To provide a solution to the problems caused to wind turbines by sudden dips in voltage in a part of the electric grid. This was the objective of industrial engineer and member of INGEPER Research Team at the Public University of Navarre, Jesús López Taberna.
The fruit of his research is a rotor model which enables anticipating how the wind power unit will behave in these situations. Mr López has patented two techniques of protection, one of which has already been transferred to a manufacturer who will exploit it at international level. This system allows the generator turbine to remain in operation during these voltage dips and thus prevent the wind energy converter from ceasing to function.
The title of the PhD is: “The behaviour of wind-powered generators with double-fed asynchronous motor during voltage dips”.
Over the past few years, the growth and development of wind energy converters has been slowed by problems that have arisen from the increase in the number of these connected to the electric grid. One of the most important problems is precisely the manner in which the wind generators behave during these voltage dips.
A voltage dip is a sudden reduction in potential in the electric grid, followed by a rapid return to its normal value. This, at times, can be caused by lightening or a tree falling on power cables but also due to a large company consuming a lot of energy in one go. This drop in voltage happens in a matter of milliseconds; “we are aware of it because the lights begin to flicker or because they go off and on momentarily – but, for a machine, this can be an eternity”, explained Mr López. In fact, an interruption of half a second in a productive process can cause the whole process to block and it may have to be reinitiated.
With wind generators, in the case of a voltage dip, the electronic part of the unit can burn out or otherwise be destroyed, unless a protection system is installed “The current system of protection, known as Crowbar, has the advantage of being able to protect the machine but the disadvantage of the machine coming to a halt”, pointed out Mr López. “For example, if a large company suddenly consumes a lot of current, the voltage drops. This causes the wind power units at El Perdon (Navarre) to disconnect and cease producing electricity. As a result, the power dip is even more accentuated and, consequently, it is even more difficult to bring the voltage up to its normal operating value”.
Taking into account that, in Spain, there are days that wind-powered energy can account for one third of electricity production, the problem can prove to be a serious one. The idea being worked on currently is focused on the generator behaving more as a conventional power station and not disconnecting during a voltage dip/power failure but helping to bring the grid voltage back up. This is why wind generator manufacturers are currently working on finding a new system of protection that is efficient and efficacious.
Two new protection techniques patented
“Before looking for a solution, the problem has to be studied from a theoretical perspective, i.e. why does this machine behave as it does when there is a voltage dip? And why, if we do not install a protection system, the machine starts to burn out”? The research produced a rotor model which was “sufficiently simple to be able to deal with without having to carry out simulations. A model in which we can see what role each parameter of the machine plays, how they interact, how the current drops if we increase the leak inductances, etc”.
Once this model was developed, it was more or less easy to propose solutions. “The most important thing is that we have achieved solutions that enhance the behaviour of the machine without any need to change anything, except the control. It’s like changing the version of a text treatment programme on the computer, without needing to change the PC. There a number of computers inside a wind energy converter and one of these – that which controls the electrical machinery – is the one the control of which we have proposed to modify in order to enhance the behaviour of the machine”.
Jesús López Taberna specifically proposed in his PhD thesis two systems of protection and both have been patented. The first, only requiring changing the control of the machine converter, has been transferred to a manufacturer for introduction into wind parks worldwide; the other requires changing elements inside the machine and continues to be developed for applications in new creation wind generators.
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