Core balance CT (also known as zero sequence CT) is a ring type current transformer used for earth fault protection.
What is Core balance current transformer (CBCT)?
Core balance current transformer (also
termed as CBCT) is a ring type current transformer (CT)
through the centre of which either three single core cables or a single three
core cable of three phase system passes. That three core cable forms primary
winding of CBCT.
In other words, We can define as: A zero sequence CT (ZCT) (also known as core balance CT) is a special type of current transformer (CT) which detects the presence of zero sequence current during short circuit fault (such as LG fault) condition that causes unbalance in 3-phase system.
[ ##eye## What is Transducer? ]
Core balance Current Transformer (CBCT) works on the concept of balancing zero sequence current in three phase system. That's why it is also known as zero sequence current transformer (ZCT). Core balance CT is generally used for earth fault protection (or earth leakage protection) for medium voltage electrical system. A typical CBCT is as shown in below figure.
Unlike
current transformer (CT), primary winding of CBCT is formed with three core
cable passing through centre of its iron core and 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 3-phase system. Hence it is also called as 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 electrical circuit will be equal to zero.
During
the normal operation of three phase system as the vector sum of its phase
current ( Īa +
Īb + Īc = 0 ) is zero. Therefore no any residual
zero sequence current will present in primary winding of CBCT. Therefore there
will not be any flux developed in the Core of CBCT. Hence no current will flow
in secondary circuit of CBCT.
[ ##eye## Different Parts of transformer and their functions]
Whereas
in abnormal condition, when sum of phase current will not zero. Then due to
unbalancing of phase current, a zero sequence current will flow in the
secondary circuit of CBCT. Hence earth fault relay connected with secondary of
CBCT, will get energized and it isolate the healthy system with the help of
circuit breaker.
Let Īa , Īb and Īc are line currents of three phase
system and Φa , Φb and Φc are
corresponding components of magnetic flux developed in the core of zero
sequence CT or CBCT. Assuming that CBCT is operating in linear
region, then magnetic flux developed in 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
proportionality constant. Since here all three phase currents produces
corresponding magnetic flux in the same core of ZCT (same magnetic material).
Hence same proportionality constant (k) is used in all phase
fluxes.
[ ##eye## Surge impedance loading (SIL) of Transmission line]
Hence
resultant magnetic flux in the core of ZCT or CBCT will be
Φr = k(Īa + Īb + Īc) ________________(1)
But
as we all know that from concept of symmetrical components
(Īa + Īb + Īc) = 3Ī0 = Īn________________(2)
Here
Īn is
neutral current and Ī0 is zero sequence current in 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
three phase system
As
we know that Īa + Īb + Īc = 0 _______(4)
Here by comparing equations (1) and (4), we got result as:
Net
resultant flux (Φr = 0) is zero. It means no current will flow in
secondary circuit of CBCT. Hence earth fault relay will not operate in normal
healthy condition.
Case 2: During earth fault (say LG fault)
in abnormal condition, phase current in three core cable passing through centre
of current transformer will not balance. Hence a zero sequence current will flow
in secondary circuit of ZCT. For example consider the case of single line to
ground fault (LG fault).
If = 3Ia0 = In _____________(5)
[ ##eye## Nilpotent matrix and its properties]
Comparing
equation (5) with equation (3) we got that the net magnetic flux in core of
CBCT will not zero. It has some finite value that will induce current in
secondary circuit of ZCT. Therefore those secondary current flows in 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 application as earth
fault protection of induction motor, that is discussed below in detail.
CBCT for motor protection
In
most of industries, CBCT is being used for earth leakage protection of
industrial motor ( 3-phase induction motor ). In this protection scheme, the
core of CBCT surrounds the power cables connected with 3-phase induction motor.
The earthing leads from the cable-sheath to earth must be taken through the eye
of ZCT core. A typical diagram is as shown in below figure.
 |
| CBCT for motor protection |
The
core of CBCT is excited by the current (Īa + Īb + Īc + Īsheath + Īearth).
The
effect of ( Īsheath + Īearth )
is cancelled when the earthing leads are taken through the eye of CBCT. Hence
the protection scheme responds only for (Īa + Īb + Īc) current in induction motor.
Advantages of CBCT
The
main advantage of using CBCT as earth fault protection scheme is that in this
protection scheme only one CT core is used instead of three core as in
conventional system. Thus the magnetic flux required for production of
particular secondary current is reduced to one third (1/3), which is biggest
advantage as the overall sensitivity of protection system is increased.
[ ##eye## Theory and working of Star-Delta Starter]
Also
the number of secondary turns of CT is not required according to rated current
of cable because no secondary current flows in normal operating condition as
the three phase system current are balanced. This allows to optimize 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 as:
- 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)
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Very good article.. thanks
ReplyDeleteWell explained...
ReplyDeleteVery good...
ReplyDeleteThis explanation is only applicable for delta connections, as I understand. For unbalanced star 4-wire connections (and practically, almost all of them are unbalanced), Ia+Ib+Ic = In. So you'd also need the neutral wire going through the core-balance transformer.
ReplyDeleteWell explained
ReplyDelete