Weak Link? Wind Turbine Step-Up Transformers

The key wind turbine generator (WTG) step-up transformer design issues that wind farm owners and developers should pay attention to include transformer loading, harmonics and non-sinusoidal loads, transformer sizing and voltage variation and special requirements to withstand faults.  The role of the WTG step-up transformers is critical and its design must be carefully and thoughtfully analyzed and reevaluated.

Wind turbines are highly dependent upon local wind and other climatic conditions, and their yearly average load factors can be as low as 35 percent.

Harmonics and Non-Sinusoidal Loads

WTG transformers are switched with solid state controls to limit the inrush currents. While potentially aiding in the initial energization, these same electronic controls contribute damaging harmonic voltages that, when coupled with the non-sinusoidal wave forms from the turbines, cannot be ignored from a heating point of view.

Normal voltage is alternating at 60 cycles per second. If the transformer operates at other voltages, the voltage peaks will not line up and you will not get the amplification you would achieve when frequencies line up. The transformer tries to pass the voltage it sees through the circuit and causes extra loading. All the electronics used today send spikes on the line and each time a frequency disturbance goes back to the transformer, the transformer must be able to handle the higher loading it sees.

When a rectifier/chopper system (the electronic controller used in wind turbines) is used, the WTG transformer must be designed for harmonics similar to rectifier transformers. These are "dirty" from a harmonics point of view, meaning they may contain high frequencies that the wind farm owner does not want to pass onto the utility power's grid, because it will affect other equipment.

If this happens, it can result in a protective equipment fault, causing transmission grid equipment to protect itself against faults by shutting down. The WTG must be able to take the additional loading into consideration and provide electrostatic shields to prevent the transfer of harmonic frequencies between the primary and secondary windings. It must be able to handle the energy and not transmit it to the grid.

Cost Risk

When purchasing a WTG step-up transformer, make sure you factor in how reliability affects the total cost of ownership. Wind farm turbines are unique because they are ganged together and dependent upon each other. For example, say one transformer fails, with a loss of revenue of about $1,000 per day. It may take two or even three days to replace the transformer, and in the meantime, the faulty generator may take down 10 to 14 other generators, not allowing them to produce. So that loss of $1,000 of revenue could turn into $30,000 in lost revenue, plus the cost of another transformer, construction labor and crane expenses. (Power Engineering, April 2012)