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Frequency, voltage & interconnected system
The frequency of the generated emf for a p polar generator is given by f = p/2 n where n is speed of the generator in rps or f = p/120 n when n is in rpm. Frequency of the generated voltage is standardized to 50 HZ in our country and several European countries. In USA and Canada it is 60 Hz. The following table gives the rpm at which the generators with different number of poles are to be driven in order to generate 50 Hz voltage.
A modern power station has more than one generator and these generators are connected in parallel. Also there exist a large number of power stations spread over a region or a country. A regional power grid is created by interconnecting these stations through transmission lines. In other words, all the generators of different power stations, in a grid are in effect connected in parallel. One of the advantages of interconnection is obvious; suppose due to technical problem the generation of a plant becomes nil or less then, a portion of the demand of power in that area still can be made from the other power stations connected to the grid. One can thus avoid complete shut down of power in an area in case of technical problem in a particular station. It can be shown that in an interconnected system, with more number of generators connected in parallel, the system voltage and frequency tend to fixed values irrespective of degree of loading present in the system. This is another welcome advantage of inter connected system. Inter connected system however, is to be controlled and monitored carefully as they may give rise to instability leading to collapse of the system. All electrical appliances (fans, refrigerator, TV etc.) to be connected to A.C supply are therefore designed for a supply frequency of 50 Hz. Frequency is one of the parameters which decides the quality of the supply. It is the responsibility of electric supply company to see that frequency is maintained close to 50 Hz at the consumer premises. It was pointed out earlier that a maximum of few hundreds of volts (say about 600 to 700 V) could be developed in a D.C generator, the limitation is imposed primarily due to presence of commutator segments. In absence of commutators, present day generated voltage in alternator is much higher, typically around 10 kV to 15 kV. It can be shown that rms voltage induced in a coil is proportional to φ and n i.e., Ecoil ∝ φ n where φ is the flux per pole and n is speed of the alternator. This can be justified by intuition as well: we know that mere rotating a coil in absence of magnetic flux (φ) is not going to induce any voltage. Also presence of flux without any rotation will fail to induce any voltage as you require rate of change of flux linkage in a coil. To control the induced voltage one has to control the d.c field current as speed of the alternator gets fixed by frequency constrain.