Smart Grid Case Study:
Increasing the Thermal Rating of
Overhead 230-kV Transmission Lines
from Niagara Falls, Canada
William A. Chisholm, Ph.D., P. Eng, FIEEE
W.A.Chisholm@ieee.org
Universidade de São Paulo
6 Oct 2015
W.A.Chisholm@ieee.org
Abstract
• A “Smart Grid” case study allowed a significant increase
in transmission line thermal rating while ensuring safe
electrical clearances below the line.
• Showed operational use of sonar clearance
measurements from the ground below the lines.
• Demonstrated use of the operating transmission line
itself as a very long (30-km) hot-wire anemometer,
compared to conductor diameter (28 to 34 mm).
W.A.Chisholm@ieee.org
Abstract
• The nature of the project involved several disciplines,
including meteorology, thermodynamics, multi-span
mechanical coupling, data acquisition and human factors.
• The seminar will be presented at an accessible, tutorial
level with strong support from field results.
• The foundation of the seminar was invited for the first
SENEV conference, sponsored by CEMIG in Minas Gerais,
Brazil.
W.A.Chisholm@ieee.org
Motivation
• In Ontario, Canada, there have been ongoing
restrictions in ability to transfer power from the
Niagara Falls stations to load centers 100 km
away.
• Capacity of a five-circuit 230-kV transmission
interface to load centers is reduced when wind
speed drops below 2 m/s.
• Plans to supplement existing lines with two new
circuits have been frustrated.
W.A.Chisholm@ieee.org
Motivation
• In the meantime, Hydro One initiated advanced
studies of overhead line clearance using:
– Helicopters to perform laser surveys of the conductor
position at a known temperature
– Sonar to measure the conductor heights continuously
– New algorithms to invert a traditional (IEEE Standard
738) overhead line thermal rating calculation.
W.A.Chisholm@ieee.org
Motivation
• The process establishes spatially averaged wind
speed along entire line sections, rather than
relying on spot readings with differing exposure
height and sheltering, measured at remote sites.
• This presentation will compare different methods
to measure low wind speeds for line rating.
• This presentation will also cover the important
mechanical response of the coupled spans.
W.A.Chisholm@ieee.org
Organization of Seminar (1), 60 minutes
• Heat Balance of Overhead Conductors
• Field Observations of Temperature, Solar Flux, Wind
Speed, Direction and their Standard Deviations
• Change of Tension, Sag and Clearance with
Conductor Temperature
– In single span
– In multiple spans with freely swinging insulators
• Field Observations of Clearance and its Predictors
W.A.Chisholm@ieee.org
Organization of Seminar (2), 30 minutes
• Ampacity Inversion
– Long-Axis Hot-Wire Method
– Inputs: Clearance, Line Current, Ambient Temperature,
Solar Radiation
– Outputs: Spatially Averaged Conductor Temperature
and Wind Speed
• Implementation by Utility and Results
• Conclusions and Discussion
W.A.Chisholm@ieee.org
Heat Balance for Outdoor Conductor
• Heat Inputs
– PJ , Joule Heating, I2R where R is the ac resistance at
the temperature of operation
– PM , Magnetic Heating from ac currents induced in the
steel core from unbalanced stranding
– PS , Solar Heating, up to 1000 W/m2 and affected by
conductor absorptivity
W.A.Chisholm@ieee.org
Heat Balance for Outdoor Conductor
• Heat Outputs
– Pc , Convective cooling
• Wind speed v > 0.4 m/s, conductor diameter D =20 mm
• Reynolds number Re =vD/ (mf/rf )about 1000
• Nusselt number Nu is a function of Re and wind angle
– Pr , Radiative cooling, varies as fourth power of
difference in absolute temperature (K)
– Pw, Evaporative cooling if the conductor is wet or
sublimation cooling if it is loaded with ice
W.A.Chisholm@ieee.org
Heat Balance Model: Solar, Radiation
Pr: Radiation Heat Loss
• Varies as (Tc4 –TA4) times
• Conductor Emissivity
PS: Solar Input
• Intensity
• Azimuth
• Elevation
•(time of day)
• Conductor
Absorptivity
W.A.Chisholm@ieee.org
Heat Balance Model: Joule, Convection
PC : Convection Heat Loss varies
as (TC-TA )(Effective Wind Speed) 0.5
Line Current
• PJ varies as I2R
• PM varies nonlinearly
Wind speed, angle to conductor
W.A.Chisholm@ieee.org
Overall Heat Balance
Radiation Heat Loss
(Tc4 –TA4 ) (T in Kelvin…)
Convection Heat Loss
Varies as (TC-TA ) (Wind Speed) 0.5
Solar Input
Line Current
Wind speed, angle to conductor
W.A.Chisholm@ieee.org
Heat Balance for Outdoor Conductor
• Steady State, Dry Conditions
PJ  PM  PS  Pc  Pr
• Unsteady State
dTav
mc
 PJ  PM  PS  Pc  Pr
dt
– m is mass per unit length of conductor
– c is the overall heat capacity of the steel/aluminum
W.A.Chisholm@ieee.org
Transient Thermal Response (IEEE 738-2012)
Step Change in Current, 800 to 1200 A
Rise in Conductor
Temperature
from 80 to 128°C
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Transient Thermal Response (IEEE 738-2012)
Step Change in Current, 800 to 1200 A
Rise in Conductor
Temperature
from 80 to 128°C
Conductor reaches 110°C
(63% of difference)
in 13 minutes
W.A.Chisholm@ieee.org
Effect of Wind Speed on
Conductor Temperature (CIGRE B2 Tutorial)
(m/s)
W.A.Chisholm@ieee.org
Effect of Wind Speed on
Conductor Temperature (CIGRE B2 Tutorial)
(m/s)
Critical Wind Speed Range: 0-2 m/s (0-7.2 km/h)
W.A.Chisholm@ieee.org
Effect of Wind Speed on Thermal Transfer:
CIGRE Technical Brochure 207
Conductor Temperature (°C)
(CIGRE Technical Brochure 207 Presentation)
Extra 150 MVA of Power Transfer at 2 m/s
(7.2 km/h) compared to 1 m/s (3.6 km/h)
Power Transfer (MVA)
W.A.Chisholm@ieee.org
Effect of Wind Angle on
Nusselt Number governing Heat Transfer
• CIGRE TB 207 Model for Wind at angle d:




 0  d  24 : Nu90 0.42  0.68  sin( d 1.08
Nud   
0.90

24  d  90 : Nu90 0.42  0.58  sin( d 
• Stranding, catenary give minimum value of 0.42
Nu90 for axial flow along line (d=0°).
• Older model mixes forced, natural convection
based on V.T. Morgan concept.
W.A.Chisholm@ieee.org
Effect of Wind Angle on
Nusselt Number governing Heat Transfer
W.A.Chisholm@ieee.org
Standard Deviation of Wind Direction
also affects Nusselt Number
Nueff
Nu90

1
2d

e
d 2 2 d2
0.05  0.95 sin d 
2
0.25
dd

W.A.Chisholm@ieee.org
Standard Deviation of Wind Direction
also affects Nusselt Number
Wind angle
standard deviation
d tends to
increase as wind
speed drops.
W.A.Chisholm@ieee.org
Standard Deviation of Wind Direction
also affects Nusselt Number
Wind angle
standard deviation
d tends to
increase as wind
speed drops.
At low wind speed,
CIGRE TB207
suggests yaw angle
of 45°.
W.A.Chisholm@ieee.org
Organization of Seminar (1)
• Heat Balance of Overhead Conductors
• Field Observations of Temperature, Solar Flux, Wind
Speed, Direction and their Standard Deviations
• Change of Tension, Sag and Clearance with
Conductor Temperature
– In single span
– In multiple spans with freely swinging insulators
• Field Observations of Clearance and its Predictors
W.A.Chisholm@ieee.org
Field Observations of Weather Parameters
Used in Transmission Line Thermal Rating
• Historical data –hourly or daily maximum basis.
• Direct observations – utility weather stations.
• Predictions – numerical models based on recent
observations and/or historical trends.
• CIGRE Technical Brochure 299 provides guidance
on how each dataset can be used.
W.A.Chisholm@ieee.org
Historical Weather Data:
Annual Variation of Ambient Temperature
W.A.Chisholm@ieee.org
Historical Weather Data:
Annual, Daily Variation of Ambient Temperature
W.A.Chisholm@ieee.org
Historical Weather Data:
Annual, Daily Variation of Ambient Temperature
𝑇𝐴 = 18.0 − 2.6 sin
2𝜋𝐷𝑜𝑌
2𝜋𝐷𝑜𝑌
2𝜋𝐻𝑜𝐷
2𝜋𝐻𝑜𝐷
− 10.2 cos
−3.6 sin
+ 2.6 cos
365
365
24
24
W.A.Chisholm@ieee.org
Historical Weather Data:
Relation of Wind Speed to Ambient Temperature
W.A.Chisholm@ieee.org
Direct Observations:
Utility Weather Stations
• Use specialized, ultrasonic or lightweight
anemometers to measure low wind speeds.
• Data sampled over 10-minute periods, recording
average and standard deviation (CIGRE TB 299).
W.A.Chisholm@ieee.org
Direct Observations: Wind Parameters with
Sonic Anemometer and Conductor Replica
Sonic Anemometer
(Vaisala)
W.A.Chisholm@ieee.org
Direct Observations : “Wind Rose”
measured with Sonic Anemometer
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Direct Observations : Wind Speed, Direction
Standard Deviations with Sonic Anemometer
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Direct Observations : Wind Speed, Direction
Standard Deviations with Sonic Anemometer
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Direct Observations : Standard Deviations at
Low Wind Speed using Sonic Anemometer
W.A.Chisholm@ieee.org
Reminder: Standard Deviation of Wind
Direction affects Nusselt Number
Wind angle
standard deviation
d of 45° at 1 km/h
suggests effective
wind yaw angle of:
48° for cross flow
(Nueff -13% )
30° for axial flow
(Nueff +55 %)
W.A.Chisholm@ieee.org
Direct Observations: Heated / Unheated
Aluminum Conductor Replicas
• Shaw ThermalRate (formerly Pike)
• Heated, unheated portions of stranded aluminum
oriented along the line direction.
W.A.Chisholm@ieee.org
Direct Observations: Shaw Sensor Rating
versus Ambient Temperature
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Direct Observations: Shaw Sensor Rating
versus Wind Speed (Ignoring Wind Angle)
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Direct Observations: Shaw Sensor Rating
versus Wind Speed (Including Wind Angle)
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Direct Observations: Shaw Sensor Rating
versus Wind Speed (Including Wind Angle)
Remaining Scatter:
• High Ampacity when Wet
• Solar Input
• Wind Direction Standard Deviation
W.A.Chisholm@ieee.org
Weather Data “Nowcasting” and Predictions:
Ambient Temperature, Humidity
NOAA, http://rapidrefresh.noaa.gov/hrrrconus/
W.A.Chisholm@ieee.org
Weather Data “Nowcasting” and Predictions:
Solar Radiation
NOAA, http://rapidrefresh.noaa.gov/hrrrconus/
W.A.Chisholm@ieee.org
Weather Data “Nowcasting” and Predictions:
Wind at 10 m and 80 m
1 kt (knot) = 0.51 m/s
NOAA, http://rapidrefresh.noaa.gov/hrrrconus/
W.A.Chisholm@ieee.org
Best Weather-Based Line Rating Practices:
Boundary Layer Modeling of Terrain Effects
W.A.Chisholm@ieee.org
Best Weather-Based Line Rating Practices:
Boundary Layer Modeling of Terrain Effects
Example of down-sampling of GFS and WRF Wind Speed Models
W.A.Chisholm@ieee.org
Organization of Seminar (1)
• Heat Balance of Overhead Conductors
• Field Observations of Temperature, Solar Flux, Wind
Speed, Direction and their Standard Deviations
• Change of Tension, Sag and Clearance with
Conductor Temperature
– In single span
– In multiple spans with freely swinging insulators
• Field Observations of Clearance and its Predictors
W.A.Chisholm@ieee.org
Changes in Sag of Single Span
W.A.Chisholm@ieee.org
Conductor Elongation Factors
Length As Manufactured
W.A.Chisholm@ieee.org
Conductor Elongation Factors
Length As Manufactured
Increase in Length from Creep after 1 hour
W.A.Chisholm@ieee.org
Changes in Sag of Single Span
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Conductor Elongation Factors
Length As Manufactured
Increase in Length from Creep after 1 hour
Increase in Length from Creep after ……… 20 years
W.A.Chisholm@ieee.org
Changes in Sag of Single Span
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Conductor Elongation Factors
Length As Manufactured
Increase in Length from Creep after 1 hour
Increase in Length from Creep after ……… 20 years
Reversible Elastic Strain from change in Wind/Ice Loads
W.A.Chisholm@ieee.org
Reversible Change: Elastic Strain
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Conductor Elongation Factors
Length As Manufactured
Increase in Length from Creep after 1 hour
Increase in Length from Creep after ……… 20 years
Reversible Elastic Strain from change in Wind/Ice Loads
Reversible Thermal Strain from change in Temperature
W.A.Chisholm@ieee.org
Reversible Change: Thermal Strain
W.A.Chisholm@ieee.org
Effect of Heat
• Extreme electrical power system load peaks in
summer, thanks to air conditioning
• Temperature rise above ambient powered by
square of current (I2R)
• Aluminum and steel expand when they get hot,
reducing clearances.
• Excessive temperature rise anneals aluminum,
damages splices
W.A.Chisholm@ieee.org
Tension Change versus Temperature for
Different Span Lengths (CIGRE TB324)
W.A.Chisholm@ieee.org
Organization of Seminar (1)
• Heat Balance of Overhead Conductors
• Field Observations of Temperature, Solar Flux, Wind
Speed, Direction and their Standard Deviations
• Change of Tension, Sag and Clearance with
Conductor Temperature
– In single span
– In multiple spans with freely swinging insulators
• Field Observations of Clearance and its Predictors
W.A.Chisholm@ieee.org
Multiple Spans – “Ruling Span” Theory
• Calculate behavior
of “Ruling Span”
End
RS 
 (span)
3
Start
End
 (span)
Start
• Transform result to
other spans
2
Sag SpanN
 SpanN 

 Sag RS
 RS 
W.A.Chisholm@ieee.org
Multiple Spans with Real Insulators
• Perfect tension equalization (infinitely long
insulator strings) is basis of ruling span theory.
• Horizontal component of tension up the
insulator string is small but important.
• The tensions each side of
an insulator are different.
Winkleman AIEE 1959;
Chisholm and Barrett PWRD April 1989;
CIGRE TB 324 p. 23-25.
W.A.Chisholm@ieee.org
Insulator Displacements for
IEEE Ten-Span Test Case (Motlis et al. 1999)
The predicted swing with infinite insulator length is 22° in
the 750’ span.
An insulator with 1.52-m length restrains swing to 8°.
W.A.Chisholm@ieee.org
Insulator Displacements for
IEEE Ten-Span Test Case (Motlis et al. 1999)
Exercise: Where should the insulator swing be
measured to have the best response?
W.A.Chisholm@ieee.org
Insulator Displacements for
IEEE Ten-Span Test Case (Motlis et al. 1999)
Span, m
 Insulator Displacement, RS Model (°)
1.52 m Insulator Displacement, SWING (°)
229
22
8
290
23
10
457
-12
-11
According to ruling span method, spans 6 or 7.
According to correct calculations, spans 7 or 8.
W.A.Chisholm@ieee.org
Multiple Spans – Sag/Temperature Slope
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Multiple Spans – Sag/Temperature Slope
More Sag Change with Temperature
for Short Spans:
15 mm/C° versus 11 mm/C°
W.A.Chisholm@ieee.org
Multiple Spans – Sag/Temperature Slope
Less Sag Change with Temperature
for Long Spans:
42 mm/C° versus 47 mm/C°
W.A.Chisholm@ieee.org
Multiple Spans – Sag/Temperature Slope
Different Sag Changes with
Temperature for Spans of
Same Length in Different Positions
W.A.Chisholm@ieee.org
Influence of Correct Tension Balance Model:
– Estimated using 100°C with Ruling Span
W.A.Chisholm@ieee.org
Organization of Seminar (1)
• Heat Balance of Overhead Conductors
• Field Observations of Temperature, Solar Flux, Wind
Speed, Direction and their Standard Deviations
• Change of Tension, Sag and Clearance with
Conductor Temperature
– In single span
– In multiple spans with freely swinging insulators
• Field Observations of Clearance and its
Predictors
W.A.Chisholm@ieee.org
Typical Clearance Limits for Overhead Lines
• Legal Vehicle Height 4.3 m.
• Added Electrical Buffer set to withstand Switching
Surge Flashover Voltage.
• Examples of Total Clearance Requirement:
– 5.5 m for 115 kV
– 6.1 m for 230 kV
W.A.Chisholm@ieee.org
Options for Thermal Rating:
Automobile Dashboard Analogy
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Clearance Measurement Analogy
W.A.Chisholm@ieee.org
Clearance-Based Thermal Observations
•
•
•
•
•
•
Measure Weather Parameters
Measure Line Current
Perform Heat Balance Calculation
Estimate the Conductor Temperature
Calculate Thermal Expansion of Conductor
Measure the Tension Change from Known State
(recent manual survey)
• Measure Catenary of Reference Span
• Estimate Catenaries of Other Spans
W.A.Chisholm@ieee.org
Echo Level (dB)
Clearance Monitoring with Sonar
Distance (m)
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Clearance Monitoring with Sonar
Echo Level (dB)
Phase Conductors
Phase Conductor Echo
Distance (m)
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Clearance Monitoring with Sonar
Echo Level (dB)
Overhead
Groundwire
Overhead
Groundwire
Echo
Distance (m)
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Sonar Equipment to measure Clearance is
Easily Calibrated from the Ground.
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Layout, Ontario Hydro Monitoring Program
2002
2003
2003/4
Permanent
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Temporary Sites for Clearance and Weather
Looking East
Looking West
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Temporary Sites for Clearance and Weather
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Temporary Sites for Clearance and Weather
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Permanent Installations to monitor
Clearance and Weather
Weather Sensors: Sonic Anemometer,
Temperature, RH
Shaw Sensor (Conductor Replica)
Sonar Clearance Sensors
on Wood Crossarms
W.A.Chisholm@ieee.org
Permanent Installations to monitor
Clearance and Weather
Standalone System on 4” ABS
Pipes with Solar Panels
Sonar Clearance Sensors
on 4” ABS Plastic Pipes
and Line Power
W.A.Chisholm@ieee.org
Permanent Installations to monitor
Clearance and Weather
Empty (new) Distribution
Transformer Case
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Typical Measured Weather Data
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Typical Measured Clearance Data, 2002
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Clearance Measurement Analogy:
Four Parallel Circuits
You
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Typical Measured Clearance Data, 4 Circuits
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Excellent Correlation:
Other Lines on Same Right-of-Way
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Very Good Correlation :
Clearance in Parallel Line, 3-15 km South
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Clearance Measurement Analogy:
Along Right-of-Way
You
2 cars ahead
………
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Clearance Measurement Analogy:
Along Right-of-Way
You
2 cars ahead
8 or 98 cars ahead
………
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Good to Excellent Correlation:
Clearances Along One Line, Especially Within Section
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Tension Measurement Analogy
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Tension-Based Thermal Observation
•
•
•
•
•
•
Measure Weather Parameters
Measure Line Current
Perform Heat Balance Calculation
Estimate the Conductor Temperature
Calculate Thermal Expansion of Conductor
Measure the Tension Change from Known State
(recent manual survey)
• Calculate Catenary of “Local Ruling” Span
• Estimate Catenaries of Other Spans
W.A.Chisholm@ieee.org
Tension-Based Thermal Observations (CEMIG)
Tension (kg)
Clearance (m)
(Nascimento et al)
W.A.Chisholm@ieee.org
Ambient Adjusted Rating Analogy
Estimate Speed Based on Fuel
Consumption, 7 liters/100 km
(equivalent: Line Current)
… adjusted for Temperature
W.A.Chisholm@ieee.org
Weather-Based Thermal Rating
•
•
•
•
•
•
Measure Weather Parameters
Measure Line Current
Perform Heat Balance Calculation
Estimate the Conductor Temperature
Calculate Thermal Expansion of Conductor
Calculate the Tension Change from Known State
(stringing, 30-60 years ago)
• Calculate Catenary of “Ruling” Span
• Estimate Catenaries of Other Spans
W.A.Chisholm@ieee.org
Ambient Temperature Alone: Mediocre
IEEE 738 Thermal Model: Good
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Conductor Temperature: Good Predictor
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Static Rating Analogy:
Line Current
Estimate Speed Based on Fuel
Consumption, 7 liters/100 km
(equivalent: Line Current)
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Square of Line Current: Mediocre Predictor
W.A.Chisholm@ieee.org
Regression Coefficient as “Grade”
• Ranked nine predictors of clearance in a span:
– Other clearances in same line section
– Tension
– Other clearances on adjacent lines within right-of-way
– Other clearances on parallel lines, 3 -15 km away
– Conductor surface temperature, 3 spans away
– Clearance, 32 km away
– IEEE 738 (Ambient, Wind, Solar, Current)
– Ambient
– Line Current
W.A.Chisholm@ieee.org
Ranking Using Linear Regression Coefficient
Predictor of Clearance
Pearson R2
Square of Load Current
Ambient Temperature
IEEE 738
(Ambient, Load Current, Wind Speed, Solar)
Clearance in Another Stringing Section
(98 spans away, different line direction)
Conductor Temperature 3 spans away
0.38-0.53
0.53-0.69
0.65-0.78
Clearance in Parallel Right-of-Way, 3-15 km Away
0.85
Clearance in Adjacent Line on Same Right-of-Way
0.94
Tension in Same Stringing Section
0.94
Clearance in the Same Stringing Section
>0.99
0.69
0.77
W.A.Chisholm@ieee.org
Organization of Seminar (2)
• Ampacity Inversion
– Long-Axis Hot-Wire Method
– Inputs: Clearance, Line Current, Ambient
Temperature, Solar Radiation
– Outputs: Spatially Averaged Conductor Temperature
and Wind Speed
• Implementation by Utility and Results
• Conclusions and Discussion
W.A.Chisholm@ieee.org
Clearance-Based Thermal Rating
• Measure Clearance in Reference Span
• Estimate Clearances of Other Spans
• Estimate Tension Change from Known State (recent
LIDAR survey of line at known temperature)
• Calculate Thermal Expansion of Conductor
• Estimate Conductor Temperature
• Measure Ambient Temperature and Current
• Perform Heat Balance Calculation
• Estimate Average Wind Speed
• Calculate Limited-Time Rating
W.A.Chisholm@ieee.org
Input Data Quality:
Measurement Error < 6 cm
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Input Data Quality: Standard Deviation of
Clearance versus Wind Speed
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Input Data Quality: Agreement of Clearance
Among Three Phases
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Clearance-Based Thermal Rating
• Measure Clearance in Reference Span
• Estimate Clearances of Other Spans
• Estimate Tension Change from Known State (recent
LIDAR survey of line at known temperature)
• Calculate Thermal Expansion of Conductor
• Estimate Conductor Temperature
• Measure Ambient Temperature and Current
• Perform Heat Balance Calculation
• Estimate Average Wind Speed
• Calculate Limited-Time Rating
W.A.Chisholm@ieee.org
5 Years, All Seasons: Stable Ta to Clr1 Relation
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5 Years, All Seasons: Stable Ta to Clr2 Relation
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5 Years, All Seasons: Stable Ta to Clr3 Relation
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Conductor Temperature from Clearance
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Four Conductor Temperature Estimates from
Individual Clearance Readings
Tconductor > Tambient
Non-Physical
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Standard Deviation of Four Temperatures
(obtained from Clearances) versus Ambient
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Standard Deviation of Four Temperatures
(obtained from Clearances) versus Currents
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Standard Deviation of Three Temperatures
(obtained from Clearances) versus dT/dt
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Organization of Seminar (2)
• Ampacity Inversion
– Long-Axis Hot-Wire Method
– Inputs: Clearance, Line Current, Ambient Temperature,
Solar Radiation
– Outputs: Spatially Averaged Conductor Temperature
and Wind Speed
• Implementation by Utility and Results
• Conclusions and Discussion
W.A.Chisholm@ieee.org
Running in Reverse, Item 3:
Wind Speed from Clearance
• With known line currents and conductor
temperatures estimated from clearances, iterate
wind speed until heat balance converges.
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Wind Speed from Clearance, 2002 Trial
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Wind Speed from Clearance, 2003 Trial
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Organization of Seminar (2)
• Ampacity Inversion
– Long-Axis Hot-Wire Method
– Inputs: Clearance, Line Current, Ambient Temperature,
Solar Radiation
– Outputs: Spatially Averaged Conductor Temperature
and Wind Speed
• Implementation by Utility and Results
• Conclusions and Discussion
W.A.Chisholm@ieee.org
Background: Power and Weather
Addition to Base Load (MW), Ontario 2004
The black line shows the
expected effect when the
weather is normal for that
time of year.
Blue lines indicate days when
the weather effect was less
than normal.
• Warmer than normal days
during the winter months
• Cooler than normal days
during the summer
The weather effect takes into account temperature, wind,
lighting, and humidity. The normal effect is determined by
analyzing thirty years of records. Source: IESO
Red lines indicate days when
the weather effect was greater
than normal.
• Below-normal temperatures
in winter
• Above-normal temperatures
in summer
W.A.Chisholm@ieee.org
Background: Why Summer?
Addition to Base Load (MW), Ontario, 2005
W.A.Chisholm@ieee.org
Power Transmission System in Ontario
Queenston (Sir Adam Beck) Generators
W.A.Chisholm@ieee.org
Power Transmission System in Ontario
Queenston Flow West (QFW) Interface
W.A.Chisholm@ieee.org
Background: Why Niagara?
Two new 230-kV circuits were constructed to
supplement five existing parallel 230-kV lines.
These would have increased limits by 800 MW and
enable Sir Adam Beck plants to deliver an additional
1.6 TWh/year. However, a land dispute delays the inservice date.
In the meantime, a significant generation upgrade was
carried out that made the line thermal rating issues
even worse.
W.A.Chisholm@ieee.org
Background: Why Niagara?
The largest hard rock Tunnel
Boring Machine in the world
finished drilling a massive
tunnel deep beneath the City of
Niagara Falls in May 2011.
W.A.Chisholm@ieee.org
Background: Why Niagara?
The new tunnel is 12.7 meters (41 feet) wide and 10.2
kilometers (6.3 miles) long.
W.A.Chisholm@ieee.org
Background: Why Niagara?
In March 2013, the tunnel was filled with water and
now provides additional 500 m3 / s to generate clean,
renewable hydroelectricity at the existing Sir Adam
Beck stations.
The increased output is still carried along the same, five
230-kV circuits forming the QFW interface, that have
been thermally limited since 2002.
The sonar clearance-based dynamic line rating system is
used to manage and maximize the energy transfer, at
times allowing 2400 MW to flow safely rather than
the static 1800 MW rating.
W.A.Chisholm@ieee.org
Organization of Seminar (2)
• Ampacity Inversion
– Long-Axis Hot-Wire Method
– Inputs: Clearance, Line Current, Ambient Temperature,
Solar Radiation
– Outputs: Spatially Averaged Conductor Temperature
and Wind Speed
• Implementation by Utility and Results
• Conclusions and Discussion
W.A.Chisholm@ieee.org
Conclusions:
Heat Balance of Overhead Conductors
• Thermal Models (IEEE 738, CIGRE Brochure 207)
balance I2R (Joule) and sunshine heat inputs with
losses from convection and radiation.
• Convection depends on wind speed at 1-2 m/s.
• Convection depends on wind yaw angle to line.
• Wind direction standard deviation increases
remarkably as wind speed decreases below 2 m/s.
This usually increases the convective cooling.
W.A.Chisholm@ieee.org
Conclusions: Change of Tension, Sag and
Clearance with Conductor Temperature
• A line between strain towers functions as a single
unit, averaging the effects of wind, solar and
temperature over distances of 5-50 km.
• Calculating tension from clearance requires the
same care as calculating clearance from tension.
– No ruling span approximation allowed!
W.A.Chisholm@ieee.org
Conclusions:
Field Results, Predictors of Clearance
• It was practical, inexpensive and reliable to measure
clearance with ±6cm accuracy from ground level with
standard industrial sonar outdoors over a wide
temperature and humidity range.
• It was better to:
– predict clearance using another clearance measurement,
taken 98 spans away on the same circuit,
• than to:
– use the IEEE Standard 738 thermal model with load
current and weather data directly under the conductor.
W.A.Chisholm@ieee.org
Conclusions:
Wind Speed Observations
• Agreement between heated conductor heat
balance (Shaw Sensor) and effective wind speed
from nearby sonic anemometer was rather good.
• Neither measurement of wind was a good
predictor of line clearance with present methods.
W.A.Chisholm@ieee.org
Conclusions: Reverse Rating Process to
obtain Distributed Wind Speed
• Calculating conductor temperature from
clearance is robust and has standard deviation
less than 3C°.
• It is possible to invert the ampacity calculation
process to establish the cross-wind speed V90
averaged over an entire 5-50 km stringing
section.
• The process gives noisy results that differ from
the measured wind speed under the line.
W.A.Chisholm@ieee.org
Conclusions: Utility Implementation
• Clearance-based thermal rating offers strong
advantages, especially when multiple lines
present a combined limitation to power transfer.
– Inexpensive and autonomous installation
– Practical in-service calibration
– Multiple vendors, standard industrial technology
– Directly useful for estimating clearance of other spans
• Distributed wind speed provides a sound basis for
thermal rating when line current exceeds 200 A.
W.A.Chisholm@ieee.org
Open Questions for Further Study
1. Measure component state (line, cable,
transformer etc) in two or more ways.
•
They will disagree.
2. What are good measures of agreement?
•
•
Static case – X tracks Y with scatter
Dynamic case – X lags Y with delay
3. What is the composite effect of:
–
–
thermal time constant of conductor (10 minutes)
mechanical time constant of coupled spans in series
(1 minute travel time end to end, poorly damped)
W.A.Chisholm@ieee.org
Open Questions for Further Study
4. Rank of Other Predictors
–
–
–
–
Ultrasonic wind speed / direction
Shaw (Hot wire) sensor
Sag at 150’ from insulator (Video Sagometer)
Insulator tilt angles (Sagometer, others)
5. Rank of Same Predictors, Other Sites
– Limited comparisons with tension
– Limited comparisons on lines with poorly
correlated load currents
– No comparisons in sheltered terrain
W.A.Chisholm@ieee.org
CIGRE and IEEE References for
Thermal Rating of Lines
• CIGRE Technical Brochure 207 (Thermal behavior of
overhead conductors)
• IEEE Standard 738 (Standard for calculating the current
temperature of bare overhead conductors)
• CIGRE Technical Brochure 299 (Guide for selection of
weather parameters for bare overhead conductor ratings)
• CIGRE Technical Brochure 324 (Sag-Tension calculation
methods for overhead lines)
• CIGRE Technical Brochure 601 (Guide for thermal rating
calculations of overhead lines)
W.A.Chisholm@ieee.org
About the Author
William A. (Bill) Chisholm, F (IEEE), Ph.D., P.Eng, managed the
Hydro-One Niagara to Hamilton Real-Time Thermal Rating
project, starting in 2001 and ending in 2006 with
commissioning of permanent clearance observation sites.
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•
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BASc in Engineering Science, University of Toronto, 1977
M.Eng (part time), University of Toronto, 1979
Ph.D. (part time) in Electrical Engineering, University of
Waterloo, 1983.
38 years of research background in power system lightning
protection, icing and thermal rating
Retired as Principal Engineer at Kinectrics, 2007
Professor, Université du Québec à Chicoutimi, 2007-8
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Wrote book, Insulators for Icing and Polluted Environments
Taught graduate course, Protection contre la foudre
Panelist, Conductors sessions at IEEE-PES meetings
Member, joint IEEE/CIGRE Task Force B2.12, Weather
Parameters for Bare Overhead Conductor Ratings
Bronze medal, 200m fly, 2010 World Masters Championships
W.A.Chisholm@ieee.org