Thursday, 14 September 2017

Fault detection method and calculation formula of different cable fault types

Cable fault search and locating work is always a big problem of the electric industry. This is mainly because most of the cable buried in the ground, laying distance from several meters to several kilometers, dozens of kilometers, coupled with the complex environment of the scene. In addition to the cable fault test equipment for high quality requirements, the technicians comprehensive quality is also requested very strict.
There are three commom types of cable faults. They are low resistance faults, high resistance short circuit ground fault and flashover fault. Three kinds of cable fault detection methods are also different for different types of cable faults.
1.  Low resistance faults
The general availability of Wheatstone bridge to loop measurement to the point of failure. The single phase earth fault point is measured by loop method, as shown in figure 1.

Single phase to ground fault measuring connection
When the fault point is connected to the X2 , the fault distance is LxAB×2L
When the fault point is connected to the X1, the fault distance is LxB÷AB×2L
Among them, A is the bridge arm unit variable readings, unit is Ω,
B is the other arm bridge readings, unit is Ω;
L is the long lines, unit is m;
Lx is the cable test end away from the fault point, unit is m.
When the two phase short circuit is used, the method is connected with a single ground, and the positive end of the power supply is connected to another bad cable core, and the end of the tested fault phase is short connected with the intact phase to form a loop.
When the three-phase short circuit is applied, the other line or temporary line is used as the loop line, as shown in figure 2.

Three phase short circuit grounding fault point measuring wiring
When the fault is connected to X2, a fault location from the end is LxABR×L;
When the fault is connected to X1, a fault location from the end is LxB÷ABR×L;
The single line resistance R is the temporary line, such as R≤A+B, can be ignored.
If there is no proper bridge, the voltage drop or current method can be adopted. The wiring is shown in figure 3.

With the voltage drop method, the fault distance is LxU1÷U1U2×2L.
With the current meter method, the fault distance is LxRx×L÷R=(I2-i1i2RL÷i1I1i2)+i2I2i1, RLRxRy
2. High resistance grounding short circuit fault
Basically, the loop method is used, but because of the large resistance to the fault, the fault current must not be too small. High voltage power supply is obtained by rectifier. In the test, the insulated platform, the distributor and the insulation rod of the bridge shall be used, and the handshake part shall be grounded. The pilot shall be grounded, and the wiring diagram shall be shown in figure 4.

High voltage grounding fault test guide wire
When the fault point is connected to the "0" , the fault distance is Lx=A÷100×2L;
When the fault point is connected to the "100" , the fault distance is Lx=2L×(1A÷100).
 A is a sliding wire resistance reading.
When detecting a high resistance fault, it can be tested by DC high voltage breakdown, and then by AC burning to make it low resistance grounding, and then test with low resistance measuring method.
3. Flashover fault
The flashover fault refers to the breakdown after the insulation resistance is still the same as the normal fault. It can be dealt with in the following manner:
(1) Repetitive breakdown of the fault point with DC voltage. It is changed into low resistance grounding, and then it is changed into stable low resistance grounding by AC burning.
(2) The acoustic method is used to measure the conduction. This is the use of capacitor C charging, after discharging the ball gap S to the fault point discharge, there is a larger sound, arrange people to listen to each part of the discharge electroacoustic, you can find the fault point, as shown in figure 5.

Acoustic test wiring diagram


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