Research On DC Autotransformer Based On DC/DC Series-Parallel Technology

In recent years,DC grid has become a research hotspot at home and abroad by virtue of its advantages in new energy integration and absorption capability.The DC transformer used for interconnecting DC lines is one of the core equipments for constructing DC grid.When it comes to power transmission,the entire power will pass through the traditional isolated DC transformer,which will double its design capacity and increase its operation loss.In order to meet the urgent demand of efficient and low-cost DC transformers for DC grid,this paper proposes a new topology of DC autotransformer(DC AT)based on DC/DC series-parallel technology,which is able to connect DC lines of different voltage levels and realize power flow control.The main contents of this paper are as follows:Inspired by AC autotransformer(AC AT)technology,a new DC AT topology is proposed based on the advantages of DC/DC series-parallel combination system,and its topology structure and working principle are both analyzed.The DC AT topology is characterized by direct electrical connections between interconnected systems.Its obvious advantage is that only part of the power is transmitted by the transformer,which can reduce the design capacity and operation loss of the transformer.The sub-modules(SM)can be flexibly connected in series and parallel according to the voltage levels of the DC systems on both sides.In order to ensure the stable operation of DC AT,the topology structure and voltage sharing control strategy of DC/DC multi-module series-parallel system are studied.This paper focuses on the input-series output-series(ISOS)combined system and proposes an adaptive voltage sharing control strategy based on dual-droop characteristics of input and output voltages,which can realize the voltage sharing and modularization of the input/output side of ISOS system.For the series-parallel compound system,the corresponding voltage sharing control strategy is designed,and the feasibility of the control strategy is verified by theoretical analysis,simulation and experiment.For the sake of smooth starting of DC AT system,the precharge control strategy is studied.Charging by current-limiting resistance is suitable for the situation where the DC AT is used to connect active power grids on both sides.When the low-voltage side is passive,the self-starting of low-voltage side can be realized by using controllable charging mode of increasing the duty ratio step by step.When the high-voltage side is passive,the constant-current charging strategy can make the voltage at the high-voltage side rise smoothly,and it has the advantage of low current impact as well.Finally,the overall control of DC AT is achieved by means of hierarchical control.Specifically,the lower layer control is accomplished by implementing phase-shifting modulation and sub-module voltage sharing strategy,and the interleaved parallel control can be realized by adding grouping carrier phase shift.Besides,the middle layer adopts the voltage and current double closed-loop control,and the upper layer can employ constant power or voltage control according to different operating modes.A DC AT model is constructed in Matlab/Simulink,and an experimental platform consisting of four isolated DC/DC converters is built in the laboratory.The proposed control strategies are simulated in MATLAB and verified experimentally.The results show that the DC AT topology can work stably and partly reduce the converter capacity.