Research On Modular Power Converters Used In DC Grids

The development of renewable energy has put forward new requirements on the structure of the power grid,and DC power grid is focused on again because of low loss and high stability.DC power grid is the integration of new energy sources,HVDC and DC distribution;witch will be conducive to large-scale collection of renewable and mutual support of various regional power grids.National DC grid is effective to meet the trend of clean energy and large power grid.As a key energy conversion device,high-power converter plays an important role on the renewable energy and DC bus interconnection,thus it is of great significance to study high power converter for DC grid.In this context,the paper focuses on DC-DC converter and modular multilevel converter based on the structural characteristics of the DC network by taking technical requirements into account.Firstly,a non-isolated soft-switching bidirectional DC-DC converter based on rotating capacitor is proposed,which can reduce the converter size and energy conversion loss.The converter has high voltage ration by controlling the switching frequency to adjust the input/output voltage.Working principle of the converter in different modes is analyzed,and corresponding mathematical model is established.Besides,modularized structural characteristics and loss characteristics of the converter are studied.Simulation results show that the proposed converter has the advantages of small volume,low loss and bidirectional flow of energy,zero current turn-off of the switches and diodes are achieved and energy conversion efficiency is more than 92%,which make the converter is suitable for DC bus interconnection.Secondly,an improved double half-bridge sub-module with less device and low loss is proposed which can block fault current directly to solve the problem of short circuit fault on DC side.Structural characteristics of the proposed submodule and fault blocking mechanism are analyzed.Device characteristics and loss characteristics of the new submodule are also studied by compared with other submodules.Simulation results verify the fault blocking capability of MMC based on improved double half-bridge sub-module,which can rapidly remove DC fault.Finally,two more new submodules suitable for MMC are proposed based on the flying capacitor structure in order to further solve the problem of fault current absorption.Both of them can absorb fault current and forcibly turning off the diode by introducing the sub-module capacitor into current path through additional device.At the same time,a control strategy based on capacitor's energy equalization and voltage balance is presented to adjust original modulation signals,which can effectively solve the problem of capacitor voltage balance of the new submodules and improve their practical engineering.Simulation results show that the proposed control strategy can realize the dynamic tracking of the capacitor voltage.Both of the proposed submodules can effectively eliminate the fault current and restart system in few seconds,thus ensuring the reliability of the transmission system.