| The use of quadruple-circuit transmission line (QCTL) on the same tower can address the problem of lack of transmission corridors, improving power transmission capacity and decreasing construction costs. However, the mutual impedance between lines and the complicate cross-country faults challenge to fault analysis, backup protection scheme and setting calculation. The development of optical fiber communication technology offers a good technology support to main protection of EHV transmission line, especially current differential protection (CDP). Therefore, the study on CDP of QCTL has important significance to improve the quick operation of protection, reliability of power supply, safety and stability of power grid. This article is the research summary on CDP of QCTL during the author'graduate study, mainly include the following contents:(1) Phase-mode transformation of QCTL is analyzed in this paper, which is the basis for QCTL study. Considering two more practical QCTL models, two phase-mode transformation methods for QCTL are proposed. The necessary and sufficient conditions of transformation matrices are analyzed and calculated. And then, the fault sequence networks of QCTL are presented with consideration of connection frame, it is the basis of theoretical analysis and numerical calculation of QCTL.(2) Distribute capacitance current on transmission line will decrease sensitivity and reliability of current differential protection. This paper analyzes capacitance current of QCTL, and the results show that the influence of distribute capacitance current to current differential protection is big due to the increase amount of transmission lines and mutual distribute capacitance. So the capacitance current compensation is essential for current differential of QCTL. Based on phase-mode transformation, a scheme of capacitance current compensation for QCTL which can enhance the performance of CDP is proposed.(3) The mode current will reflect different fault type if the transformation matrices are different. It can distinguish single line-ground fault by integrating the information of each current mode. Based on this principle, the paper presents a new mode-current differential protection for QCTL. The capacitance current compensation of this protection scheme is then simple, and the protection has sufficient sensitivity and quick tripping speed.(4) With the phase-mode transformation of QCTL, the current computation method based on Bergeron transmission line model is presented. Based on the gradient of mathematical morphology, a novel Bergeron current differential protection scheme for QCTL is promoted. The capacitance have no impact on current differential protection based on Bergeron transmission line model because it compares the currents of the same terminal and the Bergeron model has taken the distribute capacitance into consideration, so it has a good sensitivity and reliability. Finally, for all the protection schemes proposed in this paper, QCTL test models with distribution parameter are established. Simulation results verify the feasibility and validity of all the protection schemes.
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