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Frequency converter |
Tvindkraft must be utilized to its Fullest Potential: a New Frequency Converter! The board of the Foundation has in 2001 granted 350,000 Dkr for the construction of a frequency converter for Tvindkraft, which will result in Tvindkraft having a capacity of 1,000 kW instead of the current 450 kW. By doing this, Tvindkraft, as a sustainable environmental project, can produce even more renewable energy and thus save us from emitting vast amounts of carbon. The Foundation has paid 200,000 Dkr to Tvindkraft. The police has frozen the remaining 150,000 Dkr on a blocked account. What is a Frequency Converter? A frequency converter converts currents of fluctuating frequencies to a current with one specific frequency, which is necessary for sending the power to the electric grid. The generator in the mill produces current of varying frequency because Tvindkraft's number of revolutions vary - depending on the wind. Other types of mills run with a fixed number of revolutions. The advantage of having a varying number is that the wind's energy can be utilized best in this way. Allan Lund Jensen, who for many years has been the caretaker of Tvindkraft, says: "Tvindkraft already has a frequency converter of 450 kW. The new frequency converter is 1,000 kW. When Tvindkraft was built the idea was that the power, which could not be supplied to the grid when the mill produced more than 450 kW, was to be used to heat water in immersion heaters. In practice this has not proven very efficient. Especially during the summers we have not been able to get rid of the heat produced in the immersion tanks. When there is only a little wind there is no problem to utilize the power. But when the wind blows more than 8 meters per second, we have to pitch off the wings a bit - because we are only able to produce the 450 kW. We are thus able to use only part of the mill's capacity, which is a pity. We are now going ahead with the new frequency converter to be able to utilize this extra power. With a capacity of 1,000 kW the new frequency converter will also be able to cope with the stronger gusts of wind." The Application The Foundation granted the amount, and in April 2001 Tvindkraft received its first payment. So how do you build a Frequency Converter, when you have never tried it before? Allan Lund Jensen says about the project: "I have never built a frequency converter before. But I know the one we have inside out, and I love to construct things which can be used and function in real life. As a preparation for solving the task, I have looked up people with expertise within the relevant fields. I have for example visited Semikron and Fuji in Germany, two producers of the IGBTs (insulated gate bipolar transistors), which are the most important parts of the frequency converter, switching the current on and off. Generally speaking, they cut the current up into small parts. In this way it is possible to control a very large current with a very small current. First the rectifier in the frequency converter makes direct current out of the alternating current produced by the windmill. Then the IGBTs revert the direct current into alternating current again, but this time with the right frequency, 50 Hz, so that it can be transferred to the grid. If the mill had been running with a fixed number of revolutions it could have been connected directly to the grid, but then we would not have been able to utilize the full potential of the mill - added to this is the fact that the synchronous generator installed in the mill, would not have been able to cope with the task. I have been greatly helped by several other experts. I have been at Chalmer's Technical College in Göteborg and at Ålborg University, where research is being done into principles of control for frequency converters. Through discussions with them the topology of the converter is determined. It consists of two units of 500 kW each. This system has been chosen because it is simpler, practically speaking, and because we obtain redundancy, that is, we are able to run with one unit, should the other break down. Each module consists of a rectifier, a step-up-converter (increasing the voltage from low to high) and an inverter. The project is at the current stage uncovered and mapped, and the different parts of the frequency converter are being put together. The frequency converter itself is 1,60 m x 1,40 m x 0,70 m, with a built in water cooling, where the hot water is led into a radiator. At the same time I am constructing the control system. We are using a DSP-computer - a digital signal processor. This is a computer able to work as quick as lightning; it must be able to turn on and off 5,000 times per second to ensure that not too much noise is produced. Every time the computer turns on and off it has to measure the voltage and current and calculate when to turn on and off next time. It is much faster than the CPU of a normal computer, and has been specially designed to perform very fast calculations, again and again." The Project has not yet finished |
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