Detailed analysis of problems in relay protection and solutions

In the practical application and operation of relay protection, there are some problems that are easily overlooked. The analysis is as follows:

1.1 Influence of magnetizing inrush current on relay protection device
The magnetizing inrush current is caused by the fact that the magnetic flux in the iron core cannot be abruptly changed when the transformer is put into operation, and the non-periodic component magnetic flux is generated, the transformer core is saturated, and the exciting current is sharply increased. The maximum value of the magnetizing inrush current of the transformer can reach 6-8 times of the rated current of the transformer, and it is related to the capacity of the transformer. The smaller the capacity of the transformer, the larger the multiple of the inrush current. The magnetizing inrush current has a large aperiodic component and is attenuated by a certain time coefficient. The time constant of the attenuation is also related to the capacity of the transformer. The larger the capacity, the larger the time constant, and the longer the inrush current exists. The 10 kV line is equipped with a large number of distribution transformers. When the line is put into operation, these distribution transformers are hung on the line. At the moment of closing, the magnetizing inrush current generated by each transformer is superimposed on the line and reflected back and forth, resulting in a complicated In the electromagnetic transient process, when the system impedance is small, a large inrush current occurs, and the time constant is also large. The current quick-break protection in the two-stage current protection is often made smaller due to the sensitivity, especially when the long line or system impedance is large. The excitation inrush current value may be greater than the device setting value, 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 the line transformers increase. The Guiyang North Power Supply Bureau had a problem that the 10 kV line could not be put into normal operation due to the inrush current after the substation increased capacity.

1.2 Methods to prevent inrush currents from causing misoperation
The obvious characteristic of the magnetizing inrush current is that it contains a large number of second harmonics. This characteristic is used in the main transformer main protection to prevent the protection from malfunction caused by the magnetizing inrush current. However, if it is used in 10 kV line protection, the protection device must be applied. Retrofitting will greatly increase the complexity of the device, so it is of poor practicality. Another characteristic of the magnetizing inrush current is that its size decays with time. At first, the inrush current is very large. After a period of time, the inrush current decays to zero. The current flowing through the protection device is the line load current. Using the inrush current, the current quick-break protection is added. A short time delay can prevent the malfunction caused by the magnetizing inrush current. The biggest advantage of this method is that it does not need to modify the protection device (or simply modify it). Although it will increase the fault time, it is stable for the system like 10 kV. The smaller impact of the operation is still applicable. In order to ensure reliable escape of the inrush current, the acceleration circuit in the protection device also has to be added with a delay. Through several years of exploration, the time limit of 0.15~0.2 s has been added to the 10 kV line current quick-break protection and acceleration circuit. In recent years, the operation is safe, and the protection device due to the magnetizing inrush current in the line can be well avoided. Malfunction.

2.1 The effect of TA saturation on protection
The short-circuit current at the exit of the 10 kV line is generally small, especially in substations in rural power grids, often away from the power supply, and the system impedance is large. For the same line, the magnitude of the short-circuit current at the outlet will vary depending on the size of the system and the mode of operation. As the scale of the system continues to expand, the short-circuit current of the 10 kV system will become larger, which can reach several hundred times of the rated current of the TA. Some TAs that can operate normally with a small ratio may be saturated. The short circuit fault is a transient process, and the short circuit current contains a large amount of non-periodic components, which further accelerates the TA saturation. When the 10 kV line is short-circuited, the current on the secondary side will be small or close to zero due to the saturation of the TA, so that the protection device will be rejected, and the fault will be removed by the bus-coupled circuit breaker or the main transformer backup protection, which not only prolongs the fault time. It will expand the scope of the fault, affect the reliability of the power supply, and seriously threaten the safety of the operating equipment.

2.2 Ways to avoid TA saturation
TA saturation is actually the saturation of the magnetic flux in the TA core, and the magnetic flux density is proportional to the induced potential. Therefore, if the TA secondary load impedance is large, the secondary loop induced potential is large at the same current, or the same Under the load impedance, the larger the secondary current is, the larger the induced potential is. In both cases, the magnetic flux density in the core is large, and when the magnetic flux density is large, the TA is saturated. When the TA is severely saturated, the primary current becomes the excitation current, the secondary side induced current is zero, the current flowing through the current relay is zero, and the protection device is rejected. Avoiding TA saturation mainly starts from two aspects. First, when selecting TA, the ratio cannot be chosen too small. Consider the TA saturation problem when the line is short-circuited. Generally, the 10 kV line protection TA ratio is preferably greater than 300/5. On the other hand, we should minimize the secondary load impedance of TA, try to avoid the protection and measurement of common TA, shorten the length of TA secondary cable and increase the cross section of secondary cable. For integrated automation substation, 10 kV line should be protected, measured and controlled as much as possible. The integrated product is installed locally on the control panel, which effectively reduces the secondary loop impedance and prevents TA saturation.

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