Using the multi-loop fault analysis method for setting and evaluating generator protection elements

Stator winding interturn, interbranch, and series faults can result in large circulating currents in the faulted coils. Generator protection elements may not be sensitive enough to detect these fault conditions until the fault evolves into a phase-to-phase or phase-to-ground fault. Large machines have been severely damaged by delayed or failed protection system operation. Determining fault quantities for the various possible internal faults is not trivial and requires the aid of numerical models. Protection element models can then be used to determine the protection coverage provided by these elements. Certain machine modeling methods are useful for analyzing external faults or power system transients but are not appropriate for analyzing internal faults. The most commonly used machine models use dq0 transformation and assume an ideal equivalent model of the machine derived from its normal operating mode using lumped winding parameters. Consequently, these models ignore the effect of the strong harmonics that result from the internal machine asymmetry during internal faults. Alternate methods, such as symmetrical component analysis or phase-coordinate methods, are simplified models that introduce large errors during internal asymmetric conditions. The multi-loop method treats a machine as a set of loops in relative motion. The method involves a permeance analysis of the machine to calculate the time-variant electric parameters of the stator branches and rotor loops. The stator branches (including fault branches) are converted to loops via a transformation matrix corresponding to the state of the machine. The model is then solved using a numerical method. Because the multi-loop method uses machine geometry and winding design information, it preserves the harmonics that result from internal faults. The transformation matrix provides a simple and intuitive mechanism to apply internal faults in the fractional winding. In this paper, we validate the multi-loop method using test data from a scale-model machine in a lab. We then use the fault quantities obtained from the multi-loop method to determine the sensitivity and coverage provided by various generator internal-fault protection algorithms for the lab machine.