Core Balance Current Transformer

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.

22 thoughts on “Core Balance Current Transformer”

  1. Hi R Deenadayalan,

    First of all, check the unbalance current under normal operating condition. Say, this maximum unbalance current is 0.2 A (secondary current). Then the relay setting must be more than 0.2 A to prevent operation of relay under normal condition.

    • It is possible but refer CBCT manufacturer manual for other parameters which needs to be taken care.

  2. CBCT are used in LV and MV sytems. For LV system, the source usually comes with 3-phase + neutral + earth [5-wires ]. While for MV system, the source usually comes with cable with 3-phases + screen/armour (which is earthed) [3-wires].
    It would be more informative to show the installation of CBCT for both LV [5-wire] and MV [3-wire] systems. It is extremely important to point out the routing of the neutral and the earth wire.
    a) For a 5-wire system, the [neutral] MUST follow the same route as the three phases (i.e. goes through the CBCT. But the [earth wire] must NOT go through the CBCT. With the neutral going through the CBCT, the earth fault protection will have No problem irrespective of whether the load is balance or unbalance. It is Not necessary to find out what is the unbalance value on the three phases. The unbalance current must return through the neutral; when there is No earth fault.
    b) For a 3-wire system, the 3-core armoured cable goes through the CBCT. But the [armour] connected with a pig-tail Must make a ” U-turn back through the CBCT” and then earth it at the earthing bar.
    Che Kuan Yau (Singapore)

    • Ideally you can’t. But please check the relay manual if it measures neutral current and Earth Fault can be enabled in the relay by giving normal CT input.

  3. Hi,
    It is written here that “In case cables are already laid in a Switchgear, physically split core, which is also known as Slip-over type CT, are used”. What are the other types of CBCT, I think next type is fixed type. Then technically which is better in terms of characteristics.

  4. How to design the CBCT core size and secondary turns on basis of given Load capacity and expected fault current .

  5. greetings!
    it is very helpful for core balance CT for R, Y & B phase 11KV, 33KV & 132KV.

  6. Dear Sir,

    What happen if two core cable runs through the CBCT , then system leakage current remains same at relay or it become double.

    i.e. system earth fault = 6mA
    if we use two runs of cable then , relay will receive leakage : 6mA or 12mA.

    Please revert.


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