COMPENSATION IN DISTRIBUTION SYSTEM

Abstract

The aim of supply utility is to satisfy its consumer power demand uninterruptedly with good power quality. The power demand in today’s world is increasing rapidly. Also, the power demand is no longer the same as it was in the past, where maximum loads were typically linear. Now-a-days distribution system mainly supplies number of loads which are generally nonlinear, critical and voltage sensitive. Apart from these, various other connected loads like industrial drives, heat ventilation and air conditioning (HVAC), lighting, highly inductive and capacitive loads are present. These loads lead to harmonics in current and voltage, flickers as well as sag/swell in voltages and affect the power factor of supply system, performance of nearby sources and loads too. However the power quality concerns become more drastic with rapid increased use of equipment based on power electronics and solid state controllers. As a result, it is the customer's responsibility to inject as less distortion and harmonics into the system as possible in order to keep distribution system reliable and healthy. Custom power devices are used to satisfy pre-specified standards in order to eliminate power quality concerns and preserve distribution system dependability. To enhance the power quality in distribution system already various authors reported various devices such as Shunt APF, Series APF and UPQC. This thesis deals with designing of capacitor-supported shunt APF controller using Unit Vector Template (UVT) method, Synchronous Reference Frame (SRF) and Instantaneous Power Theory (IPT) and a PI-Controller is used to stabilize the capacitor DC-Link voltage and Hysteresis Current Controller to provide triggering pulses to converter of APF. The shunt APF is used to lower the harmonics in supply current by injecting compensating harmonics of same magnitude but phase shifted by 180* , such that utility will supply only active power and shunt APF will provide reactive power to load. Supply system power factor is maintained to unity. The shunt APF performance is v analyzed for above mentioned three control techniques using MATLAB -Simulink to present THD in current, power flow and DC-Link voltage respectively. The loads which consume high surge currents when they are switched ON such as heavy inductive loads, pumps, compressors, milling machines, welding machines and etc. lead to unbalance, flickers in system voltage. This affects the voltage quality of distribution system and causes mal-operation, failure of various nearby loads. In order, to preserve voltage quality devices such as DVR or Series APF are installed in series at PCC. The series APF corrects the load voltage to acceptable level by injecting and absorbing compensating voltage at PCC. In this thesis the designing, performance analysis of custom power devices is presented using UVT method and SRF theory is utilized to make reference injecting voltage for battery-supported Series APF and, hysteresis voltage controller provides the gate pulses to operate the APF’s converter. Based on the effectiveness of SRF theory controlled shunt and series APF, to mitigate power quality related concerns for both current and voltage simultaneously a device called UPQC have been developed in thesis. UPQC is a custom device which is having series and shunt APF sharing common DC-Link to preserve the balance of system power. In this PI-Controller well regulates the capacitor DC-Link voltage throughout the compensation process. It is capable of lowering the harmonics in current and abolishes the sag, swell and flicker in voltage under ideal grid conditions. Also it works well for non-ideal grid conditions i.e. when supply voltage is having distortions. This thesis presents MATLAB simulation-based findings to show the performance of UPQC during various disturbances, for %THD content in current and voltage, voltage during compensation, system power flow, study of capacitor voltage during normal operation, sag, swell, and polluted grid.