No Neutricity is NOT negative voltage. Neutricity itself can be negative voltage but that is upside down to electric measurement. A great description of negative voltage is contained in this link. Scroll down to Jan 2005. Quoted here:
Any voltage can be negative with respect to a more positive point.
Take two 12-volt batteries, connect in series – negative terminal of one battery to the positive terminal of the other battery. Now, for purposes of reference and this explaination, drive a ground rod into the ground and connect the “tied end” of the batteries (negative-to-positive connection you made earlier) to the ground rod. The ground rod is the reference or common connection for our 12-volt battery arrangement.
So, if you measure between the ground rod and negative terminal of one battery (black lead to ground rod & red lead to negative terminal) you will read “- 12 volts”. The voltage at this point is more negative with respect to ground. Conversely, you could say that ground is more positive with respect to that batteries negative terminal.
Electrons always flow from negative to positive. The positive-most point in any circuit represents a depletion of electrons and the negative-most point represents a surplus of electrons. The surplus (-) condition is always trying to satisfy or equalize the depleted (+) condition. Thus, electron current flow.
“Conventional current flow” depicts current flowing from positive to negative as a matter of conveinience. All is good and well as long as you realize that it is actually the movement of electrons doing the work.
“when and why you must use a negative voltage?.”
The reasons are as varied as the circuits themselves. One common application of a negative voltage is for biasing circuits. A negative voltage may be used to turn a device on whereas a positive voltage may be used to turn it off and visc-versa.