PI-VPI Based Current Control Strategy to Improve the Performance of Shunt Active Power Filter B.S.Nalina 1 Ms.V.J.Vijayalakshmi 2 Department Of EEE Department Of EEE 1 PG student,skcet, Coimbatore, India 2 Assistant Professor, Coimbatore, India nalinabs@yahoo.com vijik810@gmail.com Abstract- Nowadays the number of non linear loads in power systems is increasing dramatically. These non linear loads inject harmonic currents and voltage which makes the supply currents non-sinusoidal. These harmonics are eliminated via an active power filter. In this paper, a current control scheme is proposed which does not require a harmonic detector but requires two current sensors on the supply side. In order to make the supply current sinusoidal, an effective harmonic compensation method is carried out with the aid of conventional PI controller and vector PI controller. The accuracy of the APF is improved and the performance is not affected by the harmonic tracking process due to the absence of harmonic detector. The value of %THD is reduced in the proposed control scheme. The total implementation cost is reduced as the number of current sensors is reduced. Index Terms: Active power filters(apfs),harmonic current compensation, power quality, resonant controllers, proportional controllers, vector-proportional integral controller I.INTRODUCTION In recent years, the use of power electronic devices including switching devices like diodes, thyristors, IGBT s have grown tremendously. Though they offer numerous advantages, they often suffer from the problem of drawing harmonics and reactive power from the source which results in poor power factor and low system efficiency. Harmonics are generated due to introduction of non linear loads which produce non sinusoidal currents and non sinusoidal voltage drop across network impedance so that these voltages appear at the point of mains. The presence of harmonics in power lines results in distribution problem, electromagnetic interference in communication network, operational failures, protection devices, electronic equipments. It leads to overheating of lines, transformers and generators due to excessive iron losses. Due to all these problems, the quality of electrical energy delivered to the end user is an object of concern and power engineers face the challenge of solving the problem of harmonics caused by non linear loads. So to resolve the harmonic problem, various mitigation techniques are developed. In order to improve the power quality of distribution networks as well as to meet these restriction standards, two main solutions have been introduced. LC passive filters and active power filters (APFs).LC filters are not mostly preferred as they are large and heavy. Furthermore, the compensation capability of a passive filter is fixed. Hence, an active power filter (APF) is used to suppress the harmonics generated. The basic principle behind generation of compensating current by the active power filter is to generate a current equal and opposite in polarity to harmonic currents drawn by load and inject it to the point of common coupling, thereby forcing the current to be purely sinusoidal. The three phase diode bridge rectifier feeding resistive and inductive loads behaves as a non linear load in the power system. An instantaneous reactive power theory (PQ method) is used for harmonic detection to calculate the reference currents for the active power filter. Fig. 1 Typical control scheme of shunt APF 27
The design of APF is a challenging task as it has to produce non-sinusoidal currents. The various control methods that have been developed to control the APF s are dead beat control, hysteresis control and proportional integral control. PI controllers are not suitable for certain applications with high frequency signals due to the limitation of control bandwidth. In order to overcome these disadvantages, several high-performance current controllers have been developed for APFs. These current controllers consists of a proportional controller plus multiple sinusoidal signal integrators, a PI controller plus a series of resonant controllers, or vector PI (VPI) controllers.the VPI controller is used in alternate to resonant controller and it has superior and robust characteristics. The high-pass, lowpass, adaptive filters are used mostly as harmonic detector. Due to the harmonic tracking performance by the harmonic detector, it is difficult to achieve the steady state performance. The PI plus VPI controllers have some limitation on the control bandwidth and are not able to regulate high frequency signals. Assuming the supply currents to be sinusoidal, the reference currents are given. This paper proposes an advanced current control strategy with the absence of harmonic detector. The supply currents are measured directly and made sinusoidal by a harmonic compensator based on PI plus VPI controller. The absence of harmonic detector improves the accuracy of the system as it is not affected by the harmonic tracking process. II.GENERATION OF REFERENCE CURRENTS an equal and opposite compensating current(i c ) is generated and given at the point of common coupling which cancels out the harmonics and the supply currents to be sinusoidal. The instantaneous p-q theory is used for the generation of reference currents. Equation (1-2) indicates the transformation of the phase voltages V a, V b, V c and load currents from a, b, c coordinates to α β coordinates. = (1) (2) = + (3) A small high pass filter is used in the system to avoid high frequency between the source impedance. Equation (3) describes the power calculation. Each power comprises of two components, ac power component and dc power component. For harmonic compensation both the powers are used as reference powers. The reference currents in coordinates are given by equation (4). (4) Fig. 2. Basic block diagram of shunt APF Fig.2 shows the basic compensation principle of shunt active power filter. The supply current is drawn and PLL (Phase locked loop) employed in shunt filter tracks automatically, the system frequency and fundamental positive sequence component of three phase generic input signal. Proper operation of the shunt filter under distorted and unbalanced voltage conditions is made by proper and exact design of PLL. The i d -i q currents obtained after transformation is given into two low pass filters respectively. The filter to which the id current is given filter outs the positive ripples and the filter to which the iq current is given filters out the negative ripples. The main advantage of this method is that the angle is calculated from the main voltages. 28
III. PROPOSED CONTROL STRATEGY TO IMPROVE THE PERFORMANCE OF SHUNT ACTIVE POWER FILTER (A) Structure of APF: Active filters are generally used in the compensation of harmonic currents. In the front ends, diode bridge rectifiers are mostly used as loads. The order of harmonics introduced by these loads is in the order of 6n+1 of the fundamental frequency. The harmonics are compensated by APFs to improve the power quality. As shown in fig 1 a three phase voltage source inverter connected in parallel to a non linear load forms the APF. The compensating current is given to the system at the point of common coupling through an inductor L f. A large capacitor is used for energy storage and it is located at the dc-link side of the inverter. A split capacitor is used in this paper for sharing of the stored energy and a RL load is used as a non-linear load. As stated earlier, the conventional method requires a harmonic detector which causes complexity in the design process. The proposed advanced current control strategy is carried out to overcome these disadvantages. (B) Design of the proposed current control scheme Fig 4 depicts the proposed current control scheme for the control of shunt active power filter. In the proposed control scheme, the control is carried out only by using the supply currents as harmonic detector is not used in measuring the load current (i Labc ) and filter currents (i Fabc ). The control scheme implementation is carried out in two loops outer voltage control and inner current control. Fig. 4 General block diagram of proposed current control scheme Ensuring the reactive power provided by the supply to be zero, the reference reactive current (i* sq ) is set to be zero. The shunt APF supplies the reactive power caused by loads. The use of only PI controllers is not sufficient to make the supply currents sinusoidal. This is because the supply currents are indirectly controlled. PI controller has a limitation on the control bandwidth and so it is not able to implement in case of high frequency signals. To make the supply currents to be sinusoidal, number of resonant controllers tuned at different frequencies are used. This series of controllers form the VPI controller. The VPI controller is connected in series with the PI controllers and the outputs are added together forming PI- VPI controllers. In this paper, the resonant controller is tuned at 6n to regulate a pair of h=6n of harmonic currents where the harmonics with multiples of 6 are compensated. The transfer function of PI-VPI controller is given as follows = + (5) h=1,5,7,11 where and are the proportional and resonant gains respectively and K rh = K ph R F /L F, K i1 = K p1 R F /L F. Tuning the resonant controllers at harmonic frequencies, high gain can be obtained at harmonic frequencies. Fig.5 Block diagram of PI-VPI current controller The complete proposed current control scheme using the PI-VPI controller is shown in Fig. 5. The proposed current controller is designed in the fundamental reference frame, the measured supply current i sαβ first must be transformed from the stationary to the fundamental reference frame i sdq. The proposed current controller is then executed to regulate this current follow its reference The outputs of the current controllers and the feedforward supply voltage term v sdq are added together and then transformed in the stationary reference frame through an inverse rotational transformation to obtain the command voltage which is the control signal given at the point of common coupling. 29
TABLE 1 SYSTEM PARAMETERS DC link capacitor 370V voltage DC capacitor 1500 F Filter resistance 0.05 Filter inductance 2mH Supply frequency 50Hz Non linear RL load R=5Ω,L=imH Line impedance 0.02e-3 H Fig.7 Direct axis (I d) and quadrature axis (I q) current TABLE 2 CONTROLLER GAINS 100 =7.5 =20 =0.1 =2.5 =15 =0.1 IV. SIMULATION RESULTS The results of the proposed current control strategy are investigated with MATLAB/SIMULINK and the results are obtained. The harmonic currents are compensated with the control scheme and the %THD is obtained according to the IEEE standards. The value of %THD obtained after compensation is 1.87%. Which cannot be obtained only with the help of PI-VPI controller? The proposed control scheme is much cheaper and provides good performance. To show the effectiveness of the proposed control scheme, the output waveforms obtained are given as follows Fig.8 Performance of proposed current controller with RL load V. CONCLUSION In this paper, an advanced current control strategy with the aid of PI-VPI controllers is carried out. The simulation is made for the harmonic compensation of supply current and the supply current waveforms even under distorted voltage conditions. The reference currents were obtained from instantaneous p-q theory and various transformations were carried out. The compensating current was obtained and given at the point of common coupling (PCC) for reactive power compensation. The %THD has been reduced to 1.87% which comply to the IEEE-519 standards. The proposed controller is found to operate satisfactorily and good performance is obtained. REFERENCES Fig. 6 Reference currents [1]. Quoc-Nam Trinh and Hong-Hee Lee, Senior Member, IEEE An Advanced Current Control Strategy for Three- Phase Shunt Active Power Filters IEEE transactions on Industrial Electronics, vol. 60, no. 12,December 2013. [2]. L.Malesani, P. Mattavelli, and S. Buso, Robust deadbeat current control for PWM rectifiers and active filters, IEEE Trans. Ind. Appl., vol. 35, no. 3, pp. 613 620, May/Jun. 1999. [3]. S. Rahmani, N. Mendalek, and K. Al-Haddad, Experimental design of a nonlinear control technique for three-phase shunt active power filter, IEEE Trans. Ind. Electron., vol. 57, no. 10, pp. 3364 3375, Oct. 2010. 30
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