Author Topic: MMC capacitor crash  (Read 1166 times)

Offline Mads Barnkob

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Re: MMC capacitor crash
« Reply #20 on: September 16, 2022, 08:32:01 PM »
SDS00004.png shows a normal classic voltage transient inverter situation where current ramps too high and the DC bus is pulled down, which then limits current and DC bus crawls back up again as the MMC from from empty (a short circuit to the inverter) to less load and current falls. As Dave says, this is a sign of no energy transfer.

random example from the Internet(tm)


However, the latest SDS00155.jpg shows that you have a ringup of about 200 uS, 200 us additional where it falls abit and goes into steady state for 600+ us. The steady state shows a continues energy transfer to the secondary circuit, energy fed into the inverter goes all the way to the spark. It does not help the spark growth, it just keeps it on for a longer time. Your spark growth is all in the ringup.
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Online davekni

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Re: MMC capacitor crash
« Reply #21 on: September 16, 2022, 09:04:59 PM »
Quote
here generator input feedback f = 153khz
As Mads has pointed out in other posts, running open-loop can lead to IGBT failures due to excess hard-switching (IGBTs switching at high current parts of the waveform).  Certainly keep Vbus low as you have been for such testing.

I'm guessing that SDS00153.jpg is slightly below primary resonant frequency.  Running open-loop (from signal generator) below resonance causes phase lag.  That is the most risky (for IGBT failure) condition where turn-on is after current reverses.  Resulting switching spikes are clipped in scope capture, presumably because your differential probe is still set for 50x rather than 500x.

Quote
why this change in length ? instability ? vbus too low,
All three traces show operation longer than 200us.  If your interrupter is set for 200us, either your interrupter or driver board is not functioning correctly.  I suggest finding and fixing that issue first.  If pulses are lasting much longer than interrupter is requesting, that could have been the initial cause of MMC crash, much higher duty cycles than you intended.

Quote
SDS00004.png shows a normal classic voltage transient inverter situation where current ramps too high and the DC bus is pulled down, which then limits current and DC bus crawls back up again as the MMC from from empty (a short circuit to the inverter) to less load and current falls.
With respect for Mads' experience, I think this isn't the case here.  Inductance in the differential probe connection to H-bridge output confuses the image.  The individual H-bridge output steps can still be seen, and decrease in amplitude only slightly across SDS00004.png capture.  Also, primary current rise rate decreases only slightly, indicating Vbus is still close to its initial value.  Your scope captures will be easier to understand if differential probe connection can be improved.  Can you post a picture of probe connection including probe leads?

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Offline Mads Barnkob

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Re: MMC capacitor crash
« Reply #22 on: September 16, 2022, 09:48:56 PM »
Quote
SDS00004.png shows a normal classic voltage transient inverter situation where current ramps too high and the DC bus is pulled down, which then limits current and DC bus crawls back up again as the MMC from from empty (a short circuit to the inverter) to less load and current falls.
With respect for Mads' experience, I think this isn't the case here.  Inductance in the differential probe connection to H-bridge output confuses the image.  The individual H-bridge output steps can still be seen, and decrease in amplitude only slightly across SDS00004.png capture.  Also, primary current rise rate decreases only slightly, indicating Vbus is still close to its initial value.  Your scope captures will be easier to understand if differential probe connection can be improved.  Can you post a picture of probe connection including probe leads?

It was not as based on my own experience, as it was more based on similar waveforms in inverters.

I will stand corrected if that is the case :) I have never seen so bad measurements with a differential probe, but then again, I assumed the measurement here is not to blame. But definately worth checking up upon with some pictures.

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Offline JCF

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Re: MMC capacitor crash
« Reply #23 on: September 18, 2022, 05:22:31 PM »
Merci pour l'aide !
here are 2 photos that show the box with primary and details of HV probes connected to the bridge output.


with toroid on feedback I see f=  151.6khz is it  necessary to adjust the frequency of the primary to 153khz. 153khz is maximum spark lenght ? ?
the instability came from the contact on the aluminum crown : it is ok now.
sense for current is 1:300 on 100 ohms.
input fiber optic signal 200┬ÁS and 10hz sqare

it seems to be working fine ?




« Last Edit: September 18, 2022, 05:43:05 PM by JCF »

Online davekni

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Re: MMC capacitor crash
« Reply #24 on: September 19, 2022, 12:09:22 AM »
Quote
here are 2 photos that show the box with primary and details of HV probes connected to the bridge output.
The unusual H-bridge output scope traces are due to inductance of wires from IGBTs to front edge of the black box.  If it isn't too hard, run a second pair of wires (may be smaller wire) from IGBTs (H-bridge output) to the back side of the black box.  Connect scope probe to those new wires instead of to wires connected to MMC and primary coil.  That will avoid most wiring inductance that is common to both primary coil path and scope path.  Scope traces will look more like square waves.  Any subtle issues will be easier to see and correctly interpret that way.

Quote
with toroid on feedback I see f=  151.6khz is it  necessary to adjust the frequency of the primary to 153khz. 153khz is maximum spark lenght ? ?
By "toroid", I'm presuming you refer to the feedback current transformer, not the top-load aluminum toroid.  If UD2.7 phase-lead is adjusted optimally, I'd expect it to oscillate at 153kHz based on previous open-loop (signal generator driven) scope traces.  However, that 151.6kHz to 153kHz is a tiny difference, not that significant.  For maximum spark length, primary frequency needs to be a little below secondary frequency.  Enough below so that it matches secondary frequency as secondary frequency drops due to spark loading (spark capacitance).  JaveTC is a good way to model primary and secondary frequencies (and coupling factor etc.).  Include wire length in JavaTC input since your primary wire length is significant.  (In general, I'd recommend pairing primary leads closer together to reduce wire inductance.  That will increase primary frequency slightly, so might require adjusting primary coil to compensate.)

Quote
it seems to be working fine ?
Looks fine in that pulses last 200us as intended.  Still don't see as much evidence of power transfer to secondary as shows up in the one old scope capture SDS000004.  That makes me suspect secondary frequency isn't close enough to 153kHz in this test.

Quote
Merci pour l'aide !
You are certainly welcome!  (Google translate worked well on this short phrase.)

Good luck!
David Knierim

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Re: MMC capacitor crash
« Reply #24 on: September 19, 2022, 12:09:22 AM »

 


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