Protection relays detect abnormal electrical conditions and trip circuit breakers to isolate faults before equipment is damaged. This cheat sheet summarizes the relay functions, settings, and coordination checks commonly used in power systems engineering. Students need these references to connect device numbers, current transformer data, pickup thresholds, and time delays into one protection plan.
It is useful for short-circuit studies, relay setting calculations, and substation design review.
Key Facts
- Instantaneous overcurrent relay 50 operates with no intentional time delay when current exceeds the set pickup value.
- Time overcurrent relay 51 operates after a delay determined by current magnitude and the selected time-current curve.
- Relay secondary current is found from I_secondary = I_primary / CT ratio, where CT ratio is primary rating divided by secondary rating.
- Pickup in primary amperes is calculated as I_pickup_primary = relay pickup setting x CT ratio.
- Plug setting multiplier is PSM = fault current at relay / relay pickup current, using currents on the same side of the CT.
- Time multiplier setting or time dial shifts the relay curve up or down without changing the pickup current.
- Coordination time interval is CTI = upstream operating time - downstream operating time, and a common target is 0.2 s to 0.4 s depending on breaker and relay types.
- Selective coordination means the nearest protective device to the fault trips first while upstream devices remain closed unless backup protection is needed.
Vocabulary
- Protective relay
- A device that measures electrical quantities and commands a breaker to trip when an abnormal condition is detected.
- Device number
- An ANSI code that identifies a relay or protection function, such as 50 for instantaneous overcurrent and 51 for time overcurrent.
- Current transformer
- A measurement transformer that reduces primary current to a smaller secondary current suitable for relays and meters.
- Pickup current
- The minimum current at which a relay begins to operate according to its setting.
- Time-current curve
- A graph showing how long an overcurrent device takes to operate for different multiples of pickup current.
- Coordination time interval
- The time margin between the downstream protective device operation and the upstream backup device operation for the same fault.
Common Mistakes to Avoid
- Mixing primary and secondary current values, because relay settings and fault currents must be converted to the same side of the CT before comparison.
- Ignoring CT ratio and saturation, because an incorrect or saturated CT can cause the relay to see a lower current than the actual fault current.
- Setting upstream relays too fast, because they may trip before downstream devices and remove more of the system than necessary.
- Using pickup current as the only coordination check, because time-current curves and breaker clearing times must also be compared.
- Forgetting load and inrush margins, because a pickup set too low can trip during motor starting, transformer energization, or normal overload conditions.
Practice Questions
- 1 A feeder CT is rated 600:5 and the relay pickup is set to 4 A secondary. What is the pickup current in primary amperes?
- 2 A downstream relay clears a fault in 0.18 s and the upstream relay clears the same fault in 0.45 s. What is the coordination time interval, and is it within a 0.2 s to 0.4 s target range?
- 3 A relay sees 25 A secondary fault current and has a pickup of 5 A secondary. Calculate the plug setting multiplier.
- 4 Explain why a protection engineer may choose a slower upstream time-current curve even when a faster trip would reduce fault duration.