Limitations in Operation of High Voltage Equipment Resulting of Frequent Temporary Overvoltage’s
Bartosz Rusek
Abstract
The tutorial will provide an overview of limitations for high voltage equipment (Instrument transformers, transformers, power plants, cable, surge arrester, switchgear) resulting of frequent temporary overvoltages. The limitation will consider both the ageing of the insulation as well as the abilities to perform specific tasks (e.g. keep the adequacy or switch the currents). Fortunately, in majority of network conditions, the standardized equipment can withstand discussed temporary overvoltage surge without major concerns. Nevertheless, the users need to verify the application boundaries very precisely. The tutorial will provide list of possible limitations and possible remedial measures.
Content:
- Background information
- Overview of phenomena leading to temporary overvoltages
- Principles of insulation coordination, insulation design and potential insulation ageing
- Limitation of abilities for performing specified tasks for following assets
- Instrument transformers
- Transformers
- Power plants
- Cable
- Surge arrester
- Switchgear
- Summary
CV
Bartosz Rusek has received Ph.D. degree in the Electrical Power Engineering from Technical University of Darmstadt in Germany. Since 2006 he has been working in the Department of Asset Management at Amprion (German TSO) dealing with special aspects of switching phenomena, insulation coordination and dynamic ratings of assets. Presently, he manages a team dealing with scouting, evaluation and initialization of R&D projects for primary TSO technologies.
Transformer dynamic thermal behaviour, modelling and applications
Tim Gradnik
Abstract
The rapid transformation of electrical power networks—driven by the transition to low-carbon distributed generation and constrained investments in mature systems—has intensified the need for advanced methods to design, operate, and assess power transformers. Temperature remains a dominant factor in defining transformer loading capability and operational risk, making accurate prediction of dynamic thermal behaviour essential for maximizing asset performance and preventing unexpected outages. Although significant progress has been achieved through the evolution of modelling practices such as Thermal-Hydraulic Networks (THN), Computational Fluid Dynamics (CFD), and machine-learning-based approaches, field data and fibre-optic winding measurements reveal important limitations in the dynamic transformer thermal models of the IEC and IEEE loading guides. These limitations are particularly evident in the modelling of variable cooling scenarios, viscosity effects in ester-based insulating liquids, and transformer operation under sub-zero ambient conditions.
This tutorial provides a comprehensive overview of Dynamic Transformer Thermal Modelling (DTTM). It addresses steady-state and transient thermal phenomena and examines the influence of transformer geometry, loss distribution, and cooling arrangements on global liquid flow and overall thermal behaviour. Particular attention is given to liquid flow distribution among transformer components, the impact of by-pass flows on hot-spot temperature calculations, and winding-level flow phenomena, including maldistribution, stagnation, and reverse flows.
A review of the historical evolution of lumped-parameter models and the development of international standard models (IEC, IEEE, AS/NZS) highlights key modelling assumptions, mathematical formulations, and test methods for parameter determination. Based on this analysis, the tutorial identifies the principal areas where further model improvements are required.
The tutorial further examines advanced dynamic thermal modelling approaches. While static THN-based tools are widely used in the thermal design of liquid-immersed power transformers, dynamic THN models have not yet reached comparable levels of technological maturity. Dynamic thermal-hydraulic models (THM) are, however, capable of real-time execution under grid operating conditions and can function as thermal digital twins, providing time-varying distributions of liquid flow and temperature throughout the transformer. The role of CFD frameworks and emerging machine-learning-assisted thermal models is also discussed.
An open-source Dynamic Transformer Thermal Models Benchmarking Platform (DTTM-BP) is presented to enable transparent and objective benchmarking of DTTM. The platform includes Python-implemented standard and enhanced models, as well as mechanisms for benchmarking proprietary models using standardized input datasets that cover a wide range of operating scenarios and transformer types. The DTTM-BP is used to address key working group challenges, including validation of proposed improvements to the IEC model related to variable cooling states, tap-changer operation, and viscosity-driven thermal effects of different insulating liquids.
In the final part, practical applications of advanced dynamic thermal models are presented, spanning transformer loading capability assessment, network operation and emergency rating, asset management, insulation lifetime prediction, and cooling system optimization. These applications demonstrate the essential role of dynamic thermal modelling in modern power system planning, real-time operational decision-making, and long-term transformer health management. In addition, results from an international industry survey are presented, highlighting current practices in the application of dynamic thermal models for asset and network management.
CV
Tim Gradnik received his M.Sc. degree in electrical engineering from the University of Ljubljana, Slovenia, in 2004. He is employed at the Milan Vidmar Electric Power Research Institute (EIMV), in the Physical-Chemical Transformer Diagnostics Department. His professional and research work focuses on high-voltage transformers, including dynamic thermal modelling, dynamic thermal rating, on-line monitoring, dielectric and ageing analyses of insulation materials, and the development of diagnostic and monitoring methods for transformer insulation systems. He has been involved in the international CIGRÉ and IEEE ICDL communities for nearly two decades. He was a member of CIGRÉ Paris Study Committee A2 from 2005 and has served as its Secretary between 2016-2021, and as webmaster of SC D1 since 2022. Over the years he has contributed to a wide range of CIGRÉ working groups, including work on transformer procurement processes, thermal modelling, intelligent condition monitoring, moisture measurement, dynamic thermal behaviour, transformer digital twin, digitalisation of transformer information, and on-line moisture monitoring. He has been Convener of WG A2.60 since 2019 and remains an active contributor to CIGRE working groups addressing advanced transformer monitoring, modelling, diagnostics, and digitalisation challenges.
Mechanical properties of insulating materials and insulated conductors for oil insulated power transformers
Lars Schmidt
Abstract
Solid insulation materials — paper, pressboard, laminated wood, and aramid-based components — are essential for power transformer reliability. Beyond their dielectric role, they must withstand significant mechanical stresses, particularly during short-circuit events. Recent advances in materials science and modelling have provided new insights into their mechanical behavior. The work carried out within D1.65 summarizes key aspects of insulation in liquid-filled transformers, focusing on mechanical properties, innovative testing methods, and state-of-the-art modelling techniques.
CV
Lars Erik Schmidt is the Global Product Manager for Insulation at Hitachi Energy, leading the portfolio strategy for transformer insulation and driving strategic projects across global factories. Lars joined Hitachi Energy (former ABB) in 2006. He has a background in Material´s Science, holds a PhD from the Swiss Federal Institute of Technology, Lausanne, and an Executive MBA from Mannheim Business School. He is the Chair of IEC TC 15 Solid electrical insulating materials.