Parallel Generators different pitch 1

[RonShap]

What might be the expected result of two paralleled generators, one 2/3 pitch and the other different, both resistance grounded. I guess that they will not share load well.
A grounding resistor vendor claims that he will provide a fully rated grounding resistor and it will absorb circulating currents and make the installation operational.
The load to the generators will be a data center (UPS' and HVAC/chiller motors).

 

[JensenDrive]

I am not a good resource on the matter, but have heard enough to believe the analysis involves a tad more than grounding and the triplen harmonics. It involves interaction of harmonics that do not go to ground as well. The link below says about 10 times as much as I know.

http://www.bmcoi.com/CatLit/Power/TECHNICAL PAPERS/GEN. WINDING PITCH - LEKX3115.PDF

 

[RonShap]

That's a good resource.
Interesting that I think most of Cat's products now come with 2/3 pitch windings and I guess the reference was written before they made that change since it is negative to 2/3 pitch.

 

[edison123]

So CAT will change their "pitch" now ?

*Why a man thinks he outrun a chasing dog when it has twice as many legs?*

 

[alehman]

Genset winding pitch seems to be a topic of some controversy. For years, Cat advocated what they called "optimal" pitch, the theory of which was to minimize total harmonic distortion. They still recommend specifying in terms of harmonic distortion instead of winding pitch for stand-alone units. They seem to have changed to 2/3 as standard for some of their larger units. That would seem to contradict the old article. Cat has always offered 2/3 pitch as an option.

Some other manufacturers have always promoted and still promote 2/3 pitch for all applications and in fact, provide no other choice. 2/3 minimizes 3rd harmonics at the cost of very low X0 value (read high L-G fault).

I think 2/3 can be paralleled with other pitches, but you should consult the manufacturers. At a minimum, you would probably want to update voltage regulators.

Despite studying the Cat article and some of the competition's statements on the subject over the years, I remain unclear. Manufacturers' opinions are swayed by what they have to offer. Newage for example, who makes generators for Cummins, I think has only 2/3 pitch. Is anyone aware of any independent articles on this subject?

 

[waross]

Load sharing. The sharing of real power loads is dependent on governor settings and tuning.
The voltage setting controls the sharing of reactive loads.
My understanding is that the output of many generators is not a true sine wave, and different pitches produce different wave forms.
If you superimpose dissimilar waveforms of two generators with the same RMS values the waveforms will cross. On unloaded generators this will produce circulating currents between the generators. This is not serious because the magnitude is low and most loads have a power factor less than unity. When the load demands reactive power both machines will supply it but there will be variations within each cycle as whichever wave form has the highest instantaneous voltage will deliver slightly more instantaneous reactive power.
Very few generators are loaded to 100% at exactly 0.8 PF. so there is almost always some spare current capacity.
It is my understanding that in the majority of cases the increase in individual generator currents is negligible.
It is also my understanding that the circulating currents are swamped and cease to circulate as soon as load demands for reactive current exceeds the circulating currents.
remember that the driving EMF of these harmonics is the difference between fairly similar waveforms.
respectfully

 

[catserveng]

Our early experience with winding pitch issues and CAT generators was with the oil drilling industry, the SRSE and SRCR generators were 5/6 pitch to provide better service with SCR drives found on the drill rigs. As CAT expanded into building service generators, mostly standbys and some islanded power, the 5/6 and 6/7 pitch machines produced did a pretty good job, probably more from being way overbuilt for their kVA rating. CAT moved to 2/3 pitch for one primary issue, it is what the competition had and what customers requested, now most of our generators are 2/3 pitch.

Paralleling machines of different pitches can cause circulating currents, as pointed out above, depending on load factor may or may not be a real issue. The actual problem is heating of the stator core, and resulting deterioration of the winding insulation. Many times you can feel localized hot spots on the stator that don't always show up on the stator RTD's (if there any even installed).

I have always been told that neutral resistors are to limit fault current and usually have no effect in solving circulating currents caused by pitch differences, we usually use grounding reactors properly sized to mitigate circulating currents caused by winding pitch difference.

Here is a link to a company we use quite a bit to help us with these issues,

http://www.tierneytransformer.com/index.html

Also at one time ASCO had a white paper discussing neutral grounding reactors, and how to handle generator neutral grounding, with some comments regarding winding pitch differences.

Our experience is based on units from about 200 kW to about 4.0 MW, mostly CAT with some KATO and AVG. We have yet to find a "standard" solution for this issue as we always seem to be doing the system analysis on a case by case basis.

The EGSA Handbook also has some information on this issue from a general standpoint that you may find helpful. Their website is

http://www.egsa.org

Hope that helps.

 

[alehman]

One of my clients usually buys Cummins. A while back the subject of winding pitch came up. Not being a big fan of 2/3 pitch, and I pulled out the Cat article. Having not spec'd a Cat unit for a couple years, I was surprised when the Cummins guy told me Cat did an about face. Can we still get 5/6 if we want?

It seems like gensets are more and more "off the shelf", with few options - kind of like cars. Getting anything other than "standard" product seems to be difficult and time consuming.

 

[catserveng]

The "about face" came when CAT changed suppliers of generators, for years CAT generator ends were built by Century Electric, and CAT was their bread and butter. CAT made a long term deal with Emerson and now CAT is getting generators from Leroy Somer USA.

5/6 pitch is still available in several frame sizes as special order.

Newage and AVK (both Cummins companies)I believe will also provide different winding pitches for direct sale customers.

Baldor and KATO both put out some information several years ago regarding winding pitch and their oil field application generators, I do not have an electronic copy of either, but you could try contacting them directly for current information.

 

[JensenDrive]

Catserveng:
Was that the link you meant to insert? The link to Tierney xfmrs does not seem to help any.

 

[catserveng]

JensenDrive,

Sorry, the link is to get the contact info, they have a couple of engineers working there who have always been helpful in figuring out what we needed, but they don't have much published I know of, our inside engineer and our sales people call them when we have these kinds of questions.

This seems to be one of those areas where there is a lot of differnt ideas, but not a lot on paper.