Current Transformer (CT): The "Sensing Organ" of Power Systems and a Design Guide for Engineer Selection

2025-08-22 15:29:12

——Building the cornerstone of system level measurement protection from electromagnetic principles to selection traps

1、 Core function: not only "flow conversion", but also the cornerstone of system perception

Current Transformer (CT) is a sensory neuron in the power system, whose core task is to proportionally convert a large current into a standard secondary current (1A or 5A), providing isolated and accurate signal input for measurement, metering, and protection equipment.   

>Analogy: It's like a doctor listening to a heartbeat with a stethoscope - the stethoscope safely attenuates powerful heart sounds into subtle sounds that can be recognized by the human ear, without changing the sound characteristics and protecting the listener. CT also listens to thousands of amperes of current on the primary side and converts it into a safe and usable standard signal on the secondary side.   

Its design must simultaneously meet two types of requirements:

-Measurement accuracy: Provide accurate data for energy measurement and operation monitoring (with extremely strict error requirements).   

-Protection reliability: Ensure that the protective relay operates correctly in the event of a fault (anti saturation capability is key).   

2、 Working principle and key characteristics: Engineering implementation of electromagnetic theory

1. Ideal vs. Reality: Sources of Error

CT is based on the principle of electromagnetic induction, but its performance is limited by the presence of excitation current. In an ideal CT, all primary currents are used to excite secondary currents; In actual CT, a portion of the current (excitation current) is used to establish a magnetic field, resulting in differences in ratio and angle.   

Key formula prompt:

The error mainly comes from the excitation branch. The higher the precision, the greater the required excitation impedance, and the higher the requirements for the core material and cross-section.   

2. Core parameter: Engineer's evaluation dimension

-Rated primary current (Ipn): selected based on the system's long-term maximum load current (usually 1.2-1.5 times).   

-Rated secondary current (Isn): 5A (traditional system) or 1A (reducing line loss, suitable for long cables).   

-Accuracy level: Identify the error limit of CT.   

-Measurement level: 0.2S, 0.5 (energy metering), 0.5 (general monitoring)

-Protection level: 5P10, 10P20 (P represents protection, numbers represent composite error limit and accuracy limit coefficient)

-Rated load (Va): The maximum apparent power that the secondary side can carry (determining the number of instruments and cable length that can be connected).   

-Accuracy limit factor (ALF): defines the working range for protecting CT to maintain accuracy (e.g. 5P10, ALF=10).   

3、 Selection Core: The Completely Different Way of Measurement and Protection

Incorrect selection is one of the main causes of system failures. The design objectives of measuring and protecting CT are fundamentally different:

Engineering Design Checklist:

1. Determine the purpose: clearly for measurement, metering, or protection? Or a combination of the two?   

2. Calculate the current once: Select the transformation ratio based on the system short-circuit current and load current.   

3. Select accuracy level: Choose 0.2S/0.5S for measurement; Select 0.5/1 for monitoring; Choose 5P/10P for protection.   

4. Calculate load: Count the impedance of all secondary equipment+the impedance of connecting cables (copper core cable~0.75 Ω/100m). Be sure to leave some margin!   

5. Verify thermal stability and dynamic stability:

-Thermal stability verification: Ith 2 × t ≥ Qdt (Qdt is the system short-circuit thermal effect)

-Dynamic stability verification: Ies ≥ ish (ish is the peak value of the system short-circuit current)

4、 Common pitfalls and failure modes: Experience from the field

1. Saturation: Protecting the 'Blind'

-Scenario: When the system is short circuited, the CT depth saturates and the secondary current is severely distorted, resulting in protection refusal or misoperation.   

-Root cause: ALF selection is too small, secondary load is too large, or iron core residual magnetism is too high (especially after circuit breaker failure).   

-Solution: Choose a higher ALF, use TPY grade CT (anti remanence), or reduce the impedance of the secondary circuit.   

2. Secondary Open Circuit: Dangerous' High Voltage Bomb '

-Principle: CT is approximately short circuited during normal operation. Once the circuit is opened, the excitation current surges and the iron core saturates, inducing thousands of volts of high voltage on the secondary side, endangering personal and equipment insulation.   

-Handling: It is absolutely forbidden to open the circuit with electricity! The power must be cut off to replace the instrument; If live work is required, the secondary Terminal must be short circuited.   

3. Measurement disputes

-Scenario: There is a deviation in electricity bill settlement.   

-Troubleshooting: Check the CT/PT composite error, whether the secondary load exceeds the limit, and whether the three-phase CT characteristics are consistent.   

5、 Frontiers and Evolution: Intelligent CT and Optical CT

-Intelligent CT: integrates sampling, computing, and communication functions, directly outputs digital quantities, and is the core component of intelligent substations.   

-Optical CT (OCT): Based on Faraday magneto-optical effect, it is a future technology direction with no magnetic saturation, simple insulation, and wide frequency band. However, cost and stability are still challenges at present.   

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Conclusion: System level thinking

When choosing CT, always remember:

It is not an independent component, but the starting point of the chain of measurement, protection, and control systems. Its failure means the malfunction of the system's senses.   

The most reliable selection is based on the comprehensive results of system short-circuit current calculation and accurate secondary load statistics, rather than simple current ratio matching.