The Experience of using Distributed Temperature Sensing (DTS) in XLPE Power Cables

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1 9. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV Kranjska Gora 29 CIGRÉ ŠK B1 1 The Experience of using Distributed Temperature Sensing (DTS) in XLPE Power Cables Danijela Palmgren ABB AB P.O. BOX 546, KARLSKRONA, SWEDEN danijela.palmgren@se.abb.com, Phone: Johan Karlstrand ABB AB P.O. BOX 546, KARLSKRONA, SWEDEN johan.karlstrand@se.abb.com, Phone: Lars Hammarson GENAB P.O. BOX 53, 41 2 GÖTEBORG, SWEDEN lars.hammarson@goteborgenergi.se, Phone: Abstract Today s power cable systems are often provided with some kind of temperature monitoring system. Depending on the specific need, i.e. whether the temperature needs to be measured at some critical spots or along an entire route, either thermocouples or optical fibres for distributed temperature sensing (DTS) systems can be used. When a DTS system is used the optical fibres can be either integrated into the power cable or attached to the power cable. In 26 a co-operative technical program between ABB and Göteborg Energi was set up in order to gain experience and knowledge about the technical outcome and the application of DTS systems in real operation. The program was finished at the beginning of 29. The DTS system was used to monitor the power cable system that connects a combined power and heating plant with a 132 kv switchgear. The power cable was provided with both a FIMT (Fibre in Metallic Tube) integrated into the metallic screen and an external fibre optic cable attached to the power cable. The route length is about 2.6 km and different cable installation conditions such as direct burial, duct and steel pipe, apply along the route. In addition to the screen temperature, that was measured along the entire route also ambient soil temperatures at different depths and the current have been measured. This paper will address the use of a DTS system in a real installation, compare the theoretical rating for different installation configurations with the online measurements, address and discuss some uncertainties when calculating the rating and discuss the pro s and con s for the placement of the optical fibre internally or externally. Izkušnje z uporabo sistema za nadzor temperature (DTS) v XLPE energetskih kablih Povzetek Sodobni kabelski sistemi so pogosto opremljeni s sistemi za nadzor temperature. Glede na specifične zahteve kje lokacijsko se bodo meritve izvajale, ali le na posameznih kritičnih točkah sistema ali na celotni dolžini kabelske trase, se za nadzor temperature lahko uporabljajo termočleni ali pa optična vlakna v primeru sistema za porazdeljen nadzor temperature (DTS). Za nadzor temperaturnega odčitavanja se lahko uporabijo tako termočleni kot optična vlakna. Pri porazdeljenem nadzoru temperature s sistemom DTS so lahko optična vlakna vgrajena v sam kabel ali pa so nameščena ob zunanji površini kabla.

2 9. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV Kranjska Gora 29 CIGRÉ ŠK B1 1 V letu 26 sta ABB in Göteborg Energi postavila skupni projekt namen katerega je bil pridobiti izkušnje in znanje o uporabi sistema DTS v realnih pogojih obratovanja. Projekt je bil zaključen v začetku leta 29. V okviru tega projekta je bil sistem DTS uporabljen za nadzor oz. kontrolo kabelskega sistema, ki je povezoval termoelektrarno-toplarno s 132 kv stikališčem. Povezovalni kabel je imel vgrajen FIMT (Fibre and Matalic Tube), ki je bil integriran kot del kovinskega ekrana, drugo optično vlakno pa je bilo nameščeno na zunanjo površino kabla. Kabelska trase, ki je bila dolga okoli 2,6 km, je bila izvedena tako, da je bil kabel izpostavljen različnim zunanjim pogojem tekom celotne kabelske trase, t.j. direktno zakopan v zemljo, položen v kabelski kanal in položen v kabelske cevi. Poleg spremljanja temperature ekrana, ki je bila merjena na celotni trasi, so pri projektu spremljali tudi temperaturo okolice zemlje (zasipnega materiala) na različnih globinah in pri različnem obremenitvenem toku. Ta članek predstavlja primer realne uporabo sistema DTS in primerja teoretične termične obremenitve pri različnih konfiguracijah z dejanskimi meritvami v realnem času. Članek predstavlja tudi nekatere negotovosti pri izračunu termičnih obremenitev in podaja določene prednosti in slabosti vgradnje optičnega vlakna v kabel ali pritrditev na zunanjo površino kabla. I. INTRODUCTION The ampacity of an underground power cable is calculated in accordance with IEC 6287 [1]. The calculations are governed by the maximum allowable temperature on the cable conductor. However, the surrounding conditions, particularly in urban areas, can have a significant impact on the outcome. To be able to guarantee a typical lifetime of 4 years for a power cable system, the maximum allowable conductor temperature must not be exceeded. At the same time, the variations in the proximate environment are difficult to control. As a consequence, the ampacity calculations are often performed assuming rather conservative route conditions. Thanks to the use of distributed temperature sensing (DTS) systems it is possible to gain control over the environment, localize the hot spots along the cable route and hence operate the system in an optimal manner. When, in 26, ABB had the chance to supply 132 kv cables to Göteborg Energi, both parties agreed that this project was a perfect opportunity to implement a DTS system and gain both knowledge and experience of such a system in real operation. into the metallic screen of the power cable and an external fibre optic cable attached to the power cable were applied. The fibres in both cases are of multi mode type with graded index 5/125μm. The 132 kv XLPE power cables are doublecircuited and connect a new combined power and heating plant to the existing grid of the City of Gothenburg. The construction of the 132 kv power cables is shown in Fig. 1. Conductor Conductor screen XLPE Insulation Insulation screen Copper wire screen with integrated FIMT Al - laminate PE Oversheath II. SYSTEM DESCRIPTION Since the cables were installed in an urban area, the approximately 2.6 km long cable route implied: Different types of installation, e.g. direct burial, PVC ducts and steel pipes. Different laying depths, e.g. from 1 m down to 4 m. Other heat sources in the vicinity, e.g. other cables, heat pipes, road surfaces and other unknown services. Different surrounding conditions, e.g. park areas, streets, bridge installations etc. Further on, to be able to analyze which influence the placement of the fibre has on the accuracy of the temperature measurements, both a fibre integrated Fig. 1. The construction for the supplied 132 kv XLPE power cables. The DTS system is of type DTS 8 M8 with 8 km range, 1 m spatial resolution and +/- 1 C accuracy. The DTS was used to measure two independent loops with double ended measurement technique. One loop, i.e. Channel 1 included fibres integrated into the power cable and the second loop, i.e. Channel 2 included the external fibre optic cable (Fig. 2). Additional software, i.e. PC Anywhere facilitates the remote access to the temperature data. The temperature data is stored in an SQL database, but additional tools enable conversion to a variety of different formats, e.g. ASCII, Excel etc.

3 9. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV Kranjska Gora 29 CIGRÉ ŠK B1 1 Fig. 2. Schematic presentation of the system loops. The power cables were installed in 26 and the power plant was taken into operation at the beginning of 27. Since the plant is a combined power and heating plant and the economic factor is the main driver, the normal plant operation is during winter seasons. During the first operating season, e.g. winter 27/28 both 132 kv cable circuits were in operation. The system is, however, dimensioned to operate with one circuit only. Hence, with both circuits online the screen temperatures were relatively low and differences between the internal and external fibre were negligible. In autumn 28 the decision was taken to operate the system with only one cable circuit. The results given in this paper are based on measurements performed during November and December 28. In addition to the screen temperature, ambient soil temperature at different depths (1m, 2m and 3m) and load current were measured. III. RESULTS A. Localization of the hot spots and correlation to the installation conditions Fig. 3 shows a typical temperature profile for the cable route. Please note that in Fig. 3a the temperature is displayed for both cable circuits, i.e. the right half of the diagram shows the temperature on the off-line cable circuit. As can be seen, there are several hot spots along the route but the ones with the highest temperature correspond to the sections with the cable installed in steel pipes (mark 1 and 2). Additional losses induced in the steel pipe in combination with the air filling of the pipe obviously result in a hotspot that de-rates the whole system. The problem could be mitigated by filling the pipes with for example bentonite or installing a cooling system. Hotspot 3 corresponds to a cable crossing. Along section 4 other HV cables are laid in parallel. Temperature dips 5 and 6 correspond to the two cable joints and 7 is the place where the cable enters the power plant. B. Calculated versus measured values The calculations were performed in accordance with Electra 87 [2]. Since the most common installation types along the route are direct burial (49 %) and PVC duct (47 %), the calculations were performed for these conditions and for the sections without other heat sources in the vicinity. The measured ambient soil temperature (Fig. 4.) and the current (Fig. 5.), were used as input parameters. Soil Temperature ['C] Fig. 4. Measured ambient soil temperature at different depths ( ) m depth 2m depth 3m depth 8 Current [A] w. 45 w. 46 w. 47 w. 48 w w Fig. 5. Measured current ( ). Fig. 3. Temperature profile for the cable route: a) both cable circuits; b) only the online circuit. Obviously one more input parameter was needed: the thermal resistivity of the soil. There is always some uncertainty connected to the evaluation of the

4 9. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV Kranjska Gora 29 CIGRÉ ŠK B1 1 thermal soil resistivity. The thermal resistivity is highly season dependent and can vary along the route due to variations in the environment. In some extreme climate conditions or during prolonged dry periods the influence of the dry out on the thermal resistance must be taken into account. When designing a power cable system in case of insufficient data or lack of measurements the standard gives some rather conservative values for the thermal soil resistivity. For Sweden the recommended thermal resistivity of soil is 1 Km/W. However, the comparison with the measured values indicated that the thermal soil resistivity is much lower, i.e..55 Km/W. The measured and calculated values for a direct buried configuration are shown in Fig. 6. rainfalls during autumn, the assumption of water filled ducts seemed reasonable. The calculation results are shown in Fig DTS Calculation DTS Calculation Fig. 6. Calculated values versus measured values for the screen temperatures; The cables are direct buried and laid in close trefoil, laying depth 1 m, the system is cross-bonded. The calculations for the duct were performed assuming air filled ducts and the same thermal soil resistivity,.55 Km/W. Obviously the calculated values were much higher (Fig. 7.) DTS Calculation Fig. 7. Calculated values versus measured values for the screen temperatures; The cables are installed in air filled PVC ducts (Dia. 122/16) and laid in close trefoil, laying depth 1.1 m, the system is cross-bonded. Since the City of Gothenburg is situated on the Swedish west coast characterized with frequent Fig. 8. Calculated values versus measured values for the screen temperatures; The cables are installed in water filled PVC ducts (Dia. 122/16) and laid in close trefoil, laying depth 1.1 m, the system is cross-bonded. C. Placement of the fibre Having in mind that the conductor temperature needs to be hold below the maximum declared by the standard, the placement of the fibre would ideally be as close to the conductor as possible. For a short section of LPOF cable successful tests were performed by placing the FIMT in the center of the hollow conductor [3]. For extruded (XLPE) cables however this is not practical. For XLPE cables a reasonable solution is to place the fibre integrated in metallic tube (FIMT) into the metallic screen of the power cable. The integration of FIMT into the power cable increases the accuracy of the temperature measurements, but at the same time the fibres are influenced by the HV cable s manufacturing process, possible high temperatures and various bending operations. The attenuation of the fibre may be increased. Higher fibre attenuation decreases the maximum power cable length that can be monitored. Power cables for land installations are normally supplied on drums, which implies relatively short delivery lengths. The lengths of the optical fibres integrated into the power cable are also limited by the maximum drum length of the power cable. In most cases this means that the number of fibre joints will be higher compared to an external fibre installation. Each joint introduces additional losses; consequently too many joints can break the loss budget for the fibre route. With optical fibres attached to the power cable, many of the above mentioned issues could be either overcome or mitigated. The external fibres give a good reflection of the cyclic loading and the general temperature trend, but the response to load variations is somewhat delayed and the accuracy of the

5 9. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV Kranjska Gora 29 CIGRÉ ŠK B1 1 temperature measurements is lower compared to the optical fibres integrated into the power cable. An example of this is shown in Fig. 9, displaying current and screen temperature measurements performed with both internal and external fibres between and The cable section shown in Fig. 1 is installed in PVC duct. Fig. 9. Current and screen temperatures measured with the integrated and external optical fibre on a section with cable installed in duct ( ). From Fig. 1 it is obvious that sufficient information for the sections with direct buried power cables can be obtained even from the external optical fibre. Compared with the measurements performed with the fibres integrated in the cable screen the temperatures measured with the external fibre optic cable are only a few degrees Celsius lower. But in sections where the cable is installed in ducts or steel pipes the accuracy of the temperature measurements is rather poor, i.e. compared with the measurements from the internal cable temperature differences of approximately 15 C and 2 C respectively were monitored. It should, however, be noted that the external fibre was attached to the duct and not to the cable itself, implying a less good agreement between internal and external fibre measurements FIMT External FO Current Duct Duct Fibre length [m] Ch :1:32 Ch :56:2 Fig. 1. Temperature profile for the route measured with the integrated and external optical fibres Steel Pipe Duct Current [A] Obviously the type of installation as well as the length of the measured circuit will have a great impact when judging whether a FIMT or an external fibre optic cable, should be chosen for temperature measurements. IV. SUMMARY HV underground cables are a valuable part of the electrical network and as such need to be protected. One of the precautions often used is the control of screen temperature by means of Distributed Temperature Sensing Systems. The thermal resistivity of the ambient has a great influence on the cable ampacity but especially in urban areas the influences of the environment may be difficult to control. To ensure that the conductor temperature will not be exceeded, continuous temperature measurements need to be performed. For long lengths the old fashioned way of installing thermocouples might not be applicable. Instead the measurements can be performed by installing optical fibres. Whether the optical fibres should be integrated or attached to the power cable depends on several factors, such as installation conditions, cable length to be monitored etc. However the advantages and disadvantages of these two options are summarized in Table 1. TABLE I ADVANTAGES AND DISADVATAGES OF FIMT VERSUS EXTERNAL FIBRE OPTIC CABLE FIMT Quick response Satisfactory reflection of the load changes and the cyclic load behavior Risk for damage or increased attenuation during power cable manufacturing Increased manufacturing cost The maximum length limited by the power cable drum length (might mean additional joints) Once installed not influenced by the cable surrounding / installation conditions Reparation / replacement of fibres not possible (without disrupting the power cable) External fibre optic cable Delayed response The load changes and the cyclic load behavior are smoothened but the general temperature trend is satisfactory. Better attenuation since not affected by the power cable manufacturing process Low cost/risk approach Lower losses due to jointing since longer lengths (fewer joint) can be used Greatly influenced by the cable surrounding (for example not a good solution for cables in air) and installation conditions (such as duct and steel pipes) Reparation / replacement of the fibre optic cable does not disrupt the power cable Obviously, the best solution is given by a case-bycase study but the need of monitoring the cable

6 9. KONFERENCA SLOVENSKIH ELEKTROENERGETIKOV Kranjska Gora 29 CIGRÉ ŠK B1 1 circuits is evident. Not only the critical hot spots can be detected but in conjunction with other type of equipment (such as Real Time Thermal Rating) both preventive measures can be taken and the utilization of the system may be optimized. V. REFERENCES [1] IEC : Electric cables Calculation of the current rating Part 1: Current rating equations (1 % load factor) and calculations of losses [2] CIGRE Recommendations in Electra No. 87: Computer method for the calculation of the response of single-core cables to a step function thermal transient [3] S. Cherukupalli, A. MacPhail, R. Nelson, J. Jue, J. Gurney, Monitoring produces higher cable ratings, Transmission & Distribution World, 28