Power Quality Enhancement in Variable Frequency Drive using Hybrid Filters- An Shunt Passive and Series Active Combination

Transcription

1 Power Quality Enhancement in Variable Frequency Drie using Hybrid Filters n Shunt Passie and Series ctie ombination harles S, Member EEE Sri Shakthi Institute of Engineering and Technology, / Electrical and Electronics Engineering, oimbatore, India charlesme@gmail.com bstract This paper presents a new Hybrid Filter for compensating oltage/current harmonic in Variable Frequency Dries (VFD). It is a combined system of Shunt Passie Filter (SPF) and a Series ctie Filter () is a costeffectie and practical way of mitigating both current and oltage harmonics generated in a power system due to the presence of nonlinear loads. In this paper a practical VFD system installed for the ID fan in SI has been chosen as the nonlinear load for which a hybrid filter comprising of a shunt passie and series actie filter is employed for improing the power quality at the source end. The design of passie and series actie filters hae been done from scratch; their modeling has been taken up in MT / Simulink enironment and the simulation has been carried out under arying load conditions. Index Terms Hybrid ctie Filter, power quality, Voltage and urrent Harmonics, Power Factor orrection, Series ctie Filters. I. INTRODUTION Solidstate conersion of power using diodes and thyristors is widely adopted to control a number of processes such as adjustable speed dries (SD), furnaces, chemical processes such as electroplating etc., Voltage harmonic distortion, created mainly by nonlinear loads, cause serious problem to power system s installation and control equipment. The reactie power and the harmonics lead to poor power factor, low efficiency and harmful disturbance to neighbouring appliances. In addition to these, they, also create the problems of poor utilisation of distribution system, RFI and EMI noise, interference to communication systems, oltage distortion, disturbance to neighbouring consumers, poor power quality at source such as notch, sag, swell, noise, spikes, surge, flicker, unbalance, lowfrequency oscillations and malfunction of protection systems. In early days, lossless passie filters () hae been used to reduce harmonics, and capacitors hae been chosen for powerfactor correction of these nonlinear loads. In order to help mitigating the problems of harmonic pollution, many international standards, such as IEEE59992, hae been established. Passie filters alone hae been used to 227 harles S. is with the Dept of Electrical and Electronics Engineering, Sri Shakthi Institute of Engineering and Technology, oimbatore, India 64 4, address charlesme@gmail.com. eliminate the harmonics in utilities due to their low cost and high efficiency. ut passie filters hae the demerits of fixed compensation; large size and resonance with the supply system. ctie filters (Fs) [, 2, 3, and 4] hae been explored in shunt and series configurations to compensate for different types of nonlinear loads. Howeer, they hae the drawback the rating is sometimes ery close to load in some typical applications and thus it becomes a costly operation of power quality improement in a number of situations. In response to these factors, a series of hybrid filters has been inoled and extensiely used in practice as a costeffectie solution for the compensation of nonlinear loads. Moreoer, these hybrid filters are found to be more effectie in proiding complete compensation of arious types of nonlinear loads. Therefore, it is considered timely to present a broad perspectie on hybrid filter technology for the power community dealing with power quality issues [5, 6]. This paper presents a topological structure for a hybrid series actie filter which reduces power ratings of the actie filter. It consists of a series actie power filter and shunt passie filter [7]. The series actie power filter proides high impedance for current harmonics so that all the current harmonics flow into the passie filter. Moreoer series actie filter cancels the reactie power and the oltage harmonics. Furthermore, it proides a high impedance at the fundamental frequency and a ery lowimpedance shunt branch to the load s current harmonics, thus compensating effectiely all current harmonics flowing into the source. The adantages of this configuration are small rating and low cost. Furthermore, the series PF can also attenuate the resonance between shunt passie filter and source impedances. Generally, the rating of PF is 3% of the load rating when only used for compensating harmonics and will be much higher when used to compensate reactie power simultaneously. In this paper VFD system models deeloped in Matlab/Simulink enironment were simulated and the source side current waeforms were analyzed. The firing angle of the conerter has been kept at 3 degree. The Series ctie Filter is implemented by a 6pulse 3phase IGT VSI coupled with a D side battery system. The

2 bridge firing is done by a PWM pulse generator, the reference signal or the modulating signal of which is obtained by subtracting a pure sinusoidal oltage of rated magnitude from the load terminal oltage for each phase. The systems performance is ealuated in terms of source current Total Harmonic Distortion (THD), input power factor, D oltage regulation and robustness toward any type of load under distorted source conditions. stator winding into two identical windings, but isolated and phaseshifted by 3º. Typical connections of twele pulse I drie is shown in Figure 2. II. SYSTEM ONFIGURTION diode rectifier with smoothing dc capacitor creates considerable current and oltage distortion. The current harmonic amplitude is greatly affected due to ac side or source impedance, howeer, the input rectifier oltage distortion is caused by the energy storing capacitor in the dc side. Passie Filters hae been used in power system to mitigate these harmonic distortions primarily due to their low cost. Parallel Passie Filter proides low impedance to harmonic currents drawn by the load, thus reducing harmonic currents flowing into the source. In the present work a typical nonlinear load comprising an R load supplied by a 6pulse Thyristor ridge with a ariable firing angle control has been taken for analysis as a test system. Series actie and shunt passie filters hae been used to eliminate the current and oltage harmonics generated due to the nonlinear load. This system configuration is shown in Fig. It has a line side conerter fed by a 5 Hz three phase supply feeding the motor side conerter which conerts its input D to ariable frequency supply. Fig.2 Twele pulse I drie schematic diagram Interchannel communication allows one channel to be the master and the other the slae, which takes its torque reference from the master. This communication allows the two motor winding currents to be balanced. Thus the channels delier equal power, take equal current, and fire at the same relatie firing angle. This utilizes the motor and dries capabilities to the best extent and also minimizes the harmonic distortion on the power systems, as the first two harmonics present in each sixpulse channel cancel (5th and 7th harmonics). Other higherorder harmonics will be present, but their amplitudes are much smaller. The specifications of the I drie system to be listed in Table I. TE I SPEIFITIONS OF THE I DRIVE 6 pulse operation 2 pulse operation Motor data 67 kw, x7, x3 phase, x352 a, 55 rpm,.9pf, 36.7 Hz 325 kw, 2x2, 2x3 phase, 2x369 a, 74 rpm,.9pf, 46.9 Hz Fig. onfiguration of Series ctie & Shunt Passie Filter with Nonlinear oad Operation and maintenance manuals, drawings and other reference material of General Electric I dries [] had been studied. The oad ommutated Inerter (I) is a static, adjustable frequency drie system which controls a synchronous motor from near zero to rated speed. The basic system consists of a line commutated, phase controlled thyristor conerter which feeds a load commutated thyristor conerter through a dc link reactor. Dc link inductor data, r mh, 28.5 mω 2,r2 mh, 28.5 mω Snubber data R,c 4ω,. µf Transformer data Rating 9 ka H winding 66 winding 25 Twelepulse operation is like two identical, separate, sixpulse dries operating from a common source at the same current and firing angle, with the firing reference angles shifted by 3º using the isolating transformers. The two motors are combined into one frame. This reduces the motor cost, including installation, and reduces the torque pulsation amplitude while raising the torque pulsation frequency. This is achieed by separating the 228 The data will be utilized for preparing the simulation models for six and twele pulse operation of VFD. III FITER DESIGN a. Shunt passie filter The shunt passie filter is a 2nd order damped filter, which consists of a parallel combination of

3 components. The filter s impedance is inductie for lower frequencies (below the fundamental frequency) and capacitie for all higher harmonics [8]. The source inductance has much influence on the behaiour of the shunt passie filter when used alone. It moes the cutoff frequency of the latter toward lower ones. The parallel passie filters are shown in Figure 3. The passie filter impedance can be found as: In a series R circuit impedance is gien by Z = R j( w ) and at resonance frequency w ( f n ) the alue of Z reduces to R. Two basic design parameters are considered prior to selection of R, and. X Quality Factor: Q = () R X = Inductor impedance at resonance frequency. Relatie frequency deiation: w w n δ = (2) w n w = δ (3) w n Or, ( ) Where, w n = in the aboe expression w and w are dependant on system frequency, ariation of, n around ambient temperature, manufacturing tolerances and ageing. Typically the alue of δ lies between. to.2. Fig 3 Shunt Passie Filter System Designing a damped or band pass filter is based upon two parameters; ut in frequency the Value of m typically lies f = and m = (4) 2 2π R R between.5 and 2.. f being the cut in frequency of the higher order band pass filter meaning the band pass filter shall pass through the harmonic currents of order f and aboe. b. Series ctie Filter The Series ctie Filter is implemented by a 6pulse 3phase IGT VSI coupled with a D side battery system. The inerter switching frequency selected is 5 Hz. The bridge firing is done by a PWM pulse generator, the reference signal or the modulating signal of which is obtained by subtracting a pure sinusoidal oltage of rated magnitude from the load terminal oltage for each phase. 5 Hz triangular wae generator acts as the carrier signal and a switched representation of the error oltage or the harmonic oltage is obtained across the inerter output terminals. The inerter output oltage is then added to the load terminal oltage in phase opposition through a : ratio oupling Transformer (T) in order to get the required harmonic isolation to the source. passie ripple filter eliminates high frequency switching harmonics generated in the inerter and the filter out put oltage is supplied to the T primary. The leakage inductance of the series T is the critical design parameter. It has to be ery low since a higher leakage inductance shall contribute to higher fundamental oltage drop and hence higher fundamental V which is to be supported by the series actie filter inerter, thus increasing its V rating. If we consider X m, Rm Z h, Z, Z, we can write Vh V. ZshPF =. Z (5) shpf Z Z Z Z Z ( ) h shpf shpf gain, forv h = V, it may be simplified as Z h = Z Z. s we understand the alue of Z must be ery small, so quantitatiely, h Z Z Z [9], [], [] explains the design criterion of input inductor f for shunt actie filter. Where, V Equialent harmonic oltage due to nonlinear load, Z Series impedance of load, h h Z Impedance of shunt passie filter, X shpf m Magnetizing reactance of coupling transformer R m oss resistance of coupling transformer, V ompensating oltage from series actie filter Z Equialent impedance of series actie filter inerter and D source. U s f = (6) 2 6 f ˆ s wiripple Where, f = switching frequency and s w i = peak ˆripple ripple current to be compensated and U = per phase s system oltage. IV. MODEING ND SIMUTION OF THE SYSTEM The model has been simulated in MT / SIMUINK for the three load conditions. The source current and oltage harmonics are analyzed. The break up of different harmonic contents in the source current has been assessed and accordingly a modified filter design has been carried out. The source side current and oltage harmonics are again assessed. The VFD system models also simulated and the source side current waeforms 229

4 were analyzed. The firing angle of the conerter has been kept at 3 degree. We obsere that as the reactie power rating of the shunt passie filter increases, source current harmonic content (or % THD) decreases and consequently the source oltage harmonic content (or % THD) decreases as a secondary effect. Va Scope2 Ia 3 alpha_deg pulses Discrete, Ts = 5e5 s. Vabc Iabc a b c Scope lock lock t To Workspace Scope g i dd dd g z PWM Generator Pulses Signal(s) K dd2 2 3 Fig 4 6 Pulse Operation with Shunt passie and series actie filters Filter (ombination of shut passie and series actie filter) Fig 5. 6 pulse source side current and oltage Fig 7 6 pulse Source urrent Harmonic with Filter Fig 6 6 pulse Source urrent Harmonic Spectrum without filters The Six pulse source side current and THD before compensation is obtained from the simulations and shown in Figs. 5 and 6 respectiely. Fig 4 shows the simulation model of the six pulse operation with Hybrid 23

5 Fig.8 6 Pulse Source urrent and Voltage Waeform with shunt Passie, series actie and ripple filters Fig.9 6 Pulse Source urrent Harmonic with shunt passie, series actie and ripple filters Fig. 6 Pulse Source Voltage Harmonic with shunt passie, series actie and ripple filters Fig. 8 clearly shows that the source oltages and the corresponding source currents are inphase with each other after the Hybrid actie filter starts functioning. Fig 9 and shows the Total Harmonic Distortion (THD) in the source current and source oltage after compensation with shunt passie, series actie and ripple filters. This shows that shunt passie filters are sufficient for mitigation of harmonics for 2 pulse operation and Series actie filter is required only for 6 pulse operation. fter incorporating series actie filter along with shunt passie filters for 6 pulse operations the source current THD came down to.88% and oltage THD came down to.34%. V. ONUSION This paper has presented a comprehensie solution to the Power Quality Problems in Variable Frequency Dries using Hybrid Filters. ppropriate filters are designed in accordance with design procedure. The results obtained with shunt passie filters alone for 2 pulse operation and shunt passie &series actie filter combination for 6 pulse operation are found satisfactory. The THD contents in source current and source oltage are.88% and.3 to.34% respectiely which are within IEEE 59 standard limit for all load conditions. In all, an effectie solution for the power quality problem is suggested here in the form of implementation of a hybrid filter consisting of shunt passie filters and series actie filter. The work may be treated as a useful tool in mitigating power system harmonics in adjustable speed dries. The series actie filter assists the passie filter if this last compensates harmonics and reactie power. The hybrid series actie filter is effectie and economic for soling harmonics problem in large capacity nonlinear load. VI. REFERENES []. S. hattacharya, D.M. Dian, Design and Implementation of a Hybrid Series ctie Filter System, IEEEPES onf. Record, tlanta, 995. [2]. S. Ponnaluri,. rickwedde, System Design of three phase ctie Filter using Time Domain techniques, IEEE annual conference PES,2. [3]. Mohan, N., Peterson, H.., ong, W.F., Dreifuerst, G.R., and Vithayathil, J.J.: ctie filters for harmonic suppression. Proc. IEEE PES Winter Meeting, 977, pp [4] kagi, H.: New trends in actie filters for power conditioning, IEEE Trans. Ind. ppl., 996, 32, pp [5] Takeda, M., Ikeda, K., and Tominaga, Y.: Harmonic current compensation with actie filter. Proc. IEEE IS nnual Meeting Record, 987, pp [6] Fujita, H., and kagi, H.: practical approach to harmonic compensation in power systems series connection of passie, actie filters, IEEE Trans. Ind. ppl., 99, 27, (6), pp [7] hen Guozhu, Zhengyu u, Qian Zhaoming, The design and Implementation of Series Hybrid ctie Power Filter for Variable Nonlinear oads, IEEE conference PE, 995. [8] Mukul Rastogi, Ned Mohan, bdelty Edris, Filtering of Harmonic urrents and Damping of Resonances in Power Systems with a Hybrid ctie Filter, IEEE. [9] Prieto, J., Saimeron, P., Vazquez, J.R., and lcantara, J.: seriesparallel configuration of actie power filters for ar and harmonic ompensation. Proc. IEEE IEON, 22, pp [] J. H. Marks and T.. Green, Rating nalysis of ctie Power Filters, IEEE 2. [] General Electric, I drie Drawings and Manuals, 984 onwards. VII. IOGRPHIE harles S obtained his.e. degree (24) in Electrical and Electronics Engineering and his M.E. (26) in Power Electronics and Dries from nna Uniersity, India. urrently, he is a ecturer in the Dept of Electrical and Electronics Engineering, Sri Shakthi Institute of Engineering and Technology, oimbatore, Tamilnadu. His area of interest is ctie Power Filters, Power Electronics and Dries, and Power Quality. 23