Calculation of Relay Protection Setting Voltage

Relay protection voltage settings are determined by calculating the pickup voltage or current based on CT ratios, plug setting multipliers (PSM), and the relay's operating characteristics to ensure selective and reliable fault clearance.

Understanding Relay Settings

Relay protection settings define the threshold at which a relay operates to isolate faults. The key parameters include:

• Pickup Current/Voltage: The minimum current or voltage at which the relay begins to operate. For voltage relays, this is the voltage at which the relay trips.
• Plug Setting Multiplier (PSM): Ratio of fault current (or voltage) to the relay's pickup value, indicating how severe a fault must be to trigger the relay .
• Time Multiplier Setting (TMS): Adjusts the relay's operating time; higher TMS increases the delay, while lower TMS speeds up operation .

Step-by-Step Calculation

1. Determine the Rated CT or VT Values: Identify the current transformer (CT) or voltage transformer (VT) ratio. For example, a CT with a 600/5 A ratio converts 600 A primary current to 5 A secondary current.
2. Select the Plug Setting (PS): Decide the percentage of the CT or VT rated secondary current/voltage that will serve as the relay pickup. For instance, a 250% plug setting means the relay will pick up at 2.5 times the rated secondary current.
3. Calculate Pickup Current/Voltage: The pickup value is calculated as: I_pickup = Plug Setting × Rated CT Secondary Current For voltage relays: V_pickup = Plug Setting × Rated VT Secondary Voltage .
4. Adjust for CT/VT Changes: If the CT or VT is replaced, the new PSM can be calculated to maintain the same primary pickup: New PSM = (Old CT Primary × Old PSM) / New CT Primary Then, Relay Pickup (Primary) = New PSM × New CT Primary .
5. Time Coordination: Use the TMS to adjust the relay operating time according to the network coordination plan. For inverse-time relays, the operating time decreases as fault current increases, ensuring faster clearance for severe faults .

Example

• Old CT: 600/5 A, PSM = 1.05 Pickup current (primary) = 1.05 × 600 = 630 A
• New CT: 800/5 A New PSM = (600 × 1.05) / 800 = 0.7875 Pickup current (primary) = 0.7875 × 800 = 630 A This ensures the relay trips at the same primary current even after CT replacement .

Key Considerations

• Selectivity: Ensure relays closer to the fault operate first using time-graded or current-graded coordination .
• Safety Margins: Include margins to account for measurement inaccuracies and inrush currents.
• Relay Type: Settings differ for overcurrent, differential, IDMT, and voltage relays; each requires specific calculations . By following these steps, the relay protection voltage or current settings can be accurately calculated to ensure reliable and selective fault protection in electrical networks.