Core Balance Current Transformer - CBCT

 

Core Balance Current Transformer-CBCT:

Core-Balance Current Transformer CBCT also called as Zero Sequence CT.

In this type of CT a single ring or rectangular shaped core of magnetic material, encircles the conductors of all the three phases. A secondary coil is wound on the core connected to a relay circuit.

CBCT inside Cable chamber without Cable

This ground fault relay, detects the ground leakage current and trips the faulty circuit.

The topics covered in this post are

Principle of CBCT

• Mounting of CBCT
• Selection of CBCT
• CBCT for Motor protection

Principle of CBCT for Earth fault Protection:

In absence of an earth fault in the load or the supply cable, the current in the secondary of Core Balance Current Transformer is negligible. Under balanced load and without earth fault,

Ie= (Ir+ Iy+ Ib) =0

However during an earth fault on load side of zero sequence CT,

Ie= (Ir+ Iy+ Ib) =3 I0

Where I0 is the zero sequence component.

The relay connected on secondary side of CBCT (Zero Sequence CT) senses this current and thus it will operate.

Core-Balance Current Transformer Mounting:

The following is the correct procedure for the proper mounting of these CTs.

1.It is necessary to pass all the Three, Three and half or Four cores of the cable through the core of the CBCT to detect the unbalance or the ground leakage in 3-core cables and only ground leakage in 3-1/2 and 4 core cables.

CBCT with 3-Core Cable

• Three Core cable:  A 3-core cable will detect an unbalance in the three phases, whether this is the result of unequal loading in the three phases or a ground fault.

• Three and half or Four Core Cables: These cables will detect only a ground leakage as the amount of unbalance, when unbalance occurs, will be offset by the flow of this unbalance current through the return path of neutral circuit.

2. In armoured cables, armouring must be removed before passing the cable through the CBCT to avoid an induced e.m.f through the armour and the corresponding magnetizing current which may affect the performance of the current transformer.

3. As such CTs are required to detect small out-of-balance currents, the connecting leads should be properly terminated and must be short to contain the lead resistance as far as possible.

Selection of CBCT:

The required CBCT is selected based on the following factors.

• Minimum primary ground leakage current
• Nominal CT ratio. This may be such that on the smallest ground fault the current on the secondary is sufficient to operate the relay.
• Relay setting
• CT secondary current, 1A or 5A
• Minimum excitation current required at the relay operating voltage
• Knee point voltage
• Number of cables in parallel
• Limiting dimensions and internal diameter (ID) of the CT. ID will depend upon the size of the cable.

CBCT for Motor Protection:

CBCT is used in motor protection for providing Zero sequence protection. The core of core- balance CT surrounds the power cable connected to the induction motor. The earthing lead from the cable-sheath to earth must be taken through the eye of the CT core.

Motor Protection with CBCT

The CT core is excited by ir+ iy+ ib+ isheath+ iearth. The effect of (isheath+ iearth) is cancelled when the earth lead is taken through the eye of CT core. There by the protection responds only to ir+ iy+ ib value current.