Page 156 - Power Quality in Electrical Systems

138 Chapter Nine

as peak shaving, voltage and frequency stabilization, and reactive
power supply. These systems can be rated up to 20 MW and can de-
liver energy for up to 8 hours.
Requirements for uninterruptible power for specific loads can be met
by short- and medium-term systems described earlier.

Batteries
Batteries consist of one or more cells electrically interconnected to
achieve the required voltage, stored energy, and other characteristics.
Two types of operation are important: float and cycling. Float operation
describes batteries in telephone central offices where the batteries main-
tain a relatively constant voltage—for example, 48 V DC. Cycling oper-
ation describes batteries in standby systems—for example, UPS, where
the battery charge is drawn down to supply the inverter and the AC load
when the utility power fails. These batteries for UPS rated 100 kVA and
higher, are typically rated 460 V DC. The batteries are recharged when
utility power returns, or engine generators are started and run [9.8].
The specific energy and energy density of the batteries used for
standby service are shown in Figure 9.9 [9.9]. The application of these
batteries depends on additional factors besides those in the figure.
The batteries employed for standby service are described in the
following [9.8]:

■ Flooded, lead acid batteries: These have been used for UPSs since
the 1960s [9.4], and as the backup for communications power supplies
before 1983 [9.10]. This type of battery requires periodic additions of
water to comply with its specific gravity measurements. It discharges
inflammable gas, and thus requires special facilities for safety. To fa-
cilitate venting, the gas space in flooded cells is open to outside air but

Lighter
140
120
LMP
100 Li-ion
Wh/kg 80 NiMH
60
40 NiCd VRLA
20 Flooded
0
Smaller
0 50 100 150 200 250
Wh/I
Figure 9.9 Specific energy and energy density comparison of
batteries: Wh/kg and Wh/l [9.9].
[© 2004, IEEE, reprinted with permission]