ABB’s service engineers are equipped with a suite of test equipment that enables them to perform immediate on site diagnostics on the key network elements of switchgear, transformers and cables. If the fault is identified in a cable - as is often the case - and the network is interconnected, they can sectionalise the problem circuit to restore power to as much of the network as possible, bringing in additional generation if necessary. The next task is to locate the position of the underground cable fault as accurately as possible, since this makes it easier to find and repair so that the full network can be restored quickly.
Our fault location regime is very accurate in locating underground cable faults in both modern XLPE type cables and older PILCSWA (paper insulated lead covered steel wire armoured) designs. It is usually carried out on cable networks up to
11 kilovolt (kV), however the techniques can be applied on cables up to 33 kV.
The main technique employed is the SIM (secondary impulse method) that combines the use of classic high voltage surge generator thumping with low voltage TDR (time domain reflectometry).
The high voltage surge generator, or thumper, is a portable device that injects a high voltage DC (direct current) pulse (typically up to 30 kV) at the surface termination of the cable to be tested. If the voltage is high enough to cause the underground fault to break down it creates an arc, resulting in a characteristic thumping sound at the exact location of the fault.
Historically, fault location was based on setting the surge generator to thump repeatedly and then walking the cable route until the thump could be heard. The higher the DC voltage applied then the louder the resulting thump and the easier it becomes to find the fault. But if the cable is long it could take days to locate a fault. All the cable is exposed to potentially damaging high voltage thumping. So while the existing fault might be located, other areas of the cable could have been weakened in the process. Statistically, cables that have been thumped during their life tend to fail sooner than would otherwise have been expected.
TDR (time domain reflectometry) uses a pulse echo range finding technique, similar to that used by sonar systems, to measure the distance to changes in the cable structure. It transmits short duration low voltage (up to 50 V) pulses at a high repetition rate into the cable and measures the time taken for them to reflect back from areas where the cable has low impedance, such as at a fault. The reflections are traced on a graphical display with amplitude on the y-axis and elapsed time, which can be related to the distance to the position of the fault, on the x-axis.
A cable in perfect condition will not cause any reflections until pulse reaches the very end, when it encounters an open circuit (high impedance) that results in a high amplitude upward deflection on the trace. If the cable end is grounded i.e. a short circuit, the trace will show a high amplitude negative deflection.
Low voltage TDR works very well for the location of open circuit faults and conductor-to-conductor shorts. However, for shielded power cables, it becomes very difficult to distinguish faults with a resistance higher than 20 ohms. Unfortunately, the majority of faults in underground distribution cables are high resistance faults in the area of thousands of ohms or even megaohms.
The SIM (secondary impulse method) technique combines low voltage TDR and a thumper in an integrated system that makes the trace easier to interpret, with a clear indication of the fault location on a handheld display.
The process starts by running a TDR test on a healthy core. This is then stored in the SIM system memory. The thumper is then triggered to send a single HV pulse, and while the arc is forming at the fault the TDR sends a further low voltage pulse. The arc acts as a very low impedance point that causes the pulse to reflect in exactly the same way that it would from a short circuit. The handheld display combines the two traces and the fault location is shown as a large negative dip, with its distance easily read off on the x-axis.
SIM enables a fault to be located to within a few metres, even over very long cable runs of several kilometres. Underground cables do not always take the shortest or most direct route between two points, so it is important to have access to the site cable records. In cases where a map of the cable route is not available a radio-detection system can be used to find the cable, but this does add a considerable amount of time to the exercise. We always advise customers to make creating a detailed record of their underground cable circuits a priority in their maintenance planning.
Once the target area above ground has been identified, the surge generator is turned on to start thumping the cable. The operator then listens for the thump to home in on the precise location of the fault - this approach minimises the amount of time that the cable is thumped, eliminating the risk of further damage. The next step is to bring in the repair team to dig up the cable, confirm the problem by visual inspection and carry out a repair.
The time taken to locate a fault by SIM varies according to each case, but will typically take around half a day.