ANALYSIS AND CONTROL OF SHUNT COMPENSATOR IN DISTRIBUTION SYSTEM

Abstract

Electrical Power distribution systems suffer from power quality (PQ) problems such as poor power factor, load unbalancing, harmonics, distortion in voltage etc. The contribution of non linear loads incorporating power electronics devices viz. switch mode power supply, variable frequency drive (VFD), lighting loads (CFL, LED) etc. in power distribution systems is high. This leads to higher PQ problems into the system such as harmonics injection in voltage and current, high neutral current etc. Since the continued presence of PQ problems deteriorates the quality of power at the end user level, hence it becomes important to find solutions to overcome power quality problems. Conventional methods to improve PQ in distribution system include the installation of capacitors, tap changing transformers, reactors, capacitor banks etc. However, these are slow compensation techniques and do not provide active load compensation, so new custom power devices have been designed. Shunt Active Power Filter (SAPF) is one such solution aimed to provide load compensation. The SAPF can mitigate several PQ problems such as load unbalancing, current harmonics, poor power factor of load etc. A SAPF can be realized using Voltage Source Converter (VSC) with a DC link capacitor. Three leg VSC for three-phase, three wire (TPTW) and four leg VSC for three-phase, four wire (TPFW) configuration are conventionally used VSC configurations. TPFW systems may have additional requirement of neutral current compensation due to the presence of unbalanced loads. The SAPF with four legs can be realized for TPFW system, however, the system cost increases. A low cost solution is to use zigzag transformer configuration to mitigate excess neutral current and the conventional TPTW SAPF configuration. v

The proposed work has been divided into four parts and deals with compensation in TPTW and TPFW distribution systems, distorted three-phase grid systems and single –phase systems without/ with PV integration. Detailed system design, development and analysis of new control algorithms have been investigated. Simulation as well as experimental results have been analyzed and tabulated with linear as well as non-linear loads. Conventional control techniques viz Synchronous Reference frame Theory (SRFT), Power Balance Theory (PBT) and Instantaneous Reactive Power Theory (IRPT) have been initially tested on the prototype system developed in the laboratory. Three new control algorithms have been developed on TPTW distribution system which include Notch Filter, Kalman-LMS and Hopfield neural network based algorithm. These have been developed and implemented using dSPACE 1104 as Digital Signal Processor (DSP). Various PQ issues such as load unbalancing, current harmonics and supply current power factor have been considered. Detailed simulation results are recorded and verification of these results on the experimental setup is performed. Simulation has been performed in MATLAB/SIMULINK environment. PQ problems in TPFW distribution system have been studied. The control algorithms developed for this system are Self Tuning Filter (STF), Modified Recursive Gauss Newton (MRGN) and Chebyshev polynomial based algorithms. A conventional three leg configuration of VSC has been used as SAPF; however zigzag transformer has been designed and used for neutral current compensation. The experimental setup has been controlled using dSPACE 1104. PQ issues such as load unbalancing, current harmonics, supply current power factor and neutral current compensation have been considered with different loads. Experimental results of all techniques have been analyzed in details. vi

In electrical power distribution system, the presence of distortion in the grid voltages is also a major PQ problem. The supply voltages may have high distortion due to presence of large source impedance. The TPFW experimental setup with distorted grid and unbalanced linear and non linear loads has been investigated. PQ issues such as load unbalancing, current harmonics, supply current power factor and neutral current compensation have been mitigated successfully for distorted Point of Common Connection (PCC) voltages. Both simulation and hardware results using Multiple Complex Coefficient Filter (MCCF) and Second Order Generalized Integrator (SOGI) control algorithms have been analyzed and compared. Next, PQ problems in a single phase grid connected system have been analyzed and the system has been developed in the laboratory. Two aspects discussed in this chapter are the integration of PV and mitigation of PQ problems. Three control algorithms viz SRFT, Notch Filter and second order generalized integrator (SOGI) based algorithm have been developed to mitigate PQ problems. Solution of PQ problems in single-phase, TPTW, TPFW distribution systems using different SAPF configurations and new control techniques is the highlight of this thesis work.