PROPERTY OF SOLIDSTATE CONTROLS, INC. 7/1998
Although it is common for UPS larger than 10 KVA to be specified with three phase output,
industrial distributed control systems rarely consist of any component that actually requires true
three phase power. Particularly on smaller systems (under 50 KVA), there are some very real
technical advantages to single phase distribution.
Unlike a typical building distribution system which has relatively low source impedance, an inverter
is a high impedance power source. This high impedance effectively limits the amount of fault
current available to clear downstream branch faults. Although under normal operating conditions
most UPS clear downstream faults and short circuits by transferring the load to the low impedance
bypass line via the static switch, there are operating conditions that will need the inverter to be
capable of clearing a faulted branch circuit.
The most obvious of these situations would be if the bypass line were not present due to a power
failure; in which case, the inverter would be operating off the battery. A not so obvious and rarely
considered case would be if the inverter were out of sync with the bypass. Transfer is inhibited
when out of sync to prevent an out of phase transfer from occurring. The most common cause of
an out of sync condition is if the facility in question is supported by an emergency stand-by
generator. If the generator is powering motor loads in addition to the UPS, the frequency will sag or
overshoot anytime a load is applied or removed, resulting in an inverter out of sync condition.
When three phase output inverters are used to power single phase distribution, only one third of
the inverter's total fault rating is available to clear a line to neutral fault. If a single phase inverter is
used, it's entire fault rating is available to clear a line to neutral fault.
As an example, if we considered a 15 KVA application using SCI's 500% of rated (much higher
than most) fault current capability, a three phase inverter would have a total of 208 amps available
while a single phase inverter would be capable of 625 amps. If the inverter were at full load, these
would be reduced to 166 amps and 500 amps, respectively. It would be highly unlikely that the
three phase unit could clear a fast acting fuse, much less a circuit breaker, prior to current limiting.
Once the inverter goes into current limit, the voltage will decrease and the entire load will be
dropped.
In summary, a single phase output inverter of any size rating will stand a much better chance of
clearing a branch fault than a like sized three phase inverter. Other advantages such as lower
inverter initial cost and simpler distribution (no need to balance phases) greatly outweigh the
minimal inconvenience and slightly higher cost of the larger wire required for single phase.
(12 paginas)
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