Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2001-03-28
2003-03-25
Le, N. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S522000, C324S526000, C324S551000, C324S516000
Reexamination Certificate
active
06538450
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the locating of an insulation fault in an electric cable.
More particularly, the present invention relates to a method and a system for locating an insulation fault, relative to the ground, in the metallic sheath of a telecommunication cable.
Current tree structure telecommunication networks, are made using cables
1
of a high section of the type represented in
FIG. 1A
, comprising several hundred or even several thousand electric wires
2
insulated from each other by a suitable taping and arranged in twos to form “telephone pairs”. The assembly is protected from electric disturbances by a metallic sheath, or screen
3
. The screen
3
is itself covered by a protective sheath
4
made of an electrically insulating material such as polyethylene, PVC, . . .
These telecommunications cables, arranged in the ground, in the air, or immersed, are subject to various types of aggression, the most frequent of which are caused by lightning, rodents, road works, hunting, the friction of tree branches. As seen in
FIG. 1A
, these various aggressions can lead to a tear
5
in the protective sheath and to the penetration of water in the cable. From an electrical point of view, such deterioration leads to an insulation fault of the screen
3
relative to the ground, represented in
FIG. 1B
by a resistance Rd, and leads to the occurrence of a voltage Vc, or “electrochemical couple”, particularly generated by the combination of the metal of the screen
3
with the water and various metal oxides. When a fault in water tightness is not repaired in time, the deterioration of the cable extends to the taping of the electric wires and develops over a substantial length of the cable due to the spread of water.
Checking the cables is therefore a major concern of telecommunications operators and more and more needs to be automated with a view to reducing maintenance costs and repair action times on site.
As telecommunication cables are of non-negligible lengths ranging from approximately one hundred meters to a few kilometers, the detection of an insulation fault must be followed by a step of locating the fault in order for qualified staff to operate on it.
Various methods for locating insulation faults between conductors are known of, some of which are applicable to the detection of an insulation fault between a conductor and the ground. Out of these methods, the most widespread are measuring bridge methods and the voltage drop method.
FIG. 2
illustrates a classical measuring bridge method. The measuring bridge is formed by a standard resistor RA, a variable resistor RB, and the resistance Rl of a loop comprising an auxiliary conductor AUX
1
and the defective metal screen SCR, the screen being linked to the ground at a point Pd by a resistance Rd representing the insulation fault to be located. The bridge is supplied by a direct voltage E, generally in the order of 150 V, applied between the mid point A of resistors RA, RB and the ground, and comprising a balancing galvanometer G. At bridge balance, the electric resistance R
1
that the screen SCR presents between its point of origin Po and the fault point Pd is given by the relation:
R
1
=
Rl RB
/(
RA+RB
) (1)
In view of the linear resistance of the screen SCR, the distance L
1
separating the point Pd from the point of origin Po is deduced.
This locating method and other similar methods derived from the Wheatstone bridge have various inconveniences. On the one hand, the method is based on a resistance measurement and requires a conversion of the result into a length, by means of a theoretical value of the linear resistance that can prove to be inaccurate. On the other hand, the accuracy of the measurement deteriorates when the resistance Rd of the insulation fault becomes high and must be offset by a significant increase in the power supply voltage E of the bridge, that can thus reach 500V. The application of a voltage E that high can lead to a breakdown of the cable insulating materials and represents a danger for the equipment connected to the network, thus requiring the latter to be temporarily put out of operation.
The voltage drop method, illustrated in
FIG. 3
, includes injecting a current i into the defective metal screen SCR by means of a current generator GA referenced to the ground. The current i circulates in a ground loop formed by the resistance R
1
of the screen SCR between the point of origin Po and the fault point Pd, the resistance Rd of the fault, and the ground. The voltage drop V
1
at the terminals of resistance R
1
is measured by a voltmeter VM connected to the terminal point Pe of the screen by means of an auxiliary wire AUX
1
. A similar measurement is taken at the other end of the screen, to determine the voltage drop V
2
at the terminals of resistance R
2
of the screen between the fault point Pd and the terminal point Pe. As the length L of the screen is known, the relative position Pdr of the fault is deduced:
Pdr=L
1
/
L=V
1
/(
V
1
+
V
2
) (2)
This method, like the previous one, has the disadvantage of requiring a high measuring voltage when the resistance Rd of the insulation fault is high. For example, the injection of a current of 4 mA requires the application of a voltage of 4KV when the resistance Rd is in the order of one Megohm.
2. Description of the Related Art
Moreover, the.document E.C. BASCOM ET AL “Computerized Underground Cable Fault Location Expertise” presented at the conference “Proceedings of the Power Engineering Society Transmission and Distribution Conference, Chicago, Apr. 10-15, 1994” published IEEE review of Apr. 10, 1994, pages 376-382, reference XP000470557, describes on page 380, paragraph 10, a method for locating an insulation fault between a phase wire and the ground, including causing a current to circulate in the defective phase wire, by using a nondefective phase wire connected to one end of the defective- phase wire to form a conduction loop, and measuring the voltage at the two ends of the defective wire.
OBJECTS AND SUMMARY OF THE INVENTION
One object of the present invention is to provide a method for locating an insulation fault, relative to the ground, in the screen of an electric cable, which does not need a high measuring voltage, while being accurate.
Another object of the present invention is to provide a locating method that is accurate over a wide range of insulation fault values, particularly up to values in the order of one Megohm.
Yet another object of the present invention is to provide a locating method that is simple to use and that can be integrated into an automated maintenance system.
These objects, and others, are achieved by providing a method for locating an insulation fault in a conductor element relative to the ground, the conductor element being electrically accessible at least at one point of origin and at one terminal point, comprising steps of: injecting a current between the point of origin and the terminal point of the conductor element, by means of a current or voltage generator that is insulated from the ground; measuring a first electric voltage at the point of origin in reference to the ground; measuring a second electric voltage at the terminal point, the first and second voltages measured being linked by a relation that is representative of the relative position of the insulation fault between the point of origin and the terminal point.
In practice, the second voltage can be measured in relation to the point of origin or in reference to the ground.
According to one embodiment, the current or voltage generator is arranged at the point of origin of the conductor element and is connected to the terminal point of the conductor element by means of a first auxiliary wire. Also, the second voltage can be measured from the point of origin of the conductor element by means of a second auxiliary wire connected to the terminal point of the conductor element.
According to one embodiment, the current injected into the
Hamdan Wasseem H.
Le N.
Patterson, Thuente, Skaar & Christensen LLC
Socrat
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