Core Balance Current Transformer ring or zero sequence CT ring is a ring type current transformer as an earth fault protection.
So, what is the relationship between the core balance CT ring with a Core Balance Current Transformer (CBCT)? We can say that a CBCT is a current transformer with an extra core balance CT ring.
Now, the more important matter is to understand why we use this ring on a current transformer. Before that we need to understand the explanation of core balance current transformer vs current transformer.
What is a Core Balance Current Transformer (CBCT)?
Just as stated above, a Core Balance Current Transformer (CBCT) or Zero Sequence Current Transformer (ZCT) is a current transformer with the shape of a ring. This ring current transformer (CT) is passed through by a single three phase system wire or three single phase wires form a three phase system.
This three phase wire is used as the primary winding of our CBCT .
Core Balance Current Transformer (CBCT) or Zero Sequence Current Transformer (ZCT) is a special built current transformer with a shape of ring, used to detect the zero sequence current during faults (such as short circuit fault or ground fault) that generates an unbalanced in our three phase system.
This ring type current transformer is able to balance the zero sequence current in the three phase system. This is where it got its name as a Zero Sequence Current Transformer (ZCT).
We will mainly use this type of current transformer as an earth fault protection for medium level voltage distribution. The common CBCT illustration can be seen below.
Unlike the typical current transformer, the CBCT gets its primary winding from the three phase wires passing through the ring at its center of the iron core. Its secondary winding is wound to the ring shaped iron core and connected to the earth fault relay.
How Does the Core Balance Current Transformer (CBCT) or Zero Sequence CT (ZCT) Work?
From its function, you may be able to figure out how CBCT or ZCT works. The CBCT or ZCT works by balancing zero sequence current in any three phase system. This is the main principle we need to remember before jumping to the next explanation.
Since we are dealing with the current transformer, we will try to implement Kirchhoff’s Current Law here. Since a three phase system is a three AC current with shifted phase. From the KCL, we know that:
The sum of all currents at a node will be equal to zero.
The currents entering a node are equal to the currents leaving that node.
From this statement, we can apply the same to the three phase current.
In a balanced three phase system or during normal or condition, the sum of the vector of each phase current is equal to zero. Or in mathematical way:
This way, we will not find any remainder of zero sequence current in the CBCT primary winding. Since it is a balanced three phase, there will not be any flux generated in the CBCT core. Thus, the current in the secondary winding is zero.
Since the CBCT is deployed to minimize or diminish “something abnormal”, we will now talk about a condition where we need to use a Core Balance Current Transformer or Zero sequence Current Transformer.
In an abnormal condition, the sum of the three phase current is not zero. Since the system becomes an unbalanced three phase system, the zero sequence current will start flowing in the secondary winding of CBCT.
Remember, the secondary winding is connected to the earth fault relay, it will protect the rest of the circuit with an extra hello from a circuit breaker.
Core Balance Current Transformer Calculation
For a better understanding, we will try to put everything we have read above into a mathematical theory.
I1, I2, and I3 are the three phase line currents
Φ1, Φ2, and Φ3 are the three phase magnetic flux
The magnetic flux is the product generated by the core of CBCT when the system is unbalanced.
For a start, remember that:
k = constant
Since every phase current produces its own magnetic flux in the same core, the constant k is the same for everyone.
The resultant magnetic flux found in the core of the Core Balance Current Transformer (CBCT) is
Since it is a three phase system where a symmetrical components exist,
The current In is the neutral current while the current I0 is the zero sequence current generated in the transformer. From two equations above (1) and (2), we can conclude that
Core Balance Current Transformer Calculation Example
For better explanation, let’s analyze a simple example below.
First, we assume that our three phase system is a balanced system, thus
We compare the Equation.(1) with the Equation.(4) and we get that:
The resultant flux is zero (Φr = 0). This means there is no current flowing through the secondary circuit of our CBCT. Its earth fault relay won’t get operated since this is a normal operation.
Say, there is a Line-to-Ground fault as our abnormal operation, this one will get our earth fault relay protection works.
The phase current from one of three cables will have an unbalanced value. Thus the zero sequence current will occur in the secondary winding and circuit of CBCT.
The single Line-to-Ground fault will be
From Equation.(5) and Equation.(3) compared together, we get that the magnetic flux in the core of our CBCT won’t be zero. It has some value and induces some current to the secondary circuit.
Thus, our earth fault relay will get activated to prevent further damage to our circuit and system.
Advantages of Core Balance Current Transformer
There are several things to consider using a CBCT as a protection such as:
- Easy to install
- High sensitivity
Even though we have a three phase system to protect, we only need one core instead of three if we use a CBCT or ZCT. This is very convenient for installing and is simple.
As a result, the magnetic flux needed to produce a certain secondary current is lowered to one-third (1/3), which is the highest benefit given the enhanced overall sensitivity of the protective system.
Additionally, because the three phase system current is balanced, the number of secondary turns of CT is not necessary to deliver the cable’s rated current because no secondary current flows under normal operating conditions.
This enables the secondary turns to be optimized in accordance with the CT’s actual primary pickup current.
How to Choose Proper Core Balance Current Transformer (CBCT)
To maximize the use of our CBCT, we have to consider there things:
- Minimum value of primary ground leakage current
- Dimension and diameter of the CT (making sure the cable will fit)
- Knee point voltage
- Minimum required excitation current for relay operating voltage
- Nominal CT ratio, making sure the current is enough to operate the earth fault relay even if the ground fault is very small.
Core Balance Current Transformer Applications
Since the use of a CBCT is to protect, we should use it to protect sensitive electrical devices or machines. Protecting them from the earth fault is a must if you don’t want them to malfunction.
Motor Protection with CBCT
Since electric motors are sensitive to surge current that can make them break, CBCT is installed to prevent any earth fault to affect the motor. Let’s say we have a three phase motor, we install the CBCT with its core being passed through by all the three phase motor wires.
The earth connection of our motor will be passed to the core of the CBCT through the cable sheath. The most common three phase motor with a CBCT can be seen below. You can also observe what CBCT current transformer is.
The currents found in the core are
- Line 1 current, I1
- Line 2 current, I2
- Line 3 current, I3
- Sheath current, Isheath
- Earth current, Iearth
Both sheath current and earth current will cease to exist the moment we connect the earthing through the core of CBCT.
The earth relay protection only responds to line currents, I1, I2, and I3.
Core Balance Current Transformer vs Current Transformer
Now you have understood that CBCT or zero sequencer are not used just like we use a current transformer.
A current transformer is used to step up or step down alternating current, where CBCT or zero sequencer is used as fault protection.