Research On Low Voltage Ride Through Control Of Three-phase Photovoltaic Grid-connected Inverter

With the increasing penetration of photovoltaic grid-tied generation system, the low voltage ride through(LVRT) is a mandatory function according to the new grid codes or standards at home and abroad. Under the grid voltage drop, photovoltaic grid-tied inverters should remain connected to the grid according to LVRT requirements. Otherwise it will impact the grid, and lead to the expansion of the fault, affecting the stability of the entire system. Therefore, the research of LVRT capability of the photovoltaic grid-tied inverters has an important engineering value.Firstly, two stage photovoltaic grid-tied generation system was established in this paper, Boost circuit has been used to achieved to maximum power point tracking(MPPT)by front stage. Three phase inverter has been adopted to realize grid connected control in the latter stage. In the stationary frame, double-loop control strategy was used in the paper which are DC bus voltage outer loop and grid connected current inner loop, outer loop using PI regulator, the inner loop adopting quasi proportional resonant regulator(QPR).Then three-phase inverter control mathematical model is established for finding the regulator parameters. The design of the whole photovoltaic system lays the foundation for the research of the LVRT technology.Secondly, this paper analyzed several important components of photovoltaic LVRT technology. The methods of separation of positive and negative sequence and the control structure of the system were compared and analyzed. The method and structure of this paper were determined. A power flexible control method has been proposed based on the analysis of five kinds of reference current commands and three control targets. On the basis of the above, a new LVRT control has been proposed in this paper. Under preventing current-limiting condition, the point of common coupling(PCC) voltage has been supported by injecting reactive power, where positive sequence voltage was raised by positive sequence reactive power and the negative sequence voltage was reduced by negative sequence reactive power, meanwhile the unbalance degree was decreased. The correctness of the proposed control strategy has been verified by MATLAB/Simulinksimulation.Finally, the three phase inverter experimental platform based on TMS28335 has been bulit to verify the proposed strategy, and the experimental results proved that the proposed method was effective.