In today's offshore wind farms, there are one or more transformer platforms connected to the wind turbines to collect and convert the electricity into high-voltage direct current (DC) before it is transmitted via cable to land.
“These kind of converter platforms account for a significant part of the investment cost for offshore wind power”, says Torbjörn Thiringer, Professor of applied power engineering at the Department of Electrical Engineering at Chalmers. “It would therefore be a great advantage if offshore wind farms could be connected to land using fewer such platforms, which is the underlying idea of this concept.”
The alternative to free-standing platforms is to place the equipment needed for electricity conversion attached to the wind power towers themselves. One option is to put parts of the appliances in the nacelle as well as further down in the towers, or in a container on the outside of the towers.
Size and weight matter
“To be able to install it attached to the towers, the equipment must be shrunk in size. Over the past 20 years, we have worked on this problem every now and then. Thanks to the prototype designed by Mohammad Kharezy for his licentiate thesis, the solution is taking a significant step forward”, says Torbjörn Thiringer.
The key is the compact feature of the component and its comparatively low weight. The design implies that several inverters are connected in series on the high voltage side in order to obtain a high DC voltage. Because of the small physical distances combined with the large difference in potential, the electrical insulation capacity towards ground must then be very good, otherwise it will result in discharges and a short circuit.
“As insulation medium, I am using a biodegradable vegetable oil, instead of fossil-based transformer oil that is more commonly used. The latter is associated with negative environmental risks”, says Mohammad Kharezy. “Combined with cellulose, this oil implies a promising solution to the insulation problem, and in addition it brings evident environmental advantages.”
In the electric power engineering lab at Chalmers, as well as at RISE in Borås, Mohammad Kharezy has built a magnetic and a high-voltage prototype of the novel frequency transformer, in a scale of 1:200. The photo shows the measurement of the transformer's core loss using a precision wattmeter. This experiment requires a high frequency voltage generator, a California Instrument 4500LX PWM converter.
Promising for the future
The research project is funded by the Swedish Energy Agency
within the VindEL 2017 program. The purpose is to contribute to the transition to a sustainable and renewable energy system through research and development that makes wind power technology more functional and competitive.
The researchers are now hoping to get continued funding to further develop the concept of the medium frequency transformer, and to be able to study in detail how the design of the insulation can be improved to make the function more predictable.
“So far, the concept is very promising”, says Mohammad Kharezy. “I hope and believe that my research will be part of the solution required for connecting offshore wind farms to land in the future, without the need of any large and expensive free-standing platforms to house big transformers.
Text: Yvonne Jonsson
Photo: Tatu Nieminen and Mohammad Kharezy
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