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I.
Introduction
All
electrical and electronic equipment users in underdeveloped
countries face unusual difficulties in trying to cope with
ac power problems. Among the problems are:
- Frequent blackouts due to inadequate
generation capacity;
- Wide ac mains voltage variation due to
inadequate distribution infrastructure;
- Generally poor quality of the power, in
areas such as distortion, spike/noise, frequency,
sags/surges and dropouts, especially when generators are
used to supplement utility power.
Of course, these problems
also exist in developed countries. However, in
underdeveloped countries in Africa, South America and Asia,
such power problems can occur so often as to cause
unacceptable levels of system down time and catastrophic
equipment failures. Also, uninterruptible power supply (UPS)
systems, which are installed to mitigate the above-mentioned
ac power problems, often exhibit unacceptably high failure
rates. Standard UPS systems are designed to meet IEC
specifications for a mains ac voltage window of 184 to
264 vac, while actual mains voltages quite often vary between
160 to 330 vac in many developing countries.
Operating a UPS system outside of its
specified ac mains voltage window is one of the primary
reasons for premature UPS failure. When the other common
power quality problems such as surges, spikes, noise, sags,
and frequency problems are also considered, designing a
reliable UPS system for underdeveloped countries poses a
special challenge to UPS manufacturers.
II. Differences in UPS requirements
A standard UPS designed for use in a developed
country (such as in the USA) might encounter the following difficulties
when installed in underdeveloped countries:
- Because of frequent blackouts, the
batteries in the UPS, whether online or standby, will be discharged
and charged very often. Furthermore, because the duration of the
blackout is usually longer than in developed countries, the batteries
will be more deeply discharged. These two factors combine to
drastically reduce the expected lifetime of the battery. For example,
a typical lead-acid battery will last for about 1,200 discharge/charge
cycles when the depth discharge is 30% of battery capacity. However,
when the depth of discharge is 50%, this number is reduced to 450. And
when the depth of discharge is 100%, the number is reduced to below
200. A UPS in the U.S. may experience a 100% discharge/charge cycle
about 5 to 10 times a year. In underdeveloped countries, this may
occur almost every day, reducing the expected life of the UPS to less
than a year.
- Because of the frequency of power
disturbances, the power electronic circuitry in the UPS must work far
more often and under greater stress. This is especially true for
standby UPS systems, which must make the transfer to battery backup
every time there is a power disturbance. Even in double-conversion
online UPS systems (where transfer to battery is not required), the
power transistors and transformers must operate at higher current and
voltage levels during brownout and over-voltage conditions, resulting
in a shorter-than-normal operating lifetime of the UPS.
- The batteries cannot be charged fully
when the power disturbances occur so frequently. Consequently, the UPS
cannot provide the rated backup time during each power disturbance.
This can cause severe problems with mission-critical computers or
telecommunications equipment that depend on the UPS to provide the
99.99% (or higher) system availability.
- Small emergency power generators can
cause special problems for when they are used to power a UPS system.
They are known to produce wide frequency/voltage fluctuations,
surges/spikes, and highly distorted (often non-sinusoidal) outputs
which can cause continuous mains/inverter transfer operations,
shutdowns, and even catastrophic UPS failure.
- For sites subject to frequent lightning
strikes, absence of a coordinated surge protection system based on IEC
Technical Specification 61312-3 "Requirement of Surge Protective
Devices" may cause premature failure of UPS due to unmitigated
surges and spikes induced on incoming ac power line(s).
- If the ambient temperature is outside of
the optimum 10 to 25 degree C range, standard sealed lead-acid
batteries cannot be used. Standard batteries are rated to work only
when temperature is above 0 degree C, and sudden battery failure can
occur when operating temperature goes above 40 degree C. Even at 30
degrees C, expected battery lifetime is reduced by 50% compared to the
expected battery lifetime at 20 degrees C.
III.
Solutions for reliable and cost-effective UPS protection
The
above-mentioned international power quality problems provide
UPS manufacturers with a difficult challenge in trying to
design UPS systems for underdeveloped countries. It must be
recognized that no UPS system can provide foolproof protection
against all types of power problems, especially at a
reasonable cost. To provide the best solution to power
problems in underdeveloped countries requires an intelligent
application of technology, coupled with an intimate
familiarity with the nature of the problems. Involved in
international AC power protection since 1990, TSi Power has
developed a family of UPS systems which are particularly
useful in underdeveloped countries, with the following
features:
A voltage range
extended UPS (VRe-UPS) provides an input voltage range of
160 to 330 vac for a nominal 230 vac in/output UPS system.
Battery lifetime is maximized since battery discharge takes
place during blackouts only (and not for brownout or
over-voltage periods). The up-time percentage of a VRe-UPS
is significantly higher than a standard UPS since the
battery is always kept fully charged even during deep
brownouts or over-voltage conditions.
Since only regulated 230 vac is supplied
to transistors and transformers in a VRe-UPS, they are not
subjected to excessive voltage, current, or heat even
during brownouts or over-voltage periods. Thus, the
maximum lifetime of transistors and transformers can be
expected, thus increasing the reliability (and up-time
percentage) of the VRe-UPS and the connected equipment.
When a small emergency power generator is
used to power a UPS, the Precision Voltage Regulated UPS
(VRp-UPS) should be used. The VRp circuit constantly
regenerates a clean sine wave even from a highly distorted
and non-sinusoidal output typical of a small power
generator. By providing only high-quality, regulated
sinewave input to the UPS, the VRp-UPS provides maximum
UPS up-time percentage as well as maximum UPS and
connected equipment lifetime.
When a site is subject to frequent and
severe lightning strikes, an Isolation Line Conditioned
(ILc-UPS) should be used. By providing transformer
isolation, a new neutral-to-ground bond, surge protection
at the output side of isolation transformer, and excellent
noise filtering between incoming ac and the UPS, a
coordinated protection system based on IEC Technical
Specification 61312-3 is included. Therefore, the maximum
level of surge protection for the mission-critical
equipment is provided by the ILc-UPS.
If the ambient temperature is expected to
be outside of the recommended 10 to 25 degree C window, a
special class of wide-temperature rated batteries (which
are guaranteed to work from -20 to +55 degrees C) must be
used in order to obtain 5 years (or longer) of battery
lifetime.
IV.
Conclusion
There are special challenges in designing a UPS system
which can provide the requisite ac power protection and backup in
underdeveloped countries. A UPS manufacturer must be knowledgeable of all
the possible power quality problems in the country where the UPS must be
installed.
Armed with this knowledge of actual conditions in
underdeveloped countries, it is possible to provide an
enhanced-performance, yet cost-effective UPS system that can withstand
hostile conditions and fully protect the connected equipment against the
following problems:
- Wide ac mains voltage variation from 160 to 330 vac,
- Distortion, spike/noise, frequency, sags/surges and
dropouts produced by small emergency power generators in wide use in
underdeveloped countries due to the shortage of adequate power
generation and distribution infrastructures,
- Surges and spikes caused by frequent lightning
strikes nearby, or
- Ambient temperature outside of the 10 to 25 degree C
range.
Since 1992, TSi Power has developed several special
optional modules which can be added to the standard UPS which extends the
operating voltage range significantly and maximize the equipment
protection level, up-time percentage and the lifetime of the UPS.
In order to ensure reliable UPS operation in an
underdeveloped country, a specially modified UPS might be required. TSi
Power’s experienced technical staff can work with customers to arrive at
the best UPS design for underdeveloped countries.
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