Problems easily ignored in the hottest relay prote

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The easily neglected problem in relay protection

inrush current is caused by the fact that when 1 is the arc synchronous gear belt transformer is put into operation without load, the magnetic flux in the iron core cannot change suddenly, and the aperiodic component magnetic flux appears, which saturates the iron core of the transformer and sharply increases the excitation current. The maximum magnetizing inrush current of transformer can reach 6 ~ 8 times of the rated current of transformer, and it is related to the capacity of transformer. The smaller the transformer capacity is, the greater the magnetizing inrush current multiple is. The inrush current has a large aperiodic component and decays with a certain time coefficient. The decaying time constant is also related to the transformer capacity. The larger the capacity is, the greater the time constant is, and the longer the inrush current exists. 10kV lines are equipped with a large number of distribution transformers. When the circuit is put into operation, these distribution transformers are connected. At the moment of switching on, the excitation inrush generated by each transformer is superimposed and reflected back and forth on the circuit, resulting in a complex electromagnetic transient process. When the system impedance is small, there will be a large inrush current and a large time constant. The current quick break protection in the two-stage current protection, because it needs to take into account the sensitivity, the action current value is often small, especially when the long line or the system impedance is large. The magnetizing inrush current value may be greater than the setting value of the device, causing the protection to malfunction. This situation is not prominent when the number of line transformers is small, the capacity is small, and the system impedance is large, so it is easy to be ignored, but it may occur when the number and capacity of line transformers increase. Guiyang North Power Supply Bureau once had the problem that 10kV lines could not be put into normal operation due to inrush current after the substation capacity was increased

1.2 method to prevent inrush current from causing maloperation

inrush current has an obvious feature that it contains a large number of second harmonics. This feature is used in the main protection of the main transformer to prevent protection maloperation caused by inrush current. However, if it is used in 10kV line protection, the protection device must be modified, which will greatly increase the complexity of the device, so the practicality is very poor. Another characteristic of magnetizing inrush current is that its size decreases with time. At the beginning, the inrush current is very large, and after a period of time, the inrush current attenuation is zero. The current flowing through the protection device is the line load current. Using the characteristic of inrush current, adding a short time delay to the current quick break protection can prevent the maloperation caused by magnetizing inrush current. The biggest advantage of this method is that it does not need to modify the protection device (or only make simple transformation), Although it will increase the fault time, it is still applicable to places like 10kV that have little impact on the stable operation of the system. In order to reliably avoid magnetizing inrush current, the acceleration circuit in the protection device should also add time delay. After several years of exploration, a time limit of 0.15 ~ 0.2S has been added to the current quick break protection and acceleration circuit of 10kV line. From the perspective of recent years' operation, the operation is safe, and the misoperation of the protection device caused by inrush current in the line can be well avoided

2 TA saturation problem

2.1 the impact of TA saturation on protection

10kv line outlet short-circuit current is generally small, especially the substation in nongzhong, which is often far away from the power supply, and the system impedance is large. For the same line, the short-circuit current at the outlet will vary with the system scale and operation mode. With the continuous expansion of the system scale, the short-circuit current of 10kV system will increase, which can reach hundreds of times of the primary rated current of TA, and some TA with small transformation ratio that can operate normally in the system may be saturated; On the other hand, short-circuit fault is a transient process, and the short-circuit current contains a large number of aperiodic components, which further accelerates TA saturation. When the 10kV line is short circuited, due to TA saturation, the induced current on the secondary side will be very small or close to zero, so that the protection device will refuse to operate. The fault will be removed by the bus tie breaker or the backup protection of the main transformer, led by the leading enterprise, which will not only prolong the fault time, expand the fault range, affect the reliability of power supply, but also seriously threaten the safety of operating equipment

2.2 method to avoid TA saturation

ta saturation is actually the saturation of the magnetic flux in the TA iron core, and the magnetic flux density is directly proportional to the induced potential. Therefore, if the TA secondary load impedance is large, under the same current, the induced potential of the secondary circuit is large, or under the same load impedance, the greater the secondary current is, the greater the induced potential is, both of which will make the magnetic flux density in the iron core large, and the magnetic flux density is large to a certain value, TA is saturated. When TA is seriously saturated, the primary current becomes the excitation current, the induced current at the secondary side is zero, and the current flowing through the current relay is zero, and the protection device will refuse to operate. There are two ways to avoid TA saturation: first, when selecting TA, the transformation ratio should not be too small, and the problem of TA saturation should be considered when the line is short circuited. Generally, the transformation ratio of TA for road protection of 10kV lines, such as Wang Qingyuan, Secretary General of EPS special Committee of China Plastics Association, Zhang Chengming, Deputy Secretary General of polyurethane special committee, and polystyrene foaming board manufacturers, should be greater than 300/5. On the other hand, try to reduce the secondary load impedance of TA, try to avoid sharing TA for protection and measurement, shorten the length of TA secondary cable and increase the section of secondary cable; For the integrated automatic substation, the 10kV line shall be protected as far as possible (2) the bonded pipeline and the product of measurement and control shall be adopted and installed locally on the control panel, which can effectively reduce the secondary circuit impedance and prevent TA saturation

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