Page 137 - Power Quality in Electrical Systems

Methods for Correction of Power-Quality Problems 119

These steps do not include the steps the utility can take to minimize
the sags and surges that occur in the source voltage supplied by the utility
to the facility.

Power harmonic filters
Power-harmonic filters are inserted into a power system to absorb spe-
cific harmonic currents generated by nonlinear loads such as the con-
verters of ASDs. The filters can be passive, tuned to a fixed frequency
that is usually slightly below the harmonic frequency. The filters can be
built as active filters—that is, to be tunable to account for changes in
the system impedances and loads. The subject of power-harmonic filters
is addressed in Chapter 6.

Utilization-dynamic voltage compensators
The dynamic voltage compensator corrects voltage sags by inserting a
voltage component between the power source and the load to maintain
the required load voltage. The power for the correction is usually taken
from the source, but supplementary energy storage is sometimes used.
The inserted voltage component is shaped in amplitude and waveform
by a controller. Compensation is usually limited to 12 cycles for dips to
zero source voltage, and to 2 seconds (s) for dips to 50 percent source volt-
age. One circuit concept is shown in Figure 8.8 [8.9]. Because it acts for
a short time and has no energy storage, the compensator is smaller and
lower in cost than a battery-inverter UPS. However, it cannot compen-
sate for long-term outages—for example, a duration of minutes. The sub-
ject is treated in Chapter 10.

Uninterruptible power supplies
The most commonly used equipment to protect critical loads from power-
quality problems is the battery-inverter UPS. The concept is shown in
Figure 8.9 [8.10]. The basic parts of the module are the battery, the
inverter, and the input rectifier, which also serves as the battery charger.

Shunt transf Series transf
Source Load

Converter Inverter
DC link
Figure 8.8 A dynamic voltage compensator.