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Bridge methods are so important in the measurement art that it is not surprising to find them employed in the evaluation of corona or partial discharge activity. Bridges for this purpose must work at the high voltages at which discharge measurements are commonly made. In this chapter, consideration will be limited to bridges operating at high alternating voltages with frequencies in the power or audio range. Treated first is the evaluation of discharge activity from two bridge measurements, only one of which includes the discharge activity. Then a special bridge is described that uses an oscilloscope to produce a parallelogram-like trace from which more direct measurements of discharge activity may be obtained. The parallelogram trace technique has certain advantages in the sense that it may be effectively used to measure the energy loss due to pulse or pulseless type corona discharges. The measurement of the latter type of discharge can only be properly effected using the bridge approach. The term pulseless corona is commonly applied to the occurrence of true-glow and pseudo-glow discharges [I]. A conventional corona pulse detector fails to respond adequately to a true-glow discharge due to the absence of corona pulses within the glow region of the breakdown voltage wave (see Chapter 2); likewise, the pulse detector cannot respond to pulses whose rise times are unduly long such as produced by a pseudo-glow discharge. The concluding paragraphs deal with the use of the bridge method to facilitate measurement of pulses produced by corona or partial discharges.
Doble Engineering Company, Watertown, Mass.