Method for Ratio of Current Transformer in Relay Protection
The CT ratio for relay protection is selected based on the maximum expected primary current, fault current, and relay requirements, typically using a standard secondary current of 5 A or 1 A with a high Accuracy Limit Factor (ALF).
Understanding CT Ratio for Protection
A current transformer (CT) ratio is the ratio of primary current to secondary current. For example, a 400:5 CT converts 400 A on the primary side to 5 A on the secondary side, allowing relays to safely measure high currents without exceeding their input limits . Protection CTs are designed to handle high fault currents and must saturate at higher currents to ensure the relay operates correctly during short circuits .
Selection Guidelines
- Determine Primary Current: Base the CT ratio on the maximum continuous load or transformer rating, adding a safety margin (commonly 125%) for future expansion . For example, if the maximum load is 180 A, multiply by 1.25 to select a 225 A CT, then choose the next standard ratio, e.g., 250:5 .
- Choose Secondary Rating: Standard secondary currents are 5 A or 1 A. Use 1 A for long secondary runs to reduce copper losses, and 5 A for typical relay inputs .
- Accuracy and Class: Protection CTs are classified as 5P or 10P, where the number indicates the percentage composite error at a multiple of rated current (e.g., 5P20 = 5% error at 20× rated current). These CTs prioritize fault current measurement over precision at low currents .
- Accuracy Limit Factor (ALF): Ensure the CT has a sufficiently high ALF to allow the relay to measure fault currents accurately without saturation. The relay itself must also withstand high overcurrents .
- Multi-Ratio CTs: When in doubt, multi-ratio CTs provide flexibility for field adjustments, allowing the same CT to accommodate different primary currents without compromising protection .
Practical Considerations
- Burden: Calculate the total burden including cables and relay input. Exceeding the rated burden reduces accuracy .
- Saturation: Protection CTs are designed to saturate later than metering CTs, ensuring relays operate correctly during high fault currents .
- Coordination: Match the CT ratio with relay settings to ensure proper operation during overcurrent or short-circuit events .
Summary
For relay protection, select a CT ratio slightly above the maximum expected load, use a standard secondary current (5 A or 1 A), ensure the CT has a high ALF, and choose an appropriate protection class (5P or 10P). Multi-ratio CTs and proper burden calculation improve accuracy and reliability, ensuring protective relays function correctly under both normal and fault conditions .
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