Transformer bushing CTs

A transformer bushing CT (BCT) is a window-type current transformer mounted around the bushing’s flange.  They may be inside the main tank (under the cover) or mounted externally. A bushing type CT consists only of a toroidal-shaped core with a secondary winding. The bushing’s centre conductor forms the single turn primary of the BCT.  More than one ratio is provided by tapping the secondary winding at multiple increments along the winding.  The secondary winding terminals of the BCT (and the BCT’s nameplate) are accessible in the transformer’s control cabinet. 

BCTs are typically used for protective relaying purposes. Whether used for protection or metering, the saturation of a CT is undesirable.  When operating in saturation, the secondary output current is no longer a miniaturized replica of the primary current but rather a distorted version with a lower than expected amplitude.  This may result in the possible misoperation of the protection system. 

If used for protective purposes, a current transformer is designed to saturate at extremely high current levels so that the transformer bushing CT can successfully perform its important intended function of accurately capturing (high) fault current information. However, if the CT has excessive residual magnetism, it will saturate sooner than expected. Remanence flux is dissipated very little in service and requires demagnetization of the core to remove.

Testing is important to verify a BCT’s accuracy and performance characteristics, to validate that a very low level of remanence flux exists in the BCT, and to demagnetize its core if not.

Bushing CTs diagnosis