Stability Of The Photovoltaic Grid-connected Inverter Based On The Weak Grid Situation

With the increasing seriousness of energy issue, distributed generation is widely applied in the power system nowadays, because of its clean, efficient and renewable features. However, comparing with the traditional generation technologies, the added power electronic equipments used in distributed system must bring about many hidden troubles. Also, relatively high inductive grid impedance is produced due to long-distance transmission line and power electronic facilities, such situation leads to a weak grid performance, and disturbs the stability of grid-inverter interactive system.Based on the Middlebrook stability criterion, detailed production theories of unstable issues are described in this essay, as well, a stability criterion focused on the grid-inverter interactive system and analysis compared the stability margin difference among systems under different types of grid-current controllers and filters are given.The impacts brought by the grid impedance toward the low-frequency stage and high-frequency stage are provided separately from the perspective of frequency domain. To be specific, the low-frequency stage will became unstable if the harmonic compensation peck is pull out of the system bandwidth, because of the increased grid impedance. On the other hand, the high-frequency stage problem mainly performs to be an active damping control failure, since the grid impedance variation will change the position of resonance peak. According to the theoretical analysis, this essay proposed two methods to settle those unstable problems. Introducing an adaptive notching filter can stable the high frequency range, the low-frequency instability can be solved through adding a compensate link. Besides, both method relies on the accurate grid-impedance estimation, through those methods the trace of varying grid parameters can be easily realized, consequently the system stability is improved.Finally, a 7.7kW simulation module based on the Matlab/simulink platform is established, moreover a 440 W single phase grid-connected inverter is designed based on TMS320F28335. Experimental results demonstrate the feasibility and adaptability of the proposed new methods, confirm the validity of theoretical analysis as well.