I am trying to generate switching pulses for three level NPC converter using space vector modulation technique.
While referring the literature for space vector technique I came across two situations,
In first case the sector of voltage vector is found and then the triangle number inside the the sector is calculated.Once the triangle number(among 4 triangles in each sector) is calculated then switching sequence is decided.In some literature for all the triangles seven switching states are used using nearest three vectors while in some literature among 4 triangles in each sector 13 switching states are used for one triangle,9 switching states for other triangle and seven switching states for the remaining two.
Why is there a difference in number of switching states?Does the asymmetric switching has any advantage over symmetric switching in case of space vector technique?
generally speaking, the symmetric switching can reduce the number of switching times and thus reducing the switching losses.
Even though symmetrical switching having reduced switching items, it has the drawback of achieving the maximum output voltage. Switching items depends on the voltage level and switching frequency.
There can be redundant states, which can allow you to decrease the common mode voltage and the the switching sequence will give you different options in switching the devices. The number of times a the switch turns on or off depends on sequence. The switching sequence can be asymmetrical or symmetrical. So, you can select based on your application. One will decrease the switching losses possibly.
Cheers!
It depends highly on the application. You have a lot of redundant states and depending on the switching states you use in each sampling time you can get:
-Reduced switching losses.
-Lower harmonic injection.
-Reduced leakage ground current.
-Capacitor balancing.
-Temperature balancing (losses distribution)
The problem is that you should select which of them is your objective, because usually you improve one and the others become worse.
Then, the election of the switching sequence, number of switching states and symmetric or asymmetric switching is to fulfil your initial design objective.
It is difficult to answer your question without knowing the application of it, (for the 13, 9 and 7 segment switching sequence.) please provide that specific reference. Normally we use 7 segment or 5 segment (discontinuous) switching sequence design. There are a lot of papers on balancing the capacitors, distribute the switching events equally among the switching semiconductors as well as reduction of common mode voltages, in those cases the traditional switching sequence designs are altered.
You may be interested in reading this very recent survey on modulation techniques.
Article The Essential Role and the Continuous Evolution of Modulatio...
I used 7 and 5 segments. Both can do the job. It depends on how fast is your controller to handle the requested vectors. I suggest you start with 5 segments. Also I can suggest to use dwell time control if it is not required to have fast action for capacitor balancing. Use the dwell time control of you select proper patterns that itself balance the capacitors.
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