Partial Discharge Diagnostics and Electrical Equipment Insulation ...

Beginning in the early 1970’s, Kelen introduced aprimitive form of what is now called a pulse phaseanalyzer (PPA) also known as a phase-resolved pulsemagnitude analyzer [8]. The PPA is similar to thePMA, with the exception that the phase angle of theAC cycle is also digitally recorded for each PD pulse.The result is a digital representation of the informationthat was originally measured over a complete ACcycle on an oscilloscope. The output of the PPA is atwo or three-dimensional plot of the pulse count ratevs. the pulse magnitude vs. the AC phase position ofthe pulses. There are many ways to display thisoutput. Fruth [13] and Fujimoto [14] popularized atwo-dimensional colorized presentation of the PPAdata (Figure 3). Usually the pulse repetition rate isdisplayed via a color code – which unfortunately is notstandardized. Virtually all commercial PDinstrumentation made in the past 10 years produces thePPA plots.Pulse Magnitude [mV]3020100-10-20-30Bipolar Slot Total0 to 3.16 pps 3.16 to 10 pps 10 to 31.6 pps 31.6 to 100 pps100 to 316 pps 316 to 1000 pps > 1000 pps Subset 80 45 90 135 180 225 270 315 360Phase Angle [deg]Figure 3: Pulse phase analysis (PPA) plot of the pulsemagnitude versus AC phase position, with the pulse countrate in pulses per second represented as color.The widespread use of digital instrumentation, whichproduces the PMA and PPA plots has reduced the needfor human experts to be present at the PD test. Withthe older analog technology, the expert’s presenceduring the test was needed so the expert coulddetermine peak PD magnitudes from the flickeringtraces and determine if noise was present during themeasurement. With digital instrumentation, and thepermanent recording that it enabled, the expert couldreview the data transmitted by email to a lab or office.3020100-10-20-30This facilitated a considerable productivityimprovement for PD test experts, which in turnlowered test costs.Most of the commercial PD instrumentation in usetoday uses an analog-to-digital converter withsampling rates of about 20 MHz, corresponding to amaximum analog bandwidth of about 10 MHz. Theseare completely compatible with IEC 60270 narrowband and wide band detectors. However, somesuppliers are using digitizers with bandwidths up to350 MHz to enable ultra wideband PD detection, withthe consequent advantages of noise separation and PDsite location [4,14,15].2.3.2 NOISE SEPARATIONAs condition assessment has become an importantaspect of PD testing, the desire of users to do on-siteand on-line PD testing has increased. Off-line testingrequires a shutdown of the HV equipment, and oftenalso requires a substantial HV test transformer toenergize the capacitance of the test object. In contrast,on-line tests are done during normal operation of theHV equipment where the apparatus is exposed torealistic operational stresses, so no shutdown isrequired; and an HV test transformer is not required.Unfortunately, there is usually much more electricalinterference to contend with in on-site and on-linetesting, since the test object is connected or near to thepower system. Electrical interference can come fromcorona on air-insulated transmission and distributionlines, power tool operation, arc welding, poorelectrical contacts, electrostatic precipitators, etc., allof which produce sparks/discharges that create currentpulses similar to PD. Separating this noise from testobject PD is important since if the noise is mistaken asPD, a false indication of the insulation deterioration isgiven, reducing the usefulness and the credibility ofthe PD test.Experts reviewing the PD patterns in PPA plots canusually separate noise from PD – although theseparation is subjective. Automated patternrecognition (see Section 2.4) may also separate PDfrom noise. However, in the past 20 years somehardware (sensors and instrumentation) methods toseparate PD and noise have also been implemented toseparate PD from noise.