Principle Analysis And Simulation Evaluation Of Current Differential Protection

With the continuous development of power system, the level of voltage gets higher and the system expands gradually. The EHV transmission lines increase day to day and they usually carry heaver power, so it is an important issue that how to maintain the EHV transition lines work safely and steady. In many kinds of protection relaying, the current differential protection based on Kirchhoff s current law can correctly distinguish the internal and external faults using relationship of the protected element current, and also it has been one of the best options for protecting EHV transmission line with the advantages of high sensitivity, simplicity, reliability, speediness and the ability of phase-selection.Two parts are included in this paper. One is principle analysis, the other one is simulation assessment. Many different current differential criterions exist at present. This paper analyzes the criteria based on the power-frequency sine vector, the criteria based on fault component vector, the criteria based on fault component instantaneous value and the criteria based on zero-sequence current differential protection etc. In addition,This paper discusses the two important factors which influence the current differential protection such as capacitance current and CT saturation. Under long distance EHV lines, increase of distributive equivalent capacitance makes both sides currents changed, which directly gives an impact on the sensitivity reliability of differential protection. Firstly, this paper analyzes the influence of distributive capacitance current and then presents compensation methods on general vector and zero-sequence current differential protection. The problem of CT saturation is very difficult all the time. This paper exhibits the character of primary and secondary current when CT is saturation. The method how to detect CT saturation and anti-saturation are also mentioned.In simulation assessment part, a Matlab based simulation system is designed and then the four protection modules are explored on Simulink in this paper. Using these packed modules, behaviors of four protection principles have been simulated under the situation such as normal operation, inside and outside faults and power swing etc. At the same time, characteristics of four criteria are assessed. Compensation methods on general vector and zero-sequence current differential protection together with maloperation caused by CT saturation are simulated in the basis of the prior analysis.To provide the basis for analysis and study on current differential, and also provide guidance towards development on a better principle, dynamic simulation system interface are designed based on Guide.