Distributed Generation
Distributed Generation
(OP)
I am assessing the impact of a 1600 kW synchronous generator located approximately 15 kM out from a 13.8 kV substation. There is no regulation on the feeder and the utility maintains a fixed tap that maxes the voltage at the sub out at about 106% which results in 101% at feeder end at peak load. At minimum load, feeder end is around 104%. When the generation is added the voltage at the generator is around 108% at peak load and 110% at minimum load. I can get it down by absorbing more VArs (don't get me going on vars and Vars and VArs)at the generator but that seems crude.
The feeder conductor is a mix of 1/0 ACSR and 3/0 ACSR and there is one section that's #2 ACSR which of course is the root of the problem but reconductoring is not a preferred option.
Does anyone have any ideas for mitigation? I was thinking about a feeder regulator at the substation but I've seen voltage profiles for regulators that are all over the place with high voltage at low loads and not a lot of apparent intelligence. Are they any smarter now? If I could set it to sense if the generation was on or off and boost or buck accordingly this might work.
Comments?
The feeder conductor is a mix of 1/0 ACSR and 3/0 ACSR and there is one section that's #2 ACSR which of course is the root of the problem but reconductoring is not a preferred option.
Does anyone have any ideas for mitigation? I was thinking about a feeder regulator at the substation but I've seen voltage profiles for regulators that are all over the place with high voltage at low loads and not a lot of apparent intelligence. Are they any smarter now? If I could set it to sense if the generation was on or off and boost or buck accordingly this might work.
Comments?






RE: Distributed Generation
RE: Distributed Generation
That simple huh? My background is distribution where we did not have distributed generation so this DG is all new and adjusting to the notion of increasing VArs and increased losses just doesn't sit well. Guess I'll just have to get with the program.
If you got paid for all the great advice you give people on Eng Tips you'd be a rich man.
Thanks for the advice and taking the time.
RE: Distributed Generation
RE: Distributed Generation
I agree with David Beach that I don't see a big problem with absorbing vars (leading pf for the generator) but obviously the power factor must be kept within the reactive power capability curve of the generator. There are some limitations in this region. It may be necessary to use a fairly sophisticated voltage regulator that will allow operation on voltage control but with under and over excitation limiting.
Adding regulators at the substation may not help all that much. Power has to be pushed from the generators to the substation and there is a certain voltage gradient that goes along with that.
RE: Distributed Generation
Regulation will also solve the problem. The best place to put voltage regulation is about half way along the feeder. Setting it to regulate to about 102% may work quite nicely (will boost voltage if generator is not there and buck if is there). You'd have to model it to pick a good setting and also ensure there are no voltage change issues if the generator suddenly drops out.
Ian Dromey - www.dromeydesign.com
RE: Distributed Generation
You could check the Var limit of the generator and leave certain margin.
RE: Distributed Generation
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If we learn from our mistakes I'm getting a great education!
RE: Distributed Generation
As said above, know the capability curve limits of your machines, also be aware of system voltage stability. Many digital voltage regulators have underexcitation protection, you just may need to watch the system in operation and see where the actual limits of stability really are. If your system is soft or can be unstable you may want to look at loss of sync protection, stator temp monitoring. At a smaller Alaska utility the protection people added a form of rate of change protection so if things started getting unstable, they tripped DG units, got things settled down, and reclosed them back on.
Easiest way to find out if unit is getting unstable, look at the field output of the AVR, preferably with a scopemeter. I have found I can find stability problems early on looking at the excitation long before it gets bad enough to affect the system voltage.
Hope that helps.
RE: Distributed Generation
Two different stability issues there: one is the AVR control loop stability which I think is what you've been looking at. The other is the generator stability limit beyond which there is a fair chance of the generator breaking out of sync during an external electrical system fault. The simplest way of thinking of it is that the magnet created by the field is not strong enough to resist the torque from the engine and the transient torque from the fault. With a stronger magnet - higher field current - it takes more torque to break out of sync. Obviously there's more engine torque at higher power output which is why the capability curve allows a slightly more leading PF at low output than at full power giving that part of the capability curve the characteristic shape.
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If we learn from our mistakes I'm getting a great education!
RE: Distributed Generation
One thing to keep in mind with some of the latter comments is that this thing is not being dragged in on a flatbed and connected and if things don't work out, ho hum it can be dragged away. The impact study needs to predict the outcome because someone is going to spend big dollars installing this thing running on methane from land fill and the utility has to be able to live with what happens when it connects.
So everything needs to be known ahead of time including mitigation. I think between the two mitigation methods outlined above this proposed installation can be made to work.
Again. Thanks.
RE: Distributed Generation
Have you considered getting some modelling work done? There are quite a few good software packages out there but I strongly advise that you get an experienced consultant to do the modelling rather than try to do it yourself.
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If we learn from our mistakes I'm getting a great education!
RE: Distributed Generation
Actually I am using software and that's what alerted me to the problem and I'm pleased to report that when I model the suggestions above they work.
It goes something like this. If utility reduces taps to get their voltage down to 103% then at min load at generator
PF = 0.98 V = 109%
PF = 0.95 V = 107.9%
PF = 0.9 V = 106%
PF = 0.98 w reg V = 107.2%
PF = 0.95 w reg V = 105.9%
PF = 0.9 w reg V = 104.9%
PF is overexcited importing VArs
This is a bit of an oversimplified summary but the point is there's a solution and with some more tweaking of the parameters it looks like it can be made to work out.
RE: Distributed Generation
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If we learn from our mistakes I'm getting a great education!
RE: Distributed Generation
Not not suggested all the time.
"The other is the generator stability limit beyond which there is a fair chance of the generator breaking out of sync during an external electrical system fault."
Not necessary an external fault, a generator can still be O.O.S due to small disturbances depends on the characteristics of the machine ,interconnection network and the rest of the system.
In the utility I work, all the major generators (above 200mw voltage control buses)are all not suggested absorbing var under system normal.
RE: Distributed Generation
Since I work on tranmission side, there may be rules different from DG due to the different roles in the system.
RE: Distributed Generation
ScottyUK:
machine remains syn with the system is not simply a generator problem but related to the system. transiently stable after a fault does not mean the machine can remain syn.after minutes. diffferent countries have different network topologies. Some ar estrong some are not.I believe what you said based on what you experienced but maybe something you havenot seen and have not experienced.
I moved fro mone place to the other and I saw different designs, networks and events.
Have you seen people design a 500km single 13xkV line system with machines at the end. Have you seen seen 180km 2xkV network? Maybe you do but I did not in the past but gained a lot after I experienced.
RE: Distributed Generation
You are right, all depend
OP say about 1600kW generator, absorb kVar isn't problem.
of course according to gen stability curves.
For example, one of our member story to us about Spain, where he must in the night absorb Var's.
BTW, 500km of 13kV...isn't picnik, good luck to you.
Best Regards.
Slava
RE: Distributed Generation
Not so fast there. The generators from your former life (at least the ones that were CT driven) were among the largest that your favorite generator company every built. Plenty of their stuff could be and regularly is carried on platform trailers (drop deck type naturally). I think the same regarding size is true of the Circle W gas turbines you used too, or at least true at the time they were built. But there are some real petite little CT turbogenerator packages out there-nothing like the monsters that you lived with. I think I could just about fit the engine out of a GE LM 2500 in a large pickup-said tongue in cheek, naturally.
I had an opportunity to visit Falcon Works a couple of years back when one of your (formerly your) exciter armatures was in their shop and when I saw it, it was staged in the final assembly area along with some of their new product and it was every bit as large as some of the whole complete generators that they were in the process of shipping. I had to ask what that big ugly black thing was because of its unproportionality to the other stuff I was seeing in their shop. That is when they told me that it was from Teeside.
On another visit to Falcon Works, one of Teeside's rotors was there and I had just visited another generator shop on the continent and had observed a 500 MW hydrogen cooled rotor and your air cooled rotor wasn't discernably much smaller than that 500 MW hydrogen cooled rotor. That rotor by itself probably weighed as much as some of the smaller generators that your favorite generator company builds.
I once observed one of their (crated) generators in the Houston area on a drop deck type flatbed trailer high centered on a railroad track. Cute. Glad it wasn't my problem.
rmw
RE: Distributed Generation
QBplanner,
I appreciate your comments regarding transmission systems. My experience is UK-based where we have a geographically small and heavily interconnected grid, and I sometimes forget that other grids can behave very differently to ours. Thanks for the information.
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If we learn from our mistakes I'm getting a great education!
RE: Distributed Generation
Any reason for that generator to ever be islanded or will it only operate in parallel with the substation feed.
RE: Distributed Generation
Bit of a starting problem though. Would need some TLC to get it going without dragging the feeder voltage down.
But there must be a catch. What is it?
No it will never island.
RE: Distributed Generation
In a recent project, with 2 x 1000kW synchronous units the only way we could keep the sending end voltage in reasonable limits was to install a pole mounted buck and boost unit. This was a cheap and good solution allowing us to export VArs, and never import them.
RE: Distributed Generation
A "pole mounted buck and bosst unit"? Aka a regulator? So located right at the generation site, bi-directional, and what typical settings?
RE: Distributed Generation
If this is an express feeder, you can get away with this.