Part 1.3: Coordinate overcurrent protection on a distribution feeder

Task 2:  Find the minimum and maximum fault currents & feeder currents

Find maximum and minimum fault currents at each bus to select circuit breakers and current transformers (CT), and to coordinate protection. 

To do this task we need to look at two different system configurations: one configuration that leads to the maximum fault current and one that leads to the minimum fault current.  The minimum fault current configuration is the one where the line from bus 5 to 9 is out.  Etc.

(The fault current given by PW is also the phase current for all faults except the DLG.  To find the phase current for the DLG, you need to look for the “line” current.)

Task 3:   Selecting CB, CT’s, tap and time dial setting for the radial line.

a) Select circuit breakers for B11, B12, and B13.

b) Select CT ratios for B11, B12, and B13..

c) Select Tap and Time Dial (TD) settings for relays associated with B11, B12, and B13.       

(etc.)

Criteria for Coordination of Time-Overcurrent Relays in a Radial System: 

(Etc)

The calculations needed for the radial system are shown in Appendix A and a summary of the settings are shown in Table 7.