I need to design a 3 phase pwm inverter and I am not able to determine the input dc link capacitor because I don't know the standard criterion of allowable ripple voltage.
The Input DC bus capacitance is not usually selected by ripple voltage as effects of ripple can usually be removed from the output AC by fast acting feedback control loops that controls the PWM pattern pulse-by-pulse.
The DC capacitors are commonly selected by:
1) their ac ripple current capability, which causes heating and rise of internal temperature (due to internal resistance created by imperfection), leading to failure. The ac ripple current is generated by the inverter. It will be large in 1-phase due to a double-output-frequency component being reflected back, but will be restricted to PWM switching ripples in a 3-phase inverter (as fundamental frequency ripple from output cancels out mutually, assuming balanced conditions).
2) by their energy storage capability to ride through small duration dips or even small duration interruptions in input dc supply, if so needed.
In case the input is from a rectifier, there is a need to select the capacitor such that the peak-to-peak ripple voltage on the dc input is small, typically less than 20% of the dc voltage.
The upper limit of dc-link capacitor voltage ripple is 5% of the peak value. The lesser is better.
a scientific aproach to have some funded values :
"A Review of Single-Phase Grid-Connected Inverters
for Photovoltaic Modules" IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 41, NO. 5, SEPTEMBER/OCTOBER 2005 by Soeren Baekhoej Kjaer, Member, IEEE, John K. Pedersen, Senior Member, IEEE, and Frede Blaabjerg, Fellow, IEEE
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