Your Opinion - greater power risk

[Shoe]

I'm looking at a Tier 1 data center (one utility source, one generator, one UPS) that needs to have its UPS taken offline for maintenance. The data center load will be placed in bypass during this time.

The utility and generator are connected through an open-transition automatic transfer switch. A surge protection device is present on the utility source. Would you opt to keep the load on utility during repairs or place it on the generator? What's less risky in your opinion and why?

 

[steve66]

IMO, as long as the weather is good, the utility is much more reliable than a genset, unless you have parallel gensets.

I would call and try to verify the utility doesn't have any scheduled outages or work in the area that could make it less reliable than normal.

You could also parallel a couple of rental gensets. Then the main risk is during the time it takes to connect them.

 

[ron]

Depends on the utility and its track record, AND the generator and its track record.

In some locations, I'd be on the utility all day long with the generator spinning ready to go.

 

[Shoe]

There is only one generator. It is well-maintained and gives you a sense of "control" over the power source. It will already be started, running and transferred, so you don't need to worry about those failures related to the genset.

However, the engineer in me still leans to keeping it on the utility. It's statistically more reliable (i.e. given no weather conditions) and it's being performed off-hours with no utility work being done at that time.

If either source were to fail, the other source will be available... but given the open-transition ATS the load will likely drop anyway.

 

[ron]

Is the service at 120/208? If so, and both sources will be energized, is there anything that would prevent the ATS from transferring over immediately?

The servers don't care if the source transfer is out of phase, they convert to DC anyway.

HVAC may prefer a delay, but IT equipment doesn't care. I've transferred STSs out of phase before in an emergency when the UPSs wouldn't sync because they were on different utilities.

 

[steve66]

Yes, I suppose a good ATS would have a good chance of making the transfer before the power supply caps discharged.

 

[Shoe]

Is the service at 120/208? If so, and both sources will be energized, is there anything that would prevent the ATS from transferring over immediately?

The servers don't care if the source transfer is out of phase, they convert to DC anyway.

HVAC may prefer a delay, but IT equipment doesn't care. I've transferred STSs out of phase before in an emergency when the UPSs wouldn't sync because they were on different utilities.

Interesting thought. It is a 480V service, so I would have some concerns over high inrush to the saturated coils of the step down transformers possibly tripping breakers. Pretending it was 120/208, would transferring load out of phase result in a large current spike? (i.e. I proportional to dv/dt?)

 

[Ingenieur]

for how long?

your util will have outage stats for your area
https://www.eia.gov/todayinenergy/detail.php?id=27892

offhand util

have the genset running with no xfer delay
set xfer parameters very tight

 

[ron]

Interesting thought. It is a 480V service, so I would have some concerns over high inrush to the saturated coils of the step down transformers possibly tripping breakers. Pretending it was 120/208, would transferring load out of phase result in a large current spike? (i.e. I proportional to dv/dt?)

If there were no transformers or HVAC (maybe fed from another ATS), the IT power supplies will have a little (as the caps are already charged), but nothing significant.

 

[gadfly56]

If there were no transformers or HVAC (maybe fed from another ATS), the IT power supplies will have a little (as the caps are already charged), but nothing significant.

If there are other loads such as HVAC or transformers, maybe they can be manually shed for the duration of the work.

 

[ron]

If there are other loads such as HVAC or transformers, maybe they can be manually shed for the duration of the work.

Let's shed the IT equipment and be done with it, considering they are Tier 1 anyway without concurrent maintainability :happyno: