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v
dc
=
=
v
i
−
i
abc
v
abc
+
dq
dq
*
dc
PWM
v
i
i
v
v
abc
PI
dq
i
*
v
*
*
v
Q
+
+
i
+
+
−
ω
L
PI
+
i
*
−
+
+
−
*
Q
PI
ω
L
PI
−
+
+
i
Fig. 10.
Grid side controller system
2.9
Protection of Converter (Crowbar Protection)
High currents due to a fault close to the generator can damage the rotor side
converter. To avoid any damages, the rotor side converter is bypassed when the rotor
current exceeds a predetermined limit. To achieve this, an additional resistance
otherwise known as “crowbar” is connected to the rotor circuit. The thyristors are
turned on when the rotor current exceeds its preset value. The rotor circuits are then
short-circuited by the crowbar and it shunts away the rotor overcurrent. Fig 11 shows
a crowbar protection system where
c
r
is the additional resistance added for crowbar
protection. The rotor remains connected to the crowbar until the fault is cleared.
When crowbar protection is initiated as a result of rotor current exceeding the set
limit, then
vv0
==
, hence the DFIG operates as normal singly fed induction
dr
qr
generator.
Fig. 11.
Crowbar protection circuit
From the diagram the rotor voltage equation (21) and (22) will be modified to (48)
and (49)
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