Utility Voltage - What is acceptable?

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pepinbox

Member
I am inspecting a 480VAC pump station that appears to be at the end of the utility feed. The system voltage measured (unloaded) is 500VAC. When I run a single 250HP pump, the voltage sags to 480VAC. When multiple pumps are running it sags to as low as 455VAC. This is causing a problem with the solid state drives. The pumps are having a problem reaching full speed with this voltage sag. I need to find some standards to compare these findings with.

Does the NEC address power quality?

The 120VAC power derived from this source varies from 123VAC to 116VAC. Are there any standards for how much this voltage may vary?
 

bdarnell

Senior Member
Location
Indianapolis, IN
Occupation
Retired Engineer
Do these pumps sag the voltage on start, or also during run ? Around here, we have to inform the POCO if we connect anything larger than a 100 HP motor and it can not be started across the line. You said you are using drives, so I assume you are ramping up the start ?

Most POCO's that I have dealt with say 5 to 10 % is an acceptable voltage tolerance.
 

pepinbox

Member
Yes, we are attempting to ramp up the start using A/B soft starter drives. The problem when using utility power, the 2nd pump never reaches full speed and indicated by the drive automatically switching over to it's internal bypass contacts. Under generator power, it switches over in a matter of seconds, and there is negligible (3 VAC) voltage sag.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The Illinois Commerce sets the voltage requirements and specifies +/- 10% of nominal. For a 480 volt system that would mean any voltage between 432 and 528 is acceptable. The system should work at 455. You need to check your equipment.
Don
 

pepinbox

Member
I'm feeling very confident in the equipment, as it runs great on genertor power.

The voltage was measured at the switchboard and the MCC section.
 

pepinbox

Member
All indications point to a utility problem. I'm pretty sure with better equipment, the voltage sagging will turn out to be much more than 10% during motor starting.
 

derf48

Member
How close is your equipment to the utility transformer? Normally the generator is in very close proximity to the load and thus very little voltage drop is experienced. The length of the distribution of the 480 V as well as every connection will influence the efficiency of your operation. IMHO, these factors should be checked out .

Fred
 

jtester

Senior Member
Location
Las Cruces N.M.
I believe that ANSI C84 specifies a service voltage variation of +/- 5%, and utilization voltages of +/-10%. If a utility service voltage varies by 10% from nominal for a long period of time, I believe it could cause problems. I am surprised to find that public utilities vary more than 5%.

Jim T
 

pepinbox

Member
The utility transformer is located on site in close proximity (25-40 ft) from the switchgear...

...but the site is located out near the airport, which I believe to be at the furthest point from a utility substation - approximate distance tbd.
 

MJJBEE

Member
Do you happen to know the size of the utility transformer? It may be undersized for the load.

Jtester the 5% is at the Meter. The main switchboard should be between 5 and 10%. With a maximum 10% drop at the VFD.

I would talk to the distribution engineer for the area and see what solutions I could come up with. You might start by getting a voltage recorder to get a better idea of what happens when you start a motor.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
pepinbox said:
Yes, we are attempting to ramp up the start using A/B soft starter drives. The problem when using utility power, the 2nd pump never reaches full speed and indicated by the drive automatically switching over to it's internal bypass contacts. Under generator power, it switches over in a matter of seconds, and there is negligible (3 VAC) voltage sag.

the incoming voltage is going to be whatever it is. you may just have to live with it. you can probably tweak the soft starter to get it to switch to the bypass contactor even with the lower input voltage. talk to the drive manufacturer about how to do this if they failed to supply an understandable manual.
 

pepinbox

Member
:idea:

Resolution:

The problem was found to be the utility transformer for the site was undersized. It was sized for a single pump load, rather than both pumps running as shown on the plan set.

Thanks for all the comments.
 
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