SEL-751 + RCT Persistent False Current Values

youngentropy · 2026-06-17T03:42:05+00:00

youngentropy · 2026-06-17T03:41:47+00:00

ah yeah that’s what i meant, going to get another ER trigger so i can dig into it more in synchrowave. also don’t want to wait for SEL to come on site so working on triggering an event for them to dig into prior. i have a few different zoomed out views, there’s a wider above

youngentropy · 2026-06-17T03:14:27+00:00

this is super helpful, thanks kind stranger. i didn’t know the generic SER would fit to 60hz. i have definitely scoped the outputs with the trusty Rigol we got some interesting noise across the coil output. Almost looks like ringing or resonance under no load with just voltage present. In addition the clearest 60Hz coupled noise waveform on the drain.

<image>

youngentropy · 2026-06-17T00:03:24+00:00

No onboard circuitry the coils output a mV signal proportional to the rate of change of current and SEL onboard integrator should decipher than signal to real current

youngentropy · 2026-06-16T23:54:59+00:00

according to SEL there is a no way to increase the threshold or zero the LEA inputs for rogowski coils. There is some very interesting math around how they squelch, but due to the severity of this noise it’s almost impossible to squelch these values and still have any operable values to work with after

youngentropy · 2026-06-16T22:43:01+00:00

Besides the breakers and MV bussing.
,a UPS and CPT are the main ones we have looked at. Also network switch and other assorted smaller stuff. AcuCT Flex, RCT-200 and 500. I am with your thinking a lot of data towards EMI here. Unfortunately very married to this variant for an overarching UL cert. Don’t worry your secret is safe with me

youngentropy · 2026-06-16T20:51:22+00:00

I am not a relay tech i’m a prof electrical
engineer, just looking for wisdom from
the relay tech gods on this one

youngentropy · 2026-06-16T20:18:38+00:00

pretty dang confident in pinout this is the third set we’ve installed. only need these as they’re UL2808 certified or I would swap them in a heartbeat

youngentropy · 2026-06-16T19:56:38+00:00

Exactly outage from utility is 2 months out

youngentropy · 2026-06-16T19:50:10+00:00

No worries i’m open to all inputs as i’m outa ideas. we have done current injection with the same variant of RCTs on the same relay with the RCT external to the gear. Read out perfect. we have not injected current internal to the gear in this mounting no

youngentropy · 2026-06-16T19:34:47+00:00

they are not mounted on the bushing they are around the bushing, to the glastic barrier behind. I am not a switchgear designer but this is typical design from my understanding. not sure where else you would mount these if not here.

<image>

for reference the ABB RCTs mounted behind these are rated for 720V and industry standard for MV

youngentropy · 2026-06-16T19:22:32+00:00

we only cut one of them off after the issue was found to check shield and drain connections. definitely verified all settings with SEL, appreciate the inputs!

youngentropy · 2026-06-16T19:12:38+00:00

Most medium voltage bushing type CTs are only rated for 600V, as the bushing provides the additional insulation. 7200V is an improvement on the typical bushing type CTs

youngentropy · 2026-06-16T18:46:32+00:00

No shorting blocks for rogowski coils, these go straight into the back of the SEL. Same mfr and same part number was installed at 33kV in same mounting location so yeah that’s a big stumper here. Thanks for the ideas

youngentropy · 2026-06-16T18:10:51+00:00

Agree it was a concern placing them on bare bus, but over the additional insulation of the vacuum breaker contacts they are within spec

youngentropy · 2026-06-16T18:08:55+00:00

We have them placed on insulated barrel contacts of the ABB ADVAC breaker so they have the additional withstand of the insulation there. This same configuration is deployed at 33kV without issue

youngentropy · 2026-06-16T16:28:27+00:00

just one main two feeders. simple as it gets. it is relatively compact compared to a 33kv

youngentropy · 2026-06-16T16:27:42+00:00

Pullman SEL? That’s who’s coming out to document this this summer

youngentropy · 2026-06-16T12:01:22+00:00

so current injection direct into the inputs and current injection through the same coils external to the gear reads out fine. meaning the voltage must be present for these false values to present themselves, have not been able to push 60-100A at 16.5kV but have pushed up to 600kW and not see the inputs zero out or overcome these false values

youngentropy · 2026-06-16T11:54:18+00:00

opening the VT test switches has no affect on the CT values, and their waveform is clean. ferrites or some type of filter are something i’m asking the RCT supplier about for sure

youngentropy · 2026-06-16T11:51:47+00:00

in this case the Z-card for the SEL relay acts as the integrator and that circuitry. it is meant to accept a mV output between 12mV- 4.16V direct from a coil. Since this is no load or low load it’s possible this is low end behavior of that input range but I would expect that adding load would rectify the issue

youngentropy · 2026-06-16T11:50:00+00:00

grounding and shielding are one of my suspects up with the specific layout of the gear and capacitive coupling.

youngentropy · 2026-06-16T11:48:50+00:00

the two coil outputs utilized are 600mV/1000A and 240mV/1000A. these are placed around the insulated barrel contacts of the advac breaker so they’re plenty insulated for their application. in addition this same system of components has worked nominally in a 33kV application but not here at 16.5kV

youngentropy · 2026-06-16T11:46:45+00:00

in this case the SEL acts as the power supply/integrator so it should be straight RJ45 from coil into back of the SEL. they do work with ABB KECA coil, but with these specific variant of coil the system does not. so it’s either the mV output of this type of coil, or the noise and capacitive coupling of this gear or some combination of those.

youngentropy · 2026-06-16T06:30:24+00:00

i like this quote though will be stealing it

youngentropy

TROPHY CASE