# Core Balance Current Transformer (CBCT) or Zero Sequence CT (ZCT)

A core balance current transformer (also termed as CBCT) is a ring-type current transformer (CT) through the center of which either three single-core cables or a single three-core cable of three-phase system passes. That three-core cable forms the primary winding of CBCT. In other words, We can define A zero sequence CT (ZCT) (also known as core balance CT) as a special type of current transformer (CT) that detects the presence of zero sequence current during a short circuit fault (such as LG fault) condition that causes unbalance in a 3-phase system.

Core balance Current Transformer (CBCT) works on the concept of balancing zero sequence current in a three-phase system. That’s why it is also known as a zero sequence current transformer (ZCT). Core balance CT is generally used for earth fault protection (or earth leakage protection) for medium voltage electrical systems. A typical CBCT is as shown in the below figure. Unlike the current transformer (CT), the primary winding of CBCT is formed with three core cables passing through the center of its iron core and the secondary coil of CBCT  is also wounded on its ring-type iron core, and it is connected with earth-fault relay.

Working Principle of CBCT or ZCT

A core balance current transformer (CBCT) works on the principle of balancing zero sequence current in a 3-phase system. Hence it is also called a zero sequence current transformer (ZCT). The basic principle of working of zero sequence current transformer is Kirchhoff’s current low: that is the sum of current at any node of the electrical circuit will be equal to zero.

During the normal operation of three three-phase systems, the vector sum of its phase current ( Īa + Īb + Īc = 0 ) is zero. Therefore no residual zero sequence current will present in the primary winding of CBCT. Therefore there will not be any flux developed in the Core of CBCT. Hence no current will flow in the secondary circuit of CBCT.

Whereas in abnormal conditions, the sum of phase current will not be zero. Then due to the unbalancing of phase current, a zero sequence current will flow in the secondary circuit of CBCT. Hence earth fault relay connected with the secondary CBCT will get energized and it isolates the healthy system with the help of a circuit breaker.

Let ĪaĪb, and  Īc be line currents of a three-phase system and ΦaΦb, and Φc are corresponding components of magnetic flux developed in the core of zero sequence CT or CBCT. Assuming that CBCT is operating in the linear region, then the magnetic flux developed in the core of CBCT will be directly proportional to its corresponding line current. Hence it can be written as:

Φa = kIa

Φb = kIb

Φc = kIc

Here (k) is a proportionality constant. Since here all three phase currents produce corresponding magnetic flux in the same core of ZCT (same magnetic material). Hence same proportionality constant (k) is used in all phase fluxes.

Hence resultant magnetic flux in the core of ZCT or CBCT will be

Φr = k(Īa + Īb + Īc________________(1)

But as we all know from the concept of symmetrical components

a + Īb + Īc) = 3Ī0 = Īn________________(2)

Here Īn is neutral current and Ī0 is zero sequence current in the current transformer. Hence from above equations (1) and (2), we can conclude as

Φr = k.Īn ______________________(3)

Now, let’s consider two cases.

Case 1: During normal operation of a three-phase system

As we know that Īa + Īb + Īc = 0 _______(4)

Here by comparing equations (1) and (4), we got the result:

Net resultant flux (Φr = 0) is zero. It means no current will flow in the secondary circuit of CBCT. Hence earth fault relay will not operate in normal healthy conditions.

Case 2: During an earth fault (say LG fault) in an abnormal condition, phase current in three core cables passing through the center of the current transformer will not balance. Hence a zero sequence current will flow in the secondary circuit of ZCT. For example, consider the case of a single line-to-ground fault (LG fault).

If = 3Ia0 = In _____________(5)

Comparing, equation (5) with equation (3) we got that the net magnetic flux in the core of CBCT will not be zero. It has some finite value that will induce current in the secondary circuit of ZCT. Therefore those secondary current flows in the earth fault protection relay and activate it. Because of this reason, a core balance CT (CBCT) is also called zero sequence CT (ZCT).

Application of CBCT or ZCT

The core balance current transformer (CBCT) is mainly used for earth fault protection of electrical machines. One of its important applications is earth fault protection of induction motors, which is discussed below in detail.

CBCT for Motor Protection

In most industries, CBCT is being used for earth leakage protection of industrial motors ( 3-phase induction motors). In this protection scheme, the core of CBCT surrounds the power cables connected with a 3-phase induction motor. The earthing leads from the cable sheath to earth and must be taken through the eye of the ZCT core. A typical diagram is as shown in the below figure.

The core of CBCT is excited by the current (Īa + Īb + Īc + Īsheath + Īearth).

The effect of ( Īsheath + Īearth ) is canceled when the earthing leads are taken through the eye of CBCT. Hence the protection scheme responds only to (Īa + Īb + Īc) current in the induction motor.

The main advantage of using CBCT as an earth fault protection scheme is that in this protection scheme, only one CT core is used instead of three cores as in a conventional system. Thus the magnetic flux required for the production of a particular secondary current is reduced to one-third (1/3), which is the biggest advantage as the overall sensitivity of the protection system is increased.

Also, The number of secondary turns of CT is not required according to the rated current of the cable because no secondary current flows in normal operating conditions as the three phase system currents are balanced. This allows to optimization of the secondary turns according to the effective primary pickup current of CT.

Features of ZCT or CBCT

These are very attractive features of zero sequence CT (ZCT) or Core balance current transformer (CBCT), these are mentioned below:

• High sensitivity
• Good linearity
• Reliable operation
• Convenient and easy installation

Selection of CBCT or ZCT

The appropriate zero sequence CT or CBCT is selected based on the following factors.

• Nominal CT ratio: This should be such that even in smallest ground fault the current should be sufficient to operate earth fault relay.
• Minimum primary ground leakage current
• Minimum excitation current required for relay operating voltage
• Knee point voltage
• Dimensions and internal diameter of CT (internal diameter depends on size of cable) 