  Modern power systems present low levels of inertia due to the growing shares
of converter-interfaced generation. Consequently, renewable energy sources are
increasingly requested to provide frequency support. In addition, due to the
inertia loss, the requirements regarding frequency containment reserves (FCR)
are becoming tough to meet with traditional units such as hydro, whose
non-minimum phase (NMP) characteristic reduces the closed-loop stability
margins. The shortcomings of traditional synchronous generation motivates new
protocols for fast frequency reserves (FFR). In this work, we design a wind
turbine (WT) model useful for FFR. It is shown that the dynamical shortcomings
of the WT, in providing steady-power or slow FCR support, are suitably
described by a first-order transfer function with a slow NMP zero. The WT model
is tested in a 5-machine representation of the Nordic synchronous grid. It is
shown that the NMP model is useful for designing a controller that coordinates
FFR from wind with slow FCR from hydro turbines. By simulating the
disconnection of a 1400 MW importing dc link in a detailed nonlinear model, it
is shown that the wind--hydro combination not only satisfies the latest
regulations, but also presents a smooth response avoiding overshoot and
secondary frequency dips during frequency recovery.
