UPS : Input neutral requirement 1

[Universe_in_Flux]

I have requirement of 10kVA UPS which will be fed from 415V 3 phase input supply, and output is 110V AC single phase.
I have several questions as listed below about the same:
1) Which factors decides the UPS input supply neutral requirement : Several manufacturers are offering UPS with 3ph/3 wire input and some are offering UPS accepting 3 phase /4 wire supply. I want to work out which option suits my requirement better.
2) Another question is regarding coordination between fast acting fuses (protecting inverter) and MCBs in outgoing feeders of distribution board which is connected in output of UPS. Loads by nature are electronics only. Vendors recommend to keep the ratings of these MCB below some percentage of UPS full load current, but follwoing this general value of % for every type of MCB doesn't make sense to me, as MCB with different cureves will have different let through energy. Can anyone elaborate the procedure to choose Maximum MCB rating on downstream such that coordinates with inverter protective devices.
3) When should one consider use of UHF isolation transformer?

 

[Kiribanda]

1) From your input data it looks like you are having a 415V/3ph/4Wire system.
Therefore, it is your choice to decide to use 415V/3ph/3W or 4W UPS.
Since the neutral is giving troubles, 415V/3ph/3W UPS is the most suitable option for me.
But if you go for a 415V/3ph/4W UPS option then make sure that the neutral is not bonded
to the 110V/1-ph/2W side of the UPS.Neutral should be for the 415V side only.
On the 120V side, you may have 110V/1-ph/2W with one conductor grounded but that ground
point should not have any galvanic connection to the 415V side neutral.
2) Since these UPS units donot offer high sc currents for a 120V side fault,using fuses
is not a good choice. Instead mcbs (with Type B trip curve) made for IEC60898 is the
best chioce.
3) Sorry no idea about the UHF isolation transformer of a UPS unit. But it is good
to have an isolation transformer at the 415V side so that all garbage is blocked at the
415V side.

 

[itsmoked]

2) You look at the curve for the breakers and make sure they don't exceed the recommendation.

UPSes, unlike mains, can only supply a limited current into a short circuit. If that turns out to be less than what it takes to trip that circuit's breaker something will cook -thoroughly-.

The breaker sizing should also take into account that the UPSes total fault ability is partially consumed by all the other loads operating when a fault occurs on a specific circuit.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mparenteau]

If I remember correctly (I'm sure someone will jump in if I'm wrong), I believe UPSs can typically supply up to 300% fault current until they transfer to bypass. The OCPD upstream will then clear faults downstream of the UPS.

Mike

 

[FreddyNurk]

mparenteau, it might vary depending on the size of the UPS, its topology and so on, but 300% in my experience is very ambitious, it'd be more like 200% at most.

EDMS Australia

 

[mparenteau]

@Freddy...

Yep...the model I'm familiar with is good for 200% for 200ms. Thanks for the info..

Mike