Classification of current transformers

The classification of current transformers (CTs) can be based on various standards, and the following are the main classification methods and detailed explanations:
1. Classified by purpose
Current transformers for measurement
Features: High precision, but the design needs to avoid saturation and ensure accuracy within the normal current range.
Accuracy level: 0.1, 0.2, 0.5, 1 level, etc. (the smaller the number, the higher the accuracy).
Application: Connect instruments such as energy meters and power meters.
protective current transformer
Characteristics: It needs to withstand fault currents such as short circuits and has high anti saturation ability (such as high accuracy limit coefficient ALF).
Type:
P class: Conventional protection, such as 5P10 (error ≤ 5% at 10 times rated current).
PR class: with residual magnetism limitation, suitable for high residual magnetism scenarios.
TP class: transient protection, used for ultra-high voltage systems.
Applications: Relay protection devices, fault recorders, etc.
2. Classify by structure
Wound Primary
Structure: The primary winding is directly wound on the iron core, suitable for low current scenarios.
Disadvantages: Large size and high cost.
Bar Primary
Structure: There is no primary winding, and the busbar passes directly through the iron core, making installation convenient.
Application: Distribution cabinets, switchgear.
Bushing Type
Structure: Integrated into transformer or circuit breaker bushings, saving space.
Applicable scenario: High voltage substation.
3. Classified by working principle
Electromagnetic current transformer
Principle: Based on electromagnetic induction, the iron core magnetic circuit transmits signals.
Limitations: Easy to saturate, narrow frequency band, unable to measure DC.
electronic current transformer
Rogowski Coil: No iron core, measures AC or transient current, good linearity.
Hall effect type: capable of measuring DC/AC, requires external power supply, and has strong anti-interference ability.
Optical current transformer (OCT): Utilizing Faraday effect, it has excellent insulation performance and is suitable for ultra-high voltage.
4. Classified by insulation medium
oil-immersed
Features: Insulation oil cooling and insulation, commonly used in high-voltage outdoor applications (such as 110kV and above).
Disadvantages: There is a risk of oil leakage and maintenance is complex.
dry
Material: Epoxy resin casting or plastic shell, maintenance free.
Application: Indoor medium and low voltage scenarios (such as 10kV switchgear).
Gas insulated (SF6)
Features: SF6 gas insulation, compact and pollution resistant, used for GIS equipment.
5. Classified by installation method
Indoor type: lightweight structure, low protection level (such as IP20).
Outdoor style: rainproof and dustproof design (IP54 or above), strong weather resistance.
6. Classify by phase number
Single phase: commonly used in high-voltage systems or scenarios that require phase separation monitoring.
Three phase: Integrated three-phase winding, compact structure, commonly used for low-voltage distribution.
7. Special types
Low power current transformer (LPCT): outputs small signal voltage and is directly connected to electronic devices.
Self powered type: Energy is obtained from the measured current without the need for an external power source, suitable for passive scenarios.
Application selection example
High voltage transmission lines: Oil immersed or SF6 insulated outdoor CTs are often selected, paired with TP type protection.
Smart grid: using Roche coils or optical CT, supporting wideband and digital output.
DC system: Hall effect CT, capable of measuring DC components.
The above categories help users choose suitable current transformers based on measurement requirements, installation environment, system voltage, and other factors to ensure accuracy and reliability.