Current transformers

Coil current transformers 

     Reel-to-reel transformers current transformers are the simplest, and belong to the oldest types of current transformers, developed on the basis of power transformer designs. The primary and secondary windings are made in the form of coils wound on the corresponding insulating frames. Coil current transformers are very compact and, due to the possibility of mechanization of winding operations, are cheap, but they have a number of disadvantages.
First, due to the weakness of the coil insulation, the discharge voltage of such transformers is very low. Because of this, this design is used only for low rated voltages (0.5 ... 3, kV) with reduced requirements for dielectric strength. An increase in the discharge voltage in coil current transformers is achieved primarily due to a slight increase in the core window, with the primary winding moving away from the inner surface of the core window. A U-shaped barrier made of some kind of insulating material is sometimes inserted into the gap between the primary coil and the inner surface of the core window.

Bushing current transformers

    These current transformers find the widest application in switchgears for 6 ... 35, kV. The bushing design is of particular value in this case, since in closed switchgears the ability to "pass" the current transformer through the ceiling or through the wall saves the corresponding bushing. The bushing multi-turn current transformer has two bushing insulators as a base, fastened in the middle part.
    Through the inner cavities of the bushing insulators, as many turns of the primary winding are pulled as necessary to achieve the calculated ampere-turns, providing the required class of apparatus. On the middle part of the bushings, under the grounded flange, there are cores with secondary windings, which are covered with a casing. Typically, the primary lead-in is located on the top head (in relation to the grounded flange).   



  Bushing rod current transformers 

  In rod current transformers, the primary winding passes through the core window only once. Therefore, the calculated number of ampere-turns here is always numerically equal to the rated current and cannot be increased. This is due to the specific feature of rod current transformers: the higher the current, the greater the accuracy of the apparatus, and the lower the current, the lower its accuracy. is reflected in the design of the apparatus as follows: the higher the current, the smaller the cross-section of the core, and the lower the current, the larger its cross-section. Since the diametrical dimensions of the core are usually constant for a given series of devices, the following design condition follows from the foregoing: at high rated primary currents, the axial length of the core is small; at low rated primary currents, as well as with an increase in the secondary load and an increase in the accuracy class, the axial length of the core increases.
   Rod current transformers can be made with both rectangular and round cores, but in most cases a round core (in principle with the shortest magnetic path) is the most appropriate.

              Busbar current transformers  

       Bus-type current transformers are called such transformers, the design of which includes cores with secondary windings and the main insulation according to a given nominal voltage, and the primary winding as a structural element is absent. In the main insulation of the transformer, a window is provided through which the switchgear bus is passed; it also performs the functions of the primary winding.
    Thus, busbar current transformers are in principle rod-type, with all the ensuing consequences. Only at low voltages, sometimes several turns of the conductor are passed through the core window, performing the functions of the primary winding, which already gives a multi-turn transformer design (by the way, this method makes it possible to obtain several transformation ratios on one device). However, such a system should be considered an exception to the general rule. Naturally, with a multi-turn design, not a bus is used as the primary winding, but an insulated flexible conductor.
    At high rated currents, the busbar current transformer circuit is particularly advantageous, since there is no need to connect the busbars of the switchgear to the primary winding of the current transformer.
    Thus, busbar current transformers are principally devices with high rated currents - from 2000, A and above. However, the simplicity and convenience of design sometimes prompts the use of busbar current transformers at lower rated currents.

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