No neutral? How does it work?

Status
Not open for further replies.
iwire

iwire

Moderator
Staff member
Location
Massachusetts
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?
 
I'll give it a stab:

In a Y system if all phase currents are equal you have no neutral current. So assuming all three phases are supplying a balanced current there is no need for the Neutral wire.

Maybe...:confused:
 
I suspect that the invertor is correct.
An invertor is inherently a single phase device, unlike say a three phase alternator. "three phase" invertors are in fact three single phase invertors assembled in the same housing together with common control circuits to ensure they operate together as a three phase unit.

Therefore to feed energy back into a three phase 120/208 volt supply one could use either three invertors each rated at 120 volts and connected internally in Y. This would require a neutral connection, since the three invertors may not be perfectly balanced.

Or alternativly one could use three invertors each rated at 208 volts and internally connected in delta. This would not require a neutral connection, since any slight imbalance in the outputs of the three invertors would merely result in slightly different currents on the three phases.

It would appear that the manufacturer has found the second arrangement to be cheaper, or preferable in some other way.
Most invertors do not produce a perfect sine wave, but introduce harmonic distortion, this can result in extra current in the neutral if Y connected.
A delta connection cant introduce any current in the neurtal since there is no connection to the neutral.
 
080909-0733 EST

iwire:

A three phase source without a neutral (3 wire) might imply a delta or a Y source where the neutral was not connected.

If the line-to-line voltages are correct, then there is no problem feeding the grid. If the grid was always connected to the inverter, then the grid transformer and grid would do the conversion from delta to Y.

If the inverter has to be able to run off grid and supply a random Y load (meaning not perfectly balanced), then you have the wrong inverter or need a transformer to convert to Y.

.
 
Your Satcom inverter specs clearly show L-L AC output. Perhaps 120/240 but both would be L-L, not L-N.
http://www.satcon.com/power/pdf/powergate_ae30_50_7.pdf

Xantrex 250 kw grid inverters have an AC side neutral, as a reference only, with no current flow. Perhaps a reference for circuit capacity (common-mode voltage) or auto switching phase rotation to match utility.

Whatever these inverters' purpose for that neutral-reference signal, your 50 kw Satcom may monitor the EGC, if a main bonding to neutral bus is nearby.
 
Last edited:
Consider the feasibility of the following scenario...

Two transformers connected to the same supply, but one is connected delta-wye and the other connected wye-delta. For the sake of the discussion, the transformers are rated having identical primary and secondary voltages. Can the combined output be utilized?

combineddelta-wyewye-deltaxfmrs.gif
 
inverter

inverter

Bob, my first thought was "does it need a neutral"? I have never done a solar system. Keep us posted on the details! I guess the inverter is the brain of the system. I am not!!:grin:
 
080909-0904 EST

iwire:

You do need to consider fault conditions.

If your load and inverter remain connected together and the grid transformer is disconnected from them, then neutral to line voltages will depend upon the instantaneous load and under unbalanced conditions (almost certainly to exist) there may be some excessive line to neutral voltage. The neutral voltage to each line is determined by the distribution of loads when you are not connected to the grid transformer and feed from a delta source.

If the inverter can detect the unbalanced voltage to neutral and shutdown, then you maybe OK.

Check with the inverter manufacturer on what may happen with this inverter under various fault conditions in your application.

.
 
It seems to me that the source must provide a neutral to keep voltages correct with imperfectly-balanced loads.

It sure seems to me that a 208D/208Y transformer is necessary to derive a neutral.
 
iwire said:
It is.



No, it will never run off grid.


Thanks, Bob
Click to expand...

The POCO may require you to disconenct from the grid when they are off. Otherwise you could be sending power to areas they are trying to repair.

So the question would be if the Owner of the solar plant intends to use the power while the POCO is off AND does the Owner operates anything at 120V. In that case, you could install a 208V(D)-208/120V(Y) transformer to the output of the inverter.

(There is insufficent detail on the whole issue to give a complete answer since the reverse power feed and metering/isolation issue involves more components and devices on the system.)
 
weressl said:
The POCO may require you to disconenct from the grid when they are off. Otherwise you could be sending power to areas they are trying to repair.
Click to expand...

I should have been more clear, it will never run off grid.

The inverter is UL 1741 compliant which requires 'anti islanding'

So the question would be if the Owner of the solar plant intends to use the power while the POCO is off AND does the Owner operates anything at 120V.
Click to expand...

No and yes.

(There is insufficent detail on the whole issue to give a complete answer since the reverse power feed and metering/isolation issue involves more components and devices on the system.)
Click to expand...

Other then the net meter the inverter selected has all necessary components.
 
iwire said:
I should have been more clear, it will never run off grid.

The inverter is UL 1741 compliant which requires 'anti islanding'



No and yes.



Other then the net meter the inverter selected has all necessary components.
Click to expand...

Sounds like the Owner is held hostage to the POCO. Even though he OWNS the plant he can not operate it for his own use only.
 
080909-0959 EST

Larry:

The grid transformer Y secondary is what establishes the line to neutral voltages. When a delta source is connected in parallel, obviously synchronization is implied, the delta source is simply a power source to the load and grid. If the delta source supplies more current than the load requires, then the excess current flows to the grid. If the delta source supplies less current than the load requires, then all its current flows to the load and what it does not supply comes from the grid.

The equivalent circuit of the grid transformer is three voltage sources and a series internal impedance associated with each. The three voltages have a 120 degree phase relationship. In the Y connection these voltage sources all reference from the neutral, and largely define the line-to-neutral voltages of the Y connected load.

The voltage from the inverter must exceed the grid source voltage to feed power to the grid. This is automatically regulated by the inverter which is inherently current limited because it is power limited.

.
 
weressl said:
Sounds like the Owner is held hostage to the POCO. Even though he OWNS the plant he can not operate it for his own use only.
Click to expand...

That is a design decision, he could have gone with another kind of system
 
gar said:
The grid transformer Y secondary is what establishes the line to neutral voltages.

The equivalent circuit of the grid transformer is three voltage sources and a series internal impedance associated with each. The three voltages have a 120 degree phase relationship. In the Y connection these voltage sources all reference from the neutral, and largely define the line-to-neutral voltages of the Y connected load.
Click to expand...
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.

Bob said the inverters will not let the system run as an isolated system, so the transformers will never be asked to derive the neutral de-energized. Am I correct in believing that energization of the primaries is required for them to maintain a low impedance and fix the secondary voltages?
 
LarryFine said:
It seems to me that the source must provide a neutral to keep voltages correct with imperfectly-balanced loads.

It sure seems to me that a 208D/208Y transformer is necessary to derive a neutral.
Click to expand...

I was thinking this as well, because if the phase angles were not held in place, then there would probably be a phase shift problem syncing the inverter output to the utility input. However, the SATCOM inverter referenced earlier in this thread mentions it has a "high efficiency transformer" built in the same enclosure. Could this be the delta-wye transformer? Is that the inverter being used here?
 
Status
Not open for further replies.
Top