6.5 Distribution System
demand register on each feeder in addition to utility
approved relaying. Meters shall be pulse-type for
Alternatives
connection to the BAS.
Medium Voltage Conductors. Conductors shall be
copper, insulated with cross linked polyethylene (XLP) or
Where the design alternatives have been thoroughly
ethylene propylene rubber (EPR). Insulation shall be rated
evaluated, and a medium voltage (15kV) service is
at 133 percent. Conductor size shall not exceed 240 mm2
selected as the optimal utility service for the application,
(500 Kcmil).
the design professional shall request that the utility
company provide a minimum of three (3) 15 kV
(nominal) feeders to serve the facility.
is critical and where even a momentary disruption
of service due to the loss of a single incoming feeder
Primary Distribution
cannot be tolerated, then network transformers shall be
considered. Network transformers shall be liquid-filled,
The following types of primary distribution systems are
kVa rating as required, with copper primary and
listed in terms of increasing flexibility, reliability and cost:
secondary windings. Transformers shall be equipped with
Looped primary (not recommended)
provisions for fans and/or dual temperature ratings to
Radial primary
increase the rated capacity and shall be provided with
Primary Selective
sufficient contacts to permit the remote monitoring of the
status of the network protector, temperature and pressure
Primary Selective-Secondary Selective
in the enclosure and other components recommended by
Network
the manufacturer. In addition, transformers shall be
The selection of a primary distribution system shall be
provided with voltage taps 2.5% with on-load tap
based upon a study comparing the relative advantages and
changer.
disadvantages of the feasible alternatives including a life
Double-ended Substations. Where either a primary
cycle cost comparison. Where primary service is provided,
selective or primary selective secondary selective
GSA will provide, own and maintain the transformers.
(double-ended substation) is selected, the following
Medium Voltage Switchgear. Design of the medium
paragraphs apply:
voltage switchgear shall meet all of the requirements of
If reliability is critical and spot networks are not feasible,
the local utility. Switchgear shall be provided with
double-ended substations shall be used. Transformers
enclosed, draw-out type vacuum interrupter breakers.
shall be equipped with provisions for fans to increase the
Provide one (1) per each size fully equipped spare
rated capacity. The sum of the estimated demand load of
cubicle/breakers up to 1600 Amps. Provide breaker lifting
both ends of the substation must not exceed the rating of
device. Provide Ground and Test device.
either transformer, and must not exceed the fan cooling
Note: Alternate type circuit breakers will be considered.
rating. All double-ended substations shall be equipped
with two secondary main breakers and one tie breaker
181
ELECTRICAL
ENGINEERING
6.5
Distribution System Alternatives
Revised March 2005 PBS-P100
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