Research On Grid-Connected Techniques For Input-Series-Output-Series Inverter System

Due to the depletion of traditional energy sources,human beings have begun to pay more attention to the development and utilization of new energy sources,such as solar energy and wind energy.With the increasing power of new energy power generation equipment,a single grid inverter is facing a huge challenge,no doubt that this increases the difficulty of designing a single grid inverter as well as its production costs.Based on the concept of power electronics system integration,some scholars have put forward the idea of replacing a single module with a series-parallel combination system.According to the different connection methods of input and output terminals,the series-parallel combination system can be divided into four categories.Among them,the input-series-output-series（ISOS）inverter combination system is suitable for the occasions on which input voltage and output voltage are both high.If a number of small-capacity inverter modules in the form of ISOS inverter combined system can be used in high-power grid-connected power generation applications,to replace a single,high-capacity inverter,the voltage and current stress of the single inverter will greatly be reduced,which will reduce the production costs,shorten the development cycle and improve system reliability.This paper focuses on grid-connected techniques for input-series-output-series inverter system.For ISOS grid-connected inverter system,due to its special system architecture,multiple control targets need to be met,including:power balance among modules;high power factor of grid current;suppressing the resonant peak of the LCL filter.Then,capacitor current and grid current are selected as control variables based on the multiple control targets.Finally,a control strategy suitable for grid-connected ISOS inverter system is proposed.The control strategy consists of three control loops,including modular input voltage sharing loop（IVS loop）,modular inner current loop,and the common output current loop.In the common output current loop,the grid current i_Lf2 tracks the phase of the grid dynamically so that a high power factor can be obtained.The capacitive current feedback loop has two functions:on the one hand,it controls the phase synchronization of the capacitor current on the basis of the input voltage equalization to realize the power balance among the modules;on the other hand,the active damping of the capacitor current can effectively restrain the LCL resonance spikes caused by the filter to ensure the stability of the system.A two stage structure consisting of a DC-DC phase-shifted full-bridge converter and a full-bridge inverter is also selected as the main circuit topology of the system.The design process of the main parameters of the experimental prototype is given.At the same time,according to the control block diagram of the proposed control strategy,the decoupling relationship between the input voltage sharing loop and the common current loop is analyzed and the detailed process of the control loop design is given.Finally,on the basis of theoretical analysis,a 2kVA ISOS grid-connected inverter prototype system was set up in the laboratory.Simulation and experimental results prove the correctness and effectiveness of the proposed scheme.