10kV on-load tap-changing transformer based on power electronic devices and its control method

Abstract

The present invention discloses a 10kV on-load tap-changing transformer based on power electronic devices in the field of transformers. The tap-changing unit is correspondingly connected to each phase tap-changing winding of the primary side. The tap-changing units have the same structure and include four power electronic switches. Each power electronic switch is composed of two anti-parallel thyristors. One end of each power electronic switch is connected to the tap-changing winding of the transformer, and the other end is directly connected in parallel. The tap-changing unit also includes a fifth switch, a transition resistor, a varistor, and a normally closed switch. The fifth switch is short-circuited between the electrode point between the first switch and the fourth switch and the electrode point between the second switch and the third switch after being connected in parallel with the transition resistor. The varistor is short-circuited between the other end of the first winding and the other end of the second winding after being connected in parallel with the normally closed switch. The present invention solves the overvoltage protection problem of common voltage level thyristors in the application of 10kV on-load tap-changing transformers and the short circuit problem that may occur between the tap-changing windings during on-load tap-changing.

Description

Technical Field

[0001] The present invention relates to the technical field of transformers, and in particular to an on-load tap-changing transformer and a control method thereof. Background Art

[0002] Currently, mechanical or vacuum switches are often used to adjust the transformer tap in on-load tap-changing transformers. However, these solutions suffer from short service life, high failure rates, and high maintenance costs, severely limiting the transformer's on-load tap-changing capabilities. Compared to mechanical and vacuum switches, power electronic switches offer advantages such as fast switching speeds and long lifespans. Therefore, to address this issue and promote the further development of on-load tap-changing transformer technology, the use of power electronic devices to replace traditional mechanical and vacuum switches has attracted widespread attention.

[0003] During normal operation, the voltage between the taps of an on-load tap-changing transformer is low, so the voltage required of the switching transistor is not high. Therefore, using common 1200V power electronic components can meet the withstand voltage requirements during normal operation. However, in the application of 10kV power electronic transformers, ensuring that the common 1200V power electronic components are not damaged during the transformer's power-up, startup, and shutdown processes is a technical challenge.

[0004] The prior art discloses a natural non-circulating current control method for an on-load tap-changing transformer without a transition resistor, CN113300642A, which uses power electronic switches to replace traditional mechanical and vacuum switches. However, another technical difficulty is how to ensure that 1200V power electronic devices with a common voltage level are not damaged when used in a 10kV system. In order to achieve the effect of natural non-circulating current, this solution requires detecting the current of each phase of the transformer, and the control process relies on the judgment of voltage and current thresholds, making the control procedure relatively complicated.

[0005] Another existing technical solution to avoid the short circuit phenomenon that may occur between the voltage regulating windings during on-load voltage regulation is to directly remove the transition resistors in the mechanical and vacuum switch solutions. By taking advantage of the fast speed of power electronic switches and accurately selecting the voltage regulation time, the short circuit time between the voltage regulating windings during the voltage regulation process is ensured to be short enough. The effectiveness of this solution is based on the assumption that short-term short circuits will not have a major impact on the system. It does not truly solve the short circuit problem during on-load voltage regulation. Therefore, its reliability in practical applications needs further verification. Summary of the Invention

[0006] In response to the shortcomings of the existing technology, the present invention provides a 10kV on-load tap-changing transformer based on power electronic devices. By utilizing a normally closed switch and a varistor, the application of a thyristor with a common voltage level (1200V) in a 10kV on-load tap-changing transformer is realized. At the same time, the solution uses a power electronic switch to control the switching on and off of the transition resistor, thus solving the problem of short circuits that may occur between the tap-changing windings during the on-load tap-changing process.

[0007] The object of the present invention is achieved as follows: a 10kV on-load tap-changing transformer based on power electronic devices, comprising:

[0008] A voltage regulating unit is provided on the primary side of the transformer, the voltage regulating unit is correspondingly connected to each phase voltage regulating winding, and the voltage regulating units have the same structure; the voltage regulating winding includes a first winding and a second winding, and the voltage regulating unit includes a first switch, a second switch, a third switch, and a fourth switch, one end of the first winding serves as an input, the other end of the first winding is connected to one end of the second switch, one end of the first switch is connected to the middle of the first winding, one end of the second winding serves as an output, the other end of the second winding is connected to one end of the third switch, one end of the fourth switch is connected to the middle of the second winding, the other end of the first switch is connected to the other end of the fourth switch, and the other end of the second switch and the other end of the third switch are connected together;

[0009] The voltage regulating unit also includes a fifth switch, a transition resistor, a varistor, and a normally closed switch. The fifth switch and the transition resistor are connected in parallel and short-circuited between the electrode point between the first switch and the fourth switch and the electrode point between the second switch and the third switch; the varistor and the normally closed switch are connected in parallel and short-circuited between the other end of the first winding and the other end of the second winding.

[0010] As a further limitation of the present invention, the number of the voltage regulating units in each phase can be selected according to needs.

[0011] A 10kV on-load tap-changing transformer control method based on power electronic devices comprises the following steps:

[0012] Step 1: Classify the possible conduction states of each switch in the voltage regulating unit in a steady state into conduction state 1, conduction state 2, and conduction state 3 in which the transformer secondary side voltage increases in sequence;

[0013] Step 2: Divide the control program into a "startup layer", "top layer", "middle layer", "bottom layer" and "shutdown layer"; the "startup layer" is mainly responsible for regulating the process of converting the normally closed switch into the conduction of each switch after the transformer is energized; the "top layer" mainly gives an indication of whether "boost" or "lower voltage" is needed based on the current voltage amplitude; the "middle layer" mainly assigns the next state based on the "boost" or "lower voltage" indication given by the "top layer" and the current state; the "bottom layer" executes the corresponding control steps according to the state assignment result of the middle layer; the "shutdown layer" executes the corresponding shutdown control steps according to whether there is a shutdown command;

[0014] Step 3: After the high-voltage side of the transformer is closed, the transformer is energized. After the control system is powered on, it first determines whether there is a start command. If there is no start command, it waits for the start command to arrive. If there is a start command, the control system triggers the conduction state 1, and at the same time sets the previous state and next state flags to the conduction state 1. After a delay, the control system sends a normally closed switch disconnection command;

[0015] Step 4. The "top-level" program gives an indication of whether to "boost" or "lower" based on the current voltage amplitude. If the current voltage is lower than the set value, a "boost" indication is given. If the current voltage is higher than the set value, a "lower" indication is given. If the current voltage is within the set value, no "boost" or "lower" indication is given.

[0016] Step 5: If there is no "boost" or "lower" indication, the program jumps to the "whether there is a stop command" judgment program. If there is a "boost" or "lower" indication, the middle layer assigns the next state according to the "boost" or "lower" indication and the current state;

[0017] Step 6: The "bottom layer" executes the corresponding control steps according to the state allocation result of the middle layer; if the current state and the next state flag bits are the same, no control steps need to be executed and this part of the subroutine ends; if the current state and the next state flag bits are different, the corresponding control process is executed according to the state change requirements;

[0018] Step 7: After the "bottom layer" subroutine is finished, enter the "shutdown layer" subroutine.

[0019] As a further limitation of the present invention, in step 1, the conduction state 1 is that the second switch and the third switch are turned on; the conduction state 2 is that the first switch and the third switch are turned on; and the conduction state 3 is that the first switch and the fourth switch are turned on.

[0020] As a further limitation of the present invention, the set value in step 4 is 2.5% of the rated value.

[0021] As a further limitation of the present invention, step 5 specifically includes:

[0022] If the "boost" indication is given and the current state is the conduction state 2, the next state flag is set to the conduction state 3, and this part of the subroutine ends. If the "boost" indication is given and the current state is the conduction state 1, the next state flag is set to the conduction state 2, and this part of the subroutine ends. If the "boost" indication is given and the current state is neither the conduction state 1 nor the conduction state 2, it means that the current state is the conduction state 3, and the next state flag is not modified, and this part of the subroutine ends. If the "buck" indication is given and the current state is the conduction state 2, the next state flag is set to the conduction state 1, and this part of the subroutine ends.

[0023] If the "voltage step-down" indication is given and the current state is conduction state three, the next state flag is set to conduction state two, and this part of the subroutine ends. If the "voltage step-down" indication is given and the current state is neither conduction state two nor conduction state three, it means that the current state is conduction state one, and the next state flag is not modified, and this part of the subroutine ends.

[0024] As a further limitation of the present invention, step 6 specifically includes:

[0025] If the current state is conduction state 1 and the next state is conduction state 2, first turn on the first switch trigger pulse and turn off the second switch trigger pulse, then turn on the fifth switch trigger pulse after a delay, and then the state switches to conduction state 2. Finally, the current state is set to conduction state 2, and this part of the subroutine ends;

[0026] If the current state is conduction state 2 and the next state is conduction state 3, first turn off the fifth switch trigger pulse and delay, then turn on the fourth switch trigger pulse and turn off the third switch trigger pulse, and finally wait for a set time period to ensure that the third switch current passes through zero within the time period and then turns off. At this time, the state is switched to conduction state 3, and finally the current state is set to conduction state 3, and this part of the subroutine ends;

[0027] If the current state is conduction state three and the next state is conduction state two, first turn on the third switch trigger pulse and turn off the fourth switch trigger pulse, then turn on the fifth switch trigger pulse after a delay, and then the state switches to conduction state two. Finally, the current state is set to conduction state two, and this part of the subroutine ends;

[0028] If the current state is conduction state two and the next state is conduction state one, first turn off the fifth switch trigger pulse and delay, then turn on the second switch trigger pulse and turn off the first switch trigger pulse at the same time, and finally wait for the set time period to ensure that the first switch current passes through zero and then turns off within the time period. At this time, the state is switched to conduction state one, and finally the current state is set to conduction state one, and this part of the subroutine ends.

[0029] As a further limitation of the present invention, step 7 specifically includes:

[0030] First, determine whether there is a shutdown command;

[0031] If there is no stop command, jump to the "top level" program to continue to determine whether the voltage needs to be increased or decreased. If there is a stop command and the current state and the next state are both on-state 2, set the next state flag to on-state 1.

[0032] If there is a stop command and the current state and the next state are both the conduction state 3, the next state flag is set to the conduction state 2;

[0033] If neither of the above two situations is met, the next state flag will not be modified; then it is determined whether the current state and the next state flag are both in the conduction state one. If not, the corresponding control process is executed according to the state change requirements. After execution, it jumps to the program entry to determine whether the current state and the next state are both in the conduction state two. The function of this part of the program is to continuously adjust the system to the state required for shutdown, conduction state one, before shutdown. If it is true, it means that the current system is in the conduction state one, which meets the shutdown state requirements. The control system sends a normally closed switch closing instruction, delays, and ensures that the normally closed switch is closed before closing the second switch and the third switch trigger pulse to complete the shutdown process.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1) By using varistors and normally closed switches, and by properly setting the device start and stop control logic, it is ensured that thyristors of a common voltage level (1200V) used in 10kV on-load tap-changing transformers are not damaged during transformer power-up, startup, and shutdown;

[0036] 2) During the voltage regulation process, the transition resistor is controlled by a power electronic switch to prevent short circuits between the voltage regulating windings during on-load voltage regulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0038] Figure 1 This is a main circuit structure diagram of a 10kV on-load tap-changing transformer based on power electronic devices provided by the present invention.

[0039] Figure 2This is the overall control program flow of a 10kV on-load tap-changing transformer based on power electronic devices provided by the present invention.

[0040] Figure 3 This is the control subroutine flow of "adjusting the next state flag bit according to the current state flag bit" of the 10kV on-load tap-changing transformer based on power electronic devices provided by the present invention.

[0041] Figure 4 This is the control subroutine flow of "executing corresponding control processes according to state change requirements" of the 10kV on-load tap-changing transformer based on power electronic devices provided by the present invention.

[0042] Figure 5 This is the boost simulation result when switching from S2 and S3 states to S1 and S3 states using the technical solution provided by the present invention.

[0043] Figure 6 This is the boost simulation result when switching from S1 and S3 states to S1 and S4 states using the technical solution provided by the present invention.

[0044] Figure 7 This is the voltage reduction simulation result when switching from S1 and S4 states to S1 and S3 states using the technical solution provided by the present invention.

[0045] Figure 8 This is the voltage reduction simulation result when switching from S1 and S3 states to S2 and S3 states using the technical solution provided by the present invention. DETAILED DESCRIPTION

[0046] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0047] The present invention provides a 10kV on-load tap-changing transformer based on power electronic devices, such as Figure 1 As shown in the figure, in its topology, the voltage regulating winding of each phase includes the first winding (upper half) and the second winding (lower half). Each voltage regulating unit of each phase contains five power electronic switches S1, S2, S3, S4, and S5. Each power electronic switch consists of two anti-parallel thyristors. The power electronic switch S5 is connected to the transition resistor. R In parallel, the switch S5 and the transition resistor RThe main purpose is to avoid short circuit between the voltage regulating windings during the voltage regulation process. In practice, the number of voltage regulating units per phase can be set according to the voltage regulation requirements; each voltage regulating unit also includes an 820V, 20KA varistor R v The varistor, along with the normally closed switch M connected in parallel with it, has the main function of absorbing the short-term 10kV overvoltage during the process of switching from switch M to switches S2 and S3 after the high-voltage side is closed. The main function of the normally closed switch M is to prevent the 10kV voltage from being applied to both ends of the electronic switch when the high-voltage side of the transformer is closed, causing damage to the switch.

[0048] The 10kV on-load tap-changing transformer based on power electronic devices and the control method thereof provided by the present invention are specifically implemented according to the following steps (because the voltage regulation process of phases A, B, and C is the same, the specific implementation steps are described below using phase A as an example):

[0049] Step 1: The voltage regulating unit is divided into three states according to the possible conduction conditions of each power electronic switch in a voltage regulating unit in steady state: S2 and S3 are on, S1 and S3 are on, and S1 and S4 are on. These three states correspond to the transformer secondary voltage increasing in sequence.

[0050] Step 2: Reference Figure 2 Due to the complexity of the on-load voltage regulation control process, in order to simplify the control procedure, the control procedure is divided into "start-up layer", "top layer", "middle layer", "bottom layer" and "shutdown layer"; the "start-up layer" is mainly responsible for the control of the process of the normally closed switch M turning on and turning on the power electronic switch after the transformer is energized; the "top layer" mainly gives an indication of whether "boost" or "lower voltage" is needed based on the current voltage amplitude; the "middle layer" mainly assigns the next state based on the "boost" or "lower voltage" indication given by the "top layer" and the current state; the "bottom layer" executes the corresponding control steps according to the state assignment result of the middle layer; the "shutdown layer" executes the corresponding shutdown control steps according to whether there is a shutdown command;

[0051] Step 3. After the high-voltage side of the transformer is closed, the transformer is energized. Since the normally closed switch M is closed during the high-voltage side closing process, the maximum voltage that each switch tube can withstand is only related to the voltage between the voltage regulating windings. Under a 10kV system, this voltage is usually up to 500V, ensuring the safety of conventional 1200V thyristors. When the high-voltage side is closed, the control system first determines whether there is a start command after being energized. If there is no start command, it waits for the start command to arrive. If there is a start command, the control system sends a trigger pulse to S2 and S3, and at the same time sets the previous state and next state flag bits to the state "S2 and S3". After a delay of 200ms, the control system sends a disconnection command to the normally closed switch M. Since switches S2 and S3 are short-circuited at the moment before M is disconnected, they cannot be turned on even if S2 and S3 have a trigger pulse. At this time, a varistor needs to be connected in parallel at both ends of switches S2 and S3. R v To avoid damage to the power electronic switch rated at 1200V due to a 10kV voltage applied to it from the moment M is disconnected to the time when the S2 and S3 trigger pulses arrive, that is, the time when S2 and S3 are turned on (since the frequency of the thyristor trigger pulse train is 10kHz, this period is up to 100 microseconds). The varistor should be 820V, 20KA type. After the control system sends the normally closed switch M disconnect command, it needs to delay for 500ms to ensure that the "top level" program is entered only after the switch M is disconnected.

[0052] Step 4. The "top-level" program indicates whether a "boost" or "downgrade" is required based on the current voltage amplitude. If the current voltage is 2.5% lower than the rated value, a "boost" indication is given. If the current voltage is 2.5% higher than the rated value, a "downgrade" indication is given. If the current voltage is within ±2.5% of the rated value, no "boost" or "downgrade" indication is given.

[0053] Step 5: If there is no "boost" or "depressurization" indication, the program jumps to the "whether there is a stop command" judgment program. If there is a "boost" or "depressurization" indication, the middle layer assigns the next state according to the "boost" or "depressurization" indication and the current state. Figure 3If the "boost" indication is given and the current state is "S1 and S3", the next state flag is set to "S1 and S4", and this part of the subroutine ends. If the "boost" indication is given and the current state is "S2 and S3", the next state flag is set to "S1 and S3", and this part of the subroutine ends. If the "boost" indication is given and the current state is neither "S2 and S3" nor "S1 and S3", it means that the current state is "S1 and S4". At this time, the upper limit of the system boost has been reached, and the next state flag is not modified, and this part of the subroutine ends. If the "voltage reduction" indication is given and the current state is "S1 and S3", the next state flag is set to "S2 and S3", and this part of the subroutine ends. If the "voltage reduction" indication is given and the current state is "S1 and S4", the next state flag is set to "S1 and S3", and this part of the subroutine ends. If the "voltage reduction" indication is given and the current state is neither "S1 and S3" nor "S1 and S4", it means that the current state is "S2 and S3", and the lower limit of the system voltage reduction has been reached. In this case, the next state flag is not modified, and this part of the subroutine ends.

[0054] Step 6. The "bottom layer" executes the corresponding control steps according to the state allocation result of the middle layer. If the current state and the next state flag are the same, it means that the upper or lower limit of the system voltage regulation capability has been reached, and there is no need to execute the control step. This part of the subroutine ends. If the current state and the next state flag are different, the corresponding control process is executed according to the state change requirements. See Figure 4 If the current state is "S2 and S3" and the next state is "S1 and S3", the S1 trigger pulse is first turned on and the S2 trigger pulse is turned off. At this time, S1 passes through the transition resistor R The S2 current is turned on to avoid short circuit between the voltage regulating windings, and then delayed for 20ms to ensure that the S2 current is zero-crossing and turned off within this period of time, and then the S5 trigger pulse is turned on to cut off the transition resistance. R , at this time the state switches to "S1 and S3", and finally the current state is set to "S1 and S3", and this part of the subroutine ends; if the current state is "S1 and S3" and the next state is "S1 and S4", first turn off the S5 trigger pulse and delay it for 20ms to ensure that the S5 current passes through zero during this time period and then turns off, and the transition resistor is put into operation. At this time, S1 passes through the transition resistor R and S3 are turned on, then the S4 trigger pulse is turned on and the S3 trigger pulse is turned off. At this time, the switch S4 is turned on, and finally waits for 20ms to ensure that the S3 current passes through zero during this period and then turns off. At this time, the state is switched to "S1 and S4", and finally the current state is set to "S1 and S4", and this part of the subroutine ends; if the current state is "S1 and S4" and the next state is "S1 and S3", the S3 trigger pulse is turned on and the S4 trigger pulse is turned off at the same time. At this time, S3 passes through the transition resistor RThe S4 current is turned on to avoid short circuit between the voltage regulating windings, and then delayed for 20ms to ensure that the S4 current is zero-crossing and turned off within this period of time, and then the S5 trigger pulse is turned on to cut off the transition resistance. R , at this time the state switches to "S1 and S3", and finally the current state is set to "S1 and S3", and this part of the subroutine ends; if the current state is "S1 and S3" and the next state is "S2 and S3", first turn off the S5 trigger pulse and delay it for 20ms to ensure that the S5 current passes through zero during this time period and then turns off, and the transition resistor is put into operation. At this time, S1 passes through the transition resistor R and S3 are turned on, then the S2 trigger pulse is turned on and the S1 trigger pulse is turned off. At this time, switch S2 is turned on. Finally, wait for 20ms to ensure that the S1 current passes through zero during this period and then turns off. At this time, the state is switched to "S2 and S3". Finally, the current state is set to "S2 and S3", and this part of the subroutine ends;

[0055] Step 7. After the "bottom layer" subroutine is finished, enter the "shutdown layer" subroutine. First, determine whether there is a shutdown command. If there is no shutdown command, jump to the "top layer" program and continue to determine whether the voltage needs to be increased or decreased. If there is a shutdown command and the current state and the next state are both "S1 and S3", the next state flag is set to "S2 and S3". If there is a shutdown command and the current state and the next state are both "S1 and S4", the next state flag is set to "S1 and S3". If neither of the above two situations is met, the next state flag is not modified; then determine whether the current state and the next state flag are both "S2 and S3". If not, refer to Figure 4 The subroutine shown executes the corresponding control process according to the state change requirements. After execution, it jumps to the entry of the judgment program to see whether the current state and the next state are both "S1 and S3". The function of this part of the program is to continuously adjust the system to the state "S2 and S3" required for shutdown before shutdown. If it is true, it means that the current system is in the "S2 and S3" state, which meets the shutdown state requirements. The control system then sends a closing instruction for the normally closed switch M with a delay of 500ms to ensure that the switch M is closed before the S2 and S3 trigger pulses are turned off to complete the shutdown process. In this process, the purpose of ensuring that M is closed before turning off the S2 and S3 trigger pulses is to ensure that S2 and S3 are short-circuited after being turned off, so as to avoid damage to the switch tube caused by the 10kV voltage applied to both ends of the 1200V thyristor.

[0056] According to the above specific implementation method, refer to Figure 5 、 Figure 6 、 Figure 7 and Figure 8The simulation results show that the waveform continuity in the process of adjusting the transformer output voltage can be guaranteed in the boost process of switching from state "S2 and S3" to "S1 and S3", and from state "S1 and S3" to "S1 and S4", and in the step-down process of switching from state "S1 and S4" to "S1 and S3", and from state "S1 and S3" to "S2 and S3".

[0057] The above content is only for the purpose of illustrating the technical idea of ​​the present invention. For example, the "normally closed switch M" in FIG1 is only a schematic diagram provided by the present invention. In practice, various types of switches such as normally closed contactors or magnetic latching relays with a power-off self-reset function can be used. This does not limit the scope of protection of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the scope of protection of the claims of the present invention.

Claims

1. A control method for a 10 kV on-load tap-changing transformer based on power electronic devices, comprising: A voltage regulating unit is provided on the primary side of the transformer, and the voltage regulating unit is correspondingly connected to each phase voltage regulating winding, and the voltage regulating units have the same structure; the voltage regulating winding includes a first winding and a second winding, and the voltage regulating unit includes a first switch, a second switch, a third switch, and a fourth switch, one end of the first winding serves as an input, the other end of the first winding is connected to one end of the second switch, one end of the first switch is connected to the middle of the first winding, one end of the second winding serves as an output, the other end of the second winding is connected to one end of the third switch, one end of the fourth switch is connected to the middle of the second winding, the other end of the first switch is connected to the other end of the fourth switch, and the other end of the second switch and the other end of the third switch are connected together; the voltage regulating unit also includes a fifth switch, a transition resistor, a varistor, and a normally closed switch, the fifth switch is connected in parallel with the transition resistor and then short-circuited between the electrode point between the first switch and the fourth switch and the electrode point between the second switch and the third switch; the varistor is connected in parallel with the normally closed switch and then short-circuited between the other end of the first winding and the other end of the second winding, and the number of the voltage regulating units per phase can be selected as needed, characterized in that the control method includes the following steps: Step 1: Classify the possible conduction states of each switch in the voltage regulating unit in a steady state into conduction state 1, conduction state 2, and conduction state 3 in which the transformer secondary side voltage increases in sequence; Step 2: Divide the control program into a "startup layer," a "top layer," a "middle layer," a "bottom layer," and a "shutdown layer." The "startup layer" is primarily responsible for controlling the transition from the normally closed switch to the conduction state of each switch after the transformer is energized. The "top layer" primarily indicates whether "boost" or "step-down" is needed based on the current voltage amplitude. The "middle layer" primarily assigns the next state based on the "boost" or "step-down" indication from the "top layer" and the current state. The "bottom layer" executes the corresponding control steps based on the state assignment results of the middle layer. The "shutdown layer" executes the corresponding shutdown control steps based on whether a shutdown command is issued. Step 3: After the high-voltage side of the transformer is closed, the transformer is energized. After the control system is powered on, it first determines whether there is a start command. If there is no start command, it waits for the start command to arrive. If there is a start command, the control system triggers the conduction state 1, and at the same time sets the previous state and next state flags to the conduction state 1. After a delay, the control system sends a normally closed switch disconnection command; Step 4. The "top" program gives an indication of whether to "boost" or "lower" based on the current voltage amplitude. If the current voltage is lower than the set value, a "boost" indication is given. If the current voltage is higher than the set value, a "lower" indication is given. If the current voltage is within the set value, no "boost" or "lower" indication is given. Step 5: If there is no "boost" or "lower" indication, the program jumps to the "whether there is a stop command" judgment program. If there is a "boost" or "lower" indication, the middle layer assigns the next state based on the "boost" or "lower" indication and the current state; Step 6: The "bottom layer" executes the corresponding control steps based on the state allocation results of the middle layer. If the current state and the next state flag bits are the same, no control steps need to be executed and this part of the subroutine ends. If the current state and the next state flag bits are different, the corresponding control process is executed according to the state change requirements. Step 7: After the "bottom layer" subroutine is completed, enter the "shutdown layer" subroutine.

2. The 10kV on-load tap-changing transformer control method based on power electronic devices according to claim 1 is characterized in that: In step 1, the conduction state 1 is that the second switch and the third switch are turned on; the conduction state 2 is that the first switch and the third switch are turned on; and the conduction state 3 is that the first switch and the fourth switch are turned on.

3. The 10kV on-load tap-changing transformer control method based on power electronic devices according to claim 1 is characterized in that: The setting value in step 4 is 2.5% of the rated value.

4. The 10kV on-load tap-changing transformer control method based on power electronic devices according to claim 1 is characterized in that: Step 5 specifically includes: If the "boost" indication is given and the current state is the conduction state 2, the next state flag is set to the conduction state 3, and this part of the subroutine ends. If the "boost" indication is given and the current state is the conduction state 1, the next state flag is set to the conduction state 2, and this part of the subroutine ends. If the "boost" indication is given and the current state is neither the conduction state 1 nor the conduction state 2, it means that the current state is the conduction state 3, and the next state flag is not modified, and this part of the subroutine ends. If the "buck" indication is given and the current state is the conduction state 2, the next state flag is set to the conduction state 1, and this part of the subroutine ends. If the "voltage reduction" indication is given and the current state is the conduction state three, the next state flag is set to the conduction state two, and this part of the subroutine ends. If the "voltage reduction" indication is given and the current state is neither the conduction state two nor the conduction state three, it means that the current state is the conduction state one, and the next state flag is not modified, and this part of the subroutine ends.

5. The 10kV on-load tap-changing transformer control method based on power electronic devices according to claim 1 is characterized in that: Step 6 specifically includes: If the current state is conduction state 1 and the next state is conduction state 2, first turn on the first switch trigger pulse and turn off the second switch trigger pulse, then turn on the fifth switch trigger pulse after a delay, and then the state switches to conduction state 2. Finally, the current state is set to conduction state 2, and this part of the subroutine ends; If the current state is conduction state 2 and the next state is conduction state 3, first turn off the fifth switch trigger pulse and delay, then turn on the fourth switch trigger pulse and turn off the third switch trigger pulse, and finally wait for a set time period to ensure that the third switch current passes through zero within the time period and then turns off. At this time, the state is switched to conduction state 3, and finally the current state is set to conduction state 3, and this part of the subroutine ends; If the current state is conduction state three and the next state is conduction state two, first turn on the third switch trigger pulse and turn off the fourth switch trigger pulse, then turn on the fifth switch trigger pulse after a delay, and then the state switches to conduction state two. Finally, the current state is set to conduction state two, and this part of the subroutine ends; If the current state is conduction state two and the next state is conduction state one, first turn off the fifth switch trigger pulse and delay, then turn on the second switch trigger pulse and turn off the first switch trigger pulse at the same time, and finally wait for the set time period to ensure that the first switch current passes through zero and then turns off within the time period. At this time, the state is switched to conduction state one, and finally the current state is set to conduction state one, and this part of the subroutine ends.

6. The 10kV on-load tap-changing transformer control method based on power electronic devices according to claim 1 is characterized in that: Step 7 specifically includes: First, determine whether there is a shutdown command; If there is no stop command, jump to the "top level" program to continue to determine whether the voltage needs to be increased or decreased. If there is a stop command and the current state and the next state are both on-state 2, set the next state flag to on-state 1. If there is a stop command and the current state and the next state are both the conduction state 3, the next state flag is set to the conduction state 2; If neither of the above two situations is met, the next state flag will not be modified; then it is determined whether the current state and the next state flag are both in the conduction state one. If not, the corresponding control process is executed according to the state change requirements. After execution, it jumps to the program entry to determine whether the current state and the next state are both in the conduction state two. The function of this part of the program is to continuously adjust the system to the state required for shutdown, conduction state one, before shutdown. If it is true, it means that the current system is in the conduction state one, which meets the shutdown state requirements. The control system sends a normally closed switch closing instruction, delays, and ensures that the normally closed switch is closed before closing the second switch and the third switch trigger pulse to complete the shutdown process.

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