Significance and Use
These test methods are useful in research and quality control for evaluating insulating materials and systems since they provide for the measurement of charge transfer and energy loss due to partial discharges (4) (5) (6).
Pulse measurements of partial discharges indicate the magnitude of individual discharges. However, if there are numerous discharges per cycle it may be important to know their charge sum, since this sum can be related to the total volume of internal gas spaces that are discharging, if it is assumed that the gas cavities are simple capacitances in series with the capacitances of the solid dielectrics (7) (8).
Internal (cavity-type) discharges are mainly of the pulse (spark-type) with rapid rise times or the pseudoglow-type with long rise times, depending upon the discharge governing parameters existing within the cavity. If the rise times of the pseudoglow discharges are too long , they will evade detection by pulse detectors as covered in Test Method D 1868. However, both the pseudoglow discharges irrespective of the length of their rise time as well as pulseless glow can be readily measured either by Method A or B of Test Methods D 3382.
Pseudoglow discharges have been observed to occur in air, particularly when a partially conducting surface is involved. Such partially conducting surfaces may develop with polymers that are exposed to partial discharges for sufficiently long periods to accumulate acidic degradation products. Also in some applications, like turbogenerators, where a low molecular weight gas such as hydrogen is used as a coolant, pseudoglow discharges may develop.
1.1 These test methods cover two bridge techniques for measuring the energy and integrated charge of pulse and pseudoglow partial discharges:
1.2 Test Method A makes use of capacitance and loss characteristics such as measured by the transformer ratio-arm bridge or the high-voltage Schering bridge (Test Methods D 150). Test Method A can be used to obtain the integrated charge transfer and energy loss due to partial discharges in a dielectric from the measured increase in capacitance and tan with voltage. (See also IEEE 286 and IEEE 1434)
1.3 Test Method B makes use of a somewhat different bridge circuit, identified as a charge-voltage-trace (parallelogram) technique, which indicates directly on an oscilloscope the integrated charge transfer and the magnitude of the energy loss due to partial discharges.
1.4 Both test methods are intended to supplement the measurement and detection of pulse-type partial discharges as covered by Test Method D 1868, by measuring the sum of both pulse and pseudoglow discharges per cycle in terms of their charge and energy.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precaution statements are given in Section 7.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
D150 Test Methods for AC Loss Characteristics and Permittivity (Dielectric Constant) of Solid Electrical Insulation
D1711 Terminology Relating to Electrical Insulation
D1868 Test Method for Detection and Measurement of Partial Discharge (Corona) Pulses in Evaluation of Insulation Systems
IEEEStandardC57.124 Recommended Practice for the Detection of PD and the Measurement of Apparent Charge in Dry-Type Transformers
AEICCS5-87 Specifications for Thermoplastic and Crosslinked Polyethylene Insulated Shielded Power Cables Rated 5 through 35 kV, 9th Edition, 1987
bridge circuits; bridge techniques; capacitance increase; charge-voltage-trace bridge; corona; discharge energy; discharge inception level; energy; harmonics; integrated charge transfer; internal discharges; ionizing voltage; loop trace; partial discharges; pseudoglow discharge; pulse discharge; pulseless-glow discharge; pulse measurements; solid insulating materials; surface discharge; transformer-ratio-arm bridge;
ICS Number Code 17.220.20 (Measurement of electrical and magnetic quantities)
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