Cumulative-current Differential Protection for Renewable Energy Transmission Lines with Voltage Restraint

Short-circuit currents of renewable energy sources exhibit different characteristics from the conventional power grid, such as controlled phase and limited amplitude. These factors can significantly impact the sensitivity and reliability of the pilot current differential protection on AC transmission lines, which relies on the fixed restraint coefficient. Additionally, the decreased precision of Fourier algorithm calculations further worsens the adaptability of the protection. In this paper, a novel line protection based on sampled values is proposed, which is not affected by the control strategies of renewable energy sources. The analysis initially examines the disparities and correlations between the sampled current and voltage under internal and external faults. Building upon this analysis, the cumulative-current differential protection utilizing the differential sampled current for operation and the differential sampled voltage for restraint is proposed. To further enhance the sensitivity during internal faults and maintain reliability during external faults, a sampling plane is constructed to extract sampled value features, which are unaffected by the control strategies of renewable energy sources. Leveraging these findings, a self-adaptive restraint coefficient is incorporated as an additional parameter to supplement the existing restraint value. During internal faults, the operating and restraining values reflect the amplitude characteristics of the fault current and line capacitance current, respectively, making fault identification results reliable. Moreover, the self-adaptive restraint coefficient can also distinguish between internal and external faults, reducing the restraining value and enhancing the sensitivity of protection, especially in scenarios involving high impedance short circuits and weakly coupled systems. The proposed methodology utilizes sampled values, eliminating the need for Fourier calculations and effectively mitigating the influences of controlled phase and amplitude limitations of fault currents in renewable energy sources. The PSCAD/EMTDC platform is used to build models of power systems with renewable energy integration, and the effectiveness and operational speed of the proposed principle are demonstrated via a large number of simulations.