Need for generator breaker before step-up transformer 16

[njengr1]

Is there a Code or other reason to require a generator breaker between the generator and the step-up transformer. No auxiliaries are supplied at the generator voltage (4160 v). This is for a small (4MW) cogeneraton installation. A breaker is on the high voltage side. The stepup transformer is connected to a utility

 

[GerH]

No Code restriction that I am aware of. I have built a few plants this way. Apart from cost saving another advantage is that it solves a grounding issue - assuming you are using a YD traffo with the Y on the HV side the D winding would be unearthed and could float up to the HV potential unless you add an earthing transformer. The alternators were Y with earthed neutral.

A question that arose is whether the build-up in traffo magnetism would overload the alternator exciter - found not to be an issue - exciter should only start ramp up at close to 100% speed.

 

[buzzp]

The only thing that comes to mind is what if the transformer faults? You will have to shut down the generator instead of just opening a breaker. Depending on the driver, this may be a huge deal.

 

[njengr1]

Thank you for the responses. The NEC is pretty quiet re generator installations. Re transformers, the NEC states nothing about step-up transformers. The table in the protection section 450.3(A) table doesn't require "secondary" protection for a "supervised location". The code is very vague on just what side of a step-up xfmr is the "secondary". Article 240.92(D)does say that the primary is the "supply" side. Of course, with a step-up xfmr the supply is the low voltage(generator)side. A strict interpretation of the wording would imply a high voltage breaker can be omitted. Very contradictory!

 

[alehman]

NEC 450 requires the transformer to be provided with primary overcurrent protection. The primary is the input winding and the secondary is the output winding, regardless of voltage.

NEC 445-5 requires ths condcutors between the generator terminals and the "first overcurrent device" to be sized for at least 115% of the generator nameplate amps. There is an exception that where "design and operation prevent overloading" the conductors can be sized at 100%.

 

[busbar]

Consideration might be given to:

IEEE C62.92.2-1989 Neutral Grounding in …Synchronous Generator Systems
§3.1 Unit-Connected Generation Systems

IEEE Std C37.102-1995 …AC Generator Protection
§3.4.1 Unit generator-transformer configuration

ANSI/IEEE Std C37.101-1993 …Generator Ground Protection
§4. Generator connections

IEEE Std C37.91-2000 …Protective Relay Applications to Power Transformers
§3. Philosophy and economic considerations
§4. Types of transformer failures

 

[gordonl]

I don't know if this standard on all exciters now, but it gives even more concern for fast field discharge, so faults in the Gen, bus, and txfr are fed as little as possible by the generator. I'm not sure how you could trigger a brushless exciter, but with a slip ring exciter I would certainly "trip" the field by the protection, and crowbar with a semi-conductor controlled discharge resistor.

 

[electricpete]

I work at site with 2 1250Mwe generators and we have generator on low voltage side.

There are a number of advantages with the increased flexibility. The biggest one, you can use the main transformers to back-feed the plant loads through the main (stepup) and unit auxiarly transformers during period of plant shutdown. If no generator breaker, we would have to deenergize w breakers on hi-side of stepup, remove bus links at generator, reenergize to backfeed, then deenergize again to remove bus links prior to startup. I don’t know how this applies to smaller units.

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[edison123]

pete,

Is it a breaker (protection operated) or just an isolator ? Seems like a very high current for a breaker to handle.

 

[alehman]

Big generator breakers (for big $ I'm sure)-

http://geindustrial.com/cwc/products?pnlid=5&famid=8&catid=40&id=cb-gen&lang=en_US

 

[busbar]

alehman, now that's some beefy gear...

Voltages for 12 to 27kV.
Interrupting ratings up through 125kA.
Continuous currents through 44,000A.

Those ratings can garner sobering respect from operators that have 1½-foot-thick walls between their bucolic control room and screaming generator bays.

 

[jbartos]

Comment: An application of the generator circuit breaker releases the generator step up transformer for a use to power the generating station auxiliary systems from the grid. This may save an extra transmission line or power source, beside the required alternate power supply transmission line. So that savings can be significant. However, the step-up transformer has to be engineered and designed (mainly protected) for a bidirectional power flow.

 

[electricpete]

edison - yes it is a circuit breaker and not just an interrupter. If memory serves me right the interrupting capacity is 250,000 Amps (25kv system). It is made by GEC/Alsthom. Three very large garage-size pole units with dedicated air system of two cycling compressors.

njengr- A more important benefit for a steam power plant is that a generator trip does not cause loss of power to plant auxiliaries powered from the aux transformer. If steam power plant auxiliaries are abruplty interrupted there can be severe water hammer problems. This type concern may not be relevant to gas turbine plants.

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[jbartos]

Comment: NFPA 70 NEC
445.12 Overcurrent Protection.
(A) Constant-Voltage Generators. Constant-voltage generators,gexcept ac generator exciters, shall be protected from overloads by inherent design, circuit breakers, fuses, or other acceptable overcurrent protective means suitable for the conditions of use.

 

[electricpete]

That only tells us the generator needs to be protected, not whether we rely on circuit breaker on low or high side of stepup transformer.

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[jbartos]

Comment: NFPA 70 NEC does not cover generating stations, substations, ships, certain government facilities, etc. However, they may adhere to NEC. The 445.12 Overcurrent Protection covers generator applications that fall under NEC. Therefore, the generator has to have circuit breaker on its output just like any other power supply, e.g. electrical service, according to NEC. The Utility may omit it.

 

[rmw]

Electripete,

Water Hammer???? Are you quitting there??? I am not bragging about it, because in each case it was accidental, but I have had the dubious experience of putting some heavy industrial plants eg refineries, paper mills, as well as portions of and even whole cities in the black and, speaking for the generating plant situation, (because that was where I was located when I did whatever unfortunate deed caused the incident) all kinds of HELL broke loose. Frankly, I did not even notice the water hammer, but I guess it was going on all the while. We had lots else to worry about. Were the DC backup lube pumps for the turbine going to start and function (or, as too often happens, was the turbine going to coast down with dry bearings)??? Was the furnace going fuel rich with the fuel in there, and no air being supplied?? Were all those safeties going to ever reseat again??? Could we get all the (solid) fuel dragged out of the (stoker) furnace before what was left in there melted everything???

Man, are you telling me that we had water hammer problems, too??

Now, let me mention a situation that I once heard of second hand, so I cannot go into a court and place my hand on the bible, but it was told for the truth by reliable people.

Seems a plant manager at a medium sized generating station, (large for its time over 30 years ago) had heard conversations about a generator becoming a motor if the breaker was not open. So.... he disabled the breaker, how I do not know, and tripped the turbine. Well, it did motor, and with disastrous results. As I remember it, something about a phase shift, may have broken the turbine shaft, which took out the shaft driven oil pump, and the backup did not function right, and the long and short of it was that he wrecked the turbine. The only way I had knowledge of it was that I worked for the company at the time who furnished the turbine, and some of my compadres were involved in the fix.

Now, to the point of this tirade. Njengr1 asked about code and/or other reasons. You electircal gurus have dealt with the code issues well, and that is well beyond me. I threw in the above to address "other reasons" to give Njengr1 something to comtemplate with respect to his system, and the question he asked.

If he is connected to a utility, he is probably not going "black" but there may be other ramifications.

Thanks for this trip through memoryland.

rmw



rmw

 

[rmw]

PS, The plant manager at the utility got fired, although I never did over any of my exploits, because the were caused by anomolies in the controls, and the trip was the beginning of the solution.

rmw

 

[rmw]

Futhermore, Pete,

Boy, did you get the old juices flowing.

Most of this fun happened before the days of OSHA, and the current tag-out, lock-out procedures that we have today, so we, as young turbine engineers, were schooled by the old timers to come to a turbine or generator job the first day dragging a light logging chain.

If we were asked "what is that for?" the answer was "I am going to throw this chain across the generator bus to make sure it is unenergized." This exercise was more to test reactions than to get some actual throwing exercise. I never threw mine, but I did get the panicked look on the face of the plant management person I was reporting to on a couple of occasions, and watched him scurry off to do 'something'. What you were looking for was the reaction that conveyed 'go ahead, who cares, because we know the breaker is open.

So, njengr1, if you have no way to guarantee that upon accidentally closing the generator breaker, the generator can become energized, one of the 'other reasons' you are looking for is safety.

I say all this because I don't know in what country and/or technical culture (even in my country) you operate, and some of the safety procedures we take for granted in this day and time may not apply to your situation, so you have to build them in for your own protection. You may be the one working on that machine some day. How much protection do you want between yourself and that step-up X-former???

I think I am through now.

rmw

 

[jbartos]

Suggestion/question to njengr1 (Electrical) Apr 15, 2004 marked ///\\///Why not to ask a fundamental question late than never?\\Is there a Code or other reason to require a generator breaker between the generator and the step-up transformer.
///Do you adhere to NFPA 70 National Electrical Code (NEC)?\\\
No auxiliaries are supplied at the generator voltage (4160 v). This is for a small (4MW) cogeneraton installation.
///Is the project committed to the NEC in its documentation?\\ A breaker is on the high voltage side. The stepup transformer is connected to a utility
///Where is the property line box?\\\