Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
1999-08-16
2001-05-08
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
C361S078000, C250S227110
Reexamination Certificate
active
06229680
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to detecting arcing faults in electric power systems and particularly within the metal cabinets of switchgear. More specifically, it relates to apparatus and the method for detecting such arcing with optical sensors which are resistant to false indications induced by other light sources such as ambient light, tungsten bulbs, fluorescent lights, flashlights and even flash bulbs.
2. Background Information
Electric power systems incorporate switches for control and protection purposes. Distribution systems which form part of the overall electric power system include main and branch power buses and circuit breakers mounted in metal cabinets to form switchgear. Interruption of current flow in the buses of the distribution system by a circuit breaker creates an arc as the contacts of the circuit breaker open. These arcs caused by interruption are contained and extinguished in the normal course of operation of the circuit breaker.
At times, however, unintended arcing faults can occur within the switchgear cabinets, such as between the buses, or between a bus and a grounded metal component. Such arcing faults can produce high energy gases which pose a threat to the structure and nearby personnel. A common approach to protecting personnel from arcing faults in switchgear has been to design the metal enclosures to withstand the blast from the arcing fault. This has been done at great additional costs due to the heavy gauge metal used and numerous weld joints needed to prevent flying debris. Even with these precautions, the blast from an arcing fault inside the switchgear cannot be contained.
Recently, methods have been developed for minimizing the severity of the blast from an internal arcing fault. These methods include pressure sensing and light detection which sense the arcing fault within the switchgear and cause a circuit breaker to trip before significant damage can result. The pressure sensing method is limited by the insensitivity of the pressure sensors. By the time cabinet pressure has risen to detectable levels, the arcing fault has already caused significant damage. The light detection methods used to date are not selective so that any light signal can trigger the sensor. Thus, a flashlight used by service personnel, or sunlight or a photoflash can falsely trigger the optical sensor. In order to avoid such false tripping of the optical sensors, it has been proposed that optical sensing be combined with current sensing either directly or by sensing the magnetic field created by the current. Unfortunately, these methods are costly and have not proven to be reliable.
There is a need, therefore, for improved apparatus and method for detecting arcing faults in electric power systems and particularly within switchgear.
More particularly, there is a need for such improved apparatus and method for detecting arcing faults which respond before significant damage has occurred and yet are reliable and resistant to false arcing fault indications.
SUMMARY OF THE INVENTION
These needs, and others, are satisfied by the invention which recognizes that light from the arcing fault contains wavelengths characteristic of the material creating the arc. Commonly, the arcing fault occurs at a bus or disconnect which are typically made of copper. Copper has a strong line emission wavelength at about 520 nm (520.820 nm). In some applications, the buses are coated with silver which also has a strong line emission characteristic wavelength of about 520 nm (521.908 nm) and another at about 546 nm (546.550 nm). Unfortunately, other light sources which can be present have a broad band continuum of wavelengths which includes 520 nm and 546 nm. This includes the common sources of light such as ambient light, tungsten bulb light, flashlight, fluorescent light and flash bulb light. Thus, merely detecting light at about 520 nm will not distinguish an arcing fault from these others sources.
In accordance with the invention, light is gathered from the components of the electrical power system of interest. This gathered light is split into two beams. Light within a first band of wavelengths which includes a predetermined wavelength characteristic of the arcing material is extracted from the first beam as the sensed light. Light is extracted from the second beam within a second band of wavelengths which does not include the predetermined wavelength as a reference for the background light. The sensed light and the background light are compared. An output indicating the presence of an arcing fault is generated based on this comparison. If an arcing fault is present, the sensed light should be much stronger, have a higher irradiance, than the background light. While the background light will also produce light in the first band of wavelengths, the irradiance will not greatly exceed that in the second band of wavelengths. Preferably, the second band of wavelengths is selected so that for the background light the signal strength in the second band of wavelengths will be greater than in the first band. To minimize the risk of a false indication, light in the first band of wavelengths must exceed that in the second band of wavelengths by a selected threshold amount before an indication of an arcing fault is generated.
The light within the first band of wavelengths is extracted from the first beam by a filter selected to pass the predetermined wavelength. The width of this first band of wavelengths can be up to about 25 nm, but is preferably about 5-10 nm. The narrower this band of wavelengths, the more discriminating is the detection, but the sensitivity is reduced as the bandwidth narrows. Light is extracted from the second beam by another filter passing light with wavelengths not including the predetermined wavelength but having a similar bandwidth. In order to more closely balance the light extracted from the two beams of gathered light, the second beam which is passed through the filter passing wavelengths of the second wavelength band which does not include the predetermined wavelength, can be also passed through a neutral density filter.
The invention is also directed to apparatus for detecting arcing faults including light gathering means for gathering light from the monitored component and splitting the gathered light into first and second beams. A first filter means extracts from the first beam light in a first wavelength band including the predetermined wavelength characteristic of the arcing material of the component. A second filter means extracts light from the second beam in a second band of wavelengths not including the predetermined wavelength. Response means generates an arcing fault signal in response to a predetermined relationship between the light passed by the first filter means and that passed by the second filter means.
Preferably, the response means comprises a first photodetector which generates a sensed light electrical signal from the filtered first beam, and a second photodetector generating a background light electrical signal from the second filtered light beam. A comparator circuit assumes a first state indicative of an arcing fault when the sensed light electrical signal exceeds the background light electrical signal by a predetermined amount. Otherwise, the comparator remains in a second quiescent state. Output means generate the signal indicative of an arcing fault in response to the first state of the comparator circuit.
In the preferred embodiment of the invention, the light gathering means comprises a bifurcated optic fiber having a first branch which generates the first beam and a second branch generating the second beam. Where a plurality of components or locations within a switchgear are to be monitored for arcing faults, the light gathering means can include additional optic fibers each having a first end aimed at a selected component or location, and a second end coupled to the bifurcated optic fiber. The bifurcated optic fiber is a bundle of optic fibers with a comb structure which splits light gather
Eaton Corporation
Moran Martin J.
Sherry Michael J.
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