Influence of grid support functionalities on the mechanical loads and the energy yield of wind turbines

As the share of inverter connected generation increases in power systems, grid operators worldwide
increase their demand for system services by wind turbines. One of these services, grid frequency
control, is very challenging for wind turbines, as it requires a change of the active power of the wind
turbine. Furthermore, wind turbines are curtailed when the grid capacity is not sufficient to transport
the produced power: the so-called feed-in management. Hence, the control of the wind turbines has
to cope with the needs of the grid. Thus, the wind turbines are exposed to an additional external
excitation in addition to the prevailing wind conditions.
In this thesis, the influence of the providing these services on the wind turbine is analysed. The analysis
is based on time domain simulations of the wind turbine. The models were partly developed in this
project. Additionally, a focus is laid on analysing data to derive realistic scenarios of the grid signals. As
some of these signals were not available, several measurement had to be set up and maintained during
the project.
In a research project with the wind turbine manufacturer Suzlon Energy, a controller for providing
inertial response continuously was developed. In contrast to the state-of-the-art, the magnitude of the
inertial response is scaled with the operating point of the wind turbine in order to provide this service
reliably and at the same time to use the full potential of the wind turbine in strong wind conditions.
This controller is tested with grid frequency scenarios from Europe and India to identify the
consequences for the energy yield and the mechanical loads of the wind turbine. It is shown, that
neither the energy yield nor the loads are affected significantly.
During the research project, a grid islanding and eventually a blackout occurred in Flensburg. The local
grid operator provided data of the grid situation for the day of the blackout. This data allowed to
analyse the cause of the blackout and to develop fictive scenarios how wind turbines could have helped
to stabilize the islanded grid. In the analysed scenarios it was shown, that wind turbines equipped with
an inertial response and a fast frequency response controller could have stabilized the islanded grid
making a blackout less likely. However, the wind turbines were also at risk of running in overspeed, as
they had to reduce their power drastically when stabilizing the grid.
As a consequence of the overspeed problems, a feedforward loop for the pitch control was added to
the grid frequency support controller. It modifies the pitch angle signal from the feedback controller
by a signal depending on the pitch sensitivity and the magnitude of the power change for grid
frequency support. The proposed controller achieved to reduce the overspeed significantly.
In addition to the grid frequency support, a controller for continuous feed-in management was
developed at the institute. It was tested with a fictive case of a weak grid connection, which was
designed based on measurements at the campus in Flensburg. In comparison to the state-of-the-art
feed-in management, the controller allowed a higher energy yield of the wind turbine and a better
utilization of the grid components. The loads of the wind turbine were only slightly increased. Finally,
it is also shown, that feed-in management and inertial response can be provided by a wind turbine at
the same time, as the excitations and the wind turbine response are largely decoupled in the frequency
domain.
It is concluded that wind turbines can provide system services to a larger extend than today without
suffering from unduly harm for its mechanical loads and its energy yield.