Configuring & Testing Intelligent Electronic Devices (IED) Protection Relays

1. The Role of IEDs in Digital Protection

Microprocessor-based protection relays — Intelligent Electronic Devices (IEDs) — have fully displaced electromechanical and static relays in modern power systems. A single device combines multiple protection functions such as overcurrent (50/51), earth fault (50N/51N), differential (87), and distance (21), while also providing metering, event recording, fault waveform capture (disturbance recording), and SCADA communication. Protection functions are conventionally referenced by device numbers per IEEE C37.2.

This capability brings greater responsibility for the settings engineer: a single IED carries hundreds of parameters — protection settings, CT/VT ratios, programmable logic, binary input/output mapping, and communication configuration. One error, such as a wrong CT ratio or an incorrect curve selection, can cause the protection to fail to operate during a real fault or to overreach and trip healthy sections. A settings sheet verified against a coordination study is therefore a fundamental requirement of this work.

2. Reference Standards for Relay Setting and Testing

Overcurrent settings reference the inverse-time curves of IEC 60255, which defines the equations for Standard Inverse, Very Inverse, and Extremely Inverse characteristics along with timing and pickup accuracy requirements for measuring relays; North American practice uses the curves of IEEE C37.112. Curve selection must be consistent across the system so that coordination between upstream and downstream devices holds, with grading margins typically in the range of 0.2 to 0.4 seconds depending on device types and relay accuracy.

For communication, modern IEDs support IEC 61850, allowing trip and interlock signals over GOOSE in place of copper wiring and metering data over MMS directly to SCADA. Fault recordings conventionally use the COMTRADE file format per IEEE C37.111 (now jointly published as IEC 60255-24), enabling analysis in third-party software. Engineers should configure disturbance recorder trigger conditions, pre-fault length, and sampling rate with post-event analysis in mind.

• IEC 60255: inverse-time curves and relay accuracy requirements
• IEEE C37.2: protection function device numbers (50, 51, 87, 21, etc.)
• IEEE C37.112: North American inverse-time characteristics
• IEEE C37.111 / IEC 60255-24: COMTRADE recording format
• IEC 61850: GOOSE and MMS communication for IEDs

3. Setting and Testing Procedure with a Relay Test Set

Work begins with a coordination study based on short-circuit calculations and actual load data, from which pickup values, time multipliers, and curve types for each stage are determined. A settings sheet documenting every parameter, with a named reviewer, is then prepared before values are entered into the IED via the manufacturer's software. Entry should be performed by one person and independently checked by another, with the as-entered values printed from the device and compared line by line against the settings sheet before approval.

Testing uses a secondary-injection relay test set to inject simulated currents and voltages at the IED terminals, verifying pickup and dropoff of each function and timing the operation at multiple points on the curve (for example 2, 5, and 10 times pickup) against calculated values within accepted tolerances. Trip logic is proven through to the actual circuit breaker trip coil, and auxiliary functions such as breaker failure (50BF), auto-reclose (79), and SCADA signalling are exercised. All results must be recorded as a baseline report for comparison at the next periodic test.

• Always complete the coordination study before finalizing settings
• One person enters settings; another performs an independent check
• Verify pickup and timing at multiple points on the curve
• Prove tripping through to the breaker trip coil
• Retain test reports as the baseline for periodic testing

4. Frequent Errors and Long-Term Maintenance

Common IED setting errors include CT/VT ratios that do not match the actual nameplates on site, reversed polarity in current circuits — which directly affects directional and differential functions — enabling functions not in the design, and leaving test mode active after work is complete so the relay never issues a real trip. Another recurring case is firmware updates performed without assessing their impact on existing settings; some models may reset values or alter the behaviour of certain functions.

Long-term maintenance should schedule relay testing according to circuit criticality, always comparing results against the original baseline. Setting files and firmware should be version-controlled in a central repository with access control, and settings must be reviewed whenever the power system changes significantly — a new transformer, resized feeders, or newly connected generation. Cybersecurity of protection devices also matters increasingly: change default passwords, restrict access to engineering ports, and log every settings change so modifications remain auditable.