With massive renewables integration and fostering the EU's internal energy market, transmission system operator's day-to-day business requires modern tools to assure transmission capacities and system stability. Nowadays, the key question for power system operators is how to operate the power system flexibly while keeping it secure and stable yet economically efficient. Also essential are – better utilization of the existing power system infrastructure, which is more environmentally friendly than building new power lines – and maintaining power system resilience.
The Slovenian national transmission system operator, Eles, has innovatively utilized Dynamic Thermal Rating (DTR) technologies to address the aforementioned challenges. DTR is a power system operation concept aiming to maximize utilization of the equipment, like power lines, transformers, when weather conditions allow it, without compromising the safety of operation.
The most know DTR technology is the Dynamic Line Rating (DLR) which focused on overhead power lines (OHL). Conductors of an OHL are heated by Joule losses caused by current and by solar heating, but on the other hand, they are also cooled – mostly by radiation and convection. As the temperature of a conductor increases, the sag also increases, and at some point, safety clearance may be violated. As this must be prevented, static ratings are used – these are current limits to prevent the conductor's temperature from reaching critical value. As these limits are set regarding the worst-case weather conditions, most of the time – e.g. when air temperatures are low, when the wind is blowing, the power lines can be utilized more as the static ratings allow.
It all started ten years ago when Eles joined forces with research institutions Elektroinstitut Milan Vidmar, Faculty of Electrical Engineering, University of Ljubljana, and Jožef Stefan Institute. A comprehensive modular concept of Dynamic Rating System (DRS) was developed, mainly focusing on power system operation, operational planning, and improving power system resilience – the latter with an innovative approach of reversing the DLR algorithm for the application for icing prevention.
The system – called SUMO – is an indirect dynamic thermal rating system (DRS) comprising of the Dynamic Line Rating (DLR), the Dynamic Power Transformer Rating (DPTR), and the Dynamic Line Anti-icing (DLAI) subsystems. It is a software-enabled and meteorological model’s based system with the possibility to integrate also data from meteorological weather stations. DLR calculations are made for all power line spans, and the weakest line-span determines the rating for the whole line. The system supports real-time and short-term forecast operations, calculations of transmission capacities for up to two days ahead, and allows for mitigation of overloading operational situations, taking into account also contingency situations. It also features an inverse DLR algorithm for icing prevention and alarms for extreme weather conditions along the power lines. It is a modular IT system featuring an integration bus with programming APIs and templates for the integration of additional third-party modules.
As the system uses physical models to calculate results and as the main input data is weather assessment and forecasts, the main challenge has been dealing with uncertainties to operate the system with foreseeable risks. It should be mentioned that operating the system on static limits, as is the practice so far, is also not without risk. On a hot summer day with no or very low wind, the transmission capacity can also fall below the static limit! That means a reduction of transmission capacity to maintain safety clearances within permissible limits.
The weather assessment and forecast are based on mezzo and microscale meteorological models. The results from the models can be further enhanced by integrating measurements from the weather stations.
Mesoscale weather model results represent an important source of uncertainties, which can be lowered by local weather measurements at the OHL's towers. On the other hand, weather models constantly improve over time, and weather data quality may improve significantly in the next years. But the most important is that the uncertainties can be evaluated and taken into account by the SUMO uncertainty module, which calculates the lower and upper limit of the dynamic thermal current related to the chosen confidence interval.
As of December 2013, the system had been trial running with four power lines. In December 2015 additional 17 power lines and one phase shift transformer were added. Since 2017 the system has been in full operational mode and in daily use at the National Control Centre. The dynamic thermal limits are currently calculated for 29 OHL on 400 kV, 220 kV, and 110 kV voltage levels, two phase-shift, and two power transformers. The system is fully functional and integrated into the daily operation. The main applications that support real-time operation and operation planning are mitigation, overloading operational situations, and calculations of transmission capacities for up to two days ahead.
On average, 92 – 96% of the time, the system offers a higher transmission capacity with a median increase of 15-20 % of the nominal capacity. Over 500 possible overloading events are mitigated annually.
In 2021, Eles decided to further promote the solution and has established a daughter company OPERATO, with a mission to share the knowledge and experiences with other national operators. Apart from ELES implementation, the system also runs at HOPS, the Croatian national transmission system operator.
