27.2 BPS Types
All BPSes have three common elements: a
battery,
which stores electrical energy against power failures; an
inverter,
which converts DC voltage supplied by the battery to the AC voltage
required by the load; and charging
circuitry, which converts AC mains power to
the DC voltage required to charge the battery. IEEE recognizes three
categories of BPS, which they term UPS:
- On-line
-
An on-line UPS (often called a true
UPS to differentiate it from an SPS) connects the load
directly to the inverter, which converts DC voltage supplied by the
battery to standard AC voltage. The charging circuitry charges the
battery constantly while the UPS is operating, and the equipment
always runs from battery power supplied by the inverter. On-line
UPSes are not often used on PCs because they cost substantially more
than SPSes, described below. An on-line UPS has two advantages.
Because the PC runs on battery power all the time, there is no
switch-over time, and no switch to fail. Also, because the PC does
not connect to mains power, it is effectively isolated from AC line
problems. Against this, an on-line UPS has three drawbacks. Foremost
is cost, which may be 50% to 100% higher than an equivalent SPS.
Also, because the system runs from battery constantly, UPS batteries
typically require replacement more frequently than SPS batteries, and
UPS batteries are not cheap. Finally, UPS efficiencies are relatively
low. An SPS runs at nearly 100% efficiency during normal operations,
and at lower efficiency only during power failures. A UPS runs its
inverter all the time. That results in efficiency as low as 70%,
which translates to higher electric bills. This is of little concern
to most home and office PC users, but is a major issue for data
centers. An on-line UPS may also be called a
dual-conversion on-line UPS, to differentiate it
from a line-interactive UPS, described below.
- Line-interactive
-
A line-interactive
UPS, also called a single-conversion
on-line UPS,
differs from an on-line UPS in that the load normally runs primarily
from utility power as long as that power is available. Rather than
convert utility power to DC, use it to charge the battery, and then
reconvert it to AC for the load (the
"dual-conversion" part), a
line-interactive UPS feeds utility power directly to the load under
normal conditions. Minor variations in utility power are smoothed out
by the inverter using battery power. The defining characteristics of
a line-interactive UPS are that the inverter runs at all times, and
that the load is always dynamically shared between inverter and
utility power. During routine operation, utility power may support
99% of the load and the inverter only 1%. During a brownout, the
inverter may support 10% or more of the load. Only during a blackout
does the inverter assume 100% of the load. A true line-interactive
UPS has no switch-over time, because the inverter and utility power
dynamically share the load at all times, so a power failure simply
means that the inverter instantaneously assumes 100% of the load.
Although line-interactive units do not isolate the load from the AC
line to the extent that an on-line UPS does, they are quite good at
maintaining clean, steady AC to the load. Line-interactive UPSes are
common in data centers, but uncommon in the PC environment.
- Off-line
-
Any BPS used with a PC (or even a server) nowadays is almost
certainly an off-line power
supply, sometimes called a standby
power supply
(SPS). BPS marketers dislike
"standby" and downright hate
"off-line," so off-line power
supplies are always described as
"uninterruptable" power supplies,
which they are not. The defining characteristics of an SPS are that
it has a switch and that the inverter is not always running. During
normal operation the switch routes utility power directly to the
load. When utility power fails, that switch quickly disconnects the
load from the utility power and reconnects it to the inverter, which
continues to power the equipment from battery. SPSes are less
expensive than on-line and line-interactive units because they can
use a relatively inexpensive inverter, one rated for low duty cycle
and short run time.
Unlike on-line and line-interactive units, SPSes do not condition or
regenerate incoming AC before supplying it to the load. Instead, they
pass utility AC power through a passive filter similar to an ordinary
surge suppressor, which means that SPSes do not provide power as
clean as that provided by on-line and line-interactive units. In
theory, SPSes have another drawback relative to on-line and
line-interactive units. Actual switching time may be considerably
longer than nominal under extended low-voltage conditions and with
partially depleted batteries. Because the hold-up time of a PC power
supply decreases under marginal low-voltage conditions, in theory an
SPS may require longer to switch than the hold-up time of the PC
power supply, resulting in a system crash. In practice, good SPSes
have typical switching times of 2 to 4 ms and maximum switching times
of 10 ms or less, and good PC power supplies have hold-up times of 20
ms or longer at nominal voltage and 15 ms or longer during sustained
marginal under-voltage conditions, which means this is seldom a
problem. Several SPS variants exist:
- Standard SPS
-
A standard SPS has only two modes—full
utility power or full battery power. As long as utility power is
within threshold voltage limits (which can be set on many units), the
SPS simply passes utility power to the equipment. When utility power
dips beneath threshold, the SPS transfers the load from using 100%
utility power to using 100% battery power. Some standard SPSes also
transfer to battery when utility voltage exceeds an upper threshold.
That means that the SPS switches to battery every time a surge, sag,
or brownout occurs, which may be quite frequently. This
all-or-nothing approach cycles the battery frequently, which reduces
battery life. More important, frequent alarms for minor power
problems cause many people to turn off the alarm, which may delay
recognition of an actual outage so long that the battery runs down
and work is lost. Most entry-level SPS models are standard SPSes. The
American Power Conversion (APC) Back-UPS series, for example, are
standard SPSes.
- Line-boost SPS
-
A line-boost SPS adds line-boost mode
to the two modes of the standard SPS. Unlike line-interactive units,
which use battery power to raise AC output voltage to nominal,
line-boost units simply have an extra transformer tap, which they use
to increase output voltage by a fixed percentage (typically, 12% to
15%) when input voltage falls below threshold. For example, when AC
input falls to 100VAC, a line-interactive unit uses battery power to
raise it 15V to 115VAC nominal. For 95VAC input, the line-interactive
unit raises it 20V to 115VAC nominal. For 100VAC input, a line-boost
unit uses the extra tap to raise output voltage by the fixed
percentage (we'll assume 12%), yielding 112VAC
output. For 95VAC input, the line-boost unit raises it by the same
fixed percentage, in this case to 106.4VAC. That means that output
voltage follows input voltage for line-boost units, with the
resulting transients and current surges on the load side as the
inverter kicks in and out. Most midrange and high-end PC SPS models
are line-boost SPSes. The American Power Conversion (APC) Back-UPS
Pro and Smart-UPS series, for example, are line-boost SPSes.
- Ferro-resonant SPS
-
A ferro-resonant
SPS uses a ferro-resonant
transformer rather than the tap-change transformer used by
a line-boost unit. Its sole advantage relative to a line-boost unit
is that it provides some power conditioning rather than allowing
output voltage to vary with input voltage. Against that,
ferro-resonant units have several serious drawbacks. First, as a high
output-impedance source, ferro-resonant units are inherently unstable
with some loads, including the power-factor-corrected
(PFC) power supplies that are relatively common in PCs. Second, a
ferro-resonant unit can introduce severe oscillation into output
voltage even when input voltage is relatively clean and stable. Most
important, although ferro-resonant units are often claimed to have
zero transfer time, their actual transfer time can be greater than 25
ms, which is larger than the hold-up time of nearly any PC power
supply. We believe ferro-resonant units are a poor choice for use
with PCs. Best is the best-known maker of ferro-resonant units.
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