All About Core Balance Current Transformer
A ring-type current transformer (CT), also known as a core balance current transformer (CBCT) or zero sequence current transformer (ZCT), detects the presence of zero-sequence current during a short circuit fault, which results in an unbalanced three-phase system. CBCT is used to balance the zero-sequence current in a three-phase system. In most industries, CBCT is used for earth leakage protection of industrial motors. For medium-voltage electrical systems, CTs are commonly used for earth fault protection.
Unlike regular current transformers, the primary winding of a CBCT is formed by a three-core cable that passes through the center of its iron core. The secondary winding is wrapped around a ring-shaped iron core, which is connected to an earth fault relay.
Working Principle of CBCT or ZCT
Kirchhoff’s current law states that the algebraic sum of currents at any node in an electrical circuit will be zero. This is the fundamental principle on which ZCTs or CBCTs operate.
During normal operation, the vector sum of the phase currents in a three-phase system (Ia + Ib + Ic = 0) is zero. Therefore, there will be no residual zero-sequence current in the primary winding of the CBCT, resulting in no flux in the core of the CBCT. Consequently, the secondary circuit of the CBCT remains completely static.
In contrast, during abnormal conditions, the sum of all phase currents will not be zero. Then, a zero-sequence current will flow in the secondary circuit of the CBCT as the phase currents are unbalanced. As a result, the earth fault relay connected to the CBCT secondary will activate, causing the health system to disconnect through a circuit breaker.
Features of CBCT or ZCT
Some highly attractive features of zero sequence CT (ZCT) or core balance current transformer (CBCT) include:
- Extremely sensitive
- Decent linearity
- Dependable performance
- Easy and convenient installation
Selection of CBCT or ZCT
The selection of CBCT or ZCT involves the following criteria to choose the best zero sequence CT or CBCT:
- Nominal CT ratio: This should be set so that even in the case of a minor ground fault, there is enough current to run an earth fault relay.
- Minimum main ground leakage current
- Minimum excitation current required for relay operating voltage
- The voltage at the knee point
- Dimensions and internal diameter of the CT (the internal diameter depends on the size of the cable)
