IMPROVEMENT OF POWER QUALITY BY SHUNT ACTIVE POWER FILTER IN A THREE-PHASE FOUR-WIRE DISTRIBUTION SYSTEM

Power quality disturbances such as harmonics, power factor and neutral conductor current in a distribution network are the major concern as utilities are moving forward to a Smart Grid. Power quality for the digital economy in a Smart Grid distribution system is a priority with a variety of quality/price options. Meanwhile, the impact of renewal sources of energy like solar, wind, wave etc., which mainly use power electronic controllers and integrated into the Smart Grid cause increased power quality problems. The main source for high current harmonics, low power factor and excessive neutral current are high power mixed non-linear loads. International standards on power quality impose a maximum limit on Total Harmonic Distortion (THD) of 5 %. In addition the power factor is to be unity and the neutral conductor current is close to zero. In many applications the electrical power is distributed through three-phase four-wire system and as a result high neutral conductor current is produced in addition to the generation of current harmonics. The presence of harmonics in the distribution lines results in greater power losses, interference with communication lines and failure of operation of sensitive equipment. A reliable and cost-effective solution to power quality disturbances is active power filtering. This thesis addresses the above issues by using two-level, five-level and seven-level three-phase four-wire Shunt Active Power Filter (SAPF) in industrial distribution network to minimize the neutral current and to mitigate the current harmonics. The line-currents which have to be injected by SAPF are shaped by controlling the two-level and multilevel Voltage Source Inverter (VSI) based SAPF. In order to control the VSI, the reference signals are computed by using three proposed control schemes based on p-q theory, d-q-0 theory and a-b-c theory. MATLAB/SIMULINK is used as a simulation tool to develop the power system models for dynamic simulation. The proposed control schemes for SAPF are analyzed and applied to three-phase four-wire system to compensate for harmonic source currents, correcting the power factor of the supply side near to unity and to improve the source current THD within the prescribed limit of 5 % as recommended by IEEE-519 standards.