Close and Trip Relay

[alivip]

As you know, in Protection system we usually use close and Trip relay between Main relay (as commander) and circuit breaker.
These relays must have to main features, Heavy duty and high speed.
The main use of these relays is that, when main relays (for example Distance relay) sends trip to CB, about 4ampere are used by inductor of CB.
hen the relay contact wants to break this current ,it must withstand Ldi\dt voltage . this voltage usually results to destroy main relay contact.
So, based on this philosophy which above mentioned, we use trip and close relay.

After improvement in CB mechanism, the CB contact position is used in the path of relay and CB coil. So the trip current is broken by this contact (for example after CB is tripped, the CB will be opened, so the CB contact will be opened and breaks the trip current)

But we still use trip relay and close relay between main relay and CB,,why?
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For example we use 7PA27 Siemens relay as Trip relay and 7PA23 Siemens relay as Close relay.

I know just one reason.

 

[davidbeach]

I see no purpose for an interposing relay between the protection relay and the breaker. As far back as our oldest breaker (60+ years) the breaker has included an 'a' contact in the trip circuit. Interposing relay is just something more to go wrong while providing no benefit. Lockout relays to keep a breaker open are somewhat different, but can also be a single point of failure.

 

[jghrist]

The main use of these relays is that, when main relays (for example Distance relay) sends trip to CB, about 4ampere are used by inductor of CB. hen the relay contact wants to break this current ,it must withstand Ldi\dt voltage . this voltage usually results to destroy main relay contact.

That's why you put a 52a contact in series with the trip coil. It interrupts tripping current instead of the relay contact.

 

[davidbeach]

Oh, and by the way, there is also the equivalent of a 'b' contact in the close circuit. As far back as way before our oldest installation the relays have been able to handle the breakers. Trip and/or close aux relay that aren't lockout relays have probably never been justified. I'd hate to have to do the write up should something with a tripping relay fail and cause a misoperation.

 

[alivip]

Thanks All.
But one of its benefit is that in the CBF times (when CB was tripped but it was not opened.) the CB contact is still closed , so Ldi\dt will be opened by relay contact. if we not use trip relay , maybe, it leads to impair main relay contacts in that time.

on the other hand,these interposing relay are still produced by manufacture ( Siemens,ABB AREVa...) and also are in Protection Schematic drawings.

(note : the main use of these relay is not for making current, it is for breaking time )

(note 2 : trip relay are not just between main relay and CB coil, it use between every thing should command open CB, for example discrepancy switch. close relay as the same is between A\R relay and .... )

(note 3 : trip relay and close relay price is about 400$ for each CB mechanism )

by considering all above items i think it must have a reason(s).

 

[davidbeach]

Not a good one that I can think of. Sounds like a bad design.

 

[inpran]

@alivip.

1. As you have righly put in your notes and your initial explaination above, the main reason for the trip relay in the present day protection system has been reduced to that of a "multiplication" relay that multiplies the trip contact (from distance relays etc)and used further on for various reasons.

2. Further, the main relay contacts that are used for tripping a breaker are high speed trip contacts. The close / Trip coil of a breaker is rated at 200 W (approx) while that of the close / trip relay's wattage rating is at Max 10 W.
In order to maintain the High Speed feature, the available ways are either by reducing the mass or by reducing the energy requirement. Since mass of these coils have more or less remained constant to achieve the closing or opening operation, the only way to maintain the speed high is by reducing the Energy requirements. Thus by routing the trip / close command via a relay, you are reducing the energy requirement of the main relay(which are predominantly numerical these days.)
Now suppose you want to route the command directly, this would increase the energy requirement of these Main relays which inturn would increase the Rated Burden of the CTs and PTS/CVTs which consequently increase in the size of these instrument transformers and hence the overall cost of the system.
I suppose the cost of the above change will definitely be greater than 400$ and hence a Trip / close relay.

 

[davidbeach]

What????

Numeric relays run off power supplies and place almost no burden on the instrument transformers. Your whole "2." doesn't make any sense.

 

[alivip]

thanks Inpran, but the same as davidbeach said :

the main relays (numeric and others) have separate power supply (they usually supplied with DC system(battery or charger), So the load of CB coil dose not influence on them at all.

the brief conclusion : the only reason we can find till now is :
To prevent damaging contact relay just(and just) in breaking the trip current when we experience CBF faulty.

Anybody knows more reason?

 

[slavag]

yes, this is a reason, to prevent damage of digital relay contacts. But not all comapnies are used some additional relays.

 

[jghrist]

If an 86BF is used to retrip the failed breaker, it would protect the relay trip contact.

 

[alivip]

jghtist :
the CBF (when trip comes and cb will not be opened) results : the cb contact between main relay and coil dose not change position so the role for breaking trip current will be done by main relay contact. ( just in this state we need trip relay)
the CBF function (86BF) does not make sense in this topic.

 

[davidbeach]

No, it won't. Just latch trip until the breaker opens.

 

[alivip]

when we have CBF, trip remains till fault clear(latch trip)
( fault will be clear by opening all CB connected to faulty CB ). so the CB will not be opened and the relay contact disconnect trip current.

 

[lz5pl]

My opinionion is also that separate trip relays (high-speed, high DC current breaking => high expensive) are not necessary now. But they are still required in tender documents and/or in utility requirements, so we continue to put them in new projects.

Some time ago I used another, much cheaper solution. Relay protection sends trip command to the CB directly. In the same time second contact of the relay protection, set also on "General trip", operates auxiliary relay of standard "contactor" type. Contact of this contactor duplicates trip command to CB trip coil. This way we still have high speed trip (without additional delay of external trip relay), but trip circuit is cut by powerful contact of the contactor, because relay protection's contact always drops-out first. Unfortunately I cannot implement this scheme in my current projects. I am sure it will not be approved - contactor type relay in 110 kV trip circuit doesn't look serious. It seems that my African client in that old project was more ready to accept some non-standard solution.

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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

 

[alivip]

lz5pl :
your scheme is used in ABB protection scheme. AS you know ABB co. does not have trip relay and we use compounding tow set relay (one of them is high speed and for trip and other is heavy duty and used for breaking current). anyhow your scheme is cheaper.

 

[Marmite]

Interesting discussion. We've just had a problem where a Micom relay was used to close and trip an old solenoid closing 11kV circuit breaker. The C/B has a closing contactor and the making duty (for the contactor) was within the Micom relay rating. However the Micom relay contacts burned out. One of the problems was that the closing pulse wasn't of long enough duration to make sure that the C/B "b" contact interrupted the solenoid magnetising current, rather than the relay in the Micom. We would normally fit an interposing relay in a sacrificial role as it's much less expensive to repair or change than an internal relay card.

 

[jghrist]

One of the problems was that the closing pulse wasn't of long enough duration to make sure that the C/B "b" contact interrupted the solenoid magnetising current, rather than the relay in the Micom.

Why not make the closing pulse longer? In SEL relays, the CLOSE word stays asserted until either an unlatch condition (like TRIP) asserts, 52a closes, reclose initiates, or a close failure timer times out. Can similar logic be implemented in the Micom?

 

[dpc]

Surge suppression for protective relay contacts dealing directly with trip and close coils is generally a good idea.

I try hard to avoid using interposing relays on trip circuits but it worthwhile to note that the most common protective relay failure for the new microprocessor relays is output contact failure. Things happen.

These relays essentially already have an interposing relay built into the device in most cases, so adding another relay may decrease reliability instead of improving it.





David Castor

http://www.cvoes.com

 

[slavag]

David, you say about SEL surge suppersion solution?

Im use interposiong relay only by special request of customer or in some special cases.

Marmite, btw, in the laqst project with Micom relays, for close I was used interposiong relays ( it was local Areva recommendation)