Short Answer:

A Potential Transformer (PT) measures high voltage by stepping it down to a lower, safer level, while a Current Transformer (CT) measures high current by reducing it to a small proportional value. Both provide isolation and scaling, but PTs are used for voltage measurement and CTs for current measurement.

PTs are connected parallel to the power line, whereas CTs are connected in series with the current-carrying conductor. These transformers make it possible to connect standard measuring instruments to high-voltage or high-current circuits safely and accurately.

Detailed Explanation:

Difference between potential transformer and current transformer

Potential Transformer (PT) and Current Transformer (CT) are both types of instrument transformers, used to measure electrical parameters in high-voltage power systems without direct contact. Their purpose is to step down voltage or current so that it can be safely and accurately measured using standard low-range instruments.

While they look similar in design, their functions, connections, and applications are quite different.

Working Purpose:

• PT (Potential Transformer):
  • Used to step down high voltage to a lower, standardized voltage.
  • Allows voltmeters and protective devices to monitor voltage in high-voltage circuits.
  • Maintains proportional voltage ratio under normal conditions.
• CT (Current Transformer):
  • Used to step down high current to a lower, standardized current.
  • Allows ammeters, energy meters, and relays to monitor current flow.
  • Maintains proportional current ratio, usually 5 A or 1 A at secondary.

Connection Method:

• PT: Connected across (parallel) to the circuit whose voltage is to be measured.
• CT: Connected in series with the line carrying the current.

Construction and Design:

• PTs have many turns in the primary winding and fewer in the secondary, similar to regular step-down transformers.
• CTs have few turns in the primary, sometimes just one conductor passing through the core, and many turns in the secondary to reduce current.

Safety Note:

• PTs should never be short-circuited, as it can damage the equipment.
• CTs should never be open-circuited while energized, as it can produce dangerously high voltage on the secondary.

Accuracy and Use Cases:

• PTs are used in:
  • Voltage metering
  • Voltage-based protection systems (like over-voltage or under-voltage relays)
  • Grid monitoring and power quality checks
• CTs are used in:
  • Current metering
  • Overcurrent and short-circuit protection
  • Energy billing and load monitoring

Key Differences in Summary:

• PT measures voltage; CT measures current
• PT connected in parallel; CT connected in series
• PT secondary voltage standard: 110V or 63.5V
• CT secondary current standard: 5A or 1A
• PT used with voltmeters and relays
• CT used with ammeters, meters, and protection devices

Conclusion:

A Potential Transformer (PT) and a Current Transformer (CT) are essential tools in high-voltage systems, each serving a specific purpose—PT for voltage, CT for current. They both work on the principle of electromagnetic induction but differ in their design, connection, and usage. Understanding their differences helps ensure safe and correct measurement and protection in electrical systems.