Dimensioning CTs for small generator differential protection

Small generators are commonly installed to provide backup or standby power for critical facilities such as hospitals, data centers, communications centers, and the like. These generators are typically diesel generators 1 to 3 MVA in size, and are normally intended for use in applications for facilities at the 4kV to 15kV level. High-speed detection of internal electrical faults is typically provided by stator differential protection. Undesirable operation of the stator differential element due to unequal CT saturation during transformer energization or out-of-zone faults is well known. Protection engineers typically follow the guidelines outlined in ANSI C37.110 IEEE Guide of the Application of Current Transformers Used For Protective Relaying Purposes to dimension CTs for stator differential protection. The maximum accuracy class and turns ratio of the CT is typically limited by the physical dimensions of the switchgear and generator set. However, beyond the recommendations in C37.110, the protection engineer must understand, and be able to model, the performance of the entire stator differential system, including the differential relay, CTs, and CT secondary leads. This paper discusses methods to model the stator differential performance, based on the IEEE Power System Relaying Committee CT Saturation Calculator tool. The paper then goes on to suggest methods to use this tool in dimensioning CTs and CT secondary leads.