Author: Noyafa–CCTV Tester
Low-resistance and high-resistance faults in cable faults Power cable faults are caused by damage to the insulation of the cable. Generally, the types of faults are roughly divided into two categories: low-resistance short-circuit, open-circuit and open-circuit faults; high-resistance leakage faults and flashover failures. 1. Low-resistance and high-resistance faults in cable faults Power cable faults are caused by damage to the insulation of the cable. Generally, the types of faults are roughly divided into two categories: low-resistance short-circuit, open-circuit and open-circuit faults; high-resistance faults Leakage failures and flashover failures. 1. Low-resistance fault When the insulation resistance of the fault point of the cable drops to the characteristic impedance of the cable, and even the fault with zero DC resistance is called a low-resistance fault or a short-circuit fault (Note: This definition is from the perspective of using the pulse reflection method, considering It is due to the effect of different wave impedances on the polarity changes of the reflected pulses.
For the bridge method, the definition of a low-resistance fault is not limited by the concept of characteristic impedance. ) Here is a reference value of cable characteristic impedance: the characteristic impedance of a power cable with an aluminum core of 240m ㎡ cross-sectional area is about 10Ω; The characteristic impedance of a power cable with an aluminum core of 35m ㎡ cross-sectional area is about 40Ω. The characteristic impedance of the remaining cross-sectional area of the aluminum core power cable can be estimated accordingly.
Any fault that the insulation resistance of the cable is infinite or the insulation resistance value of the normal cable is the same, but the voltage cannot be fed to the user end is called an open circuit (open circuit) fault. 2. High-resistance fault The faults where the DC resistance of the fault point of the cable is greater than the characteristic impedance of the cable are all high-resistance faults. 1. Leakage fault: During the high-voltage insulation test of the cable, the leakage current increases with the increase of the test voltage.
When the test voltage rises to the rated value (sometimes far below the rated value), the leakage current exceeds the allowable value, which is called high resistance leakage fault. 2. Flashover fault: When the test voltage rises to a certain value, the value of the meter monitoring the leakage current suddenly rises, and the needle oscillates in a flashover manner. When the voltage drops slightly, this phenomenon disappears, but the cable insulation is still extremely high. resistance, which indicates a faulty cable. This kind of fault point does not form a resistance channel, and only the fault on the discharge gap or flashover surface is called a flashover fault.
The properties of general high-resistance fault points can be represented by the equivalent circuit shown in the figure below. High-resistance fault equivalent circuit diagram Although the manifestations of high-resistance faults are varied, their essence is shown in the equivalent circuit in the above figure.“High resistance leakage fault”superior,“High resistance leakage fault”The resistance value directly determines the characteristics of high resistance faults, they can be either“High resistance leakage fault”,or“High resistance flashover fault”, or a failure of both. For example: when Rs is approximately infinite, the DC voltage across the fault point Js can be increased to a relatively high level without the leakage current exceeding the rated value. It is entirely possible that Js will be broken down before the voltage rises to the rated value, resulting in a flashover fault. .
When Rs is less than a certain value, during the withstand voltage test, a large leakage current will be generated due to the existence of Rs, such a large leakage current will produce a large voltage drop on the internal resistance of the high-voltage power supply, and the voltage across Js will not be able to If the voltage is increased, Js may not be broken down. If the voltage is to be increased, the leakage current is bound to increase. Therefore, it is entirely possible that the relay protection will operate because the leakage current greatly exceeds the allowable value, and the Js will not reproduce the flashover phenomenon. When Rs is equal to zero or less than the characteristic impedance of the cable under test, the nature of the fault becomes a low-resistance fault.
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