Page 216 - Power Quality in Electrical Systems

198 Chapter Thirteen

■ Parallel Operation: Two or more E/G sets can be operated in par-
allel for redundancy, to secure more than one set’s power, and to
handle large motor starting. The governors and voltage regulators
must insure parallel operation at the required voltage and frequency,
as well as active and reactive power division.

Transfer switches
The purpose for a transfer switch is shown in Figure 13-5. The switch
connects the load, in this case a UPS, to either the utility source or
to an E/G set. The operation is usually conducted when power from
the utility line is interrupted and the output of the E/G set must
supplement the capacity of the batteries of the UPS. The transfer
switch has other functions, such as transferring between two feed-
ers or transformers, and transferring power to a motor from a failed
feeder to an alternate feeder. The subject is covered extensively in ref-
erence [13.5].

Standards. Pertinent standards on transfer switches include the
following:

■ NEMA, “AC Automatic Transfer Switches,” ICS 2-44 [13.6]
■ NFPA, “National Electrical Code,” NFPA 70-2005 [13.4]
■ UL, Standard for Automatic Transfer Switches, fourth edition, UL
1008 [13.7]
■ NFPA, “Emergency and Standby Power Systems,” NFPA 110-2005
[13.3]

Types of transfer switches. Several types of transfer switches are avail-
able and include the following:

■ Manual: The transfer switch can consist of a double-throw multi-
pole switch, or two mechanically interlocked circuit breakers or
contactors.
■ Automatic Electromechanical
■ Three-pole or four-pole to switch three phases with or without
neutral.
■ Open or closed transition, which usually requires utility permission
when switching E/G sets with respect to a utility line.
■ Controls, including voltage sensing, engine starting and shutdowns.
■ By-pass: Incorporates a by pass switch section that can connect the
preferred line to the load while the transfer switch is removed for
maintenance, as shown in Figure 13.9 [13.5].