IEEE Std C37.108:2002 pdf free download

IEEE Std C37.108:2002 pdf free download.IEEE Guide for the Protection of Network Transformers.
4.2 Application considerations of adding protection to spot-network vaults versus grid-network vaults
As noted earlier, the first network vault installations were operated primarily at 208Y/120 V and were connected to other vaults on the low-voltage side. These vaults were generally located below grade and external [mm buildings, for example, underneath sidewalks in business districts. Historical evidence indicates that 208Y/120 V faults in these vaults were generally self-clearing. That is, faults were contained within the vault enclosure and the locations of the vaults were sufficiently isolated from buildings to allow for the fault to continue until enough conductive material melted, thereby extinguishing the fault-generated arc without damaging the buildings.
As the loading in newly constructed high-rise buildings increased, the 208YIl2() V vaults were installed inside them and dedicated to serving the load of the building. Due to the economics of scale, the building vaults had enough capacity to supply the building load with at least a single contingency outage margin. Tics between vaults were no longer needed or practical and the era of the spot-network vault was born. Eventually. 480Y/277 V spot-network vaults replaced the 208Y/l20 V spot networks, and during this transition period, some small 480Y/277 V grid-network systems were installed.
These changes prompted consideration by some utilities to provide additional network vault protection. Frequently. the damage due to arcing 480 V fliults has caused lengthy power outages to buildings, more so thaii at 208 V. Since many of these vaults are inside buildings, a higher incidence of damage caused by smoke has occurred. This has prompted some utilities to provide additional network vault protection.
The principles of protection included in this guide apply equally to spot-network vaults or grid-network vaults and at any low-voltage level. The need to consider additional protection and the associated high- voltage interrupting devices should be evaluated on the basis of risk assessment and cost.
5. Operation of equipment in a network transformer vault under fault
conditions
A network system can be separated into three parts when considering operation under fault conditions:
a) The high-voltage primary supply circuits.
b) The network equipment. which consists of the network transformer, its associated primary-side switch (disconnect and grounding). the low-voltage network protector. and the low-voltage bus.
C) Secondary voltage supply to the consumer and interconnecting cables to other transformer vaults, if any.
Typical network transformer protective devices for faults in the vault and external to it are described in the remainder of this clause.