Interlocking Requirement Between Grounding Systems

The purpose of the switches is to ensure that only one neutral is ever connected to ground/earth at any time, as has been stated in this forum earlier today and many times before. The reason is to eliminate the risk of currents circulating in the neutral conductor.

If we refer to the phasor diagram of delta-start transformer, and delta zig zag transformer, we will find out that the neutral point for star grounded transformer, and zig zag transformer are at same location.

So there should not be voltage difference between the 2 neutral points, and no circulating currents at the normal conditions.

What could be the reasons that will make a voltage difference between the 2 neutral points, and thus create a circulating current?

Thanks

Distance.

As long as you can ensure that both sources cannot be simultaneously connected to the bus then there is no need for the interlock. However, since you stated that "...power can flow in both direction through the (13.8KV/34.5KV) transformer..." you have implied that both sources can indeed be connected at the same time.

If that mode of operation is allowed then the interlock is part of a larger scheme to ensure that both feeder breakers cannot be closed until a key (indicating an open ground/disconnect) is released from one of the grounding means. This is done to prevent reducing the effective grounding impedance in the doubly-fed configuration (essentially the parallel combination of the two grounding means) from becoming so low that the available short circuit current from both systems exceeds the interrupting capacity of the breakers, the mechanical withstand capacity of the bus, and/or the downstream system.

Of course the interlock system should also include logic to never allow operation if both grounding means are open.

Regarding the circulating current:

If we refer to the phasor diagram of delta-start transformer, and delta zig zag transformer, we will find out that the neutral point for star grounded transformer, and zig zag transformer are at same location.

So there should not be voltage difference between the 2 neutral points, and no circulating currents at the normal conditions.

What could be the reasons that will make a voltage difference between the 2 neutral points, and thus create a circulating current?

I need more clarification for this question.For what purpose you are using this Sig Sa transformer in Delta side of the TFRMR - 2 ?

Provides a reference for ground fault clearance on the Δ feeder side if that transformer is connected to a supply.

What could be the reasons that will make a voltage difference between the 2 neutral points (star neutral point, and the zig zag neutral point), and thus create a circulating current?

See #8⇑.