Fundamentals and improvements for directional relays

Phase and ground directional elements are relied on for fast and secure protection throughout the power system. Although directional relays have been applied successfully for many years, several new and unique applications and power system disturbances present challenges. Using field and laboratory data, this paper reviews fundamentals, discusses the limits to sensitivity, and shows how and why directional element designs have progressed. The paper also describes how directional elements are applied during loss of voltage conditions. In addition to design basics, we show several practical field examples that illustrate problems and solutions, while providing guidance on applying and setting modern directional relays.