Saturation point of current transformer

Under normal circumstances, the core magnetic flux in the current transformer is in an unsaturated state. At this time, the load impedance and excitation current are small, but the value of the excitation impedance is large, and the magnetic potentials of the primary winding and the secondary winding are in balance. However, if the magnetic flux density of the core in the transformer increases and reaches saturation, it will cause Zm to decrease rapidly as the saturation increases, and the linear proportional relationship between different excitation currents will be broken. The factors that cause the current transformer to reach saturation mainly include: excessive current; excessive load. When the load connected to the current transformer is too large, the secondary voltage will increase, causing the magnetic flux density of the iron core to increase and reach saturation.
When the current transformer reaches saturation, the characteristics are as follows: the secondary current decreases, and the current waveform has a large distortion of high-order harmonic components; the internal resistance decreases, even close to zero; if a fault occurs, the current waveform is near zero , the current transformer will cause a linear relationship transfer; at the moment of the fault, the transformer will start to reach saturation after a delay of about 5 seconds. Under normal circumstances, it is strictly forbidden to open the secondary circuit of the current transformer. Because during the operation of the current transformer, once the secondary open circuit occurs, the primary current will be converted into the excitation current, causing the magnetic flux density of the iron core to increase, resulting in rapid saturation of the current transformer. The saturation magnetic flux will produce a higher voltage, which will cause greater damage to the primary and secondary winding insulation facilities and easily pose a threat to personal safety.
1. Transformer protection impact and countermeasures
Generally, transformers have small capacity and high reliability, and are mostly installed on 10kV and 35kV busbars. The high-voltage short-circuit current is the same as the short-circuit current of the system, while the short-circuit current on the low-voltage side is relatively large. If the protection of the transformer is not in place, it will seriously affect the safe operation of the transformer or the entire system. Traditional transformers have fuse protection devices, which have the advantages of safety and reliability. However, with the improvement of system automation requirements and the increase of short-circuit capacity, traditional methods can no longer meet the needs. For some newly built or renovated substations, they are often equipped with transformer switch cabinets, and the system protection devices are similar to those of 10kV lines. However, the disadvantage is that the saturation problem of current transformers is often ignored. At the same time, due to the small capacity and primary current of the transformer, a shared transformer is used. In order to ensure measurement accuracy, the transformation ratio of the current transformer will be reduced. Once the transformer fails, it will cause the saturation of the current transformer and the secondary current speed will decrease, causing the transformer protection to refuse to operate. If a fault occurs on the high-voltage side of the transformer, the resulting short-circuit current will automatically cut off the backup protection action. If a fault occurs on the low-voltage side, the generated short-circuit current cannot reach the start-up value of the backup protection, which will make the fault unremovable, and even cause the transformer to burn out, which will seriously affect the safe operation of the system.
To solve the protection failure of the transformer, we need to start with the reasonable configuration of the transformer. When selecting the current transformer, we must take into account the saturation problem caused by the failure of the transformer. Current transformers with different functions should be distinguished from each other. For example, the transformer for measurement should be set on the low-voltage side of the transformer to ensure the measurement accuracy requirements; the transformer for protection is generally set on the high-voltage side of the transformer to ensure conservation work.
2. Current protection influence and countermeasures
After the current transformer is saturated, it will cause the reduction of the secondary equivalent current, causing the protection to refuse to operate. When it is far away from the power supply or the impedance coefficient is large, the short-circuit current at the line outlet will be small. However, if the scale of the system is expanded, the short-circuit current will increase accordingly, even reaching hundreds of times the primary current of the transformer, causing saturation of the transformer in the system that was originally functioning normally. At the same time, short-circuit current faults are transient processes, and there are a large number of different phase components in the current, which will accelerate the saturation of the current transformer. If a short-circuit fault occurs in a 10kV line, the saturation of the current transformer will reduce the current on the secondary side, causing the protection device to refuse to operate. Cutting off the switch on the low-voltage side of the busbar and the main transformer will increase the scope of the fault and prolong the time, which will affect the reliability of the power supply.
In severe cases, it will threaten the safe operation of the equipment.
According to the above analysis, when the current transformer is saturated, it will cause the primary current to change into the excitation current. At the same time, the secondary current is zero, the current through the relay is also zero, and the protection device in the equipment refuses to operate. In view of the above problems, the load impedance of the transformer should be reduced as much as possible to avoid the sharing of current transformers, and at the same time increase the cross-sectional area and cable length of the cable; the transformation ratio of the current transformer should not be too small, and attention should be paid to the saturation problem caused by the short circuit of the line.