Electrical Power Systems Technology

Power System Fundamentals 57

where:

VL is the line voltage in volts,

IL is the line current in amperes,

VP is the phase voltage in volts, and

IP is the phase current in amperes.

Sample Problem:

Given: a three-phase wye system has the following values: phase

voltage = 120 V, phase current = 18.5 A, and power factor = 0.95.

Find: total three-phase power of the circuit.

Solution:

PT =3 × VP × IP × pf

= 3 × 120 V × 18.5 A × 0.95

= PT = 6,327 W = 6.327 kW

Calculations involving three-phase power are somewhat more com-

plex than single-phase power calculations. We must keep in mind the dif-

ference between phase values and line values to avoid making mistakes.

SUMMARY

In this chapter, we have examined some of the fundamentals of elec-

trical power systems. We need to have some understanding of the three

basic types of circuits—resistive, inductive, and capacitive—in der to un-or

derstand the operation of power-producing systems, such as generators,

chemical cells, and other power-conversion systems (such as electric lights

and electric motors).

Resistive circuits exhibit similar characteristics with either applied

AC or DC. The power converted in a resistive circuit is expressed in all

cases as:

P=V × I

The property of inductance occurs in systems because of coils of wire

or windings that exhibit electromagnetic characteristics. The current de-

veloped in an inductive AC circuit lags behind the applied AC voltage

because of this electromagnetic effect or counter electromotive force (cemf).