Adding directional elements to relay protection

Directional elements in relay protection are set by defining forward/reverse operating sectors, pickup currents, and phase angles to ensure secure and selective fault detection.

Purpose of Directional Elements

Directional elements are used to determine the direction of fault currents in a network, which is critical for selectivity and security in overcurrent and distance protection schemes. They are essential in looped or networked systems, where non-directional relays could misoperate due to bidirectional fault currents. Directional elements are commonly paired with overcurrent or distance relays in schemes like Permissive Overreaching Transfer Trip (POTT) and Directional Comparison Blocking (DCB) to improve speed and reliability of fault clearance.

Key Settings for Directional Elements

Operating Sector (Forward/Reverse)Define the forward and reverse sectors in degrees relative to the polarizing voltage or current. This ensures the relay operates only for faults in the intended direction and blocks for reverse faults. Typical Relay Characteristic Angle (RCA) settings: Solidly grounded networks: +60° to +75° (lagging/inductive)Isolated networks: -90° (leading/capacitive)Compensated networks: 0° (unity/resistive), The sector can be extended or retracted using minimum/maximum forward and reverse angle settings to accommodate capacitive or resistive fault currents.

Pickup Current

Set the pickup value based on system studies and fault current calculations. For overcurrent directional relays, this is typically a percentage above the maximum load current to avoid nuisance tripping. Example: 120% of line primary current for phase overcurrent, 125 A for earth fault protection with a time delay of 0.

8 seconds.

Time Grading

Time delays are coordinated with upstream and downstream relays to maintain selectivity. Ensure a minimum grading margin after accounting for relay and breaker tolerances. Zone 2 and Zone 3 distance elements are set considering adjacent line fault clearance times and breaker operating times.

Coordination

In ring or networked configurations, only the closest relay to the fault should assert forward. Reverse elements supervise or block to prevent unnecessary remote trips. Directional elements also supervise distance elements to prevent forward elements from tripping for reverse faults due to residual currents or phase effects.

Practical Testing and Validation

Use secondary injection testing with a multifunctional test set to verify forward and reverse operation, pickup, drop-off, and time grading. Check CT/PT polarities and reference voltages before testing to ensure accurate angle measurements. Document settings including CT/VT ratios, operating sectors, and coordination tables for repeatable and auditable testing.

Summary

Setting directional elements involves: Defining forward/reverse operating sectors with appropriate angles. Selecting pickup currents based on system studies. Coordinating time delays with other relays.

Validating through secondary injection testing. Adjusting for network type (grounded, isolated, compensated) and fault characteristics to ensure secure and selective protection.