power systems: Redundancy options
major applications like major computer installations, process control in chemical
plants, safety monitors, IC units of hospitals etc., even a temporary power failure
may lead to large economic losses. For such critical loads, it is of paramount
importance to use UPS systems.
But all UPS
equipments should be completely de-energized for preventive maintenance at least
once per year. This limits the availability of the power system. Now there are
new UPS systems in the market to permit concurrent maintenance.
The computing industry talks
in terms of "Nines" of availability. This refers to the percentage of
time in a year that a system is functional and available to do productive work.
A system with four "Nines" is 99.99 percent available, meaning that
downtime is less than 53 minutes in a standard 365-day year. Five "Nines"
(99.999 percent available) equates to less than 5.3 minutes of downtime per year.
Six "Nines" (99.9999 percent available) equates to just 32 seconds of
downtime per year. These same numbers apply when we speak of availability of conditioned
power. The goal is to maximize the availability of conditioned power and minimize
exposure to unconditioned utility power. The concept of continuous availability
of conditioned power, takes this concept one step further. After all, 100 percent
is greater than 99.99999 percent.
To Continuous Availability
availability by studying four key elements:
The individual UPS modules, static transfer switches and other
power distribution equipment must be incredibly reliable, as measured by field-documented
MTBF (Mean Time Between Failures). In addition, the system elements must be designed
and assembled in a way that minimizes the complexity and single points of failure.
The UPS must be able to protect the critical load from
the full range of power disturbances, and only a true double-conversion UPS can
do this. Some vendors offer single- conversion (line-interactive) three-phase
UPS products as a lower cost alternative. However, these alternative UPS's do
not protect against all disturbances, including power system short circuits, frequency
variations, harmonics and common mode noise. If your critical facility is truly
critical, only a true double conversion UPS is suitable.
The system design must permit concurrent maintenance
of all power system components, supporting the load with part of the UPS system
while other parts are being serviced. As we shall see, single bus solutions do
not completely support concurrent maintenance.
The system must have fault resiliency to cope with
a failure of any power system component without affecting the operation of the
critical load equipment. Furthermore, the power distribution system must have
fault resiliency to survive the inevitable load faults and human error.
The two factors of field-proven critical bus MTBF in excess of one million hours
and double-conversion technology ensure reliability and functionality. With reliability
and functionality assured, let us look at how different UPS system configurations
compare for maintainability and fault tolerance.
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