The secondary load of the current transformer directly affects its correct operation. Generally speaking, the greater the secondary load, the greater the error of the transformer. As long as the secondary load does not exceed the manufacturer’s setting value, the manufacturer should ensure that the error generated by the transformer is within its accuracy level or within the range of the 10% error curve. Inside. Therefore, during the use of the current transformer, its rated secondary load and actual secondary load must be known. Only when the actual secondary load is less than the rated secondary load can the error meet the requirements.

When the error of the current transformer exceeds the specified value of the manufacturer, it will have adverse effects on secondary equipment such as relay protection and metering devices. Compensatory measures to be taken when the error of the current transformer exceeds the specified value of the manufacturer.

(1) Increase the cross-sectional area of the secondary cable or reduce the length of the cable. Increasing the cross-sectional area of the secondary cable of the current loop or reducing the length of the cable actually reduces the impedance of the secondary loop wire and reduces the secondary load.

(2) Connect the secondary coil of the backup current transformer in series to double the load. The secondary coils of two in-phase current transformers with the same transformation ratio and the same characteristics are used in series.

(3) Increase the transformation ratio of the current transformer or use a current transformer with a secondary rated current of 1A. According to the principle that the loss of the line is proportional to the square of the current, it can be seen that the loss of the line becomes smaller and the output impedance becomes larger, so the load carrying capacity is strengthened.

(4) Reduce the secondary load. Choose a relay with a large setting current as much as possible, because the wire diameter of the relay coil with a large setting current is thick and the number of turns is small, so the impedance is also small; or change the series connection of the relay coil to a parallel connection, because the impedance of the series connection The impedance is larger than the parallel connection; or use a microcomputer protection device to replace the electromagnetic relay.

Test of insulation resistance of current transformer

1. Purpose of the test

It can effectively find overall insulation defects, such as damp, dirt, penetration, insulation breakdown, etc., as well as serious overheating and aging defects. Measuring the insulation resistance of the final shield to ground can effectively detect the water ingress and moisture defects of the capacitive current transformer.

2. Test scope

Measure the insulation resistance of the primary winding to the secondary winding and the casing, and the insulation resistance of each secondary winding and the casing.

To measure the insulation resistance between the primary winding parts, but it is not necessary to measure when it cannot be measured due to structural reasons.

Measure the insulation resistance of the final stage shield of the capacitive current transformer.

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3. Selection of equipment

Measure the insulation resistance between the main insulation of the current transformer, the end shield, the secondary winding and the ground. Insulation resistance tester of 2500V and above should be used for maintenance or handover test and preventive test.

4. Risk point analysis and control measures

to prevent falling from a height

Prevent injuries from falling objects

to prevent electric shock

Before disconnecting and connecting the test line, the transformer under test should be completely discharged to the ground to prevent the residual charge and induced voltage from causing harm to people and affecting the measurement results. The metal casing of the test instrument should be reliably grounded, and the tester operating the instrument must stand on an insulating pad or wear an insulating slash to operate the instrument. The test tongs should be coordinated with the person in charge, and cross-operation is not allowed.

Set up closed shelters around the test site, hang “stop, high voltage danger” signs, and strengthen monitoring. Strengthen supervision and implement a singing system in operation.

5. Preparation before the exam

Understand the field conditions and test conditions of the equipment under test.

Complete test equipment and equipment

Take safety and technical measures at the test site

The box testers should explain the work content, live parts, on-site safety measures, on-site operation danger points, and clarify the division of labor and test procedures.

6. Field test steps and requirements

Check the megohmmeter itself before the test, put the megohmmeter level stably, first short-circuit test and then open-circuit test, when connected to the power supply of the rectified voltage megohmmeter, the wire of Uno will short-circuit “L” and “E”" terminal, the indication should be zero; when it is turned on, when the power is turned on or the rated speed is expressed in megohms, the indication should be “∞”. When wiring, connect the ground terminal first, and then connect the high voltage terminal.

The terminal “E” on the megohmmeter is the ground terminal of the test object, which is the positive pole, and “L” is the high-voltage terminal of the test product, which is the negative pole. “G” is connected to the shield terminal, which is the negative pole.

7. Insulation resistance test

Measure the insulation resistance of the primary winding of the current transformer to the secondary winding and the shell

Measure the insulation resistance between the secondary winding of the current transformer and the ground

Measuring the insulation resistance of the final shield of the current transformer

Measuring the insulation resistance of the primary winding

The primary windings P1 and P2 of the current transformer are short-circuited with short wires, all secondary windings are short-circuited to ground, and the final shield is short-circuited to ground. (If the surface of the transformer is too heavy, a shielding ring should be installed and connected to the “G” terminal of the megger with an insulated wire.)

The “L” terminal of the high-voltage insulation tester is connected to the primary winding P1 and P2 terminals of the current transformer or a short wire, and the “E” terminal is grounded

After checking the wiring, press the “Start” button, and the meter starts to work. After 1 minute, the insulation resistance value will be recorded. After the test is completed, the meter should be disconnected from the sample, and then press the “stop” button to resume the meter.

Finally, discharge the test part of the current transformer.