ABSTRACT:
In uninterruptible power supply (UPS) systems, voltage robustness to load disturbance, total harmonic distortion (THD), transient performance, and current overload protection are the main concerns. Control techniques can be applied to reduce output voltage THD and achieve output current limitation. The objective of the control system is to maintain the output voltage to approximate the desired value with limited THD under normal load current and limit the load current under overload condition through the inverter voltage control. A UPS system includes a DC bus, a DC/AC space vector PWM inverter, a three-phase transformer, and primary and secondary side inductive and capacitive filters. In this research, the UPS system is modeled as a linear time-invariant system with a reference voltage input, and the load current and the voltage harmonics are considered as disturbances. The control system has two feedback loops - the inner loop is used for current control while the outer one is for voltage control. A discrete time sliding mode controller is used for current regulation due to its fast tracking in transient and a robust servomechanism controller is used for voltage tracking and THD reduction because it can eliminate any specified voltage harmonics and achieve low steady state error. In this report, the theories behind the control algorithms are introduced, the steps to develop the control system are described, and the simulation results are presented.
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