1. Inrush Current Problem in Lines (1) The Influence of Inrush Current on Relay Protection Devices Inrush current is unique to transformers. It is caused by the fact that when a transformer is put into operation, the magnetic flux in the transformer core cannot change abruptly, resulting in a non-periodic component of the magnetic flux, which saturates the transformer core and causes the excitation current to increase sharply. The maximum value of transformer inrush current can reach 6 to 8 times the rated current of the transformer, and it is related to the size of the transformer capacity. The smaller the transformer capacity, the larger the inrush current multiple. There is a large non-periodic component in the inrush current, which decays with a certain time coefficient. The decay time constant is also related to the size of the transformer capacity. The larger the capacity, the larger the time constant and the longer the inrush current exists. A 10kV line is equipped with a large number of distribution transformers. When the line is put into operation, these distribution transformers are connected to the line. At the moment of closing, the inrush currents generated by each transformer are superimposed and reflected back and forth on the line, generating a complex electromagnetic transient process. When the system impedance is small, a large inrush current will appear, and the time constant is also large. In two-stage current protection, the instantaneous overcurrent protection needs to take into account sensitivity, so the operating current value is often relatively small, especially in long lines or when the system impedance is large. The 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 ignore. However, it may occur when the number and capacity of line transformers increase. Our bureau once encountered a problem where a 10kV line could not be put into normal operation due to inrush current after the substation capacity was increased. (2) Methods to prevent maloperation caused by inrush current. Inrush current has a distinct characteristic, which is that it contains a large number of second harmonics. This characteristic is used in the main transformer protection to prevent the protection from maloperating due to 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 its practicality is very poor. Another characteristic of inrush current is that its magnitude decays over time. Initially, the inrush current is large, but after a period of time, it decays to zero. The current flowing through the protection device is the line load current. By utilizing this characteristic of inrush current, a short time delay can be added to the instantaneous overcurrent protection to prevent false tripping caused by inrush current. The biggest advantage of this method is that it does not require modification of the protection device (or only requires simple modification). Although it increases the fault time, it is still applicable to places like 10kV where the impact on the stable operation of the system is relatively small. In order to reliably avoid inrush current, a delay should also be added to the acceleration circuit in the protection device. Through several years of exploration, our bureau has added a time limit of 0.15 to 0.2s to the instantaneous overcurrent protection and acceleration circuit of 10kV lines. In recent years, the operation has been safe and can effectively avoid false tripping of the protection device caused by inrush current in the line. 2. TA saturation problem (1) The impact of TA saturation on protection The short circuit current at the outlet of 10kV lines is generally small, especially in substations in rural power grids, which are often far from the power source and have a large system impedance. For the same line, the magnitude of the short-circuit current at the outlet varies depending on the system scale and operating mode. As the system scale continues to expand, the short-circuit current in a 10kV system increases, potentially reaching hundreds of times the rated primary current of the current transformer (CT). Some CTs with small transformation ratios that were previously operating normally may become saturated. Furthermore, a short-circuit fault is a transient process, and the short-circuit current contains a large amount of non-periodic components, further accelerating CT saturation. During a short circuit on a 10kV line, due to CT saturation, the induced current on the secondary side will be very small or close to zero, causing the protection devices to fail to operate. The fault must be cleared by the bus tie circuit breaker or the main transformer's backup protection, which not only prolongs the fault time and expands the fault area, affecting power supply reliability, but also seriously threatens the safety of operating equipment. (2) Methods to avoid TA saturation: TA saturation is essentially the saturation of magnetic flux in the TA core. Since magnetic flux density is proportional to induced electromotive force (EMF), if the secondary load impedance of the TA is large, the induced EMF in the secondary circuit will be large under the same current condition. Alternatively, under the same load impedance, the larger the secondary current, the larger the induced EMF. Both of these situations will result in a large magnetic flux density in the core. When the magnetic flux density reaches a certain value, the TA will saturate. When the TA is severely saturated, the primary current becomes entirely the excitation current, the induced current on the secondary side is zero, the current flowing through the current relay is zero, and the protection device will fail to operate. To avoid TA saturation, two main approaches should be taken. First, when selecting a TA, the transformation ratio should not be too small. The TA saturation problem under short circuit conditions should be considered. Generally, the transformation ratio of a 10kV line protection TA should ideally be greater than 300/5. On the other hand, we should try to reduce the secondary load impedance of the TA, avoid sharing the TA for protection and metering, shorten the length of the TA secondary cable and increase the cross-section of the secondary cable; for integrated automation substations, 10kV lines should use integrated protection and control products as much as possible and install them on the control panel. This can effectively reduce the secondary circuit impedance and prevent TA saturation. 3. Protection of substation transformers (1) Problems with substation transformer protection Substation transformers are a relatively special type of equipment. They have a small capacity but very high reliability requirements. Moreover, their installation location is also very special. They are generally connected to the 10kV bus. Their high-voltage side short-circuit current is equal to the system short-circuit current, which can reach tens of kA. The low-voltage side outlet short-circuit current is also relatively large. We have always paid insufficient attention to the reliability of substation transformer protection, which will pose a great threat to the safe operation of substation transformers and even the entire 10kV system. Traditional substation transformer protection uses fuse protection, which has relatively high safety and reliability. However, with the increase in system short-circuit capacity and the requirements of integrated automation, this method has gradually failed to meet the requirements. Newly built or renovated substations, especially those with integrated automation systems, are mostly equipped with station transformer switchgear, and their protection configurations are similar to those of 10kV lines. However, the saturation problem of the current transformer (CT) used for protection is often overlooked. Because station transformers have small capacities and very low primary rated currents, and often share CTs for protection and metering, the CT ratio is often chosen very low during design to ensure metering accuracy, sometimes even as low as 10/5. As a result, when a station transformer fails, the CT will be severely saturated, inducing almost zero current in the secondary circuit, causing the station transformer's protection devices to fail to operate. If the fault is on the high-voltage side, the short-circuit current is sufficient to trip the bus tie protection or the main transformer backup protection. If the fault is on the low-voltage side, the short-circuit current may not reach the activation value of the bus tie protection or the main transformer backup protection, making it impossible to clear the fault in time, ultimately burning out the station transformer and seriously affecting the safe operation of the substation. (2) Solution: To address the problem of transformer protection malfunction, the solution should start with the rational configuration of protection systems. The selection of current transformers (CTs) should consider the saturation problem during transformer faults. Furthermore, the current transformers used for metering must be separate from those used for protection. The protection CTs should be installed on the high-voltage side to ensure protection of the transformer, while the metering CTs should be installed on the low-voltage side to improve metering accuracy. Regarding setting, the instantaneous overcurrent protection can be set according to the short circuit at the low-voltage outlet of the transformer, and the overload protection can be set according to the transformer capacity.