In power system, one of the power quality problems is the voltage sag that mostly occurs in the engineering. The voltage sag is the reduction of the rms voltage at the certain period or certain cycle in a short period oftime. Voltage sag refer to decreasing ofrms voltage in between 0.1 pu to 0.9 pu in a period of0.5 cycle to 1 minute. Voltage sag commonly caused by fault within the system however it is also caused by the starting of large motor or switching of a heavy load. One of the solution that can be applied to mitigate or improvise the voltage sags is by using the Distribution Static Compensator (DST A TCOM). It is a power controller that gives a flexible voltage controlled with a fast response characteristic to provide a power system quality improvement. When the system voltage is lower than inverter voltage, D-STATCOM will inject reactive power to the system and when the system voltage is higher than inverter voltage, the D-STATCOM wiJI absorb the reactive voltage. The circuit is design in the Power System Computer Aided Design (PSCAD) software and all parameter will design base on the power system network characteristic. There are two type of Voltage Source Converter (VSC) that is voltage source inverter (VSI) and voltage source rectifier (VSR). These VSC used semiconductor switches as the switching devices and the pulses will trigger those switches. By referring to advantage and disadvantages, gate turn off thyristor (GTO) will be used as the switching device. DC capacitor play an important role in storing the energy that used to mitigate voltage sags. It is used to supply the reactive power through DST A TCOM when voltage sags occurs. In this simulation, the three-phase induction motor has been used as the load for the AC system. The data of the motor was taken from the real motor in cooling tower near the PPKSE School at UniMAP. Without using the D-ST A TCOM, the sag duration was about 0.5s for both sag occurrences and the voltage sag were slowly disappear when the induction motor reach it rated speed. The voltage sag that occurs in the simulation system has a reduction of voltage for 40% (the highest) and 28% (the lowest). From the IEEE standard, it prove that the decreasing of voltage occurrences is in between 10% - 90% in a period of 0.5 cycle to I minute. By implementing D-ST ATCOM, it shows that the voltage sag has been 99% completely being mitigated. D-STATCOM gives a huge contribution for optimizing power quality issue in unbalances distribution network.
