protection drawings standards

[powercam]

hi

i want to find out if there is a specific standard when it comes to duplicate protection. when wiring two differnet protection relays for a transmission bay, should the tripping circuit for the trip coils be seperated. i know that in my experience when we have 2 distance relays they are powered up by individaul dc supplies and their tripping contacts are used to trip both trip coils. now here where i am working it is a standard to keep thier protection completly seperate ie one relay only trips one trip coil. i need to find out whether this is a standard or a preference. it will come down to relaibilty versus standards.
thanks people.

 

[RalphChristie]

Hi

I know on transmission-systems they use duplicate protection due to the high cost of the equipment. I think it was in the beginning just for reliablity, but I'm almost sure it would be incorporated now into a standard. Sorry I have no references.

Regards
Ralph

 

[stevenal]

Individual DC circuits are used to energize the same coil? Sounds like a common failure point to me. One faulty coil could blow both circuits and leave the remaining coil useless. Keep them completely separate. Each relay energizes it's own trip coil, or each relay energizes both coils through isolated output contacts.

Might be something in the IEEE paper on trip circuit design:

http://www.pes-psrc.org/k/rlytrckt.zip

 

[mersin33]

it is not a "standard". but it is an "industry standard" or "best practice" or preference one could say. of course as stevenal says, ideally you should have separate dc coils. but if you do not have a second coil, it does not mean it is wrong. two separate dc's feeding two relays connected to one coil is still better (more reliable) than one dc system feeding one relay connected to one coil.

 

[powercam]

hey thanks guys...
this is for 400kv ac transmission so reliabilty is very imp. just for clarity:--

i have 2 different dc supplies,which powers up two different relays and the relays each have tripping contacts (one from each relay) tripping the CB. the CB has 2 trip coils and each coil gets energised by the tripping contact of the relays(positive gets switched via the contact) is this ok to keep the tripping seperate. wouldnt it be more reliable if i had two different tripping contacts(1 from each relay) tripping both trip coils? in other words i will have
a trip contact from relay 1
a trip contact from relay 2 both tripping one CB coil Y1
and then the same for coil Y2.
rather than having only relay contact 1 tripping Y1.

thanks for the input so far.really appreciate everyones ideas.

 

[stevenal]

Salton,

He does have a separate coil, he's just not using it in manner that provides much redundancy.

Powercam,

Separate them for the reasons given above. The key to effective redundancy is to eliminate the common failure points. Many modern relays provide multiple programmable outputs, just a matter of programming a second output to trip.

 

[buzzp]

What your saying could arguably be the best method:
One relay tripping both trip coils
Second relay tripping both trip coils
However, if one coil burns out (shorts) then this will render both trip coils useless unless the fusing for the coils is placed in the proper position so that both DC breakers powering these seperate relays do not trip if one coil shorts. Then you have to ask, what if the wire shorts for some reason before the fusing, well then both DC breakers trip rendering it useless.
I could see arguments for seperate trip coils and relays: one relay only operating one trip coil
second relay only operating a second trip coil

Redundancy has opportunities for one to be 'anal' (I am not saying your being anal) if this is a good word, what if your single set of CTs feeding these two relays fail? Your redundancy is rendered useless. Chances are, as well, that both DC supplies originate from the same source sooner or later (IE the plant goes dark and now your operating on one UPS system (or battery bank) so both supplies are from the same source).
Anyway, enough from me, I think your on the right track.

 

[RalphChristie]

What I understand under the term duplicate-protection is that there are two of each protective elements. Two sets of the same CTs, two sets of the same relay-schemes, two different DC-supplies, two tripcoils etc. I can not/have not seen any transmission system without at least one duplicate. The cost of the primary elements are too high compared to an extra protective scheme. That is why I think there must be some kind of standard to do it that way and it is plain stupidity not to have a duplicative protection scheme on a transmission element.

You can hit both tripcoils from one relay by wiring a N.O. tripcontact from the one relay into the other scheme(schemes electrically seperated from each other) and vice versa.

Ralph

 

[stevenal]

The only standard I'm aware of is C37.113. Redundancy gets one section four paragraphs long that doesn't say much except that there are various ways to accomplish different levels.

I know some use relays from different manufacturers. Some will use the same manufacturer as long as different models or algorithms are used. Dual DC systems, instrument transformers, and trip coils still makes little sense to me if you're still relying on the same breaker mechanics. I've seen more problems with the breakers than any other single item.

 

[mersin33]

if done correctly, shorting of a trip coil ends up in the loss of the first dc supply. It is hard to describe the schematics on text. but if you have a copy, see p.692 on ABB switchgear manual, section 14.3.3.

It is definetely the best book written on switchgear, however none of the standards match the ones in North America.

 

[busbar]

 
An important consideration in the use of dual trip coils is the possibility of a magnetic flux path common to the two coils. Some designs are such that if the two coils are energized simultaneously, but with voltages of opposite polarity, the action of the trip mechanism is defeated. Appendix A

http://www.pjm.com/services/trans/downloads/RS_stn.doc