Power cable faults can be divided into open circuit faults, low-impedance faults, and high-impedance faults.
If the insulation resistance value of the cables in phase or relative to each other reaches the required specification value, but the working voltage cannot be transmitted to the terminal, or if the terminal has voltage but the load capacity is poor, such failure is called an open-circuit fault. If the phase-to-phase or relative insulation of the cable is damaged, the fault whose insulation resistance is reduced to a certain degree is called a low-impedance fault. Relative to the low resistance fault, if the fault resistance of the cable between phases or relative to each other is large, it is called a high resistance fault, which includes a leakage high resistance fault and a flashover high resistance fault. Leakage high-impedance faults refer to faults in which the leakage current gradually increases with the increase of the test voltage and greatly exceeds the specified leakage value. Flashover high-impedance fault means that the insulation resistance value is very large, and when the test voltage rises to a certain value, the leakage current suddenly increases.
In the detection of cable faults, the nature of the cable fault must first be determined. Usually the cable is disconnected from the power supply system and measured according to the following steps:
1. Insulation resistance tester measures each relative insulation resistance, such as the insulation resistance indicator is zero, can be measured with a multimeter or loop resistance tester to determine whether it is high resistance or low resistance grounding;
2. Measure the insulation resistance between the two phases to determine if it is a phase-to-phase fault.
3. Short-circuit the other end of the three phases and measure the DC resistance of the core to determine if there is an open circuit fault.
First, cable fault detection technology The main methods used are low-voltage pulse method and high-voltage flashover method.
Low-voltage pulse method can measure open circuit faults, phase-to-phase or relatively low-impedance faults in cables;
The high voltage flash method can be used to detect high resistance faults.
The principle of low-voltage pulse measurement is based on the principles of transmission and reflection in a uniform transmission line. The cable under test is regarded as a uniform transmission line, and its characteristic impedance at each point is equal. When a low-voltage pulse wave is transmitted from one end of the cable, the impedance of the fault point changes, and the electromagnetic wave propagates to the point where it collapses. , reflection phenomenon, reflected voltage Ue and incident voltage Ui satisfy the relation:
Among them: Zc is the characteristic impedance of the cable, Z is the equivalent wave impedance of the cable fault point. For a low-resistance fault, if the fault point resistance to ground is R, then the equivalent wave impedance at this point Z = R/Zc; for an open-circuit fault, if the fault resistance is R, the equivalent impedance at this point is Z = R + Zc.
When -1<β<0, it means that there is a reflected wave at the low impedance point, and the reflected wave is opposite in polarity to the incident wave. The smaller R is, the larger β is, the larger Ue is.
When R=0 is a short-circuit fault, β=-1, Ue=-Ui: the voltage wave produces total reflection at the short-circuit fault point;
When 0<β<+1, it means that there is a reflected wave at the open circuit fault point, and the reflected wave is the same polarity as the incident wave. The larger R is, the larger β is, the larger Ue is.
When R = ∞, which is a disconnection fault, β = +1, Ue = -Ui: The voltage wave generates an open-circuit total reflection at the disconnection fault point.
When actually measuring the low-resistance and open-circuit faults with the instrument, a low-voltage pulse with a width of 0.1~2μs and an amplitude greater than 120V is generated in the device and is added to the faulty phase of the cable at time t0. At this point, the pulse propagates to the cable fault point at speed v and reaches the fault point after the same time ?t time, and a reflected pulse is generated. The reflected pulse wave propagates to the measuring terminal at the same speed v, and after the same time? t arrives at the measuring end at time t1. If the distance from the fault point to the measuring end is L
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