Wind Power in Power Systems

630 Impacts on Power System Dynamics
This chapter discusses the impact of wind power on power system dynamics. First, we
present the concepts of power system dynamics and of transient and small signal
stability, as well as the most important currently used wind turbine types. Then, we will
study the impact of these wind turbine types and of various wind farm interconnection
schemes on the transient stability of the power system. For this, we will analyse the
response to disturbances, such as voltage and frequency changes. Finally, we will deal
with the impact of wind power on the small signal stability of power systems and use
eigenvalue analysis for this purpose.
28.2 Power System Dynamics
Power system dynamics investigates how a power system responds to disturbances that
change the system’s operating point. Examples of such disturbances are frequency
changes because a generator trips or a load is switched in or disconnected; voltage
drops due to a fault; changes in prime mover mechanical power or exciter voltage, and
so on. A disturbance triggers a response in the power system, which means that various
properties of the power system, such as node voltages, branch currents, machine speeds
and so on, start to change. The power system is considered stable if the system reaches a
new steady state and all generators and loads that were connected to the system before
the disturbance are still connected. The original power system is considered unstable if,
in the new steady state, loads or generators are disconnected.
Two remarks must be made at this point. First, when a system is stable, the new
steady state can either be identical to or different from the steady state in which the
system resided before the disturbance occurred. This depends on the type of disturbance,
the topology of the system and the controllers of the generators. Second, that a
power system is unstable does not necessarily mean that a disturbance leads to a
complete blackout of the system. Rather, the system’s topology is changed by protection
devices that disconnect branches, loads and/or generators during the transient phenomenon,
in order to protect these. In most cases the changed system will be able to reach a
new steady state, thus preventing a complete blackout. However, although the ‘new’
system that results after the changes is stable, the ‘old’ system was unstable and stability
has been regained by changing the system’s topology.
There are two different methods to investigate the dynamics of a power system in
order to determine whether the system is stable or not. The first method is time domain
analysis. The type of time domain analysis that we use here is also referred to as
dynamics simulation, fundamental frequency simulation or electromechanical transient
simulation (see also Chapter 24). This approach subjects the system to a disturbance
after which its response (i.e. the quantitative evolution of the system’s properties over
time) is simulated. In this way, it can be decided whether the system is stable or not. In
the case of instability, strategies can be designed to change the system’s topology in such
a way that stability is regained with minimum consequences to loads and generators.
The second method is frequency domain analysis, also referred to as an analysis of the
small signal properties of the system or as eigenvalue analysis. Frequency domain
analysis studies a linearised representation of the power system in a certain state. The
linearised representation makes it possible to draw conclusions as to how the power
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