Equipment Damage Curves Generators

 
     
  The information presented in this application guide is for review, approval, interpretation and application by a registered professional engineer only. SKM disclaims any responsibility and liability resulting from the use and interpretation of this information.

Reproduction of this material is permitted provided proper acknowledgement is given to SKM Systems Analysis Inc.

Purpose

The purpose of this guide is to provide basic information about generator thermal limit curves and characteristic landmarks necessary for plotting on time-current curves, for the purpose of equipment overcurrent protection.

Full Load Amps (FLA)

Rated continuous current carrying capacity of a motor at a referenced ambient temperature and allowable temperature rise, see table 1.
 
  Table 1 Generator Temperature Ratings  
 
Max Amb.
Temp.
Hot Spot
Temp.
Temp.
Rise
Temp.
Rise
Total Temp.
Rise
Insul.
Temp.
Insul.
Temp.
40°C 5°C

Class A

60°C 105°C Class A 105°C
40°C 10°C Class B 80°C 130°C

Class B

130°C
40°C 10°C Class F 105°C 155°C

Class F

155°C
40°C 15°C

Class H

125°C 180°C

Class H

180°C
40°C 10°C

Class B

80°C 130°C

Class F

155°C
40°C 15°C Class F 105°C 160°C

Class H

180°C
 
  Short-Time Thermal (Overload) Capability Curve

The short time thermal capability or overload curve represents the permissible output capability under emergency conditions. This curve is not meant as an indication of continuous overload capability, and should never be used to schedule running overload operations. Repeated operation up to and beyond the running overload curve will reduce insulation life. The short-time thermal capability of cylindrical rotor generators as defined in ANSI C37.50 is listed in Table 2.
 
  Table 2 Shot-time thermal capability  
 
Time (sec.) 10

30

60 120
< #2 AWG 226 154 130 116
 
  Short Circuit Withstand Capability Point  
  The short circuit withstand capability point represents the maximum time the machine can withstand a 3-φ short circuit at its terminals without damage when operating at rated kVA and power factor, at 5 percent over-voltage, with field excitation.

Decrement Curve

The decrement curve represents the current output of the machine considering a 3-φ fault applied at or near its terminals. The current output is defined by the following equations.
 
     
   
  Example

Plot the characteristic landmarks for a 750kVA, 480V, 902A synchronous generator with the following characteristics.

Xd" = 0.107 Ω p.u., Xd' = 0.154 Ω p.u., Xd = 1.54 Ω p.u.
Td" = 0.015 sec., Td' = 0.417 sec., Ta = 0.189 sec.
If = 3 A p.u., Ifg = 1 A p.u.

Solution

In the submittal package from the generator manufacturer the overload capability at rated operating (hot) temperature is listed as
 
     
   
   
   
  The results are plotted in figure 1.  
   
   
  References