Understanding Voltage and Grounding in Three-Phase Transformer Systems

[samaaa]

hi guys i like this forum

if we have a transformer(star type),,has three phase(a,b and c) and has also neutral ,
let Vphase=220 R.M.S
if the neutral of this transformer is not grounding"neutral hasn't any contact with earth"

my question is:
if we put a voltmeter between the Earth and the phase "a"(or b or c) what the voltmeter read??

i discuss this question with my teacher in the college he said that the voltmeter will read value "220 V" but i said the voltmeter will read "0V" because the circuit is open,,

 

[NobodySpecial]

If the system has no connection to ground then the meter will read some random number

But in the real world the other end of the three phase supply is grounded at the power station so the Earth forms an effective 4th wire running along the three transmission phases so the star point on the transformer (assumign everythign is balanced properly) is 0v = ground

 

[samaaa]

If the system has no connection to ground then the meter will read some random number

why the voltmeter will read random value??
it is open circuit.

the study that was about the transmition line,
my teacher said that a capacitor"unreality" will appear between the wire of phase"from transformer" and the earth,
he said the capacitor appear because there is Voltage differential between the Earth and wire of phase.

 

[NobodySpecial]

why the voltmeter will read random value??
it is open circuit.

In the real world it isn't an open circuit - but a voltmeter between two points that aren't connected could have any value (although a real meter with finite input impedance would read zero)

he said the capacitor appear because there is Voltage differential between the Earth and wire of phase.

Yes because the three phases are generated relative to Earth at the power station the Earth is like a 4th wire between the star points of the transformer and the generator.
So there is a voltage difference between the lien and ground = a capacitor (this is one of the reasons why power lines are on the top of high towers rather than buried in the ground)

 

[samaaa]

Ok i understand that the (voltage differential between the earth"neutral not grouding" and wire) occur because in generator they use the Earth as reference,,
but if we not use the Earth as reference the voltage between Earth and wire will be equal zero.

 

[Adjuster]

Ok i understand that the (voltage differential between the earth"neutral not grouding" and wire) occur because in generator they use the Earth as reference,,
but if we not use the Earth as reference the voltage between Earth and wire will be equal zero.

Modern electronic voltmeters have very large input impedances. If you used this kind of instrument it probably would give a substantial reading, even if the only things "grounding" the secondary were the parasitic capacitances from the secondary coils to the core of the transformer.

On the other hand, if you used a very insensitive old analogue instrument it might read zero, or an extremely small value, because the currents flowing in these parasitic capacitances could be too small to give much deflection. Effectively, the lower impedance meter would bring the terminal being measured closer to ground potential.

 

[m.s.j]

It is not related to generator grounding or transformer primary and secondary coupling terms, in this case we face to distributed capacitance of transformer winding which close to core body (which usually is grounded). Indeed during mentioned measuring we measure difference voltage of that equivalent capacitance.

 

[sophiecentaur]

Ok i understand that the (voltage differential between the earth"neutral not grouding" and wire) occur because in generator they use the Earth as reference,,
but if we not use the Earth as reference the voltage between Earth and wire will be equal zero.

To understand how the unconnected neutral would, ideally, have a potential of zero, think of a simpler circuit with two resistors connected in the middle and at each ends to +12V and -12V, respectively. This is a simple potential divider and will produce a potential of zero at the mid point.
Replace the two resistors with antiphase ac signals on two series transformer windings and the centre point will also have zero volts on it.
This extends to multiphase situations - i.e. a three phase system. The way that the three phases add up, vectorially, will produce a zero potential at the centre of the star. The 'internal resistance' of the source of zero potential will be low because, if you try to drive it away from zero, loads of imbalanced current can flow through the three windings to redress the balance and pull it back (i.e. a voltage source).

 

[m.s.j]

I think, the general education level of electrical engineering is low in worldwide among U.S and Europe now. Unfortunately your conceptual understanding of basic concepts is very weak. I think it is related to explosion of information (internet) and huge relevancy to it.

 

[sophiecentaur]

I think, the general education level of electrical engineering is low in worldwide among U.S and Europe now. Unfortunately your conceptual understanding of basic concepts is very weak. I think it is related to explosion of information (internet) and huge relevancy to it.

I have to agree. People seem to expect knowledge and understanding to emerge from the very briefest exposure to a subject. Everyone seems to want a three word answer.
It doesn't help, either, when 'the blind lead the blind', perpetuating the situation.

So speaks one old gimmer (possibly to another?)

btw, it amazes me that these questions are so seldom accompanied by a diagram of the actual situation.