The reference current in relay protection is the pickup current at which a relay begins to operate, typically derived from the relay setting and the current transformer (CT) ratio.

Definition and Calculation

The reference or pickup current is the minimum current at which a protective relay will actuate to isolate a fault. It is calculated using the relay pickup setting and the CT ratio:

• Relay secondary current: $I_{\text{secondary}}=\frac{I_{\text{primary}}}{\text{CT ratio}}$
• Pickup in primary amperes: $I_{\text{pickup\_primary}}=\text{relay pickup setting}\times \text{CT ratio}$ For example, if a CT has a ratio of 200/1 A and the relay pickup is set to 150%, the relay will operate when the primary current exceeds 300 A (1.5 × 200 A) .

Plug Setting Multiplier (PSM)

The Plug Setting Multiplier is the ratio of the fault current at the relay location to the relay pickup current:

This determines how quickly the relay will operate under fault conditions. A higher PSM results in faster relay operation .

Time Multiplier Setting (TMS)

The Time Multiplier Setting adjusts the relay operating time without changing the pickup current. It shifts the relay's time-current curve up or down, allowing coordination with upstream and downstream devices. The coordination time interval (CTI) is typically targeted between 0.2 s and 0.4 s to ensure selective tripping .

Importance in Protection Coordination

• Selective coordination ensures that the relay closest to the fault trips first, while upstream relays act as backup.
• Proper reference current settings prevent unnecessary outages and protect equipment by isolating only the faulty section .
• The relay must be sensitive, reliable, and fast enough to respond to actual fault conditions while ignoring normal load variations .

Summary

The relay reference current is a critical parameter in protection schemes, derived from the relay pickup setting and CT ratio. It works in conjunction with PSM and TMS to ensure fast, selective, and reliable fault isolation, maintaining system stability and minimizing service interruptions. Proper calculation and coordination of these settings are essential for effective power system protection .