The alternator armature winding must be protected from earth faults. The earth fault protection scheme for an alternator whose armature winding is connected in a delta is more complicated as neutral is not available in the case of a delta-connected armature. An open delta transformer is required to be installed to detect the earth fault in the delta-connected armature winding.
For the star-connected armature, the neutral is formed where one end of all three winding is connected. The neutral point is connected to the neutral ground resistor. Under balanced load conditions, when the current in all the phases is equal, no current flows through the neutral.
In case of unbalance in the phase current, the unbalance current flows from neutral to earth through a neutral grounding resistor. The CT mounted for measuring the current flowing from the neutral to the earth point senses the current under earth fault conditions, and the earth fault relay gets tripped with earth fault. Moreover, the neutral grounding resistor limits the fault current to the rated current of the alternator. If the alternator armature is star-connected, it is easy to form the neutral, and the single-phase supply can be easily obtained. In the case of the delta-connected armature, to derive the single-phase supply, a transformer of vector group Dyn11 needs to be installed for a single-phase supply.
If the armature of an alternator is connected in a delta, the line voltage appears across the armature winding.
Therefore, more insulation is required for armature winding. If the armature is connected in a star, the voltage across the armature winding is 58 % of the line voltage and, thus, requires less winding insulation.
The insulation requirement with star-connected armature gets reduced by 42 % in comparison to the delta-connected armature winding.
To summarize,the reasons of armature connection of the alternator in the star are as follows.
1. In star connection, neutral is formed, which can be used to derive the single-phase supply. Also, the neutral facilitates ground fault protection of the alternator.
2. In the star connection, the phase-to-neutral voltage is about 58 % of the phase-to-phase or line voltage. Thus, the insulation requirement is reduced by 42%. The number of turns also reduces by 42 % of the number of turns in the delta connection.
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