**[b]1. Problem Statement[/b]** On power lines with different power sources on both sides, to avoid equipment damage caused by asynchronous parallel operation, the three-phase reclosing on both sides of the line typically employs a synchronization check method and a line no-voltage check method, respectively. Once a transient fault occurs on the line, the protection devices on both sides of the line activate and trip the switches. After the reclosing operation using the line no-voltage check method, if the power sources on both sides of the line have not lost synchronization and can meet the conditions for synchronous parallel operation, then the reclosing operation using the synchronization check method closes the switch on this side, and the line resumes power supply. Clearly, the necessary condition for successful line reclosing is that the power sources on both sides can still maintain synchronous operation after the line fault is disconnected. However, for most small power plants operating in parallel with the grid, since they usually adopt a single-line, single-channel grid connection method, the possibility of an isolated small power plant maintaining synchronization with the system after the tie line trips is extremely low. Therefore, after a line fault, the reclosing success rate is close to zero. At this time, the reclosing is practically useless. If the tie line has a large switching power before the line fault occurs, then after the tie line is disconnected and the system is disconnected, the isolated system where the small power plant is located will experience a large power surplus (when the small power plant sends power out through the tie line) or deficit (when the small power plant receives power through the tie line), which will greatly affect the power supply quality of the isolated system, and in severe cases, power outages may be necessary, and even damage to electrical equipment may be caused. Therefore, to improve the reliability of power supply, it is necessary to study other reclosing methods. In some cases, using busbar-detection-free reclosing instead of synchronization-detection reclosing is a good choice. [b]2 Application of Busbar-Detection-Free Reclosing[/b] As shown in Figure 1, the small power plant G is connected to system S via tie line l1 through the busbar and line l2 of substation B. Simultaneously, substation B supplies power to load P. [img=300,190]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/xbslfd/2002-1/26-1.jpg[/img] Since the fault tripping of tie line L1 does not affect the system's power supply to substation B, the reclosing methods on both sides remain in the traditional configuration. The reclosing on the substation side uses the line-no-voltage detection method, while the reclosing on the power plant side uses the synchronization detection method (given the low expected success rate of reclosing on this side, it can also be disabled). Line L2 provides a grid connection channel for power plant G and also supplies power to substation B along with it. When L2 trips, small power plant G is disconnected from system S. If small power plant G supplies power to load P of substation B alone, as mentioned earlier, when there is a large power difference between the output of power plant G and load P, the power quality of this islanded system will not meet the requirements of load P. The result could be that the low-frequency and low-voltage disconnection protection of the small power plant trips the disconnection point switch, causing a complete power outage for load P; or the low-frequency load shedding device installed at substation B trips, disconnecting part of the load. In either case, the power supply reliability of load P will be significantly reduced, which should be avoided as much as possible. Therefore, the reclosing of line l2 should not use the same method as that of tie line l1. If the reclosing of line l2 on the substation side (load side) is set to detect busbar de-voltage, its system side (power supply side) reclosing should still be set to detect line de-voltage. When a transient fault occurs on line l2 and the switches on both sides of the line trip due to protection, the reclosing on the system side of line l2 will close its own switch after detecting line de-voltage (indicating that the switch on the opposite side has tripped) and after a set time limit; simultaneously, the reclosing on the substation side of line l2 will also close its own switch after detecting substation busbar de-voltage (indicating that the small power plant disconnection protection has tripped and the disconnection point switch has tripped) and after a set time limit. In this way, power can continue to be supplied to the load P of substation B, significantly improving its power supply reliability. To prevent the failure of the small power plant disconnection protection from affecting the success rate of the busbar reclosing under no-voltage conditions on the L2 substation side, a circuit can be added to the L2 substation side protection to trip the L1 substation side switch. In this way, once a fault occurs at L2, the substation side protection will not only trip its own switch, but also the L1 substation side switch, thereby ensuring that the L2 substation side busbar reclosing under no-voltage conditions can be successfully reclosed. [b]3 Implementation of the busbar no-voltage reclosing device[/b] The no-voltage detection reclosing usually refers to the reclosing under no-voltage conditions of the line. The voltage on the switch line side (introduced to the reclosing device by the line PT) is less than the set value, which is a necessary condition for its start. The busbar no-voltage reclosing discussed in this article takes the voltage of the busbar at the reclosing device installation location as the detection object - the busbar voltage being less than the set value is a necessary condition for the reclosing to start. As shown in Figure 2, using the busbar no-voltage detection method for reclosing eliminates the need to modify the conventional reclosing device itself; only the terminals originally connected to the line voltage within the device need to be reconnected to the busbar voltage. During the reclosing device's setting, conventional no-voltage detection reclosing can still be performed, and the line PT can be eliminated, making implementation very convenient. [img=337,235]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/xbslfd/2002-1/27-1.jpg[/img] Furthermore, when using the busbar no-voltage detection method for reclosing, since reclosing is only possible after all other power supply incoming switches on the busbar have tripped, similar to the synchronization detection method, the post-acceleration section of this protection can be disabled. [b]4 Conclusion[/b] Installing busbar-detection-based reclosing on switches near the load side of certain lines in small power grid connection channels can effectively improve the reliability of power supply to the load. This reclosing method has been adopted on four 110 kV interconnected lines in the Nantong power grid and is operating well.