Topology Design And Research Of Current Limiting DC Circuit Breaker Based On Precharging Capacitor

In recent years,with the emergence of new high-power electronic switches,DC transmission can meet the control requirements technically and compete with AC transmission economically.Especially under the condition of long-distance and high-power energy transmission,DC transmission has irreplaceable advantages.With the development of distributed energy and the explosive growth of power system capacity,the traditional AC transmission system has been difficult to meet the needs of social development.However,unlike AC,there is no natural zero-crossing point in DC,so AC arc extinguishing method is not applicable,and the impedance of DC transmission system is much smaller than that of AC system,and its short-circuit current increases rapidly(millisecond level),especially in DC system composed of voltage source converter.How to accurately judge and quickly break the fault current has become a difficult problem in DC breaking field.In this thesis,firstly,the current technical situation of DC circuit breaker and the research status at home and abroad are described in detail.The topological structure,working principle,advantages and disadvantages of mechanical,solid and hybrid DC circuit breakers are analyzed.It is pointed out that hybrid DC circuit breakers can meet the needs of current DC transmission system,and have a series of advantages such as low on-state loss,fast breaking speed,strong controllability,long equipment life and compact size,which is the mainstream direction of DC breaking equipment development.Secondly,the key technologies involved in the R&D and application of DC circuit breakers are expounded,including the summary of cascade voltage-sharing and current-sharing technology of power electronic switches,the description of the principles and working characteristics of quick mechanical switches and metal oxide arresters,the summary of DC arc extinguishing technology and the timing control strategy of opening and closing,which lay a theoretical foundation and technical support for the subsequent analysis and design.At present,the research on current-limiting DC circuit breaker at home and abroad is still in the theoretical research stage.Combining the advantages of the existing hybrid DC circuit breaker topology,this thesis proposes a new current-limiting DC circuit breaker topology,in which the current-limiting part adopts semi-controlled power thyristors and the circuit-breaking part adopts full-control devices(IGBT),making full use of the advantages of large thyristor capacity,low price and quick controllability of full-control devices,so as to reduce the breaking current and improve the breaking capacity of circuit breakers.Based on the current limiting part of the pre-charge capacitor,the rising rate of the fault current can be quickly suppressed after the fault occurs,thus reducing the contradiction between the sensitivity of fault judgment and the quickness of the circuit breaker;Self-discharge/absorption circuit is formed in the current-limiting part after the action of the circuit-breaking part,which reduces the contradiction between the current-limiting level and the fault breaking speed,and shortens the time needed by the circuit breaker to isolate the fault.The introduction of grounding part has no influence in normal operation.After the action of the circuit breaking part,the inductance of the fault side discharges through the grounding resistance,which reduces the inductive energy absorbed by the arrester and objectively shortens the fault breaking time.In addition,based on the original circuit,an improved current-limiting topology is proposed,which simplifies the circuit structure and control strategy without affecting the breaking performance of the circuit breaker,has the ability of fast secondary closing,and reduces the requirements for the voltage and current resistance level of the power electronic switches in the current-limiting part.In this thesis,the working principle and action characteristics of the proposed new and improved topologies are analyzed in detail,and the equivalent modeling and detailed derivation of their working states are carried out in stages,and the basis for parameter selection of each component is given.Furthermore,the grounding short circuit model of 320kV/1.6kA DC system established in PSCAD/EMTDC power system simulation software is used to carry out computer modeling and simulation analysis on the proposed topology,which is mutually verified with theoretical analysis.The simulation is mainly divided into three parts: closing process,load current opening process and fault current opening process.The analysis results show that the proposed current-limiting topology can effectively suppress the rising rate of fault current after the fault occurs,and the peak value of current decreases by 21.5% compared with the existing scheme,which enables the circuit breaker to complete the opening process at a lower current level;And can significantly reduce the energy absorption of lightning arrester,with a maximum reduction of 83.2%;The fault breaking time is as short as 6.1ms,which can meet the requirement of DC fault breaking.Finally,according to the simulation results,the power electronic switches of each part are selected and configured.Comparing the schemes before and after improvement,it can be seen that the improved structure has a further improvement in the number of thyristors used,the complexity of control strategy and the reliability of devices under the premise of hardly affecting the breaking performance of circuit breakers.