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Calculation of Relay Operating Time

Need and Importance of Protective Systems Causes and Effects of Faults & Faults in a System In a Year Types of Faults Fault Statistics Basic Trip Circuit Zones of protection (Protective Zones) Primary and Back-up protection Types of Back-Up Relay Principles Under-laying Back-up Protection Essential Quantities of Protection (Basic fundamental requirements) Classification protective relays Classification of Protective Schemes Basic Relay Terminology Role of Engineers

Introduction of Protection of Medium Voltage Transmission Line Over-Current Relay Time – Current Characteristics Theory of Torque Formation Important terms of connection with relays. Calculation of Relay Operating Time Overcurrent Protective Schemes Induction Relay Induction Type Directional Power Relay or Reverse Power Relay Induction Type Directional Over-current Relay Induction Type Non-Directional Over-current Relay Applications of Over-current Protection Relays Comparison of Directional O/C Relay & Power Relay (self) Drawbacks of Overcurrent Relays Relays Used in Over-current Protection Numerical On Over-current Relay

Introduction of High Voltage Line Protection

Calculation of Relay Operating Time

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In order to calculate the actual relay operating time, the following things must be known:

Time / PSM curve
Current setting
Time setting
Fault current
Current transformation ratio

The procedure for calculating the actual relay operating time is as follows:

Convert the fault current into the relay coil current by using the current transformation ratio.
Express the relay current as a multiple of current setting i.e. calculate the PSM.
From the Time / PSM curve of the relay, read off the time of operation for the calculated PSM.
Determine the actual time of operation by multiplying the above time of the relay by the time setting multiplier in use.

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