Research On Prarallel Control Technology For Combined Three-Phase Inverter

Taking combined three-phase voltage sourced 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 establishes mathematical models of three-phase inverter in ABC,αβand dq reference frame, and analyzes control technology which based on combination of immediate PID closed-loop control and repetitive control in Synchronized reference frame. It also analyzes current limiting control technology based on the combination of hardware lcokout and software, which increases the reliability of inverter.Since we may regard the parallel system composed by the three-phase inverters 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 or more inverters but also inverter's output reactive and active power's characteristics in parallel system, and thus relative parallel control strategy can be obtained according to the characteristics of the actual inverter's output impedance. The paper includes how the output impedance of inverter makes an affect on restraining load's disturbance as well as circulating current in parallel system when inverter is controlled with closed-loop control of voltage.The paper includes thought, principle and realization of the parallel control method of closed-loop control of reactive power for voltage adjusting and active power for phase lagging. It presents model of parallel control system and small signal model for the strategy, analyzes influences of controllers'parameters upon the control system and circulating current's change and its convergent envelope curve caused by phase's break. It makes two three-phase inverters'parallel simulation model on the basis of the parallel strategy, and simulates their circulating currents caused by breaks of voltage and phase.It constructes discrete and continuous mathematical models about DPLL in the inverter. The models reflect vividly the principle of synchronous phase-locked technique in the inverter. DPLL control system is studied in detail. The paper establishes a simulation model of DPLL, which simulates its phase-locked process while given or output phase varies abruptly.The paper proposes a parallel control strategy based on synchronous pulse-width tracking. It analyzes the strategy's parallel control principle and its power pulse-width modulation's principle, establishes the models for closed-loop control of reactive power to adjust voltage and closed-loop control of active power to regulate phase respectively also, and thus analyzes parallel control system and adjusting voltage and regulating phase's influences with controllers'parameters in detail. In view of characteristics of inverter's output impedance in parallel system causes coupling between inverter's output power and amplitude and phase of voltage, an improved synchronous pulse-width tracking strategy for parallel control is proposed. It establishes two three-phase inverters'parallel simulation model, simulates circulating currents'convergence in parallel system by differences of voltage's amplitude and phase. The parallel strategy restrains the circulating current effectively.The paper finally establishes a parallel system with two 10kVA prototypes of combined three-phase inverters, experimentes with the parallel strategies advanced forward. We can learn by the experiments that the parallel strategies restrain system's circulating current effectively.