International Journal of Advancements in Research & Technology, Volume 3, Issue 1, January-2014 ISSN

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1 134 Single Phase Unified Power Quality Conditioner with Minimum VA requirement B. Lakshmana Nayak 1, V. Vijaya Kumar 2 1 M.TECH(APS), AMIE, Assoc.Prof., NMR Engineering College, Hyderabad, A.P, India, 2 M.TECH, Assistant.Prof., NMR Engineering College, Hyderabad, A.P, India, Abstract--This paper deals with unified power quality conditioners (UPQCs) which aim to integrate series active and shunt active filters. The main purpose of a UPQC is to compensate for voltage flicker/imbalance, reactive power, negative sequence current and harmonics. It protects the consumer at the load end from supply voltage sag, and provides unity power factor condition at the utility for different values of load power factor. During the unbalanced voltage sag/swell at the input side, the DVR maintains the rated voltage at the load side. It regulates the load voltage with minimum VA loading of the overall UPQC by an optimum voltage angle injection. Some experimental results of UPQC using MATLAB are presented. Index Terms Active power filters, Dynamic Voltage Restorer, optimized Unified Power Quality Conditioner. I. INTRODUCTION The wide application of the nonlinear and electronically switched devices in distribution systems, the problems such as voltage sag/swell, flicker, harmonics and asymmetries of voltages have become increasingly serious. Power electronics loads inject harmonic currents in the ac system and increase overall reactive power demanded by the equivalent load. The other development of the digital electronics /communications and the process control have increased the number of sensitive loads that require ideal sinusoidal supply voltages for the proper operation [1]. Spurious tripping of voltage sensitive loads such as PLCs or adjustable-speed drives due to the voltage sags is a serious power quality concern for industrial customers. In [2],the adjustable-speed drives have been reported to trip for voltage sag as small as 15% for 8 ms. IEEE Std defines voltage sag as a decrease in rms voltage at the power frequency for duration of 0.5 cycle to 1 minute, caused by faults in the electric supply system and staring of large loads like induction motors. Customers describe tripping of equipment due to disturbances in the supply voltage as bad power quality. The increased use of converter-driven equipment, such as consumer electronics, up to adjustable-speed drives has led to a large growth of voltage disturbances. The main cause here is the non-sinusoidal current of rectifiers and inverters. In view of the proliferation of the power electronic equipment connected to the utility system, various national and international agencies have been considering limits on harmonic current injection to maintain good power quality. As a consequence, various standards and guidelines have been established that specifies limits on the magnitudes of harmonic currents and harmonic voltage distortion at various harmonic frequencies.

2 135 Some of these are as follows: EN , "The Limitation of Disturbance in Electricity Supply Networks caused by Domestic and Similar Appliances Equipped with Electronic Devices", European Standard prepared by Comité Européen Electrotechnique, CENELEC; IEC Norm 555-3, prepared by the International Electrical Commission; West German Standards VDE 0838 for household appliances, VDE 0160 for converters, and VDE 0712 for fluorescent lamp ballasts; IEEE Guide for Harmonic Control and Reactive Compensation of Static Power Converters, ANSI/IEEE Std , which was revised in 1992 to CENELEC, IEC, and VDE standards specify the limits on the voltage (as a percentage of the nominal voltage) at various harmonic frequencies of the utility frequency, when the equipment-generated harmonic currents are injected into a network whose impedances are specified. The revised IEEE-519, which contains recommended practices and requirements for harmonic control in electric power systems, specifies requirements on the user as well as on the utility. Under this circumstances, a new technology called Custom Power emerged [3], which is applicable to distribution systems for enhancing the reliability and quality of power supply. Custom power devices include static shunt converter and static series-converter. Static shunt-converters, such as DSTATCOM, are mainly intended for conditioning the current flowing from the load into utility [4]. The series converters, such as DVR, are used to improve the quality of the voltage supplied by the utility to the load [5]. Since the basic compensation principles were proposed around 1980, much research has been done on active filters in the last years [6], [7], [8]. Recent research effort have been made towards utilizing a device called Unified Power Quality conditioner (UPQC) to solve almost all power quality problems [9], [10],[11]. To put in nutshell, UPQC aims the integration of series active and shunt active power filters connected through a common dc-link capacitor. This led to the development of advanced control techniques for UPQC. In [12] some works have been done based on the quadrature voltage injection by the series-converter of the UPQC. This scheme, leads to purely reactive power handling by the series-converter, does not result in a minimum VA consumption of the overall UPQC, cannot operate for supply voltage swell, and is not optimized from point of view of VA loading and efficiency. The series voltage injection at an optimized angle not only results in load voltage regulation but also in overall minimized VA loading and improved efficiency. This paper presents an optimized UPQC where the series voltage injection is controlled at an optimized angle for minimum VA loading of the overall UPQC. To verify and validate the proposed Power Conditioning equipment and the control method, a laboratory prototype was performed; the system is fully digitalcontrolled using the fixed-point TMS320F240 digital signal processor. The proposed Unified Power Quality Conditioner has the following goals and characteristics:

3 136 The overall UPQC VA loading is maintained at a minimum value for the whole range of operation; Maintains the load voltage at the rated voltage for supply voltage sag/swell; the injection voltage assured by seriesconverter is taken from the same dclink voltage controlled at a fixed level (450VDC) by shunt-converter; Compensates the load reactive power, by controlling the current of shuntconverter; in steady-state operation, at the rated supply-voltage, no active power is consumed by shunt-converter; Assures an input current with very low harmonic content, having in view the regulations of the previous mentioned standards. In this paper, the analysis, design and experimental results are presented to show the feasibility of the proposed scheme. II. BASIC CONFIGURATION OF THE UNIFIED POWER QUALITY CONDITIONER The configuration of the UPQC is presented in Fig.1. The UPQC consists of two single-phase PWM controlled converters connected in cascade through a common dc-link capacitor and a series lowimpedance transformer. The series-connected converter such as DVR, inject/extract the necessary voltage to compensate the supply-voltage sag/swell. The shunt-converter, such as STATCOM (STATic synchronous COMpensator), connected in parallel with the load takes a current from the supply to compensate the reactive power requested by the load, to reduce the harmonics and to control the dclink voltage at a desired value. Shuntconverter acts not only as an active filter, but assures the necessary active power for series-converter, as well. The shuntconverter can operate with hysteresis current control mode to force the source current Ii, in the same phase with Ui such the input power factor is always maintained unity. The series-inverter operates in unipolar PWM switching mode. When voltage sag/swell occurs, the DVR injects a voltage in such a manner so that the load end voltage is always maintained at the desired magnitude. There are two main methods for load voltage compensation. As [12] presents, when voltage sag occurs, the DVR injects a voltage in quadrature advance to the supply voltage. This method cannot be used for compensation of the voltage swell, and the overall UPQC VA loading is not minimized. Another method, as [9] presents, is the controlling of the compensating voltage in the same phase or opposite phase, depending on the supply voltage event. In this case the UPQC is capable of compensating both voltage sag and swell, being not optimized from point of view of UPQC VA loading. Fig 1. Single-phase Unified Power Quality Conditioner system. The main advantage of the shunt-series controller is that it does not require any energy storage. It is designed to mitigate any supply-voltage variation of a certain magnitude, independent of its duration.

4 137 A. PHASOR DIAGRAM Since the STATCOM is locally supplying the reactive current component of the load and V S1 is in phase with the PCC voltage (VPCC) the source current I S1 is the real component of the load current. Therefore, and (1) can be rewritten as Therefore, I L cos Ø = I S1 and (1) can be rewritten as I s2 = I L.cos( Φ) /(1 x) III. VA REQUIREMENT OF THE OPTIMUM UPQC Because the shunt-converter has the main role to assure an input current with a very low THDi, this is connected and controlled in a such way to compensate the Fig. 2 Phasor representation of various currents and voltages in distortion component of the load current UPQC (IdisL=IdisSh).The THDSh (Total Harmonic The authors [8] reported a control scheme Distortion of shunt-converter current, IC) is considering minimum energy requirement one of the main factors, which affects the only for the DVR. There is no report on the VA ratings of the UPQC. It is very overall UPQC optimization where the important to find a relationship with THDL combined operation of DVR and (Total Harmonic Distortion of the load STATCOM has been taken into account. current IL). The loading calculation of UPQC during the Series VA loading voltage sag has been carried out on the basis of a linear load for fundamental frequency at Vinj. I s2 = Vinj. I L.cos( Φ) /(1 x) rated load current ( I L Rated). From the phasor diagram of Fig. the presag source Shunt VA loading = I C2 V L2 2+ I 2 C2 Z SLC P.U voltage V S1 and load voltage ( V L2) during 2 the sag are assumed to be equal to the VA( total) = Vinj. I L.cos( Φ) /(1 x) + I c 2. VL2 + I C 2. Z slc P. u healthy source voltage V S, which is 1 p.u (1) Let the rated load current ( I L Rated) a load power factor angle be 1 p.u. with Ø 1. Equation (1) shows that at rated load current ( I L rated = 1 p.u), the total VA loading of In case of a voltage sag, the postsag the UPQC is the function of three quantities, source voltage (V S2 ) is given as V S2 = viz. x, ø and θ, and sag x and p.f. angle ø are V S1 (1-x),where x is per-unit sag. Now, to independent quantities. θ is the angle by maintain active power constant under the which leads the postsag voltage. The voltage sag condition, the following voltage is injected by the DVR at an angle L equation can be written as w.r.t postsag as shown in Fig.2. By applying any standard method for function Vs 1 I s1 = Vs2I s2 I s1 = ( 1 X ). I s 2 minimization with x and ø as two

5 138 constraints and as a variable varying from 0 to 90, an optimum angle θ can be found out, that results in minimum total VA loading. IV. SIMULATION RESULTS The results presented in this paper are for a diode bridge rectifier with the non-linear load at dc side - an inductance L in series with a parallel RC load. Fig.3.(b) - Rectifier bridge load - L=5mH, R=4Ω, C=20µF THDL=0.18; THDSh=0.94; THDi= Figure.3. Distortion compensation and power factor correction. 1 - Supply-voltage (320V/div); 2 - Load current; 3 - Input current; 4 - Compensating current; (80A/div; time - 5ms/div) In Fig.3.a and Fig.3.b there are shown from top to bottom supply-voltage, load current, input current and compensating current-shunt-converter, for two different loads. V. CONCLUSION Fig 3.(a) - Rectifier bridge load - L=3mH, R=4Ω, C=1,500µF THDL=0.37; THDSh=0.82; THDi= Figure. 4. Load Voltage compensation. Ui sag from 220V to 154V; γ = 410;UTr = 79.5V. (200V/div, 20ms/div).1-Supply-voltage; 2-Peak supply-voltage; 3-Injected voltage; 4-Load voltage Voltage compensation method both for supply voltage sag and swell also shows a very good performance. The power factor is improved at unity by compensation. The load voltage is maintained at its reference value. UPQC operates in the minimum VA optimization mode; the optimized operation can result not only in reduced overall size, but also in the increased efficiency. From the experimental results, one can say that the proposed control methods have good compensation characteristics and the proposed UPQC system can have an important role for power quality improvement.

6 139 REFERENCES 1. D. Sabin and A. Sundaram, "Quality enhances reliability," IEEE Spectrum, vol. 33, no. 2, Feb. 1996, pp M.H.J. Bollen, "Characterization voltage sags experienced by the three phase adjustable-speed drives," IEEE Transaction on Power Delivery,vol. 12, no. 4, Oct. 1997, pp N. G. Hingorani, "Introducing Custom Power," IEEE Spectrum, vol. 32, no.6, June 1995, pp P.W. Lehn and M.R. Iravani, " Experimental Evaluation of STATCOM closed loop dynamics," IEEE Transaction on Power Delivery, vol. 13. no. 4, Oct. 1998, pp T. Wunderlin, O. Amhof, P. Dahler and H. Guning, "Power supply quality improvement with a dynamic voltage restorer (DVR)," in Proceedings of EMPD'98 - Energy Management and Power Delivery,1998, vol.2, March 3-5, 1998, pp H. Akagi, "Trends in Active Power Line Conditioning," IEEE Transaction on Power Electronics, vol. 9. no. 3, May. 1994, pp T.Thomas, K. Hadded, G. Joss and A. Jaafari, "Design and performance of active filters," IEEE Industry Application Magazine,vol. 4, issue 5, Sept.-Oct. 1998, pp F. Barrero, S. Martinez, P.M. Yeves and P.M. Martinez, "Active Power Filters for Line Conditioning; A critical Evaluation," IEEE Transaction on Power Delivery, vol. 15, no. 1, Jan. 2000, pp H.J. Ryoo, G.H. Rim, T.J. Kim, D. O. Kisck, Digital-Control Single-Phase Unified Power Quality Conditioner for non-linear and Voltage Sensitive Load, in Proceedings of the 30th Annual Conference of the IEEE Electronics Society, IECON 2004, Busan, Korea, November 2-6,2004, pp M. Aredes, K. Heumann and E.H. Walandble, "An Universal Active Power line Conditioner," IEEE Transaction on Power Delivery, vol. 15, no. 1, Jan. 2000, pp H. Fujita and H. Akagi, "The Unified Power Quality Conditioner: The integration of Series and Shunt-active filters," IEEE Transaction on Power Electronics, vol. 13, no. 2, 1998, pp M. Basu, S.P. Das and G.K. Dubey, "Experimental investigation of performance of a Single Phase UPQC for Voltage Sensitive and Nonlinear loads," in Proceedings of IEEE Conference PEDS, Bali, Oct.22-25,2001,pp AUTHORS First Mr. B. Lakshmana Nayak, Born on 15 th June 1980, Anigandlapadu, Andhra Pradesh, India. He received the B.Tech. Degree in Electrical and Electronic Engineering from Sri Sarathi Institute of Engineering and Technology, Nuzvid, Andhra Pradesh, and M.Tech. Degree in Advanced Power System from the Jawaharlal Nehru technological university, Kakinada, Andhra Pradesh. Presently he has been working as a Associate Professor in the Electrical and Electronic Engineering Department in Nalla Malla Reddy Engineering College, Hyderabad, Andhra Pradesh, India. He has 8 years of experience. His main research area power systems. lakshmananayak@gmail.com Second Mr. V. Vijaya Kumar, Born on 8 th May 1983, Shad Nagar, Andhra Pradesh, India. He received the B.Tech. Degree in Electrical and Electronic Engineering from Sri Saijothi Engineering college, Mahabubnagar, Andhra Pradesh, and M.Tech. Degree in Electrical Power System from J. B. I. E. T, Andhra Pradesh. Presently he has been working as a Assistant Professor in the Electrical and Electronic Engineering Department in Nalla Malla Reddy Engineering College, Hyderabad, Andhra Pradesh, India. He has 7 years of experience. His main research area power systems. vijay.victory.259@gmail.com