Abstract：As a key component, the high frequency transformer (HFT) plays an increasingly important role in voltage isolation and power transfer in solid state transformer (SST). Unlike the power frequency transformer, HFT faces the complex stresses of non-sinusoidal high frequency and high temperature. The fast and steep pulse voltage occurs on the rising and falling edges of a non-sinusoidal square wave voltage in HFTs. Its voltage change rate in the rise period (dv/dt) is higher than 10 kV/μs, which poses great challenges to the insulation of HFT. A High dv/dt voltage contains a large number of high frequency harmonics, resulting in a significant increase in dielectric loss. The insulation suffers from cumulative impact stress at high frequency and high dv/dt voltages, leading to local electric field distortion and partial discharge in the insulation. However, insulation degradation and breakdown is difficult to understand owing to the rapidly repetitive pulsed voltages. This paper focuses on the influence of dv/dt on HFT dielectric losses and impact energy accumulation. Firstly, the step response function was used to simulate a high dv/dt square-wave voltage. It combines with the decomposition and calculation of the dielectric loss. This paper used the complex dielectric parameter, including the e¢ and e¢¢ of epoxy resin. It depends on frequency and temperature, which can be used in the dielectric loss calculation. The actual square wave voltage can be simulated and treated by frequency domain response. Then a calculation method for dielectric losses with superposition of dv/dt and square-wave voltage was proposed. This equivalent technique simplified the complex multi-frequency dielectric loss calculation. The results indicated that the dielectric loss increases with increasing dv/dt of the square wave voltage, apparently enhancing the insulation heating. The dielectric loss reaches 38.2 kW/m3 at 10 kV/μs, which is two times higher than at 0.5 kV/μs. Secondly, a finite element simulation (FEM) of 10 kW, 10 kHz, 1 000 V/750 V HFT was performed, considering the dielectric parameters of epoxy resin and calculating the dielectric loss. The insulation electric field distortion occurs at the end of the winding, and its variation within the dv/dt period is small. However, the occurrence time of Emax is short. It shows that the electric field distroation in the insulation occurs several times at high dv/dt period. For example, the insulation electric field can reach the maximum value at 0.1s (dv/dt=10 kV/ms). A high dv/dt can causea local electric field distortion in a short time, resulting in insulation deterioration. In this case, the simulated maximum temperature on the winding can reach up to 56.4 ℃. Finally, according to the energy storage of insulation under square-wave field, the impact power density of the insulation pd at high dv/dt is studied. The FEM results showed that the electric field, displacement current and energy impact power density depend on dv/dt. The impact energy density induced by the square-wave voltage (pd at 1 kV, 10 kHz) increases with dv/dt. The impact energy accumulation increases due to the high dv/dt caused by the voltage polarity reversal. The pd can reflect the insulation performance under high frequency non-sinusoidal voltages with high dv/dt. The experimental results of the 10 kW, 10 kHz, 1 000 V/750 V HFT were performed by a dual active bridge (DAB) platform. The temperature measured after 30 minutes operation under load agrees with the results of the FEM simulation. It can verify the dielectric loss calculation and the pd analysis. In combination with the dielectric loss, it indicates that the insulation is subjected to severe electro-thermal stresses, leading to insulation degradation at high dv/dt voltages (during voltage polarity reversal). The research results provide guidelines for insulation failure and the design of high capacity HFTs.
王威望, 李睿喆, 何杰峰, 张晓彤, 李盛涛. 快速陡脉冲重复电场下高频变压器绝缘介质损耗与冲击能量积聚特性[J]. 电工技术学报, 2023, 38(5): 1206-1216.
Wang Weiwang, Li Ruizhe, He Jiefeng, Zhang Xiaotong, Li Shengtao. Dielectric Loss and Impact Energy Accumulation of High Frequency Transformer Insulation under Rapidly Repetitive Pulsed Voltages. Transactions of China Electrotechnical Society, 2023, 38(5): 1206-1216.
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