Investigation On The Control Technique Of Grid-connected Inverter For Distributed Generation System Integrated With Renewable Energy

With rapid development of electrical power demand and increasingly larger scale of power rating, more and more issues in terms of security and feasibility are exposed in the traditional power system featured on centralized generation, high voltage transmission and interconnection of regional power grid. Distributed generation integrated with renewable energy is increasingly concerned with the advantage of environment friendly, high efficiency, distribution and feasibility. Integrating power grid with distributed generation is considered as the trend of power system in 21th century.Aimed at the grid-connected operation of distributed generation integrated with renewable energy, the control strategy of distributed generation under grid-connected mode and stand-alone mode, as well as the seamless transfer between the two modes were investigated from the aspects of theoretical analysis, simulation and experimental verification.Firstly, with respect to the problems on the asymmetrical grid connected current, ripples of active power, reactive power and dc-bus voltage when a traditional control strategy of grid-side converter including the dc-bus voltage loop with PI controller and the converter current loop with PI controller is adopted under unbalanced power grid, a SVPWM-based predictive current control scheme is proposed which can achieve three different control targets, including keeping converter current symmetrical, eliminating active power ripples and eliminating reactive power ripples. Therefore, the performance of distributed generation under unbalanced power grid is improved, which is verified by the simulation and experiment results.Secondly, aimed at the demand of the output power quality of the distributed generation system under the stand-alone operation mode, a dual-loop control strategy including Proportion Integration Resonance(PIR) voltage control scheme and Predictive Current Control (PCC) scheme was proposed to improve the magnitude and frequency stability of the output voltage under the unbalanced and the nonlinear load, which will be deteriorated with the traditional PI control scheme due to the low bandwidth disadvantage of the traditional PI control scheme. Associated control scheme for the distributed generation system was designed and constructed so as to confirm the feasibility of the proposed control scheme. Experimental results show that the proposed strategy is capable of implementing system output voltage with the stable amplitude and frequency under the unbalanced and the nonlinear load. As a result, the load-adaptive ability of the distributed generation system under the unbalanced and the nonlinear load is enhanced.Finally, the seamless transfer of distributed generation between the grid-connected mode and the stand-alone mode under grid fault was investigated. A three-phase programmable voltage sag generator based on a voltage source inverter was built to precisely simulate the possible grid voltage faults. Then, the transient process of the operation mode transfer is researched, and a control strategy of seamless transfer was introduced. Finally, a laboratory setup including voltage sag generator and distributed generation has been built to validate the effectiveness of the control strategy.