Why doesn't current short through ground in a grounded secondary winding of a transformer?

Question

I am reading a book on electrical practices and I've come across to a statement that says it is safe practice to ground the secondary winding of a transformer.

In general, why does grounding one terminal of a secondary winding not cause a ground fault? If, for example we hook up a load to the secondary winding side, what prevents the current from going to the path of least resistance which is the grounded point of the secondary, and ignore the high resistance load?

Especially since here in our ship, our generators wye neutral is connected to our ship's hull, and grounding the secondary winding means connecting it to the ship's hull. Why doesn't this cause a ground fault?

Edit: Addded photo.

Edit 2: I apologize for the over simplification of the drawing, let me stress that the diagram is only for the sake of simplicity and that we have the necessary circuit protection in between the generators, switchboard, transformers, and load. The ground fault current is also limited by the NER in this case, which in the drawing there is a mistake wherein if you interpret the grounding on the secondary winding it would immediately make electrical contact to the ships hull.

Answer 1

You write that at time (2) on your graph,

b is at highest potential, why does this not cause the current to flow from point b to ground? Isn't it effectively shorting one line to ground at the highest reverse peak voltage at the negative cycle of AC?

Since point b is connected to the ground, point b is always at ground potential and no current flows between b and the ground.

On the other hand, point a will rise above and fall below ground potential as the AC cycle proceeds. Specifically, the highest potential that point a will reach is approximately ground plus 311 V, and the lowest potential that it will reach is approximately ground minus 311 V. At no time will point b be at a potential different from ground potential.

(This analysis assumes that the capacitance between the secondary side of the circuit and the ground is zero. In fact, the capacitance is nonzero, but it's so tiny that it's negligible.)

Answer 2

• Current flows in loops.

• It won't flow into ground/earth because there is no conduction loop involved.

• Current will flow into the load.

Faults are only faults if they are designated as such. For instance, your ship's generator's ancillary equipment will recognize a ground fault if any of the live wires connect to ground but, given that it's highly likely that your generator's neutral is bonded to the ships hull, you wouldn't regard a neutral to ground/earth/chassis connection as a fault (and neither should it be regarded as a fault).

Answer 3

A tad strange a 450V switchboard has a neutral connected to the hull. This is usually done with higher voltages. Any ground fault, as in a hard or incindental connection of a mains line to the hull could black out the ship.

Most 450V switchboards have an insulated neutral. This allows for the ship to have a single ground fault, which will not interrupt the flow of power to the loads, i.e. black out the ship.

A single ground fault means ship hull is energized, but much like birds on bare wires, the crew is not at direct risk. But ground faults must be cleared because a second ground fault on another power line could black out the ship or shock personnel if they make contact with circuitry. So proper PPE is required to protect personnel. A ground fault and a person makes contact with a second circuit, but has insulated shoes means there is no where for the current to go. Again, birds on wires.

Ground fault detection circuitry is required to detect ground faults. Transformers magnetically isolate secondary side from primary side, so secondary side needs ground fault detection circuitry also. This is a clue to the answer. I suggest you stop here and think about your question and this paragraph.

The power system needs to be monitored, so instrument transformers step mains voltages and currents down to lower voltages (voltage transformer - 450V:120V) and currents (current transformer - rated current:1A or 5A) to create dead switchboards to protect personnel. Again, transformers provide magnetic isolation, so to detect ground faults, the secondary side of voltage transformers and current transformers is grounded to the ships hull.

This applies to an insulated neutral or a neutral grounded at the switchboard (but without hull return - load is balanced at switchboard so no current flows through hull).

The major difference being (in your case), any ground fault will trip breakers or blow fuses, with hopefully proper protection discrimination to isolate the fault to the proper sub-circuit and not black out the ship. Easier to detect, but any ground fault could put ship at risk if sailing in a channel, close to land or other vessels.

Answer 4

A transformer with a primary and a secondary winding as you drew without a connection between those windings isolates the primary circuit from the secondary circuit.

No current will flow between those two circuits, whether they are grounded or not. The two ground connections are only a line but a closed loop is needed to let a current flow. Of course a voltage or current source within the loop is needed to let a current flow. In a loop without a source the current is zero.

A ground fault detector uses a special current transformer with two primary and one secondary windings. This transformer is used to get the difference between the currents through the phase and the neutral conductors. If this difference is bigger than a few milliampere, a ground fault is detected and the phase connection to the load is switched of.

Grounding of the transformer is done in the same way on the ship and on Earth surface, detection of a ground fault works the same way. If not all current through the phase conductor returns through the neutral conductor, a ground fault is present.