Overcurrent protection on only one phase 1

[genhead]

I have a 3-phase, y-connected generator, 400volt. Overcurrent protection is provided in one phase only. Does this provide adequate protection?
Any comments?

 

[rasevskii]

It depends...If you are connected to a floating symmetrical 3 phase load without neutral that might work BUT in the real world that is almost no protection and it is hard to imagine why anyone would use such a method.

Unfortunately I cannot open the docx file. try a doc or pdf and tell us more...

rasevskii

 

[dpc]

In the US, that would be an NEC violation. If it is only for overloads, you could make a case it is OK. But if it has to detect short circuits, it obviously is not adequate.

David Castor

http://www.cvoes.com

 

[rbulsara]

Check if the LV breaker has its own built-in trip unit. The single phase CT may be primarily for metering and the OC relay is just a back up? Check with the mfr.

Otherwise it is not sufficient as dpc said.

Rafiq Bulsara

http://www.srengineersct.com

 

[VTer]

From the diagram attached it seems that CT is used for metering only.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[Skogsgurra]

No, there's a protection wired in series with the meter.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dpc]

I think your overcurrent protection is built into the circuit breaker. The CT is for metering - not sure what the device in series with the ammeter is, or what it's for.

David Castor

http://www.cvoes.com

 

[rbulsara]

The CT circuit does show an OC (overcurrent relay per the symbol list on upper right corner) and a N.O. contact of the OC relay in series with the trip coil.

Rafiq Bulsara

http://www.srengineersct.com

 

[genhead]

The circuit breaker has no thermal trip unit, only magnetic, so provides short circuit protection only.
The manufacturer supplies thousands of similar units into the Japanese/Australasian market.

 

[rbulsara]

That probably would not sell in the USA. I am not sure how does it become cheaper to add a relay, than to just by a regular LV thermal magnetic breaker or a breaker with LI function? May be it does for them.

Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

I wouldn't accept that...
There's a time and place for lean manufacturing design, this is neither the time or place.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[genhead]

The generators can be configured as 200 or 400 volt by means of the inter-winding connection links. By using the CT and overload relay, positioned as it is, the overload protection is "correct" whether 200 or 400 volt is selcted.
I just don't know why they have only used one CT, and what sort of prorection that provides. i would think that it would only provide adequate protection the the load was perfectly balanced across phases.
The manufacturer uses 2 CTs on most other (larger) of its generators.

 

[iceworm]

Hello guys and girls -
I've been lurking for a few years, enjoying the posts and interesting ideas. I finally decided today was the day to see if I could help out. I've been hanging around industrial grade generators for a lot of years and possibly I have an informed opinion.

First I'll make some guesses (and I could easily be all wet):
The generator is at least 50kw and not more than 500kw - probably closer to 100kw - 300kw.

The driver is on the small side for the rated output.

I don't see any external inputs for voltage (field current), so it is likely there is also no external governor input for frequency. The unit is likely not designed for parallel operation.

The manuals with the generator show the output connected to a circuit breaker feeding the premises distribution system.

It could meet easily NEC445. The guiding light for overload protection is 445.12 and that is pretty vague. A small driver with a under frequency trip on the governor might be enough.

As for selling in the US, there is nothing I saw in the thread that would violate UL2200. But since it is 400v and likely 50Hz - it probably would not sell many.

So, based on my guesses, and the posts from the OP, the short answer is, "No this is not adequate protection". But these are design issues, and once the installation meets these, it will be in excess of the regualtory requirements.

Assuming you are going to use the gen on a grounded wye, 3phase overload protection is a must. It is possible to overload the system heavily on one or two phases and not exceed the driver capability. Since you have to protect the conductors from the gen to the first over current device, set the first OCP small enough to trip the gen CB inside of it's damage curve.

Also you will want a under voltage/over voltage to keep the gen from damaging the loads if the AVR fails

Any protection past these has to do with the cost of replacing the generator structure and if it is cost effective to rewind the gen as opposed to replacment. The issue is that if the generator faults, how quickly do you want the fire put out?

Hope I helped.

ice

 

[waross]

We don't know the setting of the magnetic trips. They may be low enough to provide protection and given the relatively high internal impedance (compared to the transformers that shape the overload and fault current thinking of most of us) and the probability of voltage collapse in the event of a fault, the unit may be better protected than we give it credit for.
There does not seem to be a PMG so voltage collapse may be expected to provide some protection.

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