CT is a special measuring transformer designed to convert large currents in a power system into standardized, safe, small currents for measurement, monitoring and protection.CT is essentially a “step-down” transformer, but it reduces the current rather than the voltage.
The principle of operation of a CT is based on the laws of electromagnetic induction and consists of the following core structures:
Primary winding: usually only 1 or a few turns, directly connected in series in the circuit under test.
Secondary winding: a substantial quantity of turns (up to several hundred), linked to a measuring instrument or a protective apparatus.
High permeability core: provides effective magnetic coupling
The primary function of a CT is to safely reduce current, converting large currents (hundreds to thousands of amperes) that are difficult to measure directly into standard small currents (typically 5A or 1A). This conversion facilitates measurement while ensuring electrical isolation between the high voltage system and the measuring instrument, thereby safeguarding operators and equipment from high voltage hazards.
This isolation feature of the CT is particularly important in high-voltage systems, as it allows the operator to safely monitor the operation of the high-voltage line on the low-voltage side without having to come into direct contact with dangerous high-voltage conductors.
CTs can be divided into two categories: CTs for measurement and CTs for protection:
CT for measurement (Class 0.1, 0.2, 0.5, etc.):
Design focus: Maintains high accuracy within normal load range
Characteristics: saturated at overcurrent, protects downstream instruments from damage.
Applications: power measurement, meter indication, etc.
CT for protection (5P, 10P class, etc.):
Design focus: Maintain adequate output under fault high currents
Characteristics: high accuracy limit factor (ALF), strong anti-saturation ability
Applications: relay protection, fault detection, etc.
There are several technical parameters to consider when selecting a CT:
Rated primary current: should match the maximum continuous current of the line under test
Ratio: e.g. 100/5, 400/5, etc., determines the current conversion ratio.
Accuracy level: indicates the limit of error under rated conditions.
Rated load: the maximum load (VA or Ω) allowed on the secondary side under the premise of guaranteeing accuracy
Accuracy Limit Factor (ALF): Key parameter of CT for protection
A typical relay protection system consists of three key components:
Current transformer (CT): monitors line current
Voltage transformer (PT): monitors the system voltage.
Protective relay: determining faults and triggering actions based on CT/PT inputs
In this system, the CT plays the role of “system sense”, providing the relay with current signals that reflect the state of the power system. Without accurate and reliable CT inputs, relays will not be able to recognize faults correctly.
Current monitoring: The CT scales down the high currents in the main circuit in real time.
Signal transmission: The reduced safe current (typically 5A) is transmitted to the relay.
Fault determination: the relay analyzes the current size, phase and other characteristics
Protection action: When an abnormality such as an overcurrent or short circuit is detected, the relay triggers the circuit breaker to trip.
CTs for protection must be designed to ensure that adequate signals are provided to the relay even in the event of high short-circuit currents (which may be dozens of times the rated current), which is the main difference from CTs for measurement.
CT continuously monitors load current
Relay triggers circuit breaker when current exceeds set threshold value
Protects equipment and circuits from overload damage
Multiple CTs simultaneously monitor the inlet and outlet line currents of the equipment
Relay compares each CT signal to detect internal faults
Especially suitable for transformers, generators and other important equipment
Ground fault protection:
Zero sequence CTs detect three-phase unbalanced currents
Recognizes ground faults (e.g. insulation damage)
Rapid isolation of faulty segments to improve system safety
Application requirements:
For measurement: prioritize accuracy (e.g. class 0.5)
For protection: focus on ALF and anti-saturation capability
System parameters:
Maximum continuous current determines primary rating
Possible short-circuit currents influence ALF selection
System voltage level determines insulation requirements
To summarize, current transformer, as the “sense organ” of the power system, bears three core missions:
Safety monitoring: through current conversion and electrical isolation, enabling personnel to safely monitor the high-voltage system.
Accurate measurement: Provide accurate current signals for power measurement and system monitoring.
Reliable protection: working with relays to quickly identify and isolate faults to ensure grid safety
With the development of smart grid, CT technology is also constantly innovating, such as the emergence of electronic CT, optical CT and other new devices, which further enhance the measurement accuracy and system integration. However, no matter how the technology evolves, the fundamental role of CT in the power system will not change - it is always the key bridge connecting the primary high-voltage system and the secondary control and protection system.
