Use of contactors as circuit breakers

[FreddyNurk]

Folks of Eng-Tips,
I've got an interesting situation thats come up in amongst site drawing reviews for something completely unrelated. We manage the upgrades and installation for a number of remote (i.e. isolated grid) power stations in Australia. Its come to my attention recently that some of the installations do not have any circuit breakers, rather that contactors of a similar continuous current rating have been used, and PLCs have been used to detect overcurrent situations and consequently open the contactor. Each station typically has 3 generators, each about 125kW capacity.

These installations have been in operation (apparently fault free) for a good 10 years. A quick assessment of the potential instantaneous fault current gives the indication that the contactor's withstand rating may be exceeded for multiple sets online at once, a situation I'm told is typical for some parts of the day.

I've looked at some of the relevant standards, and note that 'contactor' isn't specifically excluded from such use, though I note that 'automatic disconnection' is a requirement and don't believe that a contactor meets this requirement. (AS3000)

The other counter argument that I've been presented with is that the station is likely to stall rather than present a large (or even somewhat modest) fault current, a situation that I believe is quite likely, given that the alternators don't have field forcing fitted.

Nevertheless I'm of the belief that at the very least, suitable short circuit protection is a requirement, and that contactors don't meet this requirement. There are no coordinating fuses fitted either.

Comments?

 

[waross]

3 x 125KVA Was that a typo? I think that an equally important consideration may be, do you have enough fault current available to ensure tripping breakes? Even with everthink online, 375 KVA is not a lot of power.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

No, not a typo, and thus the opposing argument to my concern of not using circuit breakers.

Waross, as I'd stated previously (and you quite rightly enquired), its quite likely that there won't be enough fault current to trip the breakers (contactors in this case).

I don't have access to records to see exactly what happens if the station experiences a fault, though I suspect that it would actually be the undervoltage trip controlled by the PLC that would open the breakers, rather than any overcurrent protection.

I'm most concerned about compliance, noting that as time goes on, larger and larger generators are being connected to the stations (though still quite small, really) and I don't have any records of bus withstand ratings either. In terms of practicality I think its a different case, as you noted Waross.

 

[Jereb]

I'm thinking you should have a look at those generators and do a fault study of your own. As i'm pretty sure you require circuit breakers and not just a PLC controlled contactor. There should be plenty of fault current there regardless of how small the generator to trip a correctly sized breaker.

 

[itsmoked]

I believe the installed contactors/PLC are specifically to allow tight control of when a fault interruption occurs.

Why do you think they couldn't interrupt all the available fault current?

Most can handle large inrushes and occasional interruptions while DOL starting so they can likely safely interrupt much larger current than the operational steady state rating.

To some extent it's hard to refute the many years of satisfactory historical operation.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[FreddyNurk]

itsmoked, its a case of practicality against standards compliance, at least in my opinion.

For a single generator set the withstand rating of the contactor is approximately 4kA (dug up from a datasheet) against the instantaneous fault level of about the same figure (it came out at about 3.5kA from the decrement curve)for a single generator set.

Put two (or more) online, and the instantaneous figure changes, but clearly it won't stay at that level for very long at all (subtransient time constant is 0.085 seconds), bringing it well back into the realm of what the contactor can open. Further to that, since there doesn't appear to be field forcing, the fault current will disappear very quickly.

In summary, I believe that the system works as a practical installation, but from the numbers against the relevant standards, I'm not sure that it complies. Hence the question on whether S/C protection would be mandatory, given that its quite unlikely that such a situation would eventuate.

Jereb, part of the review is what brought me to the question. I'd be happier with a far smaller genset that can't support 300% of rated current without stalling, but these are a little bit bigger, and some fault current is possible.

Thanks for the responses.

 

[ScottyUK]

To some extent it's hard to refute the many years of satisfactory historical operation.

"If it can go wrong, it will go wrong." We aren't supposed to make it easy for Murphy!

Contactors in this duty should normally be backed up with fuses. Your should be able to protect the installation using breakers if the CB is equipped with an electronic tripping unit but a CB with a plain thermal-mag trip will likely be inadequate. You'd need the generator decrement curves to really understand how the system will behave under fault conditions. Big machines use a 51V voltage restrained or voltage controlled overcurrent relay to deal with this situation, but it would be cost-prohibitive on such a small installation.


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If we learn from our mistakes I'm getting a great education!

 

[Pikoplatt]

The fuse clearing time curves should be plotted along with the generator decrement curve on the same graph, to determine whether fault current flow from a single generator, for a fault on the load side terminals of the contactor is of sufficient magnitude and duration, to cause fuse clearing. If maximum load current and downstream protection device sizes permit, the fuse size can be reduced to the minimum size required to allow load inrush and steady state current flow, and to coordinate with downstream protection devices. The reduced fuse size may then be small enough to be blown by fault current from a single generator. If fuse clearing can take place for a single generator, then it will take place for multiple generators, and the fuse will blow for fault currents above contactor interrupting rating, once the original fuse size was coordinated with contactor opening time and interrupting kA.In this way, the fuse becomes the interrupting device for fault levels above the contactor interrupting rating, and the fuse will blow before the contactor opens, once the fault level exceeds the contactor interrupt rating. For faults below the contactor interrupting rating, the PLC will signal the contactor to open, and if the PLC's response to the sensed fault current flow is instantaneous, the contactor will open before the fuse blows.

 

[rbulsara]

Nevertheless I'm of the belief that at the very least, suitable short circuit protection is a requirement, and that contactors don't meet this requirement. There are no coordinating fuses fitted either.

I fully agree with you. Contactors may be used for switching and interrupting the load current withing its rating, but not for short circuit current interruption/protection.


Rafiq Bulsara

http://www.srengineersct.com