Investigations on lightning impulses with well-defined overshoots

摘要

High voltage tests with lightning impulses are a fix part of type or routine test of high voltageequipment, operated in distribution and transmission networks. Especially the routine test ofpower transformers requires a detailed test procedure with lightning impulses. Thephenomenon of superimposed oscillation at the peak of the impulse provoked by parasiteinductance of the test circuit (known as overshoot) is a broadly discussed problem and wasinvestigated in detail mainly only on small scale test objects with low lightning impulseamplitudes. In order to realize realistic boundary conditions (as they appear in practical testfield) a test set up with only one generator has been chosen. Well-defined inductivities havebeen inserted into the generator in order to provide lightning impulses with overshoots with amaximum amplitude between ß = 5 % and ß = 20 % and an overshoot frequency between f =230 kHz and f = 800 kHz. The tests were performed first on air with a rood-to-plate and asphere-to-plate test set up. Positive and negative polarities have been considered as well. Asthe extensive tests have been performed on different days, a climatic correction has beenapplied as well. For negative polarity the breakdown was triggered at the peak of the impulse.The effect of overshoot can be observed at 0.5 us after the peak. Breakdowns at this conditionare not influenced by an overshoot but only by the peak voltage of the lightning impulse.A different situation can be observed for impulses with positive polarity. The breakdown istriggered in the tail of the impulse, up to 2 us after the peak. In that case overshoot frequencyand, with a bigger impact, higher overshoot amplitudes are influencing the breakdownbehavior of the insulating material. The consequence of that outcome is that each insulationmaterial requires for positive and negative polarity an individual k factor curve.