Measuring and testing – With fluid pressure – Leakage
Reexamination Certificate
1998-10-22
2001-03-27
Larkin, Daniel S. (Department: 2856)
Measuring and testing
With fluid pressure
Leakage
C073S030010, C073S030020, C340S605000, C340S632000, C340S635000, C374S142000, C374S143000
Reexamination Certificate
active
06205846
ABSTRACT:
The invention relates to a method of determining a leakage rate from a series of density values for a dielectric gas contained under pressure in a case having disposed therein an electrical apparatus carrying an electric current, each density value being calculated by a recording-and-processor unit on the basis of corresponding pairs of pressure and temperature values delivered simultaneously by a pressure sensor and by a temperature sensor fixed on the case via the outside and communicating with the dielectric gas.
BACKGROUND OF THE INVENTION
A generator or a circuit breaker for a distribution network constitutes an application of such an electrical apparatus, in which the dielectric gas contained inside the case is sulfur hexafluoride SF
6
, for example. SF
6
at a pressure of a few bars, either in the pure state or mixed with some other gas such as air or carbon tetrachloride CCl
4
, is used to extinguish an electric arc that forms inside the case when the circuit breaker is opened.
The pressure sensor and the temperature sensor are mounted from the outside on the case and they communicate with the dielectric gas via a duct passing through the thickness of the case. Each record of pressure and temperature as measured simultaneously by the sensors can be used to calculate a density value that is representative of the real density of the dielectric gas contained in the case, to within the uncertainty of measurement error.
It is known that by measuring temperature simultaneously with pressure it is possible to apply temperature compensation to take account of the way pressure can decrease merely because the dielectric gas has contracted under the effect of a decrease in temperature, without there being any loss of mass or leakage of the dielectric gas from the case.
With the above-described method, it is possible to monitor mass losses of a few percent per annum. However, in the future, standards concerning protection of the environment may impose monitoring for leakage at a rate of one part per thousand per annum, particularly in respect of SF
6
or CCl
4
.
On its own, temperature compensation turns out to be incapable of providing monitoring to that degree of accuracy, given that the pressure value as measured is corrected as a function of the temperature value as measured and not as a function of the real value of the temperature of the dielectric gas contained inside the case.
OBJECTS AND SUMMARY OF THE INVENTION
The object of the invention is to improve the accuracy with which the leakage rate of dielectric gas from the case of an electrical apparatus is measured.
To this end, the invention provides a method of determining a leakage rate from a series of density values for a dielectric gas contained under pressure in a case having disposed therein an electrical apparatus carrying an electric current, each density value being calculated by a recording-and-processor unit on the basis of corresponding pairs of pressure and temperature values delivered simultaneously by a pressure sensor and by a temperature sensor fixed on the case via the outside and communicating with the dielectric gas, wherein density value is indexed by the recording-and-processor unit as a function of a value for the electric current and of a value for a climate parameter concerning the medium in which the case is located, which values are recorded simultaneously with each corresponding pair of pressure and temperature values so as to enable the leakage rate to be determined from indexed leakage rates, themselves each obtained from density values all having the same index.
According to the invention, the electrical currents that flow through the electrical apparatus are recorded so that account can be taken of heat exchanged between the apparatus and the dielectric gas contained inside the case. Similarly, values for the climate parameter of the medium in which the case is located are also recorded so as to take account of heat exchanged between the medium and the temperature sensor fixed on the case.
Any difference between the real temperature of the dielectric gas and the temperature as measured by the sensor fixed to the case, e.g. due to a difference between the thermal inertias of the gas and of the temperature probe mounted in the sensor, remains substantially constant if it is generated by an electrical current that is substantially constant or by a climate parameter that is likewise substantially constant.
By recording an electric current value and a climate parameter value simultaneously with each pair of values corresponding to pressure and temperature, it is possible for the recording-and-processor unit to give a common index to the density values associated with electric current values that are similar to one another and with climate parameter values that are likewise similar to one another.
This gives rise to an indexed leakage rate, calculated on the basis of density values having the same index, which rate depends to a negligible extent on the substantially constant difference that exists between the real temperature of the dielectric gas and the temperature as measured by the sensor fixed on the case. Measurements based on indexed leakage rates are thus more accurate, as is the leakage rate obtained by averaging the indexed leakage rates.
REFERENCES:
patent: 3934454 (1976-01-01), Simo
patent: 5388451 (1995-02-01), Stendin et al.
patent: 5502435 (1996-03-01), Ralston
patent: 5693873 (1997-12-01), Thuries et al.
patent: 0 637 114 A1 (1995-02-01), None
patent: 0 726 630 A1 (1996-08-01), None
Dupraz Jean-Pierre
Thuries Edmond
GEC Alsthom T&D SA
Larkin Daniel S.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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