Power spikes in new building - utility won't admit fault

[gozoliet]

We've just finished construction on our new office building, and have noticed the lights flickering frequently throughout the day. Had the utility company to measure and test everything and they claim it's within our building.

On several occasions I've measured a roughly 8V drop (for example from 116V to 108V) that lasts 5-10 seconds on a120V outlet. I have visually confirmed that one of those drops happened when the lights flickered.

I'm waiting to get my hands on a data logger to measure at the panels. In the meantime I managed to convince the utility to send me their data log from when they measured just inside our meter. I'm seeing a lot of 20V spikes on the 340V (see graph) during business hours, which probably coincide with what I'm measuring at the outlet.

Utility acknowledged the spikes on their graph, but claim that they still fall within their "allowable range" so shouldn't be a problem. I concede that the power can move within a wide range, but the sudden fluctuations seem like a problem to me. Can anyone comment?

 

[rockman7892]

I just looked at the undervoltage settings on both our 2300hp motor and 6500hp motor. They both had an undervoltage setting of 83.2V on the 120V PT's which corrosponds to about 70%.

These settings were done by a consulting engineer, but they seem low based upon what was stated above.

 

[dpc]

Motors that large do not fall under typical NEMA requirements, so those settings may be fine. Contactor drop-out can be mitigated by using UPS power supply and other techniques when necessary.

Typical NEMA motors are rated to operate down to 90% of nameplate voltage continuously.

Acceptable dip for motor being started will depend on the motor and the load. If the load has high starting and accelerating torque requirements, the motor will need more voltage to get the job done before it trips (or melts). But for motors on centrifugal pumps, etc, they can start on fairly low voltage - that is the basis for reduced-voltage starting after all. The motor doesn't know if there is a reduced voltage starter or you have a lousy power system.

For large motors, you may want to consider motor starting analysis that takes into account actual motor and load inertia and speed-torque curves.

David Castor

http://www.cvoes.com