Testing method of power cable fault point
Testing method of power cable fault point
There are two types of cables commonly used in power systems, power cables and control cables. Power cables are used to transmit and distribute high-power electrical energy. According to the different insulation materials, it can be divided into oil-impregnated paper-insulated power cables, rubber-insulated power cables and PVC insulated cables. The most widely used in engineering is oil-impregnated paper-insulated power cables. Because the cables are being manufactured and laid The country has express regulations for wiring, ambient temperature, construction principles, etc., so I won't repeat them here. This article mainly introduces the possible points of power cables that are prone to failure and how to test several methods.
Power cable fault point test method
1. Types of cable faults and test methods
After the cable fails, generally use a 1500V or higher shake meter or a high resistance meter to determine the type of fault, and then use different instruments and methods to first measure the fault, and finally use the fixed-point method to accurately determine the fault point. The precise measurement methods for the fault point include induction method and acoustic measurement. Two methods.
The principle of the induction method is that when the audio current passes through the cable core, there are electromagnetic waves around the cable. Because some carry electromagnetic induction receivers, when walking along the line, you can hear the sound of the electromagnetic waves. When the audio current flows to the fault point , Sudden changes in current, sudden changes in the audio frequency of electromagnetic waves, this method is very convenient to find low-resistance short-circuit faults between disconnected phases, but it is not suitable for finding high-resistance short-circuits and single-phase grounding faults.
The principle of acoustic measurement is to use high-voltage pulses to prompt the discharge of the fault point to produce a discharge sound. The sensor is used to receive the discharge sound on the ground to measure the precise location of the fault point.
The specific fault types are tested according to the following methods.
1.1 Low resistance ground fault
1.1.1 Single-phase low resistance ground fault
(1) Test of failure point.
The single-phase low-resistance earth fault of the cable means that the insulation resistance of one core wire of the cable to the ground is less than 100kΩ, and the core wire has good continuity. This kind of fault is highly concealed, and we can use the principle of loop fixed-point method to test. The wiring diagram is shown in Figure 1a. The faulty core wire and another intact core wire form a measurement loop, measure with a bridge, one end is connected with a jumper, the other end is connected with a power supply, a bridge or a galvanometer, and the bridge resistance is adjusted. To balance the bridge, when the material and cross-section of the cable core wire are the same, it can be calculated according to the following formula
If the damaged core and the good core are interchanged on the bridge, then there is the formula: Z——distance from the measuring end to the fault point in m; L——total cable length, m; R1, R2——electricity The resistance arm of the bridge.
Under normal circumstances, the measurement results of these two wirings should be the same, and the error is generally 0.1% to 0.2%. If it exceeds 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 continuous scanning pulse oscilloscope method (MST-1A or LGS-1 digital tester) for testing. The reflected wave at the short-circuit or ground fault point will be negative reflection, as shown in Figure 1b on the oscilloscope screen. At this time, the distance to the fault point can be calculated according to the following formula: X-reflection time μs; V-wave speed, m/μs.
(2) Matters needing attention during measurement.
A. The cross-section of the jumper should be close to the cross-section of the cable core, and the jumper should be as short as possible and kept well.
B. The measurement loop should bypass the branch box or substation or power distribution station as much as possible, the shorter the better.
C. The DC power supply voltage should not be less than 1500V.
D. The negative pole of the DC power supply should be connected to the cable conductor via an electric bridge, and the positive pole should be connected to the inner sheath of the cable and grounded.
E. The operator should stand on the insulating pad and place the bridge arm resistance, galvanometer, shunt, etc. on the insulating pad.
1.1.2 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 faulty core wire can be used as a grounding wire, and the other faulty core wire can be connected to a bridge. The calculation formula and measurement method are the same as the single-phase low-resistance grounding fault point.
1.1.3 Three-phase short circuit fault point test
When a three-phase short-circuit fault occurs, other parallel circuits or temporary circuits must be used as loops during measurement. When temporary circuits are installed, the resistance of the circuit must be accurately measured. The wiring method is as shown in Figure 2. It can be calculated by the following formula, where R is the single-wire resistance value of the temporary line, and the meaning of the remaining symbols is the same as formula (2).
Power cable fault point test method
1.2 High resistance ground fault point
The high resistance ground fault of the cable is that the insulation resistance between the conductor and the aluminum sheath or the conductor and the conductor is far lower than the normal value, but greater than 100kΩ, and the core wire has good continuity.
1.2.1 Use high-voltage bridge method to find high-resistance ground fault
The wiring principle is shown in Figure 3a. Due to the large resistance of the fault point, a high-voltage DC power supply must be used to ensure that the current through the fault point is not too small. The bridge arm resistance is a 100-divided wire resistance of about 3.5Ω, the voltage applied to the bridge is 10～200kV, the microammeter indication is 100～20μA, the distance from the fault point to the measuring end can be measured by the following formula, that is, when the diagram is changed In 3, the position of the faulty core wire and the intact core wire is as follows: X——distance from fault point to measurement, m; L——length of cable line, m; C——sliding wire bridge reading.
1.2.2 One scan oscilloscope (711 type) method
The so-called one-scan oscilloscope method is to use a high-voltage one-scan oscilloscope to record the discharge oscillation waveform of the fault point to determine the fault point. The oscilloscope phosphor screen is shown in Figure 3b. The distance of the fault point can be calculated according to the following formula: V——wave velocity, m/ μs; T——Oscillation period, μs.
1.2.3 Matters needing attention during measurement
(1) Since the measurement is carried out under high voltage, it must be insulated from the ground*. The operator should wear insulating gloves, operate with an insulating rod, and keep a distance from the high-voltage lead.
(2) The same cable must be grounded without measuring the core wire to prevent dangerous high voltage from being induced.
(3) The pressure should be gradually increased during measurement. If the ammeter pointer is shaken or flashover failure is found, the measurement should be stopped immediately to avoid burning the meter.
(4) When the positive connection method is used for measurement and the wiring needs to be replaced, the voltage must be reduced and the power supply must be cut off. Only when the residual charge in the circuit is discharged, can the wiring be exchanged for reverse connection measurement.
1.3 Completely disconnected fault point
The so-called complete disconnection fault means that the insulation of each phase is good, and the conductors of one or more phases are not continuous. At this time, the same two methods can be used for testing.
1.3.1 Bridge method (capacitance bridge, QF1-A type bridge)
The wiring is shown in Figure 4a. Measure the ratio of the faulty capacitance to the standard capacitor at the two ends of the line to determine the distance to the fault point. The following formula can be used to calculate the CE and CF when the fault phase is at the E and F ends. The capacitance.
1.3.2 Continuous scanning oscilloscope method (MST-1A or LGS-1 type)
Using the oscilloscope method, the pulse is transmitted, and the reflected wave is regular reflection at the fault point of the disconnection. The screen picture of the oscilloscope is shown in Figure 4b. The distance to the fault point is calculated according to the following formula: V——wave velocity, m/μs; T——reflection time, μs.
1.4 Incomplete disconnection fault point
Incomplete disconnection points are divided into high-resistance disconnection (conductor resistance greater than 1kΩ) and low-resistance disconnection (conductor resistance less than 1kΩ). It shows that the phases are well insulated, and the conductors of one or more phases are not completely continuous. At this time, we can use the AC bridge method to measure the high-resistance disconnection, and the wiring schematic diagram is shown in Figure 5. Measure the ratio of the capacitance of the fault phase to the standard capacitor at both ends of the line. The distance is calculated according to the following formula, where CE and CF are the capacitances measured at the E and F terminals of the fault phase respectively. For low-resistance disconnection, first use low-voltage current to make it burn out, and then press the complete line fault test.
In addition to the above conditions, some faults may occur, such as: (1) Completely disconnected and grounded faults. This fault is manifested by good insulation of each phase at one end and grounding at the other end. We can use the complete disconnection fault point test method. (2) Incomplete disconnection and grounding fault. This type of fault is manifested by good insulation of each phase, one or more phase conductors are not completely continuous, grounded by resistance, and the AC bridge method can be used to test for high-blocking line faults. (3) Flashover faults. The so-called flashover faults show that the insulation resistance of each phase is good, and the continuity of the wire is also good, and the fault point has been closed. At this time, the one-scan oscilloscope (Type 711) method in high-resistance ground faults can be used, or other methods can be used for testing after burning through.