The current that flows in the primary winding of a transformer when it's secondary is open is only the current that supplies for no load losses and setting up the magnetic flux in the core, hence the name, no load current.

Current Flow in Three-Phase Transformer Primary with Open Secondary
When the secondary of a three-phase transformer is open, no current flows in the secondary winding. However, in the primary winding, a small amount of magnetizing current called the exciting current flows. This current is necessary to establish the magnetic field in the transformer core.

Function of Exciting Current
The function of the exciting current is to magnetize the core of the transformer. This magnetic field allows the transformer to transfer energy efficiently from the primary winding to the secondary winding when a load is connected.

Equivalent Circuit of Single Phase Transformers
In the equivalent circuit of a single-phase transformer, the exciting current is represented by the magnetizing branch. This branch includes the magnetizing inductance and resistance, which account for the magnetizing current flowing in the primary winding when the secondary is open.

Explanation of Equivalent Circuit
The magnetizing inductance represents the ability of the transformer to store energy in the magnetic field, while the resistance accounts for the losses associated with the magnetizing current. Together, they form the magnetizing branch of the equivalent circuit.
In summary, when the secondary of a three-phase transformer is open, a small magnetizing current flows in the primary winding to establish the magnetic field in the transformer core. This current is essential for the efficient operation of the transformer when a load is connected. The equivalent circuit of a single-phase transformer includes the magnetizing branch, which represents the magnetizing current and losses in the transformer core.

Open circuit