Voltage of the neutral of the power source is always zero, or very close to zero (compared to the true earth), (and that is the case for the earth conductor if the earthing is properly done) .. Then for a three phase balanced load each line to neutral voltage is say 240 V and the angle difference is 120 between each phase and the other. In this case voltage of the load neutral (star point) will be same as that of the source neutral. But if the transmission or the distribution lines are not balanced, or if the load is not balanced, then the line to neutral voltages at the load terminal will not be equal in magnitude nor phase angle difference. This will make phases currents different and cause different voltage drops in the load phases. In this case, the neutral voltage will shift from the supply neutral voltage to another value. The amount of shifting depends on the amount of unbalance.

Simply we can say: The neutral shift is the deviation of the potential of the load’s neutral point in respect to system neutral, which ideally, should be very close to zero volts in respect to true earth ground potential.

That's a nice explanation !

After reading through it, i do have a question -

Its a case of voltage unbalance, so both the phase angle and voltage of phases will differ from nominal values. But, how will that effect the neutral voltage, as mostly neutral point of transformer is grounded, either solidly or through ground impedance (theoretically speaking).

Voltage unbalance (in phase, or magnitude(amplitude) occurs only, if neutral is isolated or, not solidly grounded(neutral to ground voltage exist). Neutral point will be shifted(not remains at ground(zero) potential),and heavily load phase voltage, will reduce and lightly load, phase voltage will increased, which may damage the appliances connected. With unbalance load(current) there must be a path, for neural current to flow,as required, or will create,voltage unbalance. See reply," advantage of three phase four wire system, over three phase three wire system."

Force can be represented, as vector with magnitude and direction. If three forces, of same magnitude and 120 degree apart. acting on a point can shift the point? Answer is,' No'(resultant force on point is zero). Similarly voltage supplied in three phase system are 230 or 110Volts in magnitude, and are apart by 120 degree, and the common reference point at zero potential is called neutral, and if it connected to ground no current will flow from neutral to ground as there is no potential difference, as ground too, at zero potential, or we can say that, neutral is not shifted from ground. These three phase supply are received, from the mains by three phase four wire system(R,Y,B and N).

When voltage applied to load(connected between phase and neutral), current starts flowing from, phase to neutral. The current magnitude depends on size(watts) of load and phase shift on power factor(capacitive/inductive).If loads(current) on all three phases are balanced, in magnitude and phase difference of individual phase, no resultant current would be flowing from neutral to ground(or neutral is connection with ground will not make any difference, even if not connected.)as both at zero potential.

If loads on all three phases are not balanced, the resultant current will flow from neutral to ground(neutral will remain at same potential as ground)and the applied balanced voltage will not disturbed. If neutral is not connected to ground, think what would happened? Answer is,' Neutral point will shifted'. How much? Scale of voltage drop produced by resultant current(not allowed to flow). Or shifting of neutral would be such that the, heavily load phase voltage will reduce and lightly phase voltage will increased. Which can be understand by drawing exact current diagram(when neutral is connected) measurements of R,Y,B and N currents. Voltage diagram, by measuring disturbed voltage, when neutral is disconnected from ground.(voltages, RtoN,YtoN,BtoN and NtoG) and comparing with scale, both drawn, current and voltage diagrams.

Thank You sir for the detailed explanation