Calculation of Current Relay Protection Settings

Relay protection current calculation involves determining the pickup current, plug setting multiplier (PSM), and time multiplier setting (TMS) to ensure selective and reliable fault isolation.

Key Concepts

Pickup Current (Ip): The minimum current at which a relay begins to operate, overcoming the controlling force of the relay. It is influenced by the relay coil turns and the current flowing through the coil, and can be adjusted as a percentage of the CT's rated secondary current . Plug Setting Multiplier (PSM): The ratio of the fault current to the relay's pickup current. It determines how quickly the relay responds to overcurrent conditions. PSM is critical for coordinating relays in a network . Time Multiplier Setting (TMS): Adjusts the relay's operating time according to the selected inverse-time characteristic. Higher TMS values increase the delay, allowing downstream relays to operate first, ensuring selectivity .

Calculation Steps

1. Determine Load and Fault Currents: Identify the normal load current and the maximum fault current expected at the relay location.
2. Select Pickup Current: Typically set slightly above the maximum load current to avoid nuisance tripping. For example, 125% of load current is common .
3. Calculate PSM: $$\text{PSM}=\frac{\text{Fault Current}}{\text{Pickup Current}}$$ This ratio helps determine the relay's operating time from the time-current characteristic curve .
4. Set Time Multiplier (TMS): Using the relay's inverse-time curve (Standard, Very Inverse, Extremely Inverse), select TMS to achieve the desired operating time while maintaining coordination with upstream and downstream relays .
5. Compute Operating Time: $$t=\text{TMS}\times f(\text{PSM})$$ where $f(\text{PSM})$ is the relay-specific time-current function from IEC 60255 or manufacturer curves .

Example

For a load current of 150 A and a CT primary rating of 200 A:

• Pickup current = 125% of load = 187.5 A
• Standard plug setting = 200% → Pickup current = 250 A
• Fault current = 500 A → PSM = 500 / 250 = 2
• Using the relay curve, select TMS to achieve the desired operating time while maintaining coordination .

Practical Considerations

• Selectivity: Ensure the relay closest to the fault operates first. Time grading is used to maintain coordination between relays .
• Inverse-Time Relays: Operating time decreases with higher fault currents, improving response for severe faults .
• Definite-Time Relays: Operating time is constant, independent of fault magnitude, suitable for simple radial networks .
• Coordination Margin: Maintain a margin between consecutive relay stages to prevent simultaneous tripping and ensure system stability . Using online calculators ( ) can simplify these calculations, providing pickup current, PSM, TMS, and operating time based on input parameters like load current, CT ratio, and fault current.