Operation of a three phase UPS

[ruggedscot]

So looking at a system that utilises a couple of sizable UPS unit to supply various equipment through a site. The UPS is static with 3 phase output, both units have an input switchboard and output switchboard with a castell interlock so that the bypass can not be switched in unless the UPs itself is in bypass.

I had a look around at the original drawings and noted that this UPS sometimes operates with the neutral earth link out of circuit. when switching between mains and generator there is a time that the system is operating without the neutral earthed. The feeder to the UPS input board circuit breaker turns off on a power outage until the power is restored, either from a generator or from one of the other buildings on site.

I thinki it needs to be looked at and some form of isolation transformer fitted so that there can be a permanent neutral earth link on the supply to the UPS rather than what is there. the other site electrician is recommending fitting in earth relays but that well i dont think would be that great a solution, to have to depnd on another system to maintain the neutral grounding.

 

[pc1966]

What size of UPS(s) are you looking at?

We have a similar concern with our own setup, but that is a set of 5kVA single-phase UPS with two off each phase (ultimately, just now just one off each phase as 3 UPS in use). The floating neutral bothers me a bit, so looked in to an isolation transformer as well as making a current-limiting PTC resistor (like a filament light bulb, really!) to bleed N to E if isolated, and to warn of high N voltage (like an isolated supply insulation monitor, etc).

This guide to hospital power (not actually related to our work, just a useful document) has stuff on this, such as sections 11.45-48 on page 86:

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/608037/Health_tech_memo_0601.pdf

Such a local transformer can also help control harmonic currents and reduce the "supply" Ze (if zig-zag or delta-star wound) so there can be other reasons to consider that approach beyond just the floating neutral on change-over.

We got a quote from R Baker (Electrical) Ltd for a custom transformer but so far have not actually done anything about it as work not finished. But in our case I think we can get away without the transformer option.

Finally beware of some so-called "3 phase" UPS that really are a 3-phase charger and single phase inverter, if they go on bypass then ALL load end up on the L1 phase! Probably only for small 3-phase UPS like the 10kVA region (I hope!)

 

[ruggedscot]

The UPS system was around 500kva capacity, made up from two 500kva blocks of two 250kva systems. pretty large but not the largest ive seen so far. Interestingly the voltages are quite different when loadbanking it with the mains isolated, hence the further investigations.

 

[pc1966]

OK, that is a lot bigger than I have ever dealt with!

Could you do a test where you manually link N-E, maybe via a small fuse-switch or similar "just in case" so you can link/unlink safely even if it ended up on phase volts, etc, when isolated and running to see if the behaviour is much different from usual on-mains case?

My initial thought was something a 40W filament lamp as it would not allow enough current that you could ever think of it as TN-C, or (probably) to trip any big incoming RCD used for earth fault protection, as well as it providing a warning that you are seeing more than a trivial leakage.

 

[ruggedscot]

OK, that is a lot bigger than I have ever dealt with!

Could you do a test where you manually link N-E, maybe via a small fuse-switch or similar "just in case" so you can link/unlink safely even if it ended up on phase volts, etc, when isolated and running to see if the behaviour is much different from usual on-mains case?

My initial thought was something a 40W filament lamp as it would not allow enough current that you could ever think of it as TN-C, or (probably) to trip any big incoming RCD used for earth fault protection, as well as it providing a warning that you are seeing more than a trivial leakage.

looking into doing a check on it tomorrow, running on UPS for 10 minutes with mains isolated and the NE linked with a CT and a current monitor monitoring the current. I dont think we will see much current, issue we were thinking on was the voltages from the phases and neutral to earth if floating could be problematic to sensitive electronics etc.

 

[pc1966]

Would be interested to hear how you get on with that!

 

[timbobelfast]

looking into doing a check on it tomorrow, running on UPS for 10 minutes with mains isolated and the NE linked with a CT and a current monitor monitoring the current. I dont think we will see much current, issue we were thinking on was the voltages from the phases and neutral to earth if floating could be problematic to sensitive electronics etc.

Can you throw it back to whoever commissions / maintains the UPS, I am thinking this is a Eaton or Schneider or similar type setup? You could of course isolate everything 'upstream' and complete your voltage checks effectively with no load. Likely to be a shutdown of the essential equipment!

 

[ruggedscot]

Can you throw it back to whoever commissions / maintains the UPS, I am thinking this is a Eaton or Schneider or similar type setup? You could of course isolate everything 'upstream' and complete your voltage checks effectively with no load. Likely to be a shutdown of the essential equipment!

Indeed it is, complete shutdown, bee put off jusy now but is scheduled to occur when we get an operational permission to dark site.

 

[pc1966]

Indeed it is, complete shutdown, bee put off jusy now but is scheduled to occur when we get an operational permission to dark site.

Thanks for updating us.