Design and Development of FPGA Based Controllers for DSTATCOM in Grid Connected Power System

Abstract

In recent times, wide spread use of non-linear loads such as computers, laser printers, lighting ballasts, adjustable speed drives (ASD), arc furnaces, and other domestic and commercial appliances etc. has led to contamination of the power distribution source with excessive harmonic currents. These harmonic current causes power quality (PQ) problems in distribution network. Indeed, this harmonic pollution is not only harmful to the distribution network but also it affects control equipment used in various sectors. It causes the degradation of the voltage at the point of common coupling PCC, the heating of the cables and transformers, the ageing of the material and the possible abnormal operations of sensitive electronic controlled equipment. Load compensation can solve PQ problems like harmonic elimination and reactive power compensation.

Distribution Static Compensator(DSTATCOM) has been proved to be a suitable custom power device, which can eliminate harmonics and compensate reactive power in distribution network, is based on Power converter, suitable control strategy (reference generation) and current control schemes(current controller). The dynamic performance of DSTATCOM mainly rests on how quickly references are estimated. Therefore, fast and accurate reference extraction control strategy and current control schemes are expected to estimate reference current and minimize tracking error to a zero level for effective harmonic elimination and quick reactive power compensation. Several control strategies have been proposed and implemented. However, there is need of quick and robust control strategies for mitigation of power quality problems.

The work presented in this thesis involves development of novel control strategy along with new current control schemes to mitigate PQ problems in power distribution network. Three type of current control schemes such as hysteresis current control (HCC), sliding mode control (SMC) and model predictive control (MPC) with novel reference generation strategy are proposed. First, Instantaneous Symmetrical Component Active PowerProportional Integral-Hysteresis Current Control (ISCAP-PI-HCC) based system is developed and simulated in Simulink and real-time platform. The simulation performance of this system is compared with other reviewed control strategies (p-q, SRF, MSRF, IS and Fryze theory) HCC based system in normal and highly distorted utility condition. It is observed that ISCAP-PI-HCC is more effective in terms of harmonic mitigation, reactive power compensation and DC-link voltage regulation.

Subsequently ISCAP-PI-SMC and ISCAP-Fuzzy-SMC based system are developed to improve the performance metrics over ISCAP-PI-HCC based system which is simple to realize but is slow in terms of DC-link voltage stabilization. Though this problem is resolved by introducing ISCAP-Fuzzy-SMC based system but reference extraction strategy is complex due to use of more number of rules and membership functions in Mamdani FLC.

Further ISCAP-MPC and ISCAP-Fuzzy-MPC based DSTATCOM systems are developed for quick stabilization of DC-link voltage and use less hardware because modulators are not required at switching stage. These approaches use a model of DSTATCOM to predict its future behavior and choose appropriate control action based on optimality criterion. To study the efficacy of ISCAP-PI-MPC based system, a comparative assessment based on performance metrics has been made with ISCAP-Fuzzy-MPC based system in both steady state and transient conditions. It is observed that quick voltage stabilization is achieved by ISCAP-Fuzzy-MPC based system whereas efficient harmonic compensation outperformed by ISCAP-PI-MPC based system.

Finally three control structures ISCAP-PI-HCC(SDC(Ploss)) meant for sample delay compensation(SDC), ISCAP-PI-HCC(PDC(Pmax)) meant for phase delay compensation PDC) and ISCAP-PI-HCC(PPE(Pmax)) meant for PLL based phase extraction (PPE) are implemented on FPGA to make the processing faster and cost effective. FPGA based all-on-chip digital controllers of SDC(Ploss), PDC(Pmax) and PPE(Pmax) structures are proposed which include PI controller, +sequence detector, instantaneous power calculator(SDC(Ploss)), low-pass filters, PLL(PPE(Pmax)) and HCC controller. All these modules are configured on FPGA hardware resources aimed to mitigate harmonics and compensate reactive power in power distribution network. VHDL code for each module has been generated through System Generator and implemented on SPARTAN-3 XC3S5000 FPGA chip through RT-XSG toolbox in Opal-RT platform.