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Partial discharge testing
- 1. Partial Discharge Testing By DebojyotiMukherjee (001810802021) Electrical Engineering Department Jadavpur University
- 2. Partial Discharge • Partialdischarge is defined by IEC 60270 "localised electrical discharges that only partially bridges the insulation between conductors and which can or cannot occur adjacent to conductors". • Partial discharges can occur in any location where the local electrical field strength is sufficient to breakdown that portion of the dielectric material (whether it be a deteriorated piece of insulation or an air cavity). • The discharges generally appear as pulses with a typical duration of less than 1µs. While very short in duration, the energy present in the discharge can interact with the surrounding dielectric material resulting in further insulation degradation and eventually if left unchecked, insulation failure.
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- 4. Types of PD •Corona Discharge: PD around a conductor in free space is called corona discharge. Corona discharge takes place due to non-uniform field on sharp edges of the conductor subjected to high voltage. The insulation provided is air or gas or liquid. • Surface Discharge: Surface discharge takes place on interfaces of dielectric material such as gas/solid interface as gets over stressed times the stress on the solid material. • Cavity Discharge: The cavities are generally formed in solid or liquid insulating materials. The cavity is generally filled with gas or air. When the gas in the cavity is over stressed such discharges are taking place. • Treeing Channels: High intensity fields are produced in an insulating material at its sharp edges and it deteriorates the insulating material. That is responsible for production of continuous partial discharge, called as treeing channel.
- 5. Statistics of equipmentfailure due to PD
- 6. Breakdown in voids •Practical solid insulating materials contains voids and cavities. They are usually filled with a gaseous or a liquid medium of lower breakdown strength. • The dielectric constant of the void medium is lower than that of the insulation. This results in a higher electric stress in the voids compared to the solid, hence even under working voltage the voltage across voids may exceed BDV thereby causing void breakdown and discharge
- 7. PD detection methods PDDetection Non- conventional Acoustic UHF detection Optical Conventional Electrical Chemical
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- 9. Concept of apparentcharge Ck replenishes the charge discharged by void in the test object. We can measure this replenished charge (apparent charge) by integrating the discharge current of Ck
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- 11. Concept of apparentcharge
- 12. Detecting PD modelsfrom pulse shape Pulse Parameters (ns) PD type Tr Td T50% T10% Ttotal Internal Discharge 5-6 10-11 13-14 19-21 80-90 Surface Discharge 6-8 9-10 11-13 14-16 140-150 Bubble Discharge 3-4 9-11 11-13 14-15 120-140 Floating part discharge 8-9 11-12 18-19 28-30 290-300
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- 14. DGA testing • WhenPD or other kinds of incipent fault occurs in insulations, gases are generated and they remain dissolved in the oil. • Dissolved Gas Analysis is a routine test for transformers where oil sample is collected and the amount of gas is detected using gas chromatography. • The concentration of gases indicate wheater fault exists and the type of fault.
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- 17. Acoustic method • Acousticemission detection has been successfully used in order to localize the PD source inside of the test object as the acoustic signal is strongly dependent upon the geometry of the test object. This method is very efficient for localizing PD source because of its immunity against electromagnetic noise • The acoustic method is very attractive alternative for online partial discharge detection in power transformers. The transformer tank is grounded; therefore the sensors can be placed in a safe way, over any point of any wall of transformer tank in order to detect the acoustic emission of partial discharges • Because power transformers are not homogeneous devices, the waves do not travel in perfect spherical wave fronts because the source of PD is so small. • Classically, due to the many attenuation mechanisms, received acoustic signals have very low intensity and so sensors must be very responsive to small changes in signal amplitude in order to detect a PD
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- 20. UHF detection method •UHF PD detection has an advantage of strong anti-interference ability over traditional detection approaches • Application of UHF PD detection is mostly limited to GIS but in recent years transformer, cable applications are also considered • This method is ideal for joints and accessories, with sensors being located on the joints or accessories • Electro Magnetic Field detection picks up the radio waves generated by the partial discharge. And these radio waves can generate TEVs on the surrounding metalwork. More sensitive measurement, particularly at higher voltages, can be achieved using in built UHF antennas or external antenna mounted on insulating spacers in the surrounding metalwork.
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- 23. Optical PD detection •The optical partial discharge detection is based on the detection of the light produced as a result of various ionization, excitation and recombination processes during the discharge. However, the optical spectrum of different types of discharge is not the same. • The main advantages of this method are the immunity from electromagnetic interferences and high sensitivity compared to conventional electrical techniques • The amount of the emitted light and its wavelength depends on the insulation medium (gaseous, liquid or solid) and different factors (temperature, pressure …). So the spectrum of the light emitted by partial discharges depends on the surrounding medium and the intensity of the discharge. • 90% of the total energy of the emitted optical spectrum of PD is in the ultraviolet region.
- 24. Optical interferometric detection Theschematic of the Mach-Zehnder interferometer configuration with optical fibres in the reference and sensing arms.
- 25. Comparison of differentmethods Electrical Acoustic UHF Detection Optical Advantages • Intensity source, type, location of PD is assessable • The most suitable for continuous online PD monitoring • High sensitivity • Immunity against electrical noise • Very efficient for localization of PD • Immunity against electrical noise • Very efficient for localization of PD • Identification of PD source • Immunity against electrical noise • Easy to measure • Provide critical information Go/No Go decision Disadvantages • High interference • Relative expensive cost • Low signal intensity • Not good for continuous monitoring • Relatively expensive • Complex design No information about magnitude of PD
- 26. Comparison of differentmethods Electrical Acoustic UHF Detection Optical Possible sensors • Capacitive • Inductive • Piezo-electric transducers • Condenser microphones • UHF sensors • RF sensors • Antennas(Hil bert, monopole, loop antenna etc) • Optical fibre UV detector • Photomultipli er tube Main application area All High Voltage Equipment • Transformers • GIS All High Voltage Equipment • Transformer • GIS • Cables Frequency band 30-300MHz and 300MHz- 3GHz 10 kHz and 300 kHz 3-300 MHz
- 27. REFERENCES • Study ondifferent techniques of partial discharge(PD) detection in power transformers winding: Simulation between paper and EPOXY resin using UHF method--International Journal of Conceptions on Electrical and Electronics Engineering--Irfan Ali Soomro and Md Nor Ramdon • STUDY ON PULSE CURRENT OF TYPICAL PD MODELS IN POWER TRANSFORMER-- Guoli Wang, Yanpeng Hao, Yanming Li • EXPERIENCE WITH OPTICAL PARTIAL DISCHARGE DETECTION--Michael Muhr, Robert Schwarz • Fiber Optic Sensor for Acoustic Detection of Partial Discharges in Oil-Paper Insulated Electrical Systems--Julio Posada-Roman, Jose A. Garcia-Souto and Jesus Rubio-Serrano