Forced Current Commutation DC Hybrid Circuit Breaker Based On Full Controlled Power Electronic Devices

With the development of high voltage direct current transmission technology(HVDC)and the development of power electronic devices,the DC lines becomes more interested in the future transmission system.Moreover the multi-terminal DC transmission technology(MTDC)has received increasing attention.The relatively low impedance in HVDC grids is make a great challenge,when a short circuit fault occurs,the current in the transmission line rises to a level that the power grid can not withstand in a short period of time.So,the HVDC circuit breaker can isolate the fault and protect the power grid,which is more and more fast and efficiently.Hybrid DC circuit breakers,combining mechanical circuit breaker and solid circuit breaker and exhibiting low conduction losses of mechanical circuit breaker and the short breaking time of solid circuit breaker,has been believed that it become the main trend of DC circuit breaker research.At the same time,according to the different current commutation principles between mechanical circuit breaker and solid state circuit breaker,hybrid DC circuit breaker can be divided into three categories.The advantages and disadvantages of each category and the existing technical problems are also presented in this paper.The traditional forced current commutation DC hybrid circuit breaker based on full controlled power electronic devices,aiming to bi-directional power flow,need two solid-state circuit breaker modules composed by full controlled power electronic devices be connected in series in different directions,which ca?ses the increasing cost of full controlled power electronic devices.When the the snubber capacitor discharges,the current from the snubber capacitor will flow back to the mechanical circuit breaker,which will have influence on mechanical circuit breaker' openning time.In the traditional Hybrid Circuit Breaker,the power dissipation only depends on arrester,which not only increases the pressure of arrester and the cost of arrester,but also ca?ses a slow process of power dissipating,decreasing the service life of the devices and threat of reliability of the devices,resulting in a long period of time under over current.In view of the above technical issues,this thesis proposes two kinds of improvement schemes.In the first scheme,bidirectional power flow share one solid-state circuit breaker modules through the diode bridge circuit,auxiliary current transmission circuit can prevent the current from snubber capacitor discharging by series-connected diode.In the second scheme,the two mechanical circuit breaker strategy can make the current bidirectional flow by ?sing one solid-state circuit breaker modules.The snubber circuit of auxiliary current commutation circuit has been redesigned,whitch can prevent the capacitor discharging road.Both of the two schemes introduce the grounding diode modules that can make the stored energy in the transmission line inductance(including current limiting inductance)split up into two parts,one part is dissipated by the arrester,and the other part is dissipated by the grounding diode.In this paper,the topology characteristics,its working principles and application fields are introduced in details.The Equivalent mathematical models of the two schemes are carried out on fault state.The two schemes can effectively solve the above technical issues which are verified by the equivalent mathematical analysis.At the same time,contrast simulation between typical hybrid circuit breaker and improvement schemes are built in PSCAD/EMTDC,and the theoretical analysis is verified by the simulation.Finally,a low-voltage low-current laboratory prototype of the new schemes has been built in the laboratory,and the feasibility of the schemes are verified by the experiment.