Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Reexamination Certificate
1999-09-14
2001-05-22
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
C361S002000, C361S102000, C361S042000
Reexamination Certificate
active
06236548
ABSTRACT:
The invention relates to a method of discriminating between an internal arc and a circuit-breaking arc, generally of greater amplitude than the circuit-breaking arc, which can become established in the enclosure of a circuit breaker in a bay of a medium or high voltage metal-clad substation, and to do so by measuring the pressure of the dielectric gas inside the enclosure of the circuit breaker, in which the appearance of an internal arc is detected by a protection system which responds thereto by transmitting a disengagement order to the circuit breaker to cause its contacts to separate, the separation causing a circuit-breaking arc to appear.
BACKGROUND OF THE INVENTION
Such a metal-clad substation is constituted by a plurality of bays which are connected in parallel by a set of feeder busbars, each including in series with the circuit breaker a busbar disconnector (or selector switch disconnector) and an outgoing feeder. Each piece of gear in a bay is enclosed in a gastight enclosure filled with a dielectric gas under pressure for the purpose of maintaining a potential difference with a conductor that passes along the enclosure. A bay is thus constituted by a plurality of compartments constituted by the enclosures of various different pieces of electrical gear.
An electric arc which occurs between the enclosure and the conductor of a compartment of the bay is referred to as an internal arc. In conventional manner, a substation protection system is provided for detecting such a fault by measuring the current passing through the substation. However, the protection system does not make it possible to locate the internal arc, so it is not possible to identify the bay of the substation or the compartment within said bay in which the internal arc has occurred.
The compartments of each bay are provided with respective pressure sensors designed to measure the pressure of the dielectric gas to be found inside the various enclosures. If an internal arc occurs in a compartment, then the pressure detector detects an increase in pressure, thereby enabling the compartment to be identified.
Identifying the faulty compartment by an increase in pressure presents no difficulty with a disconnector or with an outgoing feeder from a bay.
In contrast, a problem of discrimination arises with a circuit breaker. When the protection system detects a current fault in the electrical substation, it issues a disengagement order to the circuit breakers which then open on receiving the order. On opening, each circuit breaker strikes a respective short circuit arc known as a circuit-breaking arc, thereby increasing the pressure of its dielectric gas.
To determine whether an internal arc has occurred in a particular circuit breaker of one of the bays of a substation, it is essential to be able to identify a circuit-breaking arc which inevitably follows the disengagement order issued by the protection system.
An apparently-satisfactory solution would base identification on the amplitude of the increases in pressure due to an internal arc as compared with those due to a circuit-breaking arc. Nevertheless, that solution is not applicable under all circumstances, in particular in the event of internal arcs that are small, since the increase in pressure that they cause is of the same order of magnitude as the increase in pressure caused by breaking a high short-circuit current.
OBJECTS AND SUMMARY OF THE INVENTION
The object of the invention is to discriminate between an internal arc and a circuit-breaking arc in a circuit breaker forming part of an electrical substation, by using a method which is applicable without restriction to internal arcs even when small, which method is simple to implement, and can be installed on existing substations for little investment.
The idea on which the invention is based is to take consideration of the length of time that elapses between the disengagement order being issued by the protection system and the instant at which the circuit-breaking arc appears when a circuit breaker opens.
More particularly, the invention provides a method of discriminating between an internal arc and a circuit-breaking arc established inside the enclosure of a circuit breaker in a bay of a medium or high voltage metal-clad substation, by means of measurements of the pressure of the dielectric gas inside the enclosure of the circuit breaker, in which the appearance of an internal arc is detected by a protection system which responds by transmitting a disengagement order to the circuit breaker to cause its contacts to separate, the separation causing a circuit-breaking arc to appear, the method comprising the following steps:
continuously measuring the pressure of the dielectric gas inside the enclosure of the circuit breaker;
recording said pressure measurements so that after the instant at which the disengagement order is transmitted, a first pressure value measured before the instant can be recovered; and
comparing the first pressure value with a second pressure value measured after the instant to identify that an internal arc is established in the circuit breaker enclosure if the second value is greater than the first value, or that the internal arc is not established in the circuit breaker enclosure if both pressure values are equal.
In a particular implementation of the method of the invention, the second pressure value corresponds to a pressure measured after the disengagement order so as to take account of the mechanical response time of the circuit breaker during which a moving one of its contacts is moved relative to its other contact which is fixed, but without actually reaching the separation point. The circuit-breaking arc is therefore still not struck, such that its contribution to the increase of pressure inside the enclosure is still zero.
According to another particular implementation of the method of the invention, the first pressure value corresponds to a pressure measured prior to the disengagement order so as to take account of an electronic response time of the protection system on the appearance of an internal arc. Since the internal arc had not yet occurred, the pressure stored in the memory represents the reference pressure of the circuit breaker.
REFERENCES:
patent: 5502435 (1996-03-01), Ralston
patent: 5910872 (1999-06-01), Marmonier et al.
patent: 5946171 (1999-08-01), Magnier
patent: 27 27 378 A1 (1979-01-01), None
patent: 31 31 417 A1 (1983-02-01), None
patent: 0 488 719 A2 (1992-06-01), None
patent: 2 205 051 (1982-11-01), None
patent: 2 731 520 A1 (1996-09-01), None
Alstom France S.A.
Sherry Michael J.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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