The CT ratio for relay protection is calculated as the ratio of the primary current to the secondary current, considering maximum load and fault currents to ensure accurate relay operation.

Understanding CT Ratio

The CT ratio is expressed as: CT Ratio = Primary Current / Secondary Current For example, a CT rated 200:5 means that 200 A in the primary circuit produces 5 A in the secondary circuit. This ratio ensures that the protective relay receives a manageable current proportional to the actual system current .

Steps to Calculate CT Ratio for Protection Relays

1. Determine Maximum Load Current Identify the maximum expected load current in the primary circuit. For protection purposes, it is common to include a safety factor (typically 1.2–1.25) to account for temporary overloads: Required Primary Current = Maximum Load × Safety Factor
2. Select Secondary Current Standard secondary currents are usually 1 A or 5 A, depending on the relay and system design.
3. Calculate CT Ratio Divide the adjusted primary current by the chosen secondary current: CT Ratio = (Maximum Load × Safety Factor) / Secondary Current Example: If the maximum load is 180 A and the secondary current is 5 A: 180 × 1.25 = 225 A → select the next standard CT ratio, 250:5, to maintain accuracy .
4. Consider Short-Circuit Current Ensure the CT can handle the maximum fault current without saturating. The CT ratio should allow the secondary current to remain within the CT's accuracy limit factor (ALF) during faults .
5. Check Accuracy Class Protective relays require CTs with appropriate accuracy class (e.g., 5P10, 10P20) to ensure correct operation under both normal and fault conditions. The selected CT ratio should keep the operating current within the CT's rated accuracy range.
6. Optional Multi-Ratio CTs If the system has variable loads or future expansion, consider multi-ratio CTs for flexibility in field adjustments .

Practical Example

• Maximum load current: 180 A
• Safety factor: 1.25 → 180 × 1.25 = 225 A
• Secondary current: 5 A
• Calculated CT ratio: 225 / 5 = 45 → choose 250:5 as the standard CT ratio
• Verify that the CT can handle the expected short-circuit current without saturation.

Key Considerations

• Avoid undersized CTs: They may saturate during faults, causing relay misoperation.
• Avoid oversized CTs: They may reduce measurement resolution, affecting relay sensitivity.
• Include burden calculation: Total burden (cables, relay input) affects CT performance.
• Follow standards: IEC 60044-1 and IEC 61869-2 provide guidelines for CT selection and accuracy . By following these steps, you can select a CT ratio that ensures accurate current measurement for protective relays while maintaining reliability during both normal operation and fault conditions.