Relay Protection Time Selection

Relay protection time is selected to ensure proper coordination, selectivity, and rapid fault clearance, typically using time-graded or inverse time characteristics.

Principles of Relay Timing

Relay protection time is primarily determined to achieve selectivity, meaning the relay closest to the fault operates first while upstream relays act as backup. This is achieved through time grading, where relays are set with progressively longer operating times as you move upstream in the network. Two main types of relay characteristics are used: Definite Time Relays: Operate after a fixed time regardless of fault current magnitude.

Inverse Time Relays: Operate faster for higher fault currents, making them suitable for radial networks with varying short-circuit currents. Time grading ensures that downstream faults are cleared quickly while minimizing unnecessary tripping of upstream devices.

Key Parameters for Timing Selection

Plug Setting Multiplier (PSM): Indicates how many times the actual current exceeds the relay pickup current.

Higher PS

M results in faster relay operation. Time Setting Multiplier (TSM): Scales the base operating time from the relay's characteristic curve.

Lower TS

M results in faster tripping, and it is used to create time grading between relays. Zone Delays: For distance or impedance relays, zone 1 is typically instantaneous, while zones 2 and 3 include intentional delays to coordinate with downstream relays. Coordination Margins: A safety margin is added to ensure upstream relays do not operate before downstream relays clear the fault.

Practical Steps for Setting Relay Time

Identify the network configuration and fault current levels at each relay location. Select relay type (definite or inverse time) based on network characteristics.

Determine PS

M based on expected fault currents and relay pickup settings.

Calculate TS

M to achieve proper time grading between downstream and upstream relays. Verify coordination using relay curves and ensure that the total fault clearance time includes relay operating time plus breaker opening time. Adjust for special conditions, such as transformer inrush, motor starting, or varying line impedances.

Standards and Guidelines

Relay timing selection should comply with IEC 60255 standards for overcurrent relays and IEEE guides for protective relay coordination. Inverse, very inverse, and extremely inverse characteristics are commonly used to optimize speed and selectivity.

Summary

Selecting relay protection time involves balancing speed, selectivity, and reliability. By using time-graded or inverse time relays, adjusting PSM and TSM, and coordinating zones with appropriate delays, engineers ensure that faults are cleared efficiently while minimizing disruption to the rest of the system.