Severe Saturation of Low-Ratio CTs
Well-established engineering practice exists for CT selection to ensure saturation freeoperation
of protection CTs at a given short circuit level, CT burden, X/R ratio and assumed
residual flux. In the context of this paper, it is assumed that this engineering technique is not
applied, and severe saturation will occur for short circuits within the protected zone (motor,
feeder, cable or bus).
Analytical analysis of a saturated CT is not practical. Only “time to saturation” may be
approximated with relative ease, and is used in some protection applications. More detailed
analytical analysis is not in the realm of practical engineering.
Computer simulations are the only efficient way to extract the required information on
secondary signals. These are burdensome for everyday engineering in the industrial domain.
This paper uses computer simulation to derive simple and practical analysis and engineering
charts to address the problem.