Research On Grid-Connected Inverters And Voltage Stability Based On Virtual-Machine Model In Distributed Generation

Distributed generation(DG) is becoming a promising complement to the traditional power grid because of many advantages such as high energy efficiency, flexible generating method, environmental compatibility and so on. As a connector of distributed power supply and grid, grid-connected inverters play an important role in joining the national power grid. In this paper, the concepts of abc natural coordinates and the virtual machine(VM) are suggested for grid-connceted inverters.By establishing accurate observations of flux linkages, it solves the problems of low power factor and voltages fluctuation when injecting distributed power into grid.Different from the traditional methods of voltage oriented control and virtual-flux oriented control, it proposed that the integral of output voltage from inverter should be regarded as virtual-machine-flux vectors, others except inverter should be regarded as virtual machine.As a result, The part of VM can be regarded as an infinity AC synchronous generator powered by inverter and running at a constant speed. Then, the mathematical models are constructed in the abc natural coordinates.Two control strategies about the technology of reactive compensation and quasi direct power control(QDPC) for grid-connected inverters in distributed power systems are proposed which are all based on virtual-machine model in natural coordinates.Virtual-machine-flux vectors serve as the oriented vectors of the inverters,the independent control for active power and reactive power is implemented in the abc natural coordinates. Lastly, rapid real-time validations about natural coordinates control of grid-connected inverters are implemented in experimental platforms which are semi-controllers and real objects based on Quaser8+dSPACE. The experiment results show that, the two methods proposed in this paper avoid the conversions of coordinates, phase-locked loop(PLL) and the AC voltage sensors. The technology of reactive compensation based on VM can output or absorb the suitable compensating currents to the system continuously, dynamically and quickly, and keeps the voltages constant at PCC and DC bus. The strategy about VM-QDPC meets the characteristics of model free control method on a small scale, so it has strong robustness and overcomes the problem of some uncertainty of sources and loads for distributed generation, and runs with negative unit power factor.