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Technical Data
Effective: May 1998
Page
12
TD.44A.01.T.E
Cutler-Hammer
(M)
(T)
(M)
Magnum DS
Metal-Enclosed
Low-Voltage Switchgear
Application
Types of Systems
A. Simple Radial
■
Simplest and least costly
■
Easy to coordinate
B. Primary Selective Radial
Similar to simple radial, with added
advantage of spare primary incoming
cable circuit. By switching to spare
circuit, duration of outage from cable
failure is limited.
C. Secondary Selective
Normally operates as two electrically
independent unit substations, with
bus tie breaker (T) open, and with
approximately half of total load on
each bus. In case of failure of either
primary incoming circuit, only one bus
is affected, and opening main breaker
(M) on dead bus and closing tie
breaker (T) can promptly restore ser-
vice. This operation can be made auto-
matic, with duration of outage on
either bus limited to a few seconds.
Since the transformers are not con-
tinuously paralleled, secondary fault
currents and breaker application
are similar to those on radial unit
substations.
If required, and equipped with the
appropriate relaying, either trans-
former can be removed from service
and isolated with no interruption of
service on either bus, by first closing
the tie breaker and then opening the
associated main breaker.
Service continuity and substation
capacity can be further improved by
substituting selector type primary
switches, shown above in B.
D. Spot Network
The transformers are paralleled
through network protectors. In case of
primary voltage failure, the associated
protector automatically opens. The
other protector remains closed, and
there is no “dead time” on the bus,
even momentarily. When primary volt-
age is restored, the protector automati-
cally checks for synchronism and
recloses.
■
Secondary voltage regulation is
improved by paralleled transformers
■
Secondary fault capability is
increased by paralleled transformers,
and the feeder breakers and bus
bracing must be selected accordingly
■
Primary switches are usually selector
or duplex type, so that transformers
can be transferred to alternate live
sources, thus shortening duration of
overloads
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