R. T. Harrold

Radio Frequency Sensing of Incipient Arcing Faults within Large Turbine Generators

Sensing of incipient faults within large turbine generators (> 500 MW) is very important because a persistent minor abnormality may suddenly develop into a major fault and cause extensive damage before conventional early warning sensors can respond. Consequently, there is a need for a reliable sensing system which will respond to the early arcing stage of a fault, when insulation surfaces are not at a sufficiently high temperature to emit particulates and activate the conventional Core or Generator Condition Monitor.

Acoustic Waveguides for Sensing and Locating Electrical Discharges in High Voltage Power Transformers and Other Apparatus

For safety and convenience, ultrasonic corona sensors are mounted on the outer surface of power transformer tanks, and consequently, can only receive ultrasound from internal electrical discharges after attenuation through the oil and tank wall. In order to reduce this attenuation and maximize the detected signal, it was thought that low-loss acoustic waveguides would be helpful as a means to transmit the ultrasonic emissions from internal discharges to an external sensor. To assess the feasibility of this idea, the attenuation and frequency characteristics of several forms of guides were determined by measuring the acoustic emissions from sparks in air and oil.

On Line Incipient ARC Detection in Large Turbine Generator Stator Winding

Early detection of developing malfunctions within large generator stator windings, of the type due to conductor fatigue failure, can result in a significant improvement in unit availability. This paper describes a radio frequency (RF) method of on-line monitoring which detects the arcing associated with conductor strands which have fractured from copper fatigue.

Ultrasonic Sensing of Partial Discharges within Microfarad Value AC Capacitors

As new dielectric fluids and materials are introduced, partial discharge (P.D.) testing of microfarad value capacitors in the factory is assuming greater importance. However, the large capacitance restricLs the use of the common electrical methods for detecting small (1-20 pC) discharges, and bridge techniques are not economical for production line tests. For these reasons, the use of ultrasonic sensors for detecting P.D. within microfarad value capacitors was studied, and this resulted in the development of a combined acoustic waveguide probe, ultrasonic sensor and indicating meter, suitable for factory P.D. testing of capacitors.

The Relationship Between the Picocoolomb and Microvolt for Corona Measurements on HV Transformers and Other Apparatus

Historically in North America, partial discharge or corona measurements in MV have been used for transformer quality control and customer acceptance, but in the future it is anticipated the pC unit may also be used.

The Spectrum Analyzer Applied to the Measurement of EHV Power Line Radio Noise

In order to reduce time and improve the quality of measurements* of EHV power line radio noise, the use of spectrum analyzers and X-Y recording techniques with radio noise meters was investigated. Comparisons were made with the usual manual readings taken simultaneously from radio noise meters used as the standard measuring instruments. With the spectrum analyzer, measurements of radio noise from conductor corona both in rain and fair weather conditions were made in the 1 MHz frequency region. In addition, the interference from a gap-type source was successfully measured over a frequency range from 10 kHz to 1 GHz.

Corona Location in Transformers by Radio Frequency Spectrum Analysis, Part I: Theory

After discussing transformer types, ?values, and location of voltage generators in capacitive ladder networks and equivalent transmission lines, a new electrical technique for locating transformer partial discharges or corona is described. It is based on the measurement of the radio frequency spectra at the transformer terminals. Provided the transforner winding can be approximated to a uniform transmission line for the frequency range considered, the method is shown to be theoretically sound. Measurements on model networks and actual transformers suggest that, at least over certain frequency ranges, the winding can be regarded as a uniform transmission line. Apart from locating corona, the method provides a quick and accurate means of measuring actual transformer ?values.

On predicting the life of polyethylene spacer-cable eroded by surface discharges in wet weather

As part of a study to evaluate the feasibility of uprating existing 15 kV spacer-cable systems to 34.5 kV, a method was developed for predicting the life of the polyethylene cable which may be eroded by electrical surface discharges in wet weather. The prediction method considers the changes in spacer surface resistance and the resulting electrical stress on the polyethylene throughout a typical year of varied surface conductivity during rain and fog, together with a measure of the quantity (Q) of surface discharges per power frequency cycle, which is related to the electrical stress. It is believed that the measurements and procedure outlined form a reasonable basis for predicting polyethylene spacer-cable life in wet weather, and for large value surface discharges the life appears to be inversely proportional to the initial Q2.

Corona Location in Transformers by Radio Frequency Spectrum Analysis II. Application of Technique and Results of Measurements

This paper gives a detailed account of the practical application of the technique of corona location in transformers by radio frequency spectrum analysis. The measurements are similar to the usual NEMA corona measurements taken at the bushing tap with a radio noise meter, except an X-Y recorder is used to plot the corona frequency spectrum and a current probe used at the neutral. The theory outlined in Part I is verified by extensive measurements taken from a model transformer using a pulse generator to represent a corona source, and by computer analysis of simulated transformer networks and by practical results.

The Interpretation of Corona and Dielectric Measurements on the Mica-Asphalt Insulation of a 30-Year-Old WaterWheel Generator at Grand Coulee Dam

It is important to have reliable field techniques for assessing the insulation quaility of large rotating machine stators in order to reveal any defects which could lead to a winding failure in service. Toward this objective, a variety of nondestructive measurements of internal discharges (corona) have been applied and compared on an old mica-asphalt insulated generator. Extensive field measurements of RIV corona and integrated corona charge per cycle (related to power factor tip-up) are reported and interpreted, and verified by tests on coils returned to the laboratory.