In the Netherlands, almost all power connections with a voltage less than 50 kV are directly buried in the Dutch soil, and more than 30% of the higher-voltage power connections also are directly buried. The increasing demand for electric energy, together with the liberalization of the Dutch energy market, is resulting in an increasingly higher loaded electrical infrastructure. Therefore, the usage of methods to fully exploit the ampacity of the electrical infrastructure is becoming more important. KEMA has developed a method to fully exploit the ampacity of the existing infrastructure, leveraging distributed optical measurement techniques.

Manufacturing Intelligent Cable Systems

Today in the Netherlands, some 250 km (155 miles) of underground cable circuit has been installed with integrated optical fibers for temperature measurements. The integration of optical fibers into power cables requires special production techniques, but it is a relatively inexpensive exercise.

In high-voltage single-phase cables, the glass fibers can either be installed in the copper wire earth screen, or if the cable is only equipped with a lead sheath, directly under this lead sheath. In medium-voltage three-core cables, the glass fibers are installed within the central filling element. In medium-voltage single-phase cables, the glass fibers are integrated within the copper wire earth screen. It is also possible to install a glass fiber cable adjacent to the outer surface of the power cable. By incorporating the glass fibers within the cable or by placing a glass fiber cable next to a cable circuit, the thermal status of the cable system can be determined while the circuit is in service. Determinations of the actual and future current carrying capacity are possible for this new generation of glass fiber-equipped power cables.

Now the Dutch utilities are able to review six years' operational experience of the sensor-equipped power cables installed on both the high- and medium-voltage grid. Distributed temperature measurements started in 1997 using commercially available Raman Optical Time Domain Reflectometry (OTDR) equipment for distributed temperature sensing with a spatial resolution: 4 m (13 ft), a temperature resolution of 2°C (35.6°F) and a maximum range of 25 km (16 miles). Temperature measurements conducted since show a location-dependent temperature profile (Fig. 2). This result was surprising because the power cables were expected to show a fairly constant temperature distribution along the complete cable route.

However, in practice, the cable environment determines the thermal behavior to a large extent, and this can change from one location to another, particularly when power cables are directly buried, which is often the case in the Netherlands.

The distributed temperature measurement results presented in this article were obtained from a power cable that is partly buried in an old polder (reclaimed land enclosed by dykes). After a soil survey, it appeared that the soil in this polder had a large peat content, resulting in significantly worse thermal properties compared to the soil outside the polder. The difference in soil properties inside and outside the polder were such that the temperature of the cable conductor within the polder was 10°C (50°F) higher than outside the polder when the cable was only moderately loaded (Fig. 2). Furthermore, it was found that small ditches, dykes and roads all influence the cable temperature. Investigations showed that most of the “noisy” character or rapid variations shown in Fig. 2 were caused by variations in the cable environment and not by noise in the distributed measurement apparatus.

When the measured temperatures are plotted as a function of time, interesting results are produced. Figure 3 shows these results for two locations, one within the polder (M1) and one outside the polder (M2).

Figure 3 also presents the values of the current flowing through the cable circuit during the period of measurement. The day-night variations in the current and the lower cable loading on weekends have a marked influence on the cable temperature. During the week, the power cable temperature at location M1 slowly increases. This behavior is caused by the cable environment, which heats up or cools down only slowly. Also, the difference in dynamic thermal behavior of the soil surrounding the cable at locations M1 and M2 is apparent from the temperature characteristics in Fig. 3.

Dynamic Thermal Model and Cable Maximum Loading Rating

The maximum loading limits of power cables are typically calculated based on Standard IEC 60287 or the Neher & McGrath approach, which both assume constant loading conditions. Because of this assumption, it is almost impossible to determine the loading possibilities for the directly buried cable circuit considered and presented in Figs. 2 and 3, in which the loading condition is not constant. To check the measured thermal behavior against the engineering calculations and, more importantly, to predict the future cable temperatures at a certain cable load, a dynamic thermal model is needed that can follow the variation in current that occurs in practice.

With such a model, the maximum calculated load of a cable circuit could exceed the load calculated using existing steady-state standards. As the current flowing through power cables is usually dynamic, the cable's temperature is dynamic (Fig. 3). This thermal behavior makes it possible to load the cable higher than the maximum constant rating without thermally overloading the cable circuit. Consequently, cables can be loaded higher than their maximum rating deduced from IEC 60287 or Neher & McGrath without exceeding the specified temperature limits for the cable insulation (or alternatively, the temperature limits imposed to prevent drying out of the soil along the cable route). The time period during which the cables can be loaded higher depends on the periodic variations in the cable circuit loading. Using a validated dynamic thermal model, it is possible to make optimal usage of these variations in circuit loading without thermally overloading the cable circuit.

KEMA's Dynamic Thermal Model

KEMA has devoted time and effort to developing a dynamic thermal model able to determine the real-time ampacity for buried power cables. The model inputs comprise circuit data, soil characteristics and time-dependent current data; the model does not depend on temperature measurements. These measurements are only used to check and validate the performance of the tailor-made model.

Using the example of the power cable in the polder, Fig. 4 shows a comparison of the calculated temperature and the actual measured temperature of the cable at location M1. At the start of the graph, the mismatch between model and actual temperatures was caused by the lack of historical loading information, but this difference decreases to less than 1°C by the end of the monitoring period.

Following confirmation that the theory (the model) and actual temperature measurements matched sufficiently, as shown in Fig. 4, the model can be used to predict cable temperatures in the future (based on an expected loading) or to calculate the maximum ampacity for the next hours or days. By using these mathematical techniques in a control room setting, for example, the following can be calculated:

• The current the cable can transport over the next 24 hours.

• The length of time the cable can transport 1200 A.

The KEMA dynamic thermal model has been comprehensively verified in many situations and in cable types, including oil-filled and polymeric cables in the voltage range from 10 to 150 kV, and installed in various cable environments. The predictability performance of the model is close to actual temperature measurement for periods that extend from six months to a year without equipment recalibration. Thus, the model is well-suited for use in a cable ampacity management system used to determine on-line circuit loading.

Utility Operating Experience

The Belgium transmission system owner, ELIA, has started a project to install a cable ampacity management system to determine the real-time calculated temperatures and the emergency loading possibilities for an important 150-kV power cable circuit in Brussels. As is usually the case, this power cable connection has several “hot spots” or locations that are already present in the circuit that cannot be easily eradicated. These locations have been identified and ranked, and the worst hot spots were subject to further investigations, which resulted in a tailor-made dynamic thermal model. The model has been verified by actual temperature measurements, which are recorded by an optical temperature measurement system using a telecommunications glass fiber located a few centimeters away from the cable system. The resulting real-time dynamic thermal model can be used to determine the overload possibilities of the cable circuit via a graphical user interface that can be accessed from different places.

ELIA can access real-time cable temperature data and the cable circuit overload possibilities (Figs. 5-7). The loading statistics, cable temperatures and overload possibilities are shown and updated every five minutes. This system has now been in successful operation since mid-2002.

Frank De Wild graduated from the University of Twente (The Netherlands) in 1997 with a MS degree in applied physics. He began his career at KEMA in 1998 and became involved in temperature and loading calculations of underground power cables in order to fully utilize the cable's possibilities. Currently, De Wild is a consultant in power cable issues.
Frank.deWild@kema.com

Gert-Jan Meijer graduated from the Technical High School of Haarlem (The Netherlands) in 1975 with a degree in electrical engineering. Prior to joining KEMA in 1981, Meijer worked with Fluor Engineers and Constructors. Since 1987, Meijer has been involved as senior specialist/project leader with research and engineering projects related to the electrical underground infrastructure. Currently, he is a consultant on power cable issues.
GertJan.Meijer@kema.com

Gustaaf Geerts graduated from the Technical High School of Mechelen (Belgium) in 1970 with a degree of engineering in applied electronics. He joined LABORELEC (The Belgian Laboratory of the Electricity Industries) in 1972 and became involved with underground power cables in 1978. Currently, he is the underground cable expert for the Belgian Transmission System Operator ELIA (www.elia.be) and is a member of the Belgian Commission for Normalization of Underground Cables.
Gustaaf.Geerts@elia.be