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
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
RE: Need for generator breaker before step-up transformer
RE: Need for generator breaker before step-up transformer
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
RE: Need for generator breaker before step-up transformer
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
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
http://geindustrial.com/cwc/products?pnlid=5&famid=8&catid=40&id=cb-gen&lang=en_US
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
RE: Need for generator breaker before step-up transformer
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
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
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RE: Need for generator breaker before step-up transformer
RE: Need for generator breaker before step-up transformer
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
rmw
RE: Need for generator breaker before step-up transformer
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
///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
My thanks to all who have provided their comments and thoughts.
RE: Need for generator breaker before step-up transformer
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
RE: Need for generator breaker before step-up transformer
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
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
RE: Need for generator breaker before step-up transformer
RE: Need for generator breaker before step-up transformer
RE: Need for generator breaker before step-up transformer
RE: Need for generator breaker before step-up transformer
RE: Need for generator breaker before step-up transformer
Not unless I work in another universe. See my post above.
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RE: Need for generator breaker before step-up transformer
RE: Need for generator breaker before step-up transformer
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
RE: Need for generator breaker before step-up transformer
RE: Need for generator breaker before step-up transformer
RE: Need for generator breaker before step-up transformer
RE: Need for generator breaker before step-up transformer
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
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
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
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
RE: Need for generator breaker before step-up transformer