The Research On Leakage Current Suppression Method Of Cascaded Transformerless PV System

As a renewable and green energy,photovoltaic(PV)power generation has taken an important role in the new energy power generation field.The grid-connected inverter combines PV panels with the grid and becomes the key device of the PV power generation which in turn determines the performance of the overall PV system.But due to its high cost,The total generating capacity of the photovoltaic is smaller than other common energy resources.so reducing the cost of the PV system and increasing efficiency as well turn to be the key technologies.Abandoning transformers in the system is one of the solution to reducing cost.Transformers can provide electrical isolation for safety consideration,however,they increase the volume and the weight of the whole system.As a result,the cost and the loss can be enlarger by using transformers,and also the efficiency of the system can be lower than that of the transformer-less system.So most of the scholars have focused on the non-isolated PV inverters.This paper focuses on the PV generation system based on the H-bridge cascaded multilevel inverter(HB-CMI),and studies it from the software and hardware parts.Firstly,based on the H-bridge cascaded inverter circuit,the generation mechanism and flow paths of leakage current in the cascaded non-isolated PV generation system have been analyzed.In order to explore the inhibitory effects of different modulation strategies on the leakage current,the simulation models based on four different strategies which are carrier phase modulation,carrier reverse phase modulation,carrier adjacent reverse phase modulation and carrier phase-shifted modulation have been built and studied in detail.Secondly,on this basis,adding electromagnetic interference(EMI)filters in the DC side of each module from the hardware perspective and improving the switching frequency appropriately have been selected to solve the problem mentioned above.Finally,the model of this system is built in Psim,comparative analysis of the leakage current waveforms and its RMS values prove the effectiveness of the methods.In consideration of the suppressing the common-mode voltage range in software ways,the common-mode voltage generated by the system fluctuates within a certain range.Allowing for the outstanding inhibitory effects of modulation strategies on the leakage current,a new improved modulation strategy based on the multi-carrier modulation for the H-bridge cascaded multilevel inverter circuit has been proposed in this paper.By comparing to the traditional multi-carrier modulation in the way of simulation models built in Psim verifies the inhibitory effects of modulation strategies mentioned above and reveals the advantages of the improved modulation strategy.