“We can now assess the condition of transformer fleets much better”

Markus Zdrallek, professor at the University of Wuppertal, and Jan Patrick Linossier, Head of Strategic Asset Management at Rheinische NETZGesellschaft (RNG), have worked together with MR to develop a new method for assessing the condition of power transformers. In an interview, they revealed what the new system is capable of.

Why is it so important to know the condition of power transformers exactly?

ZDRALLEK: It’s important to evaluate the condition in all voltage levels and, in principle, for all components. But with a mass product such as a distribution transformer, of course, you cannot put as much effort into the evaluation as with a power transformer. The more expensive the equipment is, the more important it is to know its condition. Investing in a somewhat more comprehensive assessment pays off quickly if afterwards I know that I can leave the transformer in the grid for another ten years without having to invest money in a new one.

LINOSSIER: From the perspective of the grid operator, I can only confirm that. We operate over 100 transformers, so for our strategic planning it is very important to know when and whether we have to take one off the grid, or if other measures might also make sense. Perhaps just replacing the motor-drive unit will be enough to extend the service life.

Why has condition assessment become so important in recent years?

Linossier, Head of asset management at “Rheinische NETZGesellschaft“ (RNG). (© Dirk Moll)

LINOSSIER: Large parts of our fleet are from the 1950s and 60s, and though they are still very reliable to this day, we are starting to think about their aging behavior. Now, in light of the energy revolution, the question of how long they will keep going has become considerably more important. We distribution system operators in particular are faced with enormous challenges. Many of the decentral power supplies enter into our networks. The topic of e-mobility also comes into play here. Which brings along entirely different loads for our transformers. And we also have to rethink the network structures. But we can only do that sensibly if we know the condition of our equipment well.

ZDRALLEK: RNG’s experiences are shared by grid operators throughout the world: Transformer fleets are getting older and older. The cost pressure today is simply higher, which is also due to the many investments required by the energy revolution. So operators want to use the equipment for as long as possible. That is why condition assessment, together with grid expansion and conversion for the energy revolution. has become quite a central topic at international conferences. The industry as a whole is faced with enormous challenges. More has happened in the last ten years in the world of power supply than in the previous hundred years, and in the next ten years just as much will happen.

What role did condition assessment play in the past?

ZDRALLEK: For a long time it was of lower-level importance. In the past, many operators followed the strategy of performing maintenance on the transformers every five years and then simply replacing them sequentially after 40 or 50 years. This is also reflected in research, which in recent years has been very focused on optimizing maintenance strategies. And when condition assessments were performed, they were often limited to visual inspections. And these are open to a high degree of subjectivity.

To what extent is a visual inspection subjective?

ZDRALLEK: We once set up a test for other equipment in a medium-voltage grid and sent out ten technicians to evaluate the same station. The results varied widely from one another: Some evaluated the system as very good, while others maintained the exact opposite. Everyone is influenced by his own store of experience, and personality also plays a role. Some are just pickier than others. Together with MR we have tried to bring more objectivity into the assessment.

How did you achieve that?

ZDRALLEK: Our system includes a visual inspection, too, but we have worked out a uniform checklist that technicians use to evaluate the transformer and, for example, check whether oil is leaking from particular spots. In addition, we use a small trick that we have learned from sociologists: We give the technician doing the evaluation an even number of grades, in our case from one (for very good) to four (for very bad). If there is an odd number of evaluation options, people tend to take the value in the middle. But an even number forces them to make a decision. And then, of course, there are also a number of measurements which we perform, such as oil analyses and dynamic resistance measurements. The measurements add significantly to objectivity.

What is special about the method you have developed with MR?

Markus Zdrallek is professor at the University of Wuppertal and holds the senior professorship for electrical energy supply technology. (© Dirk Moll)

ZDRALLEK: One new feature is that we evaluate the condition of the transformer from two perspectives. One allows us to make statements about the risk of failure, which is primarily important in the short and medium term. The other evaluates the transformer for the longer term and is aimed at its service life. For both perspectives, we weigh the investigated parameters differently using an algorithm.

LINOSSIER: Both approaches correspond exactly to the view that we have of our transformer fleet in Strategic Asset Management. Like almost all grid operators, we have one budget for maintenance tasks and one for investments. The short-term and long-term components therefore give us an ideal instrument for aligning our maintenance and replacement strategy. Existing processes of condition assessment were always one-dimensional and therefore too imprecise. Another advantage of the system is that it creates transparency. For example, if a transformer receives a bad grade, we can trace exactly which component or which measured value led to this result. That makes it easier for us to derive measures to be taken, because now we know whether maintenance measures will suffice or whether we will have to make a replacement instead.

To what extent were you able to benefit from the expertise of MR?

LINOSSIER: All of our transformers have on-load tap-changers from MR in them. Many different models have been installed over the years, and of course MR knows these precisely. The experience of MR experts on the subject of transformers, their monitoring expertise and the availability of a modern infrastructure are excellent prerequisites for offering a comprehensive solution. MR has a lab for oil analyses, the right IT solutions for these methods and the corresponding sensors in its portfolio.

ZDRALLEK: We researchers have well-founded knowledge about how an evaluation system is created, but we were able to learn a lot from MR in the area of power transformers. There were some measurement methods that we were not yet familiar with. On top of that, it’s also possible to measure a lot of garbage. Not everyone is able to carry out a thermographic analysis or take an oil sample correctly.

What do these results mean for your transformer fleet? What are your plans for the future?

LINOSSIER: So far, we have only analyzed nine transformers in the pilot study. Now we want to apply the method to the entire fleet. So that we can assess the condition even better in the future, we would like to boost our investment in digitalization and acquire monitoring systems. We don’t have much historic data to evaluate, because it was not documented like this in the past. But if we now start to measure continuously, in the future we will have even more data material for the condition assessment.

ZDRALLEK: I am also really excited about this. We really know very little about the aging behavior of transformers. The more data we get, the better the conclusions we will be able to draw in the future.

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