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

Table 2.2. Location of the faulty switch
Switch locations Fault indicator Condition
S1 | θ f |< 30 ind
o
S2 30 o < θ f < 90 ind
o
S3 -150 o < θ f < -90 ind
o
S4 | find
36
θ |> 150 o
S5 90 o < θ f < 150 ind
o
S6
> 0.5
-90 o < θ f < -30 ind
o
The advantage of this approach is that fault detection algorithm can be integrated in the
control unit with a single digital signal processor (DSP), and also the extra measurements
are not required. However, the subroutine software of the DSP must be modified, and this
might degrade the execution speed of the main software in the DSP.
2.3.3 Normalized dc current approach
Abramik [21] proposed a diagnostic method for a CID, namely the normalized dc current
method. The concept of the method is to use normalized dc current instead of the currents
transformed to orthogonal space. The normalized dc current is the ratio between the dc
component and the fundamental component. The fault observer is based upon the
relationship of normalized dc current in each phase. To illustrate this, for instance, the
current spectrums (ia and ib) of the open circuit fault at switch S1 are shown in Figure 2.9.