TERMINALS AND LEADS
Insulated copper bars or rods are used for the connections to the windings. High electric stress and corona at bends and corners occur in high-voltage transformers. Therefore, the shape and size of leads are most important in high-voltage transformers. Sharp edges and corners should be avoided to reduce dielectric stress and corona.
Transformers are connected to high-voltage lines. Extreme care is required to prevent flashover from the high-voltage connection to earthed bank. Bushings are used to insulate and bring out terminals of the winding from the container to the external circuit. For transformers upto 33 kV, this is achieved by using bushings of porceling around the conductor at the point of entry. For transformers above 33 kV, either oil-filled or capacitor-type bushings are used.
By changing the turns ratio of the transformer, we can easily control the voltage supplied to power networks by the transformer. To affect a change in the ratio of transformation, we provide tapping at different places in the windings of the transformer. Therefore, it is possible to get different turns ratio and thus different voltages at different tappings. Figure 1.16 shows the tapping used in a transformer.
For a three-phase, 11,000/400 V distribution transformer, there is always tapping on the high-voltage winding. If 11,000 V supply is given on the high-voltage side, 400 V is obtained on the low-voltage side. This tapping is known as principal tapping. The tapping at which the number of turns included is more than or less than the number of turns included at principal tapping is known as positive and negative tapping, respectively.