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Neara supports thermal ratings calculations using the IEEE method. Thermal ratings are dependant on the cooling and heating effects on a conductor. The main sources of heating are the solar radiation and the line current and the main sources of cooling are from radiative cooling and convective cooling. The weather conditions that are used in line ratings are ambient temperature, solar radiation, wind speed and wind angle.
IEEE Thermal Rating Functions
Neara has implemented IEEE Std 738™-2023 for thermal calculations. Note that the IEEE method of thermal ratings uses a linear interpolation between two Rac values (as it does in other competitors software). The following four functions can be used to calculate line ratings.
Function Name | Purpose |
| Calculate steady state line rating |
| Calculate steady state conductor temperature |
| Calculate short term line rating |
| Calculates solar radiation given location, date/time, line angle etc |
Note in addition to the library parameters the elevation of the span will be used in the function calculate_irradiance_ieee to calculate the solar radiation.
Function Parameters
The four IEEE functions use many of the same input parameters which are explained in the table below. Note all these examples assume that the rating formula has been add to a span report.
Parameter | Description | Example |
| This is a span object. | The easiest way to set this up is to create a spans report and then enter the value
span: self |
| This is conductor current per phase. For example a bundle with 2 conductors per phase set to 1000A will have 500A per conductor. | current: unit_value(500, "A") |
| This is the time for the short term rating calculations | Time in minutes rating_time: unit_value(5,"min")
Time in seconds rating_time: |
| This is the initial conductor temperature used for short term rating calculations | SI units initial_temperature: unit_value(50,"celsius")
US units initial_temperature: unit_value(120, "fahrenheit") |
| This is the target conductor temperature (maximum conductor temperature). Typically values might be 85oC, 100oC, 120oC. Note that if the parameter | SI units target_temperature: unit_value(85, "celsius")
US units target_temperature:
Max Operating Temperature of the section target_temperature: section.max_operating_temperature
|
| This is the ambient temperature for the calculation. | SI units ambient_temperature: unit_value(35, "celsius")
US units ambient_temperature: unit_value(95, "fahrenheit") |
| This is the amount of solar radiation. It can either be entered as a fixed value or calculated based on location, time of day, day of year and line angle | Fixed value irradiance: unit_value(1000, "W/m^2")
Calculated value irradiance: calculate_irradiance_ieee( |
| This is the wind speed for the calculation. | SI units wind_speed: unit_value(0.5, "m/s")
US units wind_speed: unit_value(1.65, "ft/s") |
| This is an optional parameter. If it is not supplied the wind is assumed to be perpendicular to each span which provides maximum cooling. | Bearing angle blowing from South to North (1) wind_direction: unit_value(0, "bearing")
|
| The amount of solar radiation that is absorbed by the conductor as a value 0-1. Note each conductor will have a default value in the library. This parameter has been provided since absorptivity is typically dependant on the age of the conductor (new conductors are more shiny so will reflect more heat than older conductors). | absorptivity_override: 0.8 |
| The amount of heat radiation out from the conductor as a value 0-1. Note each conductor will have a default value in the library. This parameter has been provided since emissivity is typically dependant on the age of the conductor (new conductors emit less of the heat than a older conductor). | emissivity_override: 0.8 |
| By default this is set to false. If set to true the difference in temperature between the core and the surface is calculated using the Radial Thermal Conductivity. If this is set to true then | use_radial_gradients: true |
| This is used when calculating the irradiance (solar radiation). | timestamp: parse_timestamp("yyyy-MM-ddTHH:mm:ssZ","2025-04-09T11:15:00+1000")
|
| This is the air quality used when calculating the irradiance (solar radiation). It can either be | air_quality: "clear" |
(1) bearing angles are measured from North in clockwise direction so an angle of 0 would indicate the wind is blowing from south to north and an angle of 90o would blow from west to east.
Library Parameters
The following conductor library parameters are used by the IEEE functions Absorptivity, Emissivity, AC Resistance 1, Reference Temperature 1, AC Resistance 2, Reference Temperature 2, Linear Heat Capacity, Radial Thermal Conductivity.
The Absorptivity and Emissivity can be overwritten in the functions which can allow the simulation to change the conductor from new to old (these values are lower for newer conductors and higher for older conductors).
The Radial Thermal Conductivity will only be used if parameter use_radial_gradients is set to true in any of the IEEE functions.
Note the library parameters: Unit Weight and Heat Capacity Coefficient are not used by any of the IEEE thermal functions. The Unit Weight is not used because the Linear Heat Capacity already takes into account the mass (heat capacity = mass x specific heat) and the Heat Capacity Coefficient is not used in the IEEE method. These parameters have been included in the conductor library still for future use when other thermal ratings standards are implemented.
calculate_steady_state_line_rating_ieee
This function will calculate the line rating (per phase) for a specific target temperature given a set of weather conditions. Note this function does not require line current as an input since it is solving what current will lead to the conductor reaching the specified target temperature during steady state conditions (equilibrium).
The parameters used by this function are: span, target_temperature, ambient_temperature, irradiance, wind_speed, wind_direction, absoptivity_override, emissivity_override and use_radial_gradients. See section on Function Parameters for explanation of each of these parameters.
The conductor library parameters used by this function are: Absorptivity, Emissivity, AC Resistance 1, Reference Temperature 1, AC Resistance 2, Reference Temperature 2 and Radial Thermal Conductivity.
Examples
All of the following examples can be added as a custom field to a spans report.
Example 1: Use the maximum conductor temperature specified on the strain section, ambient temperature of 30oC, solar radiation of 1000W/m^2, wind speed of 0.5m/s and a bearing angle of 45o (blowing from SW to NE)
calculate_steady_state_line_rating_ieee(
self,
target_temperature: section.max_operating_temperature ,
ambient_temperature: unit_value(30,"celsius"),
irradiance: unit_value(1000, "W/m^2"),
wind_speed: unit_value(0.5, "m/s"),
wind_direction: unit_value(45, "bearing"))
Example 2: Calculate the rating for a target temperature of 185oF, an ambient temperature of 85oF, calculate the solar radiation based on the location and date/time and line direction, wind speed of 1.65ft/s and assume the wind is perpendicular to the conductor (by not specifying a wind_direction) and override the conductor library values of absorptivity to 0.4 and emissivity to 0.3 (which would indicate a newer conductor).
calculate_steady_state_line_rating_ieee(
self,
target_temperature: unit_value(185,"fahrenheit"),
ambient_temperature: unit_value(85,"fahrenheit"),
irradiance: calculate_irradiance_ieee(
self,
timestamp: parse_timestamp("yyyy-MM-ddTHH:mm:ssZ",
"2025-04-09T11:15:00+1000"),
air_quality: "clear"),
wind_speed: unit_value(1.65, "ft/s"),
wind_direction: unit_value(45, "bearing"),
absorptivity_override: 0.4,
emissivity_override: 0.3)
calculate_steady_state_line_temperature_ieee
This function will calculate the individual conductor temperature assuming the line current and weather conditions remain constant. Note this function does not take into account how long or quickly the conductor will heat up as it is a time infinite calculation and hence there is no starting conductor temperature required. The current to be entered into this function is the overall phase current (the function will divide this by the number of conductors per phase).
The parameters used by this function are: span, current, ambient_temperature, irradiance, wind_speed, wind_direction, absoptivity_override, emissivity_override and use_radial_gradients. See section on Function Parameters for explanation of each of these parameters.
The conductor library parameters used by this function are: Absorptivity, Emissivity, AC Resistance 1, Reference Temperature 1, AC Resistance 2, Reference Temperature 2 and Radial Thermal Conductivity.
Examples
All of the following examples can be added as a custom field to a spans report.
Example 1: Calculating the steady state conductor temperature for a phase current of 500A, ambient temperature of 40oC, solar radiation of 1000W/m^2, wind speed 0.5m/s and wind angle perpendicular to the conductor (given the wind_direction parameter was not entered)
calculate_steady_state_line_temperature_ieee(
self,
current: unit_value(500,"A"),
ambient_temperature: unit_value(40,"celsius"),
irradiance: unit_value(1000,"W/m^2"),
wind_speed: unit_value(0.5, "m/s"))
calculate_short_term_line_rating_ieee
This function will calculate the short term rating given the specified starting conductor temperature.
The parameters used by this function are: span, rating_time, initial_temperature, target_temperature, ambient_temperature, irradiance, wind_speed, wind_direction, absoptivity_override, emissivity_override and use_radial_gradients. See section on Function Parameters for explanation of each of these parameters.
The conductor library parameters used by this function are: Absorptivity, Emissivity, AC Resistance 1, Reference Temperature 1, AC Resistance 2, Reference Temperature 2, Radial Thermal Conductivity and Linear Heat Capacity.
Examples
Below is an example of how to use this formula to calculate a short term 5 minute rating of a conductor starting at 50oC and rising to 100oC at the 5 minute mark.
calculate_short_term_line_rating_ieee(self,
rating_time: unit_value(5, "min"),
initial_temperature: unit_value(50, "celsius"),
target_temperature: unit_value(100,"celsius"),
ambient_temperature: unit_value(40,"celsius"),
irradiance: unit_value(1000,"W/m^2"),
wind_speed: unit_value(0.5, "m/s"))
calculate_irradiance_ieee
There are two ways to enter the solar radiation into the thermal rating functions. Either a fixed solar radiation value can be used or the solar radiation can be calculated used the location, day of year, time of day, line angle, elevation and sky clearness to calculate the solar radiation acting on that conductor.
Using the location, day of year and time the function calculates the suns position in the sky and using the line angle calculates the amount of solar radiation directed at the conductor. Note the elevation above sea level of the span is used in this calculation. The higher the elevation the higher the solar radiation.
Examples
Below is an example of how to use this formula to calculate the solar radiation using the span report.
calculate_irradiance_ieee(self,
timestamp: parse_timestamp("yyyy-MM-ddTHH:mm:ssZ","2025-04-09T11:15:00+1000"),
air_quality: "clear")
Note Z in the parse_timestamp formula specifies the timezone. As an example for a time in Australian Eastern Standard Time (not daylight savings) the time zone should be +1000 (as per the example above).
Thermal Ratings Theory
Steady State Line Rating
The steady state line rating is solving the question given the weather conditions remain the same, how much line current does it require to raise the conductor temperature to is maximum value at equlibrium (ie it will hold at the maximum value indefinitely if the weather conditions remain the same).
The answer to this question is that for equibrium to be reached the heat going into the system must equal the heat going out of the system. Hence the equation
Heat in = Heat out
The heat in comes from 2 sources solar and line current and the heat out comes from radiation cooling and convection cooling. Hence the equation becomes
Solar Radiation + heating from current = radiation cooling + convection cooling
Most of these terms are dependent on the conductor temperature. As the conductor temperature increases the resistance also increases. Likewise the larger the difference between ambient temperature and conductor temperature will increase the radiation cooling.