Research On Parallel Control Technology For Three-phase Inverter

The control of three-phase inverter which is made up by three single full-phaseinverters is the object of this paper, which is in the background of micro-grid. The paperanalyzes and studies the mathematical model and control technology of the single inverter.In addition, the main reason for circulating current of parallel control system, especially thefunction of digital phase-locked loop as well as the parallel control strategy is also beanalyzed in the next paragraph.The paper shows the establishment progress of mathematical models of three-phaseinverter in different frame, such as ABC, αβ and dq reference frame, and explains thereason for the choice of dq reference frame. In order to control the wave of the output,control technology for single inverter which based on the combination of PID closed-loopcontrol and repetitive control, which is taken in dq reference frame, also need to bediscussed. Due to the characteristics of parallel system made up by three-phase inverters,the system could be regarded as one that made up by paralleled three single inverters. Sowe only need to regard a single-phase inverter as one unite in order to analysis thecharacteristics of parallel system, which includes the circulating current and their powercharacteristics of output. And according to the affects of output impedance, we can chooserelative as one of our parallel control strategy.The paper explains the principle and function of the control strategy for parallelsystem, which includes SPLL(Software Phase-Locked Loop) for synchronization andclosed-loop reactive power control strategy for voltage adjusting. After the establishment ofdiscrete up to continuous mathematical models for SPLL, the paper constructs the model ofparallel control system for the strategy. In the base of the control strategy, it makes aparallel simulation model made up by two inverters, and simulates the change of circulatingcurrents caused by sudden increase of voltage and different initial phase.Experimental results are gained from an experimental bench which made up by two10kVA paralleled inverters, showing the effectiveness of the proposed approach forparallel-connected control.