IEEE DRAFT C57.96 : D4.0 JAN 99

1. Overview
      1.1 Scope
      1.2 General Information
      1.3 Transformer Life Expectancy
      1.4 Transformer Rated Output
      1.5 Aging of Insulation
      1.6 Ambient Temperature
      1.7 Influence of Ambient Temperature
      1.8 Influence of Altitude on Loading
2. References
3. Basic Loading Conditions for Normal Life Expectancy
      3.1 Basic Conditions
      3.2 Loading for Life Expectancy Under Specified Conditions
      3.3 Loading Determined by Measured Temperature
      3.4 Loading on the Basis of Short Time Loads
      3.5 Effects of Various Factors Existing at One Time
4. Loading Transformers with Moderate Sacrifice of Life Expectancy
      4.1 Factors Affecting Transformer Life
      4.2 Aging of Insulation Due to Operations Above Rated Hottest
      Spot Temperature
      4.3 Constant Loads and Life Expectancy by Measured Temperature
      4.4 Constant Loads and Life Expectancy Estimated by Calculated
      Temperatures
      4.5 Short Time Loading and Life Expectancy Determined by
      Temperatures
      4.6 Short Time Loading and Life Expectancy Estimated by Calculated
      Temperatures
5. Supplemental Cooling of Existing Self Cooled Transformers
6. Basis for Calculations of Temperatures, Loss of Life, and Loads
      6.1 Method for Converting Actual Load Cycle to Equivalent
      Constant Load
      6.2 Equations for Calculation of Temperature, Load and Loss of
      Life
      6.3 Transient Computer Program
7. Protective Devices - Thermal Relays
Tables
Table 1 Self Cooled Continuous Loading on Basis of Average Ambient
          Temperature
Table 2 Unusual Temperature and Altitude Conditions
Table 3 Rated kVA Derating Factors for Altitudes Greater than 1000m
Table 4 Daily Loads above Rating to Give Normal Life Expectancy in
          20 C Average Ambient for Transformers with a 30 Minute Time
          Constant
Table 5 Daily Loads above Rating to Give Normal Life Expectancy in
          30 C Average Ambient for Transformers with a 30 Minute Time
          Constant
Table 6 Daily Loads above Rating to Give Normal Life Expectancy in 40
          C Average Ambient for Transformers with a 30 Minute Time
          Constant
Figures
Fig 1 Life Expectancy Curve - Base 10
Fig 2 Life Expectancy Curve - Base e
Fig 3 Relative Life Expectancy as a Function of Hottest-Spot
        Temperature
Fig 4 Hottest-Spot Temperature as a Function of Time
Fig 5 Assumed Load Cycle
Fig 6 Actual Load Cycle
Annex A (normative)
A.1 Overview
A.2 References
A.3 Definitions
A.4 Insulating Aging
A.5 Temperature Limits for Loading
A.6 Ambient Temperatures
A.7 Influence of Altitude on Loading
A.8 Loading Equations

Guide covers general recommendations for the loading of dry-type distribution and power transformers that have 75 degrees centigrade, 115 degrees centigrade and 150 degrees centigrade average winding rises and insulation systems limited to 150 degrees centigrade, 180 degrees centigrade, and 220 degrees centigrade maximum hottest-spot operating temperatures respectively.