The application of dynamic line rating requires knowledge about maximal allowable temperature of
OHL conductor, which is also proportional to sag. Since the latter one have relatively small time
constant, the maximal temperature can be reached quickly (approximately 15 min). Hence DLR
requires weather forecasts in order to allocate the possible additional capacity to the market in
system operator processes (IDCF intra day congestion forecast, DACF day ahead congestion forecast,
WAPP weak ahead). Ampacity is defined as the maximum current, in amperes, that a conductor can
carry continuously under the conditions of use without exceeding its temperature rating.
In general, there are two ways to acquire the sag or temperature of OHL conductors:
Direct way:
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Conductor temperature is measured with the help of temperature sensors.
Calculation of sag through measurement of tension.
Calculation of sag based on vibration frequency of conductors.
Calculation of sag based on angle of the line at the span point.
Indirect way:
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Calculation of the conductor temperature based on locally measured weather conditions in
thermal hot spots of OHL line.
Calculation of the conductor temperature based on regionally (faraway of OHL) measured
weather conditions in thermal hot spots of OHL line.
Direct measurement of sag by optical or laser devices.
The main difference between those methods is the need for de-energising of line for sensor
installation and maintenance in case of direct measurement.
Dynamic Line Rating (DLR) - ENTSO-E
The use of actual time-varying capacities for thermally constrained assets is generally named as
dynamic thermal rating which are specifically named as Dynamic Line Rating (DLR) when lines are
considered as the case.
The thermal dynamics-based limits are more related to shorter lines (shorter than 100 km) when
longer lines are restricted dominantly by voltage and stability limits. The OHLs have also further
limitations such as sag and clearance safety limits compared to ULs regarding the thermal concerns.
Therefore, the thermal limits are the one of the major drivers for the effective utilization of the
existing line capacities.
Specifically for OHLs, the time varying nature of ambient conditions has vital impacts on the actual
ampacity of the conductor. Therefore, exploiting from the actual ampacity level of a conductor by
DLR implementation firstly relies heavily on effective monitoring the relevant magnitudes on the
site. The monitored magnitudes to estimate the time-varying conductor temperature in order to
further decide the DLR of the line reveal the basis of the categorization regarding the
implementation of DLR.
There are several approaches of measuring a lines dynamic rating:
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The ambient states-based approaches
The thermal states-based approaches
The mechanical states-based approaches
The mechanical states based approaches are mainly based on sag, clearance or tension
monitoring for the line as depicted in Figure 1. In these methods, the conductor temperature is
estimated through mechanical models defining the relationship between the conductor
temperature and sag, clearance or tension values where the increase in conductor temperature
has a directly proportional relationship with the sag and an inversely proportional relationship
with the clearance and tension. There is also a direct relation between tension and
sag/clearance depending also on the span length and unit length weight of the conductor. A
further vibration monitoring technique that can be considered in this class of approaches is also
available even this technique is not widely used. The most important advantage of the
mechanical states based approaches is that they give an estimation of the thermal status along
the whole span rather than a local data gathered from direct temperature monitoring oriented
thermal states based approaches.
A Comprehensive Overview of Dynamic Line Rating Combined with Other Flexibility Options from an
Operational Point of View - energies-13-06563-v3.pdf
The point and distributed measurements that form a “direct monitoring system” in CIGRE Brochure
299 [2] include:
• sag measurement or equivalent (clearance, inclination, tension) with a resolution of 20 mm or
equivalent 1 C conductor temperature change;
• combined error in environmental variation of 0.5% and less than 1% of the full-scale sag
measurement;
• accuracy of a net radiation temperature estimate of 1 C or 10% error in measuring solar radiation;
• maximum error of current measurement of 2%.
Sag monitoring systems:
A. Sag Derived From Travel Time The “sag stopwatch” is an instrument especially designed to assist
in transmission line sagging using the return wave method [22], [23]. A blow to the conductor near
the conductor clamp is initiated. This establishes a mechanical travelling wave that travels to the
adjacent insulator clamp and back. The return time of the reflected wave is easily measured by
feeling the conductor. Depending on the length of the span and conductor size, somewhere
between three and fifteen return waves can be accurately counted and timed. In theory, the transit
time of a mechanical impact from one end of a stringing section to a known monitoring location,
some spans or miles away, can be used to estimate the tensions, sags and clearances along the line.
Impact duration of 5 ms and time-synchronized measurement accuracy of 0.15% would be needed
to achieve 0.3% accuracy for tension. The Ampacimon and OTLM include sensitive accelerometers
that would allow some development testing of this active approach.
C. Sag Derived From Vibration Frequency: Ampacimon The Ampacimon [3], [12], [13] uses four, twoaxis accelerometers to establish the fundamental and harmonic vibration frequencies in a monitored
span. These frequencies can be measured precisely and have a fundamental inter-relationship to
span travel time and sag. With the use of a sag-tension program as well as input knowledge of the
line profile, it is feasible to estimate the conductor temperature as well as the tensions, sags and
clearances of other spans within a stringing section. The Ampacimon is thus a member of the
“tension or sag” monitors defined in CIGRE Brochure 299 [2]. Several different harmonic frequencies
of a span are excited by natural wind. The Ampacimon measures each of these Aeolian vibration
frequencies with precision of 0.002 to 0.008 Hz. Internal processing combines the multiple readings
to estimate the fundamental span frequency with a precision of 0.3%. This gives a corresponding
precision of 0.6% in the estimate of sags of any magnitude.
IEEE Xplore Full-Text PDF:
Explains why there is a current limit for OHL as well as DLR and RTLR.
To define DLR, it is necessary to understand why a current limit is designed for overhead electrical
lines. In order to maintain a minimum clearance distance between overhead electric lines and items
located under the lines, the line’s core temperature must remain lower than a defined value. Due to
the influence of the Joules effect on that temperature, a maximum current must be defined in order
to respect this constraint. Traditionally, this limit is set as static, and fixed for a given period, such as
a season: this is known as Static Line Rating (SLR). The limit can also be set using DLR, in which case,
unlike SLR, it is dynamically modified at each time step in accordance with the weather
characteristics and so the instantaneous value of the line rating, the Real-Time Line Rating (RTLR).
Note that most studies do not differentiate between RTLR and DLR, for this reason, it will be stressed
here and elsewhere the nature and importance of this difference in order to facilitate the
understanding of this paper. RTLR is the actual value of the ampacity of the circuit, it is an implicit
value, and it cannot be measured but only calculated in real time or after the event. DLR is the value
of rating that the network operator (DSO or TSO) decides to apply to the circuit before the event.
Central to this paper is the idea of setting the DLR from forecasts of the RTLR.
Overhead lines Dynamic Line rating based on probabilistic day-ahead forecasting and risk
assessment (archives-ouvertes.fr)