Relay Setting & Coordination Module

7 Hours of Training

A common misconception in power system protection is that installing advanced protection relays will automatically eliminate all protection issues. In reality, these benefits can only be realized if the relays are set and coordinated correctly. This module provides a comprehensive understanding of relay setting principles, coordination techniques, and practical applications. You will explore: 

The Protection Chain – Role of relays in the overall protection scheme, and the function of key components such as instrument transformers, circuit breakers, DC supply systems, and associated wiring. 

NEMA Device Numbers – Introduction to standard device numbers for relays and protection equipment. 

Primary and Backup Protection – Need for Backup protection, Fundamentals of zones of protection, overlap and dead zones, and the coordination between primary and backup schemes.

 The 3S Principles: Sensitivity, Selectivity, and Speed o Sensitivity – 

• Relationship to thermal capacity, setting criteria based on system grounding, and timer-hold features for arcing fault clearance. 
• Selectivity (Discrimination) – Discrimination by Time, Discrimination by Current and Discrimination by both Time and Current; characteristics of Definite Minimum Time and Inverse Definite Minimum Time relays. 
• Speed – High-speed tripping concepts, trade-offs with selectivity, the role of Critical Clearing Time (CCT) in system stability, and methods to achieve high speed fault clearance, application of unit protection. 
• Coordination intervals and discrimination times for fuse-to-fuse, fuse-to- breaker, and breaker-to-breaker arrangements.

 Relay Settings Parameters – 

Introduction to PS (Plug Setting), PSM (Plug Setting Multiplier) and TMS (Time Multiplier Setting), Numerical example to illustrate calculation of same, Criteria for selecting PS considering induction motor starting.

 Overload, IEC curves and pre-fault load flow – Distinction between overcurrent and overload protection; IEC curves and corresponding equations for Normal Inverse, Very Inverse and Extremely Inverse, Selection criteria for choosing these curves; Effect of pre-fault load current on relay coordination studies. 

Techniques for Reducing Fault Clearance Time 

• Concept of Break Point to reduce operating time of upstream relays using Instantaneous relays, Application of phase instantaneous relay (50), Numerical example to illustrate setting of 50 and back up over current relay (51) 
• Application of ground instantaneous relay (50N) for Star – Delta transformer 
• Pilot wire protection – Concept and application to tie feeder, numerical example to illustrate comparison of upstream relay operating time with and without pilot wire protection 
• Reverse Blocking Scheme – Use of Instantaneous relay on incomer to a bus without sacrificing coordination with relays on outgoing feeder. 
• Pseudo REF scheme – Advantage over Conventional Restricted Earth Fault scheme 
• Directional over current protection (67) – Operating current and polarizing voltage input required for fixing direction, 90 degrees connection, Maximum Torque Angle and demarcation of operating and non-operating regions, Example to illustrate application of directional relays for parallel feeder protection to achieve selective isolation of faulted feeder .

 Further Reading –

 References for further reading t practices. 

Duration: 7 Hours