Generator Pitch 1

[chad44]

I design a lot of installations with multiple emergency generators connected in parallel (typically 480 volt diesel engine units). For many years, Caterpillar has always promote an "optimum pitch" generator, which is usually around 4/5 pitch. Other manufacturers have claimed that in order to parallel, especially with a utility source, the generators should be 2/3 pitch. Does anyone have more knowledge on this subject that they could share?

 

[BSEE1980]

May I suggest you Google "generator winding pitch" and you will find several very useful articles. No reason to re-print them here.
One of the first hits is a CAT technical paper.

Steve

 

[alehman]

Also search this forum for previous threads.

 

[rbulsara]

I agree with "other" mfrs. That paper of CAT is correct as long as they do not parallel with US utilities. USA's utility supplies do not like anything other than 2/3rd pitch for paralleling.

I have designed and commissioned a number of paralleling gen plants, ironically mostly CAT, exclusively with 2/3rd pitch. Had no issues.

A case of a site (not engineered by us, and even that engineer was not at fault) where someone installed Six(6) 2MW gensets with some other pitch, even when 2/3rd pitch was specified, there was so big an issue with the circuilating currents that the mfr ended up having to replace all six alternators without added cost to the Owner!

 

[catserveng]

The optimum pitch claim from CAT came from the days when their EPG business was mostly SCR drives for drill rigs, and in those days the CAT tail ends were hard to beat, and the 4/6 pitch was very good for the 6 pulse drives found in the field. When the SR4 tail end came out it was available mainly in 4/5 pitch, and many units were shipped 5/6 pitch as well, again targeting the drilling and single unit industrial markets. Now most of CAT's tail ends are 2/3 pitch, a quiet shift over the years. The other pitches are still available, but mostly as special order.

We have paralleled a number of non-2/3 pitch machines with both the utility and other machines of different pitch, usually requires a neutral grounding reactor installed.

There have been more than a couple of times when an expansion or new project forgot to take that issue into consideration. Most of the local engineers I deal with where I live are famiilar with DG know the issues and account for them. But quite a few people aren't and it can cause some headaches, everything from complete unit changeouts to floating neutrals, depends on the site and application.

Cummins and Marathon also have excellent papers on generator winding pitch, the current Cummins info is probably the most up to date of any of the US manufacturers I know of.

Hope that helps

 

[alehman]

rbulsara,
I could see how that might happen if there is no isolation of zero-sequence current. I would be surprised that was a problem if there was a Dy transf between the gens and utility. Do you remember any details of the transformer connections?

 

[rbulsara]

No, we were not actively involved in that incident. But we were called to assist in complete the remaining commissioning after the replacement as the origial engineering firm went belly up overnite! So we heard the stories.

I have found no reason to deviate from 2/3 pitch so I stick to it.