Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2002-07-10
2004-08-10
Hirshfeld, Andrew H. (Department: 2854)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S535000, C324S536000
Reexamination Certificate
active
06774639
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to a system for monitoring the performance of electrical equipment, such as high voltage transformers. In particular, the system can detect the occurrence of faults in the overall insulation of such transformers and provide signals that trigger local and/or remote alarms indicative of the faults.
BACKGROUND ART
High voltage generator and transmission transformers form an integral part of any electrical power generation distribution and transmission system. Other transformers, such as rectifier transformers are, also used in industrial processes, such as smelting electro-deposition processes. Also, current transformers (CTs) are used for protection and metering of electricity distribution systems.
The most important part of the insulation for oil filled transformers comprises paper which is wound around the copper windings. There are spacers, washers, seals, lead through plates, taps and bushings, which are also part of the insulation system within the transformer. In order to enhance the insulation and stability, the paper is permeated with a dielectric, typically mineral oil or silicone oil, which fills the transformer. This insulating oil also serves as a coolant, distributing heat by convection or forced flow, and also quenches discharges. Other types of transformers include high frequency communication transformers which use solid polymeric dielectrics such as epoxy thermoset, which is vacuum back-filled into the transformer, and gas-filled transformers. Gas-filled transformers, for example those used in underground mines, are usually filled with argon or sulfur hexafluoride for safety. There are also some low voltage air filled transformers.
The operating lifetime of a high voltage transformer can be greater than 35 years. The lifetime depends on the loading, design, quality of manufacture, and materials and maintenance routines. During its lifetime, the transformer insulation can degrade, the rate of degradation being dependent upon the workload and the internal operating environment of the transformer, such as temperature, moisture content, pH and the like. Any degradation of the insulation, such as electronic and ionic plasma erosion of solid insulation surrounding an air bubble occluded due to faulty manufacture, can result in increasing levels of partial discharge within the transformer. Occurrence of partial discharges also leads to evolution of gases such as hydrogen and acetylene within the transformer. Such increased partial discharge leads to further degradation of the insulation which in turn leads to increasable levels of partial discharge. Continued degradation of the insulation can result in severe discharges, short-circuit faults or a catastrophic failure due to an explosion of the gases, for example, hydrogen, acetylene and ethylene, produced as chemical by-products of the degradation process. Such failure can result in reduction or loss of supply to the power system, incur considerable expense for the replacement or repair of the transformer and also present a serious risk to nearby personnel and the environment.
Partial discharge in transformers can also occur due to faulty manufacture and/or mechanical or electrical fatigue. For example, the movement of loose components, and creep and stress relaxation of metallic components, such as fastenings, or foreign metallic bodies within the transformer, provide an opportunity for discharges to occur even when there has been no or little degradation of the insulation.
Partial discharge in transformers can also arise due to windings becoming loose within the transformer. Wear and tear suffered by the tap connectors in the tap changer can also cause partial discharges. Faults in the bushings can also result in partial discharges.
It is known that a partial discharge can produce signals at different locations within a large transformer including a discharge current in neutral caused by imbalance, a displacement current through the capacitive tapping of a bushing, a radiated radio frequency (RF) pulse or wave and a radiated ultrasonic (US) pulse or wave.
The magnitude of partial discharge within a transformer provides one means of determining the integrity of the transformer's insulation. For example, a detected partial discharge having a magnitude of 50 pC would normally be ignored at normal voltage operations, a reading of 500 pC would be viewed with some concern, whilst a reading of 5000 pC would be considered potentially dangerous.
Power authorities typically test transformers by sampling the mineral oil within the transformer about once a year to determine the oil's dissolved gas concentration by analysis (DGA) and dielectric loss angle (DLA). If high gas readings are obtained, the frequency of sampling is increased to monthly and even weekly. However, there is always some delay between the sampling and the analysis in the laboratory. Rapid deterioration of insulation may not be detected and transformers have failed catastrophically even when DGA sampling has been carried out. Since it is known that partial discharges of a higher magnitude and/or repetition rate develop shortly before a major failure, continuous monitoring of electrical equipment, while it is kept on-line, to provide early warning, is very desirable.
Partial discharge can be measured using instruments such as Robinson, Haefly or Tettex partial discharge detectors, which detect high frequency electrical (RF) signals only, by coupling to the lower part of the bushing on the transformer or to the windings using capacitor dividers and a toroid system. These instruments are normally used in a test bay during high voltage proving tests for a new or re-wound transformer. These measurements can, however, normally not be undertaken in a substation location due to the high level of electrical interference. Making reliable readings with these instruments also requires considerable skill.
One device for detecting the occurrence of a single partial discharge event in a transformer is described in International Application No PCT/AU94/00263 (WO 94/28566). This device comprised an ultrasonic transducer and a radio frequency antenna that were mounted in the transformer wall and adapted respectively to detect the ultrasonic and radio frequency pulses generated by a partial discharge. If a radio frequency signal was detected within a pre-set time period before detection of an ultrasonic signal, a partial discharge was assumed to have occurred. While able to detect such signals, one problem with the device described in WO 94/28566 was that electrical noise within the transformer would generate randomly occurring radio signals that lead to the triggering of false alarms on occurrences of partial discharge. Shutting down a transformer based on a false alarm is clearly undesirable and costly.
DISCLOSURE OF THE INVENTION
According to a first aspect, the present invention is an apparatus for detecting partial discharge in on-line high voltage electrical equipment containing a dielectric, each partial discharge generating a radio frequency pulse or wave and an ultrasonic pulse or wave, the apparatus comprising:
at least one transducer means for detecting the ultrasonic pulse or wave generated by the occurrence of a partial discharge and subsequently outputting a signal corresponding to this detection:
at least one transducer means for detecting the radio frequency pulse or wave generated by the occurrence of a partial discharge and other radio frequency pulses or waves generated within the equipment and subsequently outputting a signal corresponding to this detection; and
a signal processing and analysing means which receives the signals corresponding to the detection of the radio frequency pulse or wave and ultrasonic pulse or wave and which, on receiving a signal corresponding to detection of an ultrasonic pulse or wave, is adapted to:
(a) determine, within a pre-set time period preceding the instance of time of detection of the ultrasonic pulse or wave, the time delay between the instance of detectio
Chau Minh
Hirshfeld Andrew H.
TransGrid
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