A bar type current transformer is a current transformer in which the primary winding consists of a solid conductor or busbar permanently built into the CT assembly. This primary bar acts as a single-turn primary winding, while the secondary winding is wound around a laminated magnetic core that surrounds the bar. The bar-type construction is one of the most robust and mechanically durable CT designs available, making it particularly well suited for high-current and high-fault-level applications.
The defining characteristic of a bar type CT is that the primary conductor is an integral part of the transformer itself rather than a separate cable or busbar that passes through a window. The primary bar is typically made of copper with a circular or rectangular cross-section sized to carry the rated primary current and to withstand the thermal and mechanical stresses of short-circuit events. The magnetic core-usually laminated silicon steel-is assembled around the bar, and the secondary winding is wound on the core to produce a current proportional to the primary current.
Because the primary bar is permanently built into the CT, bar-type transformers are factory-assembled and tested as complete units. This ensures precise control over the turns ratio and magnetic characteristics, resulting in excellent accuracy and repeatability. Bar-type CTs are commonly specified in accuracy classes ranging from 0.2 to 1 for metering and 5P or 10P for protection applications. The robust construction also allows bar-type CTs to be used in outdoor and harsh environments when properly encapsulated or housed in weather-resistant enclosures.
Bar type CTs are primarily used in heavy-duty, high-current applications. They are commonly found in utility substations, generator output circuits, large industrial switchgear assemblies, and power distribution systems where the primary current may range from several hundred to several thousand amperes. The ability of the primary bar to handle high continuous currents and to withstand the electromagnetic forces generated during short-circuit events makes bar-type CTs a reliable choice for critical infrastructure.
Installation of bar-type CTs requires that the primary circuit be de-energized and the bar be connected in series with the power circuit. This means that bar-type CTs are typically installed during new construction or during planned maintenance outages rather than in live retrofit situations. The primary bar must be properly sized and connected to the circuit conductors, and the secondary wiring must be routed to the metering or protection devices.
From a design perspective, bar-type CTs offer several advantages over wound or window types. The fixed primary bar eliminates the variability associated with field-installed primary conductors, ensuring consistent performance from unit to unit. The robust construction provides excellent mechanical stability and resistance to vibration, making bar-type CTs suitable for applications where mechanical stress is a concern. However, bar-type CTs are generally bulkier and heavier than window or split-core types, requiring more mounting space and structural support.
In summary, the bar type current transformer is a reliable and robust solution for high-current measurement and protection. Its permanent primary bar provides excellent mechanical and thermal performance, making it a preferred choice for utility and industrial applications where reliability and durability are paramount.