Partial Discharge Analysis

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MB04025 - Partial Discharge AnalysisPartial DischargePartial discharges are small electrical sparksthat occur within the electric insulation ofswitchgear, cables, transformers, and windingsin large motors and generators. PartialDischarge Analysis is a proactive diagnosticapproach that uses partial discharge (PD)measurements to evaluate the integrity of thisequipment. Each discrete PD is a result of theelectrical breakdown of an air pocket withinthe insulation. PD measurements can be takencontinuously or intermittently and detectedon-line or off-line. PD results are used toreliably predict which electrical equipment isin need of maintenance.Just as every material has a characteristictensile strength, each material also has anelectrical breakdown (dielectric) strength thatrepresents the electrical intensity necessary forcurrent to flow and an electrical discharge totake place. Common insulating materials suchas epoxy, polyester, and polyethylene havevery high dielectric strengths. Conversely, airhas a relatively low dielectric strength.Electrical breakdown in air causes anextremely brief (lasting only fractions of ananosecond) electric current to flow throughthe air pocket. The measurement of partialdischarge is, in fact, the measurement of thesebreakdown currents.Not only do partial discharge levels provideearly warning of imminent equipment failure,but partial discharge also accelerates thebreakdown process. The excessive arcingbetween ground and conductor within theinsulation will, in time, compromise thedielectric strength and mechanical integrity ofthe winding insulation. Once this happens, aground fault or a phase-to-phase fault isinevitable.Ultrasonic / Acoustic EmissionAll operating equipment produces a broadrange of sound. The high frequency ultrasoniccomponents of these sounds are extremelyshort wave in nature, and a short wave signaltends to be fairly directional. It is thereforerelatively straightforward to isolate thesesignals from background noises and detecttheir exact location. In addition, as subtlechanges begin to occur in electrical andmechanical equipment, the nature ofultrasound allows these potential warningsignals to be detected early, before actualfailure.Electric equipment can suffer from a varietyof manufacturing defects or operatingproblems that impair its mechanical reliability.The electrical insulation of motors andgenerators is susceptible to:• Thermal stresses Figure 1 An airborne ultrasound instrument.• Chemical attackAirborne ultrasound instruments, often• Abrasion due to excessive coil movement referred to as "ultrasonic translators", provideinformation two ways: qualitatively, due to theIn all cases, these stresses will weaken the ability to "hear' ultrasounds through a noisebonding properties of the epoxy or polyester isolating headphone, and quantitatively, viaresins that coat and insulate the windings. As a incremental readings on a meter. This isresult, an air pocket develops in the windings. accomplished in most ultrasonic translators by© 2004 SKF Reliability Systems All Rights Reserved 2
MB04025 - Partial Discharge Analysisan electronic process called "heterodyning",which accurately converts the ultrasoundssensed by the instrument into the audiblerange where users can hear and recognizethem through headphones.connection, it disturbs the air moleculesaround it and generates ultrasound. Often thissound will be perceived as a crackling orfrying sound; in other situations it will beheard as a buzzing sound.Although the ability to gauge intensity andview sonic patterns is important, it is equallyimportant to be able to "hear" the ultrasoundsproduced by various equipment. That isprecisely what makes these instruments souseful; they allow analysts to confirm adiagnosis on the spot by being able todiscriminate among various equipmentsounds.The reason users can accurately pinpoint thelocation of a particular ultrasonic signal in amachine is due to its high frequency / shortwavelength. Most of the sounds sensed byhumans range between 20 Hz and 20 kHz (20cycles per second to 20,000 cycles persecond). They tend to be relatively gross whencompared with the sound waves sensed byultrasonic translators. Low frequency soundsin the audible range are approximately 1.9 cm.to 17 meters in length, whereas ultrasoundssensed by ultrasonic translators are only 0.3 -1.6 cm long. Since ultrasound wavelengths aremagnitudes smaller, the ultrasonicenvironment is much more conducive tolocating and isolating the source of problemsin loud plant environments.Using Ultrasonic / AcousticEmission for Partial DischargeUltrasonic testing is often used for evaluationat voltages exceeding 1,000 volts, especiallyin enclosed switchgear. This is especiallyuseful in identifying tracking problems. Inenclosed switchgear, the frequency of trackinggreatly exceeds the frequency of seriousfaults, which can be identified usingtechniques such as infrared thermography.When electricity escapes in high voltage linesor when it jumps across a gap in an electricalThere are three basic electrical problems thatcan be detected using ultrasound:• Corona: When voltage on an electricalconductor, such as an antenna or highvoltage transmission line exceedsthreshold value, the air around it begins toionize to form a blue or purple glow.• Tracking: Often referred to as "babyarcing", follows the path of damagedinsulation.• Arcing: An arc occurs when electricityflows through space. Lightning is a goodexample.The process can be used to detect both endwinding (Phase to Phase) and slot section(Phase to Earth) discharge.High voltage applications include: insulators,cable, switchgear, buss bars, relays,contactors, and junction boxes. In substations,components such as insulators, transformersand bushings may be tested.The method for detecting electric arc andcorona leakage is similar to the procedureused to detect acoustic emissions frommechanical sources. Instead of listening for arushing or rubbing sound, a user listens for acrackling or buzzing sound. In some instances,as in trying to locate the source of radio/TVinterference or in substations, the general areaof disturbance may be located with a grossdetector such as a transistor radio or a widebandinterference locator. Once the generalarea has been located, the scanning module isutilized with a general scan of the area. Thesensitivity is reduced if the signal is too strong© 2004 SKF Reliability Systems All Rights Reserved 3
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