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 theTSOs 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.
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 thenatural 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.
Jozef Stefan Institute (JSI)
Elektroinštitut Milan Vidmar (EIMV)
P-Lab teamGregor Kosec