Voltage transformer (PT or VT) and current transformer (CT) are two key measurement and protection devices in power systems, mainly used to convert high voltage or large current into safe low voltage or small current in proportion for measurement, monitoring and protection. Their core differences are as follows:
1. Functional Purpose
Voltage Transformer (PT/VT)
Measure high voltage: Convert the system's high voltage (such as 10kV, 35kV, etc.) into a standard low voltage (usually 100V or 100/√3 V) in proportion.
Provide voltage signal: Used for voltage input of voltmeters, power meters, relay protection devices and other equipment.
Current Transformer (CT)
Measure large current: Convert the system's large current (such as hundreds of amperes to thousands of amperes) into a standard small current (usually 5A or 1A) in proportion.
Provide current signal: Used for current input of ammeters, energy meters, relay protection devices and other equipment.
2. Structure and working principle
PT/VT
Parallel connection system: the primary winding is connected in parallel in the high-voltage circuit, and the secondary side outputs low voltage.
Electromagnetic principle: Similar to a transformer, it relies on electromagnetic induction to achieve voltage conversion, and the iron core works in a high magnetic resistance state (close to open circuit).
High internal resistance: the secondary side is close to no-load operation, and if it is short-circuited, it will cause overheating and damage.
CT
Series connection system: the primary winding is connected in series in a high-current loop, and the secondary side outputs a small current.
Current conversion principle: the primary current is proportional to the secondary current, and the iron core works in a low magnetic resistance state (close to short circuit).
Low internal resistance: the secondary side must always be closed (if it is open, it will produce dangerous high voltage).
3. Secondary side characteristics
PT/VT
The secondary side voltage is a standard value (such as 100V), and the load impedance is large.
Short circuit is strictly prohibited: short circuit will cause the fuse to blow or the equipment to burn.
CT
The secondary side current is a standard value (such as 5A), and the load impedance is small.
Open circuit is strictly prohibited: when the circuit is open, the secondary side will sense high voltage (up to thousands of volts), endangering the safety of equipment and personnel.
4. Typical application scenarios
PT/VT
Voltage measurement, synchronous detection, power calculation, overvoltage/undervoltage protection, etc.
Commonly used in substations, distribution cabinets, generator outlets, etc.
CT
Current measurement, energy metering, overcurrent/differential protection, etc.
Widely used in power lines, switch cabinets, motor protection, etc.
Summary
| Characteristics | Voltage transformer (PT/VT) | Current transformer (CT) |
|---|---|---|
| Access method | Connected in parallel with high voltage circuit | Connected in series in a high current loop |
| Secondary output | Low voltage (such as 100V) | Low current (e.g. 5A) |
| Working state | Close to open circuit (high impedance) | Close to short circuit (low impedance) |
| Dangerous working condition | Short circuit on secondary side will damage | Secondary side open circuit generates high voltage |
| Core design | More turns on primary side, thin wire | Fewer turns on the primary side (even 1 turn) and thicker wire |
Precautions
Safe operation: The secondary side of PT needs to be protected from short circuit, and the secondary side of CT needs to be protected from open circuit (must be grounded).
Accuracy level: PT is used for voltage accuracy measurement (such as 0.2 level), and CT is used for current or electric energy measurement (such as 0.5S level)
If you need to know more about the product selection, technical parameters or application solutions of voltage transformer (PT/VT) or current transformer (CT), please feel free to contact us! Our professional team will provide you with technical support and services.
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