We also refer to the Core Balance Current Transformer or Core Balance CT (CBCT) as
- Zero Sequence Current Transformer (ZSCT)
- Window-Type Current Transformer
- Ring-Type Current Transformer
- Toroidal Current Transformer
Working Principle of Core Balance CT (CBCT)
A Core Balance Current Transformer (CBCT) is a special type of current transformer. Specifically, it works on the principle of imbalance detection. It monitors the imbalance current of a three-phase cable system. When balanced three-phase currents flow through a three-core cable or three single-core cables, the resultant current is zero. Because the vector sum of Red, Yellow, and Blue phase currents, which are 120° apart, becomes zero at balanced conditions.
Now, imagine a magnetic core surrounding the three cores of a cable system. Obviously, each core carries one phase current. In this condition, the net flux in the CBCT core is zero because the sum of the three currents is zero.
No Flux Condition
Each phase produces flux in the CT core:
- The Red phase produces its flux.
- The Yellow phase produces its flux.
- The Blue phase produces its flux.
Flux is directly proportional to current. Since the currents are 120° apart, their fluxes are also 120° apart. Therefore, these fluxes cancel each other. As a result, no net flux exists in the core.
If a coil is wrapped around the core, it will not have any induced EMF because there is no flux linkage. The current in the coil remains zero.
If an imbalance occurs in the three-phase cable due to earth faults or leakage currents, the sum of the three currents is no longer zero. This imbalance current produces a net flux in the core. The alternating flux induces EMF in the coil.
When we connect a relay across this coil, current flows through the relay. The relay works as an Earth Fault Relay (EFR). The relay connects to the tripping mechanism of the circuit breaker. It energizes the trip coil and trips the breaker. The breaker operates with proper annunciation and indication. In this way, a CBCT protects the cable system against earth leakage and earth faults.
Construction of Core Balance CT
The Core Balance CT (CBCT) core is ring-shaped. Manufacturers usually make it from cold-rolled grain-oriented (CRGO) silicon steel laminations. Some manufacturers also use nanocrystalline material for the cores.
They insulate the core with paper or similar insulation. Then, they wind enamel-coated copper wire over it to form the secondary winding. The number of turns depends on the sensitivity required for detecting small earth fault currents.
They enclose the entire assembly in resin cast, epoxy, or molded plastic. This enclosure provides both mechanical strength and dielectric insulation. The secondary winding terminals are brought out to a terminal block. This block allows us to connect an external earth fault relay to the CBCT.
Ratio of Core Balance Current Transformer
Manufacturers usually design CBCTs with ratios such as 50:1 or 100:1, etc. These smaller ratios ensure that very small residual currents in the primary circuit produce a measurable secondary current.
A normal current transformer steps down even a few thousand currents to 1 for measurement and protection. A CBCT, however, steps down the imbalance current from a few tens to the ampere range. Because during fault conditions, the imbalance current is much smaller than the actual load current in the cable. The CBCT detects this small non-zero current and produces a secondary current to operate the earth fault relay.
Choosing CBCT Ratio
To select the proper ratio:
- Estimate the lowest residual or earth fault current at which the breaker should trip.
- Set the pickup current of the earth fault relay.
- Example: If the relay pickup current is 100 mA, the CBCT ratio must produce sufficient secondary current at this residual level.
- Select the core balance current transformer turns ratio to ensure correct and reliable relay operation.