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