Research Of Key Control Techniques Of Bi-directional Inverter For Distributed Power Generation System

On account of the problems of environmental pollution and exhaustion of energy resources, the distributed generation system based renewable energy has been developed rapidly. Converter is the key component of the distributed generation system. Real-time phasor measurement, island detection and control strategy of seamless transfer between stand-alone and grid-connected modes,are three key techniques in converter of the distributed generation system, and they are developed in this paper. The main results are as follows:1) A new real-time phase measurement method with adaptive memory length recursive least square parameter estimation is proposed in the paper.it establishes observation equation in polar coordinates, introduces initial phase and frequency as estimate parameter, takes the minimum sum of the whole period squared phase angel errors as target function, uses adaptive memory length recursive least squares method to real-time estimate parameters. The results of simulation and physical experiments show that this method can accurately and fatly estimate phasor of Grid even under the perturbation of asymmetric voltage dropping, phase mutation, phase mutation and harmonic pollution,etc. The longest tracking time is not more than2power frequency cycles.2) A new island detection method based on non-characteristic frequency disturbance is proposed in the paper. Periodic reactive current disturbance is injected to force voltage frequency of PCC (point of common coupling) to occur cyclical fluctuation, sliding discrete Fourier transform method is used to extract the non-characteristic value in the frequency fluctuation, and the characteristic value is used to distinguish island. The results of simulation show that this method can accurately distinguish island under the worst case of IEEE Std1547and frequency disturbance.3) For Realizing the smooth transition between stand-alone and grid-connected, this paper puts forward two improvements. First, an indirect control strategy of virtual synchronous generator PQ outer loop with PQ power decoupled+capacitance voltage inner loop is proposed. It not only realizes load voltage smooth transition between stand-alone and grid-connected, enhance robustness against line impedance variation in low voltage system; Second, a new prep-synchronization control method based virtual reactance is proposed. It does not need any additional synchronization controller, and is helpful to reduce current transient process at the moment of connecting grid.4) With the support of the national "twelfth five-year"863project, the author has participated in developing100kVA bidirectional energy storage converters. The mein wrok include the power circuit design, control circuit design and software system design. And, under the current conditions of the laboratory, it presents voltage and current waveforms of typical working condition.