Author Topic: Near end of life electrolytics  (Read 572 times)

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Offline jitterTopic starter

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Near end of life electrolytics
« on: May 20, 2024, 06:25:19 am »
Recently I came into the posession of a decommissioned Alstom MiCom P127 protection relay (nowadays branded as Schneider).
It had been in continuous use in a distribution station for over 20 years protecting 10 kV power lines and it was still fully operational.

Curious as to how well the BC 136 RVI (now Vishay/BC) electrolytic caps had survived, I took them off the PCB and measured capacity and dissipation factor at 100 Hz and compared them to the values in the dataheet (https://www.vishay.com/docs/28321/136rvi.pdf).
They were all still comfortably within specifications, especially dissipation factor being still way better than the specified max. E.g. one 2200 uF/10 V cap measured 0.094, specified max. is 0.21. Capacity was also higher than expected at 2.09 mF.

This didn't tell the whole story, though, as I noticed some tiny amount of residue on the PCB where these caps had been touching it. It was nowhere else to be found on the PCB and invisible when the caps were still in place.
But only under a microscope could I see that these things were actually leaking past the seal, but only on the negative terminal.

I cut the two 2200 uF/10 V caps open, one is in the attachments, and found nothing out of the ordinary. For comparison, I cut open a new cap from my stash of the same series and size (albeit 470 uF/50 V), and about the same age, and found only slightly wetter foil when unwound.
There was so little electrolyte in there that it would not have continued to leak. I wonder how degradation would have progressed if the relay had been continued to be used, but I guess the preventive maintenance didn't come a moment too soon (given the application).

« Last Edit: May 20, 2024, 02:58:47 pm by jitter »
 
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Offline Geoff-AU

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Re: Near end of life electrolytics
« Reply #1 on: May 21, 2024, 03:33:32 am »
The two aging modes for electrolytic capacitors are loss of electrolyte (capacitance goes down, and ESR/loss tangent degrade rapidly) and oxide layer degradation.  As oxide layer degrades, the capacitance actually goes up and the voltage rating goes down.  So there are competing aging processes regarding the capacitance value.

Leaking is a definite call for retirement though.  It won't be in spec for much longer :)

 

Offline jitterTopic starter

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Re: Near end of life electrolytics
« Reply #2 on: May 22, 2024, 05:13:36 am »
Thanks for your explanation. I guess at some point there will be a snowball effect starting to take place, obviously, that had not yet happened in this device.
 


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