What Is a Current Transformer?

The current transformer ^[1] is an instrument that converts a large current on the primary side into a small current on the secondary side according to the principle of electromagnetic induction. The current transformer is composed of a closed core and a winding. Its primary windings have very few turns and are connected in series with the current to be measured.

Current

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I. Current transformer model ^[3]

Common faults of current transformers are often related to manufacturing defects, as follows:

1) The insulation of current transformers is very thick, and some insulations are loosely wrapped, and there are wrinkles between the insulation layers. In addition, the vacuum treatment is not good, and impregnation results in a gas-containing cavity, which may easily cause partial discharge failures ^[12] .

2) The size and arrangement of the capacitive screen do not meet the design requirements,

Statistics of the causes of current transformer failure ^[13]

Even less capacitive screen, the capacitor plate is not smooth and flat, or even misplaced or broken, making its voltage equalization characteristics damaged. Therefore, when the electric field strength along the surface of the local solid insulation reaches a certain value, a partial discharge will be caused.

The direct consequence of the above partial discharge is that the insulating oil is cracked, a large amount of x wax is generated between the insulating layers, and the dielectric loss is increased. This discharge has a cumulative effect. If it is allowed to continue, the analysis of gas in oil may have the characteristics of arc discharge.

3) As the insulating material is not clean or contains high humidity, creeping discharge may occur on its surface. This situation is more common when the terminal leads are discharged along the surface of the pad.

4) Loose connections or floating potentials of metal parts will cause spark discharge. For example, the primary winding support nut is loosened, which causes the primary shielding shielding aluminum foil potential to float, the terminal screen leads to poor contact or welding, or even broken wires.

5)-Secondary connection clamps, bolts, and nuts are loose, ground screen ground nuts are loose, and tap tightening nuts are loose, etc., all may increase the contact resistance and cause local overheating failure. In addition, improper site maintenance management should also be taken seriously. For example, the water and moisture of the transformer may be related to the sealing structure and sealing materials of the manufacturing plant, but also have maintenance and management problems. Generally speaking, insufficient vacuum degassing or vacuum drying is not performed during maintenance, which causes the dissolved gas in the oil to saturate easily, or residual bubbles and high moisture content in the oil-paper insulation. All these will leave security risks to the equipment. ^[13]