Hardware Reference
In-Depth Information
FIGURE 6.10 Mechanical design for cooling systems . CWS stands for circulating water
system. (From Hamilton [2010] .)
Although there are many variations deployed, in North America electrical power typically
goes through about five steps and four voltage changes on the way to the server, starting with
the high-voltage lines at the utility tower of 115,000 volts:
1. The substation switches from 115,000 volts to medium-voltage lines of 13,200 volts, with
an efficiency of 99.7%.
2. To prevent the whole WSC from going offline if power is lost, a WSC has an uninterrupt-
ible power supply (UPS), just as some servers do. In this case, it involves large diesel en-
gines that can take over from the utility company in an emergency and bateries or ly-
wheels to maintain power after the service is lost but before the diesel engines are ready.
The generators and bateries can take up so much space that they are typically located in
a separate room from the IT equipment. The UPS plays three roles: power conditioning
(maintain proper voltage levels and other characteristics), holding the electrical load while
the generators start and come on line, and holding the electrical load when switching back
from the generators to the electrical utility. The efficiency of this very large UPS is 94%, so
the facility loses 6% of the power by having a UPS. The WSC UPS can account for 7% to
12% of the cost of all the IT equipment.
3. Next in the system is a power distribution unit (PDU) that converts to low-voltage, intern-
all three-phase power at 480 volts. The conversion efficiency is 98%. A typical PDU handles
75 to 225 kilowats of load, or about 10 racks.
4. There is yet another down step to two-phase power at 208 volts that servers can use, once
again at 98% efficiency. (Inside the server, there are more steps to bring the voltage down
to what chips can use; see Section 6.7 . )
 
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