The effect of late-time high-altitude electromagnetic pulses (HEMP E3) can generate geomagnetic induced currents in the circuit between the earth and neutral points of transformers. In serious cases, it may make the transformer core half-cycle saturated, resulting in serious consequences, like distortion of excitation current, increase of reactive power loss, hot-spot heating and vibration. However, the existing literature usually adopted the results under steady-state direct current for E3 impact assessment, and the dynamic characteristics of transformer under E3 waveform were not fully considered. In order to analyze the influence of E3 characteristics, such as high magnitude and short duration, this paper builds an electromagnetic transient model of a 500 kV transformer. Based on IEC 61000-2-9 standard, this paper quantitatively analyzes the resulting variation of excitation current and reactive power loss of the Wye-Delta-connection transformer under HEMP E3, considering the amplitude, rising time, falling time of the induced geoelectric field, as well as transformer load type and other factors. The actual HEMP E3 induced electric fields are affected by the non-uniform earth conductivity and coast effect, and the waveform may change considerably. Thus, this paper calculates the E3 induced electric field under the uniform earth model and the 1D layered conductivity model. The results show that for the realistic earth conductivity models, the induced geoelectric fields change not only in amplitude but also in waveform. Therefore, this paper studies the influence of peak value, rise time and fall time of E3 induced electric field on reactive power loss of 500 kV transformer. To analyze the influence of the electric field amplitude, this paper selects 0.5 times, 1 time, 2 times, 5 times and 10 times of the IEC standard waveform, which are imposed on the 100 km transmission line respectively. The results show that as the amplitude of E3 electric field increases, the reactive power loss will be larger. It is worth noticing that there exists an upper limit of reactive power loss under HEMP E3, which depends on the air core inductance when the transformer is deeply saturated. For the rise time and fall time, this paper focuses on the first wave peak of HEMP E3. The results shows that when the fall time of induced electric field decreases by 80.8%, the fall time of reactive power loss decreases by 80.3% and the amplitude decreases by 20.3%; whereas when the rise time of induced electric field decreases by 83.1%, the rise time of reactive power loss decreases by 44.3%, and the amplitude increases by 7.3%. Therefore, the falling time of HEMP E3 has far more influence on the reactive power loss of the transformer than the rising time. In order to analyze the influence of load types on the transformer under HEMP E3, three types with the same impedance value, namely, resistance, inductance and capacitance are applied to the low-voltage side. The results show that when HEMP E3 electric field is applied, the excitation current and reactive power loss of the transformer are distorted to different levels with different load types. Because of the different phase angle of load impedance, the amplitude of reactive power loss is the largest with capacitive load and the smallest with inductive load. Besides reactive power loss characteristics studied above, HEMP E3 may lead transformer to harmonic distortion, hot-spot heating and vibration, which will be further investigated in future work. © 2024 Chinese Machine Press. All rights reserved.
