Principle of Old-style Relay Protection Devices

Understanding old-style relay settings involves analyzing parameters like PSM, TSM, EL, OL, and MF to determine how the relay detects faults, coordinates with other relays, and trips circuit breakers. Key Relay Settings1.

PS

M – Plug Setting Multiplier (Current Setting Multiplier) PSM indicates how many times the actual current exceeds the relay's pickup current. It is crucial for inverse definite minimum time (IDMT) relays, as it determines the basic operating time based on the fault current. A higher PSM results in faster tripping, while a lower PSM delays operation.

Correct PS

M selection ensures the relay responds appropriately to overcurrent conditions without unnecessary trips, following IEC 60255-151 standards. 2.

TS

M – Time Setting Multiplier TSM scales the base operating time derived from the relay's characteristic curve. It allows coordination between upstream and downstream relays, ensuring the downstream relay trips first while the upstream relay provides backup.

Lower TS

M values produce faster tripping, and higher values delay operation to maintain selectivity, in accordance with IEC 60255-3. 3.

O

L – Overload Setting OL defines the thermal overload threshold for equipment protection. It ensures that the relay trips if the current exceeds safe operating limits for a prolonged period, preventing damage to motors, transformers, or conductors. 4.

E

L – Earth Leakage / Earth Fault Setting EL sets the pickup current for detecting ground faults.

Proper E

L adjustment ensures that the relay responds to earth faults without false tripping due to normal leakage currents. 5.

M

F – Multiplying Factor MF is used for scaling or metering purposes, adjusting the relay's response according to CT ratios or system-specific factors. It ensures that the relay interprets secondary currents correctly relative to primary system currents.

Interpreting Relay Operation

Definite Time vs. Inverse Time: Definite time relays trip after a fixed delay regardless of fault magnitude, while inverse time relays trip faster for higher fault currents. Protected Zone: Each relay protects a defined section of the network.

Settings must ensure the relay operates only for faults within its zone, maintaining stability and selectivity. Coordination: By adjusting PSM and TSM, engineers can create a time grading scheme, where downstream relays trip first and upstream relays act as backup. Testing and Verification: Old-style relays require manual testing using secondary injection or simulation of fault currents to verify that the settings produce the expected trip times and coordination.

Practical Tips

Always refer to the relay's nameplate and characteristic curves to interpret settings accurately.

Cross-check C

T ratios and system voltages to ensure PSM and MF are correctly applied. Document all settings and coordination studies to maintain system reliability and facilitate troubleshooting. Consider the relay's age and mechanical condition, as electromechanical relays may drift over time, affecting pickup and timing accuracy.

By understanding these parameters and their interactions, engineers can effectively interpret old-style relay protection devices, ensuring reliable fault detection, proper coordination, and safe operation of electrical systems.