Motor starting voltage dip (MV) 1

[Power0020]

I am performing a motor starting study on a 2000 HP motor (4.16 kV), I wonder how much bus voltage dip is allowed in general?

As I remember, NEMA recommendations that the motor terminals voltage during starting shall not dip beyond 85%, what about other connected auxiliary loads? MV/LV Transformers supplying BOP loads? 10%?

 

[jraef]

That depends on the other loads. Some sensitive electronic equipment cannot take even a 5% dip. Most large power equipment is built to handle +-10%, but remember, the dip transmits through all transformers etc., so a 15% dip in MV becomes a 15% dip at the lowest voltages on the same grid, including those sensitive electronic devices.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LDKGR]

Hi Power0020,

if you want to be on the safe side no more than 10%, aux no more than 3%.
Moreover, check the motor's torque curve and the load curve.

 

[Power0020]

Thanks jraef & LDKGR, I am a bit worried about the pickup voltage for undervoltage relays as it may cause a nuisance trip.

ITIC curve can be helpful for electronic equipment against the acceleration time of the motor.

 

[Parchie]

I am performing a motor starting study on a 2000 HP motor (4.16 kV), I wonder how much bus voltage dip is allowed in general?

As I remember, NEMA recommendations that the motor terminals voltage during starting shall not dip beyond 85%, what about other connected auxiliary loads? MV/LV Transformers supplying BOP loads? 10%?

The formula is: %VD = LRM/F x 100, where: LRM = locked rotor MVA of your motor and F = the maximum available fault current at that point where the motor will be located.
It's been posted here a long time ago. Here's the LINK.

 

[Parchie]

Tried editing but I don't know how.

BTW, the definition of F should read: F = the maximum available faukt "MVA" not "current".

 

[7anoter4]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1455224497/tips/Equations_Help_Determine_Largest_Motors_on_Utility_Lines_pzrdcn.pdf[/url]