*martin.weibel@atel.ch

Overhead Line Temperature Monitoring Pilot Project

M. WEIBEL*, Atel Transmission Ltd., Switzerland

W. SATTINGER, P. ROTHERMANN, ETRANS AG, Switzerland

U. STEINEGGER, Meteodat GmbH, Switzerland

M. ZIMA, ABB, Switzerland

G. BIEDENBACH, monitec GmbH, Germany

SUMMARY

Power system operation is becoming more and more complex. The number of the market players using

the high voltage transmission system is increasing due to liberalisation and unbundling. Significant

power flow changes within a short time have become normal. As an example, the power balance of

Switzerland changed from 3000 MW import to 2000 MW export within a couple of days in March

2005.

Switzerland is located in the middle of the West European transmission system and is therefore subject

to high power transits. Several transmission lines situated in the north-south corridor from Germany to

Italy operate near their maximum transmission capacity. As some of these lines cross different

climatic zones (moderate, alpine, mediterranean climate), line thermal monitoring enhances power

system security. The aim of the pilot project is to provide a guideline for the evaluation of a system to

operate these transmission lines more efficiently while at the same time ensuring the security of

system operation.

By the use of different methods for on-line temperature measurement the authors show the

applicability of the tested methods. By comparing those methods and correlating them with the impact

of meteorology, general qualitative dependencies can be derived. The thermal line constant of the

overhead line can be extracted. The difference between hot spots versus medium temperature areas has

been determined.

KEYWORDS

Overhead line temperature monitoring – Overhead line sag measurements – High-quality

meteorological measurements in the vicinity of the line conductor

21, rue d’Artois, F-75008 PARIS B2 – 311 CIGRE 2006 http : //www.cigre.org

Offprint of paper SC B2-311 published at CIGRÉ 2006 SessionParis, 27th August – 1st September 2006.

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1. INTRODUCTION

In September 2003, Atel Transmission Ltd. in cooperation with ETRANS started a pilot project for

on-line thermal monitoring by an in-depth comparison of the state-of-the-art measurement techniques.

The results of the field test will be used as the basis for future decisions to implement permanent on-

line monitoring, in order to improve system operation.

The direct or indirect conductor temperature measurement of different methods have been compared

and correlated with line current measurements and high-quality meteorological data. The measured

values were recorded with a time resolution of 10 minutes using the following methods:

1. Thermo-vision with the help of special cameras and subsequent transformation of colour to

temperature.

2. MTS – (mechanical tension sensors) in combination with solar radiation and ambient

temperature measurements.

3. SAW – (surface acoustic wave) sensors, a new approach of the University of Darmstadt.

4. WAM – (wide area monitoring) line temperature monitoring approach – based on exact

measurements of the active power losses.

In addition to the overhead line conductor temperature measurements, the line sag was monitored for a

short time period, too. These measurements give an indication of the final impact of conductor

temperature variation. For line sag measurements the following two methods were analysed:

1. radar measurements

2. laser measurements

Meteorological measurements were performed near the conductors at the same altitude. They consist

of measurements of the ventilated ambient air temperature, solar radiation, 3-d wind speed and air

humidity. These measurements have been compared with recordings from official meteorological

stations, which collect their measurements at the high of 2 m (respectively 10 m for wind) above the

ground.

The results presented have been collected from three geographical different locations (490 m altitude –

moderate climate, 2130 m altitude – alpine climate and 190 m altitude – Mediterranean climate), see

Fig. 1-2.

Fig. 1: Stations and climate zone Fig. 2: Altitude and climate zones

As shown in the figures above, the 380 kV transmission line considered crosses three different climate

zones within only 100 km. It starts in the northern part of Switzerland, traverses the Alps and ends in

the southern part called Ticino. The selected line is located in the middle of the UCTE power system.

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0 50 100 150Distance [km]

Altitude [meters above sea level]

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2. TARGET

The aim of the project presented is to provide to the operation staff a reliable information about the

overhead line conductor temperature, in order to enable them to operate the system with a precise

security margin.

All external influences due to changes of the cooling or heating environment (warming up influence

by current flow or sun radiation, cooling down influence by rain, outside temperature or wind

activities) are therefore implicitly considered.

By the parallel application of meteorological measurements at conductor level, corresponding

correlations factors are derived.

Table 1: Transmission line conductor details

Material Aldrey

Conductor cross section 2 x 550 mm²

Number of conductor wires (Nbr. x Ø mm) 91 x Ø 2.77 mm

Conductor diameter 30.5 mm

Weight approximated 1534 kg/km

3. APPROACH

The following table gives an overview of the measuring methods used.

Table 2: Overview of measurement methods selected

Thermo-vision measures infrared part of the wave spectra.

Line conductor temperature can be obtained by mapping

the colour information into the corresponding temperature

spectra

The MTS system is based on the mechanical tension

measurements between the tower and the isolator in

combination with solar radiation and outside temperature

measurements. These measurements are hot spot

measurements.

The SAW measurement measures the surface acoustic

wave. This method is developed by the University of

Darmstadt. The sensor is fixed on the cable and sends the

data to a communication unit. Of course this is a hot spot

measurement.

The WAM/LTM is based on PMU-measurements. The

temperature can be calculated from the changes of the line

resistance. This is the only system which delivers an

average value of the line temperature and with the time

resolution of 1 second.

Meteorological measurements are performed on the level

of the line conductor. Temperature, solar radiation, wind

and humidity are recorded.

For direct sag measurement, laser and radar

measurements were tested. These measurements are also

hot spot measurements.

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3.1 MTS System

Three MTS transmission line monitoring systems have been installed in the overhead line under

consideration. Each of these systems measures the mechanical conductor tensions in the two adjacent

line sections.

The MTS monitoring system is mounted at selected dead-end structures along the 380 kV OH line

consisting of four load cells, ambient and net radiation temperature sensors, a main unit and a solar

panel. The measurement data are acquired by means of cellular phones.

Tension measuring load cells are installed between insulator strings and the crossarm of a tower pylon.

When a dead-end structure is selected for monitoring, two ruling spans can be measured

simultaneously. For this project, only one ruling span is monitored, using two load cells. The data

gathered by the two load cells with a full range of 44.5 kN and a resolution of 21 N are stored in the

main unit. In addition to the tension, the ambient temperature and the net radiation temperature are

also measured and recorded for each location. The net radiation sensor consists of a special aluminium

rod, which is placed in the same direction, at the same height and at the same location as the overhead

line conductor. This net radiation sensor (NRS) is developed in such a way that the integrated effect of

solar radiation, wind and ambient temperature is translated into a temperature. This net radiation

temperature is comparable to the temperature of the overhead line conductor when the connection is

switched off.

Every line section under consideration has to be calibrated to determine the overhead line conductor

temperature with a required accuracy [1]. The key to this calibration is a patented procedure for

determining the effective ruling span and the tension / temperature relation. The system is calibrated

by taking the line out of service at two different, known temperatures. Because the temperature

measured by the NRS is equal to the temperature of the conductor without current, the tension of the

conductor is now known at two

different conductor temperatures.

These calibration measurements

are used to accurately determine

the effective ruling span and the

actual sag conditions of the line

sections monitored by the system, and lead to a calibrated

relation between conductor

tension and conductor

temperature (see Fig. 3)

Fig. 3: Example of a calibration curve showing the relation between tension and ambient temperature

of the overhead line conductor

3.2 SAW System

The SAW elements have been applied as temperature sensors by using of physical effects such as

elongation of the conductor material or change of the propagation velocity of the surface acoustic

waves by mechanical forces or temperature. For further details see [2].

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5 10 15 20 25 30 35

Temperature [°C]

Tension [kN]

Measurement data Calibration curve

Ambient Ambient Temeperature [°C]

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3.3 METEOROLGY

Meteorological measurements were performed near the line conductors at the same altitude consisting

of (or including) measurements of the ambient air temperature, solar radiation, 3-d wind speed and air

humidity. These measurements have been compared with recordings from official meteorological

stations, which basically collect their measurements at an altitude of 2 m (respectively 10 m for wind)

above ground.

Maximum thermal ratings of overhead lines are dependent on the weather conditions experienced by

the line. In order to monitor the most important parameters, 3 different sensors are placed near the

overhead lines. For measuring air temperature and humidity, a Rotronic instrument is used. The

sensors are shielded against radiation and ventilated. Humidity sensor(s) also have to be shielded

against the electromagnetic fields in order to avoid faster aging.

The solar radiation is measured with the net radiometer NR-LITE. The WindMaster ultrasonic

anemometer is used to perform 3 axis wind measurements. Horizontal wind speed plus direction and

vertical wind speed are measured to accurately model the influence of winds both for the thermal

behaviour and for the tensions.

These sensors have been working without any maintenance for over 18 months. The data are sampled

in a 10 minutes interval on a logger. Data can be downloaded via RS232 Interface or by GSM data

transfer.

Comparisons with official meteorological measurements in the vicinity of our measuring locations

showed a good data quality, and as expected, important local differences.

The smallest differences were observed for air humidity. As for the air temperature a significant

smaller daily variation is observed compared to the conditions measured 2 m above ground.

The wind measurements in the moderate climate zone showed a higher wind speed than expected.

Especially in situations with foehn (strong downhill wind along the valley) this effect was very

pronounced resulting from jet phenomenon caused by the narrow valley foehn. In such cases a lifting

of air masses with a vertical wind speed of over 5 m/s could be measured. The wind conditions along

the observed overhead line are extremely variable in terms of space and time.

The aim of the measurements near the conductors is to access the real influence of meteorology as one

step for improving safety as suggested in [3].

3.4 WAM / LTM

Line Thermal Monitoring based on Synchronized Phasor Measurements

Several Applications based on Phasor Measurements has been cited in the literature. Besides the early

proposals related to State Estimation [4], various stability assessment methods relying on

Synchronized Phasor Measurements (SPM), sparsely distributed throughout the transmission system,

were dominant [5]. However, specific features of SPM can be used also for monitoring of line average

temperature in addition to above-mentioned applications. The approach can be explained as follows.

SPM of voltages and currents from both ends of the supervised line are collected. Their accuracy

allows for the computation of line parameters. Generally, line reactance as well as shunt capacitance

and conductance are essentially constant regardless of the ambient conditions of the line (of course,

they may change in the long term, but in a time scale of several years, e.g. shunt capacitance due to

vegetation grow etc.). But line resistance varies depending on the line temperature. The relation

between the line temperature and its resulting resistance is almost linear and is determined by the

properties of the material, from which the line conductors are constructed. Thus, knowing the line

material constant, a medium line temperature can be extracted.

However, errors introduced by the voltage and current measurement transformers may result in an off-

set in the temperature estimate. These off-sets can easily be compensated for, when a reference

temperature and corresponding resistance can be obtained. One occasion is the time when the line goes

into service, assuming that the line has the same temperature as its environment (i.e. ambient

temperature) until it is warmed up by the flowing current.

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4. RESULTS

The three main results of the ongoing measurement campaign are illustrated in the following figures.

Fig. 4 shows the measured conductor temperature over four days. At that project phase hot spot

measurements at only two locations were already installed (moderate climate and alpine climate). The

SAW measurement was acquired at the same location as the MTS (moderate climate). A shift of about

four degrees, which currently is the subject of a detailed analysis, between these two methods could be

observed.

The permanent difference of ten degrees between the moderate and the alpine climate (both MTS

measurements) illustrates the quite different cooling conditions on the same line but at different

altitudes (490 m versus 2130 m). Based on the measurement principle of average temperature

measurement the conductor temperature determined by the WAM system is located between the two

hot spot measurements.

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16.03.2005

17.03.2005

18.03.2005

19.03.2005

20.03.2005

Temperature [°C]

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400

600

800

1000

1200

Current [A]

Temp_SAW Temp_MTS_Alpine Temp_MTS_Moderate Temp_WAM_LTM Current

Fig. 4: Conductor temperature over 4 days

Based on only two available hot spot measurements at that time, Fig. 5 illustrates the result of a

comparison of the medium value measurement already included in the measurement principle with a

calculated average using two hot spots. Only for the last four days a difference of less than five

degrees was present. Meteorological recordings from those days show high temperatures for the

Mediterranean segment of the line with no hot spot measurement included at that time.

All methods used for conductor line temperatures measurement methods used deliver measurements

results with a sufficiently good accuracy required for system operation. One of the major issues is a

calibration procedure for the determination of the correct absolute value.

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19.07.2005

20.07.2005

21.07.2005

22.07.2005

23.07.2005

24.07.2005

25.07.2005

26.07.2005

27.07.2005

Temperature [°C]

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100

200

300

400

500

600

700

800

900

1000

Current [A]

WAM-LTM MTS-Medium Value Current

Fig. 5: Medium hot spot’s value versus WAM measurements.

Based on a rapid change of the line current of 240 A the line thermal constant has been determined,

see Fig. 6. It is essential to point out that during this time period the meteorological influences were

almost unchanged.

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11.1.05 19:00

11.1.05 19:10

11.1.05 19:20

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11.1.05 19:40

11.1.05 19:50

11.1.05 20:00

11.1.05 20:10

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11.1.05 20:30

11.1.05 20:40

11.1.05 20:50

11.1.05 21:00

Temperature [°C]

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1200

Current [A]

Temperature_alpine Temperature_moderate Current

∆I ≈ 240 [A]

∆t ≈ 18 [min.]

∆t ≈ 17.5 [min.]

Fig. 6: Determination of the line thermal constant

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5. CONCLUSION

Table 3 gives a comprehensive overview of the present project results. One major conclusion is that

the line conductor temperature starts to increase significantly only at line loadings above 80% of the

line thermal capacity. Direct solar radiation or foehn leads to high supplementary conductor heating.

At the same time rain is a perfect cooling agent. One of the next project steps will be to calibrate

corresponding line thermal calculation models with the acquired measurement data.

Table 3: General results

Based on the results of the latest third measurement configuration within the Mediterranean climate

zone, the maximum differences between hotspot measurements and medium value measurements will

be extracted. In the same time a forecast system for the corridor ambient temperature has been

installed in order to investigate the quality of the corresponding forecast with a five days horizon. The

aim of this system is to have available warning signals for the case of extreme weather situations and

to consequently adapt the system operation in time.

BIBLIOGRAPHY

[1] H.L.M. Boot, F.H. de Wild and A.H. van der Wey, G. Biedenbach, " Overhead line local

and distributed conductor temperature measurement techniques, models and experience at

TZH ", Cigré Session 2004

[2] R. Teminova, V. Hinrichsen, J. Freese et all " New approach to overhead line conductor

temperature measurement by passive remote surface acoustic wave sensor", Cigré Session

2006, B2-304

[3] T.O.Seppa, "Blackouts – Lessons learned", The Valley Group Inc., 3, September 2003

[4] A.G. Phadke, J.S. Thorp and K.J. Karimi, "State Estimation with Phasor Measurements",

IEEE Transaction on PWRS, Vol. 1, No. 1, February 1986, pp 233-241

[5] M. Zima, M. Larsson, P. Korba, C. Rehtanz and G. Andersson, "Design Aspects for Wide-

Area Monitoring and Control Systems ", Proceedings of the IEEE, Vol. 93, No. 5, May 2005

Influential Factor Impact

Line Current 2-4°C / 100 A (over 1000 A)

Solar Radiation ≈ 2°C / 100 W/m2 → 10°C

Ambient Temperature Base temperature (Offset)

Foehn ≈ 10°C

Wind No exact statements possible

Rain ≈ 10°C

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