The impulse current method (ICM)
In the case of very long cables, it is often not possible to use standard fault location methods because the measurement pulse, or its reflection, is too strongly damped. This is where the impulse current method (ICM) comes into its own.
In addition to the high level of measurement signal damping over long distances, the high capacitance of cables can cause a problem. With a SIM measurement, the capacitance of the surge capacitor must be significantly greater than the cable capacitance. This is why the impulse current method (ICM) is ideal for particularly long cables.
Using a surge voltage generator with the surge switch closed, the cable is charged with direct current up to the breakdown voltage, thereby making use of the cable capacitance. This increases the potential surge capacitance. The surge energy does not need to travel the entire distance from the surge voltage generator to the fault, but is added onto the cable capacitance; moreover, there is no ionization time as there is during surging.
With the impulse current method, a surge voltage pulse is discharged into the cable to activate a breakdown at the fault location. The breakdown produces a travelling wave that moves back and forth between the fault location and the end of the cable. This changes the polarity at these two reflection points, as both are low-resistive seals.
The distance to the fault location can then be determined based on the time it takes for the reflection to repeat. This works particularly well in long cables, since the pulse that propagates through the cable is very wide (i.e. very large and high-energy). In short cables, several reflections can overlap, making it impossible to determine the time interval. In long cables, the ICM method provides good pre-location results.
An inductive coupler that decouples the current from the cable sheath is used to analyse the travelling wave. The coupler signals are displayed by a time domain reflectometer (e.g. the IRG 400). The distance can be calculated based on the time between the second and third pulse, for example.
The operator only needs to mark two sequential peaks or flanks in the travelling wave displayed by the IRG. The distance from the surge voltage generator to the fault location is identical to the difference between the two peaks calculated by the device in metres.
The distance to the fault can be determined using the graphical display of the time domain reflectometer.
The distance range of the IRG should be set to a multiple of the cable length so that as many of the peaks of this travelling wave as possible can be detected on the screen.
Overview of cable fault location
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