Djokic, Sasa

Harrison, Gareth

Judge, Paul

Duan, Chengyan

2026-06-01T14:13:15Z

2026-06-01

Met Office has reported that the top 10 warmest years in the UK from 1884 have occurred since 2002, clearly indicating an emerging pattern of higher temperatures due to climate change. These high temperatures (“heatwaves”) have drawn increased attention to the impact of climate change on the thermal ratings and loading limits of power system components and joint circuits in which these components operate. Thermal rating of a given component is based on the component’s maximum allowed design/operational temperature, typically representing its maximum loading capacity, which, however, changes with both ambient/weather conditions and operating conditions. Violating or exceeding component’s maximum temperature limit can, and usually will, result in a number of negative consequences, including, but not limited to: accelerated ageing, reduced safety clearances and degradation of electrical insulation, which in turn can cause equipment failures, fires and electric shocks, leading not only to the possible permanent damage of the component, but also to potentially wider power system blackouts. When two or more power system components operate in a joint circuit, as is usually the case, thermal ratings and loading limits of the joint circuit are generally determined by the component in the circuit which has the lowest maximum loading capacity for given ambient and operating conditions for the considered period of interest. As the ambient and operating conditions change, the specific component with the lowest maximum loading capacity in the joint circuit may also change, which further complicates accurate evaluation of circuit and system loading capacities. In order to more accurately evaluate thermal ratings and loading limits of power system components, this PhD research applies thermal modelling, based on the physical models and characteristics of the modelled components at 400kV to 132kV levels (Scotland high-voltage transmission level). Thermal ratings were calculated and assessed using the developed thermal models and considering different operating and ambient conditions, as well as time periods, for the three main power system components: oil-immersed power transformers, cables, and overhead lines. Cables are further divided into directly buried cables, cables in filled troughs, and cables exposed in the air. Thermal modelling processes and thermal-electrical models are formulated for unified ambient and operational conditions, allowing for the evaluation of: static thermal rating, based on and calculated from the assumed constant (seasonal or monthly) ambient conditions; dynamic thermal rating, calculated for actual ambient conditions; short-term and medium-term overloading rating, calculated for a set of given ambient conditions, pre-fault loading levels and specific durations; cyclic thermal rating, calculated for the given ambient condition, but considering daily cyclic variations of loading profile, with periods of high and low loading conditions, and, finally, exceedance level based ratings, calculated by acknowledging acceptable risk levels for component’s operational temperature exceeding its maximum temperature limit (the exceedance ratings directly apply only to overhead lines). A detailed review of existing standards and literature identified several limitations in current models. These include incomplete treatment of transformer losses that depend on winding temperature and tap position, simplified representation of cooling-mode transitions between oil-natural air-natural and oil-forced air-forced, and limited inclusion of weather effects such as wind speed and solar irradiance. To address these gaps, the transformer model was enhanced to include temperature- and voltage-dependent losses, cooling-mode transitions, exposure differences between tank and cooling plant, tertiary windings, and effective cooling areas. Cable models were extended to cover directly buried, filled-trough, and air-exposed configurations, with explicit consideration of solar irradiance and wind speed effects. In terms of the improvements of existing models and development of new models: The inconsistencies and gaps described previously are all being addressed accordingly. The previously developed cable thermal model for directly buried cables and cables in conduits is now extended to include cables in filled troughs. Thermal models for the cables exposed to air and solar irradiance are developed, also enabling the consideration of wind speed. Finally, there is currently no comprehensive approach in existing literature for modelling and evaluating the thermal ratings of joint circuits, while only some guidance is provided for selecting and coordinating ratings for joint circuits with multiple components. However, such guidance is very simple in order to meet the ease-of-use requirements. Building on the improved component-level models, a new methodology was developed to evaluate the thermal ratings of joint circuits that contain combinations of the mentioned multiple power network components. This method couples the steady-state and transient thermal behaviour of each component under common weather and loading conditions, identifying the limiting element at each time step. Using simulation-based analysis, the presented approach determines time-varying static thermal rating, dynamic thermal rating, cyclic thermal rating, short-term and medium-term overloading rating, and exceedance level-based ratings at the circuit level, and identifies when the circuit thermal loading limit shifts between components for different ambient operating conditions, including heatwave scenarios. This thesis develops and refines methodologies for accurately evaluating the thermal ratings and loading limits of transformers, cables, and overhead lines under changing climatic conditions. This thesis improves existing thermal models by including weather-dependent and operational factors, such as wind speed, solar irradiance, tap position, cooling-mode transitions, etc., and extending these methods to joint circuits. The work enables a consistent and realistic assessment of circuit/system capability across varying ambient scenarios. The results provide power system operators and planners with a reliable means to quantify the impact of climate change and heatwave events on the network performance, identify thermally constrained assets, and inform asset reinforcement.

https://era.ed.ac.uk/handle/1842/44779

https://doi.org/10.7488/era/7293

en

and uncontrolled residential EV charging with static and dynamic thermal ratings of network components Zakaria, A., Duan, C. & Djokic, S. Z., Mar 2024, In: IET Generation, Transmission and Distribution. 18, 6, p. 1283-1301 19 p

Thermal Ratings

Loading Limits

Power System Components

Thermal Modelling

Climate change

Evaluation of thermal ratings and loading limits of power system components and joint circuits

Thesis

Doctoral

PhD Doctor of Philosophy