The occurrence of high-amplitude magnetizing inrush current during the energization of a transformer without load poses significant challenges to the stable operation of both the transformer and the power grid, potentially leading to malfunctions in relay protection devices. This paper analyzes the underlying mechanisms of transformer inrush current and presents a novel approach utilizing a soft-start-based method for effectively suppressing inrush currents. The proposed method employs an inrush current suppressor comprising anti-parallel thyristors and filters to mitigate the adverse effects caused by the inrush current. A comparative study is conducted to evaluate the filtering efficacy of three types of filters incorporated in the inrush current suppressor: an LCLC damping filter, a high-order single trap filter and a high-order double trap filter. Through careful analysis and optimization, the high-order double trap filter with parallel RC damping damper is selected as the optimal configuration. To ensure effective suppression, it is necessary to incorporate a filter at the termination point of the anti-parallel thyristor. Additionally, a closed-loop control strategy is implemented to ensure a smooth start and actively suppress the magnetizing inrush current. To validate the effectiveness of the proposed method, comprehensive simulations are performed using Matlab/Simulink. The results demonstrate the successful suppression of inrush current and the maintenance of stable operation for the transformer. This inrush suppression method does not require considering the influence of residual magnetism and the grounding mode of the transformer’s neutral point. It is also suitable for various transformer structures and wiring methods, which makes it highly applicable.

To accelerate the large-scale integration of renewable energy and support the strategic goals of “carbon peaking and carbon neutrality,” High Voltage Direct Current (HVDC) transmission technology has made significant breakthroughs. Among the various approaches, a hybrid DC transmission system that combines a line-commutated converter (LCC) and a voltage source converter (VSC) retains the inherent fault self-clearing capability of the LCC topology while mitigating the risk of commutation failure when connected to a weak grid. In this paper, based on the harmonic generation mechanisms of hybrid DC transmission systems, an improved 3-pulse harmonic source model of the LCC and a dynamic phase-sequence harmonic analysis model of the VSC are developed. The integrated harmonic model demonstrates strong adaptability in accurately calculating DC-side harmonics under the influence of power imbalances and background harmonics. Based on this model, the fundamental characteristics of DC-side harmonics in hybrid DC transmission systems are analyzed. To mitigate harmonic effects, this paper proposes an LCLC-trap2 high-order filter structure with parallel RC damping circuits and a co-optimized design of filter parameters. Finally, a ±500 kV hybrid DC transmission system is modeled using the MATLAB/Simulink platform, and the harmonic filtering performances of the conventional LC filter, the Butterworth filter, and the proposed filter are simulated and compared. The results verify that the proposed filter offers superior performance in suppressing low-order harmonics under non-ideal operating conditions.