The commonly used cables in the power system are two types of power cables and control cables. The power cables are used to transport and distribute high-power electrical energy. According to the different insulation materials, it can be divided into oil-impregnated paper-insulated power cable, rubber-insulated power cable and PVC insulated cable. It is widely used in the engineering of oil-impregnated paper-insulated power cable, due to the cable in production, and the laying of lines. The environmental temperature, construction principles, etc., have been stipulated by the State, and will not be repeated here. The following points are mainly for the possible points of power cable faults and how to test them.

Cable fault types and test methods

After the cable is faulty, the fault type is usually judged with a 1500V or higher shake meter or a high resistance meter, and the fault is determined by using different instruments and methods. The fault point is then accurately determined by the fixed point method. The precision methods of the fault point are induction and acoustic measurements. Two kinds of law.

Induction method, the principle is when the audio current through the cable core, there are electromagnetic waves around the cable, because these carry electromagnetic induction receiver, walking along the line, you can listen to the sound of electromagnetic waves, audio current flow to the point of failure The abrupt change of current and the abrupt change of the audio frequency of the electromagnetic wave are convenient for searching for low resistance short-circuit faults between the disconnected phases, but they are not suitable for finding high resistance short circuits and single phase ground faults.

Sound measurement method, the principle is to use high-voltage pulses to promote fault discharge, resulting in discharge sound, with the sensor on the ground to receive this discharge sound, to detect the exact location of the fault.

The specific fault type is tested as follows.

A low resistance grounding fault

1. Single-phase low-resistance ground fault

(1) Test of the fault point.

The single-phase low-resistance grounding fault of a cable means that the resistance of one core-to-ground insulation of the cable is lower than 100 kΩ, and the continuity of the core wire is good. This kind of fault concealment is strong, we can use the principle of the loop fixed point method to test. Wiring diagram shown in Figure 1a, the fault core wire and another intact core wire to form a measurement loop, measuring with a bridge, one end with a jumper jumper, the other end of the power, bridge or galvanometer, adjust the bridge resistance The bridge is balanced. When the material and section of the cable core are the same, the following formula can be used to calculate. If the damaged core and good core are exchanged on the bridge, the Z-measurement end The distance of the fault point m; L - the total length of the cable, m; R1, R2 - the resistance arm of the bridge.

Under normal conditions, the measurement results of the two wirings should be the same, and the error is generally 0.1% to 0.2%. If it is out of this range or X>L/2, the measuring instrument can be moved to the other end of the line for measurement.

In addition, we can also use the continuous scanning pulse oscilloscope method (MST-1A type or LGS-1 digital tester) for testing. The reflected wave at the short circuit or ground fault point will be a negative reflection. In this case, the distance of the fault point can be calculated by the following formula: X—reflection time μs; V—wave speed, m/μs.

(2) Matters needing attention when measuring.

a. The section of the jumper wire should be close to the cross section of the cable core. The jumper wire should be as short as possible and be kept good.

b. The measurement loop should bypass the branch box or transformer and power distribution station as much as possible. The shorter the better.

c. DC supply voltage should not be less than 1500V.

d. The negative pole of the DC power supply shall be electrically bridged to the cable conductor, and the positive pole shall be connected to the inner sheath of the cable and grounded.

e. The operator should stand on the insulation mat and place the arm resistance, galvanometer, diverter, etc. on the insulation mat.

2. Two-phase short circuit fault point test

When a two-phase short-circuit fault occurs, the measurement wiring method is shown in Figure 2. When measuring, any fault core wire can be used as the grounding wire, and another faulty core wire can be connected to the bridge. The calculation formula and measurement method are the same as the single-phase low-resistance grounding fault point.

3. Three-phase short circuit fault point test

When a three-phase short-circuit fault occurs, other parallel lines must be borrowed or temporary lines must be installed for the measurement. To install a temporary line, the resistance of the line must be accurately measured. It can be calculated by the following formula, that is, R is the single-line resistance value of the temporary line in the formula, and the rest of the symbols have the same meaning as the formula (2).

Second, high resistance ground fault point

The high resistance earthing fault of the cable is that the insulation resistance between the conductor and the aluminum sheath or between the conductor and the conductor is much lower than the normal value, but it is greater than 100 kΩ, and the core wire continuity is good.

1. Use high-voltage bridge method to find high-resistance grounding fault

Due to the large resistance at the point of failure, high voltage DC power must be used to ensure that the current through the point of failure is not too small. The resistance of the bridge arm is about 100 Ω. The resistance is about 3.5Ω. The voltage applied by the bridge is 10～200kV. The microampere meter is indicated as 100～20μA. The distance from the fault point to the measuring end can be calculated by the following formula, that is, when the fault is exchanged. The position of the core and the perfect core is where X is the distance from the point of failure to the measurement, m; L is the length of the cable, m; and C is the bridge reading.

2. One-scan oscilloscope (711 type) method

The so-called one-scan oscilloscope method uses a high-voltage scanning oscilloscope at a time to record the fault point discharge oscillation waveform, determine the fault point, the oscilloscope screen shown in Figure 3b, the distance of the fault point can be calculated according to the following formula V - wave speed, m / Μs; T - oscillation period, μs.

3. Considerations when measuring

(1) Since the measurement is conducted under high pressure, it must be insulated from the ground. The operator should wear insulated gloves, operate with an insulating rod, and maintain a distance from the high voltage lead.

(2) The core wire should not be measured in the same cable and must be grounded to prevent dangerous high voltages from being induced.

(3) Gradually pressurize the measurement. If you find that the pointer of the ammeter is shaking or flashover failure, stop measuring immediately to avoid burning the instrument.

(4) When the measurement is completed by the positive connection method and the wiring needs to be replaced, the voltage must be reduced, the power supply must be cut off, and only the residual charge in the loop can be exhausted. The reverse wiring method can be used to change the wiring.

Third, completely broken fault point

The so-called complete disconnection fault means that each phase is well insulated and one or more phase conductors are not continuous. At this point, two methods can be used for testing.

1. Bridge method (capacitance bridge, QF1-A type bridge)

Measure the ratio of the faulty capacitor to the standard capacitor at the two ends of the line to determine the distance to the fault point. Calculate the capacitances measured at the E and F sides of the fault phase with CE and CF as the formula below.

2. Continuous scanning oscilloscope method (MST-1A or LGS-1)

Using the oscilloscope method, the pulse is transmitted. At the break point, the reflected wave is a positive reflection. The distance of the fault point is calculated according to the following formula: V—wave speed, m/μs; T—reflection time, μs.

Fourth, not completely broken fault point

Incomplete disconnection points are divided into high resistance disconnection (conductor resistance is greater than 1kΩ) and low resistance disconnection (conductor resistance is less than 1kΩ). It shows that the phases are well insulated and that one or more phases of wires are not completely continuous. At this point, we can use AC bridge method to measure the high resistance disconnection. The ratio of the faulty phase capacitance to the standard capacitor is measured at both ends of the line. The distance is calculated according to the following formula: CE, CF are the measured capacitances of the fault phase at E and F respectively. For the low-resistance disconnection, it is first blown with a low-voltage current, and then the full-line fault test is performed.

V. Others

In addition to the above situations, some failures may occur, such as:

(1) Completely disconnected and grounded faults. This fault is characterized by good insulation at one end and grounded at the other end. We can use the complete breakage fault point test method.

(2) Incomplete disconnection and grounding faults, such failures are manifested as good insulation of each phase, one or more phase conductors are not completely continuous, and resistance grounding can be used to test the high blocking line fault according to the AC bridge method.

(3) Flashover faults. The so-called flashover faults show that the insulation resistance of each phase is good, and the continuity of the leads is also good, and the fault point is closed. A high-resistance ground fault can be used to scan the oscilloscope at one time (Model 711), or it can be tested by other methods after burning.

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