Function of current transformer

Current Transformer (CT) is a key equipment in power systems, mainly used to proportionally convert high current into low current for safe and accurate measurement and protection of power equipment. Its specific function is as follows:
1. Current measurement
Range adaptation: Convert the high current (such as several kiloamperes) on the primary side (high voltage side) to a small current (usually 5A or 1A) on the secondary side (low voltage side) in proportion, making it easy for standard instruments (such as ammeters and energy meters) to measure directly.
High precision sampling: By using precise transformation ratios (such as 1000:5) to ensure minimal measurement errors (accuracy levels are generally 0.2 and 0.5), it meets the needs of electricity metering and system monitoring.
2. Relay protection
Fault current detection: Real time monitoring of line current. When faults such as short circuit and overload occur, the current signal is transmitted to the relay protection device, triggering the circuit breaker to trip and isolate the fault area.
Differential protection: Quickly identify internal faults (such as winding short circuits) in transformers, busbars, and other equipment by comparing the current difference of CT at different positions.
Grounding protection: detect zero sequence current (such as three-phase imbalance or grounding fault) and initiate protection action.
3. Electrical isolation
High and low voltage isolation: By transmitting current signals through magnetic coupling, the primary side high voltage system is isolated from the secondary side low voltage instruments and protective equipment to ensure the safety of operators.
Suppress common mode interference: Block the interference of high voltage on measurement and control circuits, and improve the system's anti-interference ability.
4. System monitoring and automation
Data collection: Provide real-time current data for SCADA (Data Acquisition and Monitoring System), smart meters, etc., supporting power grid scheduling and load management.
Fault waveform recording: Record the current waveform when a fault occurs to assist in analyzing the cause of the fault.
Power quality analysis: Monitor abnormal signals such as harmonics and transient currents to evaluate the health status of the power grid.
5. Standardization and compatibility
Unified signal interface: The secondary side outputs standardized current (5A or 1A) to enable compatible interconnection of instruments and protective devices from different manufacturers.
Extension function: In conjunction with voltage transformers (PT), it provides basic data for energy metering and power calculation (such as power=current x voltage x power factor).
6. Safety and Economy
Reduce equipment costs: Avoid large cross-sectional wires and high cost instruments required for direct measurement of high currents.
Reduce energy loss: The secondary side current is small, and the measurement circuit loss is extremely low.
Prevent personal injury: Isolate the high-voltage system to avoid the risk of electric shock caused by direct contact with high currents.