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Need for generator breaker before step-up transformer
16

Need for generator breaker before step-up transformer

Need for generator breaker before step-up transformer

(OP)
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

RE: Need for generator breaker before step-up transformer

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.

RE: Need for generator breaker before step-up transformer

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.  

RE: Need for generator breaker before step-up transformer

2
(OP)
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!

RE: Need for generator breaker before step-up transformer

2
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%.

RE: Need for generator breaker before step-up transformer


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
  

RE: Need for generator breaker before step-up transformer

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.

RE: Need for generator breaker before step-up transformer

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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RE: Need for generator breaker before step-up transformer

pete,

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

RE: Need for generator breaker before step-up transformer


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.
  

RE: Need for generator breaker before step-up transformer

2
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.

RE: Need for generator breaker before step-up transformer

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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RE: Need for generator breaker before step-up transformer

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.

RE: Need for generator breaker before step-up transformer

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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RE: Need for generator breaker before step-up transformer

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.

RE: Need for generator breaker before step-up transformer

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

RE: Need for generator breaker before step-up transformer

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

RE: Need for generator breaker before step-up transformer

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

RE: Need for generator breaker before step-up transformer

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?\\\

RE: Need for generator breaker before step-up transformer

(OP)
I think I found the answer to the need for a generator breaker. Art. 705.21 DISCONNECTING MEANS, EQUIPMENT on page 2001 of the 2002 NEC HANDBOOK describes the exact case I was investigating. If the generator and transformer are designed to operate as a "unit" no disconnecting means is required between the generator and transformer. Once in a while the NEC catches up with the prevailing practices!
My thanks to all who have provided their comments and thoughts.

RE: Need for generator breaker before step-up transformer

(OP)
rmw,
Your right about the safety and lockout tagout, but the presence of a generator breaker really doesn't improve safety. Most units avoid generator breakers between the machine and the step-up xfmr because the price is high. The last unit I commissioned in NJ was 250MW and a generator breaker would have cost $1.25 Million. We opted for a breaker on the 230kV side. Three years before that the 150MW unit I worked on for a Louisiana project did the same. There were a few more in Japan and Italy I engineered and the breakers were again on the primary.

Now all these units were utility grade operations and exempt from the NEC. This little one I was investigating for a safety review was a peak shaver and comes under the NEC. A fellow investigator suggested a breaker would be needed per Article 450.3(A) on the protection of transformers. But Article 705.21 counters that convincingly.

RE: Need for generator breaker before step-up transformer

Comment: The transformer is supposed to be the integral part of the generator and work as a unit. How do you have the generator and transformer system groundings connected to form the integral part?

RE: Need for generator breaker before step-up transformer

jB,

A large generator-transformer module connected to the UK transmission grid is usually configured as follows:

Generator is high-impedance grounded, often through a distribution transformer. This affords a high level of protection to the stator core from winding earth faults by limiting the fault current to ~10A or so.

The generator line terminals are connected to the ungrounded LV delta winding of the generator transformer, typically through a phase-isolated bus duct. The HV winding is typically a solidly grounded star winding, perhaps with an on-load tapchanger.

UK practice is normally to have the generator CB on the HV side of the transformer.



-----------------------------------

Start each new day with a smile.

Get it over with.

RE: Need for generator breaker before step-up transformer

(OP)
ScottyUK, Your practice is pretty much universal for the big units.
jbartos, This system is a small generator (2MW) connected directly to the transformer. The transformer is a wye at 4.16kV and delta at 13.8kV. Both the wye point of the generator  and the wye point of the transformer are solidly grounded. This permits a flow of third harmonic current to flow instead of producing third harmonic voltage to appear on the 13.8kV side. While this connection splits the zero sequence flow for a phase-ground fault on the 4.16kV side, there is sufficient picup for the ground relaying. The generator winding pitch is not known to me, I suspect it is 2/3rds and the triplen currents are therefore small. That is yet to be measured, but will be.

RE: Need for generator breaker before step-up transformer

I suspect unless this is a utility owned facility or a NRTL listed unit transformer package, the local inspector will insist on a LV breaker. Most inspectors that I've worked with are not educated in such systems.

RE: Need for generator breaker before step-up transformer

(OP)
Your right about inspectors many of those I met didn't understand what an interrupting rating was on a molded case breaker. Previous NEC editions provided room for a professional engineer to make design decisions. Now the committees try to provide pre-engineered packages. We can at least be thankful U/L can't test high power equipment and put a label on it. Ambiguities in the NEC are now settled in arbitration in cases where the inspector is ignorant and the price is too costly to accommodate him. In the end the lawyers get to do the engineering for us.

RE: Need for generator breaker before step-up transformer

njengr1 - you are so right about that. There seems to be a trend toward dumbing down the NEC and trying to provide recipes for numerous specialized situations that in my opinion should be left for the P.E. to work out as best suits each case. I think this is partially a result of more engineering services now being provided by electrical contractors, who are not always adequately educated, but typically have or can find a P.E. to seal their drawings. The codemaking panels seem to want to allow for that type of engineering process. Just my opinion.

RE: Need for generator breaker before step-up transformer

The faster, cheaper attitude that seems to be becomming more prevelant in the U.S. Many owners don't plan to own newly constructed facilities for more than a few years, so whatever they can do to get it built fast and cheap is to their advantage. Life cycle cost, maintainability and reliability seem to be increasingly smaller design factors. A possible case in point - the Bellagio hotel cable failure discussed in another thread here.

RE: Need for generator breaker before step-up transformer

We have put in quite a few hydro schemes with breakers only on the HV side. Usually we use 33kV pole mounted reclosers as the HV synch breaker, its cheap and cheerful. The local service is supplied by a separate transformer

RE: Need for generator breaker before step-up transformer

Quote:

ScottyUK, Your practice is pretty much universal for the big units.

Not unless I work in another universe. See my post above.

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RE: Need for generator breaker before step-up transformer

All our breakers are on the low side of the transformer (generator side).

RE: Need for generator breaker before step-up transformer

2
First, see where each code, standard, etc. applies. In my opinion, busbar was almost right when recommended the IEEE standards. But there are some other aspects you should take into consideration.

As far as I know, the reason for the NEC is not so specific about the generation facilities is that this code does not apply to this type of facility. Power generation and transmission facilities are owned and operated by companies that have developed their own standards, guidelines, rules, etc. If you work with such a company, you should see what is the general accepted practice over there. If you work with someone else, try to find out the norms, guidelines, etc. of the local power generation company or other applicable local codes. Do not forget about the standards, policies, recommendations, etc. of the regional reliability organization the local company is part of.

Here is an example. Before 1998, in the Canadian province of Ontario it was only one power generation, transmission and distribution company, Ontario Hydro. In 1998, the local government decided to create an electricity market and OH was divided into 3 major types of companies: power generation, transmission and local distributors. All are now among the market participants and should comply with the Ontario codes and market rules. One of these codes is the Transmission System Code, which governs both generation and transmission sectors. This code was issued by the Ontario Energy Board (www.oeb.gov.on.ca) appointed by the local government and is based not only on the experience of the former OH, but also takes into consideration the requirements of NPCC (North-East Power Coordination Council, www.npcc.org/default.asp) and NERC (North American Electric Reliability Council, www.nerc.org), where the Ontario power systems are part of.

As guidance for what you need, here is what the abovementioned code states in Appendix 1, Schedule G-Technical Requirements for Generators, 1.1.1  (www.oeb.gov.on.ca/documents/Transmission.pdf , page 78 of the pdf file): ‘A high voltage interrupting device (HVI) shall provide a point of isolation for the Generator’s station from the transmission system. (...) The HVI shall be a circuit breaker unless the Transmitter authorizes another device.’ So, here, there is no doubt about the mandatory use of a CB on the HV side.

Immediately after this, at 1.2 Typical Generator Protection, it is stated: ‘The typical technical requirements for Generator protection SHOULD BE followed, ‘[my emphasizing, vic3fan] ‘as set out in Exhibit F.1 of Schedule F and Exhibits G.1 and G.2 of this Schedule G’. Well, in these G.1 (page 81) and G.2 (page 82) a circuit breaker is also used between generator and transformer. Again, there is no doubt about the necessity to comply with this rule.

As you can see, for an Electrical Designer whose job is to devise a generation facility in Ontario, the things are very clear. See your local requirements. Also, keep in mind that the designer might be responsible for further incidents during the operation of that part of the power system, so it's better to anticipate some potential troubles and to not give the cost reduction the first priority.

RE: Need for generator breaker before step-up transformer

I apologize, the correct link for North American Electric Reliability Council is www.nerc.com. Sorry!

RE: Need for generator breaker before step-up transformer

I agree with vic3fan. My experience has been that on larger units (>50MW), low-side generator breakers are less common and unit-connected transformers are used. Typically units in this size range belong to utility companies and do not fall under NEC regulation. Also, auxiliaries are large frequently enough to justify a separate transformer so the advantage of being able to separate the generator is reduced.

RE: Need for generator breaker before step-up transformer

Just worth mentioning that our units are 66MW units. I guess I feel more comfortable being able to seperate the generator from the transformer by the flip of a switch.

RE: Need for generator breaker before step-up transformer

Protective relaying must of course be configure to match the arrangement. Unit connected transformers are sometimes included in the generator protection zones.

RE: Need for generator breaker before step-up transformer

(OP)
Gents,
The project that started this thread is a small cogen installation in a plastic fabrication plant. Two 2000 kVA gas engine driven units supplement utility power. It was built without the generator breakers. The question arose during a transfer of ownership. One engineer stated it was against the code. The present owner (seller) disagreed because it was inspected by the local authorities. It is running for about 4 years. Article 705.21 of the NEC pretty much puts the issue to bed. The NEC Handbook notes around this article describes this installation like the author was the designer. It couldn't be closer.
The breaker issues have been resolved with regard to the project. The code doesn't require them.

 That said, I would agree generator breakers make maintenance & operating easier, and probably wouldn't burden the project return measurably for this small system.

On the bigger projects (50MVA and up) the cost of the generator breaker get disproportionally more expensive. This mainly because of the voltage limitations of the generator. Even very large units top out at 24kV. So, as the capacity increases, the current levels increase and the breaker costs climb. A case in point; the gen breaker for a 250MW unit I put in in 2001 would have cost over $1 million. Obviously we put in a breaker on the high side of the stepup transformer. This is, of course, usually done in the high powered units which are not covered by the NEC, as many of the contributors have said.
It has been a stimulating discussion by many knowledgable folks. thanks for your contributons.

RE: Need for generator breaker before step-up transformer

Yes i think gen.breaker its a necessary for instance, after maintance work has been done, to sychronise that unit back to the network its much easier. gen,breakers like bbc have both isolator and current breaker(arc quenching), so back-energing is very much simpler.
Obviously the breaker should operate in case of high abnormal currents from gen.

Beks C Lepee
Rotek Engineering-Eskom

RE: Need for generator breaker before step-up transformer

Your generator is considered a separately derived source which requires an oversurrent protection means.  In addition the NEC requires a secondary over current for you step-up transformer.  The primary over current protection ( ie. your generator breaker) should be sized at 125% of the transformer rating to prevent current in excess of the transformer damage curve.  See the NEC, section 240 & 450  for exceptions for industrial applications

RE: Need for generator breaker before step-up transformer


In the US NEC, facilities considered Not Covered are “Installations… under the exclusive control of electric utilities…” so it would be unusual to apply the NEC to unit-connected generation.  There are other national consensus standards that are more fitting to the application.
  

RE: Need for generator breaker before step-up transformer

In power plants where the grid available is at 220 or 400kV level, it makes economic sense not to install grid connected station transformers and go for generator circuit breaker. This way, start up power also can be drawn from the UAT through Generator transformer keeping the GCB open.
This will save cost of two nos. switchyard bays, two nos. station transformers and associated civil costs. All this could be substantial especially when one is looking at minimising the project costs.

RE: Need for generator breaker before step-up transformer

I completely disagree with the cost of a full size GCB. In 2001,  a fully rated GCB for a 250 MW plant would cost between $275,000 and $300,000 per unit. There are now at least four major manufactures of GCB's based upon SF6 interrupting technology. Perhaps njengr1 was refering to the old airblast cb's that cost $1.0 million or more.GCB's enhance the overall unit reliability and are almost standard on  the design of combined cycle powerplants that are now inuse or in the planning stages. Usually they are only applied on the GT units not the ST's.Since the cost have come down from $1.0 million they also are also a factor in the overall cost of the plant(They eliminate  the cost of a start up system).So powerplants are cheaper to build with GCB's and are also more reliable. They also make it a little harder to motor the machine. Not a good idea as explained in a previous post.

RE: Need for generator breaker before step-up transformer

Tell me if I am wrong, but this particular installation would not even use a generator breaker as defined by IEEE ANSI C37.013. If it does not, is there a specific standard that would encompass this size of a generator breaker?

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