Misconceptions in the Use of Current Transformers and Precautions in Work

One Misunderstanding When Using Current Transformers

Current transformers are widely used to monitor current in circuits, and are used in conjunction with energy meters to measure active and reactive energy. A large number of secondary relays are used to maintain motors. Factories (engaged in lathe processing) use more electricity than before. 3 LMZ-0.5 core-wearing current transformers were used to measure the energy meter, the original transformation ratio was 200/5, and the owner of the factory said that the meter reading should be multiplied by 10 to get his electricity consumption. We checked the number of turns of the transformer core, and it was clear that the winding method was incorrect. The original wiring electrician mistakenly believed that the calculation of the number of turns was based on the number of windings around the outer ring of the iron core. In fact, it should be based on the number of turns around the center of the transformer. Each time the wire passes through the "window", it is counted as one turn, so there is an error.

The result is as follows: If a CT current transformer with a primary current of 200A is changed to 50A, the number of turns of the primary winding is 200/50=4 turns. The original electrician mistakenly thought that the number of turns was based on the outer ring of the iron core, and wound it into the previous shape. The primary current became 40A (200/5) and the transformation ratio became 40/5 instead of 50/5, with an error of 10-8/8 times 1=25%, which is overcounting by 25%.

The LMZ-0.5 wearing-core CT current transformer has the advantage of a sensitive transformation ratio, but some novice electricians are prone to the above error, so we remind everyone to improve their level of expertise.

Precautions for Using Current Transformers

The secondary side of the CT current transformer must not be open-circuited during operation. Once the secondary side is open-circuited, the iron loss is too large, the temperature is too high and the transformer burns out, or the voltage of the secondary winding rises and the insulation is broken down, causing the danger of high-voltage electric shock.

Therefore, when changing the metering instrument, such as swapping current meters, active meters, reactive meters, etc., the current loop should be shorted first before exchanging the metering instrument.

After the meter is adjusted, it should be connected to the secondary loop first, and then the short-circuit wire should be removed to check whether the meter is normal. If sparks are found when the short circuit wire is removed, the CT current transformer is open-circuited at this time. It should be immediately short-circuited again. After confirming that there is no open circuit in the metering circuit, the short-circuit wire can be removed again.

When carrying out the operation of disconnecting the short-circuit wire of the CT current transformer, stand on an insulating mat, and also consider the protection equipment of the CT current transformer circuit being shut down. After the work is finished, the protection equipment can be put into operation. When the insulation resistance of the secondary winding of the CT current transformer is lower than 10-20 megaohms, it is necessary to carry out boring treatment to restore the insulation before use.

One end of the secondary side of the CT current transformer and the housing must be reliably grounded.

If the CT current transformer has humming sound, check whether the internal iron core is loose, and the iron core bolts can be tightened.

The secondary side open-circuit and grounding of the current transformers:

The secondary side must not be open-circuited during operation;
One end of its secondary side must be grounded;
When connecting the current transformers, pay attention to the polarity of its terminals.

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