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 by a tree falling on power cables. It can also be due to a large company consuming a lot of energy all at once. 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 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 re-initiated.
Lopez said that in the normal operation of wind turbine, the flux in the stator rotates synchronously, i.e. at the grid frequency. As the rotor turns near this speed, the voltage induced by this flux is small. The sudden dips on the grid cause the appearance of a new flux in the stator, which has been named the “natural flux.” This second flux, as opposed to the normal flux, is fixed to the stator, that is, it doesn’t rotate. Therefore, its relative speed in respect to the rotor is much larger and it induces voltages in the rotor much greater that those corresponding to the normal operation.
Usually, the electronic converter connected to the rotor is not able to overcome to theses voltages and the converter, as a consequence, loses the control of the currents. In this situation, there appear overcurrents that can damage, depending on the depth of the dip, the converter.
To date, one system has been in place to protect converters, however it’s not an optimal solution.
“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,” López said.
“For example, if a large company suddenly consumes a lot of current, the voltage drops. This causes the wind power units at El Perdon [in Navarre, Spain] 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 fully one-third of electricity production, the problem can prove to be a serious one.
Engineers are tackling the problem by trying to find a way that the generator will behave more like a conventional power plant and not disconnect during a voltage dip/power failure but rather help to bring the grid voltage back up.
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 that 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,” said Lopez.
Once this model was developed, Lopez says that it was more or less easy to propose solutions. “The most important thing is that we have achieved solutions that enhance the behavior of the machine without any need to change anything, except the control. It’s like changing the version of a text treatment program 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 behavior of the machine.”
In his PhD thesis, López proposed two different systems of protection and both have been patented. The first, which only requires 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.