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decreases in high wind speed. This kind of generators
always consumes reactive power and is not able to control
and regulate the voltage level. Hence capacitors close to
these generators are necessary to avoid a voltage decrease.
The concept is shown in Fig. 3.
Fig. 3. Squirrel-cage induction generator system
Doubly fed induction generators are the most widely
used for units above 1 MW. Wind turbine is coupled with
doubly fed induction generator, which has a wound rotor
with the windings being externally accessible via slip rings.
The wind turbine rotor is coupled to the generator through a
gearbox in the same way of the constant speed generator.
The rotor current is regulated using power electronics,
allowing the generator to operate over a relatively large
speed range. A doubly fed induction generator’s rotor is
connected to the grid through a back-to-back voltage source
converter. The concept is shown in Fig. 4. This concept
enables to use variable speed wind turbine and to adjust its
mechanical speed to the wind speed, which allows turbine
operation at the aerodynamically optimal point for a certain
wind speed range. Variable speed operation allows higher
efficiency in generating system.
Fig. 4. Doubly fed induction generator (DFIG) system
Direct drive synchronous generator is completely
decoupled from the grid by a power electronics converter
connected to the stator winding, as shown in Fig. 5. The
converter is composed by a voltage source converter on the
grid side and a diode rectifier (or a voltage source converter)
on the generator side. The direct drive generator is excited
by an excitation winding or permanent magnets. In direct
drive synchronous generator system, turbine and generator
shafts are coupled directly, without gearbox. This allows
variable speed operation over a wide range. Generator used
in such systems is high-pole synchronous generator
designed for low speed. Due to high number of poles this
generators are quite large. The solution is found in direct
drive concept but with single stage gear box with low ratio.
Hence, the required number of poles is lower and generator
is smaller.
Fig. 5. Direct-drive synchronous generator system
E. Power System Impacts
The wind power characteristics are reflected in a
different interaction with power system [5]. There is a
distinction between local and system-wide impacts.
Local impacts occur at each turbine or farm and are
largely independent of the overall wind power penetration
level in the system as a whole. They include following
aspects:
branch flows and node voltages,
protection schemes, fault currents, and switchgear
ratings and
power quality: harmonic distortion and flicker.
The local impact is highly dependant on the electricity
generating system used within the wind power plant. For
example, squirrel-cage induction generators do not affect
node voltages since they have a fixed relation between rotor
speed, active power, reactive power, and terminal voltage.
On the other hand, variable speed turbines have, at least
theoretically, the capability of varying reactive power to
affect their terminal voltage. Also, squirrel-cage induction
generators contribute to the fault currents, while in systems
with power electronics, due to its sensitivity, this might not
be the case. Harmonic distortion are an issue when power
electronics is involved, while for constant-speed wind
turbines based on directly grid-coupled asynchronous
generators harmonics are not the issue. Flicker could be a
consequence of wind fluctuating nature, which is directly
translated into output power fluctuations in the case of
constant speed turbines, while in general, no flicker
problems occur with variable-speed turbines, because in
these turbines wind speed fluctuations are not directly
translated to output power fluctuations.
Unlike local impacts, system-wide impacts are impacts
that affect the behavior of the system as a whole. They
include following aspects:
power system dynamics and stability,
reactive power and voltage control and
frequency control and load following/dispatch of
conventional units.
Important requirement that is imposed to wind generators is
the ability to maintain stability during normally occurring
power system disturbances. Premature tripping of numerous
wind generators due to local disturbances can be a risk for
stability of whole system. It is especially the case for
systems with high concentration of wind generating
facilities. The impact on the dynamics and stability of a
power system is strongly dependant on the wind turbine -
generator concepts and every system should be analyzed by
it self in order to simulate the fault ride-through ability. For
example, during a fault, squirrel-cage generators accelerate
due to the unbalance between mechanical power extracted
from the wind and electrical power supplied to the grid.
When the voltage does not return quickly enough, the wind
turbines continue to accelerate and to consume large
amounts of reactive power. This eventually leads to voltage
and rotor-speed instability. On the other hand, sensitivity of
power electronics in variable-speed wind turbines can cause
instability in power system operation, especially those with
high wind power penetration. Thus, system operators are
now days prescribing that wind turbines must be able to
withstand voltage drops of certain magnitudes and
durations, in order to prevent the disconnection of a large
amount of wind power at a fault. In order to meet these
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