Variable current 220kV ABB



1. Functions:

Current transformer (CT) is an electrical device used to convert high-value current and high voltage to current with standard values ​​5A or 1A, safe voltage to provide measurement, control circuits. control and protection.

The CT has a primary winding connected in series to the primary circuit. The impedance of the CT, including the resistance of the secondary side load is very small compared to the primary side impedance of the circuit.

2. Operation principle:

In alternating current circuits, the working principle of a current transformer is similar to that of a transformer (a current transformer is a static electromagnetic device, working according to the principle of electromagnetic induction, used to change the value of alternating current. pm but keep the same frequency).

With high voltage CT when I current I1 through the primary winding, the secondary side gives I2 current from the primary side but the frequency remains the same.

With low voltage CT when I pass current I1 through the steel core with secondary winding, the secondary side gives I2 current different from the primary side but maintains the frequency.

2.1 Working modes of CT:

– Short-circuit mode of primary current, secondary circuit with load Z2: Ratio between primary short-circuit current on rated current is called multiples of current transformer: n = I1 / I1đm.

When n is large, the CT error increases and it depends on the secondary current I2 or the Z2 load. Usually with a protection circuit, the multiple of the CT current must reach a value such that its error is less than 10%.

– Secondary open circuit mode of CT: When secondary open circuit, the secondary side will have induced voltage with very high amplitude endangering people and secondary equipment (saturated steel core). To prevent saturation in the magnetic circuit, it is also made current transformers with air gap, also known as linear current transformers.

CT has a current ratio inversely proportional to the number of windings. Therefore, it is possible to change the ratio of variables by changing the number of primary or secondary side windings.

Here is an example of a CT equivalent map:

From the schematic diagram we draw the following remarks:

The CT secondary load impedance hardly affects the value of the secondary current.
When the primary side has a current, do not leave the secondary side open because all the primary current will flow through the excitation circuit with a rather large impedance, which can be dangerous to people and secondary equipment.
If the values ​​of the excitation circuit impedance and the load impedance are known, the error value and phase angle of the secondary current can be determined.


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