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This post was last edited by chenjinfeng on 2020-2-8 at 15:14. In multi-power supply networks, fault isolation devices can also be installed in appropriate locations, depending on the distribution of power supplies and loads in the network. In the event of a network failure, the power grid is divided into several separate grids that continue to operate, ensuring the safe operation of critical loads. The stability of each of these separated grids also needs to be taken into consideration. For the main grid, whether it will lose large loads as a result, and whether grid oscillations will occur after such losses; for smaller grids, whether large loads will suddenly increase, whether low-frequency phenomena will arise from such increases, and whether there are appropriate load-shedding devices available in case of low-frequency conditions. It can be seen that the concepts of fault disconnection and low-frequency load shedding are completely different, and their roles in the power grid are also distinct. The standby auto-transfer device is used in substations with multiple power supply sources; it automatically activates the backup power source when the main power supply is disconnected, thereby ensuring continuous power supply. 1. Automatic transfer upon incoming power failure: Line L1 is in operation while line L2 is in standby mode; that is, switches 1, 2, and 3 are in operation, switch 4 is in hot standby, and sectionalizer 5 is in operation. Line L1 has line voltage, and the charging for standby mode has been completed. When L1 fails, the protection on the M side trips switch 1; if it is a permanent fault, reclosing will not succeed. During the check for standby operation on the N side, if the bus voltage is lost while there is voltage on line L2, switch 3 is tripped after a delay and then switch 4 is closed; this is the standby operation using line L2, referred to as standby mode 1. Similarly, when line L2 is in operation and L1 is in standby, this is called mode 2 of standby operation. 2. Sectionalized automatic transfer: Lines L1 and L2 are both in operation, that is, switches 1, 2, 3, and 4 are closed; sectional switch 5 is in hot standby, and the charging for automatic transfer is complete. Similarly, assuming a permanent failure of L1, switch 1 fails to close successfully; the backup system on the N side checks and finds that bus section I is under voltage while bus section II has voltage. Then switch 3 is tripped after a delay, and switch 4 is closed – this is when bus section II takes over from bus section I, which is referred to as backup mode three. For the same reason, Section I is prepared for use with Section II, referred to as Method 4. Based on the current grid operation mode, the standby mode is selected by switching through the handle switch. Through the analysis of the automatic transfer switch operation, it can be seen that the electrical quantities that the device must utilize are the line voltages of the two lines, the bus voltages of the two buses, as well as the position signals (TWJ) and closing signals (KKJ) for switches 3, 4, and 5. To prevent the device from operating incorrectly in the event of a break in the bus PT, the B-phase currents of switches 3 and 4 are also used to provide current-based interlock functionality. Manually disconnect switch 3; at this point, KKJ returns to its normal state and power is supplied to the backup system, preventing switch 4 from being activated automatically and causing an accident. The function of the line PT is to ensure that the line ready for switching in has voltage, so that the switching-in is meaningful. The line is de-energized; backup power is being supplied. The backup auto-reclosure time should be greater than the reclosing time of the opposite-end switch. The backup auto-transfer switch should have its output contacts connected to the open and closed positions in the switch operation circuit. The backup supply in this system is generally used as a backup for the high-voltage power supply in substations; in some special locations, especially larger customer sites, a low-voltage backup system is also employed to enhance the reliability of power supply. For example, 1G and 2G are the low-voltage main circuits of the transformer, while 3G is the low-voltage busbar. It operates in mode 1B: 1G and 3G are closed, 2B is unloaded, and 2G is in hot standby. In general, under such load conditions, there is no auxiliary power source on the low-voltage side; therefore, the direction of the low-voltage backup protection for the main transformer is set from the transformer to the low-voltage busbar, with a sensitivity angle of 258°. Therefore, technically, it is required that the backup protection of the transformer operates to lock out automatic reclosing. For example, in the event of a K2 fault on the busbar (or a fault on a low-voltage outlet with the corresponding switch failing to operate), the transformer’s low-voltage backup protection will activate and disconnect 1G. If the backup system activates at this time, the 2G switch will close directly onto the fault site. Similarly, the high-voltage side backup protection of the transformer, which serves as a far backup protection against K2 failures, must also lock out the automatic transfer switch. The main protections of the transformer, including differential protection and non-electrical protections, will cause the high-voltage and low-voltage switches of the transformer to trip in order to isolate the fault site; at this point, the standby unit can be activated.