How to Read a Ct Excitation Curve
The magnetization curve of the current transformer shows the human relationship between the secondary excitation current and secondary excitation voltage. The equivalent circuit diagram of the electric current transformer is as given below.
Schematic diagram of CT
I1: primary current.
I2 = Kn I1: secondary current for a perfect CT.
Is: secondary current actually flowing through the circuit. Im: magnetizing current. Due east: induced electromotive strength.
Vs: output voltage.
Lm: magnetization inductance (saturable) equivalent to the CT.
Rct: resistance at the CT secondary
Rp: load resistance.
How CT Works?
The current transformer has 2 sets of winding around the magnetic core. when alternating current(Ip) flows in the primary, the alternating field( H=Np*Ip) is generated and the magnetic flux flows in the core. The magnetic flux gets linked to the secondary of the CT and produces a voltage (Vs) in the secondary.
If the secondary of the current transformer is not connected to a brunt, the secondary electric current will exist nothing. In the absence of the secondary electric current(Is), the net magnetic flux in the core will be equal to the flux created by the primary electric current. The higher flux density in the core will saturate the core and the dangerously high voltage is produced beyond the secondary under this status, That is why the secondary of the electric current transformer must not exist kept open up circuited.
If the secondary of the electric current transformer is connected to a brunt, the secondary current will set up its own magnetic flux in the core, and the flux generated due to secondary current opposes the flux generated by the primary. Thus the net flux density in the core is below the rated flux density of the core and the secondary voltage is in order of the few volts.
The current transformer when connected to burdens like protection relay, free energy meter or ampere meter, the total current that pass through the CT is magnetizing electric current (Im) & secondary current (Is).
Thus, the total secondary electric current Is is the vector sum of magnetizing current (Im) and secondary load current (IL) flowing through the protection relay. During error condition magnetizing current can go above its knee bespeak voltage and CT gets saturated.
Just below the knee point voltage, all the magnets are aligned in the aforementioned direction as the magnetic field and the maximum rated flux density of the core has reached. If the primary current is further increased after attaining the maximum flux density in the core, the core tin't handle this increased magnetic field and CT is said to be saturated.
Magnetization Curve of CT
The magnetization curve of CT can be divided into three zones.
1. Non-saturated Zone
In the non-saturated zone of the magnetizing bend, the CT consumes a very low magnetizing electric current and the secondary current is almost linear to the master current. The ratio error is minimum in this zone and the current transformer used for measuring the electrical parameters like ampere, power, energy must exist operated in this zone. The metering class CT operates in this region. The class of the current transformer for the metering class is 0.1s,0.2s,0.5s & 1.0s.
In the intermediate Zone magnetizing current increases with an increase in primary current, and the secondary current is not proportional to the primary current; Non-linearity increases every bit the master electric current is increased.
The protection class current transformer is operated in this zone. During a fault, the magnitude of the primary current is in the order of kilo-amperes and it is expected that CT must non get saturated at the fourth dimension of sensing the fault current else the protection arrangement would not work.
3. Saturated Zone
The curve becomes virtually horizontal because of the increase of magnetizing electric current drastically, and the error in transformation ratio is high, the secondary current is distorted by saturation. If the current transformer operates in this zone; no protection of the electrical network can exist ensured.
Related Posts on Current Transformer
- Hysteresis Loop
- How to Calculate Knee Point Voltage of Current Transformer?
- What is the Use Of Articulatio genus Betoken Voltage?
- How to Calculate Burden of CT
- Current Transformer – Construction, Phasor and Errors
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Source: https://www.electricalvolt.com/2018/09/magnetization-curve-of-ct/
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