2xUPS and one main power source - solution advice request

[MisterPic]

Hello!

I am working on a power supply system that will provide interruptible and redundant power for rack servers and a few users. The system will be similar to what is shown in the image below - everything above the blue dotted line can be changed.

Initial idea and requirement is using two UPS.

Everything looked clear to me until I read a couple of posts here and this article about ATS https://www.apc.com/us/en/faqs/FA156201/ - that is not good idea to use ATS after two UPS which are connected to one Main Power Supply source.

I was trying to find a better design, but I just got stuck...

Could you please suggest the optimal solution for my case?  

Hello!

I am working on a power supply system that will provide interruptible and redundant power for rack servers and a few users. The system will be similar to what is shown in the image below - everything above the blue dotted line can be changed.

Initial idea and requirement is using two UPS.

Everything looked clear to me until I read a couple of posts here and this article about ATS https://www.apc.com/us/en/faqs/FA156201/ - that is not good idea to use ATS after two UPS which are connected to one Main Power Supply source.

I was trying to find a better design, but I just got stuck...

Could you please suggest the optimal solution for my case?  

[Teken]

At a high level an ATS is NOT intended to be installed and used in a single source power system as the APC FAQ calls out. The break fix called out is to program the ATS to none.

Depending upon your working environment a maintenance bypass would / could be used in its place or in concert with an ATS.

Lastly, nobody can see the image you uploaded. Please try a different image format and confirm it’s viewable in the forum. 👍

[MisterPic]

Hi,

I have updated the image in the initial topic. Please have a look. I hope this helps my question more now.

[Teken]

Appreciate the updated image of the intended wiring / hardware layout. Based on your outline I’m unsure why there are no dedicated generator outlets?!? 🤔

If they are in place there are no issues with respect to an ATS in place. If there are no dedicated (backup generator) outlets this would be the time for the facility to have them wired / rewired at the generator ATS. 

Generally speaking the generator ATS is wired to supply critical loads with power. Even when a backup generator is powering an entire building (all lights & plugs) the facility will have dedicated (generator only) outlets especially for a network communication room.

I would suggest you engage the facility to see what the wiring scenario is and validate the same. Normally these outlets are purposely identified and use a different color outlet such as red / blue.

[MisterPic]

Do you mean to connect the second UPS 2 directly to Diesel Generator?

But how this UPS 2 will be online (keep battery's charged) if it will be connected only to Diesel Generator? which is normally is not working.

And the main ATS unit is placed outside the building, nearby the Diesel Generator platform. So it is indeed requires some new constructions to bring this power cable to the building.

[Teken]

Generally speaking a building that has a backup generator system will have its critical loads panel connected to specific circuits in the buildings wiring.

These circuits will power lights, plugs, hardwired loads.

During normal operation the generator ATS will be switched to grid power from the utility.

When grid power is lost (Lights Out Condition) the generator will start, spool up, and once fully operational the generator ATS will transfer over its power to the critical load panel.

The UPS only needs to operate on its battery for the short duration of the transfer period from 1-5 minutes. Once the generator is supplying backup AC voltage it will also allow the UPS to recharge itself.

As stated earlier, normally a building will have dedicated circuits such as outlets / lights / hardwired loads that will be powered by the (Generator Critical Loads Panel).

Outlets are normally marked and highlighted by using coloured faceplates which typically are surge alarm outlets which come in various colours: Blue, red, orange, white, etc.

So again I would ask you to engage with the buildings facility manager to identify how the building is protected by a grid down event. They will indicate what loads / circuits are protected. Protected circuits should be identified if they are dedicated circuits. 

The possibilities are going to be the entire building is protected and powered. To only critical loads are protected and powered which is typical to extend the operation runtime, reduce fuel costs, and lower investment in a smaller generator etc.

[MisterPic]

Yes, that's right.
However, my question is about the concerns regarding using APC rack ATS devices that will be connected to two UPSes, and these UPSes will be connected to a single power source. I see one option for diversification: connecting one UPS directly to the utility power source and the second one to the generator's ITS.
This is probably what you meant.
Technically, will this connection be as a different power source for the APC rack ATS device, and why?

[Teken]

As stated earlier you should engage with the buildings facility manager. Have them explain to you what is present and (IF) the building has dedicated outlets for backup generator power.

This is literally step one . . . 

There are only so many choices and the most common ones are the entire building is protected & powered. To only specific circuits in the building which relates to lights, plugs, hardwired devices.

If the entire building is protected you don’t need any dedicated ATS. Because the UPS will only be running on batteries for a limited time until the generator is fully online.

If the building is only powering specific circuits it’s up to you and the team to determine if there are protected outlets in the area you need them to be. 

If the answer is no your team and facility have to decide if that is fine or not. Their decision will dictate what can be done and what they are willing to invest in for a final outcome.

Generally speaking all of this is driven by what standards & insurance coverage is in place and enforced.

A dedicated (ATS) is installed where the electrical system is designed & deployed correctly to use the same. 

[dpierce4776]

I have read though this a couple of times trying to see exactly what is happening.

I can also see what you are trying to do but with such a low load. You are only using 6Ks to power everything. I am left wondering why you want to connect both UPSs to both loads (User/Server).

As @Teken pointed out really the UPS only needs to power (battery power) the load while the generator starts up. And really that should only be about 30 seconds to a minute especially under such a little load. I have worked for places that had a 1 MW generator and it only took about 30 seconds to start and stabilize.

Also if you read (FA156201)  at the bottom it states

"Using the Rack ATS with a Generator
APC recommends placing a UPS between the generator and the ATS input. The UPS will condition the input voltage to the ATS to prevent thrashing.  A second UPS should be used between the power utility and the other input of the ATS. This UPS will power the load while the generator starts and stabilizes."

The reality is that here where APC calls out "a single source power system" that is a single circuit inside the building off of a single distribution panel. The issue being as the FAQ calls out that the switching of the UPS to battery and back again can cause the ATS on that circuit to see power fluctuations and either drop the load or have other issues. Personally I have seen the AP77xx series lock up in quick power flashes in this scenario. 

That said though having 2 circuits from the same distribution panel feeding, say a cabinet, with one circuit feeding the UPS that then feeds one side ("A") of the ATS. And the other circuit feeding the other side ("B") of the ATS. If perfectly fine. As well as you could also have (as the FAQ states) a second UPS on that second circuit in-between the outlet of the second circuit and the "B" side of the ATS. 

For your scenario. I see what you are trying to do. And having worked with Rack mount UPSs (10K and less) for years. I can tell you that they "operations" are vastly different than larger UPS 20K and above as well as symmetra UPSs. These smaller UPSs use "tricks" (if you will and is mentioned in the FAQ) to switch between power sources (battery/utility) and why it is recommended to use the double conversion style UPS in this case. The larger ones do not need to use "tricks" to make the transfer quicker as they can better maintain loads though better components.  

What I would do for your case is:

1. Remove the 230 volt ATS and have the PDUs plugged directly into the UPSs (one each). Your servers should be redundantly powered anyways. So you would have to lose both UPSs to drop the load.
2. Remove one of the step down transformers and the "User power distribution board" (of which a 120 volt ATS is not going to power anyways as it only has 10 NEMA 515 outlets). Then plug the "A" side of the 120 volt ATS into the remaining step down transformer, and plug the "B" said of the 120 volt ATS into utility/building power.

In reality from what I have read and seen what you are doing in your drawing will work. Provided what the UPSs are on each on their own dedicated circuit. As well as preferably from different distribution panels. 

The better way IMHO is to get a single UPS say the symmetra SYA12K16P that is fed from a distribution panel with generator backup to it. That then feeds a dedicated distribution panel that feeds the servers and users. The SYA12K16P can output 120 and 208/240 volt power. 

If you still want to be even more sure that nothing goes down then you can still stick a120 volt ATS between a 120 volt output from the UPS and an outlet fed from utility power. As well as a 230 volt ATS between a 208 volt circuit from the UPS and one PDU with the "B" side fed by and outlet feed from utility power.