0 Introduction Automatic transfer switch (BZT) is an automatic device that quickly and automatically starts a backup power supply or other normally operating power supply when the main power supply is disconnected due to a fault or other reasons, ensuring that users whose main power supply has been disconnected do not experience power outages. A normal, non-operating backup power supply is considered a visible backup. Most of the 110 kV substations under the Nanhai Power Bureau are terminal-type substations with dual power supply, meeting the conditions for visible backup. To ensure the reliability and continuity of power supply, it is advisable to install an automatic transfer switch. Our bureau uses the SEL-351 relay manufactured by Schweitzer Engineering & Testing Co., Ltd. of the United States to implement the automatic transfer function of the 110 kV lines. This automatic transfer scheme simplifies the on-site secondary cable wiring and reduces the cumbersome discrete components (relays) required in traditional automatic transfer methods. This article will introduce the scheme of implementing automatic transfer of backup power using the SEL-351 relay at the Lishui substation. [b]1 Analysis of the working principle of the automatic transfer switch for backup power[/b] The 110 kV main wiring diagram of the Lishui substation is shown in Figure 1, and the logic block diagram of the automatic transfer switch for backup power line is shown in Figure 2. [img=294,203]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxt2000/0005/image5/t57-1.gif[/img] Fig.1 The main connection diagram of 110 kV system of Lishui Substation [img=334,247]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxt2000/0005/image5/t57-2.gif[/img] Fig.2 Logic diagram of spare power source automatic switch in Lifa line The 110 kV Lifa line of Lishui Substation is the working power source, and the Dali line is the backup power source. In this scheme, the SEL-351 relay needs to incorporate 3 analog voltage signals, 4 analog current signals, and 12 switching signals. The analog voltage signals are the secondary voltage of the 11 kV TV line, used to determine if the busbar is without voltage; the analog current signals are the three-phase current of the Lifa line and the B-phase current of the Dali line, used to determine if the incoming line has no current. The 12 switching signals are: the normally open auxiliary contacts of 1G, 2G, 3G, 4G, 5G, and 1DL and 2DL, used to determine the position of the disconnectors and circuit breakers; the closed positions of 1DL and 2DL, used to determine whether the switch tripped manually or due to a fault, here incorporating the normally open contacts of an externally installed double-position relay; the 1QK position (1QK is the automatic transfer switch for switching on/off); and the normally open contacts of an external voltage relay monitoring the secondary voltage of the 110 kV line TV (1YDR, 2YDR), used to determine if the incoming line has voltage. 1.1 Under normal operation, both the Lifa line and the Dali line are energized, 1DL is closed, and 2DL is open. The Lifa line serves as the working power source, while the Dali line serves as the backup power source. As shown in Figure 2, in the AND gate AND1, all logic inputs are 1 except for the logic variable SV1T, which is 0. 1.2 Automatic Transfer Operation Referring to Figure 2, when the Lifa line loses power, the 110 kV busbar loses voltage, and the Lifa line experiences no current. The logic variable SV1 is set to 1. After a timer delay T1, SV1T = 1, thus setting the output SV3 of the AND gate AND1 to 1. SV3 triggers the timer, and after T3 (15 cycles), SV3T = 1. Simultaneously, the device sends a trip command to the Lifa line switch. When the Lifa line trip command has been issued and the device determines that the Lifa line switch has tripped and the disconnector lockout condition is not met, the logic variable SV5 is set to 1. After a delay T2, SV5T = 1, and the device sends a closing command to the Dali line switch 2DL, enabling the backup power source to be connected. 1.3 The logic for ensuring the BZT device automatically switches on once is shown in Figure 2. When the Lifa line loses power or 1DL disconnects for any reason, the device sends a closing command to the Dali line switch and simultaneously sets OR1 to 1 via SV5T. Since the OR1 and 1QK contacts form a closed loop, the OR1 output remains at 1, thus blocking the Dali line's backup automatic switch. Additionally, the Lifa line's voltage loss also blocks the Dali line's backup automatic switch, ensuring the BZT device only switches on once. 1.4 Backup Automatic Switch Interlocking Conditions: ① Backup automatic switch operation once; ② Disconnector interlocking condition (any disconnector from 1G to 5G is in the open position); ③ Standby line in the closed position; ④ Standby line in the closed position; ⑤ Operating line not in the closed position; ⑥ 1QK not in the active position; ⑦ 110 kV busbar TV secondary voltage disconnection. If any of the above interlocking conditions are met, the device automatically interlocks, ensuring selective operation. 2. Parameter Setting: The busbar is set to no voltage when the phase-to-phase voltage is below 25 V, and the line is set to voltage when the secondary voltage of the line TV is above 25 V. Simultaneously, when the incoming line current is less than the minimum load value, it is determined that there is no current in the incoming line, and the setting value for the transmission line is 0.25 A. The logic timer T1 is set to 100 cycles to ensure that the automatic transfer switch operation must avoid the reclosing time limit of the transmission line. 2. Conclusion This paper introduces a scheme for implementing 110 kV line BZT in a substation with a single busbar segmented configuration. This scheme uses the SEL-351 relay as its core, utilizing its powerful logic function to achieve automatic transfer switch operation, thereby improving the reliability of the power supply system.