• We will be performing upgrades on the forums and server over the weekend. The forums may be unavailable multiple times for up to an hour each. Thank you for your patience and understanding as we work to make the forums even better.

Use of a Y-D transformer to interface a 3W-D energy storage inverter to 4W-Y grid/load

Merry Christmas

RERoyA

Member
Not sure if this is the best location for this post

When a 480V 3p/3W BESS is grid-tied (GT) to a US 480V 3p/4W, and only operates in GT mode, the inverter can be connected directly to the grid, and can be used for peak shaving, etc. Under this connection the inverter operates in grid following mode, i.e., as a current source. The inverter will always provide a balanced output to the grid/load. Any neutral current in the system due to load imbalance is absorbed by the grid.

If however, the same inverter is also to be used for back up (i.e., must switch between grid-tied and standalone/microgrid mode), then a transformer must be placed between the inverter and the load to essentially convert the 3W inverter to 4W, and thereby be able to supply neutral current to the load under a grid failure scenario.

I am sure the transformer type used for this must be a Y-D with the Y-winding facing the grid and D-winding facing the inverter. With this configuration the D-winding "absorbs" any N current.

So the question on this post: could a Y-Y transformer be used for the above application if the inverter side Y-winding is left floating/unconnected - not grounded.

I do not think this is a usable connection, because when in uGrid mode, the inverter is acting as a voltage source supplying the load, and if the load is unbalanced, a N current exists on the load side. Since it is a Y-Y winding, there will also be a need for N current on the inverter side winding - else, any single phase connected loads would not operate. (??)

An additional comment: The down side of using a Y-D transformer as noted above is the inverter side of the circuit becomes an ungrounded Delta power system and therefore needs a small (zig-zag) grounding transformer (or other GF detector) on that circuit to determine a L-G fault condition. A shunt in the ZZ transformer neutral can be used to open a contactor on the load side of the transformer, removing it and the inverter from the circuit.
++
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Suppose you have any voltage 3 phase grid connection, and have a Delta (grid-side) : 480Y/277V transformer, with a 480V 3P3W BESS connected to the 480Y/277V secondary system. If the grid goes down and the BESS acts as 3P3W voltage source, with the grid disconnected on the primary side of that Delta : 480Y/277V transformer, then I believe the transformer will now act as a neutral forming transformer.

Cheers, Wayne
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
This is above my pay grade, but I believe you could also power the premises 480/277 wiring directly with the off-grid BESS if you have a neutral forming zig zag that is connected (only) when off-grid. Avoids the issue of having to deal with an ungrounded system. Also avoids transformer losses when on-grid.

...

So the question on this post: could a Y-Y transformer be used for the above application if the inverter side Y-winding is left floating/unconnected - not grounded.

I thought this would be a code violation but when I looked at 250.20(B)(2) it only requires grounding the neutral when it is used as a circuit conductor. But if you don't ground it you have to deal with an ungrounded system.
I'm also not so sure if this actually works.
 
Top