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Providing
Non-Stop Power for Critical Electrical Loads
By Robert Weir, P.E. In a society that is
ever more dependent on things electrical, failure of supply can be much
more than the inconvenience that was occasionally visited on our parents
and grandparents. Loss of electric power now means interruption of some
vital digital communications networks, advanced medical therapies,
financial transactions such as credit cards and bank operations, vital
transport such as elevator service, refrigeration of sensitive biological
experiments and other essential services. The vital aspect of such
functionality creates a demand for non-stop electric power. Choosing
Backup Power Systems The phrase “non-stop”
in this discussion is meant to describe those loads that cannot endure
even a brief loss of power. At the upper limit of these are those that
depend on microprocessors. In these digital devices even a brief
interruption causes reset of digital processes and loss of all data not
already recorded in nonvolatile memory — a class of memory devices that
can retain data through an electric power outage. There are a number of
devices that fit this category, and they can be inherently nonvolatile (no
power needed) or they can be supported by internal batteries. When a power
interruption resets a microprocessor and data is lost, the affected system
must re-boot at such time as power supply is restored. Anyone who has
waited for their desktop computer to perform that task in the morning
should appreciate what could happen if a very large and complex system was
involved. There are many ways
to provide auxiliary power for times when the public supply goes down. For
whatever reason, we seem to think of auxiliary generators in this context.
There are other devices available, including flywheel generators and
battery/inverter systems. Choosing among them is the subject of this
discussion. The
Transition, a Critical Time Issue Failure of the public
electric feeder is not usually an anticipated event. Whatever means have
been chosen for providing alternate power will ordinarily have to function
without advance warning. Since critical loads may be very intolerant of
even the briefest interruptions in power flow, this is the most critical
time. Reciprocating engine
driven generators are one of the most common sources of alternate power
during blackout periods. These can be installed with provisions for
automatic starting and automatic switching on to the load immediately upon
loss of the normal supply. Unfortunately, there is an unavoidable delay
imposed by the necessity to start the engine, allow it to get up to speed,
and then to complete the functions necessary to allow it to assume the
load. The necessary time delay is usually fairly brief, but the duration
of power loss will be sufficient to cause all connected digital control
and computer devices to shut down. The vital loads that involve such
devices now must be restarted with all of the associated delay and
annoyance that we associate with re-booting our personal computers.
Remember that loss of power for only a few cycles (1/60th of 1 second = 1
cycle) is sufficient to reset most digital processor equipment. The time delay
associated with starting the emergency power plant is not the only problem
in this scenario. The accumulated statistics of the Nuclear
Regulatory Commission concerning the starting reliability of such
generation assets serving vital safety functions in nuclear power plants
is not a source of great encouragement.1, 2 Bridging
the Time Gap There is a need,
therefore, to maintain a smooth supply of electricity to vital loads in
order to effect a “bumpless” transition to alternate supply. This can
be accomplished with a device called an uninterruptible power supply
(UPS). Originally designed for digital computers, these devices employ a
battery as a power reservoir. In a UPS, input power is reduced to a
battery-compatible voltage and then rectified to direct current (DC) and
directed into the battery as charging current. When input power is lost,
stored energy in the battery is inverted to alternating current (AC) and
continues to supply the connected loads without interruption. There are
different types of UPS devices available on the market and these differ in
the way that the electric energy stored in the battery is connected to its
load. In one type, the incoming AC power flow is split into two streams. One is employed to charge the battery by the means already described, while the other bypasses the battery and is sent on directly to the load. When a power outage occurs, the now dead AC input line is switched off and energy from the battery, flowing through an inverter and voltage adjustment means, continues to supply the load. The switching operations necessary to carry out this transition occur in a very brief time, but not always brief enough to avoid reset of digital circuits (see Figure 1 below).
In the other type, a single stream of power flows through the transformer and rectifier to the DC bus and then, via the inverter and voltage adjustment means, to the load. The battery, connected to the DC bus, is maintained at charge and ready to take over the load as the DC bus voltage may require. When normal power is interrupted there is no interruption in AC power to the load. The battery replaces the DC current ordinarily received from the AC line via the rectifier and the operation of the inverter section is uninterrupted. No switching is required. As an extra benefit, because all incoming power is rectified to DC, such devices completely suppress voltage spikes in the AC line. The only issue remaining is that of capacity: how long can the battery support the load? (see Figure 2 below)
Combining
UPS with Generator Sets In order to best
maintain the operation of vital equipment through an interruption of
normal supply, a UPS system, preferably of the latter type, can be combined with a
generator set. The UPS equipment provides the critical time necessary for
an orderly start of the generator set. If the UPS system is of the latter
type, all of the generator output is converted to DC in the rectifier/s,
and the problem of synchronizing the AC waveform with the load is neatly
avoided. The result is a truly uninterrupted transition from normal AC
supply from the public power grid to an alternate, on-site supply. If
sufficient UPS capacity is installed, even some difficulties with
generator start (such as maintenance in process at the time of an outage)
can be overcome without causing a power interruption. The UPS does provide
backup power supply as its name indicates, but, more important, it
provides a time window during which generating capacity can be brought on
line in an orderly fashion for longer term power. This should not be read
to say that a battery based power supply is to be preferred to an engine
generator scheme. What is meant is that the installation of a battery
based system adds important advantages to any emergency power supply
system, especially when the dependent electrical loads are unforgiving of
even minor interruptions of supply. Power
Quality — an Unexpected Benefit The interposition of
a battery power reservoir between the AC supply and the critical load
provides a benefit beyond that of an alternate power source. Because a
battery is a direct current (DC) device, it is characterized by constant
voltage that is presumably free of fluctuations. By the time the incoming
AC energy has been supplied to the battery, it has been rectified,
a word that means it has been converted from AC to DC. The essential
feature of that conversion is the suppression of the sine wave that
defines AC power, as well as any other voltage variations that may exist
on the AC power line. It is these latter
voltage variations that we know by the names of voltage surge, harmonics,
brownout and other terms. The point is that the power that now resides in
the battery is devoid of these disturbances. When demand for power is made
on the battery, this DC power is then inverted, another technical word meaning the conversion of constant
voltage, or DC power, to AC. The result is that the conversion of AC to DC
eliminates voltage disturbances, and the subsequent re-conversion of DC to
AC provides a clean and constant AC wave that has been stripped of
unwanted disturbances. Planning
for Equipment Failure The design of a
backup, or emergency, power supply capability should reflect how critical
the loads are to be supplied. Redundancy should be in proportion to the
level of risk. When truly critical electrical needs are at stake,
additional reliability measures should be taken. Each element of the
backup power scheme needs to be viewed as a point of failure, and the
design should provide for functional duplication of each. The failure of
electrical connections also needs to be anticipated. Redundancy must be applied in such a way as to ensure that adequate power is available to the critical load (s) even when one element is not operable. If two generators are provided, each must be sized for 100 percent of the anticipated load. UPS capacity must be similarly sized. Electrical connections should be arranged in “ring” configuration to permit continued operation even if a conductor is broken. Needless to say, the design must anticipate failure of any single element in such a way that continuity of power to the critical loads is not compromised. Footnotes:
Robert Weir, a director with The Hartford Steam
Boiler Inspection and Insurance Company, is a Professional Engineer and
has an extensive background in the design and construction of power
generation and industrial equipment and systems. A graduate of the U.S.
Naval
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©1997 Hartford Steam Boiler Inspection and Insurance Co.
