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JICABLE ' 03 - International Conference on Insulated Power Cables

Real Time Monitoring of Power Cables by Fibre Optic Technologies

Tests, Applications and Outlook

LUTON Marie-Hlne

SAGEM SA, Activit CblesRue de Varennes Prolonge77876 Montereau CedexFrancePhone : + 33.1.60.57.31.06Fax : + [email protected]

ANDERS G.J., BRAUN J-M., FUJIMOTO N., RIZZETTO S.

KINECTRICS800 Kipling Avenue, KL 204Toronto, CANADA M8Z 6C4Phone : 416-207-6874Fax : [email protected]

DOWNES John A.

SENSAGamma House, ChilworthScience Park, SouthamptonHampshire SO16 7NSUnited KingdomPhone : +44 (0) 23 8076 5562Fax : +44 (0) 23 8076 [email protected]

Abstract : The change in operating conditions of HVunderground cable links has led many utilities toconsider on-line and/or off-line temperaturemonitoring via optical fibres. This paper describes thetest program and preliminary results obtained on a400 kV test installation, including various methods ofcable installation, different kinds of temperaturesensors and the implementation of a real timemonitoring system. Simulation of different operatingscenarios (steady-state and transient overloading)was performed and the resulting ampacities wereevaluated.The paper discusses the outlook for the application ofsuch monitoring technologies.

Keywords : Distributed temperature sensing,dynamic feeder rating.

Rsum : L'volution des conditions d'exploitationdes liaisons souterraines HT a conduit de nombreuxexploitants envisager le monitoring en tempraturedes liaisons. Cet article dcrit le programme d'essaismis en uvre sur une boucle d'essais 400kV incluantsur son parcours diffrents modes de pose, diffrentstypes de capteurs de temprature ainsi qu'unsystme de monitoring spcifique. La simulation dediffrents modes de fonctionnement (rgimepermanent et rgime de surcharge transitoire) a tralise et les intensits rsultantes values. Cet article discute des perspectives d'application dece type de monitoring.

Mots cls : Mesure de temprature rpartie, contrledynamique de capacit de transport.

1. Introduction

During the lifetime of a buried power cable, typically40 years, its thermal environment can evolve due tomodifications of the network configuration andchanges in the local surroundings. This phenomenonis particularly sensitive and difficult to control in urbanareas.

The dimensioning of an extruded cable link isgoverned by the maximum allowable temperature onthe cable conductor. If this temperature is exceeded,the lifetime and reliability of the cable circuit can bereduced. This can lead to unexpected prematurebreakdowns. Moreover, economic factors are driving utilities to usetheir cable circuits to achieve maximum allowable

ampacity rating, thus using up previously allowedsafety margins.

Real time temperature monitoring tools are nowavailable to users, allowing them to maintain networkreliability even though the network is being operatedharder.

Thanks to the development of distributed temperaturemeasurement using optical fibres, it is possible :

- to monitor the temperatures reached along theentire cable route.

- to compute the maximum acceptable ampacitytaking into account the simultaneously measuredthermal conditions.


Within the study of new XLPE 400kV power cable,SAGEM, in collaboration with KINECTRICS andSENSA, decided to carry out a study which wouldcompare temperature measurements and ampacityrating calculations obtained according to variousprotocols.

This paper describes the configuration of the testloop, its instrumentation as well as the specific testsperformed during this study.

2. Presentation of the test cable loop

In order to be able to draw reliable conclusions fromthe measurement techniques and calculations used,it was essential to operate this test in a thermalenvironment which was as controlled as possible.The hereafter described tests were carried out on acable loop installed in a representative configurationof the network and in a thermal environment of whichthe main thermal characteristics were known orevaluated according to the IEC standards.

2.1 Presentation of the cable, accessories andways of laying

The test loop, of an overall length close to 200m,includes 2 main circuits :

a length of cable with voltage applied a length of cable, used as a dummy cable,

(without voltage applied) and made up of thesame cable as that of the loop with voltageapplied.

The thermal configuration of this system is equivalentto a three-phase cable system (fig. 1) and variouslaying techniques are included :

Zone A : directly buried cable (1,30m underground surface) laid in flat formation in weakconcrete,

Zone B : cable laid in PVC ducts in trefoilformation. The ducts are embedded withinconcrete.

Zone C : joint chamber with cable andpremoulded joints, also buried in weak concrete.

Fig. 1 : TEST LOOP CONFIGURATION

ZONE A +Z1, Z2, Z3

ZONE B +Z4, Z5,Z6

ZONE C +Z7, Z8,Z9

Z12, Z13,Z14 Z18, Z19,Z20

Z10Z11

Zx : temperature measurement zone

Z15, Z16, Z17


The 400kV XLPE cable used (fig. 2) is made up asfollows :

Conductor : 2000mm Coppersegmented withenamelled wires

Insulation : ,, XLPE

Metallic screen : aluminum wires aluminium foil optical fibres withinstainless steel tubesincluded in the aluminumwire layer.

External protective sheath

HDPE over aluminumfoil.

Fig. 2 : 2000mm Copper XLPE 230/400(420)kV

2.2. Temperature sensors

Different kinds of temperature sensors are installedon the test loop. Optical fibre loops are used tomeasure the temperature profile along the entirecable loop and thermocouples are also installed atspecific locations along the cable loop.

As indicated above, 2 stainless steel tubes includingoptical fibres are positioned in the metallic screen ofthe test cable. One tube comprises 50/125mmultimode fibres and another tube comprises60/125m multimode fibres. The optical fibre circuitis completed with optical splices and terminations. Positioning of the optical fibres in the cable metallicscreen has the following advantages: The optical fibres are laid simultaneously with the

power cable,

mechanical protection of the fibre is ensured bythe metallic screen and the cable outersheath.

recorded temperatures are more representativeof the conductor temperature because thesefibres are closer to the cable conductor, and itstemperature response is faster than an externalfibre fixed on the outersheath.

In addition to these integrated fibres, another externaloptical fibre circuit, was laid and fixed on the cableoutersheath for comparison purposes. Thistechnique, is an alternative method although it is lesssensitive to the changes in conductor temperature.

The dummy cable (without voltage applied) allowsaccurate temperature measurement, of each zone,on the conductor and on the external sheath of thecable. Thermocouples are used as the referencesensors for checking and comparing themeasurements and calculations made by themonitoring systems and various optical fibre loops.The thermocouples used are of a shielded design.

3. Presentation of the temperature andmonitoring systems

3.1. Distributed Temperature Sensor System

Distributed Temperature Sensor (DTS) systems andtheir application to power cable monitoring have beendescribed elsewhere [1],[2],[3]. Nowadays DTS iswidely specified as a condition monitoring and assetmanagement tool when new power cable installationprojects are implemented. Typical attributes of an 8km, multimode DTS system, using double-endedmeasurement are summarized below:

Temperature resolution -10 C Accuracy - 10 C Positional resolution - 1 m Hot spot (spatial) resolution - 1 m Measurement time (@ 8 km & 10 C) < 2 min

Typically, the temperature data is stored and isavailable either locally or via a communicationconnection to a remote site where the completetemperature profile of the cable can be visualized atany point in time. Because there are so many datapoints collected by the DTS, it is common to dividethe cable into a number of zones and to periodicallytransmit the maximum, average, or rate of change oftemperature, within the zones, in to a SCADAsystem. The DTS can also be configured to closerelay contacts associated with settings within a zone.In cable tunnels, it is quite common for the DTSsystem to provide the combined functionality of a firedetection sensor, tunnel temperature sensor (forventilation control) and as a power cable monitoring

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sensor. In these applications multiple fibre loops aresensed, using the internal optical switch of the DTS. Transmission and distribution utilities are recognizingthe benefits of continuous monitoring using DTS andare connecting the temperature monitoring systeminto control, alarm and communication schemes inorder to contribute to the automation and efficientoperation of the network.

Some of the benefits of DTS can be summarized asfollows:

Providing footprints of cable temperature profiles Confirming route design thermal characteristics Establishing the position of cable hot spots Providing early warning of cable failure Enabling network contingency management Enabling calculation of enhanced cable ratings

In the majority of DTS systems installed to date, otherthan alarm and SCADA data, evaluation of the DTSdata is done off-line. Some utilities are now lookingto increase further the automation of their networkand use the data from the DTS for purposes of on-line, real time ampacity rating of cables. This meansthat for this functionality, the DTS becomes the dataacquisition system for the cable rating computationand the output is ratings in preference totemperature.

The system used during the test is a DTS 800instrument with a specification similar to thatdescribed above. The different optical fibre loops areconnected to the DTS in a bay placed in the controlroom (fig. 3)

3.2. Real Time Monitoring System

The real time monitoring system consists of four keymodules, the temperature and load current sensingequipment, a data acquisition system, a real timerating system and a display/archival system. Apictorial overview of the system as installed at thetest site is given in fig. 3.

The data acquisition system is intended to capturethe necessary input parameters of load current andcable surface and soil ambient temperatures for inputto the real time monitoring system. The load currentis obtained from a current transducer (CT) sensormounted in a spare metering circuit.

Cable surface and soil ambient temperatures areobtained from thermocouples located strategicallyalong the cable route. A total of 22 temperaturemonitoring points (fig. 1) were installed on the testloop. All thermocouple and sensor inputs areconnected to a single, centrally mounted cabinet

housing the data acquisition hardware, as depicted infig. 3.

Fig. 3 : Real time monitoring systemRTU - data acquisition hardware placed outdoorBay in the control room including

a- DTS 800, b- Computer server

All current and temperature data is passed to acomputer server located in the control room via afiber optic connection, thereby providing completeelectrical isolation.

In parallel, cable surface and soil ambienttemperatures are obtained from the DTS equipmentdescribed above. A total of 10 locations are beingmonitored by the data acquisition system. Theconnection between the DTS and the computerserver is made via an RS232 link.

The computer server is the heart of the system,continuously gathering load and temperature dataand providing updated ampacity ratings for displayand archiving. The server stores the relevant thermalmodels of the cable loop and performs advanced realtime computations to determine the limiting rating ofthe monitored cross section. The system is presentlyconfigured to compute ratings at a single crosssection in Zone A (see fig. 1) A facility is alsoavailable to download historical information stored onthe server.

3.3. Modeling of the Thermal Environment

The computational program can perform calculationsin real time of steady-state and emergency ratingsand can display the computed conductortemperature. Calculation of the steady-state ratingsare performed using IEC 60287 procedures [4] with a

RTU - Data acquisition

BAY

DTS

Computer serverOptical fiber

RS 232


user-specified step load function. The program canalso consider recorded load variations over the last24 hours and will assume the same variations duringthe following period. The duration of this period issuch that the steady-state conditions are achieved.Time-dependent ratings are based on the methoddescribed in the IEC Standards 853-1 and 2 [5].Similarly as in the steady-state case, two modes ofload representation can be considered during theemergency period:

(1) a step function can be applied, or(2) the load curve from the last 24 hours can be

scaled.

During the steady-state and time-dependentcalculations, the thermal and electrical parameters ofthe model are continuously adjusted so that thecomputed and measured cable surface temperaturesmatch. The computational engine of the DynamicFeeder Rating (DFR) system permits simultaneousanalysis for several cable cross-sections in a multiplecable installation.The cable design is modeled by taking into accountall details pertaining to the conductors and insulatingsystems and the type of cable construction,addressing thermal and electromagnetic losses. Mostparameters evaluated by the engine (for examplethermal resistances and capacitances, loss factors,electrical resistances, heat transfer coefficient and airvelocity for fluid flows) are calculated for a givenoperating point, geometrical dimensions and selectedmaterial characteristics. The thermal circuit ismodeled by an analogous electrical circuit in whichvoltages are equivalent to temperatures and currentsto heat flows. If the thermalcharacteristics do not changewith temperature, theequivalent circuit is linear andthe superposition principle isapplicable for solving anyform of heat flow problem.

4. Program of tests

The aim of the tests consistsin performing thermal cycleswith controlled parameters(heating, cooling cycleduration, induced ampacityrating). This protocol allows theaccuracy of the calculationsto be checked.

Two test sequences areproposed :

- one representative of a steady-state operation,- the other simulating a transient operation. The analysis and calculations of ampacity ratings arecarried out in the predetermined zones Zx (fig. 1).The maximum allowable ampacity corresponds tothat of the hottest zone of the cable route measuredby the various temperature sensors (thermocouples,external or internal optical fibre). The values ofacceptable ampacity of each zone are also calculatedand compared.

To check the accuracy of ampacity calculationscapacity, the so calculated ampacity is thenvoluntarily induced into the test loop, as in the case ofa real time monitoring by software. The temperaturesare recorded over the determined duration of thecycle.

The thermal environment of the loop is modelled bythe software specifying the geometry and the thermalcharacteristics of the various components. Thoseretained for this evaluation correspond to the dataappearing in IEC standard.

5. Tests results

The tests are still in progress. The full results will bepublished in a separate document presenting thedetail of measurements and conclusions reached.

Fig. 4 is an example of DFR display which shows thezone calculation measurements and ratingcalculation.

JICABLE ' 03 - International Conference

Fig. 4 : DFR display exampleon Insulated Power Cables


Some preliminary results have been obtained withheating/cooling cycles (8 hours on and 16 hours off).producing 100C on the conductor at the hottestzone of the cable route.

It was found necessary to calibrate the DTS andthermocouple measurements, following initialanalysis. There was good correlation between theDTS and thermocouple measurements at the samepoints along the cable (fig. 5 - cooling period) and thesteady state and dynamic ratings were ofcomparatively similar magnitudes and comparedfavorably with calculated values.

6. Conclusion and outlook

A real time dynamic rating system has beenimplemented using the temperature values generatedfrom an optical fibre DTS monitoring system. Thepreliminary results from the measurements andcomputations are encouraging, however the testsequences have not yet been completed and furtherresults will be published at a later date.

The authors believe that the system described can beused to provide additional benefits over the alreadyestablished benefits of using distributed temperaturemeasurements for power cable monitoring. Thecombination of DTS measurement and real timemodelling will provide utilities with additionalfunctionality that will further assist with the economicand reliable management of utility networks andinterconnections.

7. References

[1] Hartog et al: "Distributed temperature sensing insolid core fibres", IEE Electronic Letters, Volume21,1985.

[2] Funnell and Harrison, "Remote temperaturemeasurement for 132 kV cables", CIRED, 1991.

[3] Barber, Jansen Nokes, Gatland,"Fibre optic monitoring takes the heat offcables", Modern Power Systems, Dec 1999.

[4] IEC 60287 : "Electric cables - calculation of thecurrent rating"

[5] IEC 853-1, 853-2 : "Calculation of the cyclic andemergency current rating of cables"

8. Glossary

DTS : Distributed Temperature SensorSCADA : Supervisory Control and Data AcquisitionRTU : Distributed terminal unitDFR : Dynamic Feeder Rating

Acknowledgments :

The authors wish to thank the EDF R&D team for allof their help during this project.

Fig. 5 : Temperature measurements in Zone 5(PVC ducts ) during a daily heating cycle

Tc EDF

Tc (shielded)

DTS channel 1 (1st and 2nd)Internal optical fibre

DTS channel 2 (1st and 2nd) :External optical fibre

ReferencesGlossary

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