: working principle, equivalent circuit, knee-point voltage & 3 zones: non-saturated, intermediate and saturated. Understand CT magnetization curve importance for metering and protection class CTs to ensure accuracy, safety & system reliability.){kind=link}
What is Current Transformer?
A Current Transformer (CT) is an essential component in power systems, frequently utilized for measurement and protection.
Its accuracy & performance are substantially determined by the magnetic properties of its core. CT’s magnetization curve illustrates the relationship between secondary excitation current (IM) & secondary excitation voltage (VS).
Understanding this curve becomes essential when selecting the appropriate CT for metering (or) protection purposes.
In this post, we will look at the working concept of CTs, the magnetization curve, and the three operational zones:
- Non-saturated zone,
- Intermediate zone and
- Saturated zone.
How Do Current Transformers Work?
A CT has 2 windings (primary & secondary) coiled around a magnetic core.
When an alternating (AC) primary current (IP) flows, it produces a magnetic field.
This field creates a voltage (VS) in secondary winding.
If the secondary is connected to a load (relay, meter, etc.), a secondary current (Is) flows, resulting in an opposing magnetic flux.
This opposition maintains the net flux density under control, ensuring precise current transformation.
Open-circuiting the CT secondary might result in dangerously high voltages due to core saturation, creating safety issues and causing equipment damage.
Thus, a CT’s secondary circuit must always be short-circuited (or) linked to a load.
Equivalent Circuit of CT
The equivalent circuit of a current transformer consists of:
I1 – Primary Current
I2 = Kn x I1 – Secondary current in an ideal CT
IS – Actual Secondary Current
IM – Magnetizing Current
E – Induced EMF
VS – Secondary Output Voltage
LM – Magnetization Inductance (Saturated Core)
RCT – Secondary Resistance of CT
RP – Load Resistance (Burden)
What is a Magnetization Curve?
A magnetization curve, or B-H curve, graphs a magnetic material’s magnetic flux density (B) & applied magnetizing force (H) in a magnetic field.
This curve shows a material’s magnetic properties, including its ability to get magnetized, its retentivity, its coercivity & whether it saturates beneath a strong field.
It’s essential for choosing transformer and motor materials.
CT Magnetization Curve
The magnetization curve depicts how the magnetizing current (IM) varies with the secondary voltage. It helps to determine the CT’s knee-point voltage & saturation characteristics.
The curve is divided into 3 zones:
1). Non-Saturated Zone
2). Intermediate Zone
3). Saturated Zone
1). Non-Saturated Zone
In this zone, the CT uses minimal magnetizing current.
The secondary current is approximately proportionate to the primary current.
The ratio error is low, ensuring precise measurements.
Metering Class CTs (0.1s, 0.2s, 0.5s, and 1.0s) are used in this zone.
Applications include energy meters, ammeters, power meters and precision measurement.
2). Intermediate Zone
As the primary current increases, so does the magnetizing current.
The link between the primary & secondary currents becomes nonlinear.
The CT still delivers adequate accuracy for protective applications.
Protection Class CTs are intended to operate in this zone, particularly during fault conditions where primary current exceeds kiloampere (kA) values.
Applications include protective relays, circuit breakers and differential protection.
3). Saturated Zone
The curve becomes nearly horizontal.
The magnetizing current increases significantly while the secondary current becomes distorted.
The transformation ratio inaccuracy is really high.
There is no reliable measurement (or) protection in this location.
CTs should never operate in the saturated zone under normal (or) fault conditions.
CT Magnetization Curve Importance
- Ensures exact measurement of CTs.
- Prevents relay malfunction during faults in the protection CTs.
- Assists in selecting the CT knee-point voltage for various applications.
- Reduces saturation concerns, assuring system safety and reliability.
Conclusion
A CT’s magnetization curve determines its accuracy & reliability. Operating a CT in the non-saturated zone ensures accurate measurement while the intermediate zone enables protective relays during faults. However, operating in the saturated zone is strictly prohibited since it results in distorted measurements and degraded protection.
For metering applications, always use metering class CTs & for protection systems utilize protection class CTs that can handle large fault currents without saturation.