Thermal measuring and testing – Leak or flaw detection
Reexamination Certificate
1998-12-28
2001-05-15
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Leak or flaw detection
C374S141000, C324S424000, C340S638000
Reexamination Certificate
active
06231227
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to electronic trip units. More specifically, the present invention relates to a method of determining contact wear of a circuit breaker at an electronic trip unit.
Electronic trip units (trip units) are well known. An electronic trip unit typically comprises voltage and current sensors which provide analog signals indicative of the power line signals. The analog signals are converted by an A/D (analog/digital) converter to digital signals which are processed by a microcontroller. The trip unit further includes RAM (random access memory), ROM (read only memory) and EEPROM (electronic erasable programmable read only memory) all of which interface with the microcontroller. The ROM includes trip unit application code, e.g., main functionality firmware, including initializing parameters, and boot code. The EEPROM includes operational parameters for the application code. An output of the electronic trip unit actuates a circuit breaker. The circuit breaker typically includes a pair of contacts which allows circuit current to pass from one contact member to another contact member. When the contacts open, circuit current is prevented from flowing from one contact member to the other and therefore, circuit current is prevented from flowing to a load which is connected to the breaker.
Circuit breaker contact wear, is a frequently occurring yet difficult to measure or predict problem because it is affected by a variety of factors. Contact wear is affected by the cumulative energy dissipated through arcing as breakers are opened. However, a single severe over-current fault can destroy contacts more quickly than several smaller faults, even though the smaller faults may add up to the same total energy dissipated. For example, some types of faults have more severe effects on contact wear than others, ground faults will destroy contacts more quickly than manual openings. Contacts are not generally easily inspected without costly disassembly and power down. However, if not detected, contact wear may result in loss of power. The only current solution to this is defensive preventative maintenance whether required or not.
BRIEF SUMMARY OF THE INVENTION
It is therefore seen to be desireable to detect contact wear in an electronic trip unit. In a preferred embodiment of the present invention, a contact wear detection algorithm (program) is initialized in the microcontroller of the trip unit for detecting contact wear. The contact wear detection algorithm (1) measures temperatures of arcs in close proximity to circuit breaker contacts, and/or (2) calculates and stores cumulative energy dissipated in the breaker contacts as a result of open and close operations. A variety of analysis techniques are utilized within the trip unit to determine contact wear. An accurate assessment of contact wear is yielded by these methods, separately or in combination.
The electronic trip unit of the present invention comprising voltage, current, and temperature sensors which provide analog signals indicative of the power line signals, contact temperatures, and ambient temperatures. The analog signals are converted by an A/D (analog/digital) converter to digital signals which are processed by a microcontroller. The trip unit further includes RAM (random access memory), ROM (read only memory) and EEPROM (electronic erasable programmable read only memory) all of which communicate with the microcontroller. The ROM includes trip unit application code, e.g., main functionality firmware, including initializing parameters, and boot code. The application code includes code for the contact wear detection algorithm of the present invention. The EEPROM includes operational parameters, e.g., code for setting user defined thresholds for the contact wear detection algorithm for the application code. These parameters may be stored in the trip unit at the factory and are selected to meet customers'requirements, but can also be remotely downloaded.
Temperature and electrical analysis is used to develop thermodynamic and electrical models of frame geometries of circuit breakers. These models provide the contact wear algorithm with the nominal operating parameters required to predict contact resistance and heat rise over ambient temperatures as a function of current flow through the breakers as the contacts wear. Alarms can be generated when (1) contact heat rise over ambient temperature deviates from stored nominal values, or (2) when calculated contact resistance (R=V/I phase corrected) deviates from stored specified maximum values. Thereby indicating that maintenance or replacement of the breaker is required due to contact wear.
The frame geometry of a circuit breakers may affect the rate at which heat is thermodynamically conducted away from the circuit breaker contacts and are modeled or experimentally determined for each model of breaker at rated current ranges. As contact wear resistance increases the temperature across the contacts during closed operation of the circuit breaker will increase with the contacts acting as electrical resistors dissipating electric energy as heat. This in turn has an accelerating effect on the rate of wear of the contacts. If undetected this will eventually lead to the mechanical and/or electrical failure of the breakers leading to a power outage.
The above discussed and other features and additional advantages of the present invention will be appreciated and understood by those skilled in the art from the detailed description and drawings.
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patent: 5216623 (1993-06-01), Barrett et al.
patent: 5270658 (1993-12-01), Epstein
patent: 5697705 (1997-12-01), Callewaert
patent: 5883568 (1999-03-01), Boyden
patent: 5941370 (1999-08-01), Nichols
patent: 6023036 (2000-02-01), Nichols
patent: 17326 (1983-02-01), None
patent: WO 98/11573 (1998-03-01), None
Cantor & Colburn LLP
General Electric Company
Gutierrez Diego
Horton Carl B.
Pruchnic Jr. Stanley J.
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