Mitigation of short and long term voltage variations by distribution static compensator

Abstract

Distribution Static Compensator (DSTA TCOM) is a Flexible Alternating Current Transmission System (FACTS) device based on a voltage source converter (VSC) shunt connected to the power network. It is used to improve the quality of the load current by injecting a controllable current. DSTATCOM can be used to mitigate voltage sag by injecting current at the point of connection at the distribution network. This thesis focuses on the DSTA TCOM for mitigating voltage sags and under voltages. First, characteristics of the voltage sag in a radial feeder, such as magnitude and phase angle variation of the voltage sags are studied and analyzed. Then, characteristics of the DSTA TCOM for mitigating voltage sags are studied. Under this, compensation current, active power and reactive power injected by the DST ATCOM to mitigate voltage sags are analyzed. These results are graphically interpreted. This is done for constant load impedance, varying source impedances and impedance angle. Comparing the magnitude of shunt compensation current, active. power arid reactive power for the same sag magnitude but at different impedance angles. These three quantities decrease when the impedance angle increases. Comparing the magnitude of shunt compensation current, active power and reactive power for the same sag magnitude but at different source impedances, these three quantities increase when the source impedance decreases. It shows that the reactive power requirement for mitigating a particular voltage is very much higher than the active power and it does not depend on the impedance angle. The reactive power requirement- increases significantly with decreasing source impedance. Therefore mitigating of this type of sag using DSTATCOM IS impracticable due to the requirement of the large current drawn from the VSI. In the second part of the thesis, analysis of voltage sags and under voltage mitigation using DSTATCOM is done with simulation. The MATLAB version 7.6.0 (R200Sa) is used for simulation. The results show that the voltage sags with 0.5 pu - 0.9 pu magnitude, can be mitigated using the DSTATCOM designed in this thesis. The duration of the voltage sag which can be mitigated, depends on the type of the DC supply used in the voltage source inverter. The under voltages (voltage drop with 0.8 pu magnitude and duration is more than one minute) can also be mitigated using the DSTA TCOM designed in this thesis