Ratings & Thermal Headroom
Where RenewMap's normal and emergency ratings for transmission equipment come from, and how thermal headroom is calculated and interpreted.
Transmission lines are, at their core, very long pieces of metal. Current flowing through a wire heats it up. A hot conductor sags, loses tensile strength and, past a point, is permanently damaged or comes too close to whatever is underneath it. So each piece of transmission equipment carries a rating: the maximum loading it can be operated at safely under prevailing conditions.
The same conductor can carry substantially more current on a cold, windy night than on a still 40-degree afternoon, because both temperature and wind speed change how fast it sheds heat. Network operators calculate ratings dynamically and feed them into their operational systems, which is why you will see that rating can change through the day and across the seasons.
Thermal headroom is a measure of how much more flow a piece of equipment could carry at any point in time. This tells you what the metal could technically do, but there are other reasons power can’t flow and the market operator will enforce additional rules on lines (see Constrained Headroom). Many lines don’t have explicit restrictions placed on them (yet) so no data exists on what the market operator would let happen, so thermal headroom is particularly useful in those circumstances.
The different types of ratings
Normal. The continuous rating: the loading the equipment can carry indefinitely under current conditions. This is the limit the system is planned and operated to in normal conditions.
Emergency. A higher, time-limited rating that can be tolerated for a short window, after a contingency such as another line tripping. The power system is operated so that a single credible failure won’t cause anything to exceed its emergency rating, and so that operators can bring loading back below the normal rating within the allowed time. Emergency ratings exist because the alternative of planning the entire network to survive every contingency within continuous ratings would be more expensive.
Many points publish both a normal and an emergency rating; some publish only one.
Where you see it in RenewMap
| Where | The Line flow chart overlays the normal and emergency ratings as reference lines; the Headroom chart plots thermal headroom; the Headroom and Headroom % map visualisations colour lines by available capacity |
| Granularity | 30 minutes, per measurement point, combined to the transmission line |
| Units | Usually MVA (average over the half hour). Ratings are generally expressed as apparent power |
| Series names | rating_norm, rating_emer at the measurement point; line_rating_norm, line_rating_emer, line_headroom_thermal, line_headroom_thermal_pct at the line |
| Applies to | Transmission equipment with published rating telemetry, see Data sources & coverage |
| Blank when | That rating type isn’t reported for the equipment, or wasn’t reported for the interval |
How it’s calculated
Ratings
Ratings are read straight from AEMO’s published telemetry for each 5-minute period and averaged to 30 minutes. They are the network operators’ own dynamic operational ratings; we don’t derive, model or substitute a design figure for them.
Where a line has several measurement points, the line-level rating is the smallest of them, since a line can only be operated to its most restrictive limit.
Thermal headroom
Thermal headroom is the arithmetic gap between the two:
Thermal headroom = rating - flow
Thermal headroom (%) = thermal headroom / rating × 100At the line level it pairs the most restrictive rating with the heaviest loading measured on the line (the absolute flow defined in Line flow) so the result is deliberately the conservative reading rather than an average across measurement points.
Two fallbacks apply. Where a measurement point doesn’t report a normal rating, its emergency rating is used in its place, since some points only publish one of the two. And where MVA flow isn’t reported, MW flow is used instead. Both fallbacks are noted here because they change what the number means: a headroom figure built from an emergency rating describes short-term capacity, not continuous capacity. A headroom calculated with MW rather than MVA will overstate room because it will be excluding reactive power.
Special notes
Headroom is not connectable capacity. Connection capacity is determined by detailed modelling that considers things like system strength, voltage, other upcoming connections, downstream or upstream congestion. A line may have plenty of room, but there is a congested line that the flow must pass through one or two hops along the network. Thermal headroom is just a useful screening signal for where the network is tight and where it isn’t.
Where the data comes from
Ratings are AEMO’s published operational values, from the same source as the flow telemetry described in Line flow, and they cover the same set of equipment. See Data sources & coverage.
Common questions
Why does the rating change through the day?
Because it’s a real, dynamic operational rating that responds to ambient conditions and equipment configuration, not a static design figure. Colder and windier means more capacity.
Can I treat thermal headroom as available capacity for a new project?
No, see Special notes. Use it to identify where the network is congested and where it isn’t, then look at Constrained headroom and Marginal value & Binding Hours for whether the market is actually enforcing a tighter limit. Real connection capacity requires a full network study.
Does headroom account for the other side of a contingency?
No. Thermal headroom is a measured, intact-system arithmetic difference. Contingency effects, the whole point of the constraint equations AEMO enforces, arrive via Constrained headroom.
Related pages
- Line flow: the measured loading that headroom subtracts
- Constrained headroom: headroom corrected for the limits the market enforces
- Marginal value & Binding Hours: what those limits cost
- Data sources & coverage: which equipment is covered, and why