The correct answer is: A. primary is supplied rated voltage.
During an open circuit test, the transformer’s primary winding is connected to a rated voltage source, while the secondary winding is left open circuit. This causes a current to flow in the primary winding, which in turn creates a magnetic field. This magnetic field induces a voltage in the secondary winding, which can be measured. The open circuit test is used to determine the transformer’s magnetization current and its open circuit voltage.
Option B is incorrect because the primary is not supplied full-load current. The primary is only supplied a small amount of current, which is just enough to create the magnetic field necessary to induce a voltage in the secondary winding.
Option C is incorrect because the primary is not supplied current at reduced voltage. The primary is supplied rated voltage, which is the voltage that the transformer is designed to operate at.
Option D is incorrect because the primary is not supplied rated kVA. The kVA rating of a transformer is the product of its rated voltage and rated current. The open circuit test does not measure the transformer’s kVA rating.