Mechanism of Saturation in Electromagnetic Current Transformer

The saturation mechanism of electromagnetic current transformer is mainly related to the magnetization characteristics of the iron core. The detailed introduction is as follows:

1. Basic Working Principle

Electromagnetic current transformers operate based on the principle of electromagnetic induction. They convert the large current on the primary side to a small current on the secondary side through iron core coupling, which is used for measurement, protection, and other purposes. When the primary current I1 passes through the primary winding, it generates an alternating magnetic flux Ø in the iron core. According to Faraday's law of electromagnetic induction, an electromotive force E2 is induced in the secondary winding, which in turn produces the secondary current I2.

electromagnetic induction

2. Saturation Mechanism

Non-linearity of the Magnetization Curve: The relationship between the magnetic flux density B and the magnetic field strength H of the iron core is represented by the magnetization curve (B-H curve). During normal operation, the magnetic circuit of the current transformer works in the linear region, where B and H have a linear relationship. At this time, the primary current and the secondary current maintain a proportional relationship（a~b）. However, when the primary current I1 is too large, causing the magnetic field strength H to exceed the saturation point of the iron core, B no longer increases linearly with H but tends to saturate（b~S relationship）. The growth of magnetic flux Ø also slows down, resulting in the secondary induced electromotive force E2 and secondary current I2 failing to accurately reflect changes in the primary current I1, leading to waveform distortion.

Influence of Secondary Load: An excessively large secondary load will increase the secondary current I2. According to the principle of magnetomotive force balance, the relationship between the primary magnetomotive force I1 N1, the secondary magnetomotive force I2N2, and the excitation magnetomotive force Im N1 is I1 N1 = I2 N2 + Im N1 (where N1 and N2 are the number of turns of the primary and secondary windings, respectively). The increase in secondary load leads to an increase in I_2, which in turn increases the excitation current Im, potentially causing the iron core to enter a saturated state.

Influence of Current Frequency: For a fixed transformer, the magnetic flux density B_m of the iron core is proportional to the secondary voltage E2 and inversely proportional to the current frequency f, following the formula Bm = E_2/(4.44*f*N2*S) (where S is the cross-sectional area of the iron core). When the current frequency is too low, the magnetic flux density Bm of the iron core will increase under a certain secondary voltage, which may cause the iron core to saturate.

3. Classification of Saturation

Steady-State Saturation: Caused by an excessively large steady-state symmetric current during line short-circuits. When the primary current continuously exceeds the rated value, the iron core enters the saturation region, resulting in the secondary current failing to accurately reflect the primary current.

Transient Saturation: The presence of non-periodic components in the short-circuit current and residual magnetism in the iron core can cause the current transformer to enter the saturation region during the transient process. Transient saturation may only occur during the transient period and will gradually disappear as the transient components decay.