As you know, open and short circuit tests are done for generator saturation reactance calculation. Also open circuit voltage-ampere curve of generators is non liner; therefore those mentioned test shall be done at rated voltage as nearest point to machine saturation voltage. For more information about those basic conceptions please note to following description:
The following circuit diagrams illustrate the per phase equivalent circuits of a round rotor synchronous machine in the generator mode.
In the per phase equivalent circuit model illustrated above, there are three parameters need to be determined: winding resistance Ra, synchronous reactance Xs, and induced emf in the phase winding Ea. The phase winding resistance Ra can be determined by measuring DC resistance of the winding using volt-ampere method, while the synchronous reactance and the induced emf can be determined by the open circuit and short circuit tests.
Open Circuit Test
Drive the synchronous machine at the synchronous speed using a prime mover when the stator windings are open circuited. Vary the rotor winding current, and measure stator winding terminal voltage. The relationship between the stator winding terminal voltage and the rotor field current obtained by the open circuit test is known as the open circuit characteristic of the synchronous machine.
Short Circuit Test
Reduce the field current to a minimum, using the field rheostat, and then open the field supply circuit breaker. Short the stator terminals of the machine together through three ammeters; Close the field circuit breaker; and raise the field current to the value noted in the open circuit test at which the open circuit terminal voltage equals the rated voltage, while maintain the synchronous speed. Record the three stator currents. (This test should be carried out quickly since the stator currents may be greater than the rated value).
Under the assumptions that the synchronous reactance Xs and the induced emf Ea have the same values in both the open and short circuit tests, and that Xs >> Ra, we have
For some machines, the short circuit current is too high if the machine is driven at the synchronous speed. In this case, short circuit test can be performed at a reduced speed say half synchronous speed nsyn
/2 or frated
/2. Since Ea ∞ f, the induced emf in the short circuit test is halved. Thus