Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 191

TSOs and DSOs, through the development of their associated grid codes, have
tended not to impose requirements that are retrospective, i.e. those wind farms
causing a critical further reduction in ancillary service provision will be required to
replace the shortfall. Increasingly, grid codes are being developed to be future-
proof, with a range of functionalities specified, although not all are active. It is
probable that different issues will become pressing at increased levels of wind
penetration, and so individual requirements will have to be implemented only when
the total capacity exceeds a defined limit. The objective, clearly, is to evolve a
power system that, from a system perspective, differs little from that today, with
only the original energy source(s) being changed. Issues relating to voltage
and reactive power support and the contribution of wind farms to power quality
management, system stability and transient performance, etc. were examined in
Chapter 4. Here, the focus is on the ability of wind turbine generators to provide
spinning reserve and continuous load-frequency control.
5.3.6.1 Power-frequency characteristic
Utility grid codes will generally specify that individual generating units must be
able to maintain continuous operation within certain frequency bounds, and main-
tain short-term operation over slightly wider frequency extremes (see Table 4.3).
Conventional generation is required to maintain 100 per cent of its real power
output within a defined band of the nominal system frequency. In Great Britain,
for example, the defined range is 49.5-50.4 Hz, while in Germany (E.ON) the
comparable boundaries are 49.5 and 50.5 Hz. This ensures that any fluctuations in
frequency (arising from generation-demand imbalance) are not exacerbated by
subsequent variation (excluding governor action) in generator output. At lower
frequencies, notably during emergency conditions, some license is normally given
for a reduction in generator output (see Section 5.1). In Great Britain, generators
are required to operate continuously between 47.5 and 52 Hz and for a period of
20 seconds between 47 and 47.5 Hz. Any reduction in generator output below
49.5 Hz should be proportional to the change in system frequency, i.e. at 47 Hz
(a 5 per cent reduction in frequency) the power output should be at least 95 per cent
of that available at 49.5 Hz. In Germany, since it is synchronously interconnected
to the rest of the European grid, the requirements are less stringent - generator
output may fall by 20 per cent of rated output when the frequency reaches 47.5 Hz.
Since all generating units in a synchronous power system will experience
essentially the same system frequency, it is extremely beneficial that all units are
capable of providing the same power-frequency response. Consequently, grid codes
do not in general differentiate between the requirements for conventional genera-
tion, as considered above, from those of renewable generation. The grid codes for
Germany (E.ON) and Great Britain represent good examples.
5.3.6.2 Frequency regulation and spinning reserve provision
Stall-regulated turbines comprise rotor blades that are aerodynamically shaped such
that the power output is naturally curtailed to a required maximum as wind speed
increases (see Section 3.4). In contrast, pitch-regulated wind turbines have the