Abstract - Power transfer through a short transmission line is

mainly limited by thermal rating. Accurate determination of

thermal rating can maximize power transfer which

subsequently saves utility from building new lines. Thermal

rating normally is set to a certain conductor temperature which

during operation the transmission line will not has problem

with ground clearance and the conductor material properties

will remain in its original state. The conductor temperature

varies with weather parameters and loading and it has close

relationship with line sag or line ground clearance. The

conductor temperature can be determined by monitoring the

line ground clearance and weather parameters and the actual

thermal rating can be calculated. Monitoring of the line ground

clearance can be done in real time using laser distance

measurement sensor with data logger and real time monitoring

and calculation software. Index Terms-- thermal rating, conductor temperature,

ground clearance, weather parameter.

I. INTRODUCTION

Nowadays resources such as right of way (ROW) are

scarce, materials for building transmission line are more

expensive and to build new transmission lines are sometimes

almost impossible, power utility companies has to find ways

to fully utilize their assets especially transmission lines.

Power transfer through a short transmission line which is

less than 80 km is mainly limited by thermal rating and for

long transmission line it is limited by voltage drop limit and

stability limit [1]. In normal practice, short transmission

lines are normally loaded according to static thermal rating.

The static thermal rating is calculated based on conservative

assumption of weather parameters [2]. However in actual

condition the weather parameters may differ from the

assumption hence there is opportunity to increase the rating

when the actual rating is known. To determine the actual

thermal rating of the transmission line, real time

measurement on the weather parameter or conductor

temperature is required.

There are a few techniques can be used to determine the

actual thermal rating of transmission line. The techniques

can be divided into 2 categories; using weather model and

conductor temperature model. This paper discusses on both

models and for conductor temperature model the conductor

temperature is determined using line ground clearance

technique. Dynamic thermal rating (DTR) monitoring

system also will be discussed.

TNB is a biggest power utility company in Malaysia.

II. DYNAMIC THERMAL RATING MONITORING SYSTEM

There are 3 main components to form the Dynamic

Thermal Rating (DTR) system that is remote monitoring

station, communication system and system computer. The

overview of system is shown in Figure 1. Remote monitoring

station gathers information on the environment in which the

transmission lines are located and also information on the

line clearance. The remote monitoring station includes

weather sensors and laser distance measurement (LDM)

sensors. The LDM sensors are used for measuring the line

ground clearance. The weather sensors consist of wind speed

and direction sensors, pyranometer and temperature sensor.

The remote monitoring station is placed at a critical location

along the transmission line where the wind cooling is

minimal or where there is an increased probability of contact

between the conductor and objects on the ground.

Figure 1- Dynamic thermal rating (DTR) system block

diagram

Remote monitoring station consists of a data logger, weather

sensors and 2 units of laser distance measurement sensor. The

function of this station is to gather weather data and line clearance

data in a defined interval and transmit it through a serial

connection to the system computer whenever dial-up connection is

established. Data obtained from the remote monitoring system will

be used for thermal rating calculation by the thermal rating

calculation software in the system computer. All sensors are

connected to the logger through its analog input terminals. The

connection of sensors to the data logger is shown in Figure 2.

Figure 2 - Components integration block diagram of the

Dynamic Thermal Rating (DTR) system

Weather sensors are used to measure weather data such

as wind speed and direction, ambient temperature and solar

Thermal Rating Monitoring of the TNB

Overhead Transmission Line using Line Ground

Clearance Measurement and Weather

Monitoring Techniques Azlan Abdul Rahim, Izham Zainal Abidin, Faris Tarlochan, Mohd Fahmi Hashim

The 5th

Student Conference on Research and Development –SCO

11-12 December 2007, Malaysia The 4th International Power Engineering and Optimization Conference (PEOCO2010), Shah Alam, Selangor, MALAYSIA. 23-24 June 2010.

978-1-4244-7128-7/10/$26.00 ©2010 IEEE 274

radiation. The wind speed data is recorded in meter per

second and wind direction is in degrees counter clock wise

from north. Sensors used for wind speed and direction

measurement is a three-cup anemometer and a wind vane on

a single axis. The anemometer is a contact-type wind sensor

which, when rotated by the wind, triggers a series of

momentary switch closures that are directly related to wind

speed. The wind vane uses a potentiometer to sense

direction changes. Depending on the position of the

potentiometer wiper, the output is a voltage signal that

corresponds to the position of the vane. By orienting the

vane to North (0 degree) during installation, wind direction

can be easily calculated from the output voltage. The

resolution of the wind vane is 1 degree. Ambient

temperature is sensed using a thermistor element which

changes resistance in response to temperature fluctuations.

For maximum accuracy, the sensor is isolated from the

effects of sunlight which can cause misleading temperature

and humidity measurements. The solar radiation shield is

used to give this protection. Solar radiation sensor or

pyranometer is used to measure solar radiation. The sensor

output is in voltage which changes in response to solar

radiation fluctuations. The solar radiation measurement is

recorded in watts per meter2. A complete installation of the

weather sensors is as shown in Figure 3.

Figure 3 - Weather sensors installation next to the

transmission line

Laser distance measurement sensor utilize laser light for

distance measurement. It allows accurate and contact-less

distance measurement over a wide range using the reflection

of a laser beam. The sensor measurement range is from 0.2

meter to 200 meter and the accuracy is ±1.5 millimeter.

Verification of the measurement accuracy has been done

where measurement by the laser distance measurement

sensor is compared with measurement tape. The results

confirmed that the measurement accuracy was within

tolerance of ±1.5 millimeter. The laser distance

measurement sensor is used to measure the transmission line

clearance to the ground where the distant ranges from 12 to

16 meter. It was installed on the ground directly under the

transmission line at the middle of the span as shown in

Figure 4.

Figure 4 - Laser distance measurement sensor installation

The output from the sensor is a current signal which

varies in the range of 4 to 20 milliampere depending on

distance measured. In this thermal rating system, at the

monitoring point, the line ground clearance is in the range of

13.0 to 15.5 meter. The sensor output has a linear

relationship with the distance measured, whereby 4

milliampere corresponds to 13.0 meter and 20 milliampere

corresponds to 15.5 meter. For data logging and retrieving

by the system computer, the current output is connected to

the remote station data logger through one of its analog input

points. To enable continuous monitoring of the line

clearance, the sensor was set to operate in automatic mode

which will take measurement continuously at every one

minute interval. Data from remote monitoring system is

transmitted to system computer via a wireless data

communication system. The communication system used in

this real time thermal rating system is the GSM wireless data

call system. This communication system is more suitable for

use in the DTR system compared to fix line due to

connectivity issues and cost. The GSM wireless is easy to be

implemented; it is robust and suitable for use in the outdoor

environment. Data transfer from remote monitoring station

to system computer is based on dial-up connection at every

10 minutes interval. The system computer consists of 2

software modules that is thermal rating calculation module

and thermal rating display module. It has 2 primary

functions, namely to process the weather and line clearance

data then calculate the thermal rating of the line and to

display the thermal rating in real-time. There are two

methods used for the thermal rating calculation, by using

weather based calculation and line clearance based

calculation. Weather based calculation utilized weather data

that is wind speed and direction, ambient temperature and

solar radiation data from monitoring station at site. The

calculation of the thermal rating is based on thermal balance

energy equation as described in IEEE Std 738 [3].

Line clearance based calculation utilized line clearance

data, ambient temperature data, line load data and solar data

from the monitoring site. Weather based thermal rating

calculation process as shown in Figure 5. In the first step the

program read the input data on conductor parameter such as

the conductor resistance at low temperature and high

temperature, conductor diameter, conductor weight per

kilometer, number of aluminum strand, number of steel

strand, diameter of aluminum and steel strand. Data on

weather parameter at the monitoring site such as wind speed,

wind direction, ambient temperature, and solar radiation data

is read from the real time updated text file. The line load

data is read from the SCADA database.

275

Figure 5 - Weather based thermal rating calculation program

flowchart

Based on the input data, the program calculates the

convection heat loss qc of the conductor. The equation used

in the calculation depends on the wind speed condition, for

no wind condition, the natural convection equation (1) is

used, for available wind speed the force convection heat loss

equation (2) and (3) is used where the highest value is used

for the qc.

(1)

(2)

(3)

The program then calculates the conductor radiation

heat loss qr and solar heat gain qs using equation (4) and (5)

respectively. For explanation of the equations please refer

IEEE Std 738 [3]. Next the program calculates conductor

resistance at the maximum conductor temperature as defined

by the user. Finally the thermal rating of the conductor Ir is

calculated using equation (6).

(4)

(5)

(6)

Line ground clearance based thermal rating calculation

process is as shown in Figure 6. The line clearance data

input for this program is from the remote monitoring station.

The conductor temperature is then determined based on the

conductor temperature and line ground clearance

relationship. The equation produced from this relationship is

given in equation (7). The relationship was developed by a

calibration process where line clearance and conductor

temperature was measured. Then qc is determined using

equation (2) and (3), where the qr, R(Tc) is calculated at the

instantaneous conductor temperature, the Ir is the existing

line load current and qs is based on the solar radiation

measurement.

(7)

Figure 6 - Line ground clearance based thermal rating

calculation program flowchart

Based on the qc value determined previously, the

equivalent perpendicular wind speed is calculated using

equation (2) and equation (3), the lowest value is used for

the next calculation. Next the qc, qr is calculated based on

the maximum conductor temperature and the calculated

wind speed. Then with the latest value of qc, qr , R(Tc) and qs

the thermal rating Ir is calculated using equation (6).

III. GROUND CLEARANCE RELATIONSHIP WITH CONDUCTOR

TEMPERATURE

Ground clearance has direct relationship with line sag as

shown in Figure 7. The line sag can be calculated based on

equation (8) as suggested in EPRI Increased Power Flow

Guide Book [4]. To develop conductor temperature and line

sag relationship, actual transmission line section physical

dimension including span length is measured as shown in

Figure 7. The transmission line span is modeled into sag

calculation formula. The line sag for every 5ºC increase of

conductor temperature from 20ºC up to 120ºC is calculated

and plotted.

Figure 7 – The selected transmission line span physical

dimension

For a reference, at a known conductor temperature and

horizontal tension the line sag is given by equation (8).

(8)

276

Parameters Value Unit

Linear expansion coefficient, α 1.93 x 10-05 1/°C

Elastic coefficient 7033 kg/mm2

Length between towers 362 m

Weight per length 1.18246 kg/m

Tension at 25 °C 1758.3 kg

Table 1 - Input and calculated data for the Zebra conductor

sag calculation [5]

At 25°C, reference to the input data in Table 1, the

calculated sag given as follows.

Weight, w

= 1.18246 kg/m

Horizontal tension, H

= 1758.3 kg

Span length, S

= 362 m

Sag, D

= 11.01592 m

The actual conductor length L, at the correspondence

line sag is given by equation (9).

S

DSL

3

8 2

+=

(9)

Span length, S = 362 m

Sag, D = 11.01592 m

Conductor length, L

= 362.8939 m

Subsequently equation (10) and (11) are used to

calculate conductor length and the corresponding sag for a

each conductor temperature.

( )[ ]frTT TTLL

REF Re1 −+= α (10)

( )

8

3 SLSD T −

=

(11)

Table 2 shows the calculated line sag value for given

conductor temperature.

Conductor temperature,

Tc

(°C)

Conductor length, L

(m) Sag, D (m)

25 362.8939257 11.01592

26 362.9009296 11.05899

27 362.9079334 11.10189

28 362.9149373 11.14463

29 362.9219411 11.18720

30 362.9289450 11.22962

31 362.9359488 11.27187

32 362.9429527 11.31397

33 362.9499565 11.35591

34 362.9569604 11.39769

35 362.9639643 11.43932

36 362.9709681 11.48081

37 362.9779720 11.52214

38 362.9849758 11.56332

39 362.9919797 11.60436

Table 2 - Calculated line sag for a given conductor

temperature

The relationship between the conductor temperature and

the line sag is shown in Figure 8. The graph shows that the

relationship is almost linear for the temperature between

28ºC to 120 ºC. From this relationship the conductor

temperature and line ground clearance is determined.

Conductor Temperature vs Sag

0

20

40

60

80

100

120

140

11.0

1591

7

11.1

4462

8

11.2

7187

11.3

9769

2

11.5

2213

9

11.6

4525

7

11.7

6708

7

11.8

8766

8

12.0

0703

8

12.1

2523

4

12.2

4228

8

12.3

5823

3

12.4

7310

1

12.5

8692

12.6

9972

12.8

1152

6

12.9

2236

5

13.0

3226

2

13.1

4123

9

13.2

4932

13.3

5652

6

13.4

6287

9

13.5

6839

8

13.6

7310

3

13.7

7701

3

13.8

8014

4

13.9

8251

5

14.0

8414

1

14.1

8504

14.2

8522

6

14.3

8471

4

14.4

8351

8

Sag (m)

Co

nd

ucto

r te

mp

era

ture

(d

eg

C)

Figure 8 - Relationship between conductor temperature and

sag

For calibrating the temperature and line clearance

relationship curve, a site test was conducted by measuring

the test conductor temperature and measuring actual line

ground clearance using laser distance measurement sensor,

the test setup shown in Figure 9 and Figure 10. The test

conductor is the same type of conductor used for the real

transmission line and the test conductor was exposed to the

same weather condition as at site. The same amount of load

current was injected into the test conductor to simulate the

actual line condition. Thermocouple wires were used to

measure the conductor temperature and temperature data

logger was used to record the temperature. The line ground

clearance was measured using laser distance measurements

sensor and the measurement data was logged for every 1

minute.

Figure 9 - Test setup for calibrating the conductor

temperature and line ground clearance relationship

277

Figure 10 - Photograph of the site test setup

Based on the conductor temperature and sag data from

the calculation, the corresponding line clearance was

calibrated with line ground clearance measurement using

laser distance measurement. Then the calculated ground

clearance was plotted to form a relationship between the

conductor temperature and ground clearance. Additional

data from the actual line clearance measurement and

temperature measurement were plotted onto the same

calculation data plot to verify the relationship between

conductor temperature and line ground clearance produced

by calculation. The plot is shown in Figure 11.

Conductor temperature vs ground clearance

y = -0.0027x4 + 0.1156x3 - 1.0157x2 - 37.31x + 570.05

R2 = 1

0

20

40

60

80

100

120

140

12 12.5 13 13.5 14 14.5 15 15.5 16

ground clearance (m)

co

nd

ucto

r te

mp

(d

eg

C)

Experiment

Calculated

Poly. (Calculated)

Figure 11 - Relationship between conductor temperature and

line ground clearance verified by experiment

From the experiment, the relationship between

conductor temperature and line ground clearance is given as

follow;

05.57031.370157.11156.00027.0 234

+−−+−= xxxxy (12)

Where

y = conductor temperature in °C

x = line ground clearance in meter

IV. RESULT AND CONCLUSION

Use of dynamic thermal rating (DTR) technology can

lead to modest gains in power flow capacity, typically 5% –

20%. In this research, development of DTR system has been

carried out and the system has been installed at the TNB’s

275kV transmission lines particularly at the selected line

span. The selected line is a very important line which

carrying large power from big power generations to the

national grid and potential to be overloaded during tripping

of other lines. However the transfer capabilities of the lines

are limited by its static rating which is 683MVA (or 1434

Amperes per phase at 275kV) per line. The static rating is

calculated based on the assumption of ambient temperature

35°C, wind speed 0.4469 m/sec, 850W/m2 solar radiation

and 75°C maximum conductor temperature. The system

calculation results have been verified by series of testing

including laboratory testing and site testing. The result as

shown in Figure 12 shows that; the actual thermal rating is

almost always greater than the static rating and the measured

load is always significantly lower than the static rating.

Figure 12- Dynamic thermal rating profile of the study line

The probability density of the thermal rating for a

period of 2 months is plotted using kernel smoothing density

estimate function in Matlab as shown in Figure 13. The

graphs show that the load is always below the static rating.

Only during emergencies would the load exceed the normal

static rating. The dynamic thermal rating is mostly 20%

higher than the static rating which is shown by the tip of the

dynamic thermal rating probability density graph in Figure

13. These graphs also show that the dynamic thermal rating

is sometimes below the static rating but the probability is

small about 3% as given by the gray color area on the Figure

13. It is important to note that when this does occur and the

load is high, the sag limits may be exceeded. This does

happen on real lines, but most often no one knows it. In

fact, it is impossible to know this is happening without DTR

system. This is a hidden risk with the traditional static rating

method.

Conductor

under test

278

Figure 13 - Probability density plot of the dynamic thermal

rating and line loading

The reasons for limiting the current in a conductor is to

avoid excess sag, and damage (due to annealing) caused by

operating at too high a temperature. The operating

temperature of a Zebra conductor used in the study line is

limited to 75ºC due to the sag limit. Figure 14 shows the

probability density graph of the conductor temperature

calculated by the DTR system. Most of the time the

conductor temperature is around 30ºC and the maximum is

about 50ºC which is far below the limit. However the

conductor temperature is always higher than the ambient

temperature as shown in the graph.

Figure 14 - Probability density plot of the conductor and

ambient temperature

Figure 15 shows the recorded line ground clearance in a

day. This plot shows that the line ground clearance varies

from 14.6 meter to 15.1 meter in a day, this is corresponding

to about 10ºC variation of conductor temperature. The

lowest recorded ground clearance is around 12.00a.m. to

2.00p.m. and the highest ground clearance is during early

morning between 4.00a.m. to 6.30a.m.

Figure 15 - Daily line ground clearance at the monitoring

site

Solar radiation can increase the temperature of an

overhead conductor by up to 15ºC during the mid sunny day,

depending on conductor absorptivity, emissivity, wind speed

and direction. During Mac 2009, at the monitoring site of

the transmission line, daily solar starts at about 7.20 a.m. and

end at about 7.20 p.m. The maximum solar radiation

recorded was during mid day which was about 1050 W/m2

as shown in Figure 16.

Figure 16 - Typical daily solar radiation profile at the

monitoring site

Wind speeds below 1.5 m/s are quite variable in both

speed and direction. This is also reported by other

researchers [6,7,8,9,10]. Though it is common to calculate

static ratings for perpendicular wind, field measurements

verify that winds below 1.5 m/s are not persistent in

direction. Therefore low winds are only occasionally

perpendicular to the conductor. It would be more reasonable

to assume an average wind direction somewhere between

perpendicular and parallel. A conservative assumption

would be that the wind is more nearly parallel rather than

perpendicular. Wind speed typically increases during the day

but rapid large changes in speed and direction are common.

Figure 17 shows the wind speed recorded for 48 hours

duration at the monitoring site; it shows that during day time

the wind speed is mostly higher compared to night.

279

Figure 17 - Wind speed variation at the monitoring site

Based on the DTR system data, conclusion can be made

as follows;

The dynamic thermal rating is mostly 20% higher than

the static rating. It is above the static rating 97% of the time

but in some instances it is below the static rating.

Conductor temperature varies between 25 ºC to 52 ºC

with ambient temperature varies between 22 ºC to 32 ºC

throughout day and night but most of the time the conductor

temperature is around 30 ºC. The actual ambient is lower

compared to the assumption made in the static thermal rating

calculation which is 35 ºC. This is one of the reasons why

the actual thermal rating is higher than the static thermal

rating.

The ground clearance of the line bottom conductor at

the monitoring site varies between from 14.6 meter to 15.1

meter which is corresponding to about 10ºC variation of

conductor temperature.

Solar time and solar radiation pattern is almost same

throughout the 6 months monitoring period, about 5% of the

time the solar radiation is greater than the value used for

static thermal rating calculation which is 850W/m2

however

most of the time the solar radiation is less than 850W/m2.

This is another reason why most of the time the actual

thermal rating is higher than the static thermal rating.

Wind speed is below 1.5m/s most of time but typically

increases during the day but rapid large changes in speed

and direction. DTR system data verify that winds below 1.5

m/s are not persistent in direction. However wind speed

value used in the static rating calculation is only 0.443 m/s at

30 degree angle toward the conductor axis which considered

conservative. This is another reason why the actual thermal

rating is higher than the static thermal rating.

V. REFERENCES

[1] Prabha Kundur "Power System Stability and Control", McGraw-Hill

1994.

[2] Black, W.Z. and Rehberg, R.L., Simplified Model for Steady State

and Real Time Ampacity of Overhead Conductors IEEE/PES Winter

Meeting, Paper No. 88 WM 236-5, 1985.

[3] IEEE Standard for Calculating the Current-Temperature Relationship

of Bare Overhead Conductors, IEEE Std 738-2006.

[4] R. Adapa, Technical Report – “Increased Power Flow Guidebook:

Increasing Power Flow in Transmission and Substation Circuits”,

EPRI, Palo Alto, CA, 2005.

[5] Cigre Working-group 22.12, The Thermal Behaviour of Overhead

Conductors, Section 3: Mathematical Model for Evaluation of

Conductor Temperature in The Unsteady State, Electra No.174

October 1997, pp. 59-69.

[6] Tapani 0. Seppa, Summer Thermal Capabilities of Transmission

Lines In Northern California Based on A Comprehensive Study of

Wind Conditions, IEEE Transactions on Power Delivery, Vol. 8. No.

3, July 1993.

[7] Patrick M. Callahan, D. A. Douglass. An Experimental Evaluation of

A Thermal Line Uprating by Conductor Temperature and Weather

Monitoring, IEEE Transactions on Power Delivery, vol. 3, NO. 4,

October 1988.

[8] Stephen D. Foss, Evaluation of An Overhead Line Forecast Rating

Algorithm, IEEE Transactions on Power Delivery, Vol. 7, No. 3, July

1992.

[9] Black, W.Z. and Rehberg, R.L., Simplified Model for Steady State

and Real Time Ampacity of Overhead Conductors IEEE/PES Winter

Meeting, Paper No. 88 WM 236-5, 1985.

[10] Dale A. Douglass, Field Studies of Dynamic Thermal Rating

Methods for Overhead Lines, 0-7803-5515-6/99, IEEE, 1999.

VI. BIOGRAPHIES

Azlan Abdul Rahim, received his Bachelor in Electrical & Electronic

Engineering from University Science of Malaysia in 1996. He joined TNB

Research Sdn. Bhd. in 1997. Currently, he is a senior researcher at Power

System Group, TNB Research Sdn. Bhd. and he is also a part time student

doing Master Degree at Universiti Tenaga Nasional. His research areas

include real time monitoring and control, dynamic thermal ratings of

electrical equipment, power system wide area measurement, fault detection

and location

Izham Zainal Abidin, received his Bachelor in Electrical Engineering

degree from Southampton University, UK in 1997. He obtained his PhD in

Electrical Engineering from the University of Strathclyde, Scotland, UK in

2002. Currently, he is an academic staff at Department of Electrical Power

Engineering, Universiti Tenaga Nasional Malaysia. His research interests

include voltage stability studies, artificial intelligence and fuzzy logic

application to Power Systems problems, and robotics

Faris Tarlochan, received his Bachelor in Mechanical Engineering and

Master in Science from Purdue University in 1998 and 2001 respectively

and obtained his PhD from Universiti Putra Malaysia in 2007. He joined

Universiti Tenaga Nasional 2007. Currently, he is an Associate Professor at

Universiti Tenaga Nasional. His research areas include finite element

analysis, optimization, design and applied mechanics

Mohd Fahmi Hashim, Mohd Fahmi Hashim received his Bachelor in

Electrical Engineering from University of Sheffield in 2003. Currently he

is a Researcher in TNBR. Prior to join TNBR, he works as a Research

Assistant in UTP since 2004. His research interests include fault location

and dynamic thermal rating

280