Saturation of the current transformer
As we mentioned earlier, errors of the CT are mainly caused by the field current Ie. Due to the large excitation impedance in normal operation, Ie is small, so that this error can be ignored. However, when the CT is saturated, the more serious the saturation degree is, the smaller the excitation impedance is and the greatly increased excitation current makes the error of the CT multiply and affects the correct operation of the protection.
The most serious will make all the primary current into the excitation current, resulting in the secondary current is zero. The cause of transformer saturation is generally too large current or the current contains a large number of non-periodic components, both of which occur in the case of accidents. At this time, it is required that the protection should act correctly and remove the fault quickly. However, if the transformer is saturated, it is easy to cause excessive error and lead to incorrect protection actions, which further affects the system safety. Therefore, we must take the problem of CT saturation seriously.
The saturation problem of CT is very complicated if it is analyzed in detail, so only qualitative analysis is carried out here. The so-called saturation of the transformer actually refers to the saturation of the transformer core. We know that the transformer can transmit variable current because the primary current generates magnetic flux in the iron core, which in turn generates electromotive force U = 4.44f*N*B*S * 10-8 in the secondary winding wound in the same iron core.
Where, f is the system frequency, HZ; N is the number of turns of the secondary winding; S is the cross-sectional area of the iron core, m2; B is the magnetic flux density in the core. If the secondary loop is a pathway, the secondary current will be generated to complete the current transfer in the primary secondary winding. However, when the magnetic flux density in the core reaches saturation point, the change of B with the excitation current or magnetic field intensity tends to be insignificant. That is to say, when N,S and f are determined, the secondary induced potential will remain basically unchanged, so the secondary current will also remain basically unchanged, and the proportional transmission characteristics of the primary and secondary current will be changed. We know that the essence of CT saturation is that the magnetic flux density B in the iron core is too large, which exceeds the saturation point. The amount of magnetic flux in the core is determined by the size of the current that creates the magnetic flux, that is, the size of the excitation current Ie. When Ie is too large, the magnetic flux density is too large, which will make the core tend to be saturated. At this time, the excitation impedance of the CT will decrease significantly, resulting in the further increase of the excitation current, which further aggravates the increase of the magnetic flux and the saturation of the iron core. This is actually a vicious cycle. As can be seen from Figure 1, the decrease of Xe and the increase of Ie will be manifested as the increase of the transformer error, so as to affect the normal operation.
The saturation of the iron core can be generally understood in two ways. One is steady-state saturation and the other is transient saturation. For steady-state saturation, we can analyze it with the help of Figure 1. As can be seen in the figure, Ie and the secondary current Is are proportionally shunt. We assume that the excitation impedance Ze is constant. When the primary current increases due to accidents and other reasons, Ie will inevitably increase proportionally, so the magnetic flux of the core increases. If the current is too large, it will also cause the Ie to be too large, which will go into the above cycle, and then cause the transformer saturation. Transient saturation refers to the saturation of the transformer caused by the transient component during the fault transient state. We know that when any failure occurs, the electrical volume is not abrupt. The occurrence of fault quantity must be accompanied by more or less aperiodic component. However, the non-periodic component, especially the DC component in the fault current, cannot be transmitted between the first and second transformer. These currents will all appear as excitation currents. Therefore, when the accident occurs accompanied by a large transient component, the excitation current will also increase, resulting in transformer saturation.
