UPS protection 1

[lyledunn]

Many UPS rely on switching to a bypass circuit when a load-side fault is detected. However, one primary function of the ups is to take the load when mains fails. Now I assume that the ups may not have appropriate energy levels to clear faults in the appropriate time, so what mechanism is generally employed to provide required clearance times?

Regards,

Lyledunn

 

[davidbeach]

The UPS will close the static bypass switch and allow raw line power to flow to the fault to aid in fault clearing. While running strictly on batteries (no source power available) you just take your chances.

 

[DanEE]

You post hits a problem we see often in planning, installing and supporting UPS systems. It's a multi-part problem..

Many times, what is done at new installation time to minimize the concern you bring up, is often degraded over time as additional unplanned (or unauthorized) loads are placed on the UPS leaving too little margin.

For purposes of discussion, you might find in typical large (e.g. hospital) telephone switching system room a 7 kva UPS system for the phone switch peripherals. The switch itself is running off the 48VDC battery/DC plant.

We spread the loads at the UPS distribution panel across many, small (e.g. 10amp breakers) to break up the loads so if there is an overload somewhere it will bring down only that circuit and few devices on that circuit. And, the smaller the breaker, the more difficult it is for that branch to bring down the entire UPS unless...

If the UPS overall load has grown to too close to the limit, the situation has been set up where the extra fault load will bring the UPS down. When we see this on our maintenance visits, the customer is made aware of the situation and suggest either upgrading the size of the UPS or installing a second UPS and break the load up . Ideally, we don't like to see more than 50% load on a UPS system.

The other common scenario that happens frequently is that someone plugs a vaccum cleaner, microwave oven, coffee pot, etc, etc into an outlet supplied by the UPS system and brings everything down. We suggest installing "non standard" twist loc 120v receptacles and changing the plugs on equipment authorized to be on the UPS, to make it difficult for Joe Blow to plug in unauthorized equipment.

Lastly and getting closer to your concern I believe, regarding mechanical breaker trip times, versus a microprocessor monitored UPS system (and potentially much faster overload detect and shutdown), in some UPS the parameter tables can be changed e.g. duration of overload and percentage of overload before initiating shut down. But this won't help much if there is insufficient margin.

I would suspect UPS topology plays into this also.. In many installations, we use a particular brand UPS that is ferro resonant transformer based (versus switching topology) because it is the largest kva rated UPS that can use the main battery plant 48 volt system (and thereby guarantee the phone switch peripherals will stay up as long as the switch does during an extended power outage).

My guess is a UPS that is DC powered off for example, the main PBX switch 1700AH 48v battery bank, and with the "flywheel" effect of the ferro resonant circuit vs some of the switching UPS, they might perform better with regard to clearing faults.

Maybe someone who is closer to the design and performance of equivalent kva ferro based vs switching UPS systems can shed some light on this..

 

[ScottyUK]

To add to DanEE's rather good post,

This is a problem which largely affects static (power electronic) UPS, including those with rotating energy storage (flywheels). It does not noticeably affect the true rotary UPS systems with a specialised electrical machine and a clutch-coupled diesel engine unless the breaker size involved in the fault is large percentage of the system rating. This shouldn't happen in a well-designed system unless a fault occurs in the main feeders, a situation which would be pretty catastrophic for any system. The downside of the rotary UPS is mainly size and cost. They don't exist below about 250kW and are very expensive compared to an equivalent-rated static system.

The approach I've used with static systems is to segregate loads onto many small circuits and avoid using large final distribution circuits like ring mains. This allows use circuit breakers with low current rating which gives the UPS a fighting chance of clearing a fault on a final circuit. Circuit breakers with type C and type D tripping characteristics are best avoided, especially the type D which is intended for very high inrush loads.

Install a backup generator and ATS which will restore power to the bypass in the event of utility failure. If you can arrange two utility feeds from independent sources plus the generator, so much the better. The generator will normally clear a faulted final distribution circuit even if the UPS can't clear it without going to bypass. This reduces the exposure to the period between loss of utility and the generator spinning up. Fault clearance times when on the generator will be longer than on the utility because of the lower available fault current.



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If we learn from our mistakes,
I'm getting a great education!

 

[lyledunn]

Thank you gentlemen for your kind responses. I see the kernel of what you are saying. I take it that the UPS own protection system operates in the event of a fault with no mains or gen set etc available. As such, where there are no minor downstream protective devices, how is it confirmed that the appropriate disconnection times are achieved in the event of say an earth fault in order to protect against shock?

Regards,

Lyledunn

 

[itsmoked]

DanEE; Nice post.

Does anyone provide a panel that essentially looks at or learns the normal individual circuit loads and can just shut off a particular circuit before the standard breaker reaches its 200%(or whatever) instant trip point. Essentially preventing large faults. ??

ScottyUK; Haven't I seen those smaller rotaries that look sort of like C3PO and run 2kW to 50kW???

All this clearing etc. taking down large UPi, seems to suggest that indivual UPS'es have some small advantage in that it gets harder to overload them, certainly not all of your users/sites at once, and no one is going to plug in a vacuum, etc. to them.

 

[DanEE]

Good question on the appropriate panel/CB downstream of the UPS. This discussion has prompted me to want to know more about this myself..

The thought occurred that this is a good monkey to put on our UPS manufacturer technical engineering support... e.g. recommendation of specific magnetic breakers with response times to minimize the chances of shutting down the UPS, yet not false trip with the type loads the UPS has to support.

I think we have a couple of UPS models in the warehouse that are factory configured plug and cord models.. Have to see what type breakers they are using on the back panel receptacles... and look up their response time specs...


btw, this is some interesting reading on Airpax magnetic breakers..

http://www.promoco.se/datablad/automatsakringar/design_guide0404.pdf

 

[ScottyUK]

Hi Lyledunn,

The fault clearance time is a tricky one when on battery. Frankly I'm not convinced a static UPS on battery would meet the UK spec for disconnection time for socket outlets (100mS) under all conditions. The only way I see of guaranteeing it would be to use RCD or RCBO protection on these circuits. The 5 seconds allowed for fixed equipment would see the UPS shut down well within that period. I guess hardwiring all your circuits is a technical solution if not especially practical.

ItSmoked,

Wasn't C3PO the gold one who looked humanoid? Never seen a UPS that shape...

Do you mean the types offered by Cat and, I think, Merlin Gerin? These use a rotating flywheel as the energy store, rather than a battery, but the stored energy is converted to AC by a power electronic stage which has the same vulnerabilities as the battery-based system. The 'proper' rotary types are made by the likes on Anton Piller (Pillar? I forget - it is getting late here) and have no power electronics conversion. The load is fed from a special electrical machine which serves as motor, generator and energy store within a common frame. The rotating mass holds enough energy to support the load while a diesel engine spins up to drive the machine, or if the diesel refuses to start the engine is crash-started by dumping the clutch. I bet that is exciting on a megawatt class machine - it is impressive on the 500kW sizes.


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If we learn from our mistakes,
I'm getting a great education!

 

[ScottyUK]

See also thread238-101964 which someone has dug up to discuss something unrelated to UPS systems in. Tomatoes, ItSmoked?



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[itsmoked]

Scotty; I was trying to make a point to that guy.. Man-o-man...

Yeah got my droids confused, the one shaped like a shop vac.

And yes that's what I remember, sorta under the counter rotary(flywheel) types. lyledunn has never actually stated the size he's dealing with.

Scotty, did you mean "RE-lated to UPS systems"?

 

[ScottyUK]

You have to laugh or else you cry with some of these posts. The tomatoes - I bet you say tom-ay-to too, even though we all know it's tom-ah-to - just got me laughing out loud. God only knows why he posted a load of stuff about magnetic bearings and optical strain gauges in that forum.

The small UPS you describe sounds like a flywheel type. At least, I hope it doesn't have an engine if it is under the counter.





----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[itsmoked]

{toe-may-toe} Bruahahaha!! Yeah, I've NEVER seen such a out-of-the-blue post before. It was so far off I can only hope it was an accident.

Here we go R2D2!

http://www.beaconpower.com/products/EnergyStorageSystems/SmartEnergy25kWh.htm

And a few others:

http://www.genpropower.com/powerware_pf2_flywheel.htm

http://www.oneac-powercom.com/liebert-fs-flywheel.html

http://www.pcorp.com.au/2003_products/prod_pwrstore_oview.shtml

http://www.afstrinity.com/specs.html

http://www.afstrinity.com/specs/spec_m3.html