No neutral? How does it work?

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iwire said:
OK here is my dumb question of the day.

As some of you know I am doing a grid tied solar installation. For this installation we have a 50 KW 208 volt 3 phase inverter without a neutral.

The service is 208Y/120.

This seems odd to me, I would have assumed the inverter would also have to be 208Y/120.

Is this inverter correct for the application?
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Is this part of a package system that has been designed by an engineer that understands the complexities of solar systems, or a home owner bought some equipment at HD* setup?

If it's a piece meal system, you might be better off having an engineer that's trained in solar systems sign off on it, or at least review it and make recommendations, if it's a kit system, than an engineer has already signed off on it, in theory.

Is this system grounded? That would really be all you need, if you're planning on using the earth ground as a "neutral" path back to source. I hear there are a few countries that use a single line supply system with an earth "neutral", since they have large spans without anyone tapping into them.

*Not to insinuate or insult HD or the homeowner, just used as a reference for a piecemeal system that may or may be right.
 
iwire said:
That is a design decision, he could have gone with another kind of system
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Bob, out of curiosity, did you contact the manufacturer and ask about this, and whether a neutral is internally (or should I say 'externally'?) available?
 
080909-1128 EST

Larry:
So, what you're saying is that the three utility secondaries establish a neutral point for the Delta, by the very nature of their low source impedance. As long as the primaries are supplying power, the secondaries are "locked" at 120v, and behave as a neutral-derving device. Makes sense.
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Yes.

If you have three single phase transformers connected Y to Y and open all the primary hot lines, then the secondaries all look like a high impedance, and this won't do much to stablize the secondary neutral voltage.

Do a Google search on zig-zag transformers. These can be used to establish a Y neutral from a floating delta source. However, this is not a likely direction to follow vs just using a delta to Y transformer.

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Ok, I got lost from the OP to the last post, so go ahead and call be dense.

But, according to the PV systems we've done, the manufacturer of the inverter will usually dictate that there has to be neutral isolation between their equipment and the system they tie into. To do this, they will ground their equipment but not bring out the neutral. The fact that you are tying a 3 phase 3 wire inverter into a 3 phase, 4 wire system makes no difference to the system. With the wye connection on the LV side of the service (208Y/120V), disconnecting fro the grid will not remove the LV system reference, and you could continue to serve single phase loads from the inverter power only. Assuming you have either load shedding capability, or you are set-up for island operation.

If you want to know why, you will probably have to contact the inverter manufacturer with this question.
 
080909-1300 EST

kingpb:

If you have a 3 wire floating delta or a 3 wire floating Y where the neutral is unconnected to anything, and you connect an unbalanced Y load of 1, 2, and 3 ohms, then the neutral voltage of the load will not be at the theoretical voltage of a Y source neutral. To get 120 V across each of these loads where is the unbalanced current going to flow? Either you have a neutral path or your not going to have equal voltages.

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LarryFine said:
Bob, out of curiosity, did you contact the manufacturer and ask about this, and whether a neutral is internally (or should I say 'externally'?) available?
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Heck no, not when I have access to all of you. :)

Besides, I don't mind looking dumb here, I don't want to sound dumb on the phone with the maker.
 
gar said:
080909-1300 EST

kingpb:

If you have a 3 wire floating delta or a 3 wire floating Y where the neutral is unconnected to anything, and you connect an unbalanced Y load of 1, 2, and 3 ohms, then the neutral voltage of the load will not be at the theoretical voltage of a Y source neutral. To get 120 V across each of these loads where is the unbalanced current going to flow? Either you have a neutral path or your not going to have equal voltages.

.
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I guess I'm not following where it is floating since the load, whether single or 3 phase, is connected to the bus side that is attached to a 3 phase 4 wire system, e.g. 208Y/120V. It is really no different then operating two generators in parallel, one being grounded wye and the other with a ground switch open. (This is required when operating a combustion turbine and diesel engine generator in parrallel, BTW).

Not really wanting to argue the point, it's simply the way these PV equipment manufacturers require their equipment to be installed. I would like to hear from them why.

I may have mis-spoken on disconnecting from the utility, in that you would NOT want to run in island mode because you would lose your reference.
 
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I don't have a code book in front of me right now, but seems like I read something about groundfault protection being required for the solar panels, maybe thats why it's required, I don't know.:confused:
 
Satcom Neutral

Satcom Neutral

Bob, I believe Satcom requires a Neutral but for reference purposes only. I have found reference to a neutral online and got a reply from a online request for info after about 3 weeks.
 
080909-1630 EST

kingpb:

I will try to clarify the point.

If I have a 3 wire 3 phase voltage source (it does not matter whether that source is wired as a delta or a Y), and there is no neutral connection to that source (obviously there can not be a neutral if it is a 3 wire source), then an unbalanced Y load will cause unequal voltages from the source lines to the load center point which one would normally connect to a neutral conductor.

If this voltage source is tied in parallel with a grid supplied Y secondary, the grid is supplying power (meaning the primaries are not unconnected), and the transformer neutral is connected to the load neutral, then the grid voltages primarily define the load voltages.

Obviously the power available from the solar system is small compared to the grid and thus the grid dominates in defining voltages. When operating where the inverter is tied to the grid it has to operate in a current limiting mode.

If the inverter is operating to supply the load off grid, then it has to be in a voltage mode with overload current limiting. This may not really be different than when operating on grid other than maybe the voltage setting of the inverter is less when off grid. However, if the load is a Y connection, then there must be a neutral connection to the inverters.

Note: I am not describing this from any experience with a solar system inverter, but simply from basic electrical theory.

.
 
LarryFine said:
So, what you're saying is that the three utility secondaries establish a neutral point for the Delta, by the very nature of their low source impedance. As long as the primaries are supplying power, the secondaries are "locked" at 120v, and behave as a neutral-deriving device.
Click to expand...

Are you referring to a "Zig-Zag" connection
which is a "Delta+Wye" secondary transformer combination.

See NEC 450.4 for the "Grounding Auto-Transformers" !

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080910-0603 EST

benaround:

As long as the only time the inverter is supplying power is when the grid transformer is connected to the grid, then there is no need for a neutral from the inverter. The grid transformer with its secondary Y connection and its neutral connection to the Y load establishes the neutral point.

As I described earlier the grid transformer, when powered, looks like a voltage source with a small internal impedance between the neutral and a hot line. Each of the three voltage sources is frequency synchronized, but offset in phase by 120 degrees from the others.

When the grid is active and able to supply power, then its Y secondary performs the conversion of the inverter's delta to the load's Y. Basically when connected to the grid the inverter looks like a current source. A current source supplies a constant current independent of the load voltage.

Obviously in the real world there are no voltage or current sources that can operate over an unlimited range. A battery is approximately a constant voltage source. As you vary the load on the battery over a substantial range there is very little voltage change at the battery terminals. You can approximate a current source with a large voltage source and a resistor. For example: a 1000 V voltage source and a 1000 ohm resistor produces 1 A under short circuit conditions. Go from a 0 resistance load, the short circuit, to a 10 ohm short. The load current changes from 1 A to 0.9901 A. A change of 1% in current while the voltage changed from 0 to 9.9 V across the load.

With electronic feedback circuits one can build rather good voltage and current sources within reasonable ranges.

There might be some need by the inverter to pickup a reference from the load netural to detect voltage imbalance to shutdown the inverter. This would be a nonload carrying wire.

.
 
080910-0647 EST

glene77is:

With a grid connection it does not matter what the grid primary connection is. It is only important that the grid is a moderately low source impedance and that the grid secondary is a Y connection. Simply view the grid transformer secondary as I just described in my post addressed to benaround.

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Since the grid connection provides the ground source, the inverter does not have to have a neutral. Any zero-sequence load current will be supplied by the grid.

I don't think this is any different from having an ungrounded (delta or ungrounded wye connected) generator running in parallel with the grid.
 
Bob, don?t know if you got your answer or not, I got tired of reading all the replies. If this is a grid-tied system, a neutral is not required; the service transformer will derive it. Doesn?t matter if it is single-phase (240/120) or poly-phase (208/120).
 
Extremely late reply :)

Extremely late reply :)

Bob,

I am coming into this thread really late - and I am unsure if the topic has been covered completely...

But I wish to toss in my 2? anyway! :eek:

Your Inverter may be like the many I have dealt with in the past - an extremely large version of a common UPS.

This means the input would only need to be 3 Phase 3 Wire, as the input feeds a Step-Down Transformer, which then feeds a polyphase Rectifier.
The Secondary may be 3 Phase, 6 Phase or 9 Phase.
Many of the ones I dealt with had 6 Phase Delta/Delta Secondaries, one Secondary set "offset" from the other Secondary set.

Typical DC Voltages are 120VDC

From the Rectifier & Filter Array, the DC charges the storage Batteries, and also drives the Inverter Section directly.
(In "Battery Mode", the Batteries feed the Inverter Section directly).

The output section of the Inverter assembly uses a Delta / Wye Step-Up Transformer configuration, where the Common Neutral is derived.

This is most likely why there is no Common Grounded Neutral Conductor on the Inverter's Input side, however, there is a Common Grounded Neutral Conductor on the Inverter's Output side.

I did not check the entire thread for replies, so if this information has been mentioned before, I apologize.

Scott
 
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