- The modern electrical equipment's are normally designed to operate satisfactorily at a fixed voltage level.
- On the other hand voltage variation in power system is a normal phenomenon owing to the rapid growth in industries and distribution network.
- It is, therefore, very important to keep the consumer's terminal voltage within the statutory limits.
- The transformer output voltage and, so, the consumer's terminal voltage may be controlled by providing tapping's on the transformer windings (either on the. Primary or on the secondary).
- The principle of regulating the secondary voltage is based on the changing the number of turns on the primary or secondary i.e. on changing the ratio of transformation.
- Increase in Primary turn causes increase in emf per turn, and so in secondary output voltage.
- Secondary output voltage can also be increased by increasing secondary turns and keeping primary turns fixed.
- In other words, decrease in primary turns has the same effect as that of increase in secondary turns.
What is Principle Tapping ?
- There is always tapping on the HV winding which when connected to the rated voltage gives rated voltage on the lv side. This tapping is called the principle tapping.
What is positive tapping and negative tapping ?
- The number of turns included at other taps may be either larger or lesser than the number turns used at principle tapping.
- The tapping at which the turns included are more than that of principle tapping is called positive tapping, while a tapping at which the number of turns included is less than the number of turns included at principle tapping is called the negative tapping.
Why are distribution transformer provided with tap ?
- Standard Distribution transformers have taps arranged in 2 1/2 % steps so that rated secondary voltage can be obtained when the primary supply voltage is 0, 2 1/2, 5, 7 1/2 % below the nominal primary voltage ratting.
- Transformer used in unit substations often have tap taps arranged in 2 1/2 % steps may include some over voltage taps to accommodate primary Voltage above the nominal ratting.
- Taps in 2 1/2 % steps may be used on the transformers with automatic tap changing equipment.
- most of the transformer carry rated Kva on any over voltage tap and/or rated current on reduced voltage tap.
- The tapping which cause the change in turn ratio , may be placed either on the primary or on the secondary side.
- The choice between the two sides should be based on maintaining the Emf per turn constant, as far as possible.
- Decrease in primary turns (or increase in Emf per turn) Causes in increase in core flux, and therefore , saturation of the core , increase in core loss, increase in magnetizing current and pronounced third harmonic.
- On the other hand decrease in emf per turn result in reduction in core flux, and therefore reduction in core losses and poor utilization of the core.