23
2006 by Taylor & Francis Group, LLC.
Hard to Find
Information (on
Distribution System
Characteristics and
Protection)
Jim Burke
InfraSource Technology
23.1 Overcurrent Protection.................................................. 23-1
Introduction . Fault Levels . Surface Current Levels .
Reclosing and Inrush . Cold Load Pickup . Calculation
of Fault Current . Current Limiting Fuses . Rules for
Application of Fuses . More Overcurrent Rules .
Capacitor Fusing . Conductor Burndown . Protective
Device Numbers . Protection Abbreviations . Simple
Coordination Rules . Lightning Characteristics .
Arc Impedance
23.2 Transformers ................................................................. 23-16
Saturation Curve . Insulation Levels .
D-Y Transformer Banks
23.3
Instrument Transformers............................................. 23-17
Two Types . Accuracy . Potential Transformers .
Current Transformer . H-Class . Current Transformer
Facts . Glossary of Transducer Terms
23.4 Loading.......................................................................... 23-21
Transformer Loading Basics . Examples of Substation
Transformer Loading Limits . Distribution Transformers .
Ampacity of Overhead Conductors . Emergency Ratings of
Equipment
23.5 Miscellaneous Loading Information........................... 23-24
23.1 Overcurrent Protection
23.1.1
Introduction
The distribution system shown in Fig. 23.1 illustrates many of the features of a distribution system
making it unique. The voltage level of a distribution system can be anywhere from about 5 kV to as high
as 35 kV with the most common voltages in the 15 kV class. Areas served by a given voltage are
proportional to the voltage itself, indicating that, for the same load density, a 35 kV system can serve
considerably longer lines. Lines can be as short as a mile or two and as long as 20 or 30 miles. Typically,
S
R
138 kV Distribution
Substation Transformer
13.8 kV
ISC = 10,000 A
Feeder Breaker
Peak Load = 600 A
Three-Phase, 4-Wire,
Multi-grounded Fuse Cutout
Normally