Analysis and control of paralleled rectifiers, active power filters and inverters, and grid-connected photovoltaic power generation.

摘要

Power quality and distributed generation have gained a lot of attention recently. Poor power quality due to excessive nonlinear and reactive loads overstresses the power system and causes system failure. Distributed generation on the other hand can release some burdens from the power system by generating the power locally. Moreover, distributed generation facilitates the use of renewable and alternative energy sources. This dissertation focuses on power converters for power quality improvement and renewable generation. Power factor corrected (PFC) rectifiers and active power filters (APF) can effectively improve the current quality of source side. Grid-connected inverters are desired to convert do power to ac power by injecting sinusoidal current into the grids.; For high power applications, paralleling PFC rectifiers, APFs or inverters extends their power handling capability to a much higher level and allows modular design. In parallel operation of rectifiers and inverters, current sharing and circulating current are two challenges. These phenomena within two paralleled rectifiers and inverters are studied in this dissertation, and a new control method based on One-Cycle Control (OCC) with combined vector operation and bipolar operation is proposed. As a result, current sharing, limited circulating current, low switching losses and simple circuitry are achieved. For parallel operation of APFs, the distribution of compensation current among the paralleled units and their safe operation are the main issues. In this dissertation, a peak current limitation method integrated within OCC core is proposed for each APF, so that its compensation current is below a preset limit. The cascade structure and the centralized structure of paralleled APFs both distribute the compensation currents and achieve good performance. For all rectifiers, inverters, and APF, there are only a few add-ons to the original one-cycle control core, thus the control circuit remains simple.; For grid-connected photovoltaic (PV) power generation, inverters are key elements to convert the dc power to ac power acceptable to the grids. Previous inverters employ a two-power-stage structure and intensive power calculation for maximum power point tracking (MPPT). Accordingly, they have complex circuitry and high losses. In this dissertation, firstly a single-stage single-phase OCC inverter is proposed. The inverter can inject a high quality current into grids, achieve MPPT without complex calculation, and remain simple and cost-effective. Furthermore, in order to allow the input voltage, i.e. the PV voltage, to be lower than the peak grid voltage, a three-phase boost type inverter has been proposed. It achieves dc-to-ac power conversion and MPPT within a single power stage with a high efficiency, and the control circuit remains simple, stable and cost-effective.