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.
Insulation breakdown characteristics of UHV-class oil-immersed transformer for lightning impulse withstand voltage test waveform - K-factor value and front time related characteristics
The lightning impulse withstand voltage test waveform for electrical power equipment is specified in IEC 60060-1 "High-voltage test techniques". Now that ultrahigh voltage-class (UHV-class) electrical power equipment is being introduced and increasingly used, the electrical capacitance of equipment to be tested has increased, which has eventually hampered the test using the waveform that conforms to the previous standard. To address this issue, the IEC revised the IEC 60060-1 standard in 2010 and now is studying the standard for UHV-class electrical equipment. In this revision, an evaluation method using the k-factor function (test voltage function) was introduced, whereby the overshoot waveform was converted to a test voltage waveform. To date, concerning these k-factor values, measurement results based on experiments have been reported, but most were for small-scale insulation models with breakdown voltage levels mainly around 100 kV. This paper reports the experimental result of insulation breakdown characteristics for lightning impulse withstand voltage test waveforms in the model with the largest oil gap possible assuming an actual UHV-class oil-immersed transformer. Breakdown voltages and breakdown times were measured with the superimposed oscillation frequency, overshoot rate and front time as parameters. Consequently, it was indicated that, as the overshoot rate increased, the 50% breakdown voltage was inclined to increase. Subsequently, the evaluation using the k-factor function is considered to be useful for UHV-class oil immersed transformer. The result of evaluation of the k-factor value based on the experimental result was almost identical to the result of the European Project, which is the basic data of the existing k-factor function. As a result, it emerged that the existing k-factor function itself was most likely to be applicable to UHV-class oil-immersed transformers. Furthermore, since the extension of the front time up to about 3.6 μs caused only a small change in insulation breakdown characteristics, the extension of the front time up to about 3.6 μs might possibly be allowed in the standard that assumes a UHV-class transformer.
PULSE: a high-repetition-rate linac driver for x-ray FELs
We describe a staged concept for a linac-based freeelectron laser providing coherent tuneable VUV and soft X-ray output with pulse lengths down to 10 fs or less. Use of 1.3 GHz cryomodules operated with CW RF will allow flexible pulse patterns at potentially very high repetition rates up to 1 MHz. We consider the options for FELs that may be included in a facility of this type.