Abstract
The analysis and calculation of the short-circuit fault current in the DC grid is of great significance to the design and configuration of the converter station and DC circuit breaker parameters. The existing flexible DC system not only includes the modular multilevel converter (MMC) converter, but also needs power equipment such as the fault current limiter and DC circuit breaker. Therefore, the system modeling and short circuit calculation of the multi-terminal DC system after adding the DC circuit breaker are also of great significance for the design of DC power system parameters and the grid troubleshooting ability. In this paper, firstly, the parameters of the four-terminal DC system of the modular multilevel converter (MMC) are simplified, and the analytical solution of the short circuit fault current of the multi-terminal DC system is given. Then, the external characteristics of the cascaded hybrid DC circuit breaker are modeled. Based on the equivalent circuit of the fault current in different stages, the short circuit calculation method of four-terminal MMC system with DC circuit breaker is obtained. This method can effectively describe the overall trend of fault current and provide the basis for the configuration of DC line protection settings and DC circuit breaker related parameters.
Highlights
In recent years, as one of the effective technical means to solve the new energy grid-connection and accommodation, flexible DC transmission technology has become a hot topic of academic research [1,2,3].Multi-terminal flexible DC transmission can meet the demand of multi power supply and receiving, and effectively reduce the number of converters
The half bridge submodule modular multilevel converter (MMC) topology is often used in the existing flexible DC grid projects
When the DC side of the converter with MMC topology has a short circuit fault, the submodule capacitor will discharge through the fault point, which will cause a great impact on the MMC converter and other DC system equipment, seriously endangering the safe and stable operation of the DC system
Summary
As one of the effective technical means to solve the new energy grid-connection and accommodation, flexible DC transmission technology has become a hot topic of academic research [1,2,3]. The transient stability analysis of the DC system with DCCB under the short circuit fault is mentioned in the simulation, the detailed mathematical model and combined calculation process of the DC circuit breaker are not given. The equivalent models of different transfer branches of hybrid DC circuit breakers are quite different in DC lines, so it is necessary to combine the external characteristics of DCCB into the short-circuit fault analysis of multi-terminal DC systems. In order to solve the above problems, this paper proposes a short circuit calculation and analysis method of four-terminal flexible DC system including cascaded full bridge hybrid DCCB, which provides theoretical basis for the design of DC system protection control strategy and the parameter setting of DCCB, DC FCL, and other related protection equipment.
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