase Power System Page 56

6 Figure 13: Single phase source current with PWM technique pi controller Figure 10: Simulink Moel for an q axis current controllers of APF V. RESULTS: The obtaine results of test system are Figure 14: Converter DC voltage uner operating conition using pwm Figure 11: Source current without filter Figure 15: THD with pwm technique Figure 12: THD without active filter Figure 15: Source current with hysteresis controller Page 57

7 [4] A. Bhattacharya, C. Chakraborty, an S. Bhattacharya, Shunt compensation, IEEE In. Electron. Mag., vol. 3, no. 3, pp , Sep [5] T. Ohnishi an H. Yamauchi, Active filter by instantaneous ripple line power reuction, IEEJ Trans. In. Appl., vol. 111, no. 11, pp , [6] A. Nakata, A. Uea, an A. Torii, A metho of current etection for an active power filter applying moving average to pq-theory, in Proc. 29th Annu. IEEE Power Electron. Specialists Conf., 1998, vol. 1, pp Figure 16: THD of source current using hysteresis controller VI.CONCLUSION: This project has propose a new harmonic current etection metho which can reuce the current harmonics an voltage fluctuations cause by non linear loas. The controller input signals are from c bus voltage an loa currents with changes when there are any external isturbances. by using general PWM technique to the propose etection technique the total harmonics istortion reuce to 7.90% from 15.73%. An if the pulse generation technique is replace by using hysteresis control the THD is reuce to 4.55%.from the above results we can conclue that by using hysteresis base current control technique we get better results. VII. REFERENCES: [1]S.Bhattacharya,T.M.Frank,D.M.Divan,anB.Banerjee, Activefilter system implementation, IEEE In. Appl. Mag., vol. 4, no. 5, pp , Sep./Oct [2] B. Singh, K. Al-Haa, an A. Chanra, A review of active filters for power quality improvement, IEEE Trans. In. Electron., vol. 46, no. 5, pp , Oct [3]L.Asiminoael,F.Blaabjerg,anS.Hansen, Detectionisk ey Harmonic etection methos for active power filter applications, IEEE In. Appl. Mag., vol. 13, no. 4, pp , Jul./Aug [7] P. Jintakosonwit, H. Fujita, an H. Akagi, Control an performance of a fully-igital-controlle shunt active filter for installation on a power istribution system, IEEE Trans. Power Electron., vol. 17, no. 1, pp , Jan [8] W. Merk, H. Stemmler, an J. Allmeling, Stationary-frame generalizeintegratorsforcurrentcontrolofactivepowerfilte rswithzerosteay-state error for current harmonics of concern uner unbalance an istorte operatingconitions, IEEETrans.In.Appl.,vol.38,no.2,p p , Mar./Apr [9]P.Jintakosonwit,H.Akagi,H.Fujita,anS.Ogasawara, I mplementation an performance of automatic gain ajustment in a shunt-active filter for harmonic amping throughout a power istribution system, IEEE Trans. Power Electron., vol. 17, no. 3, pp , May [10] M. E. Ortuzar, R. E. Carmi, J. W. Dixon, an L. Moran, Voltage-source active power filter base on multilevel converter an ultracapacitor DC link, IEEE Trans. In. Electron., vol. 53, no. 2, pp , Apr Page 58