Direct Power and Torque Control of AC/DC/AC Converter-Fed ...

4. Direct Power and Torque Control with Space Vector Modulation – DPTC-SVM
G
Ao1
() s
ΩmsTIU
= −
M( s )
( 1+
sT )
U
(4. 28)
4.2.2.2. Transfer Function of the AC/DC/AC Converter-Fed IM Drive with DClink
Voltage Feedback and Active Power Feedforward Calculated Based on
Mechanical Speed, Commanded Torque, and Power Losses – PF
This case takes into consideration PF control loop which uses power of the VSI
calculated based on mechanical speed, commanded torque, and power losses as
follows:
3
P
PFΩ = M
ecΩ
m
+ Plosses
= M
ecΩ
m
+
FVSI S r Sx
+
Sy
(4. 29)
2
2 2
( R + R + R )( I I )
There is a time delay between calculated power and actual power of the VSI,
defined by Eq. (4.18). Such defined system has been shown in Fig. 4. 2. The transfer
function of the system can be defined as:
G
Ao2
() s
2
( 1+
sTU
) s ( −T2
−TIT
+ TIF
− sT2TIT
)
( 1+
sT )( 1+
sT ) M( s )
mTIU
= Ω (4. 30)
IF
If the delay provided by VSR and VSI are equal i.e., T = T yields:
G
Ao2
() s
2
IU
( 1+
sTU
) s ( −T2
− sT2TIT
)
( 1+
sT )( 1+
sT ) M( s )
2
mT
= Ω (4. 31)
IF
2
IT
IF
Ω
4.2.2.3. Transfer Function of the AC/DC/AC converter-Fed IM Drive with DC-link
Voltage Feedback and Active Power Feedforward Calculated From
Commanded Stator Voltage and Actual Stator Current - PF
The active power of the VSI can be calculated from commanded values of the
stator voltages and actual stator current –
between
VSIc
PF
UI
UI
. In such a case there is smaller delay
P and P
2
. For simplification it can be assumed that T = 2
0 . The
VSIc
modified model for this case is shown in Fig. 4. 4. Hence, the transfer function can
be defined as:
G
Ao3
() s
ΩmT
=
IU
2
( 1+
sTU
) s ( −TIT
+ TIF
)
( 1+
sT ) M( s )
IF
(4. 32)
Assuming the case, when T = T . In such situation right hand side of Eq. (4.32)
IT
IF
should become 0. Hence, theoretically the DC-link voltage should not be affected by
the change of the load P INV
power. However, in the real system the unbalanced
power difference that makes the fluctuations of the DC-link voltage occurs mainly
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