Page 130 - Power Quality in Electrical Systems

Methods for Correction of Power-Quality Problems 113

voltage for 1 cycle to 30 cycles, the equipment should not be interrupted,
even though the rate of sags can be up to six per site per year.
The objective of the correction methods has been discussed in
Chapter 2 in the form of voltage versus time curves defined as CBEMA
and ITIC. The theory being that if the source voltage is corrected to
the acceptance regions of these curves, then equipment designed to
operate in these regions will operate without interruption with the
corrected voltage. An introduction to correction methods is given in
Table 8-1. Correction methods are listed for typical voltage distur-
bances and time durations.
Manufacturers offer variations of the end-use equipment, which can
withstand ranges of voltage sag and time duration. Furthermore, pro-
tected loads, like computers, can be divided and supplied by multiple cor-
rection sources—for example, UPSs [8.3].

Reliability
The reason for correcting power-quality problems is to insure the reli-
ability of the equipment supplied by electric power from the system in
which the problems occur. Although specific equipment, such as a bat-
tery-inverter UPS, is given reliability numbers, like mean-time to fail-
ure (MTBF), data centers supplied by multiple UPSs are classified in
terms of availability by using tier designations, as follows [8.5]:

■ Tier 1: Tier I is composed of a single path for power and cooling dis-
tribution, without redundant components, providing 99.671 percent
availability. (Unavailability, 28.8 hours per year.)
■ Tier II: Tier II is composed of a single path for power and cooling dis-
tribution, with redundant components, providing 99.741 percent avail-
ability. (Unavailability, 22.7 hours per year.)
■ Tier III: Tier III is composed of multiple active power and cooling dis-
tribution paths, but only one path active, has redundant components,
and is concurrently maintainable, providing 99.982 percent avail-
ability. (Unavailability, 1.58 hours per year.)
■ Tier IV: Tier IV is composed of multiple active power and cooling
distribution paths, has redundant components, and is fault toler-
ant, providing 99.995 percent availability. (Unavailability, 0.44
hours per year.)

The ultimate availability is termed, “five nines,” or 99.999 percent
availability. The unavailability is 5.26 min per year.
Additional descriptions of equipment for data centers will be described
in Chapter 9.