Advanced inverter control for uninterruptible power supplies and grid-connected renewable energy applications

ADVANCED INVERTER CONTROL FOR UNINTERRUPTIBLE POWER SUPPLIES AND GRID-CONNECTED RENEWABLE ENERGY APPLICATIONS By Shuai Jiang The advancement of digital signal processors (DSPs) and programmable logic devices in modern power electronics systems offer great control flexibility and capability, providing attractive features particularly for applications in which complex control tasks are involved. This dissertation investigates some DSP based advanced control algorithms for pulse-width modulation (PWM) inverter applications, in particular, voltage regulated inverters connected with AC loads and current regulated inverters connected with utility grids. Uninterruptible power supply (UPS) is a typical example of voltage regulated inverter applications. It is widely used to supply high quality, continuous and disturbance-free AC power to critical loads such as medical equipments, computers and communication systems. A good UPS system requires not only excellent steady state performances in terms of voltage regulation and total harmonic distortions (THD) regardless of unknown load disturbances but also a fast transient response during load step change. In this dissertation, a three-phase four-wire AC-DC-AC double conversion UPS system is first studied. Multi-loop control strategies are designed to regulate the system input currents, DC voltages, and output voltages. Next, study will deep dive into a DC-AC three-phase UPS inverter. A high performance repetitive controller (RC) for the voltage regulated three-phase inverter is proposed. The proposed control algorithm can eliminate all the periodic distortions and guarantees a high quality sinusoidal output voltage under unknown and severely distorted loads. A novel 4th-order linear phase infinite-impulse-response (IIR) filter is first used in the RC such that harmonic distortions up to the 19th order are rejected. In order to achieve fast response during step load transient while still maintaining the low THD feature, a modified synchronousframe approach with significantly reduced delay is later proposed and investigated. Grid-connected inverters utilizing renewable energy sources (e.g., photovoltaic, wind, fuel cell, etc.) are growing rapidly in recent years along with the constantly growing global demand for electricity. A grid-connected inverter injects a synchronously regulated sinusoidal current to the utility grid with required low THD and high power factor. Using an LCL filter in such a system has been recognized as a small size low cost solution due to its -60dB/dec high frequency attenuation. In this dissertation, a high-resonance-frequency LCL filter with minimal size and cost requirement is designed. A proportional plus repetitive control hybrid strategy is then proposed to achieve very low THD current regulation and high power factor. Although utility grid is often modeled as an infinite AC voltage source in inverter current control, it can introduce way more complicated resonance issues particularly when long transmission cables are used between the inverter and the main grid. A real example of an HVAC offshore wind farm system with long submarine cables is then investigated and emulated by a scaled-down 120Vac single-phase system. A systematic analysis is carried out and the key passive component parameters that cause high frequency resonances are identified. A notch filter based active damping control is proposed and implemented in the inverter. When a grid-connected inverter is sourced from photovoltaic (PV) panels, a front-end isolated DC-DC converter is usually incorporated and dedicated control must be employed based on the converter dynamics to achieve both maximum power point tracking (MPPT) and coordination with the 2nd-stage inverter control. A high performance system-level control scheme is designed for the proposed boost-half-bridge converter-inverter system in the PV application.