Overcurrent performance of bushing-type current transformers

G. B. Dodds (Duquesne Light Company, Pittsburgh, Pa.): Aside from the calculation of ratio curves and determination of leakage currents, there are other uses of excitation curves. One of the most useful and easiest to use is a quick method of determining the approximate range over which the current transformer will perform without any great deviation from normal ratio for any given combination of burden and secondary currents. This is covered in appendix III, item c (linear performance range). For example, in figure 1 of this discussion are shown excitation curves of various types of current transformers, including both wound primary and through type. Ratio curves of the same current transformer at burdens of four ohms and two ohms respectively are shown on figures 2 and 3. It can be observed that overcurrent ratio performance of various types of current transformers is in the same order as the excitation curves shown in figure 1; that is, the current transformers having the lowest voltage excitation curves perform the poorest during heavy overcurrents.