1. Main types and judgments of cable faults
No matter the high voltage cable or the low voltage cable, in the process of construction, installation and operation, often due to short circuit, overload operation, insulation aging or external forces and other causes of failure.
Cable faults can be classified into three types: ground, short circuit and broken cable. The fault types mainly include the following aspects:
Three-core cable ground with one or two cores.
2. Short circuit between two phase cores.
Three-phase core wire completely short circuit.
A broken or multiphase core wire.
For direct short circuit or broken wire fault, the multimeter can be used to directly measure and judge; for non-direct short circuit and ground fault, the Megabyte can be used to measure the insulation resistance between the core wires or the insulation resistance of the core wire to the ground, and the fault type can be determined according to its resistance value.
After the fault type is determined, it is not an easy thing to find the fault point. Here, according to the author's experience, introduce several methods to find the fault point for reference.
2. Search method of cable fault points
(1) Acoustic measurement method:
The so-called acoustic measurement method is to find the fault cable discharge sound. This method is more effective for the flashover discharge of the high voltage cable core.
The equipment used in this method is dc compression tester.
When the capacitor charge to a certain voltage value, the ball discharge gap of cable fault core wires, cable conductor at fault on the insulation layer discharge spark discharge of the "zi and zi", for Ming laying cable by hearing can be directly to find, if buried cable, first to determine and indicate the cable to, again at the time of mixed noise minimum, with the aid of deafness audio amplifier devices such as hearing AIDS or stethoscope lookup.
When looking for, the pickup should be close to the ground and move slowly along the cable. When the discharge sound of "Z, Z" is heard to the maximum, the point is the fault point.
It is necessary to pay attention to safety when using this method. Special persons should be assigned to monitor the test equipment end and cable end.
(2) Bridge method:
The method of electric bridge is to measure the direct current resistance value of the cable core line with the double-arm electric bridge, and then accurately measure the actual length of the cable, and calculate the fault point according to the direct ratio relation between the cable length and the resistance.
The method for direct short circuit or short circuit point contact resistance between the cable core is less than 1 Ω fault, general judgment error is not more than 3 m, for the failure of point contact resistance is greater than 1 Ω failure, can adopt the method of heightening voltage burn through resistance drop below 1 Ω, then measured by this method.
Firstly, the resistance R1 between the core wire A and B is measured, then R1=2Rx+R, where Rx is the one-phase resistance value from a phase or B phase to the fault point, and R is the contact resistance of the short contact point.
Is again at the other end of the cable to measure a 'and b' core direct current resistance value between the R2, the R2 = 2 R + R (L - X), type of R (L - X) as a 'phase or b' phase conductor resistance, to the point of failure of a phase after measuring the R1 and R2, then b 'and' short answer c, b and c two phase conductor is measured between the dc resistance, then the half of the value for each phase conductor resistance value, expressed in RL, RL = Rx + R (L - X), which can be concluded that the failure point of contact resistance: R = R1 + R2-2 RL, therefore, fault point on both sides of the conductor resistance value can be used under type said:
Rx = (R1 - R) / 2, R (L) X = (R2 - R) / 2.
After the three values of Rx, R(L-x) and RL are determined, the distance X or (L-x) between the fault point and the cable end can be calculated according to the scaling formula: X=(Rx /RL)L, (L-x)=(R(L-x)/RL)L, where L is the total length of the cable.
When using bridge method, the measurement accuracy should be guaranteed, the bridge connection line should be as short as possible, the wire diameter should be large enough, the connection with the cable core should be pressed or welded, and the middle and small digits should be all retained in the calculation process.
(3) Capacitance current measurement method:
During the operation of the cable, there is capacitance between the core wires and the core wires to the ground. The capacitance is evenly distributed, and the capacitance is linearly proportional to the length of the cable. The capacitance current measurement method is based on this principle to measure, which is very accurate for the determination of the cable core broken fault.
The equipment used is 1 ~ 2kVA single-phase voltage regulator, 0 ~ 30V, 0.5 ac voltmeter, 0 ~ 100mA, 0.5 AC milliammeter.
Measurement steps:
(1) First measure the capacitive current of each core wire at the end of the cable respectively (the applied voltage should be kept equal)Ia, Ib, Ic values.
At the end of the cable, the capacitance current of Ia ', Ib 'and Ic' of each phase conductor is measured to check the capacitance ratio between the intact core and the broken core, and the approximate point of the broken distance can be preliminarily determined.
According to the capacitance calculation formula C=1/2 fU, C is proportional to I with constant voltage U and frequency F.
Since f(frequency) of power-frequency voltage is constant, the ratio of capacitor current is the ratio of capacitance as long as the applied voltage is constant during measurement.
Suppose the total length of cable is L and the distance of core wire breaking point is X, then Ia/ Ic=L/X, X=(Ic/ Ia)L.
In the measuring process, as long as the voltage is not changed, the ammeter reading is accurate, the total length of the cable is measured accurately, and the measurement error is relatively small.
(4) Zero potential method:
Zero potential method, that is, potential comparison method, it is adapted to the shorter length of cable core over the ground fault, this method is applied to measuring accuracy is simple, do not need precision instruments and complicated calculation, measurement principle is as follows: compare cable fault core and long wire in parallel, in E on both ends of the applied voltage, the equivalent on both ends of two parallel uniform resistance wire connect the power supply, at this point, any aspect of a resistance wire and the other a potential difference between the corresponding points on the resistance wire must be zero.
Conversely, two points with zero potential difference must be corresponding points.
Since the negative pole of the microvolt meter is grounded at the same potential as the cable fault point, when the positive pole of the microvolt meter moves on the comparison wire to the point where the indicated value is zero and the fault point is at the same potential, that is, the corresponding point of the fault point.
The measurement steps are as follows:
First connect the battery E on the b and C core wires, and then lay a comparison wire S equal to the length of the fault cable on the ground. The wire should be made of bare copper wire or bare aluminum wire with the same section and no intermediate joint.
(2) ground the negative electrode of the microvolt meter, connect the positive electrode to a longer soft wire, the other end of the wire should be able to fully contact when sliding on the laid comparison wire.
Close the knife switch K and slide the end of the flexible wire on the comparison wire. When the microvoltmeter indicates zero, the position is the cable fault point.