Dissimilar AC Generation Source Wave Profiles

[crshears]

Hello all,

I've been reading up on generator pitches and how it's not [generally?] a good idea to have AC generators with dissimilar pitches interconnected/paralleled at a given site since this can induce undesirable circulating currents.

Would the same concern apply to dissimilar generation sources to sub-transmission / distribution feeders at, say, 44 kV, such that if a new generator wishes to connect to a feeder it would be necessary to determine the waveforms of other generators on the feeder [windfarms, PV arrays, etc.] before deciding on what pitch of generator to use? My thinking is that the impedance of the secondary windings of the grid-tie transformer will serve as the single "gate" between all of the feeders on that bus and the greater system beyond and confine any circulating currents between the generators.

Correct thinking on my part, or flawed?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[crshears]

Just so you know, I'm posing this question in regards to the possible conversion of the legacy steamship where I volunteer into a small-scale combined-cycle power plant, and not just from sheer academic interest [I've posted on this several times before].

Being a jack of all trades and master of none [although power system operation is probably my strongest field], I continually research as many of the facets involved as I can think of; I could never serve as a consulting engineer, but in the event we ever engage one to design our plant, I hope to at least have some kind of yardstick series of questions and/or areas of concern ready that we can use to determine what kind of bang we're getting for our buck.

Thanks for listening.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

Your thinking is correct: the GSU transformer impedance will present a fairly large impedance which will substantially reduce the interaction between machines.

The circulating currents tend to reach problem levels when dissimilar machines are directly connected to a common bus where the harmonic currents arising from the imperfect output waveforms are constrained by the positive or negative sequence impedances of the machines depending on the harmonic order (5th -ve, 7th +ve, 11th -ve, 13th +ve...).

 

[crshears]

Thanks, ScottyUK, although I'm beginning to wonder if I was less than clear...

GSU: generator step-up, right? In my scenario I'd be looking at having two of those, 8 kV > 44 kV ratio, one to each of two separate feeders fed from opposite buses at the transformer station several kilometres away [yes, I know this will involve remote/transfer tripping schemes]. The grid tie I was referring to was the pair of 230 kV/44 kV step-down transformers at that TS, and the other generators I was referring to are PV plants, some on the same 44 kV feeders we'd hope to export on, some on other 44 kV feeders.

Based on your answer, I'm surmising there wouldn't be a lot of concern, if any at all, that unbalanced / circulating currents from other generatings sources connected at the 44 kV level would have any noticeable effect on the 8 kV side of our GSUs...which, now that I think about it, only makes sense, since such circulating currents would only flow between dissimilar machines on a common bus, or possibly [depenfding on relative impedances] to some degree in the low-side windings of the GSUs, but not in the high-side windings.

As Homer Simpson would say: "Doh!"

In future I will try to think things through further before posting. Thanks for your patience.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]