Inverter for 3-phase fully controlled Thyristor converter

[cnh1995]

I am helping a group of my friends for writing microcontroller code for their final year project in EE. They are building a small model of HVDC system (fed from 3 phase supply), with microcontroller based converter and inverter.

We are done with the coding for firing of the SCRs in the converter (it is line commutated SCR converter). Now, for the inverter stage, which one should we use? VSI or CSI? I know that the line commutated converter is also called as 'current source converter'. Does that mean we should use a CSI? What is done in practice?

 

[Baluncore]

You rectify the 3PH to DC with SCRs. The DC output voltage will vary between half Vpk and Vpk as it follows the AC supply phase voltages. You draw as much current as you want whenever you want from the low impedance supply. It does not matter if you have a big reservoir capacitor. It will appear as a voltage source, so you will use a VSI.

 

[cnh1995]

You rectify the 3PH to DC with SCRs. The DC output voltage will vary between half Vpk and Vpk as it follows the AC supply phase voltages. You draw as much current as you want whenever you want from the low impedance supply. It does not matter if you have a big reservoir capacitor. It will appear as a voltage source, so you will use a VSI.

Thanks for the reply.

The dc voltage will vary but we are adding one inductor in the dc line to minimize the ripple in the current. Does this mean that the dc input voltage to the inverter is almost constant irrespective of the load across the inverter output?
Why is it that we don't need a capacitor?

 

[Baluncore]

They are building a small model of HVDC system

You probably know what you mean but there are many other interpretations.
What precisely do you mean by "HVDC system" ?
Is it single or full wave rectification? Why do you need to control the rectification? What strategy do you use to decide when to switch the rectification?
Without a circuit or block diagram it is hard to tell what you are doing.
If you plot the three phases with 120° separation, you will see that the rectified voltage never falls below ½ Vpk. So it would seem that there is no need for a big storage capacitor as used in a single phase supply. You will need a small capacitor, (with high ripple current rating), to keep HF switching noise from the inverter out of the SCRs.

A series inductor in the DC will reduce the ripple voltage, but the average voltage will remain the same or slightly less. A big inductor might make the rectifier look like a current source.