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

 

[njengr1]

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.

 

[njengr1]

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.

 

[jbartos]

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?

 

[ScottyUK]

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.

 

[njengr1]

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.

 

[alehman]

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.

 

[njengr1]

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.

 

[alehman]

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.

 

[alehman]

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.

 

[Rodmcm]

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

 

[electricpete]

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

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

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[buzzp]

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

 

[vic3fan]

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 (

http://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,

http://www.npcc.org/default.asp)

and NERC (North American Electric Reliability Council,

http://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 (

http://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.

 

[vic3fan]

I apologize, the correct link for North American Electric Reliability Council is

http://www.nerc.com.

Sorry!

 

[alehman]

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.

 

[buzzp]

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.

 

[alehman]

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

 

[njengr1]

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.

 

[CBLEPEE]

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

 

[jrhaus]

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