Practical EHV reactor protection

Shunt reactors are applied to long, high-voltage transmission lines to offset the impact of line charging capacitance to prevent high voltage during lightly loaded conditions. Shunt reactors are becoming more prevalent associated with the construction of long transmission lines to connect remote wind energy sources to load centers. During conditions when wind generation is at a minimum, the system is more likely to need reactive compensation to control high voltage on the transmission system. It can be difficult to design a protection system that provides adequate sensitivity to the extremely low levels of fault current for in-zone reactor faults, especially turn-to-turn faults, while remaining dependable for high-level terminal faults that are not limited by the impedance of the reactor. Current transformer (CT) selection criteria must balance sensitivity with performance during switching and internal faults. Often, standard equipment ratings put further restrictions on CT sizing selection. This paper discusses the different reactor types currently used, their characteristics, CT selection and performance issues, and different types of reactor faults. The paper also provides guidelines to practicing engineers to evaluate reactor protection design and determine protection elements and relay settings for a high-voltage transmission line shunt reactor. The discussion of various factors that affect the sensitivity and dependability of the protection elements helps in understanding the challenges encountered while determining relay settings. Detailed examples of CT performance calculations and guidelines are provided for protection element settings calculations.