Analysis of STATCOM for voltage dip mitigation

Abstract

Static Synchronous Compensator (STATCOM) is a Custom Power Device based on a Voltage Source Converter (VSC) shunt connected to the grid. By injecting a controllable current, it can improve the quality of the load current, e.g. compensating harmonic currents or fluctuating currents. However, a STATCOM can also mitigate voltage dip by injecting current at the point of connection with the grid. This thesis focuses on the STATCOM for mitigating voltage dips. First, characteristics of the STATCOM for mitigating voltage dips are studied, such as the required shunt compensation current, injected active and reactive power for given voltage dip magnitude. This is done for different grid impedances and load characteristics (i.e. constant impedance load and constant current load). The results of this study show that the shunt compensation current, injected active and reactive power decrease as the impedance, which is the source impedance in parallel with the load impedance, increases. It also shows that reactive power is the main requirement for voltage dip compensation by STATCOM. All results are verified in MATLAB and simulated in PSCAD/EMTDC. Then, a Dual Vector Controller of the STATCOM, incorporating a vector voltage controller (outer loop) and a vector current controller (inner loop), is designed for mitigating the voltage dip and tested in simulation. The simulation models are implemented by the PSCAD/EMTDC. Three controller simulation models are reported in this thesis, which are Vector Voltage Controller with a simplified VSC, Dual Vector Controller with a simplified VSC and Dual Vector Controller with a real VSC. Three controllable current sources and three controllable voltage sources are used to represent the STATCOM and the VSC in the first two models, respectively. The Pulse-Width Modulation (PWM) strategy is implemented in the third model. The vectors are implemented by using the Clarke and Park transformations to realize the controller system. In order to obtain the phase and frequency information of the grid voltage, Phase-Locked Loop is used in the controller system. The voltage dip can be mitigated in 5 ms by using the reactive power control in the vector voltage controller and the deadbeat gain in the vector current controller for the second simulation model. The third simulation model is verified both in 400 V system and 10 kV system. When applying the STATCOM in a 10 kV system, the STATCOM configuration must be changed by replacing the L-filter with LCL-filter. It is shown that the STATCOM with LCL-filter has robust performance, but the reactive current requirements are very high.