Research On Dc Switchyard In High Voltage DC Grid

Flexible direct current(DC)transmission technology based on voltage source converters has many advantages,such as low output harmonic content,convenient power adjustment,and power transmission to passive networks.It is widely used in DC grid and multi-terminal DC transmission.Due to particularity of structure and operation mechanism of the HVDC transmission system,the fault current rises rapidly when fault happens in DC side,and the fault propagation speed is fast,which will have a serious impact on safe operation of the DC grid.Therefore,the fast and effective DC side fault removal and isolation technology has been receiving widespread attention.Using DC circuit breakers to remove faults in DC side is a trend in the future.Hybrid DC Breaker based on current transfer principle is the most concerned type of DC Breaker,for it has the advantages of good quickness,low loss and economic operation.However,when it is applied to large-scale DC power grid,there are a large number of expensive main breakers with high cost and poor economy.By adopting a common "main breaker",DC switchyad can effectively reduce the number of main breakers in topology.Different DC switchyard topologies that have been proposed have their own advantages and disadvantages.Classification analysis and summary of different DC switchyard topologies are helpful for subsequent research on DC switchyard,and provide guidance for the actual type selection in future engineering applications.Based on different kinds of current transfer mode,the proposed DC switchyards can be divided into three types: selective transfer DC switchyard,single transfer DC switchyard and all transfer DC switchyard,and following research work is carried out:1.Topological structure and working principle of three types of DC switchyards are analyzed,and the characteristics of various types of DC switchyards are analyzed and summarized.Considering the basic requirements of DC switchyards under high voltage DC network in the future,performance of various types of DC switchyards in practical application is compared and analyzed.2.The proposed single transfer DC switchyard topology has a main-break grounding process when any line fault occurs,and it does not have failure protection function for a fast mechanical switch.Based on the single transfer DC switchyard topology,this paper proposes a unidirectional current all transfer DC switchyard topology.Working principle of action process during the normal operation,such as opening and closing,closing,reclosing and fast mechanical switch failure during line fault is analyzed.Also,improvement of unidirectional current all transfer DC switchyard topology and adaptability of multi-line cascading faults are analyzed.Finally,the performance of the proposed topology is simulated and verified by PSCAD/EMTDC.3.Internal component faults of all transfer DC switchyard are divided into two categories: breakdown of internal power electronics components and DC bus grounding,and influence of different internal component faults of the full-transfer DC switchyard is analyzed.Based on this analysis,improved topology scheme is proposed.The working principle of the improved topology during different DC bus ground faults is analyzed and verified by PSCAD/EMTDC simulation software.