Difference between Measuring & Protection

Описание: Understanding the Difference Between Metering and Protection CTs:

1. Metering CT (For Measurement):
A metering CT is designed to accurately measure normal load current and provide data for energy meters monitoring systems. It focuses on precision under standard operating conditions, not during faults or
overloads.
Metering CTs typically have accuracy classes of 0.1, 0.2, 0.5, or 1. For example, a CT with class 0.2 means it will have a maximum error of 0.2% in current measurement under normal conditions.
Metering CTs are only accurate during normal operating conditions. During faults (overload), they may saturate and fail to provide correct current readings.
Used for energy billing (with energy meters) and monitoring load, power factor, and system voltage.

2. Protection CT (For Relay Protection):
A protection CT is designed to provide accurate current readings even during fault conditions, so protection relays can act correctly and quickly (e.g., by tripping breakers).
Protection CTs typically have accuracy classes like 5P, 10P PS (Protection Special), or PX (Protection Extended). A CT with class 5P10 means it can handle up to 10 times the rated current with a maximum error of 5%. PS/PX Class is used for sensitive protection
systems like differential protection. These CTs are highly precise and do not saturate during faults.
Protection CTs are defined by their knee point voltage (Vk), which ensures that the CT does not saturate under high fault currents. A higher Vk indicates better performance during faults.
Used for Overcurrent Relays (50/51), Differential Protection Relays (87G), and Backup and Earth Fault Protection Systems.
Selection_Guide_in_Detail:
1. For Metering CTs:
If you need a CT for energy measurement or billing, select one with a lower accuracy class like 0.2 or 0.5.
Example:
CT Rating: 100/5 (Primary: 100A, Secondary: 5A).
Accuracy Class: 0.2.
Burden: 15 VA.
2. For Protection CTs:
For relays or fault protection, choose a CT with 5P or PS class to ensure accuracy even during high fault conditions.
Example:
CT Rating: 100/5.
Accuracy Class: 5P10 (5% error at 10 times the rated current).
Knee Point Voltage: 400V.
Burden: 15 VA.

Metering CTs are meant for normal operation where high accuracy is needed for measurements, like energy billing. Protection CTs are designed for fault conditions and ensure reliable operation of relays even at high fault Currents.