Recent experience with directional couplers for partial discharge measurements on hydrogen cooled rotating machines in a noisy environment

Manitoba Hydro has used directional couplers for online partial discharge measurements on hydrogen-cooled rotating machines since 1981. Two 80 pF capacitive couplers per phase are permanently installed with one usually being located on the air side of the line end gas seal bushing, with the other at least 2 m towards the step-up transformer and with both connected to instrumentation by means of a coaxial signal cable. Originally the partial discharge analyzer was connected differentially between the outputs of the two couplers so as to obtain common mode cancellation of noise, but this was incomplete due to the mismatch in surge impedances between the isolated phase bus and the winding. A new partial discharge analyzer was introduced in which noise recognition is achieved based on the time of arrival of pulses from the two couplers. The paper describes an improvement in the sensitivity of the coupler system to partial discharges in the winding which can be obtained by relocating one of the couplers to the hydrogen side of the gas seal bushing. This is especially useful when there are substantial noise sources external to the machine. Details of the implementation are included.