Accommodating High Levels of Variable Generation - NERC

Characteristics of Power Systems & Variable Generation
wind turbine-generators also decouple the turbine-generator drive train from the electric power
grid, controlling the dynamics of the wind turbine-generator during grid disturbances. In
common with Type 3 wind turbine-generators, this decoupling means that in the standard design
inertial response can be a programmed feature during a frequency event 23 and the Type 4 wind
turbine-generators can provide comparable inertial response/ performance to a conventional
generator. The converter system also reduces dynamic stresses on drive train components when
grid disturbances occur. Finally, the output current of a Type 4 wind turbine generators can be
electronically modulated to zero; thereby limiting its short-circuiting current contribution and
reducing the short-circuit duty of standard protection equipment.
2.4.1.2. Control capabilities of wind turbine generators
Because of the rapid growth of variable generation and the resulting impacts on power system
performance, variable generation must actively participate in maintaining system reliability
along with conventional generation. In combination with advanced forecasting techniques, it is
now possible to design variable generators with the full range of performance capability which is
comparable, and in some cases superior, to conventional synchronous generators: 24
• Frequency Control and Power Management: Many modern wind turbines are capable of
pitch control, which allows their output to be modified (curtailed) in real-time by adjusting
the pitch of the turbine blades (i.e., “spilling wind” or “feathering the blades”). By throttling
back their output, wind plants are able to limit or regulate their power output to a set level or
to set rates of change by controlling the power output on individual turbines, as shown by the
multiple red traces in Figure 2.5 and 2.6. This capability can be used to limit ramp rate
and/or power output a wind generator and it can also contribute to power system frequency
control.
Turbines without pitch control cannot limit their power output in the same fashion.
However, a similar effect can be realized by shutting down some of the turbines in the wind
plant (sometimes known as a “wind farm”). Some Type 3 and Type 4 wind-turbine
generators are also capable of controlling their power output in real time in response to
variations in grid frequency using variable speed drives. This control feature could be useful
or required for islanded systems or in interconnections with high penetration scenarios when
the turbine can operate below the total available power in the wind.
23 Lalor, G., Mullane, A., and O’Malley, M.J., “Frequency Control and Wind Turbine Technologies,” IEEE
Transactions on Power Systems, Vol. 20, pp. 1903-1913, 2005.
24 Morjaria, M., Grid Friendly Wind Power Plants, European Wind Energy Conference. Brussels, Belgium, March,
2008.
Accommodating High Levels of Variable Generation 18