A robust voltage limit

New decision support tools can help assess voltage stability

Autorin: Stephanie Bos

The energy system is changing from the ground up as part of the energy transition, resulting in various challenges for all stakeholders in the energy sector. One of these challenges is voltage stability. As large, conventional power plants disappear, ensuring voltage stability will become more difficult and place demands on system operation. In the future, new decision support tools could help with the assessment of voltage stability by indicating potential courses of action. Katharina Kaiser devoted her Master’s thesis at ETH Zurich to the topic, with guidance from Swissgrid. Her work represents the first step towards the development of such tools. In the course of an interview, she tells us exactly what she examined and how the results could be taken further.

What exactly is your Master’s thesis about?
Katharina Kaiser: In my Master’s thesis, I worked on a method that makes it possible to determine a robust lower voltage limit. By robust, I mean valid for a variety of different operating conditions. We looked at the grid from a purely static point of view. In other words, we considered individual operating points rather than dynamic changes. First of all, we had to define the decisive operating point for determining the lower voltage limit. And then, of course, we had to work out how to find this specific point. Voltage stability is often illustrated with the help of curves known as nose curves for a simple system with a generator, a line and a load. In this case, the critical voltage can easily be determined as the voltage at the maximum transmissible active power. In a real grid, however, it is much more complex. Here, too, a critical voltage value can be determined for a specific load scenario, but there are a large number of combinations of how much active and reactive power is fed into or drawn from the grid at a certain point. There are too many possibilities to analyse each of them individually. We therefore developed an iterative algorithm for finding the decisive operating point.

Voltage limits are needed for planning purposes and to monitor the grid in real time to ensure that it remains stable.

Katharina Kaiser

Why is the lower voltage limit important for grid operation?
Voltage instability can lead to large-scale failures and thus endanger the secure supply of electricity. Limits are therefore needed for planning purposes and to monitor the grid in real time to ensure that it remains stable. An upper voltage limit is defined so that the components can withstand the voltage without becoming damaged. A lower limit, on the other hand, is needed so that the grid doesn’t reach its limit in terms of maximum transmissible power. Increasing demand for electrical energy and changes in the structure of power generation are just two of the current developments that are leading to changes for grid operation and voltage stability. Grid stability must also be ensured for future generation, transit and consumption patterns. That’s why it’s important for the lower voltage limit to cover the variation of possible operating points.

How could the results of your Master’s thesis be developed further, and is this already the case?
In my work, the focus was very much on methodology. After defining the operating point we were looking for, the next step was to develop the algorithm and prove that it worked. Given the time frame, we had to make assumptions that simplified the problem and the implementation. For example, we assumed that there were no active power losses in the grid. The voltage values I determined through my work are only valid under the assumptions made. To obtain a conclusive value for the lower voltage limit, missing functions would have to be integrated and assumptions for certain parameters would have to be checked, e.g. using historical data.

Many challenges face us in the area of power grids, both now and in the future, especially with regard to the energy transition.

Katharina Kaiser

Electrical engineering is a broad field. What sparked your interest in the subject?
The topic of my thesis had to be related to power grids. Many challenges face us in this area, both now and in the future, especially with regard to the energy transition. This is something I would like to help address, and that’s why I wanted to focus on the area in my Master’s thesis. The fact that it’s about voltage stability just came about without me explicitly looking for it. I saw the potential to apply what I had learned in lectures and to delve deeper into the theory. The fact that it was possible to work in cooperation with Swissgrid was another positive aspect. I saw it as a chance to gain insights into the company and to make contacts along the way.

How did you come into contact with Swissgrid?
It was quite straightforward. I’d been looking around the Power Systems Laboratory at ETH for possible topics for my thesis. Johanna Vorwerk, who later also helped and mentored me from the ETH side, made contact with Swissgrid. After a short meeting with the people involved at Swissgrid and a discussion about the background and goals of the work, it was clear that this would be my topic.

What did you think of the cooperation with Swissgrid?
It was very positive! I must admit I’d imagined the corporate culture to be rather conservative. So it was a pleasant surprise to find that the opposite was true. The Swissgrid employees were very open, I was integrated into team and department events, and I always enjoyed coming to Aarau. As far as the supervision of my Master’s thesis is concerned, I can also consider myself very lucky. In Marc Hohmann, Stavros Karagiannopoulos and Fabian Streiff, I found three colleagues who regularly took the time to discuss possible solutions and give me feedback. I learned a great deal from them. Thank you again for the opportunity to write my Master’s thesis at Swissgrid.


Stephanie Bos
Stephanie Bos

Communication Manager

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