Discussion on transformer protection and current p

2022-08-24
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Discussion on transformer protection and current protection of current transformer saturation point

to study the working characteristics of current 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. However, for the protection level current, its saturation point may exceed 15 ~ 20 times the rated current. When the current transformer becomes relatively large, it will be difficult to carry out this test on site

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in addition, the saturation point of current transformer can also be measured through volt ampere characteristic test. As mentioned above, the saturation of current transformer is caused by the excessive magnetic flux density of iron core, which can be reflected by the induced electromotive force of 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: 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 primary current transformer, without the demagnetization of the primary current, the iron core is easy to be saturated under the action of a small current. Therefore, the construction period is 3 to 5 years, and the volt ampere characteristic test is constructed in three phases. It does not need to add a large current, and it is easy to realize on site

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 numerical value of excitation impedance GB 8626 (8) is large, and the magnetic potential of primary and secondary windings is in balance. However, if the 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 that cause the current transformer to reach saturation include: excessive current; The load is too large. When the load connected to the current transformer is too large, the secondary voltage will increase, resulting in the increase of the magnetic flux density of the iron core, reaching saturation

when the current transformer reaches saturation, it has the following characteristics: the secondary current decreases, and the current waveform has large distortion of high-order harmonic components; The internal resistance decreases, even close to zero; If a fault occurs and the current waveform is near zero, 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 strictly forbidden to open the secondary circuit of current transformer. Because during the operation of current transformer, once the secondary open circuit occurs, the primary current will be converted into excitation current, which will increase the magnetic flux density of iron core and lead to 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 it is easy to pose a threat to personal safety

1. Transformer protection impact 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, as the automation system reduces the variability of measurement technology for operators, and the requirements of system automation and short-circuit capacity increase, the traditional methods have been unable to meet the needs. For some newly-built and reconstructed substations, transformer switchgear is often configured, and the protection device of the system is also similar to that of 10kV lines, but the disadvantage is that the saturation problem 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, reduce the secondary current speed, and cause the protection of the transformer to refuse to operate. If the high-voltage side of the transformer fails, the short-circuit current generated 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 the backup protection, which will make the fault unable to be removed, and even cause the burning of the transformer, 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 failure when selecting current transformer. Current transformers with different functions should be different from each other. For example, the transformer used for measurement should 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, it will cause the reduction of secondary equivalent current and cause protection failure. When it is far away from the power supply or the impedance coefficient is large, the short-circuit current at the outlet of the line will be small. However, if the scale of the system is expanded, the short-circuit current will increase, even up to hundreds of times the primary current of the transformer, which will cause saturation of the physical characteristics of the mutual analysis test samples in the system. At the same time, the short-circuit current fault is a transient process, and there are a large number of different period 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, 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 the power supply, and even threaten the safe operation of the equipment in serious cases

according to the above analysis, when the current transformer is saturated, it will cause the primary current to change into excitation current. At the same time, the secondary current is zero, and the current through the relay is also zero, and the protection device in the equipment refuses to operate. In view of the above problems, we should try to reduce the load impedance of the transformer, avoid the sharing of current transformers, and increase the cable cross-sectional area and cable length; The transformation ratio of current transformer should not be too small, and attention should be paid to the saturation problem caused by line short circuit

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