As the number of Submarine Power cables grows alongside the growth of Offshore Renewable Energy installations and Island Interconnects, now seems a good time to offer some comments on Fault Location Techniques.
Very few submarine cable assets are 100% reliable, planning for failure is essential in order to manage faults when they do occur. Accurate fault location is critical to successfully complete cable repairs, but it does require time and dedication to achieve a good result.
Outage time for the cable asset and repair vessel time are both significant costs, as is cutting in at the wrong position or utilising more stock cable than is absolutely essential. A few tens of thousand pounds spent on accurate fault location can save hundreds of thousands or even millions of pounds when compared with the cost of failure associated with poor fault pin-pointing.
It is beyond debate that investing in good resource and data acquisition to provide the most precise fault location possible is just good (business) sense. Experience has shown that no submarine fault is the same, accurate confirmation of the fault position often requires multiple measurements and the use of several different techniques, a few of the more usual techniques are discussed below.
Fault Location Techniques
Time Domain Reflectometer (TDR)
The most widely used device for locating faults on Power Cables is the Time Domain Reflectometer (TDR). If a fault occurs on a cable there is an impedance change at the location of the fault. This may be an open circuit, short circuit or a resistance change (usually low insulation)
The TDR sends pulses of electrical energy along the cable and measures the time between the launch of the pulse and the reflection being returned to the instrument from any anomaly along the cable. This time measurement can be converted to distance if the characteristics of the cable are accurately known.
Significant events like short circuits or open circuits can be easily identified but a breakdown in the insulation can be far more difficult to see. Whilst TDR is the preferred technique for locating these faults, it should be noted however that these instruments will only locate faults to an accuracy of approximately 1% of the distance to the fault and then only if a narrow pulse-width can be used. Faults that are further away from the location instruments will require more energy to "see" the fault. Providing greater levels of energy into the cable requires a longer duration pulse which in turn leads to less accurate distance measurement as the pulse-width widens and it is more difficult to pinpoint the details of the reflection. Capacitance within the cable also causes the pulses to be rounded and attenuated as distance increases.
The best results are obtained where precise "reference data" is available. This could be a set of detailed measurement taken when the system was installed, after a previous repair or maybe by measurements on an adjacent "good" core in a multi-core cable structure. Utilising the A-B feature built into most modern machines will give you a straight line if there has been no change or the two cores are identical, but if a fault is present in the cable, will show the anomalies. Detailed analysis can identify the location of joints or previous repairs and this data can be used to "calibrate" the instrument. Such detailed measurements will require many averages to be taken and will therefore take a long time. However there is usually a long period of time between the fault occurrence and the availability of a suitable repair vessel so this is unlikely to be an issue.
On land the initial TDR location of a short-circuit fault is usually followed by higher energy "thumping" of the cable and the use of acoustic "listening" devices to pinpoint the actual position at which the breakdown is occurring. This can be difficult when the cable is below water and often buried within the seabed sediments, although these techniques are improving.
Electromagnetic Field (Electroding)
An electromagnetic field is produced around a cable when current is flowing through it. This can be detected and used as a location tool but, unlike telecom cables, the resistance of Power Conductors is very low so a much larger current is required to produce any significant field around the cable. Additionally, power cables generally have a variety of semi-conducting sheaths which can mask the fault location. Instruments used for telecom cables generally do not produce sufficient signal to be easily detected so a new generation of location machinery is required to be able to produce and detect an electroding signal on Subsea Power Cables.
Electroding is only useful when an approximate location has already been established. It can be used to identify the location of a shunt fault but, in the case of a power cable fault, it would require the fault to have a very low resistance value to enable a large current to pass through the fault. The distant end should be left Open-circuit so the only path for the current to flow is across the fault.
Fibres within Power Cables
Many of today's Power Cables have integral optical fibres. These fibres can be used for telemetry as well as other uses such as temperature monitoring, but can also be very useful for locating cable faults. Fibres are sensitive to bending which can be detected using a multi-wavelength Optical Time Domain Reflectometer (OTDR). The sensitivity to temperature can be used to detect any "hot spots" within a cable which provides an excellent tool for monitoring and locating heat generation within a core which might be causing a breakdown. Contaminants such as water ingress and hydrogen (which can be caused by electrolysis around a fault where water has entered the cable) can also be detected, as well as other variables such as fibre strain, or vibration, which can also assist pin-pointing the fault to high levels of accuracy.
Fibres have been used to identify the location of faults on a number of occasions and they can provide a more precise position as OTDR's can measure to within a few metres over a 100 km cable section or greater depending on the instrument used and the fibre characteristics.
Fibres can be safely monitored whilst the cable is still loaded and so no system outage is required whilst any investigation takes place. There are usually "unused" fibres available within a cable and these can be monitored to identify any change.
Identifying the root cause of failure
When a low insulation fault occurs there may be many reasons that could have caused this breakdown. It can be caused by a manufacturing fault, by poor cable handling during installation, by external aggression to name but a few. It is therefore good practice to minimise high energy techniques during testing as these, whilst making the fault easier to locate, may cause additional damage to the cable making it more difficult to confirm the root cause of the failure.
Regular monitoring of integral fibres may provide warning of an impending failure of the power conductors. OTDR measurements showing changes along the fibres may well lead to a very precise location of any failure.
Digital Temperature Sensing (DTS) can also show the possibility of an impending failure as "hot spots" can be identified and enable the loading of the system to be readjusted to prolong its life and enable a planned repair to take place prior to any catastrophic failure.
It is understood that such monitoring would be impractical on many Wind Farm installations but regular monitoring of the Export Cables could prove to be beneficial.
- Locating faults on Subsea Cables can be very difficult.
- The use of multiple techniques may be required to refine & pin-point the fault location.
- Inaccuracy can prove very expensive as outage and repair ship times are extended. Additional "Stock Cable and accessories" may be required to effect a repair if faults are not accurately located.
- Investment in good fault location will save costs before, during and after the repair.
- Current 'Telecom' Electroding techniques are prone to issues on power cables. The instruments are designed for use on Telecom cables and require further development to be reliably effective on Power cables
- Regular monitoring of integral fibres could identify potential problems before they cause failures.
- Many new cable owning companies may not have sufficient internal experience to fault locate subsea faults effectively, and staff churn may make it difficult for owners to keep skilled and proficient employees in post.
- Specialist techniques may help locate the failures more accurately, cost effectively and quickly.
Measuring, storing and routinely refreshing detailed TDR & OTDR "fingerprints" of new or repaired cables may assist in locating future failures.
Accurate and detailed optical fibre data may help to predict the location of Electrical faults before they fail and pin-point the failure position when they do.
Routine monitoring of fibres to identify potential issues before they become failures allows owners some choice to manage and potentially dramatically reduce the cost of failure allowing 'planned' repairs to be considered instead of suffering more significant costs associated with repairing an 'unplanned' fault.
Digital Temperature Monitoring (DTS) is a technology which should be carefully considered for all "new builds", and its retro-fitment on existing assets may prove beneficial. It may be useful to manage the loading of the cable during operation and may be especially useful if "Hot Spots" are identified.
When failures do occur in Subsea plant it will be expensive, investment in suitable experienced specialist help may save significant costs.