The potential transformers are used for measurement of high voltages by means of low range voltmeters or for energizing the potential coil of wattmeter's and energy meters. These are also used for energizing relays and other protective devices.
The potential transformer operates on the same principle as the power transformer. Both primary and secondary winding are wound on a high grade steel-low voltage winding is kept next to the earth core and the high voltage winding is on the outside. The potential transformers are always Step down transformers , since they are meant for reducing the voltage to a reasonable operating value.
As a rule , the potential transformer are of the shell type because such a construction develop a high degree of accuracy.
The input voltage may be as high as 1,38,000V or higher even, and in this case the pressure exists between the primary terminals as they are connected across the supply system and not in series with it as in the case of a CT. The secondary circuit is extremely well insulated from the high voltage primary and that, in addition, it is grounded from the point of view of safety. Such grounding serves to protect the operator from the high voltage hazard.
Upto voltage of 5000 V PTs are usually of dry type, between 5000 V and 1,38,000 V, they may be either dry type or oil immersed type, although for voltage exceeding 1,38,000 they are always oil immersed type.
The output voltage of potential transformers has been standardizes at 110 V irrespective of the high input voltage. this comparatively low voltage has been chosen for safety and reduce the necessary insulation of the windings and terminal of the instrument. Further , as the voltage is standardized, the design of the voltage coil portion of the instrument may also be standardized.
Why is it not necessary to design potential transformer for heavy currents ?
In a step down power transformer, the primary or high voltage winding usually has a high voltage but of small current, while the secondary has heavy current at low voltage. The potential transformers, however differ in that currents in both windings are comparatively small. This is because the load on a potential transformer is an instrument having a high impedance(mostly resistance). since only a very small current is required to operate the potential coil of the instruments , so it is not necessary to design potential transformers for heavy currents. In other words PTs behave as ordinary step down two winding transformers operating on low load.
The very high input voltages necessitate a special arrangement of coils. for example, assuming that the high voltage was connected to 33,000 V supply, and the winding considered of two layers of turns, the first layers running from left to the right and the secondary layers from the right to the left, the voltage between first and the last turn would be 33,000V.
This pressure between adjacent turns would need very high insulation. For reducing the insulation and the risk of breakdown, high voltage winding are divided into sections. The voltage between two turns where there are two layers of turns, cannot exceed one eighth of 33,000V, as there are eight such a coils. The greater the number of layers per section. the lower the voltage between adjacent turns. Heavy insulation in the form of oil ducts is provided between the coils, between sections and between the layers.