This is the project documentation for a masters thesis at the Institute of Electrical Energy Systems (EES) at Friedrich-Alexander-Universität Erlangen-Nürnberg.
First supervisor: M.Sc. Ilya Burlakin (Main topics behavior of transformers, OLTCs, control mechanisms, stability assessment)
Second supervisor: M.Sc. Georg Kordowich (Python related topics and general simulation framework "diffpssi")
Professors: Univ.-Prof. Dr.-Ing. Matthias Luther and Univ.-Prof. Dr.-Ing. Johann Jäger
Date of submission: 02nd May 2025
Date of presentation/colloquium: 16th May 2025
The thesis counts seven chapters, and 122 pages in the official version. The unofficial format has 132 pages, due to the different settings, as the content is identical.
Modeling of Transformers with On-Load Tap-Changers for Voltage Stability Studies in Python
Fast-switching modules controled transformers respectively their on-load tap-changers, enable a more flexible voltage stabilization. But on the other hand, stability assessments and control strategies are not known. Because of the missing possibility of modelling such a fast-switching module in most grid simulations, using an own python-based simulation tool can be beneficial. An improved ability for analysis and optimization in Python is supporting this approach. Therefore this master thesis is expanding an existing Python simulation framework diffpssi from Georg Kordowich with an alternative transformer modelling, including continouos regulated OLTCs, looking into its control, and testing scenarios on system stability. The thesis defines following research questions:
- How do different control types and characteristics of transformers with On-Load Tap Changers (OLTCs) influence the voltage stability of a given system?
- Can the already existing tap changer control of the Fast Switching Module (FSM) be improved towards a more operation oriented control?
In this thesis, a new control concept for transformer tap changers is investigated, which takes into account an increase in dynamic capabilities by shortening the switching times. This reduction is realized by a Fast Switching Module (FSM), which is based on power electronics and extending the classical On-Load Tap Changer (OLTC), resulting in a hybrid solution. A transformer model with variable transformation ratio and the associated control scheme for a tap changer are modeled in an existing Python package for electrical power grid simulation. The implemented extensions are evaluated and compared with results of the commercial grid simulation software DIgSILENT PowerFactory. In addition, tools for the evaluation of simulation scenarios are implemented. The toolset includes the calculation of P-V Curves and a Trajectory Violation Integral (TVI). The effects of the various controls on voltage stability are tested as part of an application study. In addition, improvements to the controls are derived and outlined. The implemented models are validated using commercial software. The results show a stabilization of the bus voltages through the application of the FSM. The study illustrates a feedback of the fast tap-changing to the power and speed fluctuations of a synchronous machine. In addition, a voltage band can be identified in which one of the controllers shows no reaction. As future outlook, the discussed control improvements can be realized. Futher, an investigation a FSM applied to a phase shifting transformer seems useful.
For acessing a quick overvies, the thesis is structured as follows:
- Chapter 2: Fundamentals, illustrates and recalls fundamentals for stability assessments, modeling, and discussions;
- Chapter 3: Methodical Modeling, describes the process of modeling in the tool diffpssi, and the implementation of voltage stability tools;
- Chapter 4: Validation Setup and Result, shows the validation of the implemented models and tools with the help of common and simple test systems;
- Chapter 5: Application Study, looks at the novel control methods from different perspectives and evaluates with an exemplary simulation set-up;
- Chapter 6: Discussion, discusses the FSM control strategies, considering the fundamentals, verification, and case study results;
- Chapter 7: Summary, summarizes with regards to the research questions, and looking towards research potential and future developments.
The summary slide of the thesis presentation is depicted down below. As the presentation language is german, the slide is as well.
As comparative tool for validating the implemetation, DIgSILENT PowerFactory 2023 with SP1 is used. Currently, the Code of the implemtation is not published yet, but planned to be.
Additionally, a replication package ia planned, and will be linked here. With that, one can easily reproduce the obtained results locally. As the base package diffpssi is based on Python, the code runs cross compatible, also with the to be used scripts. Futher details and instructions will be included in the replication package.
This Master Thesis by Maximilian Köhler is licensed under CC BY-SA 4.0
This license requires that reusers give credit to the creator. It allows reusers to distribute, remix, adapt, and build upon the material in any medium or format. If others modify or adapt the material, they must license the modified material under identical terms.
Feel free to use the thesis within the license frame with the given citation.
@mastersthesis{kohler_2025,
title = {Modeling of {{Fast-Switching On-Load-Tap-Changers}} for {{Voltage Stability Studies}} in {{Python}}},
author = {K{\"o}hler, Maximilian},
year = {2025},
address = {Erlangen},
langid = {english},
school = {Friedrich-Alexander-Universit{\"a}t Erlangen-N{\"u}rnberg}
}
The bibliography entry in IEEE-Format would then look like the following:
M. Köhler, “Modeling of Fast-Switching On-Load-Tap-Changers for Voltage Stability Studies in Python,” Master Thesis, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, 2025.