Core Balance Current Transformer or CBCT is a ring-type current transformer through the center of which a three core cable or three single core cables (forms the primary winding) 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 the fig.1.
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 Core balance Current Transformer
Let Īa, Īb and Īc be the three line currents and Φa, Φb and Φc be the corresponding components of magnetic flux in the core. Assuming that the current transformer is operating in the linear region, 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 the magnetic flux in the same core i.e. magnetic material.
Thus the resultant magnetic flux (Φr) 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 the neutral current. Hence we can write as
Φr = kĪn …………………………(2)
Now let us consider two cases:
Case-1: 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.
Case-2: 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 a current in the secondary circuit due to which earth fault relay will operate. Because of this reason, a Core Balance Current Transformer is also called Zero Sequence Current Transformer (ZSCT).
Advantages of Core Balance Current Transformer
The advantage of using CBCT for earth fault protection is that only one current transformer core is used instead of three core as in a conventional system where the secondary winding of three cores is 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 a 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.
Applications of Core Balance Current Transformer
The Core-Balance Current Transformer is used for Earth Fault Protection.
An earth fault relay, connected to the secondary winding, is energised only when there is residual current in the primary system.
The advantage in using this method of earth fault protection lies in the fact that only one current transformer core is used in place of three-phase current transformer’s whose secondary windings are residually connected. In this way, the current transformer magnetising current at relay operation is reduced by approximately three-to-one, an important consideration in sensitive earth fault relays where a low effective setting is required. The number of secondary turns does not need to be related to the cable rated current because no secondary current would flow under normal balanced conditions.
This allows the number of secondary turns to be chosen such as to optimise the effective primary pick-up current.