Normal Power Source
The normal power distribution system includes the
electrical utility service equipment, circuit breakers,
feeder conduit and conductors, distribution boards,
transformers, panel boards, and surge protective
equipment. Except for circuit breakers and surge
protective equipment, most equipment associated with
the normal power service require minimal maintenance
and have reliability ratings comparable to utilities.
Circuit breakers and surge protective equipment must be
serviced on a scheduled basis to ensure reliability.
Location
The service equipment should be located within a secure
room, separate from the emergency power equipment,
and protected from outside influences such as weather or
flooding. The utility power should be routed underground,
and not on overhead power poles, to obtain maximum
reliability. Colocation data centers with dual services fed
from separate utility substations will provide the highest
reliability.
Transformers and associated panelboards should be
located as close to the loads as physically possible to
limit voltage potential between neutral and grounding
conductors.
EMERGENCY POWER SOURCES AND EQUIPMENT
Emergency power is necessary for those times when
normal power is lost. We are considering generators,
automatic transfer switches, and uninterruptable power
sources.
Generators
Generators work in concert with UPSs to provide
emergency power to a data center. Multiple
configurations such as 2N (multiple generators rated
for twice the load) and N+1 (multiple generators with an
extra generator) exist. The more multiples (2N) or extra
generator (N+1) that exist, the greater the reliability.
Fuel Source(s)
Generally, either diesel or natural gas is used as a fuel
source. As noted in the computer-room-design article,
either source can be impacted by a natural disaster
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which would impact the infrastructure as is the case with
natural gas, or delivery as is the case with diesel.
Fuel Storage Capacity
The Uptime Institute, 2014, notes that the minimum fuel
storage capacity for tier-defined data centers is 12-hours.
Redundancy of fuel storage needs to be considered.
For example, the capacity of (2) 24-hour tanks claiming
redundancy is not 48 hours, but 24 hours.
Automatic Transfer Switches (ATSs)
Sandberg, 1999, notes that the function of an ATS,
independent of specific type and manufacturer, is
to monitor utility power, and upon loss of power,
automatically transfer to an alternate source of power,
which could include generators or an uninterruptable
power supply. Some types of ATSs are listed below:
Open Transition – Open transition switches are also
known as break-before-make and are among the most
common transfer switches. An advantage to this type of
transfer switch is that the sources will be fully isolated,
however, transfer will require up to several seconds
to complete. That is to say, for a few seconds the
downstream system will be without power.
Closed Transition – Sandberg, 1999, summarized the
closed transition switch, or make-before-break switch,
as momentarily permitting multiple sources to supply
power to the downstream system. The advantage is an
increase in system reliability. Most utility companies
require coordination and review of these systems since
other momentary sources of power may pose potential
problems to the utility grid.
Solid State – The solid-state transfer switch is a
combination of the open transition and closed transition.
Utilizing solid-state technology, a solid state ATS
maintains the concept of a “clean” power break but does
so in terms of cycles rather than seconds.
Uninterrupted Power Supplies (UPSs)
The website www.electricalengineeringtoolbox.com, 2017,
describes UPSs as providing loads with tight voltage
tolerances through use of a battery, rectifier, inverter,
and static switch with no transfer time and can be
equipped with a bypass switch.