Grid connection response of Nordex MW wind turbines equivalent to that of conventional power stations

Nordex AG has managed to raise the grid connection response of its multimegawatt turbines to a level comparable to that of conventional power stations. Simulation calculations as well as practical field testing using a 2.5 MW turbine prove that the electrical and mechanical protection systems of the turbines comply with the strict requirements of German grid operator EON-Netz and additionally safeguard Nordex’s technological leadership in the French market. The results of field testing of the entire system have also been confirmed by an independent measuring institute for the first time.

The reason for the work to achieve further enhancements in the grid response of Nordex’s multi-megawatt turbines is the stipulation by grid operators according to which wind turbines must exhibit asimilar grid response to thermal power stations. Thus, in the event of any grid errors, wind turbines must not automatically disconnect from the grid but, on the contrary, provide additional support.

To comply with these requirements, Nordex has modified a number of key components fitted to its MW turbines such as hydraulics, gen-erator, converter, coupling, azimuth system, pitch system, switch cabinet, connections as well as the wind turbine control system. Us-ing an artificially triggered short circuit in a wind park it was possible to demonstrate that the Nordex turbines feed in reactive power im-mediately – i.e. within 20 milliseconds – after the occurrence of an error in accordance with the grid connection rules, thus supporting the voltage in the public grid.

This solution is based on a sophisticated fault-ride-through (FRT) system. “In the event of grid short-circuiting, the frequency trans-former performs a defined sequence of activities permitting swift de-magnetization of the generator and then synchronizing it to the resid-ual voltage remaining after the error has occurred,” explains Thomas Schubert, head of Turbine Management and Electrical Engineering at Nordex. “This eliminates any random current and torque peaks.”

During the short circuit, the turbines remain on line for a defined pe-riod and supply reactive power to support the grid. Within 5 seconds of the error being remedied, the turbines go back to supplying 100 percent of their potential. As a result, Nordex turbines are able to meet short-circuit response requirements.

To support the voltage in the public grid, the turbine has a reactive power range of 0.90 ind. – 0.05 cap. at the clamps in steady state operations without any restrictions in output. In accordance with the grid operator’s requirements, the reactive power can be regulated at the grid connection point. Automatic reactive power supply is also possible depending on the voltage at the turbine clamps (AVR auto-matic voltage regulation).

At the same time, the turbines’ effective power response has been modified to ensure grid compatibility in a frequency range of 47.5 – 51.5 Hz. This ensures automatic voltage regulation in the event of any change in frequency. As a result, the turbines power curve can be maintained depending on frequency. Similarly, variable output regulation is possible in line with the grid operator’s stipulations.

Nordex is implementing a wind farm management (WFM) system, which actively regulates the individual turbines, to ensure that the entire wind farm can be operated as required. The WFM system measures and regulates the system, with the individual turbines pro-viding the final control. In addition, external components, such as an additional central reactive compensation unit, can be integrated in the system. Various signals can be exchanged with the grid operator to enhance wind farm management in the light of requirements con-cerning the grid connection. Accordingly, the wind farm management system enables fully automatic operations allowing effective power and reactive power to be managed according to a wide range of dif-ferent models.