Research on Capacitance Current Compensation Scheme of Current Differential Protection of Complex Four-Circuit Transmission Lines on the Same Tower
Current differential protection is the main protection of transmission lines which include multi-circuit lines on the same tower, and whose sensitivity and reliability of differential protection is mainly affected by the distributed capacitive current. For the four-circuit line on the same tower, due to the influence of coupling between the loop road, the distributed capacitance current increases significantly when compared with ordinary lines, affecting the sensitivity of the current
... e current differential protection, especially for different voltage levels throughout the four-circuit lines on the same tower. The relationship of the electrostatic coupling between the circuits is more complex, and increases the difficulty of the compensating the distributed capacitance current. This paper is based on the electrostatic coupling principle of four-circuit lines on the same tower, establishes the distributed parameter model of four-circuit transmission lines on the same tower, and discusses the effect of circuit operation mode on the compensation of capacitance current differential protection when different faults occur on the complex four-circuit transmission lines on the same tower. A new compensation scheme suitable for capacitive current compensation is proposed. Simulation results show that this capacitive current compensation scheme can effectively improve the performance of current differential protection. The main factor that affects the reliability and sensitivity of current differential protection is the presence of distributed capacitance current which is closely related with the length of lines, tower structure and operation mode, so how to reasonably compensate the capacitance current is an important issue to be solved in the practical applications of current differential protection . Many studies    have analyzed the influence of capacitive current on the line current differential protection, and proposed a corresponding compensation scheme. However, these are mainly aimed at whole transmission lines, but in the actual project the segment multi-circuit lines on the same tower exist, and the relationship of electrostatic coupling between the wires is more complicated. This occurs because the phase conductors of each line are close, so capacitive parameters vary when line operation mode is changed, therefore effective measures need to be taken to reduce any adverse effects of the capacitance current which due to changes in operating mode for the current differential protection in a complex line on the same tower. In this paper, the effects of different operation modes (OM) on the capacitance current compensation of current differential protection of four-circuit transmission lines on the same tower were analyzed, through theoretical analysis and simulation calculations. The capacitor compensation scheme was proposed for the segregated phase current differential protection and the zero-sequence current differential protection which are suitable for the complex four-circuit lines on the same tower under different operating conditions. Based on the PSCAD/EMTDC (Manitoba HVDC research centre, Winnipeg, MB, Canada) and MATLAB (MathWorks, Natick, MA, USA) simulation software, the model of four-circuit lines on the same tower was established, and the effectiveness of the proposed method was verified. The Distributed Capacitance Current Compensation Method of Current Differential Protection Influence of Capacitive Current on the Current Differential Protection The inter phase capacitive current and the capacitive current between parallel lines outflow from the lines, which form part of the tripping current of differential protection. Under normal operation or external fault conditions, the distributed capacitive current will reduce the security of the current differential protection, and even cause malfunction. In particular, for long parallel extremely high voltage/ultra-high voltage EHV/UHV transmission lines, the inductive reactance of bundled conductors decreases, and the distributed capacitance increases, this leads to the effect of distributed capacitive current on current differential protection becoming more apparent. Thus, effective measures need to be taken to reduce the adverse effects of the capacitive current. The principle of current differential protection is based on Kirchhoff's current law, which has good selectivity, and is the chosen main protection of transmission lines. Figure 1 shows the influence of capacitance current on current differential protection.