Relay Protection Curve

A relay protection curve graphically represents the operating time of a protective relay versus the magnitude of current, allowing coordination and selective tripping in electrical networks.

Overview of Relay Protection Curves

Relay protection curves, also known as time-current characteristic (TCC) curves, define how quickly a relay will operate in response to overcurrent or fault conditions. The curve plots operating time on the vertical axis against fault current magnitude on the horizontal axis. These curves are essential for ensuring selective protection, where only the relay closest to the fault operates first, while upstream relays act as backup.

Types of CurvesIEC standards define several common curves for overcurrent and earth fault relays ( ): Normal Inverse (NI): Slower at low fault currents, faster at high currents; commonly used in distribution feeders. Very Inverse (VI): More responsive to higher fault currents; suitable for overhead lines with widely varying fault currents. Extremely Inverse (EI): Very fast operation for high fault currents; often used for transformer feeders.

Long Time Inverse (LTI): Allows longer delays, useful for backup protection or ignoring transformer inrush currents.

Key Settings Affecting the Curve

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

Higher PS

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

Lower TS

M produces faster tripping, while higher TSM provides coordination with upstream relays. Overload (OL) and Earth Leakage (EL) Settings: Define thermal and earth fault thresholds for relay operation.

Coordination and Grading

Relay curves are coordinated to maintain a grading margin between downstream and upstream relays. This ensures that the relay nearest the fault trips first, while upstream relays provide backup if the primary relay fails. Inverse time relays are particularly useful in radial networks, as their operating time decreases with increasing fault current, allowing faster clearance for severe faults while maintaining selectivity for lower currents.

Practical Application

Radial networks: Time-graded or inverse-time relays are preferred for selective protection. Transformer feeders: Extremely inverse curves allow rapid fault clearance to protect sensitive equipment. Backup protection: Long time inverse curves or upstream relays with higher TSM ensure reliability if downstream relays fail.

Relay protection curves are fundamental for safe, reliable, and selective operation of electrical systems, balancing speed of fault clearance with coordination across the network.