Page 287 - Practical Power System and Protective Relays Commissioning

Chapter 19

Protection Relays Settings

INTRODUCTION
This chapter introduces relay settings for different types of relays used in
power system, with real worked examples.

Subchapter 19.1

Overcurrent Protection Settings

19.1.1 INTRODUCTION
Overcurrent protection is used in extra-high voltages and high-voltages
networks as the main protection, and in medium networks it is also used as
backup protection for power transformers.
Time grading for overcurrent relays is used in the radial system.
The minimum possible time delay is selected for the relay which protects
the line farthest from the source. The grading time between the successive
relays will be in the order of 0.4 0.5 seconds, and with modern relays it can
be of the order of 0.35 seconds.
Current settings for an overcurrent relay should consider the following
factors:
1. Maximum possible loading;
2. Transient currents due to switching of transformers, inrush current, heavy
motor restarting currents after clearing external faults;
3. Drop off/pickup ratio, which is called the reset ratio.

I set 5 K 3 Rð Þ=F 3 I L Max
where:

K 5 safety factor 5 1.2 R 5 restarting factor, which depend on the load
profile
Set to 2 for industrial loads

Practical Power System and Protective Relays Commissioning.
DOI: https://doi.org/10.1016/B978-0-12-816858-5.00019-8 281
© 2019 Elsevier Inc. All rights reserved.