Thermal rating (.ohlta)#

OpenOHL computes the current a conductor can carry (its ampacity) for a maximum conductor temperature, the conductor temperature for a current, the temperature after a step of current, and — for a whole line — the highest conductor temperature its clearances allow, by IEEE Std 738-2012, CIGRE TB 601 or the older CIGRE TB 207. The formulas are described in the theory manual.

What the conductor needs#

The cable's [cable.electrical] data in the wire library (Wire library files): at least one AC resistance (two or more at different temperatures — linear between and beyond them), and preferably the emissivity and solar absorptivity (0.8 each when neither the cable nor the ambient case gives them). The diameter comes from the cable; the heat capacity for transients is estimated from its components (aluminium outer wires, steel core) and its mass.

[cable.electrical]
ac_resistance = [
    { temperature = "25 °C", resistance = "0.07283 Ω/km" },
    { temperature = "75 °C", resistance = "0.08688 Ω/km" },
]
emissivity = 0.8
solar_absorptivity = 0.8

The shipped example acsr-drake-26-7 has such data (the IEEE 738 example's values; typical, not a manufacturer's).

Ambient cases (.ohlta)#

The weather around the conductor, one [[case]] each, with units in the values (examples/synthetic-line/thermal-ambient.ohlta):

format = "ohl-thermal-ambient"
version = 1
name = "Summer"

[[case]]
name = "summer noon"
description = "hot, sunny, light wind across the line"
air_temperature = "35 °C"
wind_speed = "0.6 m/s"
wind_angle = "90 °"       # between wind and conductor: 90 ° across (default);
                          # any angle, folded to 0–90° (180 ° = along the line)
elevation = "500 m"       # of the conductor above sea level (default 0)
latitude = "47 °"         # north positive
date = "2023-07-15"       # or day_of_year = 196
time = "13:00"            # local solar time
line_azimuth = "90 °"     # from north; leave out for the sun across the line
atmosphere = "clear"      # or "industrial"

[[case]]
name = "winter night"
air_temperature = "5 °C"
wind_speed = "1 m/s"
solar_flux = "0 W/m²"     # instead of the sun's position: irradiance on the conductor

A case gives either solar_flux or the position, never both; unknown keys are errors (a typo never goes unnoticed).

Command line#

Ampacity at a maximum temperature, per ambient case:

ohl thermal ampacity --cable acsr-drake-26-7 \
    --ambient examples/synthetic-line/thermal-ambient.ohlta --max-temp 80

Drake 795 kcmil 26/7 ACSR — IEEE 738-2012, 3 ambient cases
  summer noon                   840 A at 80 °C
    Joule 62.28, solar 23.18 (sun altitude 61.8°, azimuth 210.6°, 76.1° to the line), convection 59.29 (forced), radiation 26.16 W/m
  …

• --current 900 instead of --max-temp: the steady-state temperature;
• --current 1000 --from-current 500: the temperature after switching from 500 A (steady) to 1000 A, every --print-every s (300) for --duration s (3600), in --time-step s steps (10, or the default values'); --defaults FILE.ohldef (Criteria › Default values exported): the emissivity, absorptivity, roughness, albedo and time step where the cable or the case gives none; --heat-capacity J/(m·K) instead of the estimate;
• --method cigre601 or cigre207 (default ieee738); --case NAME (wildcards, repeatable) for some cases; --csv FILE for the results (the transient: every step); --library for another wire library.

Highest temperature the clearances allow#

cd examples/synthetic-line
ohl thermal clearance-limit synthetic-line.ohl.json --terrain synthetic-survey.csv \
    --clearance 5 --weather weather-cases.ohlwc --case "max temperature" \
    --ambient thermal-ambient.ohlta --method cigre601 --csv limits.csv

clearance-limited temperature of 3 sections (0–150 °C, ±0.5 °C; base case "max temperature"; 1.8s):
  S1           acsr-drake-26-7      above 150 °C
      summer noon              at least 1424 A (CIGRE TB 601)
  …
  S2           acsr-drake-26-7      69.1 °C            ground span 2 vertical 4.99 m < 5.00 m at 69.4 °C
      summer noon                  655 A (CIGRE TB 601)

Each section is solved at other temperatures in the base weather case (--weather and --case; still air without them) and checked by the ground and point rules of --rules (and --clearance as a vertical ground rule) that apply to that case, against --terrain, --lidar or --pointcloud — the same options as ohl line. The result per section:

• the limiting temperature (to --tolerance, 0.5 °C) with the governing span, rule and clearance just above it;
• above the range (--max-temp, 150 °C): nothing fails — the ampacity shown is at the top of the range, a lower bound;
• below the range (--min-temp, 0 °C): the section misses a clearance even then;

and the ampacity at the limit for every ambient case (--ambient, --ambient-case). --step (10 °C) sets the scan before the bisection. The CSV has one row per section: limit, status, governing rule, span, measure, clearance, requirement, then one ampacity column per ambient case.

Wire-to-wire rules are not part of this check. A long line with LiDAR takes up to about a second per section and limit.

Desktop application#

Checks › Thermal rating… opens a window:

• Ambient cases — the project's .ohlta file (next to a line file) is taken at once; Choose… for another; each case in a few words below.
• Method and maximum temperature — the table shows at once the ampacity of every section in every ambient case (hover a value for the heat terms).
• Weather case heated and the temperature range — then Clearance-limited temperature runs the search in the background with a progress bar (Cancel stops it), using the rule file chosen under Checks › Wire clearances and the ground and LiDAR points of the checks. The table gains the limit per section, the governing span and the ampacity at the limit per ambient case.

Nothing in the model changes.