Correctly tieing in a generator to the Utility Grid

[jljrandall]

Need some help. (this application is on a dairy) (this is a privately engineered design). I have a generator that used to be a 3 phase motor, that has been rewound to be a generator? It is driven by a 454 methane fueled engine. (they are using the cow manure), The name plate says 240/480V, 50 KW,60 Hz. The Utility company installed a 3 phase 120/208 bank for this dairy. The 3 Phase wye is brought all the way to the transfer switch, (I could attach a hand sketch if you like), They bonded at the first means of disconnect, ( a sqare D I line panel 600A) and the Nuetral of the wye in on an insulated lug and dead ends in the transfer switch. (most gen systems I have seen installed have been single phase and the Nuetral of the gen is tied with the Nuetral of the Permenant power in the the auto transfer switch. Well my question is is that the I am being told that the dairy uses the utility power to apply the field and maintain the field because the supplier of this home made contraption has not supplied the exciter package yet. (So when the gen is on line it is always connected to the Utility grid, their is no sync control, no meter watching voltage, freq, or amps) Well, that seems a little dangerous to me in my gut, with the influx in frequency in by utility and out by the gen?, and then I get to looking at this thing and they have only 3 phase wires and 1 equipment ground going to the head of the motor? The dairy seems to think that if they had there exciter package they could operate as a stand alone unit, ie, run a few lights, operate their HMI controls for milking, some pumps ect., my question also is where are they going to get there Nuetral? They are under the assumption that if the Utility has a power outage they will just use the Utility system nuetral from the the 120/208 bank out at the pole. The big 3 phase gens I have hooked up before had 3 phases, a nuetral and a gound. A lot of things do not seem right, electrically here and I can not quite put my finger on it or have the code knowledge with these home made generation systems people are wanting to install weather it be wind, solar, water, or methane fueled to lower their KW usage from the power company and in some cases customers are wanting to do a net metering agreement with the local Utility. I have dug in 705 and 445 and I am stuck, and I need your help.

thanks alot.

Jason

 

[Cold Fusion]

The motor is operating as an induction alternator. It is not a synchronous generator. It has to be over synchronous speed to generator power. If it is a 2 pole mptor (3600 rpm nominal) then it will have to be turned 3700 - 3800rpm to generate.

It also has to be connected to a grid to work. The grid supplies the magnetizing current to the stator and sets the voltage.

As for the exciter package, it likely is an inverter. I have not seen one set up like that but have often though that it should work. The inverter would have to have batteries. The alternator will not self start.

This is an exciting project. Let us know how it turns out.

cf

 

[iwire]

Jason, I would run from this, I would not even admit I was there.

Back feeding the utility without the utilities active participation is a major problem and also a dangerous one.

If you did have the right equipment to be always tied into the grid you would have a control system that would prevent back feeding a dead utility line.

As far as the neutral, I think using the power companies transformer to provide the neutral would work but it is certainly a bad idea and I am sure a violation of both the NEC and power company rules.

 

[drbond24]

I 2nd iwire's opinion. This is not something you want to get involved with.

The only way to do it right would be to heavily involve the local POCO so they know what is going on, but I'd bet a large sum of money that they would just flat out tell you that you can't do it. They (I could actually say 'we' since I work for a POCO) don't like random generators connected to the line.

From the plentiful references to this being a hack job on the generator construction, I would assume that your customer is not the type of person who wants to do this right, so I'd get out of there. When somebody gets hurt you'll be glad you weren't there.

 

[growler]

Need some help. (this application is on a dairy) (this is a privately engineered design).

Jason, I would run from this, I would not even admit I was there.

I don't know that I would run from it but I sure as heck wouldn't get involved without seeing lots of paperwork. First I would have to see some signed and sealed drawing from a PE that had been submitted for approval from the local AJC. If they can come up with everything that will allow for a permit then I would contact a field engineer from the power company for a site visit and a consulation.

It really should be the engineers that designed this system working with the power company engineers to come up with and installation method that's accaptable.

Who knows, there may be some money to be make from this BS, I mean cow manure. If they can come up with and approved method of installation that's both legal and safe. This sort of thing may catch on.

 

[zbang]

I don't think I'd touch it unless I was a PE with a signed consulting contract, and then only to offer my opinion that they're nuts.

OTOH, -if- they had an approved grid-tie-in converter, then I might change my mnd.

 

[winnie]

I concur that systems that supply power to the utility really need proper engineering and coordination with the utility. If the customer doesn't want to deal with the proper authorities, then I agree: run.

On the theory side of things:
As cf said, an ordinary induction motor will naturally operate as a generator, with no rewinding required. All that is needed is an 'overhauling load', something mechanical that is pushing the motor to operate at greater than synchronous speed.

An induction motor is always consuming VARs to supply its magnetic field, and this reactive current remains pretty constant across the entire load range. An unloaded induction motor simply sits and consumes VARs, with very little real power consumption. When you apply the overhauling load, the induction motor continues to _consume_ VARs, but supplies real power to outside loads.

One very nice safety feature about this approach for a small scale generator is that if the source of VARs goes away, then the generator output falls to zero. If the utility grid supplies the VARs, then if there is a utility outage, then the VARs go away, and the generator output goes to zero; no backfeed.

However there are other ways to get the needed VARs. Given the description of the system, I'd be worried that the 'exciter' is a very sketchy and poorly regulated capacitor bank, which can certainly supply VARs to keep an induction machine generating......

As far as deriving the neutral, a transformer separate from the generator can certainly provide a neutral, but using the utility's bank for this purpose is a bad idea. Separate transformers are sometimes used to derive a neutral in order to ground previously ungrounded delta systems. These transformers are either 'zig-zag' grounding transformers, or they are conventional delta-wye transformers with nothing connected on the delta side. In the latter case, currents circulating on the delta side are needed to maintain the neutral point with unbalanced loading. A simple 'wye' secondary connected to a delta system will not give you a reliable neutral.

If they use the utility transformer bank to derive their neutral, then they are relying on current flow on the _primary_ side to maintain their neutral.

-Jon

 

[mivey]

I'm surprised it went this far. Before we even start the review process, there are over 20 pages of signed documents that have to exchange hands, most of it dealing with avoiding this very problem.

 

[Cold Fusion]

jlj -
Don't let the nay-sayers get you down. From the tone of their posts not one of them understands what the job entails.

Yes, the system will have to be engineered - and not somebody's homebrew engineering. This is not something that can be designed using the NEC as a cookbook.

Yes, you will have to follow the poco's requirements for interconnection. It will be a pretty small or poorly run poco that does not have a list.

Yes, the design will include a D-Y transformer between the alternator and the on-site distribution.

You said it was "privately engineered". If that doesn't mean "engineered" then what does it mean? If it means that the system was speced and designed by someone that doesn't understand the science or physics then it likely won't be safe, reliable, or meet regulation.

cf

 

[mivey]

Yes, you will have to follow the poco's requirements for interconnection. It will be a pretty small or poorly run poco that does not have a list.

And if they don't, I would suggest you find some from another utility as a starting point to having your own guide. We don't want to hurt anybody or damage anything.

The customer may not even be aware of the dangers.

 

[Cold Fusion]

A couple of posts had some technical statements that I don't understand. Perhaps the posters could elaborate.

--- On the theory side of things:
--- I'd be worried that the 'exciter' is a very sketchy and poorly regulated capacitor bank, which can certainly supply VARs to keep an induction machine generating......

jon -
Do you have any references about how a cap bank can supply vars to an asynchronous induction alternator and get it to self start? I've never heard of that. Nor do I know of any science or math model that would support that happening.

---if- they had an approved grid-tie-in converter, then I might change my mnd.

z -
What is a "grid-tie-in converter" and how would it apply to an asynchronous induction alternator?

cf

 

[Cold Fusion]

I don't think I'd touch it unless I was a PE with a signed consulting contract, and then only to offer my opinion that they're nuts. ---

Well, I am a PE and I'd be interested in completing the system. It would be an interesting project. The physics concepts are sound and will work fine. They just need some real engineering. This is not a house wiring job. As I said earlier, NEC cookbook does not cover it.

cf

 

[iwire]

jlj -
Don't let the nay-sayers get you down. From the tone of their posts not one of them understands what the job entails.

Yeah, OK.:roll:

What I get from the first post is this is some sort of home brewed set up without Power company involvement.

If that is the case I would run from it.

But hell, I don't know anything, let the guy go ahead, perhaps hurt himself, or linemen, or maybe just damage some equipment, or maybe he will just get in a legal bind. But CF says it's OK.:grin:

I have worked with power company interconnections and the power companies around here get more than a bit uppity when you talk about back feeding them until the PEs stamped drawings run by them.

 

[iwire]

Well, I am a PE

Cool, must be a great gig, I don't think the OP is a PE and I doubt he is in a position to hire one for the project.

and I'd be interested in completing the system. It would be an interesting project.

And I would be more than happy to do the installation with your signed prints in my hand.

They just need some real engineering. This is not a house wiring job. As I said earlier, NEC cookbook does not cover it.

I did not see anyone suggest that the NEC is an engineering handbook.

 

[drbond24]

A couple of posts had some technical statements that I don't understand. Perhaps the posters could elaborate.


jon -
Do you have any references about how a cap bank can supply vars to an asynchronous induction alternator and get it to self start? I've never heard of that. Nor do I know of any science or math model that would support that happening.


z -
What is a "grid-tie-in converter" and how would it apply to an asynchronous induction alternator?

cf

It just has to be a flux capacitor. All is well unless you let the rotation get faster than 88 revs/sec in which case you'd see some serious....well if you know the movie you know what goes there.

The grid-tie-in converter is very much like the staples 'easy' button. It magically makes all of the problems go away.

Well, I am a PE and I'd be interested in completing the system. It would be an interesting project. The physics concepts are sound and will work fine. They just need some real engineering. This is not a house wiring job. As I said earlier, NEC cookbook does not cover it.

I'm a PE too, but I don't go around handing people axes and then sticking my neck on a block of wood.

Ok, seriously now. I agree that with the proper engineering work done in advance this project would work. My problem is that from the description in the OP, I would give it about a snowball's chance in h-e-double-hockey-sticks that anyone involved is interested in actual engineering work. The OP needs to hire a PE and pay him good money to come up with a plan that will work, but he is asking us here on this forum instead. Not exactly the same thing even if there are PEs here.

 

[winnie]

Do you have any references about how a cap bank can supply vars to an asynchronous induction alternator and get it to self start? I've never heard of that. Nor do I know of any science or math model that would support that happening.

Unfortunately I have no experience with these systems; just something that I read awhile ago, and it seemed to make sense at the time. Because the simplest possible approach involves a few fixed capacitors and a motor, it is easily adoptable as a jack-engineered scheme'; see for example: http://www.redrok.com/cimtext.pdf complete with a hand drawn schematic showing 3 120V circuits tapped from midpoint and 'hot' point of wye connected coils (run away fast!)

A google search for 'capacitor excited generator' or 'capacitor excited induction alternator' or 'self excited induction generator' brings up quite a few hits, including respectable research on the topic, eg: http://www.mssanz.org.au/MODSIM97/Vol 2/Su.pdf

As I understand it, with no loads connected, most induction machines will probably have enough residual magnetism on the rotor to get just enough current flowing in the capacitors to get induction generation to start. Clearly a perfect induction machine will have zero residual magnetism, because it is made with perfect 'soft' magnetic material.

-Jon

 

[Cold Fusion]

--- I'm a PE too, but I don't go around handing people axes and then sticking my neck on a block of wood.---

Dave -
Good point. I don't need to do that.

I wish to show that the concepts are nothing to be afraid of - one just has to understand them. And other than jon, I can't tell anyone here does.

I agree with you in that I also think the OP doesn't understand the concepts, but he said he had "private engineering" - maybe that means that someone connected with the project does understand.

--- The OP needs to hire a PE and pay him good money to come up with a plan that will work,---

No, the customer, the dairy company, needs to hire this out - I would not expect the installing electrician to do the engineering or selection of the engineering.

Aside:
If this just happened to be a Double Fed Induction Alternator, with a vfd feeding the wound rotor, and with a battery bank and inverter for self starting, I'd really be interested in working on that. That would be exciting.

cf

 

[Cold Fusion]

Unfortunately I have no experience with these systems; just something that I read awhile ago, and it seemed to make sense at the time. Because the simplest possible approach involves a few fixed capacitors and a motor, it is easily adoptable as a jack-engineered scheme'; see for example: http://www.redrok.com/cimtext.pdf complete with a hand drawn schematic showing 3 120V circuits tapped from midpoint and 'hot' point of wye connected coils (run away fast!)

A google search for 'capacitor excited generator' or 'capacitor excited induction alternator' or 'self excited induction generator' brings up quite a few hits, including respectable research on the topic, eg: http://www.mssanz.org.au/MODSIM97/Vol 2/Su.pdf

As I understand it, with no loads connected, most induction machines will probably have enough residual magnetism on the rotor to get just enough current flowing in the capacitors to get induction generation to start. Clearly a perfect induction machine will have zero residual magnetism, because it is made with perfect 'soft' magnetic material.

-Jon

Maybe it can work. However, I don't see how the voltage and frequency are regulated.

cf

 

[Cold Fusion]

Yeah, OK.:roll:

What I get from the first post is this is some sort of home brewed set up without Power company involvement.

If that is the case I would run from it.

But hell, I don't know anything, let the guy go ahead, perhaps hurt himself, or linemen, or maybe just damage some equipment, or maybe he will just get in a legal bind. But CF says it's OK.:grin:

I have worked with power company interconnections and the power companies around here get more than a bit uppity when you talk about back feeding them until the PEs stamped drawings run by them.

(emphasis by cf)

I'm not sure how to answer this.

I didn't advocate backfeeding. The OP didn't either. You and a couple of others are the only ones that have mentioned backfeeding.

So, I would have to say, "Bob (and others), don't backfeed, the pocos don't like it. Even if you had stamped drawings the poco wouldn't like it."

cf

 

[iwire]

I didn't advocate backfeeding. The OP didn't either. You and a couple of others are the only ones that have mentioned backfeeding.

OK, I am out of this, knock yourself out. But given the OPs post I stand by every letter of each of my posts in this thread. :roll: