A ring-type current transformer (CT), known as a Core Balance Current Transformer (CBCT), Also known as a Zero Sequence CT (ZCT), detects the existence of zero sequence current during a short circuit fault situation resulting in an unbalanced 3-phase system.
The CBCT uses to balance zero sequence current in a three-phase system.
In most industries, CBCT is used for industrial motors’ earth leakage protection. For medium-voltage electrical systems, CT is typically utilized for earth fault prevention (earth leakage protection).
In contrast to a current transformer (CT), a CBCT’s primary winding is made of a three-core cable that runs through the center of its iron core, and its secondary coil is wrapped on a ring-shaped iron core that is linked to an earth fault relay.
Working principle of CBCT or ZCT
Kirchhoff’s current law states that the total current at every node of an electrical circuit will equal zero is the fundamental tenet governing how zero sequence current transformers operate.
The vector sum of a three-phase system’s phase current (Ia + Ib + Ic = 0) is zero during regular operation. Therefore, the primary winding of the CBCT will not include any leftover zero sequence current. As a result, no flux will emerge in the Core of CBCT. As a result, the secondary circuit of the CBCT will be completely static.
In contrast, the total phase current will not be zero under abnormal circumstances. A zero sequence current will then flow in the CBCT secondary circuit because the phase current is out of balance. As a result, the CBCT’s secondary-connected earth fault relay will become activated, isolating the health system with a circuit breaker.
CBCT or ZCT features
The following are some extremely appealing characteristics of zero sequence CTs (ZCTs) or core balance current transformers (CBCTs):
- Extremely sensitive
- Decent linearity
- Dependable performance
- Easy and convenient installation
Selection of CBCT or ZCT
The following criteria are used 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. - Main ground leakage current at a minimum
- Relay operating voltage requires a minimum excitation current.
- The voltage at the knee
- The CT’s dimensions and internal diameter (internal diameter depends on the size of the
cable)
