Evaluating the Precision of the DC Decay Test Method for Characterizing Wound-Field Synchronous Machines in Power Plants

The sudden short-circuit test is the gold standard for determining the equivalent diagram of wound-field synchronous machines (WFSMs). Only a single measurement is needed while the machine is in saturated mode. However, this destructive test can significantly reduce the lifetime of the stator winding. Moreover, determining the equivalent diagram in the q-axis is more complicated and is often not performed. DC-decay tests are low-power alternatives that allow for the determination of the equivalent diagram in both axes without damaging the machinery. Until recently, they required the rotor to be aligned with the axes, which is not feasible in large power plants. A recent breakthrough eliminated the need for rotor alignment by proposing a DC-decay test that can be performed with the rotor in any position. However, numerous independent measurements are needed to obtain the equivalent diagram in both axes. This paper addresses the question of the minimum number of independent measurements needed for the DC-decay test to be practical for industrial use. Around 10 measurements are sufficient for reasonable precision, while 5 of high quality are sufficient using the symmetry of the root locus. Finally, a comparison against the short-circuit test shows that the DC-decay test is a valid alternative.