Voltage stabilizing device and control method for long-distance power supply equipment side of coal mine excavation working face

Abstract

The present invention belongs to the technical field of underground power supply in coal mines, and specifically relates to a voltage stabilizing device and a control method for long-distance power supply equipment side of an underground excavation working face in a coal mine. In the device, the input end of a multi-tap three-phase transformer is connected to the output 3300V AC busbar of a preceding transformer through a long-distance transmission cable; the output of the multi-tap three-phase transformer leads to n groups of three-phase taps, which are respectively connected to the input ends of three-phase AC switches S1k~Snk, and the output ends of the three-phase AC switches S1k~Snk are connected in parallel and then connected to the input end of the three-phase AC switch S8; the input end of the intelligent voltage regulator is connected to the input end of the multi-tap three-phase transformer through the three-phase AC switch S0, and the output end is connected to the input end of the three-phase AC switch S8 through the three-phase AC switch S7; the voltage stabilizing controller is used to perform switch control on the three-phase AC switch, and is also used to control the intelligent voltage regulator. The present invention can improve the working efficiency of excavation equipment and reduce costs.

Description

Technical Field

[0001] The present invention belongs to the technical field of underground power supply in coal mines, and specifically relates to a voltage stabilizing device and a control method for long-distance power supply equipment on an underground tunneling working face in a coal mine, which is used for stabilizing the voltage at the end of a complete set of tunneling equipment under long-distance AC power supply conditions. Background Art

[0002] As the requirements for intelligent coal mine construction continue to increase, the demand for coal resources is also gradually increasing, and the speed requirements for underground coal mine excavation equipment are also gradually increasing. At present, the complete set of excavation equipment is limited by the power distribution system settings of the underground excavation working face. Usually, when the excavation distance exceeds 1 kilometer, it is necessary to move the transformer in the distribution system forward and shorten the length of the connecting cable so that the distance from the excavation equipment does not exceed 1 kilometer. This is because as the power supply distance increases, the active power loss and reactive power exchange on the power supply line will cause the voltage on the equipment side to drop, and the equipment cannot start normally. For this reason, it is necessary to dig a new transformer chamber at the destination after the move, and connect and retract the temporary support and power supply cables at the same time. When the excavation tunnel is long or inconvenient to move, the operation of moving the transformer forward will delay more working hours and reduce the efficiency of excavation work.

[0003] In order to improve the efficiency of excavation operations and achieve the goal of excavating a tunnel to the bottom without moving the transformer, it is necessary to perform voltage steady-state control on the excavation equipment side to reduce or eliminate the workload caused by canceling the movement of the transformer and improve production efficiency. Summary of the invention

[0004] The present invention is based on the shortcomings of the existing power distribution system of the tunneling working face, and the technical problem to be solved is: to provide a voltage stabilizing device and a control method for the long-distance power supply equipment side of the tunneling working face, to cooperate with the operating process requirements of the tunneling equipment, and to add a continuously adjustable voltage stabilizing device in front of the combination switch of the tunneling equipment, so as to stabilize the terminal voltage of the tunneling equipment, so that it can be smoothly started and operated within the entire length of the tunnel.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a voltage stabilizing device for long-distance power supply equipment side of coal mine excavation working face, comprising: a multi-tap three-phase transformer, an intelligent voltage regulator, three-phase AC switches S1k~Snk, S0, S7, S8, and a voltage stabilizing controller;

[0006] The input end of the multi-tap three-phase transformer is connected to the output 3300V AC busbar of the previous transformer through a long-distance transmission cable; the three-phase output of the multi-tap three-phase transformer leads out n taps respectively, forming n groups of three-phase output taps, which are respectively connected to the input ends of the three-phase AC switches S1k~Snk, and the output ends of the three-phase AC switches S1k~Snk are connected in parallel and then connected to the input end of the three-phase AC switch S8, and the output end of the three-phase AC switch S8 is used to connect equipment; wherein n represents an integer greater than 2;

[0007] The three-phase AC switches S1k~Snk are in an interlocked state, with only one switch closed at the same time, and each switch corresponds to the required voltage level of the equipment at a power supply distance;

[0008] The input end of the intelligent voltage regulator is connected to the input end of the multi-tap three-phase transformer through the three-phase AC switch S0, and the output end is connected to the input end of the three-phase AC switch S8 through the three-phase AC switch S7; it is used to provide the equipment side with transitional power during the gap time of switching between switches S1k to S6k;

[0009] The voltage regulator controller is used to control the switching of the three-phase AC switches S1k~Snk, S0, S7, and S8 according to the relationship between the current output voltage of the multi-tap transformer and the target voltage, and is also used to control the intelligent voltage regulator to output a voltage that is the same as the current output voltage of the multi-tap transformer.

[0010] The n=6, and the three-phase AC switches S1k~Snk correspond to the working conditions where the power supply distance between the equipment and the preceding transformer ranges from 1km to 6km.

[0011] The intelligent voltage regulator includes a Vienna rectifier arranged at the front stage, a three-level Buck buck arranged in the middle and a three-phase bridge inverter arranged at the rear stage. The Vienna rectifier is used for controllable rectification of unity power factor, the three-level Buck buck is used for closed-loop voltage reduction, and the three-phase bridge inverter is used for phase-locking the current output voltage of the multi-tap transformer and assigning the phase information to the output voltage of the rear stage to generate a three-phase AC voltage value with the same amplitude and phase as before the multi-tap three-phase transformer is switched.

[0012] In addition, the present invention also provides a control method for a voltage stabilizing device on a long-distance power supply equipment side of a coal mine excavation working face, comprising the following steps:

[0013] Step 1. After starting up, the front-stage transformer outputs voltage; the three-phase AC switches S1k~Snk maintain the last operating state, and the three-phase AC switch S8 is closed;

[0014] Step 2. The voltage regulator controller is powered on, and the input voltage Us7 of the three-phase AC switch S7 and the output voltage Us8 of the three-phase AC switch S8 are collected, and then the three-phase AC switch S8 is disconnected;

[0015] Step 3. Determine the relationship between the voltage Us8 and the working voltage range. If the working voltage range is met, keep the state of the three-phase AC switches S1k~S6k unchanged, and turn on the three-phase AC switch S8; if the upper cut-off working voltage range is met, go to step 4; if the lower cut-off working voltage range is met, go to step 5; if neither is met, go to step 6;

[0016] Step 4. Turn on S0, start the intelligent voltage regulator, and when the inlet voltage Us7 of the three-phase AC switch S7 is consistent with the outlet voltage Us8 of the three-phase AC switch S8, turn on the three-phase AC switch S7, then turn off the current three-phase AC switch, close the three-phase AC switch corresponding to the output tap of the next higher level, and turn off the three-phase AC switches S0 and S7, and finally turn on the three-phase AC switch S8;

[0017] Step 5. Turn on S0, start the intelligent voltage regulator, and when the inlet voltage Us7 of the three-phase AC switch S7 is consistent with the outlet voltage Us8 of the three-phase AC switch S8, turn on the three-phase AC switch S7, then turn off the current three-phase AC switch, close the three-phase AC switch corresponding to the output tap of the lower level, and turn off the three-phase AC switches S0 and S7, and finally turn on the three-phase AC switch S8;

[0018] Step 6. Turn on S0, start the intelligent voltage regulator, and when the inlet voltage Us7 of the three-phase AC switch S7 is consistent with the outlet voltage Us8 of the three-phase AC switch S8, turn on the three-phase AC switch S7, then disconnect the current three-phase AC switch, and finally turn on the three-phase AC switch S8.

[0019] The upper cut working voltage range < working voltage range < lower cut working voltage range;

[0020] In step 3, the operating voltage range is 93% to 107% of the target voltage; the upper cut operating voltage range is 90% to 93% of the target voltage, and the lower cut operating voltage range is 107% to 110% of the target voltage.

[0021] The target voltage is 1140V.

[0022] In the steps 4 to 6, the inlet voltage Us7 of the three-phase AC switch S7 and the outlet voltage Us8 of the three-phase AC switch S8 are consistent, which means that the phase and amplitude of the voltages are consistent.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. The present invention provides a voltage stabilizing device and control method for long-distance power supply equipment side of a tunneling working face, comprising a multi-tap three-phase transformer, an intelligent voltage stabilizer, a voltage stabilizing controller and a plurality of three-phase AC switches. The voltage stabilizing device inputs an AC voltage of 3300V, which is respectively output through the multi-tap transformer and the intelligent voltage stabilizer to supply power to the equipment side. The outputs of the multi-tap transformer and the intelligent voltage stabilizer are complementary. Under the control of the voltage stabilizing controller, the voltage stabilizing device can output a voltage suitable for the operation of the tunneling equipment. The output electric energy is connected to the power supply combination switch of the tunneling equipment, and the voltage stabilizing controller of the voltage stabilizing device draws power from the input 3300V system.

[0025] 2. The voltage stabilizing device of the present invention is arranged on the input side of the complete set of tunneling equipment, which can realize continuous adjustable output voltage, ensure that the input voltage of the tunneling equipment is stable within a suitable working range, so that the tunneling operation can be continuously and stably operated, which not only eliminates the need for a forward transformer, but also takes into account the advantages of low cost and continuous adjustment, thereby improving the working efficiency of the tunneling equipment and reducing the labor intensity of production personnel.

[0026] 3. Compared with the solution of simply increasing the output voltage of the power distribution system of the tunneling working face and manually adjusting the working voltage of the tunneling equipment, the present invention saves the cost of upgrading the electrical system of the tunneling equipment.

[0027] In summary, the present invention can demonstrate great technical and economic advantages. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A topological wiring diagram of a voltage stabilizing device for long-distance power supply equipment at a coal mine excavation working face provided in the first embodiment of the present invention;

[0029] Figure 2 is a topological diagram of the intelligent voltage regulator in the first embodiment of the present invention;

[0030] Figure 3 A flow chart of a method for voltage stabilization control on the long-distance power supply equipment side of a tunneling working face provided in the second embodiment of the present invention. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0032] Embodiment 1

[0033] like Figures 1-2As shown, an embodiment of the present invention provides a voltage stabilizing device on the side of a long-distance power supply equipment of a tunneling working face, which is arranged at the front stage of a combined switch of a complete set of tunneling equipment, including: a multi-tap three-phase transformer, an intelligent voltage regulator, three-phase AC switches S1k, S2k, S3k, S4k, S5k, S6k, S0, S7, S8, and a voltage stabilizing controller; the input end of the multi-tap three-phase transformer is connected to a 3300V AC bus; six taps are drawn out from the three-phase output of the multi-tap three-phase transformer to form six groups of three-phase output taps; the six groups of three-phase output taps are respectively connected to the input ends of the three-phase AC switches S1k~S6k, and the output ends of the three-phase AC switches S1k~S6k are connected in parallel to the three-phase AC The input end of the three-phase AC switch S8 is connected, and the output end of the three-phase AC switch S8 is used to connect the equipment; the three-phase AC switches S1k~S6k are interlocked, corresponding to the required voltage levels of the equipment at different power supply distances, and only one is closed at the same time; the input end of the intelligent voltage regulator is connected to the input end of the multi-tap three-phase transformer through the three-phase AC switch S0, and the output end is connected to the input end of the three-phase AC switch S8 through the three-phase AC switch S7; it is used to provide transitional period power to the equipment side during the gap time of switching between switches S1k to S6k, and its output voltage is consistent with that before the switch is switched. After the switching is completed, the intelligent voltage regulator is powered off and locked to save energy to the greatest extent.

[0034] The voltage regulator controller is used to control the switching of the three-phase AC switches S1k, S2k, S3k, S4k, S5k, S6k, S0, S7, and S8 according to the relationship between the current output voltage of the multi-tap transformer and the target voltage, and is also used to control the intelligent voltage regulator so that it outputs a voltage that is the same as the current output voltage of the multi-tap transformer.

[0035] Specifically, in this embodiment, each group of three-phase taps corresponding to the three-phase AC switches S1k~S6k respectively corresponds to the working conditions where the power supply distance between the equipment and the previous transformer is from 1km to 6km. For example, if the power supply distance is within 1km, the corresponding switch is the three-phase AC switch S1k, and if the power supply distance is in the range of 1km~2km, the corresponding switch is the three-phase AC switch S2k, and so on. In addition, in this embodiment, the number of three-phase output tap groups of the multi-tap three-phase transformer can also be greater than or less than 6, then it is only necessary to set the corresponding number of three-phase AC switches S1k~Snk according to the concept of the present invention, where n is equal to the number of output three-phase tap groups.

[0036] Specifically, in this embodiment, the multi-tap three-phase transformer and the three-phase AC switches S1k, S2k, S3k, S4k, S5k, S6k, S8 are encapsulated in one explosion-proof housing; the intelligent voltage regulator, the voltage stabilizing controller and the three-phase AC switches S0, S7 are encapsulated in another explosion-proof housing. The two explosion-proof housings are electrically connected by explosion-proof cables and communicate through optical fibers.

[0037] Figure 2 The topology diagram of the intelligent voltage regulator in the embodiment of the present invention. The intelligent voltage regulator includes a Vienna rectifier arranged at the front stage, a three-level Buck buck arranged in the middle, and a three-phase bridge inverter arranged at the rear stage, wherein the Vienna rectifier is used for controllable rectification of unity power factor, the three-level Buck buck is used for closed-loop voltage reduction, and the three-phase bridge inverter is used for phase-locking the current output voltage of the multi-tap transformer, assigning the phase information to the output voltage of the rear stage, and generating a three-phase AC voltage value with the same amplitude and phase as the multi-tap three-phase transformer before switching.

[0038] In this embodiment, after receiving the start-up command, the device turns on the three-phase AC switch S0, the control system of the intelligent voltage regulator is powered on, and then the Vienna rectifier of the front stage performs controllable rectification of the unity power factor, and the three-level Buck buck of the middle stage performs closed-loop voltage reduction. After the three-phase bridge inverter of the rear stage phase-locks the current output voltage of the multi-tap transformer, the phase information is assigned to the output voltage of the rear stage to generate a three-phase AC voltage value that meets the phase and amplitude requirements, and then the three-phase AC switch S7 is turned on according to the instruction of the voltage regulator controller to supply the power load equipment; after receiving the shutdown command, the three-phase AC switch S7 is cut off, the voltage regulator is locked, and the three-phase AC switch S0 is disconnected.

[0039] Embodiment 2

[0040] like Figure 3 As shown, the second embodiment of the present invention provides a control method for a voltage stabilizing device on a long-distance power supply equipment side of a tunneling working face, comprising the following steps:

[0041] Step 1. After starting up, the front-stage transformer outputs voltage; the three-phase AC switches S1k~S6k maintain the last operating state, and the three-phase AC switch S8 is closed;

[0042] Step 2. The voltage regulator controller is powered on, and the input voltage Us7 of the three-phase AC switch S7 and the output voltage Us8 of the three-phase AC switch S8 are collected, and then the three-phase AC switch S8 is disconnected;

[0043] Step 3. Determine the relationship between the voltage Us8 and the working voltage range. If the working voltage range is met, keep the state of the three-phase AC switches S1k~S6k unchanged, and turn on the three-phase AC switch S8; if the upper cut-off working voltage range is met, go to step 4; if the lower cut-off working voltage range is met, go to step 5; if neither is met, go to step 6;

[0044] Step 4. Turn on S0, start the intelligent voltage regulator, and when the inlet voltage Us7 of the three-phase AC switch S7 is consistent with the outlet voltage Us8 of the three-phase AC switch S8, turn on the three-phase AC switch S7, then turn off the current three-phase AC switch, close the three-phase AC switch corresponding to the higher-level tap, and turn off the three-phase AC switches S0 and S7, and finally turn on the three-phase AC switch S8;

[0045] Step 5. Turn on S0, start the intelligent voltage regulator, and when the inlet voltage Us7 of the three-phase AC switch S7 is consistent with the outlet voltage Us8 of the three-phase AC switch S8, turn on the three-phase AC switch S7, then turn off the current three-phase AC switch, close the three-phase AC switch corresponding to the lower-level tap, and turn off the three-phase AC switches S0 and S7, and finally turn on the three-phase AC switch S8;

[0046] Step 6. Turn on S0, and when the input voltage Us7 of the three-phase AC switch S7 is consistent with the output voltage Us8 of the three-phase AC switch S8, turn on the three-phase AC switch S7, then turn off the current three-phase AC switch, and finally turn on the three-phase AC switch S8.

[0047] In this embodiment, after the three-phase AC switch S8 is turned on, it can supply power to the subsequent equipment.

[0048] Specifically, in this embodiment, a higher-level tap refers to a tap with a higher output voltage, and a lower-level tap refers to a tap with a lower output voltage.

[0049] Specifically, in this embodiment, the upper cut working voltage range < working voltage range < lower cut working voltage range;

[0050] Specifically, in the present embodiment, in step 3, the operating voltage range is 93% to 107% of the target voltage; the upper cut operating voltage range is 90% to 93% of the target voltage, and the lower cut operating voltage range is 107% to 110% of the target voltage.

[0051] Specifically, in this embodiment, the target voltage is 1140V.

[0052] In addition, in this embodiment, by setting the upper cut-off operating voltage range and the lower cut-off operating voltage range, it can be ensured that within a relatively small voltage regulation range, by adjusting the output tap of the three-phase transformer, the output voltage can be adapted. If the required voltage regulation range is large, the voltage can be stabilized by directly switching the voltage stabilizer, and then the tap is adjusted step by step. This method can take into account both voltage stabilization efficiency and cost. Compared with the solution of using continuous closed-loop voltage regulation, the control device and method have the advantages of low cost and high reliability.

[0053] Specifically, in the steps 4 to 6, the inlet voltage Us7 of the three-phase AC switch S7 and the outlet voltage Us8 of the three-phase AC switch S8 are consistent, which means that the phase and amplitude of the voltages are consistent.

[0054] In summary, the present invention provides a voltage stabilizing device and a control method for the long-distance power supply equipment side of the tunneling working face, which is mainly aimed at improving the voltage regulation and reactive power compensation capabilities of the long-distance power supply equipment side of the tunneling working face in underground coal mines. It can also be widely used for other similar equipment designed and upgraded on the equipment side, including long-distance power supply scenarios with different forms, voltage levels and capacity levels.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A voltage stabilizing device for long-distance power supply equipment on a coal mine excavation working face, characterized in that: include: Multi-tap three-phase transformer, intelligent voltage regulator, three-phase AC switches S1k~Snk, S0, S7, S8, voltage regulator controller; The input end of the multi-tap three-phase transformer is connected to the output 3300V AC busbar of the previous transformer through a long-distance transmission cable; the three-phase output of the multi-tap three-phase transformer leads out n taps respectively, forming n groups of three-phase output taps, which are respectively connected to the input ends of the three-phase AC switches S1k~Snk, and the output ends of the three-phase AC switches S1k~Snk are connected in parallel and then connected to the input end of the three-phase AC switch S8, and the output end of the three-phase AC switch S8 is used to connect equipment; wherein n represents an integer greater than 2; The three-phase AC switches S1k~Snk are in interlocked state, with only one switch closed at the same time, and each switch corresponds to the required voltage level of the equipment at a power supply distance; The input end of the intelligent voltage regulator is connected to the input end of the multi-tap three-phase transformer through the three-phase AC switch S0, and the output end is connected to the input end of the three-phase AC switch S8 through the three-phase AC switch S7; it is used to provide the equipment side with transitional power during the gap time of switching between the switches S1k and Snk; The voltage regulator controller is used to control the switching of the three-phase AC switches S1k~Snk, S0, S7, and S8 according to the relationship between the current output voltage of the multi-tap transformer and the target voltage, and is also used to control the intelligent voltage regulator so that it outputs a voltage that is the same as the current output voltage of the multi-tap transformer.

2. A voltage stabilizing device for long-distance power supply equipment at a coal mine excavation working face according to claim 1, characterized in that: The n=6, and the three-phase AC switches S1k~Snk correspond to the working conditions where the power supply distance between the equipment and the preceding transformer ranges from 1km to 6km.

3. A voltage stabilizing device for long-distance power supply equipment at a coal mine excavation working face according to claim 1, characterized in that: The multi-tap three-phase transformer and the three-phase AC switches S1k~Snk, S8 are packaged in an explosion-proof housing; the intelligent voltage regulator, the voltage stabilizing controller and the three-phase AC switches S0, S7 are packaged in another explosion-proof housing.

4. A voltage stabilizing device for long-distance power supply equipment at a coal mine excavation working face according to claim 1, characterized in that: The intelligent voltage regulator includes a Vienna rectifier arranged at the front stage, a three-level Buck buck arranged in the middle and a three-phase bridge inverter arranged at the rear stage. The Vienna rectifier is used for controllable rectification of unity power factor, the three-level Buck buck is used for closed-loop voltage reduction, and the three-phase bridge inverter is used for phase-locking the current output voltage of the multi-tap transformer and assigning the phase information to the output voltage of the rear stage to generate a three-phase AC voltage value with the same amplitude and phase as before the multi-tap three-phase transformer is switched.

5. The control method of the voltage stabilizing device on the long-distance power supply equipment side of a coal mine excavation working face according to claim 1 is characterized in that: The following steps are involved: Step 1. After starting up, the front-stage transformer outputs voltage; the three-phase AC switches S1k~Snk maintain the last operating state, and the three-phase AC switch S8 is closed; Step 2. The voltage regulator controller is powered on, and the input voltage Us7 of the three-phase AC switch S7 and the output voltage Us8 of the three-phase AC switch S8 are collected, and then the three-phase AC switch S8 is disconnected; Step 3. Determine the relationship between the voltage Us8 and the working voltage range. If the working voltage range is met, keep the state of the three-phase AC switches S1k~Snk unchanged and turn on the three-phase AC switch S8; if the upper cut-off working voltage range is met, go to step 4; if the lower cut-off working voltage range is met, go to step 5; if neither is met, go to step 6; Step 4. Turn on S0, start the intelligent voltage regulator, and when the inlet voltage Us7 of the three-phase AC switch S7 is consistent with the outlet voltage Us8 of the three-phase AC switch S8, turn on the three-phase AC switch S7, then turn off the current three-phase AC switch, close the three-phase AC switch corresponding to the output tap of the next higher level, and turn off the three-phase AC switches S0 and S7, and finally turn on the three-phase AC switch S8; Step 5. Turn on S0, start the intelligent voltage regulator, and when the inlet voltage Us7 of the three-phase AC switch S7 is consistent with the outlet voltage Us8 of the three-phase AC switch S8, turn on the three-phase AC switch S7, then turn off the current three-phase AC switch, close the three-phase AC switch corresponding to the output tap of the lower level, and turn off the three-phase AC switches S0 and S7, and finally turn on the three-phase AC switch S8; Step 6. Turn on S0, start the intelligent voltage regulator, and when the inlet voltage Us7 of the three-phase AC switch S7 is consistent with the outlet voltage Us8 of the three-phase AC switch S8, turn on the three-phase AC switch S7, then disconnect the current three-phase AC switch, and finally turn on the three-phase AC switch S8.

6. A control method for a voltage stabilizing device on the side of a long-distance power supply equipment at a coal mine excavation working face according to claim 5, characterized in that: The upper-cutting operating voltage range < the operating voltage range < the lower-cutting operating voltage range.

7. The control method of the voltage stabilizing device on the long-distance power supply equipment side of a coal mine excavation working face according to claim 5 is characterized in that: In step 3, the operating voltage range is 93-107% of the target voltage; the upper cut operating voltage range is 90-93% of the target voltage, and the lower cut operating voltage range is 107-110% of the target voltage.

8. A control method for a voltage stabilizing device on the side of a long-distance power supply equipment at a coal mine excavation working face according to claim 7, characterized in that: The target voltage is 1140V.

9. The control method of the voltage stabilizing device on the long-distance power supply equipment side of a coal mine excavation working face according to claim 5 is characterized in that: In the steps 4 to 6, the inlet voltage Us7 of the three-phase AC switch S7 and the outlet voltage Us8 of the three-phase AC switch S8 are consistent, which means that the phase and amplitude of the voltages are consistent.

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