# Why value of DC capacitor in VSC-HVDC converter is (Vdc/2) in some cases and (2Cdc) in other cases?

Joined Apr 22, 2019
5
Why 2 capacitors are used at the output side of VSC-HVDC converter and why value of DC capacitor in VSC-HVDC converter is (Vdc/2) in some cases and (2Cdc) in other cases?

#### mvas

Joined Jun 19, 2017
538
Why 2 capacitors are used at the output side of VSC-HVDC converter and why value of DC capacitor in VSC-HVDC converter is (Vdc/2) in some cases and (2Vdc) in other cases?
Can you provide a URL Link to specific examples.
VSC-HVDC = Voltage Source Converter - High Voltage DC ?

Are you inquiring about the "Voltage Rating" of the capacitor or the actual Voltage developed across the capacitor?

I assume it has to do with the Topology of the converter
because I would never specify a capacitor with a DC Rating of ( Vdc / 2 )
if Vdc is the voltage expected across that capacitor.

Unless the capacitors are stacked in series,
and Vdc is actually the total voltage across all of the caps in series,
which can be tricky.

Your question is a little too vague and needs more detail.

MMC ( Modular Multilevel Converter ) is a newer & more efficient topology.

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• #### TechWise

Joined Aug 24, 2018
53
It's hard to say without seeing a schematic but sometimes the DC side will be shown as two capacitors with Vdc/2 across them as this provides a convenient neutral point for ease of analysis.

It's all a question of how you DEFINE the voltages. I very much doubt that one converter produces twice the output of the other.

• Joined Apr 22, 2019
5
Can you provide a URL Link to specific examples.
VSC-HVDC = Voltage Source Converter - High Voltage DC ?

Are you inquiring about the "Voltage Rating" of the capacitor or the actual Voltage developed across the capacitor?

I assume it has to do with the Topology of the converter
because I would never specify a capacitor with a DC Rating of ( Vdc / 2 )
if Vdc is the voltage expected across that capacitor.

Unless the capacitors are stacked in series,
and Vdc is actually the total voltage across all of the caps in series,
which can be tricky.

Your question is a little too vague and needs more detail.

MMC ( Modular Multilevel Converter ) is a newer & more efficient topology.
@mvas Dear Brother, Many thanks for your reply. I have attached a picture of VSC-HVDC model where 2 capacitors are installed on the output side and the value of each capacitor mentioned here is (2Cdc). I want to know why 2 capacitors are used here instead of 1 capacitor? I also want to know that how the value of each capacitor is (2Cdc) here because in case cases, the value of capacitor is (Udc/2)? Can you kindly explain this?

#### Attachments

Joined Apr 22, 2019
5
It's hard to say without seeing a schematic but sometimes the DC side will be shown as two capacitors with Vdc/2 across them as this provides a convenient neutral point for ease of analysis.

It's all a question of how you DEFINE the voltages. I very much doubt that one converter produces twice the output of the other.
@TechWise Dear Brother, Many thanks for your reply. I have attached a picture of VSC-HVDC model where 2 capacitors are installed on the output side and the value of each capacitor mentioned here is (2Cdc). I want to know why 2 capacitors are used here instead of 1 capacitor? I also want to know that how the value of each capacitor is (2Cdc) here because in case cases, the value of capacitor is (Udc/2)? Can you kindly explain this?

#### BR-549

Joined Sep 22, 2013
4,936
"I want to know why 2 capacitors are used here instead of 1 capacitor?"

Because both caps are referenced to ground. How can you do that with one cap?

" I also want to know that how the value of each capacitor is (2Cdc) here because in case cases, the value of capacitor is (Udc/2)? Can you kindly explain this?"

Not sure of you terms and context. What does 2Cdc mean? What does Udc/2 mean?

One wants the voltage rating of a cap.......to be well over any expected voltages.

#### TechWise

Joined Aug 24, 2018
53
As I said before, the two capacitors are there to provide a convenient neutral in the middle. The value of the capacitors is 2C because two of those in series gives an equivalent value of C.

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