High voltage flashover test principle of cable fault tester
The impact high voltage flashover method can test the high resistance leakage fault, high resistance flashover fault, low resistance short circuit fault and wire break fault of cable. It is an efficient, reliable and widely adaptable means of cable fault detection.
Impact high pressure flashover test principle
An impact high voltage is applied at the beginning of the fault cable to break down the arc at the fault point. The voltage jump at the moment of fault click is used as the test signal. Observe the signal between the fault point and the beginning of the cable round trip time ranging. There are many signal sampling methods of shock high voltage flashover method, and the common methods are voltage sampling method, terminal voltage sampling method, current sampling method and so on. At present, due to safety reasons, voltage sampling method is gradually phased out. Current sampling method has been widely used at home and abroad.
The current sampling method uses the principle of electromagnetic induction and uses current transformer to pick up the current signal on the ground line to get the electric wave current reflection signal in the cable. There is no electrical connection with high voltage generator and mains, so it is safer. The waveform obtained by current sampling method has many periods and clear inflection points, which is especially beneficial to fault distance analysis and location.
The low voltage pulse method is triggered by the cable fault tester, while the flashover method is triggered by the external high voltage. This phenomenon can be reflected by pressing the sampling key under different circumstances. When the low-voltage pulse is pressed, the sampling waveform will be displayed immediately, while when the high-voltage flashover is pressed, the waveform will not be reflected immediately, but will be displayed after the external high voltage triggers the ignition.
High voltage flashover fault test waveform
In terms of the nature of most faults, they basically belong to the damage of insulators. The high resistance fault is caused by the decrease of electrical strength of insulating medium. Because of the high resistance and small measurement current at the point of failure, it is difficult to measure even with a sufficiently sensitive instrument. For the pulse method, since the equivalent impedance at the fault point is almost equal to the cable characteristic impedance, the reflection coefficient is almost equal to zero, and cannot be measured because the reflected pulse is not available. However, starting from the electrical breakdown phenomenon of the medium, as long as the cable is applied with a high enough voltage, the breakdown phenomenon will occur at the fault point. At the moment of breakdown, the fault point is short-circuited by the discharge arc, so before and after the discharge at the fault point, the voltage jump will occur.
Due to dielectric breakdown, the ionization process takes a certain amount of time, and arc discharge generally lasts hundreds of microseconds to a few milliseconds, so the jump voltage is reflected back and forth between the fault point and the cable end in the form of waves during the discharge. If the instantaneous jump voltage and the waveform reflected back and forth are recorded at the end of the cable (beginning or terminal), the time of the wave reflected back and forth can be measured. The distance from the point of failure to the end can be calculated from the speed at which the waves travel through the cable. Based on this physical mechanism, flashover test method was developed.
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