Discussion on Transformer Protection and Current Protection of Current Transformer Saturation Point

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

In addition, the saturation point of current transformer can also be measured through volt ampere characteristic test. As mentioned earlier, the saturation of the current transformer 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: open the primary side, apply current from the secondary side, and measure the voltage drop on the secondary side winding. Due to the open circuit of the original square of the current transformer, without the demagnetization of the original square current, the iron core is easy to be saturated under the action of small current. Therefore, the volt ampere characteristic test does not need to add a large current, which is easy to realize in the field.

Under normal conditions, the magnetic flux of the iron core in the current transformer is in an unsaturated state. At this time, the load impedance and excitation current are small, while the value of excitation impedance is large, and the magnetic potential of primary winding and secondary winding is 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 main factors causing the current transformer to reach saturation include: excessive current; Excessive load. When the load connected to the current transformer is too large, it will cause the increase of secondary voltage, resulting in the increase of magnetic flux density of iron core and saturation.

When the current transformer reaches saturation, the characteristics are as follows: the secondary current decreases, and the current waveform has large distortion of high-order harmonic component; The internal resistance decreases, even close to zero; If a fault occurs and the current waveform is near the zero point, the current transformer will cause linear relationship transmission; At the moment of failure, the transformer will lag about 5 seconds before reaching saturation. In general, it is forbidden to open the secondary circuit of current transformer. Because in the operation process of current transformer, once the secondary open circuit occurs, the primary current will be converted into excitation current, resulting in the increase of magnetic flux density of iron core and the rapid saturation of current transformer. Saturated magnetic flux will produce high voltage, which will cause great damage to the insulation facilities of primary and secondary windings, and is easy to pose a threat to personal safety.

1. Influence of transformer protection and Countermeasures

Generally, transformers have small capacity and high reliability. Most of them are installed on 10kV and 35kV buses. 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 whole 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, the traditional methods can not meet the needs. Some newly-built and reconstructed substations are often equipped with transformer switchgear, and the protection device of the system is similar to that of 10kV line, but the disadvantage is that the saturation of current transformer is often ignored. At the same time, due to the small capacity and primary current of the transformer, the common transformer is adopted. In order to ensure the accuracy of measurement, the transformation ratio of current transformer will be reduced. Once the transformer fails, it will cause the saturation of the current transformer and the decrease of the secondary current speed, resulting in the failure of the transformer protection. If a fault occurs at the high-voltage side of the transformer, the generated short-circuit current will automatically cut off the backup protection action. If a fault occurs at the low-voltage side, the short-circuit current generated cannot reach the starting value of backup protection, the fault will not be removed, and even the transformer will be burned, which will have a serious impact on the safe operation of the system.

To solve the protection failure of transformer, we need to start with the reasonable configuration of transformer, and consider the saturation problem caused by transformer fault when selecting current transformer. Current transformers with different functions shall be different from each other. For example, the transformer for measurement shall be set at the low-voltage side of the transformer to ensure the measurement accuracy requirements; The transformer for protection is generally set at the high-voltage side of the transformer to ensure the protection of the transformer.2. Influence of current protection and Countermeasures

After the current transformer is saturated, the secondary equivalent current will be reduced and the protection will 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, even hundreds of times of the primary current of the transformer, resulting in the saturation of the transformer that can normally operate in the system. At the same time, short-circuit current fault belongs to transient process, and there are a large number of different period components in the current, which will accelerate the saturation of current transformer. If a short-circuit fault occurs in a 10kV line, the saturation of the current transformer will reduce the current at the secondary side, resulting in the failure of the protection device. The removal of the switch on the low-voltage side of the bus and the main transformer will increase the scope and time of the fault, affect the reliability of power supply, and threaten the safe operation of the equipment in serious cases.

According to the above analysis, when the current transformer is saturated, the primary current will be transformed into excitation current. At the same time, the secondary current is zero and the current through the relay is 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, the sharing of current transformers should be avoided, and the cable cross-sectional area and cable length should be increased at the same time; The transformation ratio of current transformer shall not be too small, and attention shall be paid to the saturation problem caused by line short circuit.