Hello,
I have a question on PWM Technique for Power Electronics Inverter.
For 3-level inverter and sine PWM, modulation index is evaluated by
m=(V(line-lineRMS)/sqrt(3))*sqrt(2))/(Vdc/2)
Now, when I change the technique to Third Harmonic Injection PWM (THIPWM), the DC bus utilization increased by 15.5% which is evident.
Can anyone tell, how the equation to find modulation index will change in THIPWM? How one can calculate analytically the modulation index in THIPWM technique?
as a simple logic, for THIPWM, I reduce modulation index by 15.5% of value which I obtained for SPWM. I am not sure is it the engineering way or not.
Regards
Aalok
Hello Aalok,
Some references explain the analysis of PWM overmodulation strategies for VSI (voltage source inverters) such as:
A. M. Hava, R. J. Kerkman and T. A. Lipo, "Carrier-based PWM-VSI overmodulation strategies: analysis, comparison, and design," in IEEE Transactions on Power Electronics, vol. 13, no. 4, pp. 674-689, Jul 1998.
D. G. Holmes and T. A. Lipo, "Pulse Width Modulation for Power Converters-Principle and Practice," IEEE Press/Wiley-Interscience, New York, 2003.
Bin Wu, "High-Power Converters and AC Drives," IEEE Press/Wiley-Interscience, 2006.
PS: THIPWM is not an overmodulation strategy because its modulation index still lower than unity, but it increases the fundamental line-to-line voltage amplitude [Bin Wu, page 99, 2006]
The calculation can be made by using interactions, (A. M. Hava’s paper) which uses the Fourier analysis. This paper can be very helpful.
Another way is presented in D.G. Holmes’s book, pages 226-229, where he calculates the optimum injection level through an analytical way and obtains the modulation index of THIPWM equal to 1.155 (or Mi = 2/sqrt(3)).
The last way is calculating through space vector, plotting the space vector diagram, calculating the dwell times and modulation index (see pages 99-107 in Bin Wu's book). This produces almost the same results in 3rd harmonic injection PWM.
Best regards,
Davi
This is an optimization problem that can be solved as follows:
cost function is ma*cos(60wt)+0.15ma*cos(180wt)-1
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