Subsea cable overheating risk reduced by monitoring

A 278km-long highvoltage power cable was recently installed between Ireland and Wales. Gary Parker of Sensa, a Schlumberger company, describes how a distributed temperature sensor (DTS) system is monitoring the performance and safety of critical parts of the cable in real-time.

Gary Parker is a technical sales manager at Sensa, a Schlumberger company, focusing on providing solutions to the power transmission industry in Europe and the CIS. His responsibilities include developing opportunities in emerging markets. Parker holds a Bachelors degree in electrical engineering from the University of Lincoln, UK, and a Masters degree in electronics from the University of Lyon, France. He is a member of the Institution of Engineering & Technology(IET).

Large subsea cable installations such as the country-to-country interconnectors and cables used to power subsea oil fields, are high value assets costing hundreds of millions of dollars. If these assets fail due to overheating, the results can be catastrophic. Knowing the temperature of the cable allows operators to extend its life by reducing the power load in the event of an over-temperature.

Additionally, with the assurance that the cable is operating within safe temperature limits, operators can increase and optimize cable utilization.

Power cable temperature

Although rare, there have been several examples of failures in power cables due to operation at excessive temperatures caused by high loads.

This can result in catastrophic damage to the cable and possible blackouts.

Every shutdown has a major impact on revenue.

A classic example of overtemperature occurred in 1998 in Auckland, New Zealand, when a 110kV cable overheated and caused a blackout in the city for about a month. It is probable that this failure could have been avoided by monitoring cable temperature. Monitoring provides other advantages, including:

In 2011, a failure occurred in the power supply provided by a cable running through an umbilical line to the Kupe offshore platform located in the Taranaki Basin, offshore New Zealand. The operator of the field lodged a warranty claim due to the cable failure. In addition, an insurance claim for US$95 million was lodged relating to the failure.

If the operator had used cable temperature monitoring, it could have established in court whether the cable was truly at fault.

Temperature detection

There are two measurement methodologies for measuring temperature: point measurements and distributed measurements.

Point measurements can provide the most accurate method of monitoring temperature, particularly when directly measuring the copper core of the conductor. However, when the conductor carries large electrical currents, direct measurement is only suitable for safe laboratory applications and is not practical for field installations. The alternative is to install a temperature sensor on the external surface of the cable.

Thermocouples are the most commonly used external temperature sensors. They are simple to operate and are reasonably accurate. Resistance temperature detectors (RTDs) are another common type of sensor. They offer higher precision than thermocouples, but are more expensive and are relatively fragile. In cable-rating, a resolution of ±1°C is often more than adequate, therefore higher resolution provides no benefit.

One disadvantage of thermocouples and RTDs is that they are susceptible to electromagnetic interference (EMI), which may distort the temperature reading. An alternative is to use thermo-optical temperature sensors. Semiconductor-based temperature sensors, which can also use infrared, are not affected by EMI and are more suitable for cable monitoring.

A key disadvantage inherent to monitoring the temperature of a long cable with point measurements is that they only detect the temperature at discrete locations. The cable temperature is unknown where there is no sensor. If, for example, seabed sand was deposited on part of the cable causing a localized hotspot that could lead to permanent damage, this would not be detected unless a sensor was located on that particular part of the cable.

Distributed temperature sensing (DTS) systems use an optical fiber that can provide a temperature reading every 1-2m along the entire cable length, overcoming gaps in a point measurement temperature profile. The location of the fiber is important; the closer it is to the cable conductor, the more accurate the measurement will be. An advantage of optical fiber is that it can be located inside the insulation of the power cable, which is close to the core.

If this is not possible, good results can still be obtained when the fiber is strapped to the outside of the cable.

DTS systems can measure temperatures over very long distances. No electrical power is used around the measurement zone; therefore they are intrinsically safe. The fiber is a practical and robust material that can be installed to measure temperature in a wide variety of locations.

Figure 1. The DTS was installed during 2012 and was commissioned last October. in Ireland, it includes an optical amplifier. The HVDC cable is in black and the DTS system is in red.

Cable between Ireland and Wales Ireland is currently taking some of its older coal-burning power stations offline. A subsea cable has been installed to allow the country to purchase base-load power from the UK. Additionally, when local weather conditions lead to one or the other country producing an excess of renewable energy, it will be possible to sell that energy to the other country.

ABB Sweden was awarded a turnkey contract by EirGrid, operator of Ireland's national electricity grid, to supply and install the highvoltage, direct-current (HVDC) power cable between Ireland and Wales. Known as the East-West Interconnector, this underground and undersea link has the capacity to transport 500MW – enough energy to power 300,000 homes. ABB asked Sensa to provide a solution to monitor temperature at critical points along the cable, which is about 278km long. Sensa is the inventor of DTS technology and has been designing, manufacturing, and supplying systems to power transmission and distribution markets worldwide since 1991. For the East-West Interconnector, the company deployed its Sensa Prime integrated long-range power cable monitoring system for temperature and strain to provide a temperature reading at 2m intervals all along the cable, which is a much finer sampling interval than can practically be achieved with classic temperature probes.

Figure 2. DTS data is collected and displayed as a 3D function of temperature, distance, and time.

The maximum measurement range of a single-fiber optics DTS system is currently about 50km. On the Irish side, ABB wanted the DTS control hardware to be within an electrical substation located 45km inland and to be able to measure the temperature of the cable for a further 25km subsea from the shoreline. This would mean a single measurement length of 70km, which was beyond the range of a normal DTS system. The solution from Sensa was to include an optical amplifier that extended the range of the DTS system by the required 20km.

The amplifier has no electrical or mechanical parts; it simply transfers light from a donor fiber to the signal fiber, thereby boosting the signal. The amplifier itself is compact, with dimensions similar to a DVD disc. Two optical fibers enter the amplifier and one fiber leaves. The amplifier had previously only been supplied by Sensa for use in monitoring oil and gas pipelines, but the experience gained in the oilfield proved to be transferrable to this commercial power transmission project.

Specific requirements for monitoring the East-West Interconnector included storing three months of hourly cable temperature measurements taken at 2m intervals along the route. Graphs of the cable temperature profile can be plotted for any given set of readings. The DTS can be remotely programmed and interrogated. The system, which is self-monitoring, was successfully designed and installed, and was commissioned in October 2012 (Figure 1).

Figure 3. This DTS example from another installation demonstrates how easily hotspots can be identified to guide workers to within 2m of any problem.

The DTS system provides the 3D function of temperature, distance, and time. The actual temperature data of the East-West Interconnector cable is confidential, so the example DTS data presented in this article are from other installations. However, the data demonstrate how hotspots can be easily identified so that, if necessary, an investigation team can be guided to within 2m of the correct location (Figures 2 and 3).

The DTS system is providing EirGrid with valuable data on the health of its East-West Interconnector cable. The measurements enable the cyclic (daily, monthly, and yearly) behavior of the cable to be established, providing a basis from which variations in its usual thermal behavior can be analyzed. The system can be programed to trigger an alarm message if the cable temperature in a particular zone exceeds a predetermined temperature limit. The DTS information will allow the company to extend the life of the cable by reducing the power load in the event of an over-temperature. Alternatively, with continuous monitoring, utilization of the cable can be increased with the assurance that it is operating within safe temperature limits.

Realizing the value of assets such as power cables requires using them at their optimum capacity. Real-time temperature monitoring enables the prediction and detection of possible problems such as thermal runaway. Appropriate action can then be taken to reduce the load. DTS systems are also useful for locating hot spots on the cable. Once identified, these can be rectified, so that the cables can be operated at their nominal temperature. Temperature trends identified from the DTS measurements enable the power distributor to maximize the use of a circuit during periods of peak loading such as mid-summer.

Future considerations

The hardware already installed in this DTS application has the potential for several enhanced functions.

One of these is a real-time rating system that would allow the operator to automatically generate cable power/load calculations, based on the real-time cable temperature. Using the same fiber optics, it is also possible to install intervention monitoring systems that will inform the operator when a third party is performing an action that may damage the cable. For example, it could detect if a ship has dropped its anchor above the cable.

 

Maximizing power cable utilization: Cables are typically rated conservatively; often well below their effective capacity. If temperature is continuously monitored, it may be possible to safely load a cable above its nominal loading rating.

  • Hot spot detection: The appearance of new hot spots in a cable can indicate changing conditions that should be investigated.
  • Power overload management: Systems in which one cable can be safely loaded above its usual rate while another cable is out of service.
  • Plotting seasonal/yearly trends: Cable temperature data can be compared to historical records.
  • Deferring capital expenditure on new cables.
  • Detecting and locating faults in a cable.

Similar systems have been used to find short circuits, and can inform the operator where the fault is to within a few meters. OE

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