Page 127 - Power Quality in Electrical Systems

110 Chapter Eight

on-time, as described in Chapter 8.4. Power-quality standards are
discussed in Chapter 2. Each problem may require a different correc-
tion method that will be discussed in this chapter, and in the following
chapters.
Power-quality problems, particularly line-voltage disturbances, can
originate at four levels of the system that delivers electric power, namely
[8.1]:
■ Bulk power: Power plants and the entire area transmission system
■ Area power: Transmission lines, major substations
■ Distribution network: Includes distribution substations, primary,
and secondary power lines, and distribution transformers
■ Utilization equipment: Includes service equipment and building
wiring

In addition, the problems can be caused by the equipment supplied
with electric power—for example, power-electronic converters.
Redundancy at all levels of the electric-power system reduces the incidence
and duration of line-voltage disturbances.

Correction Methods

Correction methods include the following:
■ Design of load equipment
■ Design of the electric-power supply system
■ Installation of power-harmonic filters
■ Use of dynamic voltage compensators
■ Installation of uninterruptible power supplies (UPSs)
■ Reliance on standby power—for example, engine-generator (E/G) sets

Load equipment, such as switch-mode power supplies, can be designed
to reduce harmonics in the load current, and also to reduce sensitivity
to voltage disturbances. The supply system can be designed to reduce
source impedance, to separate loads, and to avoid harmonic resonances.
Power-harmonic filters are installed to correct continuous voltage dis-
tortion produced by nonlinear loads in power systems, such as adjustable
speed drives. Dynamic voltage compensators function to correct short-
time voltage waveform sags. The available time duration of the correc-
tion depends on whether supplementary energy storage means, such as
batteries, are incorporated in the compensator. A UPS provides an
independent power source to the load from an electronic inverter. When
utility power is interrupted, batteries serve as the energy source.