Study On DC Transformer For Medium Voltage DC Distribution Network

At present,most of the DC transformers proposed for medium voltage DC distribution networks use multi-transformer series-parallel systems,because the system can effectively reduce the electrical stress of the converter module,thereby reducing the development difficulty of the system,reducing the development cost,reducing development cycles,increasing system reliability and redundancy.On the one hand,the traditional DC transformer topology mostly uses the directly series input capacitors.This series method makes the fault-clearing more difficult when the DC bus meets short circuit.On the other hand,most of the current high-frequency converter topologies also have problems such as low efficiency and low voltage regulation flexibility.In order to improve the modularity of the ISOP DC transformer system to improve system reliability,efficiency and voltage regulation flexibility,two improved DC transformer topologies are proposed.The main parts of this paper are as follows:1.The comparative analysis is carried out on the ISOP type DC transformer based on the DAB converter and the series resonant converter.At present,there are two kinds of topological schemes for commonly used DC transformers.One is the ISOP type DC transformer based on the open-loop operation of the LC resonant converter under the resonant frequency,the other is the ISOP type DC transformer of the DAB converter under the single phase shift control strategy.In this paper,two basic components of the ISOP type DC transformer are compared in detail.Digital simulation and experimental models were established.2.Based on the research results of the second part,a combined DC transformer based on DAB converter and LC resonant converter is proposed.The DC transformer combines the advantages of DAB DC transformer and LC series resonant DC transformer to realize transmission power regulation while maintaining high efficiency over the full load range.And the effectiveness of the topology is proved by simulation and experiment results.3.An indirect input series output parallel full-bridge DC-DC converter based on asymmetric PWM control strategy is proposed.There is no centralized capacitor on the input side of the topology,so the input capacitors will not discharge when the DC bus is shorted,thus avoiding current overshoot.On the other hand,when one or several submodules fail,the faulty module can be completely removed from the system by simple switching.The effectiveness of the topology is verified by simulation and experiment results.