In this research project, possible repercussions of new grid-stabilizing generator controls have been investigated on the structural dynamics and loads in the drive train and support structure of a wind turbine. The goal was to provide grid inertia from the turbine rotors. The entirety of the case studies shows that moderate scenarios demanded by grid operation are uncritical from the point of view of the mechanical turbine components. However, in order to control individual special grid faults, it may be necessary either to make significant adaptions to the mechanical turbine design to increase overload capability, or to introduce additional components such as electrical short-term storage devices, e.g. super capacitors (figure 1).
In particular, the sudden occurrence of large power deficits or surpluses can under certain circumstances lead to loads that exceed the conventional design envelope of mechanical components. This is the case in the event of a large power plant failure or the spontaneous formation of an island network due to a so-called "system split". If and to what extent turbine oversizing is necessary depends decisively on the specific fault scenarios to be controlled, that are defined by the grid operator.
Other excitation patterns from the grid, such as grid oscillations (Figure 2) or the switching of step transformers, only lead to marginal increases in extreme or fatigue loads on mechanical turbine components.
Using the methods developed in the project, the analysis of coupled oscillation modes during turbine operation showed that dynamic interaction between electrical and mechanical components does take place via the control systems (Figure 3).
However, the overall system dynamics generated in this way are not critical, and in particular, no instability phenomena occur. Only active drive train damping, if present, may require a coordinated control design of the damping and network inertia functions.
Overall, the project results support the hypothesis that wind turbines are capable of providing grid inertia using the energy stored in their rotors. This means that they can contribute significantly to maintaining the stability of the electrical grid during the transformation of the energy system. The project strengthens an important aspect of the ongoing debate among experts on this topic: It depends very much on the grid-side requirements specification whether wind turbines can provide this contribution only with changes to the power electronics and associated controls, or whether more drastic changes to the turbine design are required. If the total inertia required is divided among a large number of individual turbines according to the reserves available during normal operation, the mechanical loads can be effectively limited.