Neutral Voltage - What if Phase A is Shorted in Three-Phase?

For a little while there might be a voltage. It will stop when either:

1. The circuit protective device operates, or
2. The generator self-destructs.

Obviously there will be a neutral voltage.

Assume it is a good generator (balanced , nice one )

The generator will generate voltage (EMF) irrespective of the current is there or not,

So the voltages are say V(0^o), V(120^o) and V(240^o), neutral being at 0.

Now as you conect an impedance Z_aon A phase say I take it as the first one for simplicity

The total impedance on that phase will be

Z = Z_in+Z_a,

Current Ia(Φ) = V(0^o)/Z(θ) will flow

If you do the sort of the back tracing you will land up in a Voltage at phase A at V'a (ø).

I think a phosor diagram may make it a bit clear

The neutral will now float to a voltage as phasor sum of V'a (ø)+ V(120^o) + V(240^o)

So this is irrespective of the current flowing or not.

You're right, I meant to say : is neutral high compared to power source ground? Thanks -PITB

I was in Canada starting a couple induction hydro-generators. There was an existing 3MW synchronous hydro-generator at the same site. There was a couple mile long transmission line to town. A tree branch fell on one phase of the transmission line a half block from the hydroplant, all of the generators tripped off-line.

Then the utility from town remotely closed the 3MW unit back into the transmission line, -they had some protective relay out for test- and the phase voltages went beserk.

The LCD voltage meter face blew about 8ft across the room, a voltage protective relay blew a 1 watt resistor and several other problems including a blown PLC.

I was told the line voltage between the remaining two phases went at least 1.73 times normal.

The major question is here: relative to which circuit point this voltage would be measured? If it is measured, as usualm compared to neutral point - any change in voltage value will be occured. On the contrary of huge current will flow across the shortening impedance

The major question is here: relative to which circuit point this voltage is measured. If it will be measured compare to the neutral point, as usual, any change will occure place. On the contrary of huge current will flow across the shortening impedance.

The voltage is measured relative to the ground, the neutral being isolated. The current is proportional to the electrical capacity of the circuit - it is a capacitive current in nature. The generator has a small winding electrical capacity so the current should be small. If the neutral is grounded then a huge current will flow across the impedance, and that is called shortcircuit. Having isolated neutral and only one phase grounded is not dangerous for the consumer but for the system's pahse to insulation. The phase to phase voltage does not change significantly.

Answer No3 by 'North of 60' sums it up.

If the generator is not grounded (fully insulated from earth) then there is no complete circuit back to the generator from any phase at the outlet via the earth loop - so no current flows.

If any phase is shorted to earth, and you connect a voltmeter between neutral and earth, you will get a reading of that phase voltage.

If the generator is not grounded (fully insulated from earth) any of current will flow across impedance which ia shortening one of phases to a ground. Two remain phases of course are opened. For current producing as minimum two phases have to be connected by their impedances to a ground. In this case the same question remains: between which circuit point the voltage is measured. Thus, a voltage more than zero may be measured between ground and generator neutral if as minimum two phases are connected to ground. In the case when a generator neutral is grounded the same question: between which point the voltmeter will be connected. To answer this question the equivalent electric circuit is to be drawn and is to be calculated.

As Guest - post 11 says, you need to draw the equivalent circuit.

In doing so, it will help to imagine the ground system as being a single conductor - an earth-wire.

The phase wire (via an impedance ?) is 'shorted' to one end of the earth-wire. The other end of the earth wire running back to the generator (which is fully insulated and not grounded) will thus not be connected to anything. Hence no circuit, and hence no current.

A meter connected across the open circuit of 'earth' and 'neutral' will show phase voltage.

If the neutral is not connected to ground then connecting phase A to ground has no affect on the generator and perhaps not to any load the generator is wired to, if the load is not grounded anywhere. The BIG problem is that the generator frame may be connected to the neutral so anyone standing on the ground and touching the generator framework is not going to like the result!!!!!! Lethal is the operative phrase!