CT（Current Transformer）versus PT（Potential Transformer）

Although both current transformers (CT) and voltage transformers (PT/VT) are classified as transformers, their functions and application scenarios differ significantly. In this issue, we will start with their core concepts to explore their respective roles, functions and applicable scenarios.

 

CT(Current Transformer)

 

Concept

A current transformer (CT) is an inductive electrical device. By electromagnetic induction, it steps down the high primary current to a standardized low secondary current (1A or 5A) at a fixed ratio, while providing galvanic isolation between the high-voltage main circuit and secondary control and

measurement equipment.

 

 

Functions

• Measurement & Metering:Steps down high load current into standardized signal current for ammeters, power meters and energy meters, enabling accurate measurement of current and electric energy.

 

• Relay Protection:Provides current signals for microcomputer protection devices to realize overload, short-circuit and fault protection of power lines and equipment.

 

• Electrical Isolation:Isolates high voltage and heavy current, ensuring safe operation of instrumentation, control components and on-site maintenance personnel.

 

• Standardized Interface:Unifies secondary output specifications to achieve universal compatibility among measurement, monitoring and protection devices.

 

Application Scope

It is widely applied in 10kV high-voltage switchgear, ring main units, box-type transformers and low-voltage distribution cabinets. It is also commonly used in primary and secondary circuits of power transformers, motor feeders, and all power supply circuits that require current monitoring and protective functions.

 

Voltage Transformer (PT/VT)

Concept

A voltage transformer (VT/PT) is a dedicated step down electromagnetic device. It proportionally steps down high voltage to a standardized low voltage at a fixed transformation ratio. The primary winding is connected in parallel to the high-voltage main circuit, with the secondary side delivering a standard output of 100V. Adopting the same operating principle as conventional transformers, it functions for both voltage reduction and high-voltage electrical isolation.

 

• Voltage Measurement:Supplies voltage signals to voltmeters and frequency meters for real-time busbar voltage monitoring.

 

• Energy Metering & Billing:Cooperates with electric energy meters to measure power consumption of high voltage lines.

 

• Relay Protection & Monitoring:Provides voltage feedback to activate protection against overvoltage, undervoltage, phase loss and zero-sequence voltage faults.

 

• High-Voltage Isolation:Steps down high voltage to a safe 100V secondary voltage, enabling secure operation and convenient maintenance of secondary circuits.

 

• Insulation Monitoring:Detects line earth faults in high-voltage distribution systems.

 

Application Scope

It is widely adopted in 10kV busbar cabinets, PT cabinets, metering cabinets, high-voltage incoming and outgoing switchgears, substations and box-type transformers. This device is suitable for all high-voltage facilities that require voltage monitoring, high-voltage metering and comprehensive voltage protection.

 

 

To provide a clearer overview, the table below compares CT and PT based on the same criteria.

 

Comparison Items	CT (Current Transformer)	PT/VT (Voltage Transformer)
Core Function	Collect current signals and convert high current into low current	Collect voltage signals and convert high voltage into low voltage
Wiring Method	Connected in series in the main circuit	Connected in parallel between busbars or two phases
Primary Side	Connected in series with the load phase line, carrying load current	Connected in parallel with high-voltage phase lines, bearing system voltage
Standard Secondary Output	Rated: 1A / 5A	Rated: 100V
Main Applications	Current measurement, energy metering, overload/short-circuit protection	Voltage monitoring, high-voltage metering, overvoltage/undervoltage/grounding protection
Operation Taboo	The secondary side is strictly forbidden to open circuit (will generate high voltage and damage equipment)	The secondary side is strictly forbidden to short circuit (will burn out the coil)
Insulation Focus	Withstand high current impact	Withstand long-term high-voltage insulation
Common Installation Positions	Incoming and outgoing cabinets, compensation cabinets, high and low voltage sides of transformers, motor circuits	10kV PT cabinets, busbar cabinets, metering cabinets, high-voltage ring main units