The major portion of all electric power presently used in
generation, transmission, and distribution uses balanced three-phase systems.
Three-phase operation makes more efficient use of generator copper and iron.
Power flow in single-phase circuits was shown in the
previous section to be pulsating. This drawback is not present in a three-phase
system. Also, three-phase motors start more conveniently and, having constant
torque, run more satisfactorily than single-phase motors.
However, the complications of additional phases are not
compensated for by the slight increase of operating efficiency when polyphase
systems other than three-phase are used.
A balanced three-phase voltage system is composed of three
single phase voltages having the same magnitude and frequency but
time-displaced from one another by 120°.
Figure 2.5(a) shows a schematic representation where the
three single-phase voltage sources appear in a Y connection; a Δ configuration
is also possible. A phasor diagram showing each of the phase voltages is also
given in Figure 2.5(b).
Phase Sequence
As the
phasors revolve at the angular frequency ω with respect to the reference line
in the counterclockwise (positive) direction, the positive maximum value first
occurs for phase a and then in succession for phases b and c.
Stated in a different way, to an observer in the phasor
space, the voltage of phase a arrives first followed by that of b and then that
of c. The three-phase voltage of Figure 2.5 is then said to have the phase
sequence abc (order or phase sequence or rotation are all synonymous terms).
This is important for applications, such as three-phase
induction motors, where the phase sequence determines whether the motor turns
clockwise or counterclockwise.
With very few exceptions, synchronous generators (commonly
referred to as alternators) are three phase machines. For the production of a
set of three voltages phase-displaced by 120 electrical degrees in time, it
follows that a minimum of three coils phase-displaced 120 electrical degrees in
space must be used.
It is convenient to consider representing each coil as a
separate generator. An immediate extension of the single-phase circuits
discussed above would be to carry the power from the three generators along six
wires.
However, instead of having a return wire from each load to
each generator, a single wire is used for the return of all three. The current
in the return wire will be Ia + Ib + Ic; and for a balanced load, these will
cancel out. If the load is unbalanced, the return current will still be small
compared to either Ia, Ib, or Ic.
Thus the return wire could be made smaller than the other
three. This connection is known as a four wire three-phase system. It is
desirable for safety and system protection to have a connection from the
electrical system to ground. A logical point for grounding is the generator
neutral point.
