Core Balance Current Transformer or CBCT is a ring type current transformer through center of which a three core cable or three single core cables of three phase system passes. This type of current transformer is normally used for earth fault protection for low and medium voltage system. A typical Core balance Current Transformer is shown in figure below.
Secondary of CBCT is connected to Earth Fault Relay. During normal operating condition as the vector sum of three phase current i.e. (Īa + Īb + Īc =0) is zero therefore no residual current in the primary will be present. Here residual current means zero sequence current. Therefore there will not be any flux developed in the CBCT core and hence no current in the secondary circuit of CBCT.
Working Principle of CBCT:
Let Īa, Īb and Īc be the three line currents and Φa, Φb and Φc be corresponding components of magnetic flux in the core. Assuming that the CT is operating in the linear region (Read B-H Curve to get idea of linearity), magnetic flux because of individual phase current will be directly proportional to the phase current and hence we can write as below,
Φa = kIa
Φb = kIb
Φc = kIc
where k is constant of proportionality. Mind here that same constant of proportionality is used as all the three phase current are producing magnetic flux in the same core i.e. magnetic material.
Thus the resultant magnetic flux in the CBCT core,
Φr = k(Īa + Īb + Īc) …………………..(1)
But we know from theory of symmetrical components,
Īa + Īb + Īc = 3Ī0 = Īn
Where, Io is zero sequence current and In is neutral current. Hence we can write as
Φr = kĪn …………………………(2)
Now let us consider two cases:
Case1: During normal condition
Īa + Īb + Īc = 0
Hence from equation (1),
Net resultant flux in the CBCT Core, Φr = 0 which means no secondary current and therefore the Earth Fault Relay won’t operate.
Case2: During earth fault, three phase current passing through the center of Core Balance Current Transformer will not be balanced rather a zero sequence current will flow. For example for single line ground fault,
If = 3Ia0 = In
Thus from equation (2),
Net magnetic flux in the CBCT core, Φr will have some finite value which in turn will induce current in the secondary circuit due to which earth fault relay will operate. Because of this reason, a Core Balance Current Transformer or CBCT is also called Zero Sequence Current Transformer.
Advantage of Core Balance Current Transformer:
The advantage of using CBCT for earth fault protectionis that only one CT core is used instead of three core as in conventional system where the secondary winding of three cores are connected residually. Thus the magnetizing current required for the production of a particular secondary current is reduced by one third which is a great advantage as the sensitivity of protection is increased.
Also, the number of secondary turn does not need to be related to the cable rated current because no secondary current flows under normal operating condition as the currents are balanced. This allows the number of secondary turns to be chosen to optimize the effective primary pick-up current.
Core Balance Current Transformer is normally mounted over a cable at a point close to the cable gland of the Switchgear. In case cables are already laid in a Switchgear, physically split core, which is also known as Slip-over type CT, are used.