scope:

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.