| 1. |
- Bergman, Anders, et al.
(författare)
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Characterisationat low voltage of two reference lightning impulse dividers
- 2017
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Konferensbidrag (refereegranskat)abstract
- An effort is pursued by several European National Measurement Institutes to lower the uncertainties in calibration of UHV measuring systems for lighting impulse. To this end, several reference dividers are investigated as regards their accuracy both for amplitude and for time parameters. At SP - RISE Research Institutes of Sweden, a 500 kV resistive reference divider has been in use since 2000. Additionally an 800 kV resistive divider is investigated as a possible reference divider for UHV lightning impulse measuring systems. The best uncertainty for the 500 kV reference measuring system is 1 % for voltage amplitude and 3 % for time parameters. The present work aims at lowering these uncertainties by means of better characterisation and evaluation of the possibilities to apply corrections for known errors. The scale factor and dynamic behaviour of a resistive divider can be conveniently determined at low voltage and frequency. Further experiments such as linearity tests and augmented by scientific work is needed to ascertain the performance at high voltage. Step response plays a major role in the characterisation of dividers, and in this work much effort has gone into gathering step responses and evaluating them for various circuit layouts to characterise the variation of the step response due to circuit dimensions and diverse proximity effects. The step applied to the divider is generated by a mercury wetted relay based step generator with an output voltage of 200 V. The step rise-time is a few ns, and thus appreciably faster than the response of the divider. Apart from inspection of the step response itself, evaluation of measurement errors is performed by convolving an ideal curve with the step response of the divider, including its transmission cable. The convolved signal is evaluated with impulse evaluation software and the parameters compared to the ideal input. The difference is a measure of the errors introduced by the divider. This procedure follows IEC 60060-2: 2010.
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| 2. |
- Bergman, Anders, et al.
(författare)
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Characterization of a fast step generator
- 2017
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Konferensbidrag (refereegranskat)abstract
- Lighting impulse measurements are made as a matter of routine in high voltage testing of high-voltage electrical equipment. The test is often decisive for acceptance of the equipment under test, and consequently proper and precise calibration of the measuring system is needed. The present work centres on the need to quantify the errors of reference measuring systems for lightning impulse. Scale factor determination at low frequency (or DC) is the starting point for this determination. The extrapolation from this frequency domain to the domain where microsecond pulses must be faithfully captured requires application either of methods in the frequency domain or in the time domain. Radio frequency measurements are only well defined for coaxial structures and at impedances in the range of 50 O or thereabouts, making them difficult to apply to the large structures of high-voltage measuring systems. The converse method in the time domain is to apply a Dirac impulse to the system and calculate the response to an assumed input signal by convolution. A true Dirac pulse is not readily available and in practice the applied pulse is a step voltage, which is then derived with respect to time and convolved with the applied signal to obtain the response of the measuring system. The step generator used for this purpose should have very fast front without oscillations. The intent is to achieve a close approximation of an ideal step function, which when derived with respect to time, yields the impulse response of a tested system. A necessary prerequisite is that the step is much steeper than the lightning impulse, and is flat after the step on times much longer than the impulse. The ideal switch element in such a step generator should have infinite resistance and zero capacitance in the off-state, very fast switching to on-state and very low resistance in on-state. The mercury wetted reed switch has often been used for this purpose since it has good characteristics in all these respects. Few, if any, electronic components exhibit competitive advantages compared to the reed switch. The relative lack of parasitic effects means that it is close to being an ideal device. Based on earlier experiences by the authors, a new design has been developed with focus on electrical screening and coaxial design in order to realise a step generator that works into a high impedance instrument. Considerable work has been performed to characterise the new device with regard to steepness of step and most importantly, to voltage stability after the step. The most demanding part of this work has been to separate the performance of the switch from that of the oscilloscope. Findings indicate that the step rise-time is less than 0.5 ns, and settling to within 0.5 % within 10 ns.
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| 3. |
- Bergman, Anders, et al.
(författare)
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Evaluation of step response of transient recorders for lightning impulse
- 2017
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Konferensbidrag (refereegranskat)abstract
- High voltage equipment will be subjected to several types of electrical stress during operation. A battery of factory tests is defined to ensure that the equipment will perform satisfactorily in service. One of the crucial tests is to apply a simulated lighting impulse as standardised to a double-exponential impulse with at front time of 1.2 µs (± 30 %) and a time to half value of 50 µs (± 20 %). Although this wave-shape only approximates natural lightning, there is a solid body of experience within industry, proving that reliability of equipment in service is adequately proven by the standard waveform. It is however crucial for consistency of results that the both voltage level and wave-shape are correctly measured. This paper discusses the requirements and performance of the recording instruments used, leaving the properties of high voltage impulse dividers outside the discussion. The requirements for the recording instrument – transient recorder – are given in IEC 61083-1. The standard provides requirements for, and/or tests to verify, that the recorder has moderately fast response, fast settling time, high resolution, linearity under dynamic conditions, high accuracy and reasonably low internal noise. This is partly in contrast to major trends in transient recorder development, where fast sampling and fast step response are prioritized ahead of high accuracy and fast settling without creeping response. We have therefore evaluated several commercially available recorders in order to find one with respectively flat and reasonably fast step response. In this campaign, a proprietary step generator based on the use of a mercury reed relay has been used. Evaluation of this device is submitted to ISH 2017. It has been found that the measured flatness of the step response directly after the step is a good first indicator of the performance of the transient recorder. This is identified in IEC 61083-1 clauses 1.5.2 and 1.5.3, as a requirement on stability of the recorded step from 0.5 T1min to T2max. For lightning impulse this means from 0.42 µs to 60 µs. For approved transient recorders the requirement is to be within 1 %. For reference transient recorders, a limit of not more than 0.5 % should be applied. Further proof of the accuracy of the transient recorder can be achieved by convolution of an ideal waveform with the recorded step response and analysing the resulting curve with lightning impulse parameter software. A third possibility is to make direct calibration of the transient recorder, using a calculable impulse calibrator. Several state-of-art transient recorders have been evaluated and the results show that only a few are suited for measurement of lightning impulse. Also, the variation of the performance between the ranges and channels of one instruments are significantly large. Both direct assessment of step response as well as result of convolution with a theoretical 0.84/50 µs impulse will be reported. The agreement with results obtained with a calculable impulse calibrator will be illustrated.
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| 4. |
- Bergman, Anders, et al.
(författare)
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Influence of coaxial cable on response of high-voltage resisitive dividers
- 2017
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Konferensbidrag (refereegranskat)abstract
- An effort is pursued by several European National Measurement Institutes to lower the uncertainties in calibration of UHV measuring systems for lightning impulse. To this end, several reference dividers are investigated as regards their accuracy both for amplitude and for time parameters. During these investigations a deterioration of step response was identified when longer coaxial cables were inserted in the measuring circuit. The measured front time T1 was also affected, in one observed case by 2.5 % elongation of front time as another 25 m cable was inserted. Compared to the intention to calibrate front time measurement to better than 5 % uncertainty for front time, this contribution must be well known, or preferably be eliminated. This paper presents the experimental findings from these investigations. The investigated cables included selected coaxial, tri-axial, and cables with a corrugated screen. The effect of cable length was also studied. The influence was first discovered when applying a very fast step (rise-time < 4 ns) to the high voltage arm of a resistive divider and convolution of this step with the time derivative of an ideal lightning impulse with 0.84/60 µs impulse. The calculated output was analysed with IEC 61083 compliant software to evaluate the front time. Subsequently, these analyses have been augmented by additional comparative measurements where two reference dividers were connected to the same impulse generator, and varying the cable length of one of them. The summarized changes in front time calculated for different combinations of cable and impulse voltage dividers are shown and discussed. It is noted that a change in T1 error depends both on length of cable and its type. The results show that non-negligible front time errors may be introduced when the cable length is increased. To support these findings, further tests have been carried out with two reference impulse dividers connected in standard calibration configuration in accordance with IEC 60060-2. One divider was used as reference, while the cable for the other was varied. In this way, the change of error between configurations could be measured. A theoretical study has also been performed, calculating the distortion of a lightning impulse on a coaxial cable. The results agree qualitatively with experiments, but the detailed results show discrepancies that need further investigation.
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| 5. |
- Havunen, Jussi, et al.
(författare)
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Using deconvolution for correction of non-ideal step response of lightning impulsedigitizers and measurement systems
- 2017
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Konferensbidrag (refereegranskat)abstract
- Lightning impulse measurements can be highly influenced by measurement arrangement, characteristics of high voltage divider, digitizer (transient recorder) performance, and algorithms used for parameter evaluation. The main sources of measurement errors are the non-ideal step responses of digitizer and voltage divider. This paper discusses the use of deconvolution to correct for the non-ideal step response of a digitizer, and of a large mixed divider. Correcting the step response of the complete measuring system by one part at a time is desirable because it allows to evaluate the effectiveness of the correction with trustworthy methods. Step response describes the output of a system as function of time when its input changes between two levels infinitely fast. Real life impulse digitizers and impulse voltage dividers have a finite rise time, and the response does not immediately settle to final value. Slow rise time is often the cause of error for front time parameter. Creeping response is often the cause of error for time to half-value parameter. Step response of an instrument can be determined by applying a stable, known direct voltage, which is then shorted to ground by a mercury-wetted relay. The mercury-wetted relay is assumed nearly an ideal switch, which creates almost an ideal voltage step for input of the instrument. Convolving the derivative of the measured step response with an ideal input gives a measure of distortion caused by the non-perfect step response, and conversely deconvolving the measured step response with the measured signal gives the original input signal. This paper presents an FFT-based method for step response correction using deconvolution. Deconvolution is a mathematical process, which is used to reverse the non-ideal effects of measuring instrument on recorded data. Effectiveness of the method is demonstrated by two examples. In the first example, the non-ideal step responses of the different ranges of an impulse digitizer are corrected. Functionality of the step response correction is evaluated by comparing the results against a calculable impulse voltage calibrator. Results showed that the step response correction reduced errors in lightning impulse parameters. Stability of the step response correction was analysed by studying several impulse calibration results that have been performed for the instrument within a year. The second example corrects the response of a 2400 kV impulse voltage divider. The effectiveness of the correction is evaluated by comparing its results to a 400 kV reference divider.
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