Motor Undervoltage Protection

[SidiropoulosM]

In a transient stability study I noticed that induction motors are modeled with undervoltage protection. The model shows a voltage pick-up setting of 0.60 pu and a time delay of 30 cycles. Do these values seem right? What is the range of typical settings? Any info would be greatly appreciated.

Michael Sidiropoulos

 

[DougMSOE]

The first question to ask is what is the load torque that is required to start the motor and load rolling.
The next question is what does the damage curve of the motor look like and what is the maximum time that the motor can stand to get the load rolling. With this information and some homework you should be able to calculate the settings of the protective relay.

 

[SidiropoulosM]

The stability model that I referred to above shows dynamic data for hundreds of motors around the system, all with the same voltage setting of 0.60 pu and time delay of 30 cycles. I find the uniform modeling surprising but it's done, I guess, for simplicity or lack of specific data. My question is: are these settings within the range of typical values? Are they reasonable?

Michael Sidiropoulos

 

[jbartos]

Suggestion: .6 pu is too low since some motors will stall at .75 pu, others at .85 pu. Therefore, the answer is: It depends.

 

[busbar]

IEEE Std 399-1990 {obsolete} Chapter 9 Table 16 suggests a qualified 80% “at terminals of starting motor,” with 71% “at terminals of other motors that must reaccelerate.”

The chapter may have applicable general material for this subject. Current version of the “Brown Book” is IEEE Std 399-1997 Recommended Practice for Industrial and Commercial Power Systems Analysis