Nyheder Tvindkraft

42-Driver-boards-for-IGBTsThe frequency converter is an electronic machine which is placed between the generator in the nacelle and the electric grid. The schools in Tvind get the electricity from the windmill through this frequency converter. When the windmill produces more electricity than the schools can use, we deliver it to the utility company (RAH).

We have always had a frequency converter. It was one of the pioneering inventions that was made during the building of Tvindkraft. The frequency converter is needed because the windmill is pitch-controlled and running with variable speed. (Also pioneering technologies). Therefore the frequency and voltage of the electricity from the generator changes all the time when the wind changes. It is not possible to connect this variable electricity directly to the electrical grid, which need a fixed frequency (50 Hz) and voltage (400 V). The frequency converter takes care of changing the variable electricity from the generator to fixed electricity, which can be connected to the electrical grid.

The original frequency converter was built by a team of students from the Danish Technical University (at that time DTH). The design was made by their professor Ulrik Krabbe, and the details and the physical construction were done by the students as an exam project. In was then installed and tested in the control house next to the windmill, and has been running reliably ever since.

However it was only possible to deliver 400 kW out to the public electrical grid, and therefore the converter was constructed to this capacity. But the windmill had a capacity of 1000 kW. The solution was to install water heaters for the central heating system in some of the buildings in Tvind. These water heaters can work fine with variable voltage and frequency, so they get the power directly from the generator. However, at times we get too much hot water, and in the summer time it is not needed at all.

When the commercial wind turbines reached Tvindkraft’s size in the late 1990’s the electrical grid was reinforced which made it possible to deliver the full capacity of 1000 kW.
In the 2000s we started contructing a second converter of 500 kW. The new one is now connected in parallel with the old, so together they will deliver 900 kW. We still have the water heaters, so we can decide if we want to produce hot water or only electricity according to the demand.

When we started the construction, we did it the same way as when we built the windmill: we realized that we didn't know much about it, so we had to ask the people who did know. And we have got a lot of support. Mainly from Aalborg University, which has an advanced department for power electronics. They gave a huge help with the control program and lent us measuring and test equipment. Some of their students have been working here, both in semester projects and as student jobbers. And the professors at the university are always ready the answer questions. Also Chalmers technical High-school in Gothenburg has been helping in the early phase along with the suppliers of the main components: Fuji-Electric in Frankfurt and Semikron in Nürnberg . In addition several other people and companies have been supportive.

tvindkraft-elektrikal-system

We have, of course, used more modern technology for the new converter: it is computer-controlled, and the main components (IGBT's) produce less reactive energy than the tyristors used in the old converter. And we have made good use of simulations. We have built the whole system in a simulation programs, so we can see what happens without risking to blow the components. And we can modify the circuits and see what difference it makes. Also the programming is simulated, so we can see how it works together with the hardware. To check how the two converters (which use different technology) work together, we also needed to build a simulation model for the old converter, and then connect it with the model for the new. Much of this simulation work has been done by the students from Aalborg University and Chalmers Technical High School.

After the new converter started running on its own we made various protection installations before connecting them together in parallel. We installed over-current and over-voltage protection, and then we started the two converters together.

This we did April 15, 2015, and it worked fine at once. We found that we get the most benefit when the wind is above 7 - 8 m/s, which it is a lot of the time. With this wind-speed the old converter hits it limit. We have made the system so that the new one switches in when the old one reaches its limit. This way we can increase the power with higher wind-speeds, whereas before we had to limit it. A huge benefit is also that we can keep the (RPM) of the windmill much more constant,  thereby stabilizing the production.

The finishing work is to get rid of current spikes, which probably can be done by tweaking the control program for the new converter. Also the start and stop procedures have to be integrated in the control system. When everything is as we want it to be, we will move the whole machine from the temporary shed where we built it into a new room we have prepared for it in the control-room building.

 

 

Frequency converter diagrams

42-Diagram-of-both-converters
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