Study On Dual-mode Inverter Control In The Micro-grid

Recently, as the shortage of conventional energy and the environmental pollutiongetting worse, new energy development and utilization is getting more and moreattention. At the same time, as the intense conflicts between power system anddistributed generation, micro-grid system is becoming the hotspot in the field of newenergy. Micro-grid system can not only bring the economic benefits of distributedgeneration to the power system and the user, but also improve the flexibility andcontrollability of the power system. For micro-grid system, most of the distributedgeneration needs the power electronic conversion devices to access to power system,this paper focuses on the dual-mode inverter which operates in the stand-alone modeand the grid-tied mode. As the pivotal techniques of the dual-mode inverter, thetopology, control strategy, transfer strategy and the switch are analyzed in detail.Firstly, according to the actual needs and the research priorities, the appropriatetopology is selected and its working principle is described. That is, according to theparticular cases of the system，related control strategies is applied under differentmodes．Under the grid-tied mode，the load voltage is determined by the grid, and theoutput current is directly controlled by double current closed-loop control strategy.Under the stand-alone mode, the load voltage is supplied by the inverter, and its voltageis controlled by current voltage closed-loop control strategy.Secondly, the detection system of the serious dangers of islanding is analyzed, twodifferent methods are applied to eliminate the active detection method’s non-detectionzone. One is active frequency drift islanding detection method based on phase jump.This method is appropriate for distributed generation systems, which not only improvethe detection capability under different load conditions, but also reduce the detectiontime. The other is current-disturbing method in the positive half voltage period. Theislanding can be identified by judging whether the positive and negative half voltageperiod changes alternately in the form of the positive and negative signs. This method issuitable for micro-grid system and has a small side effect on the grid.Based on the above analysis, this paper discusses a seamless transfer method toachieve the smooth switching between the two operation modes. By reasonable controlthe switching time and the controller conversion time, the load voltage mutation duringthe conversion process can be avoided. Finally, a TRIAC is used as a static transfer switch during the seamless transferprocess. With the TRIAC, the grid can be disconnected from the load within half a linefrequency cycle. Therefore, a converter control strategy is introduced. The simulationresults show that the control strategy can improve the response time and achieve theseamless transfer process.