# Selecting capacitor value for two-three level inverter

#### almotions

Joined Feb 6, 2009
46
Hi all,
Assuming i'm trying to build a 3 phase 138kV 100MVA, how do i select the capacitor value if a two level is used? how about if a three-level NPC inverter is used? Thanks.

Regards.
alvin

#### Thav

Joined Oct 13, 2009
82
138kV? What devices are you trying to use? I don't think you can get silicon SCRs or GTOs that block that high voltage for two-level operation. If you're thinking of connecting devices in series and gating them together to get two level operation, you're better off I think building a multi-level converter to save on filter costs.

For clarification, are you talking about DC bus capacitors or filter capacitors?

#### almotions

Joined Feb 6, 2009
46
Yeah,i am talking about the DC bus capacitors. Actually i'm doing a simulation and the 138kV which is the line to line voltage is just example.I just want to know how do i calculate the value of the capacitors for two-three level inverters.THanks

#### Thav

Joined Oct 13, 2009
82
Do you need any holdup time if your source goes out? I'm a UPS designer so that's usually my prime consideration with our electrolytic DC bus caps. Basically if you're normally running at .8 modulation index and you lose your rectifier, the bus caps will discharge and your controller can handle it without too much output distortion until M falls to ~.95 I'd guess.

What I would do in that case is figure out the voltage difference between the modulation indexes you're comfortable with, assume a balanced full load (or maybe overload, depending on how you intend to rate the system) consider all that current coming from your bus caps and estimate the hold up time using $i_C = C \frac{\mathrm{d}v_C}{\mathrm{d}t}[\tex]. That will be an approximation, but constant power drain, as opposed to constant current drain, from a capacitor is a little more involved to calculate. EDIT: don't know why my tex is borked. I = C dV/dt$

#### almotions

Joined Feb 6, 2009
46
Does anybody has a SIMULINK model example of a two level inverter using capacitors?PLs HelP!

#### Thav

Joined Oct 13, 2009
82