Fault Diagnostic System for Cascaded H-Bridge Multilevel Inverter ...

Current (A)
30
20
10
0
-10
-20
Ic Ib
Ia
-30
0 50 100 150 200 250 300 350 400 450
Sampling points
Figure 2.8. Line currents during open circuit faults at S1.
Figure 2.8 shows the plot of line currents during open circuit at switch S1. Obviously, the
currents in orthogonal space are related with the switch locations as shown in Figure 2.7.
The several locus curves at the same plot of each current trajectory are the current cycles
of the motor. Obviously, slope detection and Park’s average vector method may have a
problem to detect the faults when the CID is running at light load or at low modulation
index. As represented in Figure 2.7, the vector angle in orthogonal space is dependent
with the load currents; for instance, if the fault occurs at S6 during light load operation,
the vector angle of S3 and S6 is located at the same interval as depicted in Figure 2.7 (d)
and (f). Also, the switch S3 and S6 are placed in different inverter legs so that the
detection system using slope detection and Park’s average vector method would be
incorrectly detected.
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