Discussion on tolerance of relative overshoot magnitude of lightning impulse voltage waveforms in impulse test of power transformers

In transformer impulse tests, it is challenging to suppress waveform overshoot due to the residual inductance of the test circuit and the large capacitance of the transformer under test.

IEC 60076-4:2002 specifies that the overshoot should be less than 5% of the test voltage, with up to 10% permitted if this cannot be achieved. The referenced IEC 60060-1 standard, however, has evolved: the 1989 edition defines the “overshoot ratio” (𝛽") relative to the test voltage, while the 2010 and 2025 editions introduce the “relative overshoot magnitude” (𝛽′), defined as the ratio of overshoot to the maximum value of the recorded curve, and they employ digital evaluation methods, including curve fitting and the test voltage function (k-factor).

To clarify the relationship between these definitions, this study analyzes 97 sets of actual lightning impulse test data provided by four Japanese transformer manufacturers. 𝛽′ is calculated using software compliant with IEC 61083-2:2013. Theoretical relationships between 𝛽" and 𝛽′ are derived, considering the differences between the peaks of the mean curve and base curve (∆𝑈 ), and the recorded curve and test voltage curve (∆𝑈 ). The influence of the test voltage function is shown to increase as the overshoot duration decreases.

Empirical results demonstrate that the relationship between 𝛽′ and 𝛽" in the collected data closely follows the theoretical relationship (assuming ∆𝑈 + ∆𝑈 = 0). The overshoot limit set in IEC 60060-1:2010 (𝛽 = 10%) is shown to be consistent with the previous standard (𝛽" = 5%) for power transformer withstand voltage tests. However, for waveforms with shorter overshoot duration, the influence of the test voltage function becomes more significant, indicating that further investigation may be necessary for power equipment whose impulse voltage test waveforms have short overshoot durations.

In conclusion, this study confirms the consistency of overshoot limit across different IEC standard editions for transformer impulse tests and highlights the need for additional research when applying these criteria to equipment with short overshoot durations. The findings contribute to a better understanding of overshoot evaluation and support the consistency of international standards in impulse high-voltage testing.