| With the depletion of fossil fuels and the deterioration of environment,China and the world are developing and utilizing renewable energy on a large scale.The flexible DC grid based on modular multilevel converter(MMC)not only possesses the technical features of controlling the power flow fast and flexiblely,but also can make the best use of various renewable energy.Consequently,the flexible DC grid has become one of the most significant hotspots in the field of DC transmission.However,compared with the AC system,the DC grid is a "low-damping" system.When there is a fault in the dc line,the fault current develops faster and the wider areas are affected.The protection schemes adopted in the existing two-terminal HVDC transmission project can't meet the protection requirements of multi-terminal DC grid due to the slow speed of operation and the loss of selectivity when the transmission lines form a network.Hence,DC line protection is one of the key technologies to be solved in the development of the flexible DC grid.The DC line protection of the DC grid mainly includes two aspects:the rapid detection and the reliable isolation of DC fault.Focusing on the above problems to be solved in DC grid protection,this thesis studies the protection principle of DC grid on DC fault detection and isolation.The thesis is organized as follows:Firstly,the basic structure of the symmetrical bipolar MMC system is analyzed.It is pointed out that the DC reactors installed at both ends of the DC line are of great significance to the DC line protection.According to the analysis of the DC reactor voltage under pole-to-pole fault and pole-to ground fault,a boundary protection principle based on the DC reactor voltage is proposed.The protection principle utilizes the voltage magnitude and direction of the DC reactors to construct the fault line criterion.And the fault type and fault pole can be judged based on the difference of the DC reactor voltage between the positive and negative pole of the fault line.The scheme can provide fast DC fault detection and identification only by the voltage of the single-ended DC reactor.Therefore,the proposed scheme not only can meet the protection requirement of the DC grid,but also is simple and easy to realize.For the local fault detection of the proposed scheme,no telecommunication is required.The simulation results in PSCAD/EMTDC demonstrate that the proposed protection scheme can identify internal and external faults reliably,and it will not lead to maloperation under power reversal.Secondly,the characteristics of the fault current component under internal and external faults are analyzed.The included angle cosine is calculated by using the fault component of current at both terminals of the line.The pilot protection principle based on included angle cosine of fault current component is proposed and the protection scheme which is suitable as backup protection of the DC grid is designed.In the proposed protection principle,the included angle cosine of fault current component is used to construct fault identification criterion,and the voltage gradient is used to detect faults rapidly.In addition,the ratio of the voltages of positive pole and negative pole is adopted to identify the fault pole.The simulation results show that the proposed protection principle can identify internal and external faults reliably.In addition,the proposed method owns ability to respond to faults with high transition resistance and is independent of distributed capacitive current.Finally,the basic structure and working principle of DC circuit breaker are discussed.The fault isolation characteristics of DC fault isolation technology based on hybrid DC circuit breaker are mainly studies.The fault isolation characteristics of the primary protection based on the DC reactor voltage and the backup protection based on the included angle cosine of fault current component are simulated and verified respectively.The simulation results show the effectiveness of the hybrid DC circuit breaker in DC fault isolation. |
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