Why UPS closes bypass and not shunt trip the breaker? 2

[AusLee]

Hello,

I was reading the following: "In case of short circuit, the UPS shall close the parallel bypass switch to fetch more energy from the network in order to open the circuit breaker more quickly, hence reduce the damage cause by excessive heat from the short circuit current."

My question is: can't the main UPS breaker be simply equipped with a shunt trip relay (like MX relay in Schneider Electric-Merlin Gerin beakers), and hence in case of short circuit, the breaker will be "Ordered" to trip instead of being "Forced" to trip?

What do you think?

 

[davidbeach]

This design is to aid in tripping downstream circuit breakers. Assume that you have a fault two circuit breakers away from the UPS. You want enough fault current supplied to the fault to trip the circuit breaker nearest the fault, you certainly don't want to take the UPS off when you could simply remove the offending branch circuit.

 

[ScottyUK]

Expanding a little on what David said, most electronic UPS modules can not deliver enough short circuit current for long enough to trip a breaker of any size. For example a 100kVA module will quite probably fail to clear a fault downstream of a 32A Type C breaker if it can not trip to bypass. The UPS relies on the low impedance of the utility bypass supply to clear faults quickly. This is one of the weakest aspects of an static electronic or flywheel-electronic UPS, and one of the strengths of rotary UPS designs.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[flexagon]

more on this issue.
quoting a manufacturer's technical specification:

"During a fault within the load or a short circuit, the UPS will limit the amount and the duration of the current supplied (current limited), in order to protect the UPS inverter output(short circuit current).
The amount of current available will depend upon the operating mode of the UPS at the time of the fault.

UPS in Normal Operation mode: The load is instantly switched onto the by-pass supply, thus guaranteeing the values indicated in the technical data section, before the fuses intervene.

UPS in Battery Operation mode: The UPS protects itself by supplying an output current of approximately 2 times the rated current for 0.5s."

now what if...

A)
the short-circuit rating of the bypass circuit on the component with the lowest rating — or the manufacturer's specified rating — is lower than the (calculated) large fault current of the network.

->One would need to externally address short-circuit current reduction.

B)
a fault downstream occurs during a network outage (i.e. UPS is in battery operation mode)


So, how can a power systems engineer achieve both goals:
-reduce the fault current to an acceptable value
-complete coordination among circuit protection devices