Evaluation of overshoot rate of lightning impulse withstand voltage test waveform based on new base curve fitting methods - study on overshoot waveform in an actual test circuit
In a lightning impulse withstand voltage test for large-sized electric power equipment, voltage waveforms sometimes contain overshoot due to the capacitance of the test equipment and the residual inductance of the test circuit including the power source. The authors have been studying fitting methods for extracting a reasonable base curve from this overshoot waveform. In the previous study, each of the fitting methods was applied to the overshoot waveform simulated using a mathematical equation to evaluate the respective merits and demerits. As the test waveform, the present paper used the overshoot waveform actually generated using a large-scale test circuit. This actual waveform was determined as one over which an oscillation waveform of several hundred kHz, which was actually at issue, was superimposed. The correct base curve of the actual waveform was derived through detailed analysis of the equivalent circuit of the test circuit and the overshoot rate was calculated based on this correct value. This correct overshoot rate was compared with that calculated using each of the fitting methods to evaluate which was an optimum fitting method. Consequently, it was found that an overshoot rate close to the correct value could be derived using the new base curve extraction method for the actual waveform. Meanwhile, it should be noted that the kfactor filtering scheme was implemented after the application of various fitting methods, and then the shape parameters of the test voltage waveforms, such as the crest value, wavefront duration, and wavetail duration, were almost identical, which confirmed that fitting methods had only a minor influence on the test voltage waveform.
A physical approach of the test voltage function for evaluation of the impulse parameters in lightning impulse voltages with superimposed oscillations and overshoots
The introduction of the test voltage function or k-factor in IEC 60060-1 Ed. 3 has been an important step to ensure reproducibility and traceability in high voltage tests techniques for lightning impulses with superposed oscillations and overshoots. The standard recommends computed fitting methods to extract a base curve from the oscillating recorded voltage curve and, applying the test voltage function, it is possible to calculate the impulse parameters. Testing external insulations with positive polarity lightning impulses at 50% breakdown voltage or lower level, the disruptive discharge usually arrives after ten microseconds. For these cases the test voltage function and procedures for calculating impulse parameters proposed by the standard may lead to divergent results. In this paper a new approach is proposed for fitting the base curve for metric air gaps based on the relevance of the leader propagation phase, highlighting the influence of the instantaneous voltage between the impulse crest and the time-to-breakdown. A comparison of three calculation methods for test curves with long time-to- breakdown is discussed and a new test voltage function obtained by measurements on a 1 m rod-plane air gap is proposed. Compared to the standard test voltage factor, this approach evidences a negligible influence of oscillations and the relevancy of the impulse tail voltage for testing external insulations with moderately distorted lightning impulses. Finally, a generalization of the voltage test function is proposed for several types of insulation considering mainly the average breakdown times.
Evaluation of breakdown characteristics of oil-immersed transformers under non-standard lightning impulse - insulation characteristics for non-standard lightning impulse waveforms with oscillations
To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of oil-immersed transformers and thus reduce equipment cost while maintaining high insulation reliability, it is important to identify the insulation characteristics under non-standard lightning impulse waveforms that are associated with actual surge waveforms in the field besides the standard lightning impulse waveform and quantitatively compare them with the characteristics under the standard lightning impulse waveform. In a preceding paper, field overvoltages in the lightning surge time region were analyzed, and four typical non-standard lightning impulse waveforms were identified. Two of these including pulses were used to measure the breakdown voltages and the partial discharge inception voltages while changing the parameters, on three models of the winding insulation elements of oil-immersed transformers. This paper investigates the insulating properties of the other two types: "damped-oscillation waveform" and "rising-oscillation waveform", and evaluates the resultant average breakdown voltages in terms of the overvoltage duration, permitting their formulation in a unified way. In the examined range, the dielectric breakdown values under non-standard lightning impulse waveforms were up to 50% higher than those under standard lightning impulse waveforms in all cases. Further investigations will be expected for the reduction in insulation specifications.