Pre-breakdown behaviour of oil-board-arrangements under lightning impulse stress

Standard lightning impulse tests with oil-board- arrangements were performed by applying impulses with the specific shape (e.g. 1,2/50mus) on the test object and measuring the peak value with a divider and a peak voltmeter and controlling the shape with an oscilloscope. The test object passed the lightning impulse test if there occurs no breakdown or flashover which can be controlled by scope and usually noticed acoustically. But what about pre-breakdown mechanism? In this paper it will be shown that even in uniform oil-board-arrangements pre-breakdown mechanism under lightning impulse stress occurs. These pre-breakdowns produces bubbles and a charging of the board surfaces which influence the electrical strength of the oil-board-arrangement. With the conventional impulse measuring technique (divider and oscilloscope) these pre-breakdowns cannot be detected. An optical observation is not always possible because for big oil- board-arrangements you need steel vessels and the pre-breakdown channel can be covered by other board barriers. With the charging measurement after each impulse a new procedure for the certain pre-breakdown recognition is presented.