Grid Tie Question

[Laurie Forbes]

I gather form reading here and elsewhere that grid tie wind turbines usually (always?) operate through an inverter, one of the inverter functions apparently being to disconnect if grid power is lost.

However I wondering if one used a "grid excited" induction motor for the alternator in place of the usual PM type, could it be connected directly to the grid and, would an auto disconnect still be required? If so, advantages as I see it would be considerably less cost as an off-the-shelf induction motor would replace a PM type and, no inverter would be needed.

As far as autodisconnect is concerned, as I understand it an induction alternator requires grid excitation to generate voltage so, if the grid is down, the alternator also stops generation. I gather however that islanding may still, at least in theory, occur if there is sufficient capactance in the remaining connect load (IOWs the alternator would keep generating). However, I wonder if that is a real world possibility as most grid loads are neutral (eg. resistance heating/lighting) or inductive (electric motors)?

Putting it another way, are there any electrical authorities anyone knows of who would allow such a grid tie and, if not, why not?

BTW, I have a particular interest as I have recently discovered my local power utility now is allowing grid ties for micro generators.

Thanks for any insight........

[Logan Bryce]

Yes this is done, and a couple of "new" turbines are doing this. check out www.windmission.dk and Claus can take care of any questions in the induction generation field.

Logan

[Rob Beckers]

It is my understanding that the 'big guys' do exactly what you describe: Run a generator in sync with the grid. They use a bag full of tricks to keep it in sync, even if the turbine's rotor is running a little bit faster (gearbox, rotor-slip and such). There's an excellent tutorial on all of this at the Danish Wind Power Association (http://www.windpower.org/en/tour/wtrb/electric.htm) site.

As to automatically disconnecting because the stator coil doesn't get energized: I don't think that'll work. The stator only needs power to get started, once the generator is up to speed it'll supply its own stator current and will keep going quite happily when disconnected from the grid.

UL1741 spells out what a grid generator needs to do in case the grid goes off. The main purpose is to prevent electrocution of linemen (when they disconnect part of the grid to work on it they assume it's off, you can imagine what happens if a turbine would be back-feeding it). Any device you make that had the potential of back-feeding the grid will have to comply with UL1741 or your utility will simply not allow it. That pretty much rules out any DIY interties, as you propose. It's too expensive to get it UL listed. This may vary by utility, but the turbine itself does not need to be UL listed. Here in Ontario they'll "field certify" equipment, if you fork over a few hundred $$. The actual part that hooks it up to the grid (ie. inverter) will always have to be UL listed though, the utility wants to be really sure nothing can go wrong with that.

-Rob-

[Laurie Forbes]

It is my understanding that the 'big guys' do exactly what you describe: Run a generator in sync with the grid. They use a bag full of tricks to keep it in sync, even if the turbine's rotor is running a little bit faster (gearbox, rotor-slip and such). There's an excellent tutorial on all of this at the Danish Wind Power Association (http://www.windpower.org/en/tour/wtrb/electric.htm) site.

It is an excellent tutorial - thanks for pointing it out. In regards to sync, it was my understanding that an induction alternator automatically syncs with the grid (as the RPMs go above the synchronous frequency, the current output increases but the frequency remains at the grid frequency; same with voltage which is a good thing).

As to automatically disconnecting because the stator coil doesn't get energized: I don't think that'll work. The stator only needs power to get started, once the generator is up to speed it'll supply its own stator current and will keep going quite happily when disconnected from the grid.

I won't dispute the point but I'm sure I read otherwise elsewhere. One caveat cited was possible continued generation if sufficient capacitance exists elsewhere in that portion of the grid not disconnected (an induction motor can be used as an alternator if enough capacitance is connected across the terminals).

UL1741 spells out what a grid generator needs to do in case the grid goes off. The main purpose is to prevent electrocution of linemen (when they disconnect part of the grid to work on it they assume it's off, you can imagine what happens if a turbine would be back-feeding it). Any device you make that had the potential of back-feeding the grid will have to comply with UL1741 or your utility will simply not allow it. That pretty much rules out any DIY interties, as you propose. It's too expensive to get it UL listed. This may vary by utility, but the turbine itself does not need to be UL listed. Here in Ontario they'll "field certify" equipment, if you fork over a few hundred $$. The actual part that hooks it up to the grid (ie. inverter) will always have to be UL listed though, the utility wants to be really sure nothing can go wrong with that.

OK but you would not use an inverter to connect an induction alternator to the grid, or would you? I wonder if some other type tie-in is available (one which would simply disconnect and not do the AC/DC/AC conversions)? One other point, how does the hardware detect a grid shutdown if the alternator keeps on generating?

One other thing I have wondered about - do linemen depend on there being no backfeed present when working on downed lines or do they always jumper the lines to ground as a precaution?

[Rob Beckers]

It is an excellent tutorial - thanks for pointing it out. In regards to sync, it was my understanding that an induction alternator automatically syncs with the grid (as the RPMs go above the synchronous frequency, the current output increases but the frequency remains at the grid frequency; same with voltage which is a good thing).

Yup, I think you're right. Current would increase to the point of blowing something.

I'll bow out of alternator/generator discussion. While I'm an electrical engineer, my training was in electronics. Power electrics is decidedly not my thing. So I'll gladly leave this to those that know more.


I won't dispute the point but I'm sure I read otherwise elsewhere. One caveat cited was possible continued generation if sufficient capacitance exists elsewhere in that portion of the grid not disconnected (an induction motor can be used as an alternator if enough capacitance is connected across the terminals).

We're talking about different types of generators/alternators. What I had in mind is the type used in cars for example. Once that's started, requiring external power to magnetize the rotor (and not the stator as I erroneously mentioned), it'll happily keep working when the external power source (the battery in this case) is removed.

The type used in large wind turbines (I did a little reading up) is the asynchronous generator or induction generator. It requires magnetizing the stator to start producing, and to do so there has to be current flowing through the stator. If the load (the grid) were to fall away I would think the stator current would collapse, and with it the magnetic field that powers the rotor. So no more production. As I said, this isn't my field, just my current understanding after some reading up.


OK but you would not use an inverter to connect an induction alternator to the grid, or would you? I wonder if some other type tie-in is available (one which would simply disconnect and not do the AC/DC/AC conversions)? One other point, how does the hardware detect a grid shutdown if the alternator keeps on generating?

The point I was trying to make is that you, as a micro producer, would have to comply with UL1741. The big guys probably have an entirely other set of norms they have to comply with, tailored to their specific way of interacting with the grid. So, whatever you want to hook up to the grid will have to comply with UL1741.


One other thing I have wondered about - do linemen depend on there being no backfeed present when working on downed lines or do they always jumper the lines to ground as a precaution?

I'll ask my dad next time I speak to him... ;)
He worked for the electricity company from age 17 to age 65.

Actually I think they do jumper them. But even so, an energized grid when they are sure they just pulled the switches is just asking for accidents. UL1741 stresses this for a reason.

-Rob-

[Laurie Forbes]

The type used in large wind turbines (I did a little reading up) is the asynchronous generator or induction generator. It requires magnetizing the stator to start producing, and to do so there has to be current flowing through the stator. If the load (the grid) were to fall away I would think the stator current would collapse, and with it the magnetic field that powers the rotor. So no more production.

Yes - that's what I was refering to (and simply using an induction motor as the gen). It appears to me that all else you would need, apart from the grid disconnect stuff, would be a cut-in switch that would connect the gen once it reached sync speed. I suppose a 1200 RPM motor/gen would be more desirable than 1800 (or 900 would be better still) but would cost (and weight) a lot more.

The point I was trying to make is that you, as a micro producer, would have to comply with UL1741. The big guys probably have an entirely other set of norms they have to comply with, tailored to their specific way of interacting with the grid. So, whatever you want to hook up to the grid will have to comply with UL1741.

Understood - I will have to check with the local power company to see what they recommmend for induction gen setups.

I'll ask my dad next time I speak to him... ;)
He worked for the electricity company from age 17 to age 65.

Actually I think they do jumper them. But even so, an energized grid when they are sure they just pulled the switches is just asking for accidents. UL1741 stresses this for a reason.

Please ask him, I would be interested in his comments. Come to think of it, I have seen jumper wires used on such occasions - just not sure if that is standard practice although I would be v surprised if not.