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A common method to examine the current-limiting performance of saturated iron-core fault current limiter (SI-FCL) in high-voltage direct-current transmission based on voltage source converter (VSC-HVDC) systems is to solve differential equations based on the system fault transient characteristics and the equivalent inductance calculation equation. This method analyzes the fault current of the VSC-HVDC system in the time domain. However, it is computationally complex and cannot directly reflect the relationship between parameters and the currentlimiting effect of the SI-FCL. In this paper, the relationship between the magnetic flux density and magnetic field energy of the SI-FCL is analyzed. The energy exchange between the DC capacitor and the SI-FCL in the DC short circuit fault process is analyzed. From the perspective of electromagnetic energy conversion, the criterion for determining the current-limiting ability of the SI-FCL in the transient process is given based on the parameters of the SI-FCL and VSC-HVDC system. On this basis, the characteristics of the DC side fault current and the capacitor voltage when the SI-FCL has current-limiting ability are examined. Based on the parameters of the SI-FCL and VSC-HVDC system, a method for calculating the fault current peak value and capacitor voltage drop time is given. Finally, the accuracy of the analysis of the SI-FCL in the VSC-HVDC system based on the electromagnetic energy conversion mechanism is demonstrated through a case study and simulation results of the VSC-HVDC system with different SI-FCLs.

A bipolar short-circuit transient fault identification method for overhead transmission lines in modular multi-level converter-based high voltage direct current (MMC-HVDC) grid based on the hybrid HVDC breaker is proposed. The circuit breakers on both sides of the positive and negative overhead transmission lines will trip after a bipolar short-circuit fault in the MMC-HVDC grid. In order to identify whether the fault is a transient fault, the isolating switch of the DC breaker on one line side is reclosed first, and the voltage change on this line side in the case of transient and permanent faults is analysed. By comparing the electrical characteristics of the voltage on the line side under transient and permanent faults, a transient fault identification method is proposed, and the calculation method of the setting value of the criterion is obtained. Finally, by building the MMC-HVDC system model for simulation in power systems computer aided design, the accuracy and effectiveness of the proposed method for transient fault identification are verified.This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

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In order to study the influence of the clustering aggregation of doubly-fed wind turbines (DFIG) in the wind-fire bundling system on the system subsynchronous oscillation (SSO), the expression of the DFIG output active power on the electrical damping De of thermal power units is first derived. Then the wind-thermal bundling system is built based on the IEEE first bench mark, and the wind farm is aggregated to several groups according to the rated power. The complex torque coefficient method is used to analyze the two cases of the same running state of the wind turbine and the difference of wind speed between the turbine and verified by time-domain simulation. The results show that the difference in wind speed within the DFIG groups has little effect on the system oscillation frequency before and after the aggregation of the wind farm, but there may be a slight error in the electrical damping coefficient before and after the aggregation,but the error does not affect the trend and analysis of damping characteristics of system. When studying the influencing factors of wind-fire bundling system SSOthe aggregated model can be uesd to improve simulation efficiency.