DTR in natural convection regime

The development of Dynamic Thermal Rating (DTR) systems for transmission lines in recent years led to a better and safer utilization of the existing transmission network. In the past, line ratings were usually set to a constant value, determined by a set of unfavorable weather conditions that were considered as worst case scenario, i.e. ambient temperature of 35 °C, low wind speed of 0.6 m/s and high solar irradiation of 800–900 W/m2. With the introduction of DTR, line ratings can surpass the conservative static values by a significant margin for the majority of the year. However, on-site measurements in the Slovenian system have shown that many sites are subject to low wind speeds, which frequently fall below 0.6 m/s, causing the line ratings to fall even below the statically determined values. In low wind conditions the

Schematic representation of the simulated and CIGRE predicted cooling power.
Schematic representation of the simulated and CIGRE predicted cooling power.
TSOs might thus operate their networks with overestimated thermal rating. In case of no or very low wind speeds, which is a regular occurrence in Slovenia, the forced convective cooling is lower than the natural convective cooling. Moreover, references have emerged claiming that the cooling due to the natural convection is the same as with forced convection at 0.6 m/s crossing wind. The goal of this project was to clarify the thermal behavior of power line in regimes of no external wind by means of thermo-fluid simulation, the laboratory measurements, and predictions offered by CIGRE, IEEE and IEC guidelines.

Experimental setup for measurements of conductor temperature
Experimental setup for measurements of conductor temperature
CIGRE, IEEE and IEC rely on empirical relations for assessment of the convective cooling, where IEEE and CIGRE differ between low wind and high wind regimes, and specially treat the natural convection. The IEC does not model natural convection and assumes zero cooling in no wind regimes, which is obviously wrong and hence was not considered within the paper. In CIGRE guidelines, natural convection is calculated at zero wind speed and depends on the range of the product of Grashof and Prandtl numbers. Similarly to CIGRE, the IEEE guidelines calculate the
Comparison of measured and simulated conductor temperature.
Comparison of measured and simulated conductor temperature.
natural convection with a separate set of equations for zero speed. In this project we modeled the natural convection cooling by means of thermo-fluid simulation in the vicinity of the power line instead of relying on empirical relations.

In order to validate numerical solution a closed indoor laboratory experiment was set up. A conductor was connected to a laboratory class 0.1 % precision current transformer with 2500 A : 5 A ratio forming a closed current loop, operating at low voltage. The AC current output of the current transformer was determined with a regulation transformer which was in turn set with a hybrid DTR controller. The regulator measures the line current and temperature and can either hold the current constant or can control the line temperature to a constant value. The former principle was used in our measurements.

Te project was carried out in collaboration with Elektroinštitut Milan Vidmar (EIMV), the leading Slovenian engineering and scientific research organisation acting in the area of electric power engineering.

Simulated temperature field in conductors and its vicinity for no and low crosswind.
Simulated temperature field in conductors and its vicinity for no and low crosswind.
ResearchGate
ResearchGate

Partners

Jozef Stefan Institute (JSI)

Elektroinštitut Milan Vidmar (EIMV)

P-Lab team


Funding

Related publications

M. Maksić, V. Djurica, A. Souvent, J. Slak, M. Depolli, G. Kosec; Cooling of overhead power lines due to the natural convection, International journal of electrical power & energy systems, 2019 [DOI: 10.1016/j.ijepes.2019.05.005][COBISS: 32388135] ::
V. Djurica, K. Jevnikar, M. Maksić, G. Kosec; Laboratory measurements of thermal behavior of overhead power lines due to natural convection, Innovation in the Power System Industry : special issue of the 21st International Symposium of High Voltage Engineering (ISH) 2019, August 2019, Budapest, Hungary, Cigre Science & engineering, vol. 16, 2019 [COBISS: 32992807]
G. Kosec, J. Slak; RBR-FD based dynamic thermal rating of overhead power lines, AFM 18, WIT transactions on engineering sciences, 2018 [COBISS: 31829287]
G. Kosec, J. Slak; Numerical simulation of overhead power line cooling in natural convection regime, ECT2018, 2018 [COBISS: 31832615]
G. Kosec, J. Slak; Numerical simulation of natural convection from a heated cylinder, Proceedings of the International Conference on Computational Methods, ICCM2018, 6th - 10th August 2018, Rome, Italy, 2018 [COBISS: 31642919]
G. Kosec, M. Maksić, V. Djurica; Dynamic thermal rating of power lines : model and measurements in rainy conditions, International journal of electrical power & energy systems, vol. 91, 2017 [DOI: 10.1016/j.ijepes.2017.04.001][COBISS: 30421287] ::