Prebreakdown 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. 1.2/50 /spl mu/s) 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 occurred no breakdown or flashover. In this paper, it is shown that even in uniform oil-board-arrangements, a prebreakdown mechanism under lightning impulse stress occurs. These prebreakdowns produce 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 prebreakdowns cannot be detected. An optical observation is not always possible because for big oil-board-arrangements you need steel vessels and the prebreakdown channel can be covered by other board barriers. With a charging measurement after each impulse, a new procedure for certain prebreakdown recognition is presented.