Page 244 - Power Electronic Control in Electrical Systems

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH006.3D ± 232 ± [177±262/86] 17.11.2001 10:23AM

232 Power electronic equipment

Uninterruptible power supplies systems require an energy storage device to be able to
supply the load under power line failure. In most cases, a battery bank is used. The
selection of battery is not a hard task although many technologies are available.
Specifically, alkaline batteries of the nickel±cadmium type, lead±acid and other more
exotic technologies can be theoretically considered. However, the high cost of all the
technologies previously mentioned make the lead±acid battery the most common
choice for commercial applications. There are a number of drawbacks associated with
the lead±acid battery technology including maintenance requirements and environ-
mental concerns. In recent years, new systems based on technologies such as flywheels
have been commercially developed even for relatively medium power level applications.
These systems are presented in further detail in Section 6.6.1.

6.5 Dynamic voltage restorer (DVR)

Overthelastdecadeandinthetwenty-firstcentury,theelectricitysectorhasbeengoingand
will go through further deregulation and privatization in the developed world. Competi-
tion therefore amongst electricity suppliers with the increased use of power electronics in
everyday activities has resulted in increased attention to the issue of power quality.
In Section 6.4, the UPS systems were discussed. In this section, the concept of
custom power (CP), proposed to ensure high quality of power supply will be pre-
sented briefly. The DVR is such an example. It can be designed to have excellent
dynamic performance capable of protecting critical and/or sensitive load against
short duration voltage dips and swells. The DVR is connected in series with the
distribution line as shown in Figure 6.56. It typically consists of a VSC, energy

Fig. 6.56 Schematic representation of a dynamic voltage restorer (DVR).