3 Phase IGBT Inverter - LC Filter and Capacitor ground selection

[suri007]

Hello,

I am working on 3 phase IGBT inverter project in unbalanced condition so that i can use the Phases independently. As the Load would not be a Motor and i am using 3 phases independently to control 3 resistive loads (bulbs) I know to generate a Neutral point at the output common for three phases.


Below are my queries

1) From where should i take the Neutral point / How to generate the Neutral point if there is any pre-existing method

2) Design of LC filter - How to select it for an Output freq of 50HZ

3) The capacitor one end will be connected to Inductor, Other end of capacitor should be connected to which point?

I hope you guys can help me over come this problem


Regards,
Suresh

 

[FvM]

to 1) Either a transformer or a fourth IGBT half bridge generating the neutral point.

 

[suri007]

Hi FVM,

Can u please elaborate on your solution regarding both transformer and Fourth IGBT half bridge

Regards,
Suresh

 

[FvM]

Use wye-delta transformer as isolation transformer, delta connected to inverter, wye as output with star point (neutral).

Or connect wye parallel to inverter and use the star point as neutral. Delta winding must be present and acts as a stabilizing winding. In the latter circuit, the transformer carries only asymmetrical components, thus lower VA rating compared to isolation transformer circuit.

The star point of a three-phase inverter hasn't a neutral voltage, because a common mode system is superimposed to the three phase modulation to increase the utilisation of DC bus voltage. If you use a fourth pwm controlled half bridge to generate a artificial star point, it must be modulated with the same common mode system rather than staying at constant 50% duty cycle.

 

[suri007]

Hi FVM,

Thanks for the prompt response. Mine is a 2 level 3 phase IGBT inverter and I am using 180 degree conduction mode to switch the 3 half bridges from (S1 to S6). Now u are saying i need to include fourth ha;f bridge which would be (S7, S8) Switches. In the switching sequence what i followed based on the SVPWM modulation how do i switch S7 and S8 to generate the artificial Neutral point.

If you can give me some reference to any doc where i can get a practical view of what should be done with the fourth half bridge that would be great.

Regards,
SUresh