Research On Resonance Mechanism And Suppression Strategies Of Photovoltaic Paralleled String Inverters

With the continuous capacity expansion of the grid connected photovoltaic power plant, string inverters are widely used in new energy power generation system. However, its non-isolated topology may lead to establishment of electrical coupling when the inverters are directly connected in parallel. Meanwhile, the parallel connection of the output high order filter forms a complex high order electric network, which causes resonance possibility of the system, and restricts the development of large scale grid connected power generation technology. In this paper, for the string inverter parallel system resonance problem, from many aspects that may lead to parallel resonance, the influence of output filter hardware structure parameters, software control parameters, differential mode and common mode circuit and grid characteristics on string inverter parallel resonance is researched, and corresponding suppression schemes for various resonant types are proposed to lay a foundation for its large scale application in PV system. The main research and innovation of this paper are as follows:1) Aiming at LCL type inverter parallel resonant problem, the conventional parallel equivalent small signal model is modified according to the phase-frequency characteristics of inverter output current combined with carrier synchronization, to obtain a model based on carrier synchronization for resonance analysis. The influence of hardware and software parameters on resonance characteristics is studied afterwards. At the same time, the unified partial state feedback scheme and multi sampling technique are proposed to applied for resonance suppression. On the other hand, "resonance stability" is put forward and adopted for evaluation of resonance degree of parallel inverter system and the effectiveness of resonance suppression.2) Aiming at LC type inverter parallel resonant problem, based on the analysis of high frequency characteristics of the LC filter, the 2 paralleled inverter system circulating current and its characteristics are obtained combing its switching transient state. Taking into account the influence of the inverter output parasitic parameters, the high frequency resonance characteristics of the LC type inverter are derived, and the high frequency resonance suppression scheme is introduced.3) Aiming at the effect of grid characteristics on resonance,’’centralized equivalent model" is put forward and analyzed, and the parallel number and output power of inverters in the plant is concentrated to single equivalent model.4) Aiming at the common mode resonance problem of string inverters, the common mode circuit impedance model of single inverter is established, and the characteristic of each circuit is analyzed. On this basis, the common mode circuit model of parallel inverter is established, and the difference between parallel and single inverter common mode loop is compared, and afterwards, the influence of factors such as PV module to ground parasitic capacitance on the common mode loops and the possibility of resonance is considered.Finally, the experimental platform of 4*20KW grid connected photovoltaic inverters is set up. Through the experimental platform, the LCL type inverter parallel resonant characteristics and suppression scheme, the LC type inverter parallel resonant characteristics and suppression scheme, the influence of gird characteristic on inverter parallel resonant characteristics and suppression scheme, as well as the parallel inverter common mode resonance suppression scheme are verified.