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.
Evaluation of overshoot rate of lightning impulse withstand voltage test waveform based on new base curve fitting methods
The k-factor, which evaluates an overshoot of the withstand voltage test waveform in a lightning impulse withstand voltage test, is about to be adopted in IEC 60060-1 that specifies high-voltage test techniques for electric power equipment. In its procedure, it is defined that the recorded waveforms are fitted with a double exponential function to derive a base curve. For some waveforms, however, an extracted base curve sometimes deviates upward from the central lines of the recorded waveforms in the wavefront area. In particular, the degree of such dissociation is significant in waveforms with a high overshoot rate and a low frequency, and further it can not be said that the base curve is appropriate in terms of insulating properties. Accordingly, such significant dissociation may result in an irrational calculation of the overshoot rate. To resolve these problems, a numerical equation resulting from a relationship of a solution between an equivalent electric circuit and dominant equation was used, and fitting methods were overviewed in order to extract a more reasonable base curve in previous studies. In this paper, various options from these fitting methods were narrowed down to several options, which were then applied to simulated recorded-waveform; for which the overshoot rate and frequency of superimposed oscillating served as parameters, and a fitting method for extracting a more reasonable base curve was examined. Consequently, the calculation results of the overshoot rate were brought closer to their actual values than with the existing method by using a new base curve extraction method by which the fitting can be performed with removing the oscillatory part from the recorded waveform. While there is significant dissociation in the existing method, especially in the waveform on which a relatively low frequency oscillatory wave is superimposed, evaluation results of the overshoot rate were significantly improved as a result of the use of the new base curve extraction method. In the meantime, it should be noted that the k-factor filtering scheme was implemented after the application of various fitting methods resulted in the shape parameters of the test voltage waveforms, such as crest value, wavefront and wavetail durations, emerging as virtually identical, alongside confirmation of the minor effect of the fitting methods on the test voltage waveform.
An electric model of gas-insulated shunt reactor and analysis of re-ignition surge voltages
The authors proposed a simple electric model to simulate the windings used in gas-insulated shunt reactors up to a high frequency range. Its applicability was evaluated by comparison with measurements of the full-scale apparatus model. The impedance of the high frequency electric model is well simulated over a frequency range of several kHz to 10 MHz. In this paper, the proposed high frequency electric model was used for analysis of the re-ignition surges at interruptions of the reactor of a model substation. The analysis waveform gave a faster rise and lower peak value than those of the low frequency electric model, and the cause was clarified.