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motors, transformers, switchgear and cables has notdeteriorated due to electrical, thermal, mechanical orenvironmental stress during operation.The aim of each of these tests is to ensure that theequipment has the expected service life – that is, theequipment is reliable. If PD is detected, or the PDlevels are too high, it is likely that the insulationsystem will fail prematurely.Although there have been many researchers, thepioneers of practical methods to measure PD duringthe 1950s to 1980s were Bartnikas [6], Kelen [7] andKreuger [8]. These researchers developed the PDdetection methods and data display methods that arestill widely used today in laboratories and factories.Most of their work was focused on developing testmethods for Design or QA PD testing. Out of theirwork came some basic methods to suppress electricalinterference using differential measurements, as wellas approaches to display PD data using pulsemagnitude analysis and phase-resolved pulsemagnitude analysis (also known as pulse phaseanalysis).In the past 20 years, the main focus of research hasbeen to expand PD technology so it can be morepractically applied to diagnostic testing to determinethe condition of the insulation that has seen operation.That is, insulation condition assessment in cables,transformers, switchgear, etc, usually in the plantwhere the apparatus is installed (i.e. on-site).Condition assessment can include both on-line PDtesting, as well as variations of the off-line testing thatis used for Design and QA testing. On-site testingrequired developing better and better ways ofsuppressing electrical inference (which can lead tofalse indications), as well as developing tools to makethe interpretation of PD pulse phase analysis patternsmore objective. The most prolific researchers for thepast 20 years are perhaps Bartnikas, Boggs, Fruth,Gulski, Hampton, Montanari and Okamoto. These andother researchers have advanced the science of PDmeasurement technology to the point that off-line andon-line PD measurements at site are now routinelyapplied to a high percentage of transformers, cables,machines and switchgear that are operating inindustrialized countries. The impact of theseadvancements is the improved reliability of highvoltage equipment since degrading apparatus can berepaired or replaced before catastrophic in-servicefailure occurs. Of course these advances have alsobeen applied to Design and QA testing. It is importantto note however, that diagnostic testing cannot give areliable indication of remaining life. Instead thepurpose is to warn of an impending problem, andperhaps identify the root cause of the insulationproblem.This paper will first summarize the key features ofmodern PD measurement systems, and review some ofthe technology advancements in the past 20 years.The paper then addresses the salient features ofmodern PD methods that have been developed foreach type of electrical apparatus, and what questionsremain.2 FEATURES OF PD MEASUREMENTSYSTEMSPD theory and a broad overview of PD sensors andmeasurement technology has recently beensummarized by Bartnikas [1]. The followingconcentrates on the sensors, instrumentationtechnology and data displays that have, in the past twodecades, become the most popular for equipmentcondition assessment.2.1 SENSORS (COUPLING DEVICES)When a partial discharge pulse occurs, there is a veryfast flow of electrons from one side of the gas filledvoid to the other side. Since the electrons are movingclose to the speed of light across a small distance, thepulse has a very short duration, typically a fewnanoseconds [1]. The electrons carry a charge, thuseach individual discharge creates a current pulse (i =dq/dt). In addition to the electron current flow, therewill be a flow of positive ions (created when theelectrons are ionized from the gas molecules) in theopposite direction. The PD current in the void createsa disturbance and results in pulse current and voltagethat flows away from the PD site. A Fourier transformof a current pulse indicates that frequencies up toseveral hundred megahertz are created [1].Any sensor (also called a PD coupling device)sensitive to high frequencies can detect the PD pulsecurrents. In an off-line PD test, the most commonmeans of sensing the PD current is to use a high
voltage capacitor connected to the high voltageterminal of the test object (Figure1).coupler, has also been used to detect PD in statorwindings. It works in the 10-1000 MHz range [4].If discharges are occurring on the surface of aninsulation system, then the rapid flow of electrons andions create a gas pressure wave, which can be detectedacoustically. The acoustic pulse created by eachdischarge is concentrated in the 40 kHz (ultrasonic)range [10]. By using directional ultrasonicmicrophones, the location of surface PD on bushings,air-insulated switchgear and stator windings can oftenbe identified.Figure 1: Typical off-line PD test arrangement.Typical capacitances are 80 pF to 1000 pF. Thecapacitor is a very high impedance to the high ACvoltage, while being a very low impedance to the highfrequency PD pulse currents. The output of the highvoltage capacitor drives a resistive or inductivecapacitiveload called a ‘detector’ (see Section 2.2).In addition to capacitors, high frequency currenttransformers (HFCTs) can be employed to detect thePD pulse currents. HFCTs typically measure the highfrequency current that may flow in the ground leadfrom a test object. They may also measure the PDpulse currents from the test object to the HV supply.The HFCTs are often wound on split ferrite cores thatcan respond to frequencies in excess of 30 MHz. ForHFCTs applied to HV leads, a large electricalclearance is needed, thus air-core HFCTs (usually aRogowski coil) are used. Rogowski coils areconsiderably less sensitive than ferrite core HFCTs,and have a much lower bandwidth due to high interturncapacitance.PD can also be sensed from the following physicalattributes of discharge:• Electromagnetic radiation• Acoustic noise• Visible and UV light.Electromagnetic radiation can be detected with RFantennae. The TVA [4] and Lemke [9] probes tolocate PD sites are good examples of this. Theantennae are usually tuned to the 5 to 100 MHz range.Another antennae-like device, called the stator slotThe final popular method for sensing PD uses the factthat light is emitted from excited molecules, whichlose their energy after the initial discharge. The mostintense light occurs in the ultraviolet spectrum.Specialized imaging devices have been constructed todetect the UV components of the light [11]. Otherwisethe naked eye can see the light from surface PD, if thetest object is energized with the ambient light levelvery low (black out test).Although many types of PD sensors are now available,HV capacitors are by far the most widely used sensorsfor diagnostic testing. The remainder of this paperwill concentrate on the electrical measurement of thePD pulse currents, rather than on acoustic or UVdetection. In the past 20 years, there has been littleinnovation in current pulse PD sensors.2.2 DETECTORSPD detection normally consists of converting the PDpulse current from the sensor to a voltage signal, sincemost measurement instrumentation is sensitive tovoltage rather than current. The conversion occurs viaa resistor or more elaborate detection impedance. Aresistor yields a wide band detection system with theoutput normally measured in mV or V. However, ifan RLC detection impedance is used, then thecharacteristics of the detector can be manipulated tointegrate (low pass filter) the PD pulse current to yielda signal magnitude that is proportional to the apparentcharge transfer in each discharge [1,6,7]. The RLCdetection impedance is very popular since the rate ofdeterioration of organic insulation is often directlyproportional to the total number of electrons and ionsthat bombard the insulation surface. That is, if thedischarges are larger (consists of more electrons andions) the insulation will degrade faster.
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