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SG
20 / 400 kV
Turbine
B
B
B
PCC
B
0.69 / 20 kV
20 / 400 kV
DFIG
Fig. 12. The system under study
The wind farm (WF) is made up of 40 wind turbines of 2MW, 0.69kV each
modelled as an aggregate wind turbine. It is assumed that the wind farms are located
far from the point of common connection (PCC) where the wind resources are
abundantly located as the case for most real wind farms. The WF is connected to the
PCC through two 20km line (to allow disconnection of a line) and 69/20KV
transformer. The WF is feeding a 60MW, 25MVAR local load connected to bus2
(B2). Another 100MVA, 30MVAR load is connected to the high voltage bus (B4).
The whole system is connected to a strong grid through a two 400kV, 100km
transmission lines.
4
The Simulation Results and Discussions
Different scenarios were created to get an insight into the response of WF to
disturbances from the grid. First, the response of the wind farm was studied when
there is a step change of 20% in the local load connected to B 2 at 1s. The results with
the rotor controller in place and out of place are depicted in Fig 13. From the figure it
can be observed that with the controller in place, the active power (the negative values
indicate a power injected into the grid) and the electrical torque are immediately
returned to the pre-disturbance level. The step increase in the local load resulted in a
dip in the terminal voltage and an increase in the speed of the generator; however, it
stabilizes to a new value almost immediately. This is as a result of a change in the
system configuration. With the rotor controller out, the system is stable but it takes
about 3s for the wind farm to stabilize.
Fig. 14 shows the response of dr
i , crowbar protection and the pitch angle ( β )
to a 20% step increase in local load. With this disturbance, the maximum d i current
reaches 0.71pu from the prefault value of 0.65pu. The crowbar protection was set to
operate at 1.5pu and therefore could not be inserted as the rotor current is lower than
the crowbar predetermined value. The pitch angle controller acts to limit the speed of
the generator as a result of the disturbance.
i
, qr
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