CT Bushing

Bushing CT’s

Applications Instrument transformer Bushing CT’s
Bushing CT’s
A bushing CT (BCT) is a window-type current transformer mounted around the bushing’s flange. They exist both on dead-tank circuit breakers and on power transformers. On the later 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 on the asset nameplate.

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 its circuits diagnosis



Ratio/phase analysis; an assessment of the CT’s accuracy

Saturation test; identification of core type; protection or meter

Residual magnetisation/demagnetisation; a test to determine the level of residual magnetism in the CT and to demagnetize

Winding resistance; used for calculations of saturation voltage and accuracy with specific burden

CT burden test; ensures that burden is within the CT’s VA limits

A bushing current transformer is a window-type current transformer. It is mounted around the bushing’s flange. It may be either inside the main tank or mounted externally. A bushing type current transformer contains only a steroidal-shaped core with a secondary winding. The role of the bushing’s center is to form a single turn primary of the BCT.

More than one ratio can be provided by tapping the secondary winding at multiple increments the winding.
The secondary winding terminals of the transformer can be accessed in the control cabinet of the transformer.
Bushing Current Transformers are used for protective relaying purposes.
A current transformer is made for saturating at very high current levels. The reason behind this is to allow the bushing current transformer to perform its required function of accurately capturing (high) fault current information. Nevertheless, if the current transformer has excessive residual magnetism, it can saturate sooner than expected.

Use of bushing current transformer

The bushing current transformer is designed to be used to relay and meter on existing power transformers or circuit breakers. Bushing is an important component when it comes to using transformers to withstand mechanical, voltage and environmental stresses while making external input and output connections of transformers.

It is always advisable to keep the sensing equipment (CT) as close as possible to the transformer that needs to be protected. It allows preventing the reactance or losses that would generally come in the picture if CT’s were connected to the transformer by an overhead conductor or tubular bush bars.

Bushing transformers are made of insulators depending on the amount of voltage being transmitted. A disc of bushing can provide insulation of about 11KV. They are also used for preventing direct contact between the tank of the transformer and the bud. Direct contact between these two makes the corrugated tank of the transformer changed which leads to causing electrocution or damaging the transformer itself since heat will be generated. This generated heat can also degrade the transformer oil insulation property. In addition, bushings have the quality to provide a conduction path for current from the transformer windings to the transmission lines.

Below are some of the benefits of 11 KV Double Bushing Current Transformer

It reduces the risk of fire
The oil leakage is stopped
Mechanically rigid design
They do not need oil supervision
Reduces the downtime in case of major failures
Lightweight and compact
Easy transport and stored
Option to install at any angle
Can be energized soon after the installation