Understanding the Bushing CT of a Transformer: Sizing, Operation, and Applications

A bushing current transformer (CT) is a toroidal window-type sensor that slips over the grounded flange of a high-voltage transformer bushing to step down primary current for metering and protective relaying. In a real substation installation, it changes the physical footprint and cost structure by eliminating the need for separate, freestanding CT pedestals, utilizing the bushing's existing insulation to handle the high-voltage isolation. Beginners commonly confuse the bushing CT with the bushing itself or with standalone wound-type CTs; remember that the bushing is just the insulated conductor path, while the bushing CT is the measurement ring wrapped around its base.

How a Bushing CT Works Inside the Flange

Unlike a wound-type CT where the primary and secondary windings are physically wrapped around a shared core, a bushing CT is a ring-core (toroidal) device. The transformer's main high-voltage lead acts as the single-turn primary winding. The bushing CT's secondary winding consists of hundreds or thousands of turns of fine wire wrapped around a grain-oriented silicon steel or nanocrystalline core.

This toroidal core is housed in a weatherproof, oil-filled, or epoxy-sealed compartment located precisely at the grounded base flange of the transformer bushing. Because the flange is at ground potential, the bushing CT only needs to be insulated for low voltage (typically 600V class) on its secondary side, while the primary high-voltage insulation is entirely handled by the porcelain or polymer bushing itself.

A critical mechanical feature of the bushing CT installation is the shorting test switch. Because the secondary terminals are often routed to a terminal box on the side of the transformer, opening that secondary circuit while the transformer is energized is lethal. The test switch is wired to automatically short the CT secondary leads the moment the terminal box cover is removed or the test plug is pulled, protecting technicians from high-voltage transients.

Worked Numeric Example: Sizing a 15kV Bushing CT

To understand how these are selected in practice, let us size a bushing CT for a standard 10 MVA, 13.8 kV distribution transformer feeding a substation bus. We need to ensure the CT will accurately measure full load current without saturating during a fault.

Step 1: Calculate Primary Full Load Amps (FLA)
Using the three-phase power formula: I = S / (√3 × V)
I = 10,000,000 VA / (1.732 × 13,800 V) = 418.4 A

Step 2: Select the CT Ratio
We select a standard 600/5A bushing CT. This gives us roughly 25% overhead above the 418.4 A full load current.

Step 3: Calculate Secondary Current at Full Load
I_secondary = 418.4 A × (5 / 600) = 3.486 A
This is well within the continuous thermal rating of a standard 5A secondary relay.

Step 4: Verify Accuracy Class Against Fault Burden
Assume a fault occurs on the bus, pushing primary current to 12,000 A (roughly 28x FLA). The secondary current attempts to reach 100 A. The connected relay, wires, and test switch present a total burden of 1.5 Ω.
Required secondary voltage to drive this current: V = I × R = 100 A × 1.5 Ω = 150 V.
If we specify a C400 accuracy class bushing CT (meaning it can deliver up to 400V to a standard burden without exceeding 10% ratio error), our required 150V is well below the 400V knee-point. The CT will not saturate, and the protective relay will see the exact fault current to trip the upstream breaker.

Where You Meet This in Practice

You will rarely encounter a bushing CT of a transformer in residential or light commercial work; they are the domain of medium and high-voltage power systems. Here is where they appear on the grid:

• Pad-Mounted Distribution Transformers: In dead-front padmount transformers (the green boxes in commercial parking lots), bushing CTs are often slipped over the 15kV or 25kV class bushings inside the oil-filled compartment to feed utility revenue metering.
• Generator Step-Up (GSU) Transformers: Power plants use massive GSU transformers to step voltage up to the transmission grid. Bushing CTs on the high-voltage side provide differential protection, comparing current entering the transformer to current leaving it to detect internal winding faults.
• High-Voltage Circuit Breakers: While technically a 'breaker bushing CT', the exact same toroidal technology is mounted on the grounded flanges of SF6 or oil-filled circuit breaker bushings in switchyards, eliminating the need for separate freestanding CT towers.

Bushing CTs vs. Standalone Freestanding CTs

When designing a substation, engineers must choose between integrating CTs into the equipment bushings or buying standalone pedestal-mounted CTs. Here is how they compare across critical installation criteria, as outlined in standard substation design practices aligned with IEEE C57.13 Instrument Transformer standards.

Choose a Bushing CT when: Space is constrained, budget is tight, and the required accuracy class and ratio fit within the physical dimensions of the transformer's existing flange.
Choose a Standalone CT when: You need multiple secondary cores (e.g., one for metering, two for redundant differential relaying) that simply will not physically fit inside a single bushing flange, or when retrofitting an existing substation where the transformer bushings were not originally ordered with CT pockets.

Frequently Asked Questions