Calculation of bus voltage for relay protection

The voltage across a bus protection relay is calculated based on the total fault current flowing through the relay's series resistance, with settings adjusted above the maximum expected voltage to account for CT saturation and safety margins.

Basic Principle

In bus differential protection, the relay voltage $V_r$ is generated by the current transformers (CTs) connected to all terminals of the bus. Under normal conditions, the sum of currents entering the bus is zero, so no voltage develops across the relay. During a fault, the currents no longer sum to zero, producing a voltage across the relay proportional to the fault current and the relay's series resistance .

Calculation Steps

1. Determine Maximum Fault Current: Calculate the maximum possible fault current for each CT circuit, assuming total CT saturation. This ensures the relay setting accounts for worst-case conditions .
2. Compute Relay Voltage: For a high-impedance relay, the voltage across the relay is given by: $$V_r=I_f\times R_r$$ where $I_f$ is the fault current through the CT and $R_r$ is the series resistance of the relay circuit .
3. Include CT Characteristics: Assume the CT is toroidal with negligible leakage reactance. If non-toroidal CTs are used, the leakage reactance must be included in the calculation, as it affects the voltage developed across the relay .
4. Apply Safety Margin: Set the relay voltage above the calculated maximum by a typical margin (often 1.5 to 2 times the expected voltage) to prevent misoperation due to CT saturation or measurement errors .
5. Voltage Limiting: For very heavy faults, voltage limiting devices such as Thyrite resistors can be connected across the relay. The Thyrite exhibits an inverse resistance characteristic, limiting the voltage to safe levels while allowing the relay to operate correctly. An overcurrent relay in series with the Thyrite can provide additional protection against external faults .

Practical Considerations

• CT Placement and Ratio: Ensure CTs are correctly oriented and rated to handle the expected fault currents. Differential protection algorithms can compensate for CTs with different ratios .
• High vs Low Impedance Relays: High-impedance relays are sensitive to CT saturation and require careful voltage calculation, while low-impedance numerical relays can handle dynamic bus configurations with numerical compensation .
• Bus Configuration: The complexity of the bus (single, double, or breaker-and-a-half) affects the relay voltage calculation and protection scheme design . By following these steps, the bus protection relay can be accurately set to detect internal faults while avoiding false trips due to CT saturation or external disturbances.