General methods for cable fault location in underground cables

14.02.2016

Having located the cable fault operator must:

a) determine the type of fault in accordance with previous section (Classification of faults and damages in power cables);

b) determine the distance to fault with the help of a low-voltage, or high-voltage time-domain reflectometer TDR (ISKRA-3M);

c) trace the cable line with a sound frequency generator (GZCH-2500) and the induction receiver (POISK-2006M or P-806) (ie, to determine the route of underground cable on the ground surface) and to limit fault area (ie, the area within which located a FP);

d) locate and pinpoint the FP within area of fault.

The solution of this task depends on type and characteristics of equipment available and qualifications of operators. Next we will consider some techniques of the methodic.

1. Induction search method 

Induction method for cable fault location lies in the fact that during passing current through a cable from the sound frequency generator (GZCH-2500) around the cable formed magnetic field. The nature of distribution of this field depends on many factors (cable structure, the connection method of the generator, a type of fault, presence of a cable sheath contact with ground etc.).

The induction sensor of receiver (POISK-2006M or P-806) transforms this induction (magnetic) field into electrical signal, which is being amplified, selected on frequency and converted into sound by headphones. Under certain conditions, the magnetic field of cable fault decreases sharply, which is indication sign of fault during cable fault location process.

To find a route of a cable line and point of fault with induction method the receiver package includes a remote inductive sensor with hand holder, and inside the receiver has resonant narrowband amplifier.

Inductive sensor consists of a cylindrical multi-turn coil, winded on ferrite core. It is important to understand that such sensor, with respect to the variable magnetic field, has directional properties, namely: the output signal of the sensor will be maximized when the magnetic field lines are perpendicular to the coil plane (or, in other words, the magnetic vector field coincides with the axis of the sensor). The signal will be minimal or absent, if the magnetic field lines are parallel to the plane of the coil (ie, the magnetizing force vector is perpendicular to the axis of the sensor).

2. To find a route of underground cable GZCH-2500 is being connected between core and sheath, according to scheme (Figure 1). 

If the cable has no signs of the single-phase short-circuit fault of cable (SSC), then the other end of connected core at point B is connected to the sheath (Figure 2).

This connection of the generator GZCH-2500 and a cable called "route mode".

Note on the current distribution (see Figure 1): the current i1, generated by GZCH-2500 flows to FP (Point X) and then divides into currents i2 and i3, which flows through the cable sheath to the opposite ends, and i1 = i2 + i3. This is due to the fact that both cable ends are grounded, i.e. have close potentials.

The current, which flows through the sheath, called called "route current" or "single current". Exactly this current creates alternating magnetic field outside the cable, which reacts on induction sensor of receiver. Magnetic force lines of "single current" have the form of concentric circles covering the cable (Figure 3).

In section of A-X, field will be created by a difference of currents i1-i3, while in section X-B – by a current i2, where i1-i3 = i2, therefore, a magnetic field value before and after fault point does not change.

Because the field of a single current exists throughout the cable and has the same value, phase and direction, then FP in route mode cannot be determined, and can be determined only the route itself (Figure 3).

3. To determine the FP with induction method, there is another way to connect GZCH-2500 to a cable, so-called "Loop mode".

In this mode GZCH-2500 is connected to two damaged cores of the cable (Figure 4).

As shown in Figure 4, for implementation this regime, two cores must be short-circuited in a FP. Connection of cores with sheath in the FP occurs most often.

It should be noted that even a small (5 ÷ 10 cm.) distance between cores connection with sheath lead to single current appearance, which field masks the "loop" signal and does not allow to determine the FP. Formation of a single current contributes also various resistances between damaged cable cores and sheath.

If you experience such effects, you must either "burn" cores in FP or "destroy" their contact with the sheath and locate FP with acoustic method.

Figure 4 shows that the current of GZCH-2500 does not spread on a sheath, ie there is no single current in this mode. Consequently, a magnetic field that exists outside a cable and which is result of currents i1 and i2 (the so-called loop field) ends in the immediate vicinity of the FP..

It should be noted that, unlike the single current field, which strength is almost constant along the length of the cable, loop field changes its strength along the length of the cable with a strand pitch (see Figure 5). For power cables, 6-35 kV strand pitch is 0.5 - 1.5m.

Thus, location features of loop mode are undulating changing of field strength along the length of the cable with a strand pitch before the fault and no field after the fault.

Due to the shielding effect of a cable sheath and a small area of field-creating (distance between conductors in power cables approx. 1 cm) loop field 50-100 and more times smaller than the field of route at the same route currents.

In addition it should be noted that the field of loop sharply decreases with distance, and it is practically possible to register on the distance not more than 1-2 meters far from cable, whereas the field of route can be registered at a distance of 5-10 m.

4  Method of locating of solid single-phase short-circuit in cable with electroacoustic method.

Method of locating of solid single-phase short-circuit in cable with electroacoustic method see Figure 6

Figure 6 shows the cable to which surge wave generator SWG is connected between faulty core and sheath - periodically discharge capacitor to the cable 300-400 uF, charged to voltage 5-10kV.

Current pulse passing all over the distance from the point of connection of SWG till the point of fault,  will occur pulse acoustic signal due to electrodynamic forces between  two electrical conductors (core and sheath). Sheath shaking and this shaking transfers through the soil to a ground surface. After the damage point there will be no such signal (shaking of sheath or its swelling), because current does not flow after the damaged core and sheath.

In addition, flowing of current through a cable causes an electromagnetic field pulse, which starts the receiver.

Therefore, if the operator will move along cable route with the receiver and the acoustic sensor, then throughout the cable length from the beginning to the point of fault, he will register the same acoustic signal (at constant receiver sensitivity and depth of the cable), and right after the damage the signal decays rapidly and disappears.

This effect is a search feature, and based on it, should be searched the single-phase damage. The first device, which was implemented this method of finding the damage was Receiver "POISK-01", developed and tested in 2000 year.

 On the principle of the instrument was issued the patent. Later the unit was improved and modernized several times under the trade name "POISK".