Study On Control Strategy And Common Mode Current For Photovoltaic Grid-connected Power System

In recent years the international community attaches great importance to tides, wind,solar, geothermal and other new energy. However subject to certain grid-connectioncommunication interface, the power of the distributed power can not be directly suppliedfor the AC load, so voltage source type inverter usually used as parallel interface. Controlperformance of grid-connection inverter directly affects the output power quality ofphotovoltaic power generation system. Therefore, the grid-connection control researchesof inverter have significance and important application prospect.common mode currentproblem is very important to grid-connected photovoltaic (PV) system and in Germanstandard DIN VDE 0126-1-1, the stipulations related to the common mode current arespecified. When common mode current exceeds specified value, he connected PVgeneration system should be cut off within the stipulated time. This paper focuses oncontrol strategy of grid-connected inverter and method to suppress common mode current.Firstly, in this paper, parallel interface inverter output current need operationcharacteristics of fast dynamic response, adopt the hysteresis current control methods tocontrol the inverter grid, For drawbacks of the high frequency of hysteresis current controlswitch, a new type of hysteresis control method based on low switching losses, thethree-phase grid-connected inverter control grid, and analysis its working principle indetail. Compared with traditional hysteresis control methods, while ensuring the quality ofthe grid current, and reduce the average switching frequency significantly, therebyreducing switching losses.Secondly, considering the importance of common mode current to the connection ofphotovoltaic generation to power grid, a common mode current is built. It is knownthrough the analysis that common mode current is related to modulation technology andthe position where the inductance locates. In allusion to above-mentioned features, a newsingle-phase grid-connected PV system that can suppress common mode current isdesigned. The designed system is a full-bridge single-phase grid- connected PV systemwith AC bypass circuit and in which inductances are symmetically arranged at both phase line and neutral line. Comparing with traditional full-bridge inverter systems adoptingunipolar or bi-polar modulation technology, the structure of the proposed single-phasegrid-connected PV system not only can eliminate common mode current produced in thegrid-connected PV system, but also improve system efficiency and quality of current sentinto power grid, thus the overall performance of the grid-connected PV system isimproved.At last, this article briefly analyzes common mode current for three-phasegrid-connected PV system, common mode current are simulated and analyzed by using theany of several common control ways. Through comparing the simulation results, theproposed method effectively reduce common mode current.