Research On Parallel And Grid-Connected Technology For Voltage-Controlled Three-Phase Inverter

Taking combined three-phase voltage-controlled inverter which is composed by three single full-phase inverters as object of study, the paper analyzes and studies its mathematical model, control technology, parallel control system, and thus studies specially in detail the parallel control system, the digital phase-locked loop(DPLL) as well as the parallel control strategy. The paper also discusses the grid-connected control strategy for voltage-controlled inverter.The paper establishes mathematical models of three-phase inverter in ABC,αβand dq reference frame, and analyzes control technology which based on combination of PID closed-loop control and repetitive control in Synchronized reference frame.The parallel system composed by the three-phase inverters could be regarded as three single inverters'parallel system, the paper carries on the analysis to inverters'parallel system by taking a single-phase inverter as one unite. It analyzes not only circulating current's characteristic of parallel system which consisted of two inverters but also inverter's output reactive and active power's characteristics in parallel system, and thus relative parallel control strategy can be selected according to the characteristics of the actual inverter's output impedance. The paper includes how the output impedance of inverter making affects on restraining load's disturbance as well as circulating current in parallel system when inverter is controlled with closed-loop of voltage.The paper includes thought, principle and realization of the parallel control method of PLL for synchronization and closed-loop control of reactive power for voltage adjusting. It presents model of parallel control system for the strategy. It makes two three-phase inverters'parallel simulation model on the base of the parallel strategy, and simulates their circulating currents caused by breaks of voltage and phase. It constructs discrete mathematical models about SPLL in the inverter. SPLL control system is studied in detail.The control strategy of PLL for synchronization and closed-loop control of active power for voltage adjusting is used to minimize circulating current in grid-connected inverter followed by parallel control strategy. Principles and realization of the control strategy is presented in this paper. Some potential problems are also discussed.Experimental results are presented from a two-10-kVA parallel-connected inverter system, showing the feasibility of the proposed approach.Experimental results are also presented from a 10-kVA grid-connected inverter system, showing the feasibility of the proposed approach in some sense. Since the circulating current is not small enough, more research and experiments are supposed to take in the future.