BEHAVIOR OF BUSBAR DIFFERENTIAL RELAYS WITH AIR-GAP CORE CURRENT TRANSFORMERS
Protective relays normally estimate the magnitude and phase angle of current. Since the level of current is normally too high to permit a direct connection to the power system, a Current Transformer (CT) is used to scale down the current value. A CT should faithfully replicate waveform of the primary current. Power utilities normally use air-gap core and solid-core CTs. Air-gap core CTs are expensive and hard to maintain but they are able to reproduce the primary current without becoming saturated unlike solid-core CTs. Nowadays relay manufacturers claim that no matter what type of CT is used in the power grid, protective relays can intelligently sense a saturated waveform produced by a CT, and facilitates a correct decision, based on a unique algorithm. Therefore, continued use of the air-gap core CTs is being questioned. To verify relay manufacturer’s claim, behavior of protective relays when subjected to solid-core and air-gap core CT output waveforms need to be analyzed. Therefore, a mathematical model for both CT types is needed. Output waveform of a solid-core CT is already simulated by IEEE Power System Relaying Committee. In this thesis, a mathematical model of an air-gap core CT is developed and simulated on the Excel platform. Output waveform of the proposed model is then verified using the IEEE PSRC CT Simulator. Then, two commercially available busbar differential relays with CT saturation detection logic were subjected to the output waveforms of solid-core and air- gap core CTs. After testing about hundred scenarios on each relay, it is concluded that first, there is no difference in the relays’ performances for internal faults. Besides, for external faults, an air-gap core CT renders the trip output less sensitive to the relay setting. Further, the relay might be considerably slow in operation for an evolving fault, if a solid-core CT is involved.
Instrument current transformer, air-gap core, remanence, saturation, differential protection, dynamic simulation, RTDS
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