For ∆-Y power transformer Y-∆ CT connection is used. The Low voltage side is connected is connected with Current transformers that are star connected . Why this cross connection is done and how it prevents from operating on external faults and zero sequence current.
Power transformer gives a nominal voltage depending upon the rating of the power transformer. The differential relay detected the differences in the current detected by the current transformers. It is connected to both sides of the power transformer so any high power arise from the overcurrent on either sides of the power transformer windings can be detected by the differential protection relay with a stable nominal voltage. Any current fluctuation through the power transformer is detected by the differential current circuit protection relay.
Due to the fact that there is a 30 degrees phase shift between the delta and Y connections. That arrangement compensates the phase shifting.
To compensate the phase balance of 30 degrees between star and delta connections. But the present digital protective relays will do everything on their own.
In digital relay, just Feed type of transformer connections and group. Rest the relay will do it.
Dear syed taha Ahmed
you can see the attach file for more details
https://www.sciencedirect.com/topics/engineering/current-transformer
Article Transformer Monitoring and Protection in Dynamic Power Syste...
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