Current Transformer（Current Transformer, CT）It is an important equipment used for measurement and protection in the power system, mainly used to convert large currents into standard small current signals (usually5Aor1A）For the convenience of instrument measurement and the use of relay protection devices. The installation, use, and wiring methods must strictly follow electrical specifications to ensure safety, accuracy, and reliability.

1、 Installation method

1. Installation location selection

It should be installed on the main circuit of the tested circuit, usually on the outgoing side of the circuit breaker or busbar, for easy series connection of primary conductors. The installation location should be dry, well ventilated, and avoid high temperature, strong magnetic field interference, and mechanical vibration. For enclosed switchgear, it should be confirmedCTMatch the size with the cabinet and reserve sufficient heat dissipation space.

2. Mechanical fixation method

·Piercing styleCTFix on the bracket or insulator with bolts or clamps to ensure stability and prevent loosening.

·Pillar or casting typeCTThe base bolts should be directly fixed to the structural components of the distribution cabinet, with a flat installation surface and a tightening torque that meets the manufacturer's requirements.

·When the busbar is penetrated, it should be ensured that the conductor passes through the centerCTWindow, to avoid eccentricity causing magnetic circuit asymmetry and affecting accuracy.

3. Primary conductor connection

The primary side is the tested main circuit conductor (cable or busbar), which directly passes throughCTPrimary winding (i.e“One turn”）. If the actual current is small, the transformation ratio can be adjusted by increasing the number of through turns, for example, the rated primary current is100A/5AWhen the actual load is50AWhen worn twice, equivalent to50A/5A.

4. Installation direction and polarity identification

AttentionCTPolarity markings on the casing (usually marked withP1TheP2），P1The end should face the power supply side (incoming end),P2Facing the load side, ensure correct polarity to prevent reverse measurement or protection misoperation.

2、 Usage requirements

1. Variable ratio selection

Reasonably select the rated primary current value based on the system operating current, generally according to the long-term load current1.2~1.5Multiple selection, while considering the short-circuit current tolerance capability. Common secondary rated current5Aor1AIt needs to be matched with the input of supporting instruments or protective devices.

2. Accurate level matching

·For measurement purposesCT: Selection0.2SThe0.5SLevel, ensuring high precision even under low loads;

·For protection purposesCT: Selection5PThe10PLevel emphasizes the linear transmission capability and dynamic response characteristics under high fault currents.

3. Load capacity verification

CTThe total impedance of instruments, relays, etc. connected to the secondary side shall not exceed their rated secondary load (such as15VAThe30VA）Otherwise, it may cause an increase in error or even saturation. The actual total impedance of the secondary circuit (including line resistance, contact resistance, and equipment internal resistance) should be calculated with a certain margin.

4. Prevent open circuit operation

It is strictly prohibited to operate under live conditionsCTOpen circuit on the secondary side. An open circuit can generate thousands of volts of high voltage, endangering personal safety, damaging equipment, and potentially causing magnetization of the iron core. Before replacing the meter or conducting maintenance, the secondary terminal must be short circuited first.

5. Grounding and Insulation

·CTThe shell must be reliably grounded to prevent dangerous voltage from occurring during insulation breakdown;

·The secondary winding is grounded at one point, usually achieved at the terminal block in the control room to avoid interference caused by circulating currents due to multiple grounding points.

3、 Wiring method

1. Basic wiring principle

CTThe primary winding is connected in series with the main circuit, and the secondary winding is connected in series with measuring instruments or protective devices. Typical wiring includes single-phase connection and two-phase connectionV形、三相星形等。

2. Single phase wiring

Suitable for single-phase circuits or situations where only one phase current needs to be monitored.P1Connect the power input line,P2Connect the wire to the load;S1Connect the instrument“+”End,S2pick up“−”End or public end. Suitable for electric energy meters, ammeters, overcurrent relays, etc.

3. two-phaseVShape wiring (two element method)

Three phase three wire circuit commonly used in neutral ungrounded systems. InATheCInstall one for each of the two phasesCTThe secondary winding is connected end-to-end to form an artificial neutral point, which can reflect the sum of three-phase currents (zero sequence components cannot be detected). Widely used in high-voltage motor protection and line protection.

4. Three phase star connection（YShape wiring)

Three phase installationCTAll secondary windingsS2Connect the ends to form a common neutral point（N），S1Connect to the corresponding measurement or protection units respectively. It can fully reflect the current of each phase and zero sequence current (determined by three-phase vector sum), and is suitable for grounding protection, differential protection, and accurate energy metering.

5. Special wiring for differential protection

Used for longitudinal differential protection of important equipment such as transformers, generators, and busbars. both sidesCTThe transformation ratio and characteristics should be consistent, and attention should be paid to polarity coordination when wiring: same name end（P1/S1）Corresponding connection, making the normal operation time difference zero. Any internal fault causing current imbalance will trigger protection action.

6. Multi windingCTWiring allocation

modernCTEstablish multiple independent secondary windings for measurement, protection, and metering purposes. Wiring should be isolated according to function to avoid mutual interference. For example:

·winding1（0.2SGrade)→Electric energy meter;

·winding2（0.5Grade)→ DCS/PLCCollection;

·winding3（5P10Grade)→Relay protection device.

7. Secondary Circuit Wiring Specification

·The cross-sectional area used shall not be less than2.5mm²The copper core insulated wire should be increased to4mm²To reduce pressure drop;

·Separate the laying of strong and weak electrical signal lines to avoid parallel routing with control and communication cables and reduce electromagnetic interference;

·All wiring terminals should be securely crimped, labeled with clear numbers for easy maintenance.

4、 Key points of debugging and acceptance

1. Polarity test

Using DC method to check polarity: Instantaneously touch the positive electrode of a dry batteryP1Negative pole connectionP2Simultaneously observe the secondary sideS1andS2The directional deviation of the pointer type milliampere meter connected between them. ifS1If the pointer deviates in the positive direction when connected to the positive pole of the meter, the polarity is correct.

2. Variable ratio test

Apply adjustable AC current to the primary side, measure the output current of the secondary side, verify whether the actual transformation ratio meets the nameplate value, and the error should be within the allowable range (e.g±1%Within).

3. Loop impedance test

Disconnect the secondary load and measure fromCTCalculate the total circuit resistance between terminals and terminal devices, convert the success rate loss, and determine whether it exceeds the rated load.

4. Insulation resistance test

Measure the insulation resistance of the first to second, first to ground, and second to ground using a megohmmeter, generally requiring a minimum of1000MΩ（1000VMegohmmeter).

5. Live trial operation

Before putting into operation, conduct a load test to verify the balance and reasonable amplitude of the three-phase current, and compare it with the data from the backend monitoring system to confirm that the signal transmission is correct.

5、 Common problems and solutions

1. Secondary open circuit phenomenon

Performance: abnormal noise, smoking, no indication on the instrument panel, protection misoperation.

Solution: Immediately cut off the power, check if the terminals are loose, if the fuse is blown, and repair before powering on.

2. Inaccurate measurement

Reason: Incorrect transformation ratio, reversed polarity, overload, iron core saturation.

Countermeasure: Recheck parameters, correct wiring, and replace with appropriate onesCT.

3. Abnormal fever

Reason: Poor contact, overload operation, internal short circuit.

Solution: Stop the operation and check the temperature of the connection point once. If necessary, perform infrared temperature analysis.

In summary, the installation of current transformers must be accurately positioned and securely fixed; Attention should be paid to transformation ratio, accuracy, load matching, and safety protection during use; Wiring must follow polarity rules and system requirements to ensure accurate measurement and reliable protection. Strictly implementing relevant regulations is a key link in ensuring the stable operation of the power system. Huayuan Electric specializes in producing current transformer testers with excellent quality and service. Welcome everyone to come and purchase!