Study On Voltage/Current Mode Of Single-phase Inverter With Mode Adaptive And Grid-connected Control

With the rapid development of global economy and the increasing of population,energy crisis and environment problem have become increasingly prominent. Theconcern of the development and utilization of renewable energy become the widespread.Distributed generation integrated with the renewable energy is gradually applied in theworld with the environment friendly, high effieieney and feasibility.The performance of inverter has an important influence on the performance of thedistributed generation system. However, the control output of the inverter is only onecomponent and the inverter itself is not able to hold the output when the load changes,which depends on the corresponding control system. So the controller’s parameters maybe modified online when the load changes greatly, which may result in the system goingto unstable. Therefore, it is theoretical significance and practical value to study theinverter which has two operation modes (voltage or current mode).Firstly, LCL circuit is analyzed in detail and a novel design principle about LCL ispresented. The LCL inverter system constitute by LCL circuit and inverter bridge.Besides working as the filter, the LCL circuit may work in voltage/current mode, whichis referenced that the output fundamental voltage/current of LCL circuit depends on theinput voltage only and the load change has no effect on the output voltage/current whenthe input fundamental voltage is the constant. The switching condition of the two modesis proposed. And the mode switching of the system is realized by changing thecapacitance of the LCL. Furthermore, the corresponding control strategy of the twomodes has also been proposed, in which the amplitude and phase of the outputfundamental voltage (or current) is extracted in order to improve the performance of thecontrol system.A mode adaptive control has been proposed in this thesis, in which the operationmode of LCL is decided in autonomous according to the load status. The systemoperates in the voltage mode, which controls the output voltage equal to the ratedvoltage. When the output current of the load less than or equal to the rated current, theoperating mode of the system keeps in the voltage mode. On the contrary, the operatingmode of the system changes from the voltage mode to the current mode. Therefore, inthe voltage mode the system can avoid the overload. In the current mode, if the loadequivalent impedance is greater than the threshold, the output current is controlled to equal the rated current. When the load output voltage is less than or equal to the ratedvoltage, the system mode holds in the current mode. On the contrary, the system modechanges from the current mode to the voltage mode. In the current mode the system canalso avoid the system overload. Therefore, when the system goes into the overload, itcan automatically adjust the operating mode, which controls the output voltage (orcurrent) equal to the rated voltage (or current), and prevents system overload. If the loadequivalent impedance is less than or equal to the threshold, then the system operates inthe current mode and controls the output current into a smaller value (valid value0.5A),which reduces the impact of load short circuit fault current on the system, protect thepower system and other loads.The grid-connected model of the inverter and the cause of voltage mutation in theprocess of grid-connected have been analyzed in this thesis. The grid-connectedcontroller of the system has been proposed which controls the exchange power betweenthe power system and the proposed system. The proposed control method is verified bysimulation results.Finally, the simulation model of the single phase inverter system and itscorresponding control system is developed in Saber. The normal load, overload andshort circuit state have been simulated. And the design principle is verified byexperiment results.