Short Answer:

A protection CT is designed to work accurately during high fault currents, ensuring proper operation of protective relays, even under extreme conditions. It must remain unsaturated during faults to supply reliable current to the protection system.

A metering CT, on the other hand, is built for high accuracy under normal load conditions. It may saturate during fault conditions because it is not intended for protection but for precise energy measurement. The key difference lies in their design purpose, accuracy range, and behavior under fault conditions.

Detailed Explanation:

Difference between protection CT and metering CT

Both protection CTs and metering CTs are types of current transformers used in electrical systems to step down high currents to measurable levels. However, they are designed for different applications, and their internal characteristics are tailored to suit those purposes. Understanding these differences is crucial for ensuring that the right CT is used for either protection or metering, as using the wrong type may lead to inaccurate readings or failure in protection.

1. Purpose and Function:

• Protection CT:
  • Used in conjunction with protective relays.
  • Detects abnormal conditions like short circuits or overloads.
  • Must operate accurately even during very high fault currents.
• Metering CT:
  • Used for energy billing and monitoring.
  • Designed to provide high accuracy under normal load conditions.
  • Not required to perform during extreme fault conditions.

2. Core Saturation Behavior:

• Protection CT:
  • Built to avoid core saturation even under high fault current.
  • Maintains a linear response up to many times the rated current.
• Metering CT:
  • May saturate at 5 to 10 times the rated current.
  • Once saturated, it cannot provide accurate readings, which is acceptable for metering.

3. Accuracy Requirements:

• Protection CT:
  • Emphasis on accuracy at high currents.
  • Example: Class 5P10 (accurate up to 10 times rated current).
• Metering CT:
  • Emphasis on accuracy at low and normal currents.
  • Example: Class 0.2 or 0.5 for precise billing.

4. Burden Handling:

• Protection CT:
  • Can handle higher burdens (connected relay loads).
• Metering CT:
  • Handles lower burden typically associated with meters.

5. Application Use:

• Protection CT:
  • Used in switchyards, substations, transformer protection, circuit breaker tripping.
• Metering CT:
  • Used in energy meters, control panels, commercial buildings, and billing systems.

Conclusion:

A protection CT is designed for reliable operation during high fault currents, ensuring protection systems activate correctly. A metering CT, however, focuses on precise current measurement under normal load for accurate energy monitoring and billing. The difference lies in core design, saturation point, burden capability, and accuracy class, and choosing the right type is essential for safety and performance in electrical systems.