Voltage-source converter controls for distorted microgrid operation and provision of additional services

Abstract

This thesis investigates the role played by three-phase grid-tied voltage-source converters in the operation of the microgrid under optimal and distorted grid conditions. Enhanced control schemes are developed to improve power quality by providing active-power filtering and grid frequency support. In Chapter 2, an outline of the typical elements of the voltage-source converter is presented, along with the development of a linearized small-signal model of the three­phase LCL filtered VSC. The design of a vector current-control scheme and associated synchronization controls is presented. The performance of these controls is shown to be significantly degraded when operating under distorted grid conditions. In Chapter 3, a novel small-signal analysis of the VSC, accounting for the effects of the vector controls and the PLL, is de\'eloped. This analysis presents paths for the voltage harmonics at the PCC to distort the output current of the VSC. These effects are shown to be a consequence of the converter PWM delay and the non-ideal PLL estimates of grid frequency and phase. These feedthrough effects are also shown to impact on control stability when operating in weak grid conditions. Validation of this analysis is presented through simulation and experimental results. In Chapter 4, a new current control scheme is designed to maintain optimal performance of the VSC when operating under polluted grid conditions. An enhanced PLL structure is developed based on the use of a self-tuning band-stop filtering stage as part of the SRF- PLL, thus improving the estimated grid-phase and frequency through selective harmonic mitigation. The improved frequency estimate is employed as part of a resonant current­control scheme, to adapt the resonant gains of the resonant vector proportional integral controllers when operating in a variable frequency grid. The capability of the proposed adaptive selective harmonic-compensation current-control scheme is demonstrated for use in an active-power filter application in the microgrid. Chapter 5 proposes a VSC control scheme based on the virtual-synchronous machine concept. Unlike conventional vector controlled VSCs, this approach facilitates synchronization without a PLL, enabling operation under weak grid conditions. The development of an outer-loop power regulation scheme is presented, known as an inertial­droop controller. This scheme provides voltage phase and magnitude reference for an inner-loop a cascaded voltage-current controller, regulating the output voltage of the LCL filter capacitor. Experimental results demonstrate the capability of this scheme to regulate output active and reactive power. to perform microgrid islanding, and also the provision of dynamic grid-frequency support. Experimental results throughout the thesis are presented based on testing at a microgrid laboratory featuring 90 kV A and 15 kV A VSCs.