| Abstract |
This paper proposes an improved multi-loop control scheme for a three-phase voltage source inverter (VSI) for the island/microgrid operation. The constant voltage constant frequency (CVCF) pulse-width modulation (PWM) inverter can be used to regulate the output voltage with lower total harmonic distortion (THD). The output voltage is regulated under different load conditions, such as linear and rectifier loads for a CVCF for the uninterruptible power supply (UPS) inverter in stand-alone operation. An improved plug-in repetitive controller (RC) with the proportional-resonant (PR) control is used in the outer voltage loop to regulate the output AC voltage, and a simple proportional control is used in the inner current control loop for active damping and improving the transient performance. The instantaneous reference voltage of the converter is used as a feed-forward signal at the output of the converter to robust the system performance and simplify the controller design. This paper proposes a step-by-step design procedure of the voltage and current controllers, an analysis of the overall system stability from the frequency-response viewpoint, and the implementation of the PR with the improved plug-in RC for the three-phase stand-alone inverter supplying linear and non-linear loads. The improved plug-in RC combined with PR control offers high-quality sinusoidal output voltages, robustness to the parameter uncertainties, fast response, and the need for fewer sensors. Moreover, the modified plug-in RC is effective and simple to be implemented on a digital signal processor (DSP). The three-phase VSI with the proposed multi-loop control is simulated in MATLAB/Simulink and experimentally implemented on a 7.5 kW system on TMDSDOCK28379D 32-bit floating-point DSP from Texas Instruments to validate the excellent steady-state, dynamic and transient performance of the proposed control scheme with better harmonic mitigation. |
