Abstract: For substations with small power sources connected to the grid, when the main power supply line of the substation fails, if the protection settings are not configured properly and the operation mode is not arranged correctly, there is a possibility that the automatic transfer switch (ATS) may not operate correctly. This article, based on actual conditions, introduces the selection of the correct operation mode to solve the existing problems. Keywords: Small power source; Automatic transfer switch; Setting; Operation 1. Inappropriate Protection Settings Configuration In substations with small power sources connected to the grid, when the main power supply line fails, if the protection settings are not configured properly and the operation mode is not arranged correctly, there is a possibility that the automatic transfer switch (ATS) may not operate correctly, affecting normal power supply. Figure 1 shows the system diagram of Yanghe substation with small power sources connected to the grid. On February 21, the 736 Yangsi line and the No. 2 transformer of the 110kV Yanghe substation tripped. The zero-sequence protection of the line and the transformer protection operated. The reason for the tripping was the theft of the line guy wire. Following the accident, the automatic transfer switch at Yanghe Substation activated, supplying power via the 735 Yangli line. At this time, the entire Yanghe Substation lost power. Prior to the accident, the 736 Yangsi line was in operation, the neutral point of transformer #2 at Yanghe Substation was ungrounded, transformer #1 was on hot standby, the 35kV 396 Yangdian line was supplying power to the Yanghe Winery thermal power plant, and the 35kV 394 Yanglin line was charging the power lines. All other lines were powered by feeders. After the accident, due to the lack of operating power at Yanghe Substation, power was diverted from the Nanlin line to the 35kV section of the Yanghe Substation, and power was gradually restored from transformer #1. Throughout the process, transformer tripping prolonged the time required to restore power. Figure 1 shows the system diagram of Yanghe Substation with a small power source connected to the grid. After a line ground fault occurred, the 110kV Yangsi 736 circuit breaker at Siyang Substation tripped, clearing the fault on the 736 Yangsi line. The Siyang Substation 736 reclosing circuit should have operated, but because a small power plant connected to the Yangdian 396 line below was generating electricity, the power plant's circuit breaker had a time limit for operation during the 110kV automatic transfer switch setting. During this time, the small power plant continued to charge the main transformer, causing phase A of the faulty line to ground, resulting in a rise in the neutral point voltage exceeding the protection setting value. The No. 2 main transformer protection tripped, clearing the circuit breaker and disconnecting the 396 Yangdian line. Analysis of the accident situation shows that the entire accident was related to the protection settings and the operating mode at the time. The main problem to be solved is how to prevent the main transformer protection from failing to operate when a small power source is connected to the grid and a line fault occurs, while the automatic transfer switch operates correctly. 2. Solutions 2.1 Protection settings should consider the coordination of settings and time limits. For transformers with small power supply grid connection and neutral point grounding, protection settings should consider the coordination of settings and time limits, enabling zero-sequence current protection and disabling gap protection. Since the zero-sequence current protection time is used as a backup protection time for the transformer, it can be set to be longer, while the gap protection, which protects against transformer faults, has a specified time of only 0.5s, making it difficult to coordinate with other protections. Protection settings should consider the coordination of settings and time limits, and also the setting of the disconnection time of the underfrequency low-voltage disconnection device, generally 48Hz/0.5s for frequency and 70V/5s for voltage. The provincial dispatch center has set requirements for the settings of low-frequency and low-voltage disconnection devices in power plants. For power plants primarily engaged in power transmission, the settings for low-frequency and low-voltage disconnection devices should be adjusted to 47.0Hz/0.5s and 70%Uφ/5s; for power plants primarily engaged in power reception, the settings should be adjusted to 48Hz/0.5s and 70%Uφ/5s. All power supply companies and power plants (companies) should adjust according to these settings. The overcurrent protection on the power plant side should have sufficient sensitivity to 110kV system faults, with a time not exceeding 1.5s. The protection time on the power plant side should be coordinated with the protection time of the main grid on the opposite side. This time should also be coordinated with the transformer backup protection time, and should be one level lower than the power plant side backup protection time. The zero-sequence overcurrent protection time of the transformer neutral point should be greater than 2.5s. The time should be increased to ensure sufficient level difference to coordinate with the zero-sequence protection action time of the line, preventing the transformer protection from acting first. When a fault occurs in the 110kV main power supply line (whether it is a ground fault or a phase-to-phase fault), after the main system circuit breaker opens, the low-frequency low-voltage disconnection device on the power plant side will operate first to disconnect the power plant from the system. If the disconnection device malfunctions, the power plant-side protection will operate to disconnect the power plant, with the transformer neutral point zero-sequence current protection serving as the last line of defense against ground faults. 2.2 Strengthen the operation and management of automatic transfer switch (ATS) devices. For substations with small power sources connected to the grid, their ATS devices must meet the requirements of the "Technical Regulations for Relay Protection and Automatic Safety Devices," which states that "the automatic transfer device should operate when the voltage on the working power source or equipment disappears for any reason." That is, when the grid-connected circuit breaker is used as a tie-line, the ATS operation must disconnect the small power source line to prevent asynchronous closing. When used as a feeder line, the ATS operation must be able to maintain the operation of the grid-connected line. 3. Solution 3.1 Analysis of Relevant Data: Analyze the load of each transformer and the total load of the substation; the capacity of connected small power sources; relevant impedance diagrams (including generators, transformers, and lines of small power sources); the protection configuration of relevant equipment (main transformer, lines; automatic device configuration); and the protection and automatic device settings of relevant equipment. Through analysis and calculation, a limit list was determined for the protection on the small power source side, requiring sufficient sensitivity to 110kV side faults. The limit list lists the operating values ​​of underfrequency and low-voltage devices, as well as the operating values ​​and time limits of overcurrent protection. Supervise timely implementation and feedback to ensure the coordination between the power plant protection settings and the main grid. 3.2 Establishing Requirements for Automatic Restart Operation: Based on the actual power generation of several small power plants, Yanghe Distillery normally operates in the morning and shuts down at night; during the summer, there are approximately two months of unit maintenance, during which it does not generate electricity; Zhengzhuang Substation's 122 Zhengjuanxian Substation supplies the silk spinning mill's thermal power plant, operating in the morning and shutting down at night; whether the pumping station connected to the 361 line of Chengnan Substation generates electricity depends on the season and water volume, generating electricity for periods and not generating electricity for periods. Analyzing the power generation of Yanghe Distillery and the silk spinning mill, since both substation equipment is operated by the monitoring center, if operators were to go to the substation daily to operate and frequently switch the pressure plates, the manpower and material resources would be too great. Therefore, the normal arrangement is to directly disconnect the small power supply during automatic restart, and this operation is notified to both small power stations in advance. During the summer maintenance period of Yanghe Distillery's units, the automatic restart operation will not disconnect the 396 Yangdian line or the 197 Yangjiu line. For the pumping station's 361 line, based on its seasonal power generation, operators will go to the site in advance to switch the pressure plates on and off. A special automatic restart operation instruction sheet for substations has also been developed. • Dispatchers and operators are required to strictly adhere to the following: • Dispatchers may only order the automatic transfer switch (ATS) to be engaged or disengaged, not the switch plates themselves. • When the operating mode of a small power supply grid-connected line changes, the dispatcher must notify the on-site operators. • On-site operators must determine the engagement/disengagement of the relevant switch plates based on the operating mode of the grid-connected line and record this in the on-site operating procedures or typical operation tickets. Through these measures, it is ensured that the ATS in substations with small power supply grid connection within Siyang County, Jiangsu Province, operates correctly in the event of a main power supply line failure, thus not affecting power supply to users.