What is the difference between CT in measurement and protection?

  Current transformers (CTs) are widely use in power systems for measurement and protection, but their functions and requirements in these two applications are different. Here are the main differences:

1. Current transformer for measurement
Main function: Convert large currents into smaller currents in proportion to facilitate accurate measurement of current values ​​by measuring instruments (such as ammeters, power meters, energy meters, etc.).
Accuracy requirements: Current transformers for measurement have high accuracy requirements and usually have small errors at rated currents to ensure the accuracy of measurements. Common accuracy levels are 0.2, 0.5, etc.
Overload capacity: The overload capacity of the measuring transformer is small and it is generally not require to withstand currents exceeding its rated value for a long time.
Secondary load: Mainly measuring instruments with low power and low load impedance.
2. Current transformer for protection
Main function: Used to provide current signals to protection relays to monitor the operating status of the power system and trigger protection actions (such as circuit breaker tripping) under fault or abnormal conditions.
Accuracy requirements: For currents during normal operation, the accuracy requirements are relatively low, but under fault conditions (such as short-circuit currents), the changes in current need to be accurately reflect. Usually there are accuracy levels such as 5P10 and 10P10, which means that the error is still within the allowable range at 10 times the rated current.
Overload capacity: The protection transformer must be able to withstand very high overload currents for a short period of time to ensure that it can still operate correctly in the event of a fault.
Secondary load: Mainly the protection relay, which may require greater power, so the load impedance is relatively high.
3. Design difference
Core design: The core design of the measuring current transformer allows it to operate in the linear range of the magnetization curve at the rated current to ensure high accuracy. The core of the protection current transformer is design to saturate under high overload conditions, but still provide accurate current signals to the protection relay.
4. Application scenarios
Measurement: Used for daily current monitoring, metering, data recording, etc.
Protection: Used to protect equipment and lines to prevent equipment damage or power system fault expansion caused by overload or short circuit.
In general, although the working principles of current transformers in measurement and protection are similar, they are significantly different in accuracy, overload capacity, design, etc., and the appropriate transformer type must be select according to the specific application.

What is the difference between CT in measurement and protection?

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