Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Patent
1991-12-20
1993-05-11
Harvey, Jack B.
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
324521, 324551, G01R 3128
Patent
active
052104974
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to cable fault tracing systems. It is concerned particularly, but not exclusively, with the detection of faults in underground cables where the outer insulation is damaged, giving rise to a conduction path from an inner metallic element to ground.
2. Summary of the Prior Art
It is known that where the insulation of an underground cable is damaged, the short between the metallic element of the cable and ground causes the signals on the cable to be attenuated or for such signals to fail to be transmitted. This problem has been described and an attempted solution offered in U.S. Pat. No. 3,991,363. Improvements to this technique have been described in UK Patent Application No. 2211621.
In the known techniques low and high frequency signals are applied simultaneously to the line. The low frequency propagates by the resistive path through the fault to the ground. The high frequency system carries synchronising information in some form of modulator.
One of the injected signals is also used to locate the buried cable and trace its route.
SUMMARY OF THE INVENTION
With the known techniques discussed above there is a significant problem of attenuation of the high frequency signals over long distances, due to signal current leaking to ground through capacitive coupling. Overcoming this restriction simply by increasing the strength of signal applied would present a significant hazard to health and existing equipment connected to the line and the power required would make the system commercially unviable.
It is possible significantly to restrict the capacitive loss of the high frequency synchronisation signal by reducing the frequency to a few hundred hertz. However, the modulation techniques employed in U.S. Pat. No. 3,991,363 and UK Patent Application No. 2211621 require wide bandwidths that cannot be translated to these frequencies while still maintaining effective operation of such a system for effective propagation on very long lines.
It is the aim of this invention to overcome or at least ameliorate these limitations, and permit the location of faults many miles from the access point to the line, as routinely occurs with fibre optic telecommunication lines.
The present invention considers the tracing of faults in hidden electrical conductors and proposes a low frequency component and two higher frequency components be generated the low frequency signal being an integer multiple of the difference between the high frequency signals. Then a receiver may be placed parallel to the assumed or known path of the conductor so that the low and high frequencies can be detected separately. The high frequencies are then processed to obtain a synchronous low frequency reference to compare with the detected low frequency. Substantial changes in phase between the reference and detected low frequency signal indicate the location of a fault.
At low frequencies airborne interference increases due to signals derived from power transmissions. A system employing a high frequency component which is in the order of a few hundred hertz must be able to reject this interference to maintain effective operation.
One advantage of the system is the narrow bandwidth used by the transmitted high frequency component. In previous systems the type of high frequency modulation employed has resulted in a bandwidth many times greater than the value of the low frequency signal. In this invention the transmission bandwidth of the high frequency signal is simply equal to the value of the low frequency signal. This allows band filtering techniques to be employed in the receiver, thereby increasing the ability to reject airborne interface.
In a preferred embodiment the low frequency is 8 Hz and the higher frequencies are 512 Hz and 520 Hz. These may be applied simultaneously or sequentially. With sequential application, receiver synchronisation is maintained by phase locking stable local oscillators. In practice, the high frequency signals should both preferably be less than 8
REFERENCES:
patent: 3299351 (1967-01-01), Williams
patent: 3991363 (1976-09-01), Lathrop
patent: 4528427 (1985-07-01), Gadonna et al.
patent: 4896117 (1990-01-01), Floweredew et al.
Dreger Walter H.
Harvey Jack B.
Radiodetection Ltd.
Regan Maura K.
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