Comparison of Current Transformers (CT) and Potential Transformers (PT) for Power System Measurement and Protection

Current Transformers (CT) and Potential Transformers (PT) are instrument transformers used for measurement and protection in power systems, yet they differ in functions and application methods. Their main differences are as follows:

1. Different Purposes The current transformer converts the large current in the primary circuit proportionally into a standard small current for ammeters, relay protection and metering devices. The voltage transformer converts the high voltage in the primary circuit proportionally into a standard low voltage for voltmeters, metering and protection devices.

2. Different Primary-side Connection Modes The primary winding of a current transformer is connected in series with the measured circuit, and its current equals the line current. The primary winding of a voltage transformer is connected in parallel with the measured circuit and bears the line voltage.

3. Different Operating Conditions on the Secondary Side The secondary side of a current transformer must be connected to a load. Open-circuit operation is strictly prohibited, otherwise an extremely high hazardous voltage will be generated. The secondary side of a voltage transformer can operate under nearly no-load conditions, but short-circuiting is strictly forbidden.

4. Different Standard Secondary-side Parameters The rated secondary current of a current transformer is normally 5 A or 1 A. The rated secondary voltage of a voltage transformer is normally 100 V or 110 V.

5. Different Safety Hazards The greatest risk for a current transformer is secondary open circuit, which may damage equipment and endanger personal safety. The greatest risk for a voltage transformer is secondary short circuit, which may burn out the transformer.

6. Different Application Scenarios Current transformers are commonly used for current measurement, overcurrent protection, differential protection, etc. Voltage transformers are commonly used for voltage measurement, power metering, directional and distance protection, etc.

Brief Summary

A current transformer measures current, is connected in series, and must avoid open circuit. A voltage transformer measures voltage, is connected in parallel, and must avoid short circuit.

 Lily