Judgment method for common cable faults
The method to determine the type of cable fault is to measure the insulation resistance of each phase at one end of the line with an insulation resistance meter. Generally, the fault type is determined according to the following conditions:
(1) When the insulation resistance of one core or several cores of the cable is shaken to the ground, or the insulation resistance between the cores is lower than 100 kohms, it is a low-resistance grounding or short-circuit fault.
(2) When the insulation resistance of one core or several cores of the cable is shaken to the ground, or the insulation resistance between the cores is much lower than the normal value, but higher than 100 kohms, it is a high-resistance grounding fault.
(3) When the insulation resistance of one core or several cores of the shaking test cable to the ground is high or normal, a conductor continuity test should be carried out to check whether there is a disconnection. If there is, it is a disconnection fault.
(4) When one or several conductors of the shaker cable are discontinuous and grounded by resistance, it is a disconnection and ground fault.
(5) Flashover faults mostly occur in preventive withstand voltage tests, and most of them occur at cable terminals and intermediate joints. Flashovers sometimes occur multiple times in succession, with intervals ranging from seconds to minutes each time.
failure test method
The instruments and equipment used in the past include QF1-A cable detector, DLG-1 flash tester, cable path meter and fault point detector. At present, the more popular test method is the flash test method, which includes flash and direct flash, and the flash method is commonly used. The flash test has high precision, simple operation, safety and reliability. Its equipment is mainly composed of two parts, namely the high voltage generating device and the current pulse instrument. The high-voltage generating device is used to generate DC high voltage or impact high voltage, which is applied to the fault cable to force the fault point to discharge and generate a reflected signal. The current pulse meter is used to pick up the reflected signal to measure the fault distance or directly use the low voltage pulse to measure the open circuit, short circuit or low resistance fault.
The following is a brief description of the test method based on the fault point resistance:
(1) When the resistance of the fault point is equal to infinity, it is easy to find the open circuit fault by low-voltage pulse method measurement. Low resistance faults coexist.
(2) When the resistance of the fault point is equal to zero, it is easy to find the short-circuit fault by measuring the low-voltage pulse method, but this kind of fault is rarely encountered in actual work.
(3) When the resistance of the fault point is greater than zero and less than 100 kΩ, it is easy to find the low-resistance fault by measuring with the low-voltage pulse method.
(4) The flashover fault can be measured by the direct flash method. This kind of fault generally exists inside the joint, and the resistance of the fault point is greater than 100 kΩ, but the value changes greatly, and each measurement is uncertain.
(5) High-resistance faults can be measured by the flash method, and the resistance of the fault point is greater than 100 kohms and the value is determined. Generally, when the test current is greater than 15 mA, the test waveforms are repeatable and can overlap, and at the same time a waveform has one emission, three reflections and the pulse amplitude gradually weakens, the measured distance is the distance from the fault point to the cable test end; otherwise Test the distance from the fault point to the opposite end of the cable.
To improve the technical level of cable fault testing, different methods should be adopted for different fault properties, and new technologies and new equipment should be continuously introduced. At the same time, experience should be explored on new equipment to develop new functions.
