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 = 0.93 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 		 
     
 
Current (%) Time (sec.)
1.45 1080.0
1.65 420.0
2.00 180.0
 
  The manufacturer also lists a sustained short circuit capability of 300% line current for 5 seconds. The decrement curve will be calculated using equation (1) from 0.01 to 5 seconds.  
 

Time(sec.)
AC DC Total

0.01
8.3 13.1 15.5

          0.1
          6.1           8.1          10.2

0.2
5.5 4.8 7.3

0.4
4.6 1.7 4.9

0.6
4.1 0.6 4.1

0.8
3.8 0.2 3.8

1.0
3.6 0.1 3.6

2.0
3.3 0.0 3.3

3.0
3.2 0.0 3.2

4.0
3.2 0.0 3.2

5.0
3.2 0.0 3.2
 
  The results are plotted in figure 1.  
   
  Fig. 1 750kVA generator damage curves  
  References  
  •   Other Application Guides offered by SKM Systems Analysis at www.skm.com
•   Electrical Transmission and Distribution Reference Book, ABB Power T&D Company, Raleigh, North Carolina, 1997
•   Protective Relaying Theory and Applications, 2nd Edition, Marcel Dekker, New York, 2004 		 
  The latest revision of:  
  •   IEEE Std 242, IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book)
•   IEEE Std C37.102, IEEE Guide for AC Generator Protection
•   IEEE Std C37.101, IEEE Guide for Generator Ground Protection
•   ANSI C50.13, Cylindrical-Rotor Synchronous Generators
•   NEMA MG-1, Motors and Generators 		 
  Insulating Materials  
     
 
Insulation Class Maximum Temperature Insulating Materials
Y 90°C Cotton, silk, paper, wood, cellulose, fibre without impregnation or oil-immersion
A 105°C Class Y impregnated with natural resins, cellulose esters, insulating oils, etc., also laminated wood, varnished paper
Hybrid A 110°C Insuldur® Insulation Kraft paper with epoxy binders activated under pressure
E 120°C Synthetic-resin enamels, cotton and paper Laminates with formaldehyde bonding
B 130°C Mica, glass fibre, asbestos, etc., with suitable bonding substance; built-up mica, glass-fibre and asbestos laminates
F 155°C The materials of Class B with more thermally-resistant bonding materials
H 180°C Glass-fibre and asbestos materials, and built-up mica, with appropriate Silicone resins
C >180°C Mica, ceramics, glass, quartz, and asbestos without binders or with silicone resins of superior thermal stability
Hybrid H 220°C NOMEX® insulation, varnish dipped and vacuum pressure impregnated (VPI)
 
     
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