UPS-Generator Problems

[tkb]

I went out to look at a generator problem for a customer?s data center and found the following problems.

There are two 75kVA UPS systems and two CRAC units, one 15 ton and one 20 ton.
250kW generator and a 400 amp ATS. About 200 amps of load on the 480 volt system.

Both UPS system were able to accept the generator source. During the 1st test they got on right away. During the 2nd test it took a while, but it turned out that the natural gas pressure to the genset was dropping considerably. Once the gas pressure came back, the generator leveled out and the UPS systems transferred over. The gas line appears to be shared with two boilers for the building, so this might need to be changed.

We could not get either CRAC unit (one 15-ton and one 20-ton) to restart on the generator source, unless we removed one UPS from the generator. The blower fan contactor would start to pull in and then immediately drop out. If we removed one UPS from the generator (open the input breaker and force it onto its batteries), the blower fan contactor would close and the fan would run. The compressor contactor would then try to close and immediately drop out as the blower fan contactor had done.

The line voltage stayed fairly stable throughout the testing, never dropping below 273V per phase (277V nominal). So, the theory that the voltage was sagging and causing the contactor not to pull in didn?t seem to apply. We metered the control voltage to the contactor coil as well and it seemed to stay at 26V (24V nominal).

We recorded all of this with a Dranetz PX5.
I have seen the UPS systems not like being on a generator but not the CRAC units.

I am not sure if the generator is not large enough, the gas supply is not consistent enough or if it is a harmonic issue.

Anyone have any suggestions?

 

[satcom]

I went out to look at a generator problem for a customer?s data center and found the following problems.

There are two 75kVA UPS systems and two CRAC units, one 15 ton and one 20 ton.
250kW generator and a 400 amp ATS. About 200 amps of load on the 480 volt system.

Both UPS system were able to accept the generator source. During the 1st test they got on right away. During the 2nd test it took a while, but it turned out that the natural gas pressure to the genset was dropping considerably. Once the gas pressure came back, the generator leveled out and the UPS systems transferred over. The gas line appears to be shared with two boilers for the building, so this might need to be changed.

We could not get either CRAC unit (one 15-ton and one 20-ton) to restart on the generator source, unless we removed one UPS from the generator. The blower fan contactor would start to pull in and then immediately drop out. If we removed one UPS from the generator (open the input breaker and force it onto its batteries), the blower fan contactor would close and the fan would run. The compressor contactor would then try to close and immediately drop out as the blower fan contactor had done.

The line voltage stayed fairly stable throughout the testing, never dropping below 273V per phase (277V nominal). So, the theory that the voltage was sagging and causing the contactor not to pull in didn?t seem to apply. We metered the control voltage to the contactor coil as well and it seemed to stay at 26V (24V nominal).

We recorded all of this with a Dranetz PX5.
I have seen the UPS systems not like being on a generator but not the CRAC units.

I am not sure if the generator is not large enough, the gas supply is not consistent enough or if it is a harmonic issue.

Anyone have any suggestions?

How is the gen when you run it on the load bank?

 

[tkb]

How is the gen when you run it on the load bank?

We didn't try that yet.
That was one of my suggestions.

Also I suggested that we bring in a 500 kW roll-up and connect it to the ATS to see if the generator is too small. But that may not give us what we want because it would be NG vs. diesel.

This seems to be troubleshooting by committee.
I was brought in to set up the Dranetz.
We had one of our UPS techs and one of our generator techs there along with the EE (who is my boss) and there were six people from the company that owns the system. The only guy missing was a HVAC tech.

 

[brian john]

Did you do a gas pressure test while loading the system?

Did you watch voltage and frequency while trying to load the generator?

 

[tkb]

Gas pressure drooped significantly when the generator started.
The gas line for the generator is shared with the boilers.
There is about 80'-100' of 4" gas piping to the generator.

I think the gas dropped down to 2"H2O from about 6"H2O
I think the generator needs more pressure and it's own dedicated piping to it's own regulator from the street.

We know there is a gas supply problem, but the generator was running pretty smoothly and we had a steady voltage at 475 L-L.
The frequency wavered between 59.8 and 60.1

 

[rich000]

I would suggest writing a procedure for testing this and have everyone agree. You said this was troubleshooting by committee? Then the committee needs to sit down and get all the tests they want done written down.

Have you considered a third party for testing? Such as a commissioning company? Was commissioning ever done on the data center? If it was, there should be baseline test data to compare to to see where the problem is.