Bus Coupler:
It is a breaker which connects two different buses.
Purpose of bus coupler in double bus bar system:
It can allow the power transfer from one bus to another bus if surplus power is available in one bus.The bus coupler is always on. In case of failure of one bus the bus coupler must be made off to isolate the faulty bus.
Purpose of bus coupler in switchgear:
Bus coupler is using with two incoming buses. In this case under normal conditions the bus coupler is made off. In case of failure of any incoming bus the bus coupler is turn on for charging that portion from the healthy incomer.
Interconnecting Transformer:
It is a transformer which connects two different substations through buses which are in same premises.
It interconnects two different system voltage levels. For example 132 kV to 66 kV, 220 kV to 132 kV and 400kV to 220kV and so on.
Purpose:
In case of failure of supply to any one bus, to maintain continuity of supply, it takes power from another bus to which it is interconnected.
This transformer is an auto-transformer and it is bi-directional.