Most reactor faults begin as low-grade turn-to-turn faults (i.e., with one turn or very few turns shorted) and, when undetected and uncleared, may involve more turns or evolve into phase-to-ground faults or even phase-to-phase faults. Detecting and clearing a reactor fault quickly is essential in limiting damage, especially in oil-immersed reactors with possible fire hazards. This paper evaluates the limits of shunt reactor protection by considering the magnetic and physical characteristic differences between air-core and iron-core reactors. For turn-to-turn faults in air-core reactors, the magnetic flux generated by the faulted turns may only partially couple with the healthy turns. In contrast, the core in iron-core reactors substantially couples the magnetic flux between the faulted turns and the healthy turns. Because of the lower coupling in air-core reactors versus iron-core reactors, the fault current is reduced, which consequently lowers protection sensitivity. On the other hand, the saturation characteristics of an iron-core reactor result in inrush currents that have sequence quantities, which impact protection security. For turn-to-turn fault protection, the paper discusses the use of, and presents settings guidelines for, a new scheme using directional sequence overcurrent elements. Due to the large number of turns in an extra-high-voltage (EHV) reactor, it may not be possible to detect faults involving a single turn. However, the paper presents equations and tools that approximate the percentage of shorted turns that can be detected. Based on the reactor model and protection scheme settings, turn-to-turn faults can be cleared within a few cycles with sensitivities of around 0.1% for iron-core reactors and 0.2% for air-core reactors. The paper provides CT selection criteria, primarily driven by the sensitivity requirements for turn-to-turn faults. Settings guidelines to ensure protection security during reactor energization are also provided. A novel method for protection security during line de-energization is presented. The use of special timers to improve dependability for intermittent ground faults on an ungrounded transformer tertiary bus connected reactor is also discussed. The paper uses a new electromagnetic transient (EMT) model and field events of reactor inrush and turn-to-turn faults to demonstrate the security and dependability of the protection schemes discussed. These schemes are comprehensive, providing optimal protection for phase-to-phase, phase-to-ground, and turn-to-turn faults for different reactor applications. They have been applied to protect an air-core reactor at a utility, Avista, with great field experience.
Ritwik Chowdhury and Normann Fischer, Schweitzer Engineering Laboratories, Inc. Douglas Taylor, Avista Utilities David Caverly, Trench Limited Ali B. Dehkordi, RTDS Technologies Inc., presented at the 49th Annual Western Protective Relay Conference, October 2022.