A CONTRIBUTION TO THE EVALUATION OF LIGHTNING IMPULSES WITH OSCILLATIONS OR OVERSHOOT NEAR THE PEAK

After an explanation of the basic assumptions used at the evaluation of the test voltage for impulses with oscillations or overshoot near the peak the influence of the different evaluation procedures are shown. The main goal of the new proposal is the increase in the reproducibility of the test voltage evaluation for a manual as well as for automtic evlaution. The proposed methods using a single exponential fitting of the tail and the filtering of the measured curves lead to good results regarding the reproducibilty and the experimental results. INRODUCTION The actual IEC Publ. 60060-1 [1] describes the evaluation of the test voltage for lightning impulses with oscillations or an overshoot near the peak by some figures, based on the evaluation of oscillograms. The procedure given in the standards needs a mean curve through the oscillations or overshoot as shown in Fig. 1. Figure 1: Lightning impulses with oscillations Evaluating oscillograms from analogue oscilloscopes this procedure gives reliable and reproducible results. When the digital technique came into the high voltage test business a problem arises because also the evaluation should be done by a computer and it was more or less impossible to introduce a mean curve in the automatic evaluation due to the weak definition. In particular the stringent frequency limit of 500 kHz generates a large variation in the test voltage for impulses with oscillations close to this frequency. This situation leads to a number of publications how to solve this problem [2 5] and finally to an European Research Project in which the physical background of the frequency limit as well as suitable voltage parameters were investigated [3]. RESEARCH RESULTS Within the already published research work a large number of different proposals were made to overcome the above described problem. In the European Research Project a questionnaire shows that around the world different test and calibration laboratories use different method to evaluate the test voltage and that fundamental research work is needed. The main results out of this programme are the following: • oscillations or overshoot have the same influence on the 50 % breakdown voltage • the frequency of the oscillations or the corresponding frequency of the overshoot have an influence on the 50 % breakdown voltage • the amplitude of the oscillation or overshoot have an influence on the 50 % breakdown voltage The results were gained with breakdown tests of different insulating materials like air, SF6, PE, oil and oil-paper. The test set-up, the measuring devices and the individual test results are described in details in [7, 8]. Finally a proposal for the introduction of an test voltage factor k has been made which takes into the account the frequency dependent influence of the oscillation on the breakdown voltage and which can be used to evaluate the test voltage [9]. Anyhow, the problem of the mean curve still exist with the actual standard and therefore the following comments should contribute to a more reproducible evaluation of the test voltage keeping the actual standard in mind. BASIC ASSUMPTION The proposed evaluation procedure is based on the assumption that the basic impulse shape is double exponential and that the oscillations or overshoot are superimposed on that basic impulse near its peak. This assumption was also used for the experimental work within the European Project. The oscillation or overshoot are near the peak and their influence on the breakdown voltage should be taken into account assuming that the amplitude of the oscillations or overshoot will not exceed a given limit. The actual IEC 60060-1 limits this to 5 %. From the influence of the oscillations or overshoot on the 50 % breakdown voltage it is derived, that the test voltage is influenced in the same way. The oscillations or overshoot are caused by the test circuit and/or by the test object and they are part of the test voltage stressing the test object. DEFINITIONS In order to use unambiguous terms the following definitions will be used. A smooth impulse is a double exponential impulse without any oscillation or overshoot. If oscillations or overshoot are superimposed on the double exponential impulse they are placed near the peak. To the question if an impulse has oscillations or an overshoot it is necessary to draw a mean curve, because otherwise the evaluation procedures can not be applied. The highest amplitude of the measured impulse is called extreme value according to an proposal of a joint Task Force between CIGRE WG 33-03 “High Voltage Testing and Measuring Technique” and SC 12 “Transformers”. The mean curve is defined according Fig. 1 and the amplitude of the oscillation or overshoot is defined as the maximum difference between the measured curve and the mean curve and should not exceed 5 % of the extreme value. The evaluation of the test voltage depends on the frequency of the oscillation or on the duration of the overshoot resp. corresponding frequency and it is according to the actual standard the extreme value for frequencies lower than 500 kHz or the peak value of the mean curve for frequencies equal or higher 500 kHz. Here again a mean cuvre is necessary to make the decision according to the existing rules. EXTREME VALUE EVALUATION The time parameters front time and time to half value can be evaluated in most cases without a mean curve, because the relevant voltage is the extreme value used as 100 % reference. In cases of oscillations near the 30 % and 90 % levels also a mean curve or interpolations are necessary, but this influences only the time parameter and not the test voltage. PROPOSAL BACKGROUND The proposal for the evaluation of the test voltage at lightning impulse with oscillations should give the same test results as the already used procedure but it should improve the reproducibility. Furthermore the procedure should be suitable for manual and automatic evaluation of the impulses. It should be easy to perform and reflect the physical background of the test. Even if the philosophy of the standardisation prefers definition of limits it seems to be necessary to define a procedure similar to the definition e.g. of the front time of a lightning impulse. According to the already mentioned definition the extreme value of the measured curve represents the 100 % reference value. This value can be evaluated manually or by an automatic algorithm and even the determination of the extreme value by hand from an oscillogram has a good reproducibility. A test with five engineers which have an oscillogram of about 12 x15 cm size for the determination of the extreme value gives a standard deviation of about 0.5 % and a difference between the average value and the highest digital recorded value of about 0.2 %. This confirms the high reproducibility of the determination of the extreme value. AMPLITUDE OF OSCILLATION OR OVERSHOOT The next step for the evaluation of the amplitude of an oscillation or overshoot is new compared to the existing procedure. The proposal is to evaluate the exponential function which represents the tail of the impulse by hand using a curve template or a freehand drawn line. Using a regression procedure the tail parameter can also be developed automatically but the starting and end points may have some influence. Fig. 2 shows a impulse from the IEC 61083-3 (case 9) with a double exponential curve fitting (DE) and a single expontenial extrapolation (SE). Figure 2: Impulse with DE and SE fitting In order to demonstrate the influence of the fitting procedures more in detail Fig. 3 shows the part of the impulse near to the peak. Figure 3: Zooming of the peak part of the impulse It can be clearly seen that the fitting length of an exponential function (SE) influences the value of ∆U which is the difference between the extreme value and the value of the fitted curve at this time. In all evaluations the determination of the test voltage was done by using the frequency dependence 930 950 970 99