Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1995-06-07
2001-01-23
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S551000
Reexamination Certificate
active
06177803
ABSTRACT:
The present invention relates to a multi-phase measuring system to be used for the determination of certain electrical characteristics of electrical power apparatus. As a particular embodiment of the technique, the present description pertains to a method for the evaluation of the quality of high voltage insulation systems. The procedure covers an application for the measurement of changes in capacitance and/or power factor of either transformer bushings or other electrical apparatus in which the line-to-ground insulation may be considered to be part of a capacitance system. The measurements are made while the apparatus is energized during normal service operation.
For background, reference is made to V. Sokolov, On-Line Monitoring System Bushing Condition Evaluation, Part 1: Summary of Bushing Monitoring Experience in the Former USSR, Minutes of the Sixty-Second Annual International Conference of Doble Clients (1995); V. Sokolov & B. Vanin, On-Line Monitoring System Bushing Condition Evaluation, Part 2: Comparison of Power Factor Measurement with Leakage Current Imbalance Measurement, Minutes of the Sixty-Second Annual International Conference of Doble Clients (1995), P. Vujovic and R. K. Fricker, Development of an On-Line Continuous Tan (&dgr;) Monitoring System, Conference Record of the 1994 IEEE International Symposium on Electrical Insulation (1994); and D. M. Allan, K. J. Boyd and D. D. Hinde, A Continuous Insulation Monitor for High Voltage Transformer Bushings, 7th International Symposium on High Voltage Engineering, Technische Universitat Dresden (1991).
According to the invention, a method of monitoring a group of three elements, such as bushings in a three-phase alternating current network connected in parallel to a three-phase voltage supply includes producing a current output from a tap of each bushing and adding the current outputs from the bushing taps vectorially to obtain a vector sum bushing current. Apparatus according to the invention includes three reference devices connected in parallel to the three-phase voltage supply to produce a current output from each reference device, and a vector adder that vectorially adds the current outputs from the reference devices to obtain a vector sum reference current. Differentially combining the vector sum reference current with the vector sum bushing current provides a difference current representative of a bushing condition. The reference devices may be other groups of bushings, or potential transformers, or coupling capacitor voltage transformers (CCVTs), or bushing potential devices, or high voltage current transformers, or optical voltage transducers. If potential transformers or other voltage devices are used as references, they may be loaded with resistors so that the currents through the resistors are substantially equal to the currents derived from the taps of the bushings. Further, the vector sum bushing current may be delayed by ninety degrees before differentially combining the vector sum potential transformer current to correct for a phase difference between the two vector sum currents.
Additionally, successive vector difference currents may be differentially combined to provide a sequence of incremental vector difference currents. That is to say, the group of bushings and reference devices may be periodically scanned. The vector sum bushing currents may be converted into a signal suitable for transmission for remote monitoring. The current outputs of each bushing may be scaled, by electronic gain or through taps in the primary windings of a current transformer. The current outputs from the bushings and the reference devices may, if necessary, be corrected for temperature effects.
Apparatus may include a detector for obtaining the bushing current outputs and a means for combining the bushing current outputs into a vector sum bushing current (resultant) and comparing the vector sum bushing current with a vector sum reference current. The detector may be a portable device. For a plurality of groups of bushings on different voltage buses, a multiplexer may be used to simultaneously scan pairs of bushing groups and reference voltage devices connected in parallel on the same bus. The multiplexer and detector may be located at a remote station. The current outputs and the reference current may be transmitted to the remote station in digital form.
A method of diagnosing the conditions of a group of bushings on a multi-phase alternating current network may include calculating a differential value indicative of a change of capacitance and/or power factor of the bushings, and providing a message to the user if the differential value exceeds a threshold value. The difference between the differential value and the threshold may be determined to be large or small, which may be indicated in the message to the user. The threshold value may be determined from prior stored measurements taken from the bushings. The initial values of the capacitance and power factor of the bushings may be stored. The approximate values of the capacitance and power factors of the bushings may be estimated based on detected changes and the stored initial values of the capacitance and power factor.
An advantage of the present invention is to permit monitoring of bushings in which the capacitance and/or power factor of the bushings change.
Another advantage is to provide an economical monitoring system since only one signal from each group of bushings is monitored and analyzed.
The method and apparatus of the invention may be used to monitor the degradation of high voltage bushing insulation systems during normal operation. The bushings of high voltage power transformers account for a high percentage of transformer problems. When a transformer bushing fails, it may result in the destruction of the entire transformer and other damage. The invention may also be used to monitor the degradation of high voltage current transformer insulation systems. Such transformers are frequently provided with taps in the high voltage insulation structure and, on three-phase networks, the current transformers can be monitored by comparing a set of three transformers against another reference set on the same parallel voltage bus. Alternatively, the first set of current transformers may be compared against a set of transformer bushings or against a set of reference voltage devices.
Periodic diagnostic measurements made on transformer bushings provide evidence of a trend in the measured quantities which would be indicative of deteriorating insulation. The apparatus and method of the present invention allows for on-line monitoring of transformer bushings.
REFERENCES:
patent: 1945263 (1934-01-01), Doble
patent: 2146069 (1939-02-01), Higgins
patent: 3710242 (1973-01-01), Povey
patent: 4757263 (1988-07-01), Cummings et al.
Meyer et al., “On-Line Monitoring of Insulation Systems: Theory, Design and Field Data”.
Square D Company, “InsAlert Insulation Current Monitor”, Clearwater, FL, 1995.
Vujovic et al., “Development of an On-Line Continuous Tan(&sgr;) Monitoring System”, 1994 IEEE International Symposium on Electrical Insulation, Pittsburgh, PA, Jun. 5-8, 1994.
Sokolov “On-Line Monitoring System Bushing Condition Evaluation”, Part 1, 1995.
Sokolov et al. “On-Line Monitoring System Bushing Condition Evaluation”, Part 2, 1995.
Letter from Victor Sokolov to Jerry Jodice who received it May 5, 1995.
Information on KIV application. Is valid from Jan. 8, 1983 to Jan. 8, 1988.
Sokolov et al. “On-Line Monitoring of High Voltage Bushings”, 1995.
Allan et al. “A Continuous Insulation Monitor for High Voltage Transformer Bushings”, 7th International Symposium on High Voltage Engineering, Dresden, Aug. 26-30, 1991.
Allan, Blundell, Boyd and Hinde, New Techniques for Monitoring the Insulation Quality of In-SErvice HV Apparatus, IEEE Transactions on Electrical insulation, V. 27, No. 3, Jun., 1992.
Boylestad, Introductory Circuit Analysis, 5th edition, Merrill Publishing, Columbus, OH 43216, pp. 463-477 (No month), 1987.
Cavigelli George A.
Train David
von Guggenberg Philip A.
Brown Glenn W.
Doble Engineering Company
Fish & Richardson P.C.
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