Underground cable monitoring with DTS and RTTR: a measured rating instead of a catalogue value
This article covers how DTS and RTTR replace that assumption with a measurement, what the switch changes in the control system and in grid planning, and why a filing deadline on 31 October 2026 has put a date on the whole question.
Real-Time Thermal Rating (RTTR) is the continuous determination of what an underground cable may carry, derived from measured temperatures rather than from a design calculation. The measurement comes from Distributed Temperature Sensing through an optical fibre in the cable route: up to 70 kilometres per run, 50 centimetres to 2 metres of spatial resolution, roughly 1 Kelvin accurate, with a result every 15 minutes. The people affected are asset management and grid planning at distribution operators on the high-voltage level. The date comes from German law: the first grid expansion plan under section 14d of the Energy Industry Act is due with the regulator on 31 October 2026, and it calls for a congestion analysis of existing line sections together with the measures planned for them, ordered as optimisation, reinforcement, expansion. File a calculated catalogue value there and you have documented an assumption.
What the grid expansion plan asks for by 31 October 2026
German distribution system operators file their grid expansion plan under section 14d of the Energy Industry Act for the first time on 31 October 2026, then every two years on the same date. The plan is not a wish list for new circuits. It is an account of the grid that already exists.
It applies to operators with more than 100,000 customers and to operators curtailing a relevant share of wind and solar. Its predecessor, the 2024 round, came from 82 large distribution operators and covered voltage levels 3 to 6 with reference years 2028, 2033 and 2045. The basis is the set of six regional scenarios the operators published on VNBdigital on 28 January 2026.
One required item is worth dwelling on: the congestion analysis of existing line sections, together with the measures planned for them. Optimisation, reinforcement, expansion, in that order. Plan a reinforcement and you have to explain why optimisation will not do.
That is where a measurement question turns into a planning question. The statement "this section is overloaded from 2031" goes into the plan. Where the loading limit came from usually does not.
Why a cable's catalogue rating is almost never right
The permissible current of a high-voltage underground cable comes from a calculation under IEC 60287. That calculation fixes soil temperature, soil thermal resistivity, laying depth and the loading of neighbouring circuits, and it picks each of them on the pessimistic side. The result is a number that holds all year.
Except the soil moves. Thermal resistivity tracks moisture, a dry August makes it markedly worse and a wet spring makes it better. What matters is also not the route as a whole but its hottest point: a crossing with a district heating pipe, a duct block under a road, a stretch laid too shallow. A cable is as strong as its worst metre.
The cyclic rating factors in IEC 60853 are regarded in the literature as conservative. Depending on conditions, cables carry more than the catalogue allows. So the number is blurred in both directions. In winter it leaves capacity unused. During a heatwave, at the wrong spot, it overstates what is there.
The catalogue rating is not a property of the cable. It is the output of assumptions about the ground, and nobody knows those precisely until somebody measures.
How DTS and RTTR measure the rating
One run reaches roughly 70 kilometres. Spatial resolution sits between 50 centimetres and 2 metres, accuracy around 1 Kelvin. Instead of isolated measuring points the operator gets a profile across the whole length, which means finding the hot metre rather than guessing at it.
RTTR is the calculation layered on that profile. It infers conductor temperature from the measured sheath, then the current the section can still take. The thermal models rest on IEC 60287, IEC 60853 and CIGRE TB 640, the same standards used for the design. What differs is the input: measured rather than assumed.
Results usually reach the operator every 15 minutes. Present conductor temperature, present rating, plus a forecast of how long an overload could be tolerated. Fault location comes as a by-product, because the system shows the hot spot to the metre. Anyone who knows medium-voltage cable diagnostics will spot the difference at once: that answers how healthy the insulation is, this answers how much current fits today.
| Aspect | Static design rating | RTTR with DTS |
|---|---|---|
| Inputs | assumed soil parameters, a design load case | measured temperature profile, actual load |
| Location | one value for the whole section | a profile along the length, hot spot named |
| Currency | set once, stands for the asset's life | recalculated roughly every 15 minutes |
| Value during a fault | none, the route gets searched | the heating point located to the metre |
| Prerequisite | laying records | an optical fibre in the route |
From a measured value to the congestion analysis
A measured value is worth nothing while it lives in the vendor's portal. The benefit shows up in two places inside the utility, and neither of them is the measuring kit.
In the control system, a time-varying limit replaces a fixed one. That sounds like a number. It is really a change to limit logic, alarming and acknowledgement. Whoever is on shift has to know which value applies when something goes wrong.
In grid planning, a measured annual time series replaces the assumption. "This section is overloaded from 2031" becomes a statement with measurement history behind it. For the congestion analysis that does not necessarily drop the section from the plan. It moves the measure: from reinforcement to optimisation, and with it the spend to a later year. AI-assisted grid simulation in congestion analysis works on the same problem from the calculation side rather than the measurement side.
This pays off where it hurts. When extending an HV/MV substation runs into a space problem and needs more than five years of lead time, time bought on the cable is worth more than amps gained.
What the plans have shown so far
The review of the 2024 plans makes uncomfortable reading. Optimisation measures barely appear. Peak shaving shows up here and there, flexibility not at all. Operators put that down to uncertainty and missing regulatory guidance.
The framework is there, though. Section 11(2) of the Energy Industry Act lets planners assume that up to 3 percent of forecast annual output from onshore wind and solar is shaved, and VDE FNN has set out the reporting and documentation duties that go with it. The numbers on the other side of the ledger are equally well known: more than EUR 55 billion of investment to 2028 in the 2024 plans, around EUR 207 billion to 2045 in the more recent SMARD assessment, and 38,600 kilometres of high voltage, 218,400 of medium and 245,300 of low voltage to build or reinforce.
An evidenced optimisation measure in the plan is therefore more than a technical footnote. It is the only argument against a spend that otherwise sails through.
The question in October is not whether a section gets tight. It is how you know.
innobuWhere it gets awkward
A measured rating is not a licence. Making the limit move shifts risk out of the design calculation and into operations, and somebody there has to carry it.
Retrofitting is the expensive case. Without a fibre in the route there is no RTTR, and nobody digs up a cable to measure it.
A moving limit is also only as good as the model under it. Wrong laying parameters or an unrecorded crossing produce a rating that is too high, and that only surfaces when something fails. More sustained load means a hotter conductor means faster ageing of the insulation: part of the capacity gain is life brought forward.
Operationally you need a rule for who releases the measured value and which value applies in the n-1 case. Without it the control room keeps using the old fixed number when in doubt. Then the investment bought a display.
One note on the figures circulating in sales decks. Review papers on dynamic thermal rating report gains of up to about 45 percent, heavily dependent on climate, laying method and load profile. That is no basis for planning.
What grid operators should do now
The coming weeks are not about choosing a measuring system. They decide whether the plan filed in October carries an assumption or a measurement.
Five steps before the end of October
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Pull the candidates out of the congestion analysis
You are looking for sections listed for reinforcement whose bottleneck rests on a calculated catalogue value rather than on measured loading. That is a few days of list work, and it sets the order for everything after it.
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Check where a fibre already runs
On high-voltage cables laid in recent years it is often built into the sheath and sitting unused.
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Make the sensing fibre a standing requirement
For every new laying and every renewal, whether or not anyone measures today. The fibre costs little during laying and a great deal afterwards.
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Write the operating rule before the first instrument
Who releases the measured value, which value applies in the n-1 case, how the release is documented? Those three sentences decide whether the control room ends up using the number at all. Without them the system stays a display.
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Put the interfaces in the specification
Control system and grid planning, both of them. A vendor portal without an interface creates one more island, and every utility has enough of those.
Write the sensing fibre into the laying standard today and the plan after next rests on data rather than assumption. Until then the congestion analysis carries whatever the records say.
Further Reading
Frequently Asked Questions
DTS (Distributed Temperature Sensing) turns an optical fibre into one continuous temperature sensor. A laser pulse travels down the fibre, the backscattered light carries the temperature of the point where it scattered, and the travel time says where that point is. A single measuring run covers up to roughly 70 kilometres, spatial resolution sits between 50 centimetres and 2 metres, and accuracy is around 1 Kelvin.
DTS delivers the measured temperature profile along the route. RTTR (Real-Time Thermal Rating) is the calculation on top of it: it infers conductor temperature from the measured sheath, then the current the circuit may carry right now. The thermal models rest on IEC 60287, IEC 60853 and CIGRE TB 640. Results usually reach the operator every 15 minutes.
German distribution system operators file their grid expansion plan under section 14d of the Energy Industry Act for the first time by 31 October 2026, then every two years on the same date. The plan includes a congestion analysis of existing line sections with the measures planned for them, the flexibility required, and the extent of planned peak shaving under section 11(2).
Only where a fibre already runs along the route. On high-voltage cables laid in recent years it is often built into the sheath and sitting unused, which makes the job small. Without a fibre there is no RTTR: nobody digs up a working circuit to measure its temperature. That is why the sensing fibre belongs in every new laying and every renewal, even when nobody plans to measure yet.
It depends on the case. Review papers on dynamic thermal rating report gains of up to about 45 percent in favourable conditions, heavily dependent on climate, laying method and load profile. Numbers like that are no basis for planning. The dependable benefit runs the other way: the measured value shows whether the assumed bottleneck is one at all, and moves the measure from reinforcement to optimisation.