23
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 -2423.1 Overcurrent Protection
23.1.1 Introduction
The distribution system shown inFig. 23.1illustrates 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,