Arc Reflection Technique (ARC) for Cable Fault Locating

The Basics of Arc Reflection

Cable Fault Locating system

The technique of Arc Reflection (ARC) combines the surge wave generator (thumper) and the pulse echo technique (TDR) for the purpose of locating high resistance shunt faults and intermittent shunt faults.

Although the pulse echo technique is a very good tool for a low voltage cable application, a problem occurs when this method is used on medium voltage electrical power cables.

To locate vented trees, which basically form a hole in the insulation, a breakdown voltage is typically required. The pulse echo tool by itself is very low voltage in nature. So, if the fault requires some level of breakdown voltage, the low voltage pulse echo energy will travel straight past the fault never seeing it. The TDR will just show the reflection of energy off the terminating event or the end of the cable.


Yet, if a high voltage breakdown surge is applied at the same time that the pulse echo pulses are sent, when the fault breaks down due to the high voltage surge and the arc jumps through the faulty insulation, the pulse echo wave will reflect off that arc.

For the ARC, a coupling device (a filter, or a power separation unit) is introduced into the circuit allowing the high voltage surge generator and low voltage time domain reflectometer to be combined simultaneously to the faulted cable. The function of the surge generator is simply to transmit a high voltage surge into the faulted cable. This surge momentarily reduces the resistance of the shunt fault. During the high voltage impulse time interval the pulse echo technique is applied.

Complete fault locating system SWG-12/1100R

In other words, the momentary reduction in the shunt resistance established by the high voltage impulse, or arc created at the fault will create a point of reflection for the low voltage pulse transmitted by the time domain reflectometer and help to identify the fault.

Advantages and Disadvantages of Arc Reflection

The strong point of the ARC is that it helps localize any fault that will sustain an arc during the thumper pulse. Even though a surge wave generator is used, this method significantly reduces the amount of stress that is traditionally associated with the thumping technique because only one or two surges into the cable are needed in order for the pulse echo to be able to find the fault, display that image and measure the distance.

So, the amount of stress to the cable is considerably decreased by reducing the number of surges it's going to take to find this overall fault, and the output voltage is regulated by the reflection filter or power separation unit.

One disadvantage of this method is that long cables with very highly attenuated dielectrics or a lot of corrosion on the neutrals can very quickly absorb the reflected pulses of the pulse echo technique. Thus, if the TDR does not have enough transmitted pulse energy to overcome these very long cables or high corrosion is bleeding down the signal, it may not be possible to see the end of the cable or the actual fault event just because the signals attenuate before they get back to the receiver.

Also, if it's a complex circuit with many branches and therefore a lot of reflections, the waveform can be difficult to interpret. In this case, a differential technique called differential arc reflection can be applied.

The first step is to get a standard low voltage pulse echo waveform and then capture that image. Then, the high voltage surge is applied to the cable, the arc or flashover is created and the time domain reflectometer displays a new waveform showing the reflection from the arc.

Since the result is two wave forms which are really just an accumulation of data points, the computer or the analyzer will cancel everything common, getting rid of all the unwanted reflections. This will clearly show points of difference where the arc took place, allowing to very quickly visualize and measure the distance to the flash point, or reflection point of the arc.

Alexei Tiatiushkin

Marketing manager

KharkovEnergoPribor Ltd.

marketing@keppowertesting.uk

http://www.kep.ua