Motor Impedance Xd''

[rusk]

Hi, I am currently working on a short circuit study and a load flow study. Can anyone tell me what typical Xd'' values would be for a 3ph 11100kW 13.2kV motor? So far I have seen that the value of 0.17 would be an ok assumption? Please provide some insight on this issue.

Thanks

 

[dpc]

This will depend on the motor type, induction or synchronous, as well as the motor speed and construction. Your value of 0.17 pu is probably reasonably, and matches Table N1.1 in IEEE Standard 141 (Red Book). But this is quite a large motor, so the value could be somewhat less than this.

If this is an induction motor, the motor contribution will decay rapidly, within a few cycle. If it is a synchronous motor, then not quite so rapidly.

The X"d value is at best an approximation of a complex electromechanical system's transient response characterisitics.

For a motor this large, you may be able to obtain more accurate data from the motor manufacturer, if they are still in business.

 

[jbartos]

Suggestion: IEEE Std 141-1993 "Red Book" Table 4A-1 on page 174 "Typical Reactance Values for Induction and Synchronous Machines, in per unit of machine kVA Rating. The value could be lower or higher, depends on a type of the motor and a number of poles.

 

[rusk]

Thanks for the help. For right now I think I will stick with 0.17.

jbartos - You said that the value could be lower or higher depending on number of poles. The motors I am dealing with have 4 poles. Some motor drive pumps and others are just fans. Given this info will my values would be lower or higher than what is shown in IEEE Std 141-1993?

Once again guys, thanx for the help.

 

[jbartos]

Comment: Reference:
1. IEEE Std 399-1997 IEEE Recommended Practice for Industrial and Commercial Power System Analysis, (Brown Book),
Table 7-3 - Typical values of motor impedances and kVA ratings to use when exact values are not known.
on page 180.
"Individual large induction motors, usually medium voltage = Xd"=0.67"
I do not see that 0.67 realistic. This could be inquired about at IEEE:

http://www.ieee.org

There is also the following Note:
"Motor impedances are in per unit on motor voltage and kVA rating. Xd" for induction motors is approximately equal to the locked-rotor reactance." (Agreed.)
"For induction motors, the locked-rotor reactance is the reciprocal value of the locked-rotor current." (Agreed.)
Now, if you check a manufacturer literature, e.g. Baldor Motors and Drives AC Motor Data Catalog 502 April 1992, you will see that higher pole (lower RPM) induction motors have smaller Lock-Rotor amps. This implies that the Xd" is higher for the higher-pole squirrel cage induction motor. If you consider often used LRA/FLA=6.0 p.u., you will get Xd"=1/6=0.167~0.17p.u. as you indicated in your posting. If the large machine has LRA the same you can use 0.17pu=Xd". Some motors have on their nameplates Nema Code Letter, e.g. G that has kVA / HP range from 5.6 to 6.29; therefore, LRA/FLA = 6.0 is approximately right. If your large motor has a different LRA or NEMA Code Letter, then you need to adjust Xd".