Secure Electrical Systems

 
Another part of security and good network management is to make certain that high-quality
electrical power will be available at all times even in the event of a power outage. This involves
several distinct measures.
The AC power provided by many electrical utilities is often remarkably inconstant, exhibiting
long-term and short-term voltage sags as well as overvoltage conditions and occasional
spikes where voltage levels may exceed the standard voltage by many multiples. The AC may
also be troubled by the presence of harmonics, distortions in the AC waveform that can disrupt
the functioning of many kinds of electrical or electronic components if sufficiently severe. All
these conditions are undesirable, and some may be catastrophic, and the network operator
must guard against them by appropriately selecting power conditioning and power backup
equipment.
Power conditioning devices take a number of forms.
Passive systems consist of high-frequency filters (of limited usefulness because they cannot
raise or lower voltage or eliminate harmonics), constant voltage transformers, and
switched tap autoformers. Constant voltage transformers and switched tap autoformers are
devices that will maintain constant voltage within certain values, say, 5 percent over and under
the nominal value. While both are essentially passive in their operation, switched tap autoformers
contain logic circuits and relays that select among output taps on the autoformer coil
to compensate for changes in input voltage. Constant voltage transformers operate on a different
principle; the transformer core is partially saturated at the nominal line voltage and will
grow more or less saturated as the input voltage goes up and down, which in turn will cause compensatory changes in the output voltage. Constant voltage transformers are marginally
more reliable than switched tap autoformers, but they also tend to be more expensive.
A better solution is an uninterruptible power supply (UPS), which is always on and is
series-connected between the wall power and the devices being powered. Carrier-grade UPSs
are examples of active power-conditioning devices. Such a unit usually consists of an isolation
transformer interfacing with the wall current, an array of rectifiers that transform the alternating
current into direct current, a bank of storage batteries to hold the DC charge, and a set of
high-frequency solid state switches that converts the DC back into 50- or 60-cycle AC. Higherquality
UPSs have provisions for maintaining a steady output voltage regardless of input
voltage and also perform power factor correction, a process that eliminates harmonics in the
output of the UPS caused by reactive electrical loads associated with switching power supplies
in computers, with cathode-ray tube flyback circuits, and with most electrical motors.
The better UPSs produce a smooth sine wave output, and lower-quality units produce a
coarse, stair-step waveform. Since stair-step waves are rich in noise and distortion components,
they are undesirable.
Some UPSs recently introduced into the marketplace utilize fuel cells either in lieu of batteries
or to supplement them. Although generalizations must be made with caution in this
area, it is safe to say that most types of fuel cells produce several times the amount of energy
per kilogram as most types of secondary batteries, though the lowest energy density fuel cells
and the highest energy density batteries almost overlap in this regard. Fuel cells are currently
very expensive, minimally $5,000 per kilowatt, but prices may begin to decline in the near
future. Fuel cell backup power may begin to become the norm toward the end of the decade.
High-velocity flywheel generators are also beginning to appear in some central offices for providing
highly reliable, though relatively short-term, backup power.
The central office facilities may also use backup generators. In most cases, these will use
ordinary diesel or gasoline reciprocating engines, but a growing trend is to use devices called
microturbines, which usually run on natural gas. Microturbines are made by such firms as Capstone,
Allied Signal, and Ingersoll-Rand, and they are derived from the turbine designs used in
jet aircraft. Currently microturbines are much more expensive than diesel engines.
Diesel generators should be routinely tested because often diesel engines will fail to start
when they have not been operated recently.
In all cases, backup power must come on automatically and instantaneously in the event
of a power failure. The subscriber should experience no interruption of service whatsoever.