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Description / Abstract:
Overview
Network transformer vaults are fire retardant enclosures
normally within or adjacent to buildings or underneath streets and
alleys. They typically contain two or more power transformers.
These transformers are supplied from different subtransmission or
distribution lines and are paralleled on their low-voltage side
through circuit interrupting devices called network
protectors. Typically, high-voltage current-interrupting
devices have not been applied within the network vault. The
low-voltage bus of a network vault may be electrically tied to a
number of other vaults to form a network secondary distribution
system, which will be called a low-voltage network grid in
this guide, or each individual vault may stand alone as a spot
network. A one-transformer vault can be considered a network
vault if connected to other such vaults via low-voltage cables.
Low voltage in this guide implies 600 V or less, and high
voltage implies 2400 to 34 500 V. Typical low voltages are 208Y/120
V, 480Y/277 V, and 600Y/347 V. A typical high voltage is 12 500
V.
Low-voltage network systems have been used since the 1920s as a
method of providing a highly reliable source of electrical power to
densely populated commercial areas, such as office buildings.
Equipment protection within the network vaults is typically
limited. Historically, users have depended upon the physical design
of the vault to limit the risks of fault damage for faults within
the vault. They have relied upon remote detection and interruption
for transformer faults and low-voltage devices, such as transformer
fuse links and low-voltage cable limiters, to provide a measure of
low-voltage bus fault protection.
This guide is intended to aid those engineers who have
reevaluated problems associated with faults within network vaults,
particularly for those network vaults located within or near
high-rise buildings. It will also identify currently available
devices that are being used in network transformer protection
schemes. These devices should act to sense the fault and initiate
fault interruption locally or remotely, thereby minimizing damage
and restoration time. These devices will be described as to their
fault detecting capabilities.
Several annexes are included with this guide. Annex A describes
network protector response for highvoltage feeder faults. An
example utilizing a number of protective schemes is presented in
Annex B. Annex C addresses other concepts that are currently being
investigated. Annex D provides bibliographic references, such as
IEEE ® Conference papers and IEEE Transaction papers,
e.g., Anderson [B1]. 1, 2
1The mark IEEE is a registered trademark belonging to
the Institute of Electrical and Electronics Engineers, Inc.
2The numbers in brackets correspond to those of the
bibliography in Annex D.