Abstract Low frequency transmission technology enhances the transmission distance and capacity by reducing the transmission frequency, which is a new type of transmission technology taking into account the advantages of traditional AC and DC transmission, and has a good application prospect in the scenarios of medium and distant sea wind power transmission, new energy pooling and urban flexible interconnection. Flexible low frequency transmission system is double-ended power electronics systems, when the low frequency transmission line occurs asymmetrical faults, M3C (modular multilevel matrix converter) will use the fault control strategy to achieve the corresponding control objectives, and the characteristics of the fault current will be affected by the control strategy, which in turn affects the performance of the traditional line protection. For this reason, this paper proposes a current differential protection performance improvement scheme for flexible low frequency transmission system based on control and protection synergy, which improves the action performance of current differential protection by actively regulating the magnitude of negative sequence current in the line through the M3C. First, according to the power relationship during the asymmetrical fault on the low frequency side, the bridge arm active power imbalance component is theoretically deduced, the reasons for the M3C capacitor voltage imbalance are analysed, and for the problem that capacitor voltage balance through negative sequence current control limits the degree of freedom of the M3C negative sequence currents, a capacitor voltage balance strategy based on the M3C circulating current control is adopted, which realises the M3C capacitor voltage balance control with the mutual decoupling of negative sequence current control. Secondly, based on the action characteristics of the current differential protection, the influence of negative sequence current on the action performance of the current differential protection is analysed, and it is pointed out that the action performance of the current differential protection can be changed by regulating the negative sequence current in the line through the M3C to change the relationship between the action amount and the restraint amount. Finally, with the goal of improving the operational performance of the differential current protection, and under the constraint of ensuring that the M3C bridge arm doesn't overcurrent, the reference value of negative sequence current is theoretically calculated and selected. In the simulation analysis, this paper establishes a flexible low frequency transmission system simulation model, through a large number of simulation experiments, in order to verify the feasibility and effectiveness of the proposed scheme. The simulation results in this paper show that: (1) The capacitor voltage balance strategy based on M3C circulating current control adopted in this paper is able to achieve the capacitor voltage balance of M3C during asymmetrical faults. (2) The negative sequence current reference value selected in this paper can ensure that the line fault current does not exceed the safe range and avoid the overcurrent problem of M3C bridge arm. (3) By comparing the action performance of current differential protection under different negative sequence currents, it is verified that the negative sequence current reference value selected in this paper effectively improves the action performance of current differential protection, and makes the current differential protection act correctly under various fault conditions.
Zheng Tao,Kang Heng. Improvement of Current Differential Protection Performance of Flexible Low-Frequency Transmission System Based on Control and Protection Cooperation[J]. Transactions of China Electrotechnical Society, 2025, 40(7): 2162-2177.
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2025, Vol. 40 
