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.
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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.
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