Research On Control And Protection Of Capacitive Energy Transfer Principle Based Buck-Boost DC/DC Converter

The rapid development of high voltage direct current(HVDC)technology has provided effective solutions to problems,such as long-distance large-capacity power transmission and large-scale renewable energy access and delivery.Because the voltage levels of various DC lines are inconsistent,and the DC power grid cannot be formed.Therefore,it’s urgent for a high-voltage large-capacity DC/DC converter to realize DC transformation and interconnect DC transmission lines of different voltage levels.On the other hand,reduced DC voltage in the HVDC systems is common practice used under the extreme weather conditions.This requires the DC/DC converter,which interconnects two HVDC systems with stable voltage under steady states,also be capable of step-down(buck)with a wide DC voltage operating range during scheduled voltage decrease on either DC side.Thus DC circuit breaker(DCCB)is often used to block DC faults.However,due to its extremely high cost and increased losses,high-voltage and large-capacity DC/DC converters with fault-blocking capabilities have become the demand hotspots for DC grid construction.Firstly,so as to meet the needs of connecting various DC voltage levels,the capacitive energy transfer(CET)principle based buck-boost DC/DC converter is proposed,which is composed of low cost and high robustness thyristors and high controllability half-bridge submodules.The operation principle and parameter design of the converter are analyzed in detail,and the applicable modulation method is also proposed.Secondly,this paper proposes appropriate control strategy.This control strategy adds alternation frequency control,which can not only relize the wide range of voltage levels,but also adaptively reduce the alternatin frequency as the power decreases.Besides,the coupling mechanism of the traditional energy storage control is revealed,thus the energy decoupling control strategy is proposed to balance the energy between the three phases of the converter,without causing the DC current ripple.Afterwards,because of low impedances and no natural current zero-crossing of DC grid,this paper studies the fault blocking method of CET Buck-Boost converter to avoid installing DCCB.Finally,on the basis of 150 MW,300k V/200 k V simulation verification,a 4.5k W,300V/450 V experimental prototype are built to verify the proposed theory effectively.The results are helpful for the application of the CET Buck-Boost converter,which is a new type of DC transformer in HVDC.