Research On High Efficiency Isolated DC-DC Converter In Energy Storage System

The introduction of energy storage technology in energy distribution and dispatch systems such as distributed generation,uninterruptible power supplies and electric vehicles can effectively improve energy utilization efficiency and power quality.Generally,the front-end output of a typical distributed energy storage system is low-voltage DC(photovoltaic cells or fuel cells)or AC(wind power).If the output is AC,the AC voltage will usually be converted to DC voltage before other conversion.Due to the intermittent nature of distributed energy,the voltage at the output terminal exhibits a fluctuating characteristic.In order to meet the load power supply requirements,the electric energy should be converted and matched accordingly,and the energy must be stored and released through the energy storage devices to maintain the sustained stability of load power supply and the bus voltage.In view of the characteristics and req uirements of distributed energy storage systems,in order to improve the effic iency of power utilization,this paper focuses on key technologies such as high gain DC-DC converter,high gain bidirectional DC-DC converter and active equalization topology of battery pack.In the traditional schemes,the two-stage independent circuit structures are usually used to achieve the high voltage gain under the condition of large voltage difference,and the conventional single-stage isolated converters have a relatively large turn ratio,which leads to a decrease in the coupling efficiency of the transformer and a large parasitic parameter.To address these issues,an isolated high step-up DC-DC converter with integrated cascade structure is proposed in this paper.Compared with the existing two-stage cascade converters,the proposed topology in this paper has higher integration characteristics and dynamic performance in terms of main power and control circuits,and achieves high voltage gain.At the same time,compared with the traditional single-stage isolated converters,the transformer has a lower turns ratio,which can reduce the voltage spike caused by the leakage inductance and reduce the circulating current caused by the distributed capacitance,thereby improving the efficiency of the converter.Aiming at the problem that the dual-active-bridge converter under the existing phase shift control scheme mainly relies on increasing the transformer turns ratio to increase the voltage gain under the condit ion of large voltage difference,a method for improving the voltage gain for the dual-active-bridge converter is proposed.The control method adopts different control methods in the buck and the boost mode s,respectively,so that the converter achieves higher voltage gain under the same transformer turns ratio,which improves the bidirectional conversion efficiency of the converter under the condition of large voltage difference.Aiming at the limitation of the traditional dual-active-bridge topology,in order to further improve the bidirectional conversion efficiency of energy under the condit ion of greater voltage difference,a high-gain isolated bidirectional DC-DC topology with integrated cascade structure is proposed.This topology can be equivalent to a cascade structure consisting of a conventional isolated converter and a non-isolated converter in both buck and boost mode s,which achieves bidirectional high voltage gain under the condition of low turns ratio and expands the ZVS range.At the same time,the comparative analysis and research are carried out from the single-level topologies and the commonly used two-level topology in the existing literatures.Theoretical analys is and experimental results show that,compared with the exist ing bidirectional converters,the proposed topology can achieve higher voltage gain under the same circuit parameters,and is characterized by the simpler control system and higher reliability.Aiming at the problem of large losses on the balanced topology in the centralized equalization technology of the battery pack,a quasi-resonant ZVS isolated bidirectional DC-DC converter topology is proposed.In both boost and buck modes,the converter can achieve ZVS operations of all switches through quasi-resonant technology,which greatly reduces the switching losses.At the same time,the energy in the leakage inductance realizes the cyclic absorption and utilization,which reduces the voltage stress of the component s.Compared with existing centralized balanced topologies,this topology has fewer components and wider ZVS range,and avoids additional snubber circuits or windings.The battery pack is subjected to equalization test,and the experimental results verify the effectiveness of the proposed converter and show that the equalizer has good performance in terms of equalization speed and efficiency.