Test method for fault points of power cables
The commonly used cables in the power system include power cables and control cables, among which power cables are used to transmit and distribute high-power electrical energy. According to the different insulation materials, they can be divided into oil-immersed paper insulated power cables, rubber insulated power cables, and polyvinyl chloride insulated cables. The most widely used in engineering is oil-immersed paper insulated power cables. Due to the clear regulations of the country in the production of cables, as well as the laying of lines, environmental temperature, and construction principles, they will not be repeated here, This article mainly introduces the possible points where power cables are prone to faults and several methods for testing.
Test method for fault points of power cables
1. Types and testing methods of cable faults
After a cable fault occurs, a megger or high resistance meter above 1500V is usually used to distinguish the type of fault, and different instruments and methods are used to preliminarily test the fault. Finally, the fixed point method is used to accurately determine the fault point. There are two precise testing methods for the fault point, namely induction method and acoustic measurement method.
The principle of induction method is that when audio current passes through the cable core, there are electromagnetic waves around the cable. Some carry electromagnetic induction receivers and can hear the sound of electromagnetic waves when walking along the line. When the audio current flows to the fault point, the current suddenly changes and the frequency of electromagnetic waves changes. This method is very convenient for finding low resistance short circuit faults between broken wires, but 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 stimulate the discharge of the fault point, generating a discharge sound. A sensor is used to receive this discharge sound on the ground to measure the precise location of the fault point.
The specific fault types shall be tested according to the following methods.
1.1 Low resistance ground fault
1.1.1 Single phase low resistance ground fault
(1) Testing of fault points.
The single-phase low resistance grounding fault of a cable refers to the insulation resistance of one core wire of the cable to the ground being less than 100k Ω, while the continuity of the core wire is good. This type of fault has strong concealment, and we can use the principle of loop fixed point method for testing. The wiring diagram is shown in Figure 1a. The faulty core wire and another intact core wire form a measuring circuit, which is measured with a bridge. One end is connected with a jumper wire, and the other end is connected with a power supply, bridge or Galvanometer. Adjust the resistance of the bridge to balance the bridge. When the cable core wire has the same material and section, it can be calculated according to the following formula
If the damaged core and the good core are swapped on the bridge, there is a formula where Z - the distance m from the measurement end to the fault point; L - total length of cable, m; R1, R2- Resistance arm of the bridge.
Under normal circumstances, the measurement results of these two types of wiring should be the same, with an error of 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 the continuous scanning pulse oscilloscope method (MST-1A or LGS-1 digital tester) for testing. The reflected wave at the point of short circuit or ground fault will be negative reflection, as shown in Figure 1b of the oscilloscope screen. At this point, the distance from the fault point can be calculated using the following formula, where X - reflection time μ S; V - Wave velocity, m/ μ S.
(2) Precautions during measurement.
a. The cross-section of the jumper wire should be close to the cross-section of the cable core wire, and the jumper wire should be as short as possible and kept in good condition.
b. The measurement circuit should avoid branch boxes or substations as much as possible, and 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 through an electric bridge, and the positive pole should be connected to the inner protective layer of the cable and grounded.
e. The operator shall stand on the insulating mat and place the bridge arm resistance, Galvanometer, shunt, etc. on the insulating mat.
1.1.2 Testing of Two Phase Short Circuit Fault Points
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 Testing of three-phase short circuit fault points
When a three-phase short circuit fault occurs, other parallel circuits or temporary circuits must be used as circuits for measurement. When installing temporary circuits, the resistance of the circuit must be accurately measured, and the wiring method is shown in Figure 2. It can be calculated according to the following equation, where R is the single line resistance value of the temporary line, and the meaning of other symbols is the same as that of equation (2).
Test method for fault points of power cables
1.2 High resistance grounding fault points
A high resistance grounding fault in a cable refers to the insulation resistance value between the conductor and the aluminum sheath or between the conductor and the conductor, which is much lower than the normal value but greater than 100k Ω, while the continuity of the core wire is good.
1.2.1 Using High Voltage Bridge Method to Find High Resistance Grounding Faults
The wiring principle is shown in Figure 3a. Due to the high resistance at the fault point, it is necessary to use a high-voltage DC power supply to ensure that the current passing through the fault point is not too small. The bridge arm resistance is about 3.5 Ω, which is equal to 100 parts of the sliding wire resistance. The voltage applied to the bridge is 10-200kV, and the microampere meter indicates 100-20 Ω μ A. The distance from the fault point to the measurement end can be calculated using the following formula, that is, when the position of the faulty core wire and the intact core wire in Figure 3 is replaced, there is an equation where X - the distance from the fault point to the measurement, m; L - Cable line length, m; C - Sliding wire bridge reading.
1.2.2 Single sweep oscilloscope (Type 711) method
The so-called one time scanning oscilloscope method uses a high-voltage one time scanning oscilloscope to record the discharge oscillation waveform of the fault point and determine the fault point. The fluorescent screen of the oscilloscope is shown in Figure 3b, and the distance from the fault point can be calculated according to the following formula: V - wave velocity, m/ μ S; T - oscillation period, μ S.
1.2.3 Precautions during measurement
(1) Due to the fact that the measurement is carried out under high voltage and must be insulated from the ground, operators should wear insulated gloves, use insulated rods for operation, and maintain a distance from the high-voltage lead.
(2) The core wires in the same cable that are not measured must also be grounded to prevent the induction of dangerous high voltage.
(3) The pressure shall be gradually increased during measurement. If the pointer of the Ammeter shakes or flashover fault is found, the measurement shall be stopped immediately to avoid burning the instrument.
(4) When the measurement is completed using the direct connection method and the wiring needs to be replaced, the voltage must be reduced, the power supply must be cut off, and only after the residual charge in the circuit is discharged can the wiring be replaced for the reverse connection method measurement.
1.3 Complete disconnection fault point
The so-called complete disconnection fault refers to the condition where the insulation of each phase is good and one or more phases of the wire are discontinuous. At this point, two methods can also be used for testing.
1.3.1 Bridge method (capacitive bridge, QF1-A type bridge)
The wiring is shown in Figure 4a. The ratio of the faulty capacitance to the standard capacitor is measured at the two ends of the line to determine the distance between the faulty point. CE and CF can be calculated using the following formula, where they are the capacitance measured when the faulty phase is at the E and F ends.
1.3.2 Continuous scanning oscilloscope method (MST-1A or LGS-1 type)
By using an oscilloscope method, pulses are emitted, and at the point of a broken line fault, the reflected wave is a positive reflection. The oscilloscope screen diagram is shown in Figure 4b, and the distance from the fault point is calculated using the following formula: V - wave velocity, m/ μ S; T - Reflection time, μ S.
1.4 Incomplete disconnection fault points
There are two types of incomplete disconnection points: high resistance disconnection (conductor resistance greater than 1k Ω) and low resistance disconnection (conductor resistance less than 1k Ω). It exhibits good insulation in all phases and incomplete continuity in one or more phases of the wire. At this point, we can use the AC bridge method to measure high resistance wire breakage, and the wiring schematic is shown in Figure 5. Measure the ratio of the capacitance of the faulty phase to the standard capacitor at both ends of the line, and calculate the distance according to the following formula. CE and CF are the capacitance measured at the E and F ends of the faulty phase, respectively. For low resistance disconnection, use low voltage current to burn it out first, and then test it as a complete line fault.
In addition to the above situations, some faults may also occur, such as: (1) complete disconnection and grounding fault, which manifests as good insulation of one end and each phase, and grounding of the other end. We can use the complete disconnection fault point testing method. (2) Incomplete disconnection and grounding fault, characterized by good insulation of each phase, incomplete continuity of one or more phase wires, and grounding through resistance, can be tested using the AC bridge method for high blocking line faults. (3) Flashover fault refers to a condition where the insulation resistance of each phase is good, and the continuity of the wire is also good. The fault point has been closed. At this point, the single sweep oscilloscope (Type 711) method in high resistance grounding faults can be used, or other methods can be used for testing after burnout.