The Principle and Use of Through-core Current Transformers

1. The principle of through-core current transformers

The feed-through current transformer itself is not a primary winding, and the current-carrying (load current) wires from L1 to L2 pass through a circle (or other shape) made of silicon steel sheet rolling to play the role of a primary winding. The secondary winding is directly and evenly wound on the circular dead center, and is connected in series with the secondary loads of the current coils such as instruments, relays, and transmitters to form a closed loop.

Since the feedthrough current transformer does not have a primary winding, its transformation ratio is determined according to the number of turns that the primary winding passes through the dead center of the current transformer. , the larger the transformation ratio, the ratio of the rated current: in the formula I1——a rated current when one turn through the center; n——the number of turns through the center.

Compared with the common CT current transformer, the wiring of the through-core current transformer, the primary side passes through the P1 side of the transformer, and the P2 side comes out, and the secondary side wiring is the same as that of the common current transformer.

During the measurement, the circuit impedance of the secondary circuit of the current transformer is very small, so the secondary circuit of the current transformer is close to a short circuit, and in order to unify the standard, the rated value of the secondary winding is generally designed to be 5A. In practical applications, for safety consideration, the secondary side S2 of the current transformer generally needs to be grounded.

2. Correct use of through-core current transformers

Feed-through current transformer is a common electrical device with simple wiring and convenient installation. It is widely used in metering, testing and maintenance lines. If you are not careful during use, it will cause great errors, resulting in inaccurate measurement, maintenance failure, and electrical accidents. Therefore, you should pay attention to the use method.

The transformation ratio of the current transformer should meet the requirements of the load current, and the normal load current should be 1/3 to 2/3 of the primary rated current of the current transformer. A current transformer with an accuracy level of 0.2S or 0.5S should be selected.

When piercing the heart, the terminal marked "L1" or "+" in the primary circuit should be connected to the incoming power line, and the terminal marked "L2" or "-" should be connected to the outgoing wire. The terminal marked with "K1" or "+" in the secondary circuit should be connected with the incoming terminal of the current coil of the electric energy meter, and the terminal marked with "K2" or "-" should be connected with the outgoing terminal of the electric energy meter.

The key to the use of feed-through current transformers is that the number of turns of the feed-through must be correct, not only the conversion ratio indicated on the nameplate, but also the number of turns of the feed-through. The number of feed-through turns is equal to the ratio of the primary side current when the feed-through turns is one turn to the primary side current corresponding to the conversion ratio.

The terminal, shell and iron core of "K2" or "-" of the secondary circuit of the current transformer must be grounded stably.

Related Articles about Acrel

Understanding DC Digital Multifunction Meters: Features and Applications The Principles and Working of DC Current Shunts Usage Scenarios of Multi-Rate Multi-Function Energy Meter Working Principles and Wiring of Various Types of Electric Meters