Power problem, pls, help me to explain about it in theoratically or show me some website to read it.
A 440Vac and 3Phase generator and its neutral point (star point) is not grounded (insulated neutral). OK, then Phase to Phase is 440Vac and Phase to neutral point is 250Vac. I would like to know that how many V should have between on Phase to Ground (Earth) when we measure with a V meter. Why?
Pls, be noticed, this is not totally same with our home electrical circuit as a star point of distribution transfomer is already grounded.
Between 0 and 250VAC. If reading is 0, it means the installation is ok. If it is 250V, you have a fault.
This voltage is defined by the impedances between lines (phases+neutral) to ground. If they are equal (that is, the ohmic resistance of the insulation and the capacitance wire-ground), they form a star. If one of the phases have a fault - then one of the impedances become ~0.
This is called IT system, and is generally used where the voltage should not stop in case of fault (e.g. operating theatre). The first fault does not create dangerous situation, but it should be detected and removed, as the second one is dangerous (actually, a short current). That's why these systems are used with a special insulation monitor, that detects the fault. Using such a system requires special expertise, and special (expensive) devices.
That voltage is undefined, as the entire system is floating with respect to earth. It could measure anything between zero and 440 volts. It may in fact be close to 250 VAC due to equal capacitive coupling from each phase, but nothing is forcing it to stay there there. Picture a second "load" consisting of three capacitors connected in star with that star point connected to earth.
In fact, a low impedence analog voltmeter would lower the voltage to the phase being measured, while a digital meter probably would not. In the extreme case, you could ground one line and that voltage to earth would be zero, while the other two would measure 440.
My mistake - I was speaking about the NEUTRAL, sorry. You are right concerning the phase-ground voltage. In good system (no faults) it should be about 250V for each phase. If there is a fault (phase connected to ground) - then about 0 for the faulty phase, and about 440 for the good phases.
Are you asking about the phase-neutral voltage or the neutral-ground voltage?
Here are both answers:
Phase-neutral voltage is controlled by the generator windings and will not vary, regardless of whether or not the neutral is grounded.
Neutral-ground voltage is undefined, since the entire system can shift with respect to ground, as I described previously, but the neutral voltage is fixed with respect to the phases by the generator. Therefore, you could measure anything between zero and 250 volts between the neutral and ground. Picture the entire system, including the neutral, lifting off of ground potential by some amount, with a maximum being the phase-neutral voltage if one phase is grounded.
It depends on the capacitance (mainly) and leakage resistance between phases and each conductor and ground. I once fielded a call from one of the plants operated by my employer. In a large ungrounded 480V system, with ground detecting lamps, a couple 800hp motors lots of heavy cable and a couple long runs of bar bus, voltages to ground were 960, 960 and 480 or thereabouts when one of the 800hp motors was running. The voltages were probably measured with a Simpson 260 or similar meter. All meggered ok. The problem manifested itself as 2 bright lamps and one seemingly normal lamp on the ground detector. Everything else was perfectly normal. The ground lamps were normal when that one motor was off. That motor circuit included a few hundred feet of bar bus. (Actually, 2 lamps were very bright and one was merely bright.)
I found that over the years the ground detector lamps had been replaced by low wattage bulbs, 15 watt I think. The voltages returned to normal when we replaced the lamps with 75 (or maybe 100) watt bulbs.
I never did try to draw a symmetrical components sequence diagram to figure out what was going on but it was clear from the bar bus construction that 2 phases, which had bars next to the housing, had much higher capacitance to ground than the 3rd phase between the other 2. I suspect that they formed a resonant circuit with the transformers in the ground detector.
In your system, it's possible to get just about any reading from phase to ground (assuming no ground detector). It all depends on the phase capacitance/resistance to ground and the meter impedance. A low impedance meter will give low readings (maybe even 0) and a high impedance meter will give higher readings. Three identical meters, each one connected to a phase, should read nearly the same *IF* the leakage resistance and ground capacitance is balanced. Lower impedance meters will be less sensitive to any unbalance. Really low impedance 'meters' (such as the 100 watt lamps) may swamp out the unbalances between phases and ground and give nearly balanced voltages from phase to ground.
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