What is a Transformer Bushing CT (BCT)

A transformer bushing current transformer (BCT) is a window-type device shaped like a donut (toroidal core). It is placed around the flange of a transformer bushing, either hidden inside the main tank or mounted on the outside.

The center conductor of the bushing acts as a single-turn primary winding, while the BCT itself holds the secondary winding. To give you different measurement ratios, the secondary winding is tapped at multiple points. You can easily access these terminal connections and the BCT nameplate right inside the transformer’s control cabinet.

Why Are Bushing CTs Important for Power Systems?

BCTs are mainly used to protect electrical systems by feeding accurate current data to protective relays.

For safety systems to work correctly, the BCT must not hit “saturation.” If a CT saturates, its output signal becomes distorted and drops lower than it should be. When this happens, the protective relays receive bad data, which can cause the whole protection system to fail or misoperate.

What Causes a Bushing CT to Fail or Saturate Early?

While BCTs are built to handle very high fault currents before they saturate, a hidden problem called residual magnetism (remanence flux) can ruin their performance.

If a BCT traps too much residual magnetism inside its core, it will saturate much faster than expected during a power fault. This trapped magnetic force does not go away on its own during normal operation; it requires specialized demagnetization to clear it out.

How Do You Test and Diagnose a Bushing CT?

Regular maintenance testing is the only way to ensure your BCT accurately captures dangerous fault currents and is free of trapped magnetism.

Standard diagnostic tests include:

• Ratio and Phase Analysis: Checks the overall measurement accuracy of the CT.

• Saturation Test: Finds the point where the CT can no longer replicate the primary current.

• Residual Magnetization & Demagnetization: Measures trapped magnetic flux and clears it from the core.

• Winding Resistance: Checks the physical condition of the secondary coils.

• CT Burden Test: Ensures the connected relay circuit doesn’t overload the CT.

FAQs: Frequently Asked Questions About Transformer BCTs

Q1: Can a transformer bushing CT be mounted outside the tank?

A: Yes. Depending on the transformer’s design, BCTs can be installed internally under the main tank cover or mounted externally around the bushing.

Q2: What happens if a bushing current transformer saturates?

A: When a BCT saturates, its secondary output current becomes a distorted version of the real current with a lower amplitude. This can trick protective relays into failing to trip during a real electrical fault.

Q3: How do you remove residual magnetism from a CT core?

A: Residual magnetism does not dissipate on its own during normal service. It can only be removed by performing a formal demagnetization process on the core during routine testing.