Practical Power System Protection
Providing both the underpinning knowledge and basic calculations needed to understand, specify, use and maintain power protection systems, this book also covers the practical techniques required on a daily basis.
TABLE OF CONTENTS Practical Power System Protection
Chapter 10: Coordination By Time Grading
10.1 Protection Design Parameters on Medium- and Low-Voltage Networks
Although not appreciated by many engineers, the widespread use of inverse definite minimum time overcurrent and earth fault (IDMT OCEF) relays as the virtual sole protection on medium- and low-voltage networks requires as much detailed study and applications knowledge as does the more sophisticated protection systems used on higher-voltage networks.
10.1.1 Introduction
Traditionally, design engineers have regarded medium- and low-voltage networks to be of lower importance from a protection view, requiring only the so called simpler type of IDMT overcurrent and earth fault relays on every circuit. In many instances, current transformer ratios were chosen mainly based on load requirements, whilst relay settings were invariably left to the commissioning engineer to determine. Most of the times, the relay settings had been chosen considering the downstream load being protected without an effort to coordinate with the upstream relays. However, experience has shown that there has been a total lack of appreciation of the fundamentals applicable to these devices. Numerous incidents have been reported where breakers have tripped in an uncoordinated manner leading to extensive network disruption causing longer down times or failed to trip causing excessive damage, extended restoration time and in some cases loss of life.
This chapter reviews some of the fundamental points for the design engineer to watch for in planning the application of IDMTL OCEF protection to medium-voltage switchboards and networks.
10.1.2 Why IDMT?
Though it may be possible to grade the relay settings based on the fault...
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