Why generation side transformer are of delta type and distribution side transformer are of star type
Answers
A transmission line has two ends to it.
The source end (nearest the generator) will be connected to the secondary of a step-up transformer. This is most often star-connected, with the star point connected to ground through an earth fault monitor. The reason: It can detect and protect against faults due to grounding of a transmission line.
The load end (nearest the consumer) will be connected to the primary of a step-down transformer. This is most often delta connected. The reason: All practical transformers have a small non-linearity in the excitation curve of their iron cores. This will cause third harmonic currents to flow. This current will circulate in the delta connection and will not flow in the transmission line, which is a good thing. Otherwise it contributes to the RMS current in the line without carrying any power to the consumer.
This delta-star sequence in the transformers proceeds all the way from the generator to the final consumer. The drawing below shows a representative distribution system for Europe / Australasia. The North American system will be similar apart from voltages at various stages and the final consumer distribution system.