Good time of the day, friends!

Today we will talk about absolute methods for determining the location of a power cable fault.

1. acoustic method.

The acoustic method is based on listening to sound vibrations caused by a spark discharge in the damage channel above the cable line damage site. The acoustic method is practically universal and in most cases is the main absolute method. They can determine damage of a different nature: single-phase and phase-to-phase short circuits with various transient resistances, breaks in one, two or all cores.

In some cases, it is possible to determine several faults on one cable line.

Spark discharges obtained at the point of cable damage are formed in two ways.

In "floating breakdown", which, as a rule, is detected during control tests, the damage mainly occurs in the couplings.

The resistance at the site of damage is large - units and tens of megaohms.

With the help of a DC test setup (), a voltage is applied to the damaged core (no more than 5Unom, where Unom is the operating voltage of the cable).

As soon as a breakdown occurs at the fault, the distance to the fault is determined, for example, using the oscillatory discharge method.

After the first breakdown, the resistance in the damaged cable core is restored and the voltage from the DC test set rises again to the breakdown voltage. Such periodic breakdowns can last for a long time. In the area of ​​the measured distance to the fault, the operator, moving along the cable line, clearly captures the acoustic signals caused by breakdowns at the fault.

In case of short circuits having a transition resistance at the point of damage from a few ohms to tens of kOhms, a high-voltage direct current installation is used, with the help of which the capacitor is charged, after which a breakdown occurs through the arrester (the arrester can be either controlled or uncontrolled air) at the place of damage. , causing an acoustic signal. In mobile measurement laboratories, there are usually two groups of high-voltage capacitors. One group for an operating voltage of up to 5 kV with a capacitor capacitance of up to 200 μF (low-voltage acoustics), the other group for an operating voltage of up to 30 kV with a capacitor capacitance of up to 5 μF (high-voltage acoustics).

Installations for charging capacitors of the first group have a large power, which is necessary for fast charging of large capacitors (units of seconds).

If, when using the first group of capacitors, it is impossible to create a breakdown due to the high resistance at the fault site, then it is necessary to use the second group of capacitors. The operator, moving along the cable line route in the expected damage zone, measured by the impulse or wave method, can accurately determine the location of the damage in the following way.

When using a cable locator, PK-100, which has one amplification channel, the signal from the acoustic transducer is amplified by the receiver and fed to the dial indicator and headphones. Moving along the cable line route, the operator listens to the signals with the help of headphones, and only in the place of direct damage to the cable, when the acoustic signals are clearly recorded, it is necessary to use a pointer indicator to identify the point on the route with the maximum deviation of the arrow, where the damage is located.

When using a cable locator, for example, KAI-90, which has two amplification channels (one for amplifying the signals of the acoustic transducer, and the other for amplifying the signals induced in the induction transducer), the search is carried out as follows.

When moving along the cable line, the signal induced in the induction transducer goes through the amplifying path of the receiver to the pointer indicator, and the signal from the acoustic transducer goes through its amplifying path to the head phones.

In the area of ​​the damage site, when an acoustic signal is heard in the headphones, you should switch to the acoustic search mode.

In this case, the acoustic signal will come through the amplifying path of the KAI-90 receiver both to the headphones and to the dial indicator, by which, with its maximum deviation, you can find the exact location of the damage.

When determining the place of stretching (break) of the cores in the cable, the high-voltage DC test facility is connected in turn to one of the cores or to all three cores of the cable at once (Fig. 8).

When the test voltage rises to 5 Unom due to weakened insulation, a breakdown occurs at the point of rupture between one of the cores and the cable sheath. If the breakdown does not occur at the point of damage, it is necessary to install a jumper at the far end of the cable between all the cores and the cable sheath.

In this case, when the test voltage is raised, the breakdown occurs at the point of breaking the cable cores.

In both cases, the damage site is located by the acoustic method.

Rice. 8. Connection diagram of a high-voltage test facility when stretching the cores in the cable:

1 - high-voltage test facility; 2 - damaged cable; 3 - jumper between the cores and the cable sheath

2. Induction-pulse method.

The induction-pulse method is used to determine the location of damage of the "floating breakdown" type on the cable line route. The location of the breakdown in the cable is determined by the method of controlling the direction of propagation of electromagnetic waves that have arisen at the location of the breakdown.

Since during a breakdown, electromagnetic waves arise, directed from the place of damage to the ends of the cable line, the place on the route of the cable line, in which the direction of the waves changes, corresponds to the place of damage.

To determine the location of the “floating breakdown” of the cable line, a high-voltage installation is connected to the damaged cable core and the constant voltage is gradually increased until periodic breakdowns in the cable are ensured.

The oscillatory discharge method measures the distance to the damage site.

An accurate search for the fault location in the found zone is carried out by an induction-pulse cable detector KII-83 or KII-89, carried along the route when periodic breakdowns are created in the line.

With each breakdown in the line, a voltage is induced in the inductive transducer (sensor), the polarity of which is fixed by a cable finder (deviation of the device arrow).

If the fault location is passed, then the device will fix a different polarity sign, which is the basis for returning back and pinpointing the exact location of the cable fault.

Cable detectors KII-83 and KII-89 allow you to unambiguously determine in which direction you should search along the line route in order to get closer to the damage site.

This eliminates the erroneous actions of the operator. On the route of the cable line in the zone of the alleged damage site (when the sign of the indicating device changes), it is advisable to use the acoustic method to more accurately determine the damage location.

3. Induction method.

The induction method for determining the fault location is based on the principle of determining the nature of the change in the magnetic field above the cable through which the current is passed from the audio frequency generator. Current frequency from 480 to 10000 Hz. The method provides high accuracy in determining the location of damage and is widely used.

By induction, you can determine:

· cable line route;

the depth of the cable line;

the desired cable in the bundle of cables;

· phase-to-phase damage of the cable line;

single-phase cable damage.

3.1. Determination of the cable line route.

When determining the route of the cable line (Fig. 9), the audio frequency generator is switched on according to the phase-to-ground scheme.

When using a generator with an output frequency of 1000 Hz (Fig. 9 a), a jumper is installed at the far end of the cable line between the core and the cable sheath.

When using a generator with an output frequency of 10,000 Hz (Fig. 9 b), installing a jumper at the far end of the cable is not necessary. The sound signal will be generated by the capacitive current flowing through the distributed capacitance of the cable Sk.

The determination of the cable line route is based on the change in the level of the audio signal, which is induced in the induction transducer (IP) and amplified by the receiver.

The operator, moving along the cable line route with a horizontally located induction transducer (Fig. 9d) (parallel to the ground plane and perpendicular to the cable line), hears the maximum signal in the headphones directly above the cable, and when the transducer moves to the right or left of the cable axis, the signal will be weaken.

With a vertically located induction transducer (Fig. 9e), the operator hears a weak signal in the headphones above the cable, which is amplified when the transducer is moved to the right or left of the cable line route.

Thus, when moving in the direction of the maximum (with a horizontally located IP) or minimum (with a vertically located IP) signal, the route of the cable line is determined. Sometimes, due to breaks in the cable sheath and couplings, the current from the generator flows through the sheaths of adjacent cables that are under operating voltage.

In this case, the minimum of the sound signal is obtained over the cable, through the sheath of which the current flows. As a result, the cable line route will be determined incorrectly. In this case, in order to exclude a false determination of the cable line route, the generator is switched on between two cable cores (Fig. 9 c) (bifilar circuit). The operator, moving along the cable line, listens to the maxima and minima of the sound of signals in the headphones, caused by the pitch of the spiral of the cable cores (the pitch of the spiral of cores in power cables can vary from 0.5 to 1.5 m, depending on the cross section of the cable cores). The cable line route is determined by the level of these sound signals.

A) scheme for determining the cable line route at a frequency of 1000 Hz; b) scheme for determining the route of the cable line at a frequency of 10000 Hz; V) scheme for determining the route of the cable line at a frequency of 1000 Hz or 10000 Hz when the generator is connected to two cable cores;

G) EMF induced in a horizontally located induction transducer when moving it to the right and left of the cable axis; e) EMF induced in a vertically located induction transducer when moving it to the right and left of the cable axis; e) the location of the induction transducer when determining the depth of the cable line;

1 - generator; 2 - cable line; 3 - jumper; 4 - distributed capacitance of the cable Sk

Rice. 9. Generator connection diagram when determining the route and depth of the cable line:

3.2.Determining the cable depth.

To determine the depth of the cable line, the same generator connection scheme is used as for determining the cable route.

In the place where it is required to determine the depth of cable laying, it is necessary to accurately determine the route of the cable line with the vertical position of the axis of the induction transducer (Fig. 9 f).

Then the inductive transducer must be set at an angle of 45° to the ground plane using a fixing device.

By moving the transducer perpendicular to the track, they find a point on the surface of the earth at which the sound of the signal in the headphones disappears.

The distance from this point to the route is equal to the depth of the cable laying.

3.3.Determination of the desired cable in the cable bundle.

After excavation of trenches in the area of ​​the alleged damage site, it is necessary to identify the damaged cable in a bundle of other cables under operating voltage.

To determine the required cable, the generator is tuned to a frequency of 1000 Hz (Fig. 9c) and connected to two intact cable cores, which are shorted at the opposite end with a jumper.

At the excavation site, the induction transducer is set in a vertical position and, moving it perpendicular to the cables, the desired cable is found by a sharp change in the sound level of the signal in the headphones on both sides of the found cable. For a more accurate determination of the desired cable in the bundle, it is necessary to use an overhead induction loop, which is connected to the input of the cable detector.

If, when it rotates around the target cable cleared from the ground, two maxima and two minimums of a 1000 Hz signal are heard in the headphones, then the target cable is determined correctly.

3.4. Determining the location of phase-to-phase faults in a cable line.

Line-to-line faults of cable lines, as a rule, are obtained from single-phase faults by destroying the insulation of an undamaged core.

If it is difficult to determine the location of a single-phase damage (poor audibility of acoustic signals, there is no clear change in the signal when determining a single-phase damage by the induction method, there is no clear reference to the cable line route, etc.), it is transferred to interphase damage using a burning installation.

It should be noted that the resistance between the cores and the sheath or between two cores should be close to zero.

In the event that the resistance at the point of closure of two wires is a few ohms, it is difficult to determine the fault location, especially at a frequency of 10000 Hz due to the capacitive current that will flow behind the fault location.

In this case, along the route of the cable line behind the place of damage, signals will be heard in the headphones, due to the helicity of the cores.

After transferring a single-phase fault to an interphase fault and measuring the distance to the fault using devices using the pulse method, the generator is connected to two damaged cable cores (Fig. 10 a).

Rice. 10. Determining the location of phase-to-phase damage by the induction method:

A) audio frequency generator connection diagram:

1 - audio frequency generator; 2 - damaged cable; 3 - the place of phase-to-phase damage to the cable;

b) curve of change in the intensity of the electromagnetic field along the route of the cable with phase-to-phase closure of the conductors (residual resistance at the place of damage is tenths of an ohm): d is the helicity pitch of the cable conductors; c = d at the location of the couplings; V) damaged cable route

With this connection scheme, direct and reverse currents flow from the generator to the fault site, which create a magnetic field. This magnetic field rotates around the axis of the cable due to the helicity of the cores.

Due to this, the EMF induced in induction transducers and the sound signal in headphones will have a minimum and maximum value.

The distance between the maxima and minima is determined by the pitch of the spiral and can vary from 0.5 to 1.5 m. Above the place of phase-to-phase damage with low resistance between the cores, the audibility of the received signal increases, and behind the place of damage, the signal is practically inaudible. When moving over the cable at the locations of the couplings, the length of the interval with maximum sound increases, while the audibility of the signal will be higher due to the large distance between the cores in the coupling (Fig. 10 b).

Based on these features, the location of the cable sleeves is determined. When moving along the cable line, the audibility of the received signal may change due to changes in the depth (Fig. 10 c) of the cable; the audibility changes if the cable crosses communications or highways (at the same time, the audibility of the signal stops on the segment of the cable laying in a metal pipe). It should be noted that when the cable line passes along the route through sections with different types of cables (for example, the ASB cable is connected with the AAB cable using a coupling), the EMF induced in the induction converter will be different: it will be less over the AAB cable than over the ASB cable or SB. This is due to the fact that the AAB cable has better shielding.

In addition, the decrease in signal after the coupling gives the impression that the fault has been found. To avoid an error, after reducing the signal, increase the sensitivity of the receiver and listen to the zone of the cable line with a reduced signal.

If the highs and lows of the received signal are heard in the headphones, then the damage should be looked for further along the cable line route.

When working in a zone of strong electromagnetic interference caused by currents of industrial frequency of 50 Hz (overhead lines, transformer substations, operating cable lines, etc.), you should switch to a frequency of 10,000 Hz, while the effect of a 50 Hz frequency field will be reduced.

3.5. Determination of single-phase cable damages (method of "zero anomaly").

The “zero anomaly” method is used in cases where it is impossible to determine the location of a single-phase fault by other methods, for example, due to the large depth of cable laying, due to strong acoustic interference, etc., as well as the impossibility of converting a single-phase fault into an interphase one.

This method can determine the location of damage in about 50% of cases. When using this method with the help of a burner, it is necessary to obtain a resistance of several tens of ohms at the point of damage, but at the same time, the core must not be welded to the cable sheath. In some cases, the “zero anomaly” method can be used to determine single-phase faults that have a resistance at the defect site that is close to zero (“deaf earth”).

The generator at a frequency of 1000 or 10000 Hz is connected to the damaged core and cable sheath.

The operator, moving along the route of the cable line in the area of ​​the fault with a vertically located induction transducer, hears the minimum signal in the headphones.

To the right or to the left of the cable line route, the signal increases.

Using the indicator sensitivity adjustment knob, the minimum indicator reading is set exactly above the cable line route. Its pointer should be in the range not exceeding 20% ​​of the scale length.

When moving exactly over the cable line route, above the fault point, the indicator reading will increase sharply, while the audibility of the signal in the headphones will not change. After passing through the damage site, the indicator readings will be the same as before the damage site.

When using this method, you must know exactly where the couplings are located, as they tend to give a false increase in the signal.

An increase in the signal can also be in the undamaged part of the cable line, while you should go further along the line, where increases and decreases in signals can also alternate, which are measured by the indicator of the device.

In this case, the damage is at the last signal increase point.