Can any other harmonic (other than 3rd) be used for increasing dc bus utilization ? If so, what is the relative advantage or disadvantage ?
Other harmonic like 6th can be used to increase the fundamental voltage at output of inverter, too. Certainly the maximum magnitude at this case should be: dcos(π/12) ≤1; Therefore, its effect is a little inferior to the 3rd harmonic which is: dcos(π/6) ≤1. If you use other higher harmonics, their effect will be more inferior to 3rd harmonic although they can still increase the fundamental voltage output of the inverter.
Thanks, but 6th is an even harmonic. Any reference to published work avaiiable ?
For example, 9th harmonic, the maximum magnitude at this case should be:
dcos(π/18) ≤1. The fundamental voltage can still be increased a little compared with Sine-Triangle modulation.
9th is understandable since it is an odd harmonic and triple--n. But 6th is even, which will cause asymmetric distortion of the modulating waveform. Can it not result also in 6th harmonic current flow in 3-phase system ?
I agree with your opinion. In order to improve the magnitude of the fundamental voltage and keep the waveform quality, triple-n odd harmonics can be used to realize this requirement.
When you add the 3d harmonic, the voltage from one pahse to another is absolutely sinusoidal, without harmònics added. This is the nice thing of 3d harmònic injection. You could only see the 3d harmònic if you measure between one phase and a neutral, if it exists.
Yes and no! You can add any harmonic or a set of harmonics components in such a way the reference signal becomes flat or square. However, the advantage of the utilization of the third harmonic or its multiple components remains on the fact that third harmonic generated in one phase of the inverter will be cancelled by the third harmonic generated in the other due to its zero sequence characteristic. The third harmonic included in the reference signals for the generation of the pwm pattern are easily canceled by Delta-Wye of the windings of a transformer or motor.
I would be interested in why you would want to do this. Can't you just increase the set point of the inverter? You can't increase the output voltage of an inverter connected to the grid, the grid has a far lower input impedance than the inverter has an output impedance. If you want to consider the stand alone case, be careful of adding harmonics into the inverter output. Most loads might not like them, although by adding odd harmonics you can increase the delivered power (you're getting closer to a square wave which delivers maximum power for a particular voltage). In particular the third harmonic is very troublesome if you are driving induction motors. Get the phase and amplitudes wrong and the rotors can spin backwards at 3 times normal speed. Not a desirable condition!
If you are simply aiming to increase the inverter efficiency, there are many good articles on this (lower impedance and faster switching semiconductors, larger gauge wiring in inductors and transformers etc).
I am using the inverter for single phase SPV applications. The modulations are to find an alternative method to get the highest fundamental voltage at the output. Harmonics may be blocked by a series connected resonant filter.
I think its is worth you read the enclosed paper, which shows that no voltage improvement can be obtained with 3d order harmonics higher than the 3d.
I can add that is has been shown that high frequency harmonics and losses are lower than using pure sinusoidal modulation. I staarted to use 3d harmònic injection for AC motor drives in 1986.
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