High entrance capacitances of ultrahigh-voltage (UHV) transformers make it difficult to generate impulses with standard front time during the lightning impulse tests. In this paper, we discuss the feasibility of examining the longitudinal insulation design of a transformer via impulses with a front time longer than the standard value. We consider the following features: transient voltage distributions and insulation characteristics of longitudinal insulation. The transient voltages of two UHV windings are measured with a capacitive voltage sensor under impulses with different front times. The results demonstrate that the front time of the applied impulse has little effect on the gradient voltage distributions and the oscillatory component of the transient voltage, but it determines the front time of transient voltages. Therefore, the relationship between breakdown characteristics of the turn-to-turn insulation and the front time is studied. Experimental results revealed that whatever the insulation thickness, an increase in the front time of the applied impulse decreases the breakdown voltage, but this variation is within acceptable tolerance when the front time changes from 1.2 μs to 6 μs. Thus, as long as the front time is restricted within 6 μs, we can ensure the validity of lightning impulse tests on the examination of longitudinal insulation designs.

中文翻译：

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延长超高压变压器雷电冲击试验前沿时间的有效性分析

超高压（UHV）变压器的高入口电容使得在雷电冲击试验期间难以产生具有标准前沿时间的冲击。在本文中，我们讨论了通过前沿时间长于标准值的脉冲来检查变压器纵向绝缘设计的可行性。我们考虑以下特征：瞬态电压分布和纵向绝缘的绝缘特性。两个特高压绕组的瞬态电压是在不同前沿时间的脉冲下用电容式电压传感器测量的。结果表明，施加脉冲的前沿时间对梯度电压分布和瞬态电压的振荡分量影响不大，但决定了瞬态电压的前沿时间。所以，研究了匝间绝缘击穿特性与前沿时间的关系。实验结果表明，无论绝缘厚度如何，施加脉冲前沿时间的增加都会降低击穿电压，但当前沿时间从 1.2 μs 变为 6 μs 时，这种变化在可接受的容差范围内。因此，只要将前沿时间限制在 6 μs 以内，就可以保证雷电冲击试验对纵向绝缘设计的检验的有效性。但当前沿时间从 1.2 μs 变为 6 μs 时，这种变化在可接受的容差范围内。因此，只要将前沿时间限制在 6 μs 以内，就可以保证雷电冲击试验对纵向绝缘设计的检验的有效性。但当前沿时间从 1.2 μs 变为 6 μs 时，这种变化在可接受的容差范围内。因此，只要将前沿时间限制在 6 μs 以内，就可以保证雷电冲击试验对纵向绝缘设计的检验的有效性。