Stator-Rotor Differential Protection For Wound-Rotor Generators

This paper addresses turn-fault protection for both synchronous generators and wound-rotor asynchronous generators, often referred to as doubly fed induction generators (DFIG). The paper presents a new method that allows protecting both the stator and the rotor. A wound-rotor asynchronous generator has a wye-connected three-phase rotor winding connected via slip rings to an ac/ac converter that controls the rotor currents and produces an electromagnetic field that rotates relative to the rotor. The rotating field in the rotor acts as a “gearbox” to match the synchronous velocity of the power system and the mechanical velocity of the rotor. Historically, this decoupling was key in wind-powered generation, and recently, it was proposed for optimizing pumped storage hydropower generating systems. Allowing the two fields to move with respect to each other is effectively introducing an “electrical spring” that reduces mechanical stresses in all applications. By selecting the rotor speed, we maximize turbine efficiency in wind-powered applications. The ability to select the rotor speed also maximizes turbine efficiency in the generating and pumping modes of pumped storage hydropower applications. Additionally, a wound-rotor asynchronous generator maintains synchronous operation over a wide range of rotor velocities. This property makes such a generator attractive for synchronous condenser applications because the generator can slow down significantly, sending more energy into the power system when needed. The rotor ac/ac converter allows smooth re-acceleration to the near-synchronous velocity after system events. Because the size of wound-rotor asynchronous generators will increase from a few MWs in wind-powered applications to hundreds of MWs in pumped storage hydropower applications, these machines will require comprehensive protection schemes, including protection against stator and rotor turn faults. Turn-fault protection of generators is challenging. Stator differential protection does not detect turn faults because it is based on Kirchhoff’s current balance between the currents at both ends of each phase winding. Without split-phase protection, turn faults must progress and evolve into phase or ground faults before the standard generator protection scheme can detect them. Protecting the rotor winding against turn faults is extremely challenging in both synchronous and asynchronous generators. In asynchronous applications, rotor differential protection is not possible because currents are measured only at one end of the rotor winding. As a result, not only is the rotor not protected against turn faults, but it is also not protected against phase faults. Overcurrent elements can provide some protection but with low sensitivity, and the very low and variable frequency of the rotor currents creates additional complications. This paper introduces and explains a new kind of differential protection element that balances ampere-turns between the stator windings and the rotating rotor windings. Because the differential element monitors ampere-turns, it is capable of detecting both turn and phase faults in both of the windings. The presented differential element uses only currents and does not require voltages or encoders for rotor position and velocity. The differential element is instantaneous and can operate on the order of one to two power cycles.