Dual trip coils

[oldfieldguy]

I have a bit of experience in the southern US with industrial and utility power systems, but of late I have had a recurring conversation concerning the employment of dual trip coils on 5-15 kV circuit breakers in industrial environments.

My experience is that they are relatively rare in heavy industry, quite common in utility transmission, relatively rare in utility distribution.

Some of those in my company seem to think they are a good idea. I tend to look at them as an unnecessary complication. I back my position up with the experience of working in dozens of petrochemical and utility facilities. I don't know where they got their position except that they live in fear because of a couple of bad experiences where breakers failed to trip for undisclosed reasons.

I would appreciate comments.

old field guy

 

[catserveng]

I did s couple of cogeneration projects with 2 MW 12kV units, owner's engineer insisted on dual trip coils, seemed to add a lot of complexity and some extra steps at startup. A few years later the site was having some problems, which appeared to be due to lack of mechanical service on the breakers. When I asked plant operations staff I was told that the engineer hadn't felt maintenance was needed because he had dual trip coils.

I guess if they are specified and installed for the right reasons, like in critical utility systems, then they make sense. I have also had a few times when a breaker didn't trip when it was supposed too, in none of those cases a redundant trip coil would have helped. One was a 125 DC supply issue, the others were no maintenance, bad environments and piss poor planning.

My two cents, Mike L.

 

[dpc]

They seem to be standard equipment on transmission voltage circuit breakers. I've never seen or specified them for 5 or 15 kV metal-clad switchgear breakers - industrial or utility, and this includes some very large fossil plants. Nuclear plants may be different.

Unless the protection system includes redundant or backup relaying and battery systems, the dual trip coils are probably not adding all that much to the overall reliability. A lot of the new relays can provide monitoring of trip coil continuity,

David Castor

http://www.cvoes.com

 

[DTR2011]

I second DPC comment. Most IEDs can have some kind of trip coil monitoring function implemented to detect a failed trip coil.

 

[mgtrp]

When I worked for a proper generating utility, we would fit these to all our major circuit breakers - both 110/220 kV transmission level and 11 kV generator cb's. This was the final part of fully duplicated main protection systems, from the relays to the trip circuits, cabling, battery banks and chargers, etc. I don't think that it's hugely expensive, and most manufacturers seem to offer it as an option even at the medium voltage level.

I don't think I'd be so concerned working in an industrial site or utility distribution, where there isn't the same level of redundancy designed into the rest of the protection system.

 

[oldfieldguy]

You guys have pretty much mirrored my argument. Before I began working with my present employer, I knew only ONE industrial client who specified dual trip coils, and just like the present employers, they'd had ONE bad incident where breakers didn't trip. Even though the root cause of that incident was a failure to properly function check a new installation, their knee-jerk reaction was to go with dual trip coils.

With the present bunch, it's the same thing. A couple of bad incidents, and even though I can just listen to a cursory retelling of the sad stories and trace the cause to a) poor design and b) poor maintenance, they still go with dual trip coils.

In my former employment with a large electric utility, I was accustomed to dual trip coils on transmission breakers because we also had primary and backup relaying schemes.

I appreciate the input. It assuages my feelings, but I don't think my arguments are going to sway my (non-electrical) project management to change their minds.

old field guy

 

[JshC]

For the industrial project that I am currently working on, the dual trip coils is specified on the 72kV circuit breakers (which is pretty standard).

And in the 15kV switchgear feeder breakers for mining application, we specified an undervoltage release device in the CB in addition to the the trip coil. This was mainly for compliance with the mining code as shunt trip is not classified as failsafe.

Not the same as dual trip coils I know. But that's pretty much the most we did to ensure redudant protection against trip coil failure, and this is in addition to trip coil monitoring by the relay.

 

[lz5pl]

From my experience - only one case when 20 kV CB had 2 trip coils. It was step-down substation for industrial customer, where 110/20 kV transformer was connected in "Block line-transformer" scheme, with line differential protection covering 110 kV cable line and 110/20 kV transformer. So on 20 kV side we had real redundant system (87L and 51 protections in separate hardware) - hence there was a sence in dual trip coils for CB in 20 kV transformer cubicle. CB's in all other 20 kV cubicles had only one trip coil.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[bacon4life]

Dual trip coils may even be getting rarer in utility transmission. In response to the new testing requirements in the NERC protection system maintenance standard, we are considering eliminating the use of redundant trip coils. With only one trip coil, the required trip coil testing can be documented via event reports, but having the second trip coil will require trip coil testing during a maintenance outage.

 

[davidbeach]

Watch out though, redundancy standards are looming and may make dual trip coils highly desirable in some/many transmission applications.

 

[dpc]

I think modifying long-standing design practices to "game" the NERC requirements is a fool's errand. It's the ultimate tail wagging the dog scenario. And besides, NERC is free to revise, update and/or clarify their requirements at any point. I can't believe some of the nonsense I'm hearing about putting fuses ahead of circuit breakers and reducing transmission voltage, etc.

(Just my $0.02)

 

[bacon4life]

David,
Do you know which standards or which voltage levels they are looking at to require redundancy?

We have only been using dual trip coils for about 10 years and there is still internal debate whether the extra complexity is worth it when only one station battery is providing the DC. PRC-005-2 is just one more thing in the minus category.

 

[davidbeach]

My guess is that it will eventually apply to the BES, but will also likely be performance based so that each circuit could use different approaches. I don't see PRC-005 as a reason to not use dual trip coils.

Set one relay 5 cycles slower on phase and the other 5 cycles slower on ground with each relay only tripping one trip coil. Then some times you get a TC1 trip and other times you get a TC2 trip. Or use a latch to alternate which TC gets the trip and which one gets a automatic retrip 3-5 cycles later. One trip, reclose, trip cycle would have both trip coils tripping.

 

[bacon4life]

Thanks for pointing me towards finding this technical paper on redundancy that NERC commissioned.

http://www.nerc.com/filez/standards/Project2009-07_Reliability_of_Protection_Systems.html

 

[rhatcher]

oldfieldguy, I'm with dpc.

I have never encountered an industrial grade 5-15kV circuit breaker with dual shunt trip devices. We even have a customer who has industrial grade, metal enclosed, 38kV vacuum circuit breakers and switchgear. Only single shunt trip devices are used. This experience includes large coal fired power plants, steel mills, paper mills, etc.