The testing procedure or test steps of power cable faults can be simply described as three steps: analyzing the nature of the fault, roughly measuring the fault point, and locating the fault point. However, in order to better master the cable fault testing technology, the following procedures can be followed.
Understand the structural characteristics, parameters and laying conditions
Understand the voltage resistance level, insulation medium type, structural characteristics, relevant technical parameters, line connection conditions, laying methods and paths of the faulty cable under test.
Voltage resistance level of the tested cable
The fault test methods of cables with different voltage resistance levels, the level of the maximum test voltage, and the matching use of test equipment will be different and cannot be generalized. For example, for most single-phase faults of low-voltage cables below 1kV,
if the "pulse reflection" principle method is used for testing, it will not be very effective; for faults above medium and high voltage, with high fault resistance (more than tens of megohms), if the traditional "bridge method" is directly used for testing, the location of the fault point will not be measured. For example, the maximum test voltage of 10kVXLPE cables generally does not exceed 35kV, while the maximum test voltage of 35kVXLPE cables is 80kV. For example, if the "high voltage pulse method" (flashover method) is used to test cable
faults, the output voltage and output capacity of high voltage equipment (such as high voltage transformers and high voltage energy storage capacitors) for cables below 6kV, 6~35kV, 66kV and above cannot be configured and used according to one model.
Insulating medium type of the tested cable
Knowing the insulating medium type of the tested cable has two meanings:
(1) The insulating medium of the cable is related to the maximum test voltage of the cable. For example, the maximum test DC voltage of a 10kV oil-impregnated paper medium power cable is 50kV; while the maximum test DC voltage of a 10kV XLPE power cable is 35kV.
Therefore, when using impulse high voltage or DC high voltage to measure cable faults, the test voltage cannot be higher than the maximum DC test withstand voltage of the cable.
(2) When using the "pulse reflection" principle to test cable faults, the rough measurement accuracy of the cable fault is directly related to the insulating medium of the cable (see 3.5.1 for detailed analysis), and has nothing to do with the thickness, shape and withstand voltage level of the cable.
Structural characteristics of the tested cable
From the classification and structural characteristics of the cable, we can see that the structural types of power cables with different withstand voltage levels and different insulation materials are not exactly the same. Due to the different structures, there are also great differences in the connection methods of fault testing. For example, the phase-to-phase connection method of low-voltage cables and oil-immersed paper dielectric cables is not exactly the same as the connection method of single-phase to ground faults; for cables of 6kV and above, their insulation damage faults are almost all manifested as relative to ground faults. The choice of ground wire is the only copper shield. This aspect is also described in detail in the following sections.
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