FI84108C - FOERFARANDE OCH ANORDNING FOER LOKALISERING AV KABELFEL. - Google Patents

Description

84108

METHOD AND APPARATUS FOR LOCATING A CABLE FAULT FÖRFARANDE OCH ANORDNING FÖR LOCALISERING AV KABELFEL

The present invention relates to a method and apparatus for locating a cable fault, in which the location of a fault in a conductor, such as a leak, short circuit or break, is detected by a detector in situ.

Currently used cable fault location devices of this type require that the fault point has been burned across by applying high voltage to the faulty line. The fault location is then searched for by listening to the high voltage spark at the fault location with a sensitive listening device.

The prior art has a few serious drawbacks; first, the success of cross-burning in the event of a failure depends on the type of failure. On the other hand, finding a sparking fault by listening is virtually impossible due to traffic and other background noise other than at night. For these reasons, the cable often has to be excavated from an unnecessarily long distance to reveal the fault location, and the exposed cable has to be examined once again with special methods and equipment to find the exact location of the fault. The rough location of the fault (distance to the fault point measured along the cable) can generally be determined in a known manner by connecting a device resembling a resistor bridge in principle to the ends of a faulty and intact pair of conductors, as described, for example, in Finnish patent application 852542.

It is an object of the present invention to provide a method and apparatus by which any fault can be accurately located "in the field" from the ground. To achieve this, for a method according to the invention, in which a pair of cable conductors, one of which is the faulty conductor to be examined, a conductor loop is formed, the intact branch of which is supplied with a sufficiently high voltage according to the fault condition to cause a faulty operation such as a breakdown. characterized in that a high-frequency signal, i.e. a carrier, is added to the breakdown current to create a magnetic field by mutual inductance between the faulty and intact supply conductor of said voltage pulse, and the end of the magnetic field at the short-circuiting fault point is sensed by the frequency of said carrier.

Thus, with a sufficiently high voltage, a current loop is formed in the defective cable, with which a high-frequency, e.g. RF, signal travels. The fault forms a short circuit to this current loop, after which no RF signal occurs. By detecting the RF signal with a detector precisely tuned to its frequency, an interference-free and reliable indication of the fault location is obtained.

Other preferred embodiments of the method according to the invention are characterized by what is stated in the claims below.

The device implementing the method according to the invention, comprising means for providing a breakdown voltage at the fault point and an antenna or similar detector for detecting a breakdown in the conductor in situ, characterized in that the means for providing a breakdown voltage consist of the carrier is modulated by a low frequency signal and added to the output of the power supply to provide a magnetic field between the defective and intact supply conductor, and a receiver unit having said magnetic field detector, one or more amplifiers and filters for detecting the presence of said carrier;

II

3 84108 sex, a demodulator for separating the modulating low frequency signal from the carrier and means for detecting the low frequency signal to the user.

Other preferred embodiments of the device according to the invention are characterized by what is stated in the claims below.

The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which

Figure 1 shows an overview of the measurement situation,

Figure 2 shows the principle of a transmitter of a device according to the invention in block diagram form,

Figure 3 shows a basic solution for a receiver according to the invention,

Figure 4 shows an embodiment of a receiver for a device according to the invention.

In Figure 1, below the ground 1, for example, the conductors 2,3 included in the telephone cable, of which the conductor 3 has a fault at point 4 According to the invention, a direct voltage source 5 is connected to the conductor 2, which is suitably selected from the intact conductors of the cable, the voltage provided by which is selected such that a sufficient amount of direct current is passed through the defective cable 3. This may require, for example, a supply voltage of 5 kV. From the unit 6, which will be described in more detail below, an amplitude-modulated high-frequency (5-500 kHz) RF or HF signal is fed to the conductor 2 via a current transformer 7, which signal is thus summed or extrapolated to the DC voltage supplied by the voltage source 5. The result is high-voltage, high-frequency pulses with an amplitude proportional to a modulating, e.g. 1-10 kHz, frequency identification signal.

Since the overshoot potential at the fault point 4 has already been reached at the DC voltage, it is easy to cause a substantial part of the RF or HF signal to travel back along the branch A of the faulty conductor 3. The fault point 4 thus forms a substantially short-circuit-like point for the modulated signal. The signal circulating in the branch A of the conductors 2 and 3 generates a magnetic field due to the mutual inductance of the conductors 2 and 3, which ends at the fault point 4 and is therefore not detectable in the conductor branch B. By detecting the magnetic field with its resonant circuit tuned to its frequency.

Figure 2 shows an example of a transmitter of a device according to the invention in block diagram form. The transmitter consists of its main components a direct current source 6 (adjustable 0-5 kV), a current transformer 7, an RF power amplifier / modulator 9, a low frequency signal generator 10 which generates a modulating signal, 1-10 kHz, and an RF oscillator 11 which generates a carrier signal e.g., 100 kHz. As described above, the DC power supply 6 provides a low-current arc at the fault point between the conductors 2 and 3, on which the modulated signal produced by the RF amplifier 9 "rides".

Another alternative to the structure of the transmitter is to use an amplifier 9 with a sufficiently large and adjustable power, which provides a sufficiently large amplitude (e.g. 1 kV) to cause an overshoot or "spark" at the fault point. In this case, a DC power supply 6 and a current transformer 7 are not required, on the other hand, an impedance arrangement is required between the amplifier 9 and the conductors 2,3.

For various situations, such as the laying of cables dug in variable soil or laid on the seabed.

II

5 84108 to locate the devices, it is advantageous if the frequency of the carrier and / or the identification signal has to be adjusted. In a carrier oscillator, for example, three selectable fixed frequencies provided by three crystals in a suitable frequency range are sufficient. If the transmitter does not use separate power control at the output, the gain of the identification signal must be infinitely adjustable so that the amplitude modulation and thus the breakdown at fault point 4 can be controlled.

Figure 3 shows a basic solution for a receiver according to the invention. The RF frequency magnetic field generated by the transmitter in the conductor branch A is detected by an antenna, i.e. a resonant circuit 12. The antenna signal is amplified in an antenna amplifier 13 with automatic level control (AGC). The automatic level adjustment can be switched to manual adjustment with a switch, in which case, under certain conditions, the faulty point can be located more precisely, as a kind of fine adjustment. Interference cancellation takes place by filtering out the carrier from the detected signal, e.g. in the form of a frequency band below 100 Hz. This is done as shown in Figure 3 by two bandpass filters 14 and 16, followed by compensating RF amplification by amplifiers 15 and 17. This provides effective attenuation outside the carrier. The pre-filtered and amplified signal is applied to a signal detection circuit 18, which is in fact a demodulator, filtering out the modulating low frequency signal from the carrier. A level control feedback to the antenna amplifier 13 is also provided from the detection circuit 18. The low frequency signal is amplified in the amplifier 19 and converted by a speaker, headphones, indicator lights or the like into a human perceptible shape at 20, thus representing one or more of the above components. In principle, this is about receiving an AM signal, which is obvious to a radio professional, for example.

6 84108

Once the faulty point of the cable has been found and the cable has been excavated, the location of the fault is usually not yet known so precisely that it would be worthwhile to open the protective sheath and insulation layers of the cable for repair. There are devices on the market that can determine the exact location of a fault by moving the device along the surface of the cable until it hits the fault. The detection of RF frequencies by the device according to the invention as such is not the best possible solution at such short distances. Instead, if the RF portion of the receiver is bypassed in signal detection, i.e., the identification signal is passed directly from the resonant circuit 12 to the low frequency amplifier 19, the weak low frequency magnetic field In this way, the fault can be located with an accuracy of a few centimeters, and the cable sheath can now be opened. This feature is not part of the actual invention, but forms an advantageous additional feature, since a separate special device for accurately locating the fault point on the cable is not required.

If it is desired to add to the receiver according to Fig. 3 the reception frequency control possibility, which is necessary e.g. when using a frequency adjustable transmitter (Fig. 2), simply by adding a mixer between the antenna amplifier 13 and the first fixed bandpass filter 14 to the antenna signal and oscillator , frequency adjustable signal. By changing the oscillator frequency according to the changed carrier frequency so that the frequency of the signal seen by the filter 14 is always the same, the device can be used within the oscillator control range to detect different carrier frequencies, and the detected frequency value can be arranged to be read directly from the oscillator frequency control knob.

Il 7 84108

Another way to achieve frequency control of the receiver is to use the circuit of Figure 4 with a frequency adjustable digital filter to achieve very narrow passbands and steep attenuation outside the band, e.g. Q = 10000. In principle, the connection is the same as in Fig. 3, with LC circuits 21, RF antenna amplifiers 22, RF intermediate amplifiers 23 and 27, detector circuits 28, automatic level control devices 29 and low frequency signal reproducing devices 30. A digital filter known to the person skilled in the art, consists of a frequency-adjustable oscillator 24, a frequency divider 25, a decoder circuit 26 and capacitors C. An oscillator 24, the frequency of which is generally set to 2n times the carrier under test, and a divider 25 (n. e.g., JK flip-flop dads) input pulse sequences such that all four of its outputs are up simultaneously at the frequency of the carrier under study. In this case, the signal of this frequency travels directly to the next RF amplifier stage 27, otherwise, when the outputs of at least one decoder 26 are low, the entire signal current goes very accurately to charge one or more capacitors C.

It will be apparent to those skilled in the art that the various embodiments of the invention are not limited to the examples set forth above, but may vary within the scope of the claims set forth below.

Claims (6)

8 84108 A method for locating a cable fault, which method detects the location of a fault in a conductor (4), such as a leak, short circuit or break, on the detector, and in which a pair of cable conductors (2,3), one wire (3) cm -din loop, the intact branch (2) of which is supplied with a sufficiently high voltage according to the fault situation to cause a faulty function such as a breakdown and the resulting return current in the faulty conductor (3), characterized by adding a high-frequency signal to the breakdown current as a magnetic field between a defective (3) and an intact (2) conductor, and the termination of the magnetic field at the fault point (4) shorting it is sensed by means of said carrier frequency detecting device (8; 12-20; 21-30). A method according to claim 1, characterized in that the magnetic field generated by said carrier is detected by an induction coil resonantly coupled to the field and one or more bandpass filters (14, 16; 24-26) connected thereto. Method according to claim 1 or 2, characterized in that said carrier consists of an HF or RF signal with a frequency of 5-500 kHz. Apparatus for carrying out the method according to claim 1, comprising means (6; 9) for generating a breakdown voltage at the fault point (4) and an antenna (8; 12; 21) or a similar detector for detecting a breakdown in the conductor in situ, characterized in that the means for generating a breakdown voltage consist of a transmitter unit with an adjustable power supply (6; 9) and a modulator (9) connected to its output, by which the high frequency signal or carrier can be modulated by a low frequency signal and added to the output of the power supply (6; 9) to provide a defective magnetic field (3) 2) between a conductor, and a receiver unit with said magnetic field detector (12; 21), one or more amplifiers (13,15,17; 22,23,24) and filters (14,16; 24-26) for the presence of said carrier demodulator (18; 28) for separating the modulating low frequency signal from the carrier s and means (19, 20; 30) for detecting a low frequency signal to the user. Device according to Claim 4, characterized in that the transmitter unit consists of a separate DC voltage source (6) connected to the secondary winding of the current transformer (7), an RF frequency oscillator (11) and a low-frequency signal source (10). , and an AM modulator and an RF power amplifier (9) connected to the primary winding of the current transformer (7), by means of which the RF carrier modulated by the low frequency signal can be fed to the conductor loop (2,3) to be examined. Device according to Claim 4, characterized in that the transmitter unit consists of an adjustable RF-High power amplifier and a modulator (9) supplying it, an impedance matching transformer connected between it and the conductor loop (2,3) to be tested, an RF frequency oscillator (11) and a low frequency signal source (10). 10 84108