Abstract: This paper briefly describes the main wiring and operation mode of automatic transfer switching (AAT) of backup power supply in transformer low-voltage side sectional circuit breakers, which is commonly used in engineering. It also discusses the principles to be followed to ensure the safe, reliable, and stable operation of the power grid. Keywords: Automatic transfer switching; sectional circuit breaker; operation mode In a power system, an automatic control device that automatically and quickly puts a backup power supply, backup equipment, or other power source into operation after the working power supply is disconnected due to a fault or other reasons, allowing users to restore power supply as quickly as possible, is called an automatic transfer switch (AAT). Using an AAT can improve power supply reliability, simplify relay protection configuration, limit short-circuit current, and improve bus residual voltage. With the increasing demands of users for power supply reliability, AAT has been widely used and is an important means for power departments to ensure continuous and reliable power supply to users. There are many configuration methods for backup power supplies, and they are complex in form. Generally, there are two basic methods: explicit backup and implicit backup. When the system is operating normally, the backup power supply is not working; this is called explicit backup. When the system is operating normally, the backup power supply is also in operation; this is called implicit backup. Implicit backup is actually mutual backup between two working power sources. There are three main automatic transfer schemes: low-voltage busbar sectionalizing circuit breaker automatic transfer, internal bridge circuit breaker automatic transfer, and line automatic transfer. This paper discusses the commonly used automatic transfer scheme and principles for transformer low-voltage side sectionalizing circuit breakers in engineering. 1. Principles of Automatic Transfer for Transformer Low-Voltage Side Sectionalizing Circuit Breakers There are four automatic transfer modes for transformer low-voltage side sectionalizing circuit breakers. During operation, the basic requirements for each are the same, and certain principles must be followed to ensure the normal operation of the automatic transfer device and the safe, reliable, and stable operation of the power grid. The automatic transfer device must be activated when the working power supply is lost and the backup power supply is normal. When the backup power supply does not meet the voltage conditions, the automatic transfer device should not activate but should immediately discharge. Simultaneously, it should be able to send a TV disconnection signal for the backup power supply line. In the event of a momentary loss of voltage in the backup power supply, it should be able to delay discharge for a certain period of time. Regardless of the cause of voltage loss due to the working power supply or equipment, the automatic transfer device should activate, including voltage loss caused by operator error. This allows the backup power supply to automatically start working, ensuring uninterrupted power supply. When the working power supply busbar loses voltage, a no-current check must be performed on the working power supply before the automatic transfer switch (ATS) can be activated. This is to prevent voltage loss caused by a break in the secondary voltage of the voltage transformer, which could lead to a false trip of the ATS. If the working power supply busbar temporarily loses voltage and then recovers, the charging time of the ATS should be reset to zero before resetting the charging time. The backup power supply can only be activated after the working power supply has been confirmed to be disconnected. After the working power supply loses voltage, regardless of whether its incoming circuit breaker is open or not, even if the incoming current has been measured to be zero, the circuit breaker must still be opened first, and the position of the circuit breaker must be confirmed to be open before the backup power supply can be activated. This is to prevent the backup power supply from being activated on a faulty component, which could escalate the accident and worsen the equipment damage. If the working power supply fault causes the protection to fail to operate, but the fault is cleared by the upstream backup protection, the backup power supply will be connected to the faulty working power supply after the ATS activates, which will escalate the accident. Before the backup power supply is automatically activated, the circuit breaker disconnecting the working power supply must be delayed. The delayed disconnection of the working power supply incoming circuit breaker is to avoid the busbar voltage drop caused by a fault in the working busbar outgoing line. This delay time should be greater than the longest external fault clearing time. Simultaneously, the operating time of the automatic transfer switch (ATS) should be designed to minimize the load outage time. A short outage period between the loss of voltage on the working bus and the automatic transfer of the backup power supply is beneficial for the self-starting of user motors. Operational experience shows that the operating time of the ATS should ideally be 1–1.5 seconds. The ATS should not operate when the working power supply is manually disconnected. After the working power supply incoming circuit breaker is closed locally or remotely, the contacts output by its operating circuit close, serving as the input to the ATS and charging the device. When the circuit breaker is tripped by the protection mechanism, the contacts remain closed and do not change position. When the circuit breaker is tripped locally or remotely, the contacts open, and the ATS immediately discharges, thus automatically deactivating. The ATS should have a lockout function. Each ATS should have a logic circuit to lock out the automatic transfer of the backup power supply to prevent the backup power from being transferred to a faulty component, thus preventing the escalation of the accident. As shown in Figure 1, with main transformers #1 and #2 operating separately, if a fault occurs on busbar I or a 10kV outgoing line, and its protection fails to operate, the backup protection of main transformer #1 will trip 1DL, causing a voltage loss on busbar I. In this situation, busbar II, supplied by main transformer #2, should not be connected to the faulty busbar I via 3DL. Instead, the switch output from the backup protection of main transformer #1 should block the automatic transfer switch (ATS) from operating. The ATS is only allowed to operate once. If, after a voltage loss and the ATS operates, the relay protection device trips again and disconnects the backup power, it indicates a possible permanent fault. Therefore, it is not allowed to reconnect the backup power to avoid repeatedly connecting to faulty components, causing unnecessary impact on the system and potentially more serious accidents. The ATS can only start or operate after the charging conditions are met and all preparatory work is completed. The ATS will not operate a second time after discharging. For 10kV small hydropower lines connected to the grid on the low-voltage side busbar, when the automatic transfer switch is implemented as a transformer automatic transfer switch, the small hydropower line circuit breakers on both busbar sections should be disconnected simultaneously; when it is implemented as a sectional circuit breaker automatic transfer switch, the small hydropower line circuit breakers on each busbar should be disconnected. 2. Application Example of Automatic Transfer Switch for Sectional Circuit Breakers on the Low-Voltage Side of Transformer The main wiring diagram of the automatic transfer switch scheme for the 35kV Gushan Substation of Chun'an County Power Supply Bureau in Zhejiang Province is shown in Figure 1. Figure 1: Main Wiring Diagram of Automatic Transfer Switch Scheme for Sectional Circuit Breakers on the Low-Voltage Side of Transformer As can be seen from Figure 1, when main transformer #1 and main transformer #2 are not operating simultaneously, and 3DL is closed, main transformer #1 and main transformer #2 are mutually exposed backup power supplies; this scheme is called transformer automatic transfer. When main transformer #1 and main transformer #2 are operating simultaneously, and 3DL is open, the 10kV I-section busbar and II-section busbar are mutually concealed backup power supplies; this scheme is called sectional circuit breaker automatic transfer. 2.1 Automatic Transfer Mode 1 (No. 1 Main Transformer in Operation, No. 2 Main Transformer on Standby) When No. 1 main transformer fails, its main protection device 1DL trips; or when the high-voltage side of No. 1 main transformer loses voltage, causing both 10kV Section I and Section II busbars to lose voltage, and current transformer TA1 has no current, then circuit breakers 1DL, 4DL, and the small hydropower line circuit breakers of the 10kV Section I and Section II busbars are disconnected, and No. 2 main transformers 2DL and 5DL are closed, supplying power from No. 2 main transformer. The automatic transfer start conditions for No. 2 main transformer are that both 10kV Section I and Section II busbars lose voltage, current transformer TA1 has no current, and 1DL is indeed in the open position. Checking that current transformer TA1 has no current is to prevent false transfer caused by a break in the secondary voltage of the voltage transformers of the 10kV Section I and Section II busbars. 2.2 Automatic Transfer Mode 2 (No. 2 Main Transformer in Operation, No. 1 Main Transformer on Standby) When No. 2 main transformer fails, its main protection device 2DL trips; or when the high-voltage side of No. 2 main transformer loses voltage, causing both 10kV Section I and Section II busbars to lose voltage, and current transformer TA2 has no current, then the circuit breakers of 2DL, 5DL, and the small hydropower lines on the 10kV Section I and Section II busbars are disconnected, and No. 1 main transformer 1DL and 4DL are closed, supplying power from No. 1 main transformer. The automatic transfer starting conditions for No. 1 main transformer are that both 10kV Section I and Section II busbars lose voltage, current transformer TA2 has no current, and 2DL is indeed in the open position. Checking that current transformer TA2 has no current is to prevent false transfer caused by a break in the secondary voltage of the voltage transformers on the 10kV Section I and Section II busbars. 2.3 Automatic Transfer Mode 3 (Main Transformer #1 in operation, Main Transformer #2 in operation, 10kV Sectional Circuit Breaker 3DL disconnected) When Main Transformer #1 fails, its main protection device 1DL trips; or when the high-voltage side of Main Transformer #1 loses voltage, causing the 10kV Section I busbar to lose voltage, and the current transformer TA1 has no current, while the 10kV Section II busbar has voltage, then 1DL, 4DL, and the 10kV Section I busbar small hydropower line circuit breaker are disconnected, and 3DL is closed to ensure the power supply to the 10kV Section I busbar. The automatic transfer starting conditions for the sectional circuit breaker are: 10kV Section I busbar loss of voltage, current transformer TA1 no current, 10kV Section II busbar voltage, and 1DL in the disconnected position. 2.4 Automatic Transfer Mode 4 (Main Transformer #1 in operation, Main Transformer #2 in operation, 10kV sectionalizing circuit breaker 3DL in open position) When Main Transformer #2 fails, its protection mechanism 2DL trips; or when the high-voltage side of Main Transformer #2 loses voltage, causing the 10kV II busbar to lose voltage, current transformer TA2 has no current, while the 10kV I busbar has voltage, then the circuit breakers 2DL, 5DL, and the 10kV II busbar small hydropower line are disconnected. 3DL is then closed to ensure power supply to the 10kV II busbar. The automatic transfer starting conditions for the sectionalizing circuit breaker are: 10kV II busbar loss of voltage, current transformer TA2 no current, 10kV I busbar voltage, and 2DL in the open position. In actual engineering, when the substation's highest load is lower than the capacity of one of the main transformers, the main transformers operate in a mode where one main transformer is in operation and the other is on standby. In this case, the transformer automatic transfer scheme should be adopted, i.e., Modes 1 and 2 in the engineering example. For situations involving numerous small hydropower stations with 10kV outgoing lines connected to the low-voltage busbar, when the main transformer automatically switches on, the 10kV small hydropower line circuit breakers on both busbars should be disconnected simultaneously. (Source: http://www.tede.cn) When the substation load exceeds the capacity of one of the main transformers, the main transformers operate in a split-operation mode, meaning both main transformers operate simultaneously with the low-voltage side sectional circuit breakers in the open position. In this case, the sectional circuit breaker automatic switching scheme should be used, i.e., methods 3 and 4 in the engineering examples. For situations involving numerous small hydropower stations with 10kV outgoing lines connected to the low-voltage busbar, when the sectional circuit breakers automatically switch on, only the 10kV small hydropower line circuit breakers on their respective busbars should be disconnected. After the sectional circuit breakers close, there should also be an overload tripping function, disconnecting the corresponding 10kV outgoing line circuit breaker to limit the load below the capacity of the currently operating main transformer, preventing damage to the main transformer due to overload. 3. Conclusion Through analysis of the principles, methods, and operating procedures of automatic transfer switching (ATS) for low-voltage side sectionalizing circuit breakers in transformers, Chun'an County Power Supply Bureau implemented transformer ATS in the 35kV Linqi and Danzhu substations in 2006, and sectionalizing circuit breaker ATS in the 35kV Fenkou and Gushan substations in 2007. Based on the simulated tests of the protection drive belt circuit breakers over the past 2-3 years and the actual operating results after implementation, the ATS has consistently operated correctly, effectively improving power supply reliability. These engineering practices demonstrate that the principles, methods, and operating procedures for ATS of low-voltage side sectionalizing circuit breakers in transformers are reasonable and correct.