Operation and Design of Multilevel Inverters Dr ... - MotorLab.com

3.4.3 Reactor current sharing
For the parallel topology shown in Figure 3.2-8, the redundant states can be used to
ensure current sharing in the reactor. This application is an example of per-phase
RSS where the control has the structure shown in Figure 3.4-4. The digital flag
input for the a-phase is
I
aLR
⎧1
= ⎨
⎩0
Copyright 2005. K.A. Corzine 44
i
i
asL
asL
> i
≤ i
asR
asR
(3.4-4)
Similar current sharing flags are defined for the b- and c-phase. In this topology, the
lowest s a = 0 and highest s a = 2 switching states do not affect the reactor current
balance. This is fortunate, since there is no redundancy available for these states.
The middle state affects the reactor current sharing, but has redundancy according to
Table 3.2-6. The redundant choice to be placed in the RSS table is straightforward
according to the discussion in the previous section about the inter-phase reactor.
*
Specifically, the transistors can be set to T aR = I aLR and TaL = 1 − I aLR when s a = 1.
Figure 3.4-5 shows the three-level inverter simulated performance based on the RSS
rules described above. The dc source voltage was 660 V and the inverter was
controlled with voltage-source modulation using a fundamental frequency of 100 Hz,
a switching frequency of 10 kHz, and a modulation index of m = 1.
13 . The load
was a 20 kW induction motor operating at rated power. As can be seen, the left and
right side of the reactor share the phase current equally. At one point in the study,
the RSS is turned off and only one of the redundant states is used. As can be seen,
the reactor currents become unbalanced until the RSS is turned on again. Another
method of switching this type of inverter is to alternate back and forth between the
redundant states. In this case, the balance will be decent, but there will be some drift
from the perfect current sharing case. Therefore, an active RSS control is
recommended [7].
3.4.4 DC source current control
For the cascaded H-bridge inverter, it is reasonable to utilize the redundant states to
ensure that the power from the dc sources is equalized. This goal can be realized
through per-phase RSS. The two-cell topology shown in Figure 3.2-7 with equal dc
source voltages can operate with five voltage levels [22]. In this case, the per-phase
redundant states can be used to ensure that each cell supplies the same average
current [19].