There are some very smart young minds working in the renewables sector these days – witness the work of Jesus Lopez Taberna, who very recently successfully defended his PhD thesis on how to protect wind turbines against sudden dips in grid voltages.
Taberna is a member of the INGEPER research team at the University of Navarra, in northern Spain, which is a powerhouse of renewables development and has been visited by Energy.
The outcome of his research is a rotor model which enables anticipating how the wind power unit will behave in these situations.
Lopez has patented two protection techniques, one of which has already been taken up by a turbine manufacturer. Essentially, this system allows a generating turbine to remain in operation during voltage dips and prevent stalling, and entails alterations to the way a turbine’s control systems.
The second technique requires changing elements inside the machine and continues to be developed for applications in the next-generation machines which are currently under development.
Over the past few years, the growth and development of wind energy converters has been somewhat dogged by problems associated with the sheer number of turbines now being connected to grids across Europe and elsewhere in the world.
And, according to Lopez, 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 can, for example, be caused by a lightning strike or tree falling on power cables; equally, it can be due to a slug of heavy power demand from a large consumer.
“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,” said Lopez.
“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. Worst case, there can even be burn-out if a machine is not adequately protected.
“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.
“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.”
It should be borne in mind that Spain has a lot of wind generation capacity. Indeed, there are days when wind-based energy can account for one-third of electricity production. So the problem that Lopez has been working on can be a serious one.
The basis of his work is about getting a wind turbine to behave more as a conventional power station when stressed by voltage fluctuations and not disconnecting during a voltage dip/power failure, but rather 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.
Lopez said: “Before looking for a solution, the problem has to be studied from a theoretical perspective – that is, why does this machine behave as it does when there is a voltage dip? And why, if we do not instal a protection system, the machine starts to burn out? The research produced a rotor model that was sufficiently simple to work with without having to carry out simulations. 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 a 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 are 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.”