Performance Evaluation of Shunt Active Power Filter (SAPF) connected to Three Phase Four Wire Distribution Networks

This paper presents an Inductor-Capacitor-Inductor (LCL) filter with Sinusoidal Pulse Width Modulation (SPWM) based switching control strategy for four-leg Voltage Source Inverter (4LVSI) in Shunt Active Power Filter (SAPF) to compensate load current harmonics and fundamental frequency voltage unbalances at point of common coupling (PCC). The use of four-leg VSI allows independent control of the load neutral current without having any dc-link capacitor voltage balancing arrangement. Interface LCL filter between 4LVSI and PCC is used to minimize the penetration of the switching frequency component generated by VSI into the grid. Passive damping using resistance in series with the capacitor is provided to avoid resonance oscillations and to improve the stability of SAPF. Dual Second Order Generalized Integrator based Frequency Locked loop (DSOGI-FLL) is used for accurate estimation of synchronous frame angle even under unbalanced and distorted grid operating conditions. Positive Sequence synchronous reference frame ('d-q-0') Proportional-Integral (PI) and Resonant (R) based current controllers with grid voltage feed forward are used for fundamental reactive power and harmonic load current compensation, up-to 25th harmonic order of the fundamental frequency. Concept of harmonic unbalance in load current is introduced and conventional current control scheme is modified in order to compensate unbalance harmonic currents. To increase the speed of compensator reference current generation, positive sequence component of load current is extracted using SOGI (Second Order Generalized Integrator) based QSG (Quadrature Signal Generator) and positive sequence extractor. PI and R based current controllers in stationary reference frame are designed to compensate harmonic zero sequence component of the neutral current. Both balanced and unbalanced voltage sag, swell and flicker at PCC are compensated using two PI based voltage controllers in positive synchronous reference frame (PSRF) and in negative synchronous reference frame (NSRF) respectively. A zero sequence R (resonant) based voltage controller, tuned at fundamental frequency (50 Hz), is designed to compensate zero sequence PCC voltage. Simultaneous compensation of load current harmonics and unbalance PCC voltages are performed by introducing multi-functionality feature into SAPF. Performance of proposed active power filter and the effectiveness of the associated control scheme are demonstrated through MATLAB simulation and validated with experimental results obtained in a 10 kVA laboratory prototype.