Page 175 - Power Quality in Electrical Systems

Power Quality Events 157

300

Percent voltage 200

Voltage
breakdown 115%
concern 106%
Computer voltage
100
tolerance envelope
87%
Lack of stored energy in some
manufacturers equipment
30%
0
0.001 0.01 0.1 0.5 1.0 6 10 30 100 1000
Time in cycles (60Hz)
Figure 11.1 Early CBEMA curve. Typical computer voltage tolerance envelope. Source,
IEEE Std 446-1987 [11.3].
[© 1987, IEEE, reprinted with permission]

Power-quality characteristics
Personal computers both produce disturbances to the power system and
are affected by voltage disturbances from the system. Each PC incor-
porates a switch-mode power supply to convert power from the AC
supply line to low-level DC voltages for the internal circuits. The older
PCs utilized an input diode bridge with a DC capacitor filter. Newer PCs
are designed with an input PWM circuit that shapes the line current to
a sinusoidal waveform in phase with the line voltage—the so-called
unity power factor operation.
The waveform of the line current of older PCs is shown in Figure 11.4
[11.6]. The dominant harmonic is the third. When a group of older PCs
is supplied from a three-phase, 120/208-V power panel, the third-harmonic
currents return from the panel to the source in the neutral conductor.
As a result, facilities have to be wired with oversize or “double” neutrals.
The unity power factor circuit in the new PCs reduces the requirement.