Showing papers on "Partial discharge published in 1988"

Diagnostic measurements at UHF in gas insulated substations

Abstract: The presence of free metallic particles and some other conditions which might lead to breakdown in 420 kV gas insulated substations (GIS) can be detected by measuring their partial discharge activity. The discharge pulses excite the GIS chambers into many modes of resonance at ultra high frequency, which may be picked up by couplers built on to hatch cover plates. Signals from the couplers can be displayed in both the time and frequency domains. Because the oscillations spread throughout the GIS, only a few couplers are needed to cover a complete substation. The method has excellent sensitivity; for example, particles smaller than those likely to cause breakdown can be detected from many tens of metres. Moreover, the nature of the discharging source can be determined, and its position located with good accuracy by timing the wavefronts arriving at couplers on either side of it. Tests using various discharge sources have been made both in the laboratory and on site, and the results are reported. This technique is intended in the first instance for use during GIS commissioning tests. However, simpler equipment operating on the same principle has been developed to monitor continuously the partial discharge level of the GIS in service.

Partial discharge measurement and interpretation

Abstract: An overview is given of partial discharge patterns, their evaluation, and determination of their origin. Some brief comparisons of test equipment considered to be of general interest are made. The intent is to assist practitioners in the interpretation of results as well as in the accurate discrimination between true partial discharges within an insulating structure and other unwanted conduction or radiation-induced interference signals. >

The ability of diagnostic tests to estimate the remaining life of stator insulation

Abstract: Stator windings from three generators and one motor were subjected to a wide variety of diagnostic tests. Diagnostic tests included insulation resistance, polarization index, capacitance, dissipation factor tip-up, partial discharge magnitude, and discharge inception voltage. The destructive tests included breaking down individual coils with either AC, DC, or impulse voltages. Since the results of the diagnostic tests did not correlate with the breakdown voltages, it is concluded that remaining life cannot be predicted on the basis of diagnostic tests alone. Equations and other relationships recently developed in Japan to predict the remaining life of the stator groundwall insulation systems could not be confirmed. Diagnostic tests are thought to be most useful for indicating the trend in insulation aging in a particular machine. >

Experience with PDA diagnostic testing on hydraulic generators

Abstract: This paper describes the capability of the partial discharge analyzer (PDA) test to distinguish between the various operative mechanisms of stator winding insulation deterioration (if any). The ability to distinguish between the various mechanisms yields significant benefits in scheduling priorities for inspection, repairs, and possible locations of deterioration. Two normalized quantity number charts are presented: one for polyester and epoxy stator insulation systems, and one for asphalt-mica and mica folium stator insulation systems. Their stated insulation damage relationships are based on Ontario Hydro's operating experience over the past 15 years. >

Threshold voltage of luminescence and electrical tree inception in low-density polyethylene

Abstract: The characteristics of luminescence observed during the initiation of an electrical tree have been studied for polyethylene subjected to high electrical stresses. Use of a light detection system which is at least three orders‐of‐magnitude more sensitive than conventional partial discharge detectors shows that luminescence occurs during tree initiation and this luminescence is not due to partial discharges. Applying a constant high voltage to the polymer or interrupting the voltage several times does not change the behavior of luminescence. The light starts at about the same voltage and its intensity returns to the same value as before the interruption. This behavior is contrary to that observed for partial discharges. The luminescence inception voltage is the threshold voltage at which the polymer starts to degrade. Thus, it is important that the insulation of electrical equipment be protected from transient overvoltages which exceed the threshold level.

Method of and apparatus for assessing insulation conditions

Abstract: A method of and apparatus for the detection of insulation deterioration in an electrical power supply system utilizes the detection of a progressive wave generated by a corona discharge or partial discharge due to such insulation deterioration. Sensing of said progressive wave may be effected by an apparatus comprising: a first coil having two primary windings, wound with the same number of turns and in the same direction, and a secondary winding, the primary and secondary windings being wound on a core whose magnetomotive force and magnetic flux density are roughly proportional to each other; a second coil composed of a primary winding and a secondary winding both wound on a core whose magnetomotive force and magnetic flux density are roughly proportional to each other; and an impedance circuit. Determination of the position of insulation deterioration may be effected by comparing the phase of the progressive wave at a reference point in a common bus with the phase of the progressive wave in each of the power supply lines branching off from the common bus.

The study of acoustic emission method for detection of partial discharge in power transformer

Abstract: Six kinds of typical oil gaps were designed and tested to simulate partial discharges (PD) in the insulation of power transformers. The frequency spectra of acoustic emission (AE) of PD in those gaps were presented and investigated with a computer-aided measuring system. The attenuation of AE and the variation of its frequency spectrum in oil and in oil-impregnated paper were studied. Peak frequencies in the frequency spectra of AE of PD in these gaps were distributed in the 70-150-kHz range. Based on test results, the frequency bandwidth of 70-180 kHz was recommended for the AE measuring system of PD detection in power transformers. >

Method for partial discharge detection and breaking spark measuring in dynamo-electric high-voltage machines, and an apparatus for performing the method

Abstract: A method for partial discharge detection and breaking spark measurement in a dynamo-electric high-voltage machine, especially for monitoring insulation status during operation of a stator winding and connected electrical output lines in a turbo-generator. The generator includes a stator having a stator winding and a stator housing, a rotor with an exciter winding and a shaft, defining an air gap between the rotor and the stator. At least one oil lubricated slide bearing supports the shaft of the rotor with a bearing shell being electrically insulated from the ground. The method includes decoupling the frequency spectrum of high-frequency electromagnetic oscillations resulting from the occurrence of partial discharges or breaking sparks and delivering the frequency spectrum of high-frequency electromagnetic oscillations to a partial discharge measurement apparatus. The frequency spectrum of high-frequency electromagnetic oscillations introduced into the machine shaft through the air gap in the event of partial discharges inside the stator winding or the output lines are decoupled while utilizing the machine shaft insulated by a film of lubricant as a antenna. Then the frequency spectrum of high-frequency electromagnetic oscillations are delivered to the partial discharge measurement apparatus by using the bearing shell of the oil lubricated slide bearing as transmission element.

Flow-enhanced corona discharge: the corona torch

Abstract: An experimental investigation has been conducted to enhance and control corona discharge in two small diameter cylindrical hollow electrodes by an external gas flow. The results show that the time‐averaged corona current for fixed discharge voltages increases with increasing gas flowrate of pure nitrogen, and pressure drop is not significantly influenced by corona discharge currents.

Discharge measurements in cables using a solid state 30 kV bipolar low frequency generator

Abstract: A design of a 30 kV, 10 mA VLF power supply has been tested which uses solid state components to produce a device which is low noise, portable and suitable for partial discharge testing of high voltage cables up to 33 kV. The device is designed for use in testing installed high voltage cables. The power supply is sufficiently quiet that wide-band partial discharge measurements on polymeric cables may be performed with sensitivities of a few picoCoulombs. No account has been taken of any external noise. The high voltage current stacks can be controlled to operate at any frequency which the HV rails can supply current. In practice this means DC-10 Hz is the useful operating range for the power supply, depending on cable capacitance. Typical frequencies for 1000 m of installed cable would be in the range 0.1-0.5 Hz. >

Partial discharge, microcracking, and breakdown in BaTiO3 ceramics

Abstract: Pure barium titanate ceramics was subjected to partial discharge and dielectric breakdown test. Local field enhancement at the pores lowered the partial discharge inception field and breakdown strength. After applying 50% of the breakdown voltage, microcracks were formed by the shear stress set up at blocked ferroelectric twin bands. When the discharge current at the pores under high voltage exceeds a critical value, it results in immediate thermal destroy of BaTiO3 ceramics. Morphology of the breakdown channel in barium titanate ceramics was observed.

Fast measurements of partial discharge currents in solid dielectric samples containing voids

Abstract: The mechanism of partial discharges inside a void is investigated using a uniform-field electrode system. To allow fast measurements of the partial discharge current, one of the electrodes is split into a central measuring disk and surrounding ring which acts as a coupling capacitor. The minimum permissible diameter of the disk depends on the gap width (Ramo-Schockley effect). A 400-MHz Tektronix oscilloscope 7844 or a digitizer (7912AD) is used, allowing measurements with rise times down to about 1 ns. Fast current measurements in polyethene sheets at 50-Hz AC voltages are reported. The rise time (10-90%) is about 1 to 1.5 ns. The measured charge does not differ greatly from theoretical calculations of the apparent charge (67-76 pC range as opposed to a calculated value of about 66 pC). This indicates that the entire void area discharge and not just part of it. >

Partial discharge energy measurements on insulation systems and equipments

Abstract: Equipment for partial-discharge energy measurement is described. The versatility of the computer interface and the various uses of the equipment (energy, apparent charge, integrated charge, quadratic rate, number of discharges) are pointed out. Measurements on insulation systems show that there is a good correlation between life curve and the evolution of discharge power vs. applied voltage. The slope of the life curve can be deduced from the log P vs. log V plot slope. The ability of the equipment to operate in an industrial environment is verified. >

On testing methodology of thermosetting dielectrics subjected to partial discharges: effect of temperature

Abstract: Following the idea of performing tests in environmental conditions as similar as possible to the real working ones, many sets of partial discharge tests carried out on an epoxy resin within a broad environmental temperature range are reported on. Life tests were carried out on specimens with a calibrated, enclosed cavity and with embedded electrodes. In the tests performed at temperatures higher than ambient, an interesting strong decrease of data scattering resulted and emphasis on the subject is given. Actually, a strong and unexpected effect was found of the residual mechanical casting stress inside the resin on time-to-breakdown, depending on the difference between the material glass transition temperature and the temperature used in the test. Further data such as discharge amplitude distributions, discharge repetition rate and inception and extinction voltages at the investigated temperatures are also reported and commented upon. >

Partial discharge generating device

Abstract: PURPOSE:To freely vary the electrostatic capacity between 1st and 3rd electrodes by stacking plural electrode plates at specific intervals with the 1st electrode and making them conductive to one another, and providing the 3rd electrode which is moved. CONSTITUTION:When the 3rd electrode 13 exceeds a partial discharge start voltage Vi in a partial discharge generating circuit owing to a voltage applied between a metallic tank 10 and a conductor 12, the insulation of a gap 4 is broken down and partial discharge is caused between the 4th electrode 15 and metallic tank 10. When the extent of the partial discharge is varied, a ferromagnetic body 19 is attracted 19 by operating an opening/closing tool 21 arranged outside the metallic tank 10 and then moving the opening/closing tool 21 along the surface of the metallic tank 10, thereby moving the electrode plate 14. Thus, the 3rd electrode 13 is moved to vary the area of the 3rd electrode 13 facing the metallic tank 10 and vary the electrostatic capacity without discharging gaseous SF6 11 from the metallic tank 10, thereby varying the extent of the partial discharge.

Infrared thermography of negative DC and negatively enhanced AC point-to-plane corona discharge in air

Abstract: Corona discharge for point-to-plane gaps in air was investigated with emphasis given to thermographic analysis of the air gap using an infrared scanning system. Corona discharge was generated under two conditions: using negative voltage on the point and a grounded plane, and using 60 Hz AC voltage on the point and positive DC voltage on the plane (negative enhancement of AC corona). Changes in the infrared images of the air gap were observed as the corona discharge current increased. These images showed the region of the gap with the elevated apparent temperature indicating a source of infrared radiation in the gap. The relationship between the pattern in the air gap and corona discharge current was studied. It was observed that the intensity of the infrared signal generated in the gap increased with the discharge current. Temperature distribution on the anode surface was measured. In the case of the negatively enhanced AC corona, fluctuation of the region with elevated apparent temperature was observed. >

Element and device for detecting internal faults in an insulating gas charged electrical apparatus

Abstract: An element for detecting internal faults in an insulating gas charged electrical apparatus comprises a substrate disposed in the electrical apparatus charged with an insulating gas, a pair of electrodes disposed on the substrate, and a thin metal film covering the pair of electrodes and the exposed surface of the substrate. This film is capable of producing fluorides with low conductivity upon reacting with a decomposed gas produced by internal faults of the electrical apparatus. Since the thin metal film exhibits high response characteristics readily in reaction with even a trace amount of a decomposed gas, it is possible to promptly detect faults occurring in the electrical apparatus, such as a partial discharge or local heating. In addition, a device for detecting internal faults in an insulating gas charge electrical apparatus using the internal fault detection element utilizes an optical signal for transmitting signals. Accordingly, the device is free from electromagnetically induced interference and is capable of remote monitoring because of the low level of loss in signal transmission.

An experimental study of the damaging effects of harmonics in power networks on the capacitor dielectrics

Abstract: The damage in power capacitor dielectrics by harmonics in power networks is investigated. Five typical distorted waves generated by an HV (high-voltage) harmonic testing installation were recorded in tests of four model types of capacitors. Results show that all waves containing harmonics of different order have damaging effects on the capacitor dielectric. The third harmonic which makes the voltage shape sharp at its top has the most serious effect on the partial discharge and voltage-time characteristics of capacitor dielectrics. >

Interaction of partial discharge streamers in cryogenic fluids with polymeric films

Abstract: Erosion by partial discharge and the layer effect on breakdown characteristics of polymeric films immersed in cryogenic fluids are examined. Meshlike patterns were observed on the polymeric film eroded by partial discharges in liquid helium, which may lead to poor life characteristics. The layer effect depends on the waveform and the magnitude of the applied voltage. This phenomenon is discussed in terms of the discharge characteristics of cryogenic liquid between film layers. >

The design and application of a novel high-speed partial discharge diagnostic system

Abstract: A high-speed partial discharge diagnostic system that can monitor transient and fast-risetime (on the order of several-nanosecond) partial discharge pulses in capacitor insulation with good sensitivity and fidelity has been developed. This system is referred to as the fast discharge detector (FDD). The key performance specifications include a detection sensitivity of 6+or-2 pC, minimum risetime of 4+or-1 ns, and temporal resolution of less than 0.1 mu s. The sensitivity is maintained for insulation test specimens with capacitances as large as approximately 10 nF, which is considerably greater than the limits of most conventional apparatus. Some representative test data concerning the characteristics of high-speed partial discharge phenomena in mica capacitors are presented to illustrate the diagnostic capabilities of the FDD. Several research applications for the FDD that could yield information about partial discharge mechanisms and their effects on the insulation system of modern power capacitors are stated. >

Simultane Erfassung und Verarbeitung von Teilentladungs-Kenngrößen zur Beurteilung elektrischer Isolierungen / Simultaneous registration and processing of partial discharge signals for the evaluation of electrical insulations

Abstract: Die TE-Messung hat als Mittel zur Beurteilung elektrischer Isolierungen in den letzten Jahren immer mehr an Bedeutung gewonnen. Zur besseren Deutung der im Inneren der Isolierung ablaufenden und im allgemeinen schädlichen TE-Prozesse ist man vor allem bei Grundlagenuntersuchungen bestrebt, möglichst viele Informationen aus den Messungen zu erhalten. So werden neben üblichen Meßgrößen, wie scheinbare Ladung, auch TE-Energie bzw. daraus abgeleitete Größen, wie TE-Strom und -Leistung, zur Bewertung herangezogen. Eine Erhöhung der Aussagekraft von TE-Messungen wird erwartet, wenn möglichst viele TEKenngrößen unter Berücksichtigung ihres stochastischen Charakters zeitgleich erfaßt und vielseitig bewertet werden.

Device for determining internal malfunctions in electrical equipment

Abstract: The device for determining internal malfunctions in electrical equipment determines the occurrence of a partial discharge in the equipment to be tested, by picking up an electrical signal induced on an electrically conducting sleeve (1) of the electrical equipment. Upon determining a partial discharge, the device begins sampling the output signal of a vibration detector (4) for the determination of mechanical vibrations which are caused in the electrical equipment by the partial discharge. In this way a precise determination is made of a partial discharge, independently of the cycle period of the voltage fed to the equipment to be tested.

A comparative evaluation of partial discharge inception voltage of power transformer liquids

Abstract: Partial discharge inception voltage (PDIV) tests on different oils used in power transformers were carried out using a multichannel analyzer (MCA). The results are presented in terms of the partial discharge amplitude distributions from which it is possible to evaluate the PDIV for any intensity threshold. A more sensitive, comparative evaluation of the partial discharge behavior of different transformer oils is therefore possible. >

Computer-aided partial discharge amplitude distribution measurement system for large electrical machines diagnosis

Abstract: The authors describe a distributed intelligent measurement system for partial discharge amplitude distribution (PDAD) detection for large rotating machinery, such as power generators. The equipment design is based on a modular adaptive technique, which allows the flexibility required for applications to different types of generators. The partial discharge acquisition and computing system is based on two main units. The remote unit is located close to the system to be tested and is designed to supervise the dynamic conditioning process as well as the acquisition and fast preprocessing of the probe signals. The second unit has no specific location and is linked with the first one by means of a fiber-optic data link. The second unit mainly consists of a PC (personal computer) having properly designed software tools to assist acquisition and perform real-time information analysis and diagnostics. This measurement system was used to perform tests on a nonloaded hydrogenerator equipped with a directional coupler. Experimental results are presented. >

Prediction of internal faults of GIS by frequency spectrum of typical vibration on the enclosure

Abstract: The authors report several typical frequency characteristics of vibrations of the metallic enclosure of a GIS (gas-insulated substation), especially the characteristics of vibration caused by free conducting particles and partial discharge under alternating voltage. An examination is made of contaminants under different SF/sub 6/ gas pressures, the shapes of the electrodes, and their electrical strength. An inspection system which can predict internal latent faults of GIS from the vibration frequency spectrum is described. >

Circuit arrangement for acoustic partial-discharge detection

Abstract: The invention relates to a circuit arrangement for acoustic partial-discharge detection on the insulation of electrical equipment, especially on solid-material and gas-insulated systems, cable terminations and voltage converters during normal operation. The aim of the invention is to reduce the influence of radiated interference fields on acoustic partial-discharge detection. The object of the invention is the detection of partial-discharges in the insulation of electrical equipment externally on the earthed parts of the equipment during normal operation. The object is achieved according to the invention in that a main amplifier, a high-pass filter and, after it, a secondary amplifier with a demodulator, are connected downstream of a piezoelectric vibration sensor, which is in a housing together with a preamplifier, and in that the noise level which is made visible and/or audible represents a measure of the partial discharges in the insulation of the measured electrical equipment.

Computer aided partial discharge evaluation about the surface material of the PD-source in gas-insulated substations

Abstract: A PD (partial-discharge) diagnosis system is presented that gives information about the location of any detectable PD source in a GIS (gas-insulated-substation) testing unit. For every detected impulse the system measures and processes the apparent charge, impulse duration, phase angle, test voltage V/sub pp/, actual test voltage, impulse energy, and cycle number. A high-speed (60-MHz bandwidth) electronic system has been developed for online measuring and digital conversion. The data are transmitted to a personal computer by means of an electro-fiber-optic interface unit and analyzed by a statistical algorithm. This procedure runs automatically and provides information about the surface materials on which the partial discharges are active. Two types of defects representing sources of partial discharging have been implemented in gas-insulated switchgear. Some of the results are presented and discussed, showing the potential of computer-aided partial discharge evaluation. >

Partial discharge measurements for testing of low-voltage equipment

Abstract: The application of PD (partial discharge) measurements to solid insulation within electronic components and assemblies in the low voltage range (e.g. optocouplers, small transformers, and printed wiring boards with coating) for the verification of insulation integrity, is reported. Such measurements are of special interest if such insulation is used to reduce electrical separation. In order to simulate the aging process during the normal life of equipment, different aging procedures were applied. The results of PD measurements after aging are compared with the data obtained before. >