1. Introduction

The Current Transformer (CT) is an electrical device that operates based on the principle of electromagnetic induction. It is primarily used to proportionally convert high currents in three-phase circuits to smaller currents, thereby providing suitable current signals for equipment such as measuring instruments and protective devices. Widely applied in power systems for measurement, metering, protection, and monitoring, it effectively ensures the safe and stable operation of power systems.

2. Principle

When a three-phase alternating current flows through the primary winding of a current transformer (CT), it generates an alternating magnetic flux in the iron core. This flux passes through the secondary winding, according to the law of electromagnetic induction, induces a proportional secondary current in the secondary winding relative to the primary current. The ratio of the magnitude of the secondary current to that of the primary current equals the inverse of the ratio of the number of turns in the primary winding to those in the secondary winding. This principle enables the conversion of large primary currents to smaller secondary currents.

3. Function

3.1 Current measurement
• Universal for high/low voltage systems
Directly measuring high currents in either high-voltage or low-voltage circuits is hazardous. Three-phase current transformer (CT) can proportionally step down high voltages or large currents to low-voltage or currents, enabling safe and accurate measurements. For example, in a 10 kV high-voltage distribution system, currents may reach hundreds or even thousands of amperes. By using a current transformer to convert these to a secondary current of 5A or 1A, standard ammeters can easily measure and display the values.
• Multi-Device compatibility
The current transformer (CT) is compatible with various measurement instruments such as ammeter, power meter, and energy meter, providing them with appropriate current signals to accurately measure parameters like current, power, and energy in three-phase circuits. For example, in a factory’s power distribution room, connecting the current transformer to an energy meter enables precise measurement of electricity consumption, offering a reliable basis for billing and cost settlement.
3.2 Protective relaying
• Fault current sensing
When a short circuit or overload occurs in a power system, the current surges abruptly. The three-phase current transformer (CT) promptly and accurately transmits the fault current signal to protective relays, enabling them to act swiftly and trip the faulted circuit. This critical function safeguards the power system by isolating faults and ensuring its safe and stable operation
• Precision protection actuation
Various protection functions rely on specific current signals as decision criteria. Three-phase current transformers (CT) provide accurate current data to diverse protective relays, enabling them to initiate appropriate actions based on the fault type and severity. Examples include Overcurrent Protection, Overload Protection and Differential Protection
3.3 Electric Energy Metering
• Precision Billing Basis
The three-phase current transformer is essential for electric energy metering in power systems. It provides the energy meter with a secondary current signal proportional to the primary current,enabling the meter to accurately measure the electrical energy consumed by users, ensuring fair billing for industrial, commercial, and residential users
• Multi-Type Metering Compatibility
Provides accurate current signals for active and reactive energy metering, meeting the power system's requirements for monitoring and managing critical indicators such as power quality and power factor
3.4  System monitoring and control
• Operational status assessment
Monitoring secondary currents from three-phase CTs enables real-time assessment of system health, including phase currents and balance. Detecting imbalances helps identify faults or load mismanagement, allowing timely corrective actions to boost grid efficiency and reliability
• Basics of automated control systems
Current transformers supply vital current data for power system automation. In smart grids, their signals enable autonomous control—adjusting generation, balancing loads, and compensating reactive power—to enhance grid automation and stability

4. Installation Instructions

4.1 Inspection before installation 
• Inspect the current transformer for physical damage or deformation, Verify that the
nameplate parameters align with design specifications.
• Measure the Insulation Resistance of the Secondary Winding，The insulation resistance
value of the secondary winding must comply with specified requirements, If the insulation
resistance is abnormally low, perform drying treatment or inspect for insulation damage.
4.2 Installation steps
• Select the appropriate installation method based on the actual installation location and wiring requirements, such as bracket or busbar mounting.
• Ensure secure installation to prevent vibration or displacement during operation.
• Connect the primary and secondary winding according to the correct wiring configuration. The primary winding should be connected in series with the three-phase circuit, The secondary winding must be correctly connected to measuring instruments and protective relays.
4.3 Precautions for Use
• Open-circuit is strictly prohibited during the operation of the secondary winding, otherwise high voltage will be generated at both ends of the secondary winding, endangering equipment and personnel safety. Fuses and other protective devices should be installed in the secondary circuit to prevent short circuits
• The secondary load of a current transformer (CT) must remain within its rated load range. Excessive secondary load may lead to increased measurement errors or impair the CT’s normal operation
• Regularly inspect the operational status of the current transformer during operation, including temperature, sound, odor etc. If any abnormalities are detected, immediately shut down the equipment for inspection, diagnose and rectify the fault