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In simple terms, the PD signal is "decoupled" from the primary system voltage signal by use of a capacitor that only allows relevant and optimized PD frequencies to be measured and evaluated. IEC 60270 determined that by and far the most sensitive and relevant frequency of PD to measure are between 100 and 500 kHz. This created a challenge for the rotating machine PD monitoring community.
The response was a gradual movement toward larger coupling capacitors. Initially, decoupling was done with much smaller capacitors, generally 80 pF. The primary advantage of using 80 pF couplers was to measure high frequencies (> 50 MHz) outside the range of lower frequency power electronic noise. The problem is that high frequencies are attenuated much more than lower frequencies. The end result was PD sensing that did not penetrate very deeply into the winding and could not provide an indication of actual charge levels. With time and improvements in gating technologies, larger capacitors are being used in the order of 2 nF, such as the Doble LF Coupler. Some even include HV
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Generator stators alter PD signal transmission properties and can cause changes to the PD pulse shape and attenuate the measured signal. Internal PD, originating from the slot, has to pass a complex signal path to the terminals, which leads to a limitation of the inherent signal bandwidth. On the other hand, PD appearing at the end of the slots closer to the terminals, and external PD signals, are travelling through a different path and the pulse shape is altered less. The described limitation of PD pulse bandwidth by the individual discharges cross-coupling and attenuation characteristic of the stator might lead to unacceptable measuring deviations concerning the slot- and slot-end PD, if not taken into account. To reduce the impact on the measuring sensitivity, and to maintain the comparability of PD measuring results, the PD decoupling time constant, as well as the integration behavior of the PD processing unit, play an essential role. One approach to overcome this obstacle is the insertion of internal slot couplers to capture the slot-PD signal close to the source of its appearance. This will consequently lead to higher costs for the installation of such special sensors and also to a higher risk of undesired deterioration of the insulation behavior by couplers attached directly to the slot insulation.
A better approach is to apply the simple and established capacitive coupling method combined with measuring impedance in accordance with IEC 60270. In order to maintain the transmission of the shaped slot PD pulses without additional loss of sensitivity, couplers that sense lower frequencies are necessary. The lower cut-off frequency is determined by the value of the coupling capacitor. Couplers between 1-4 nF are suitable for this application and frequency requirement. Practical experiences have shown that this method represents a feasible way to decouple slot end PD, as well as critical slot PD, with acceptable attenuation and sensitivity. Another advantage is that the results of on-line PD measurements compare well with laboratory tests using test circuits described in IEC 60270.
Data acquisition and software management
With greater depth of analysis comes more data. A great deal of raw and statistical data needs to be recorded to properly characterize and analyze PD activity. In the past, just overall indications of PD analysis were quantified into single values such as NQN and repetition rate. Pattern and frequency analysis were not necessarily part of the solution, but important for characterization and diagnosis of the fault.
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