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*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.
2
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.
0
500
1000
1500
2000
2500
0 50 100 150Distance [km]
Altitude [meters above sea level]
3
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.
4
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].
11
12
13
14
5 10 15 20 25 30 35
Temperature [°C]
Tension [kN]
Measurement data Calibration curve
Ambient Ambient Temeperature [°C]
5
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.
6
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.
5
10
15
20
25
30
35
40
16.03.2005
17.03.2005
18.03.2005
19.03.2005
20.03.2005
Temperature [°C]
0
200
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.
7
0
5
10
15
20
25
30
35
40
45
50
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]
0
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.
0
5
10
15
20
25
30
35
40
11.1.05 19:00
11.1.05 19:10
11.1.05 19:20
11.1.05 19:30
11.1.05 19:40
11.1.05 19:50
11.1.05 20:00
11.1.05 20:10
11.1.05 20:20
11.1.05 20:30
11.1.05 20:40
11.1.05 20:50
11.1.05 21:00
Temperature [°C]
0
200
400
600
800
1000
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
8
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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