Talking about Transformer Protection and Current Protection of Current Transformer Saturation Point

To study the working characteristics of the current transformer, confirm whether it will saturate when the external fault passes through a large current and affect the correctness of the protection action. It can be tested by some test methods. Obviously, the most direct test method is to carry the actual load on the secondary side, input current from the primary side, observe the secondary current to find the saturation point of the current transformer. However, for a current-rated current transformer, the saturation point may exceed 15 to 20 times the rated current. When the current transformer becomes larger, it is difficult to perform the test on site.

In addition to this, the saturation point of the current transformer can also be measured by a volt-ampere characteristic test. As mentioned above, the current transformer saturation is caused by the excessive magnetic flux density of the core, and the magnetic flux density of the core can be reflected by the induced electromotive force of the current transformer. Therefore, the saturation current of the current transformer can be calculated from the saturation voltage value on the volt-ampere characteristic curve. The test method of volt-ampere characteristics is as follows: the original open circuit, the current is input from the secondary side, and the voltage drop on the secondary winding is measured. Due to the original open circuit of the current transformer, there is no demagnetization of the primary current, and the iron core is easily saturated under the action of a small current. Therefore, the volt-ampere characteristic test does not require a large current to be added, which is easier to implement in the field.

Under normal conditions, the core flux in the current transformer is in an unsaturated state. At this time, the load impedance and the excitation current are small, and 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 iron core in the transformer increases and reaches saturation, Zm will decrease rapidly with the increase of saturation, and the linear proportional relationship between different excitation currents will be broken. The factors causing the current transformer to reach saturation mainly include: the current is too large; the load is too large. When the load connected to the current transformer is too large, the secondary voltage is increased, causing the magnetic flux density of the core to rise and reach saturation.

When the current transformer reaches saturation, the secondary current is reduced, and the current waveform has a large distortion of the higher harmonic component; the internal resistance is reduced, even close to zero; if a fault occurs, the current waveform is near zero. When the current transformer causes a linear relationship to be transmitted; at the moment of failure, the transformer will start to reach saturation after about 5 seconds of lag. Under normal circumstances, it is strictly forbidden to open the circuit of the current transformer. Because during the operation of the current transformer, once a secondary open circuit occurs, the primary current is converted into the excitation current, causing the magnetic flux density of the core to increase, resulting in rapid saturation of the current transformer. The saturation flux will generate a higher voltage, which will damage the insulation facilities of the primary and secondary windings and cause personal safety threats.

Generally, the transformer has small capacity and high reliability. Most of them are installed on the busbars of 10kV and 35kV. The high-voltage short-circuit current is the same as the short-circuit current of the system, and 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 that are safe and reliable. However, as system automation requirements increase and short-circuit capacity increases, traditional methods are no longer sufficient. For some newly built and modified substations, transformer switchgear are often equipped. The protection device of the system is similar to the 10kV line, but the disadvantage is that the saturation of the current transformer is often ignored. At the same time, due to the capacity of the transformer, the primary current is small, and a common transformer is used. In order to ensure the measurement accuracy, the ratio of the current transformer is reduced. Once the transformer fails, it will cause saturation of the current transformer, and the secondary current speed will decrease, causing the protection of the transformer to refuse. If a fault occurs on the high voltage side of the transformer, the generated short circuit current will automatically remove the backup protection action. If the low-voltage side fails, the short-circuit current generated cannot reach the backup protection starting value, 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 and rejection of the transformer, it is necessary to start from the reasonable configuration of the transformer. When selecting the current transformer, the saturation problem caused by the transformer failure should be taken into account. Current transformers with different functions should be distinguished from each other. For example, the transformer for metering should be placed on the low voltage side of the transformer to ensure the accuracy of the metering; and the transformer for protection is usually placed on the high voltage side of the transformer to ensure the transformer. Protection work.

After the current transformer is saturated, it will cause the secondary equivalent current to decrease, causing the protection to refuse. When away from the power supply or the impedance coefficient is large, the short-circuit current at the line exit will be small. However, if the scale of the system is enlarged, the short-circuit current will increase, even reaching hundreds of times the primary current of the transformer, causing saturation of the originally functioning transformer in the system. At the same time, the short-circuit current fault is a transient process, 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 the 10kV line, the saturation of the current transformer will reduce the current on the secondary side, causing the protection device to refuse. The removal of the switch on the low-voltage side of the busbar and the main transformer will increase the range of faults 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.

It is known from the above analysis that when the current transformer is saturated, a primary current is converted into an excitation current. At the same time, the secondary current is zero, the relay current is also zero, and the protection device inside the device is rejected. In view of the above problems, the load impedance of the transformer should be reduced as much as possible to avoid the sharing of the current transformer, and the cross-sectional area of the cable and the length of the cable should be increased; the ratio of the current transformer should not be too small, and attention should be paid to the saturation problem caused by the short circuit.