A method for commutation of a modular multilevel converter in a quasi-two-level mode

Abstract

This invention discloses a commutation method for a modular multilevel converter in quasi-two-level mode, comprising: determining the commutation stage and output current polarity, identifying the delayed control object, and determining the operation mode based on the phase voltage level switching direction; directly executing the operation of the non-delayed control object according to the determined operation mode and constructing two candidate paths for the delayed control object: direct operation and delayed operation; calculating the circulating current prediction value and the absolute error with the reference value corresponding to the direct operation and delayed operation respectively, and determining whether the delayed control object should operate directly or delayed based on minimizing the error; introducing a piecewise linear trajectory synthesis fine-tuning mechanism based on duty cycle, determining the delay time of the bridge arm submodule by calculating the duty cycle, and repeating the steps until the commutation process ends; this invention quickly completes the commutation of the upper and lower bridge arms by adding a delay, and at the same time, the piecewise linear trajectory synthesis mechanism based on duty cycle eliminates the steady-state error caused by the whole-cycle delay.

Description

Technical Field

[0001] This invention relates to the fields of medium-voltage drive and power electronic energy conversion technology, and more specifically to a commutation method for a modular multilevel converter in quasi-two-level mode. Background Technology

[0002] Modular multilevel converters have received widespread attention in the field of medium-voltage motor drives due to their modularity and ease of expansion. However, due to inherent defects in the topology, modular multilevel converters can cause large capacitor voltage fluctuations when driving motors at low speeds, resulting in unstable system operation.

[0003] Traditional solutions involve injecting high-frequency circulating current and common-mode voltage. However, this method cannot effectively suppress capacitor voltage fluctuations when the system operates under low-speed, high-load conditions. A more effective way to suppress voltage fluctuations is to make the modular multilevel converter operate in quasi-two-level mode. However, the difficulty in achieving quasi-two-level operation mode lies in the switching process of the upper and lower bridge arms, i.e., how to ensure that the corresponding bridge arm current reaches the reference value when the bridge arm switching is completed.

[0004] To address this, the academic community has proposed two solutions to achieve a quasi-two-level operating mode for modular multilevel converters:

[0005] The inner-loop high-frequency pulse width modulation method uses a faster inner loop to control the circulating current to complete the commutation process. The circulating current is controlled by a proportional or proportional-integral controller to obtain the corresponding bridge arm voltage reference value, which is then synthesized by the high-frequency pulse width modulation method to finally realize the commutation process. However, this method cannot guarantee that the commutation will be completed in the fastest way, which will cause large fluctuations in the capacitor voltage during the commutation process.

[0006] Open-loop delay control method: This method achieves the commutation process of the upper and lower bridge arms by delaying the insertion or bypass time of a sub-module of one bridge arm (upper or lower bridge arm). Essentially, this is an open-loop control method. By pre-calculating the number of delay cycles required and which sub-module to delay, the commutation of the upper and lower bridge arms is achieved while the corresponding bridge arm reaches the reference value. However, because it is an open-loop control method, it cannot adjust in real time according to the current bridge arm current during the commutation process, which will lead to a decrease in system control performance.

[0007] Therefore, how to provide a commutation method for a modular multilevel converter in quasi-two-level mode that can ensure the fastest commutation completion while achieving accurate current tracking after commutation is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0008] In view of the above problems, the present invention aims to provide a commutation method for a modular multilevel converter in quasi-two-level mode that overcomes or at least partially solves the above problems.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A commutation method for a modular multilevel converter in quasi-two-level mode includes: S1. Determine the current stage of the commutation and the polarity of the output current, identify the object to be delayed, and determine the operation mode of the upper arm submodule and the lower arm submodule based on the switching direction of the output phase voltage level of the upper arm or the lower arm, including bypass operation and access operation. S2. Perform the operation directly on the non-delay control object according to the determined action mode, and construct two candidate paths for the delay control object based on the single delay assumption: direct action and delayed action. The delayed action path is to keep the number of sub-modules invested unchanged at the current time and postpone the execution of the access action or bypass action determined in step S1 to the next time. S3. Based on the candidate paths constructed in step S2, calculate the circulating current prediction values ​​corresponding to direct actions and delayed actions, as well as the absolute error between the circulating current prediction values ​​and the reference values. Based on the principle of minimizing the error, determine whether the delayed control object should execute a direct action or a delayed action at the current moment. S4. Introduce a piecewise linear trajectory synthesis fine-tuning mechanism based on duty cycle. Determine the delay time of the bridge arm sub-module by calculating the duty cycle. Repeat steps S3-S4 until the commutation process ends.

[0010] Preferably, in step S4, if the delayed action is determined to be better than the direct action through rolling optimization, that is, when the absolute error between the predicted circulation value and the reference value corresponding to the delayed action is small, and the absolute value of the predicted circulation value of the delayed action exceeds the absolute value of the reference value, the duty cycle is introduced to finely adjust the timing of the action and determine the specific time of the delayed action.

[0011] Preferably, the duty cycle in step S4 The calculation is based on the principle of linear interpolation, specifically as follows:

[0012] in, This is a reference value for the circulation; the value varies depending on the direction of the circulation. The two can be switched. For phase output current, These represent the three phases of the inverter. These are the predicted circulation values ​​under direct action. This represents the predicted circulation value under delayed action.

[0013] Preferably, the piecewise linear trajectory synthesis fine-tuning mechanism based on duty cycle is achieved by sequentially combining the current change rates of two discrete physical states in the time domain within one control cycle, specifically including: The high-gain acceleration phase, i.e. In the initial stage of the control cycle, the control delay locks the switching state of the control object from the previous moment and does not operate. By utilizing the high gain characteristic, the current reduces the difference from the reference value at the fastest physically permissible speed. The low-gain steady-current state stage, i.e. During the phase, at t=k+dT d At any given moment, the control delay object performs the only switching action within this cycle to complete the switching of the bridge arm submodule.

[0014] Preferably, in step S1, when the polarity of the output current is positive, the lower bridge arm is the object of delay; when the polarity is negative, the upper bridge arm is the object of delay. When the output phase voltage level of the upper or lower bridge arm shows a downward trend, the corresponding submodule is determined to be in bypass operation mode, while the opposite bridge arm submodule is in access operation mode. When the output phase voltage level of the upper or lower bridge arm shows an upward trend, the corresponding submodule is determined to be in access operation mode, while the opposite bridge arm submodule is in bypass operation mode.

[0015] Preferably, in step S2, for non-delay control objects, the number of sub-modules deployed is updated according to the action mode determined in step S1; for delay control objects, the number of sub-modules deployed corresponding to the direct action path is updated according to the action mode determined in S1, while the number of sub-modules deployed corresponding to the delayed action path remains unchanged.

[0016] Preferably, in step S3, based on the discrete mathematical model of the converter, the system state variables at the current moment and the number of sub-modules to be deployed corresponding to the direct action and the delayed action determined in step S2 are used to calculate the circulating current prediction values ​​corresponding to the direct action and the delayed action, respectively.

[0017] Preferably, in step S3, the absolute error between the predicted circulation value and the reference value is:

[0018]

[0019] in, Predicted circulating current for delayed actions Compared with reference value The absolute error, Predicted circulation values ​​for direct action Compared with reference value The absolute error; like This indicates that the delayed control object will perform a delayed action; if This indicates that the direct action of the controlled object is delayed.

[0020] Preferred, circulation for Output current for , For the upper bridge arm current, For the lower arm current, the control objective of commutation is to reduce the circulating current. Guided to the steady-state value corresponding to the target conducting bridge arm.

[0021] Preferably, when the output current When the target conduction arm is positive and the upper arm is the control arm, Tend to When the output current When the target conduction arm is the lower arm, control Tend to When the output current When the value is negative and the target conducting arm is the lower arm, the control circulation is... Tend to When the output current When the value is negative and the target conducting arm is the upper arm, the control circulation is... Tend to .

[0022] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a commutation method for a modular multilevel converter in quasi-two-level mode, which has the following beneficial effects: This invention determines whether the current submodule should be bypassed (or inserted) or remain inactive by adding a delay, and makes the delay occur as early as possible in the commutation process, thereby maximizing the change in circulating current and enabling the circulating current to reach the reference value as quickly as possible, thus completing the commutation process of the upper and lower bridge arms quickly and minimizing the energy fluctuations brought about by the commutation process. This invention utilizes a piecewise linear trajectory synthesis mechanism based on duty cycle d to calculate the optimal balance point between delayed action and direct action using a simple and accurate formula. This allows the current to accurately follow the reference value, effectively solving the steady-state error problem caused by the whole cycle delay and avoiding continuous charging and discharging of the capacitor due to the current not reaching the reference value.

[0023] The closed-loop control method of this invention has better control performance and greater flexibility than the open-loop fixed-operation mode commutation method, and can effectively solve the problem of performance degradation of the control system caused by existing open-loop control. Attached Figure Description To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of a commutation method for a modular multilevel converter in quasi-two-level mode provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the modular multilevel converter provided in the embodiment of the present invention in quasi-two-level operating mode; Figure 3 This is a schematic diagram of the commutation discrimination trajectory for the first control cycle provided in this embodiment of the invention; Figure 4 This is a schematic diagram of the commutation discrimination trajectory for the next control cycle provided in this embodiment of the invention; Figure 5 This is a schematic diagram comparing the quasi-two-level commutation trajectories with introduced duty cycle provided in an embodiment of the present invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] This invention discloses a commutation method for a modular multilevel converter in quasi-two-level mode, such as... Figure 1 ,include: S1. Determine the current stage of the commutation and the polarity of the output current, identify the object to be delayed, and determine the operation mode of the upper arm submodule and the lower arm submodule based on the switching direction of the output phase voltage level of the upper arm or the lower arm, including bypass operation and access operation. S2. Perform the operation directly on the non-delay control object according to the determined action mode, and construct two candidate paths for the delay control object based on the single delay assumption: direct action and delayed action. The delayed action path is to keep the number of sub-modules invested unchanged at the current time and postpone the execution of the access action or bypass action determined in step S1 to the next time. S3. Based on the candidate paths constructed in step S2, calculate the circulating current prediction values ​​corresponding to direct actions and delayed actions, as well as the absolute error between the circulating current prediction values ​​and the reference values. Based on the principle of minimizing the error, determine whether the delayed control object should execute a direct action or a delayed action at the current moment. S4. Introduce a piecewise linear trajectory synthesis fine-tuning mechanism based on duty cycle. Determine the delay time of the bridge arm sub-module by calculating the duty cycle. Repeat steps S3-S4 until the commutation process ends.

[0027] To further implement the above technical solution, in step S1, when the polarity of the output current is positive, the lower bridge arm is the delay object; when the polarity is negative, the upper bridge arm is the delay object. When the output phase voltage level of the upper or lower bridge arm shows a downward trend, the corresponding submodule is determined to be in bypass operation mode, while the opposite bridge arm submodule is in access operation mode. When the output phase voltage level of the upper or lower bridge arm shows an upward trend, the corresponding submodule is determined to be in access operation mode, while the opposite bridge arm submodule is in bypass operation mode.

[0028] by Figure 2 Figure (a) shows a modular multilevel converter x-phase with 3 sub-modules per arm when the output current is positive (where...). Taking the commutation process as an example, when the current is negative, according to the symmetry of the MMC topology, the control logic is completely mirror-symmetric, that is, the delay control object changes from the lower bridge arm to the upper bridge arm. The upper bridge arm executes a similar delay action / direct action judgment logic, while the lower bridge arm executes a bypass action. Output current When positive, it is represented as circulation After the first stage of commutation is completed, the desired state is the lower arm current. When the current drops to 0, the output current is entirely supplied by the upper bridge arm, i.e., the upper bridge arm current... Strain , The value is constant, and the control objective is to reduce the circulating flow. from Control as .

[0029] In this embodiment, the change in circulation The total insertion voltage of phase x and DC bus voltage express:

[0030] in, The unit delay period, i.e., the control period of the inner commutation loop, is the total insertion voltage of phase x. When the voltage is lower than the DC bus voltage for a period of time, the circulating current flows from... Become ; Represented as:

[0031] in, , The number of insertions for the upper and lower bridge arms, when the output current... When it is positive, the upper bridge arm experiences The time required to complete the insertion or bypass of all submodules is N, where N is the number of submodules contained in each bridge arm, and the lower bridge arm takes longer than the upper bridge arm to help complete the phase-to-phase commutation process.

[0032] In this embodiment, the behavior pattern of the upper arm submodule is determined: each submodule will bypass gradually in each delay cycle. Within each control cycle, it is determined whether the lower arm submodule should be directly inserted or delayed (i.e., no action in the current cycle). Assuming a one-cycle delay before insertion, the decision is made based on the final circulating current value at the end of the commutation process. The principle behind this decision is the error between the final circulating current value and the reference value under the two operations. The operation with the smaller error is the control method to be applied in the current control cycle. For example... Figure 3 It can be seen that in the first control cycle, the final circulating current value obtained by delaying for one cycle before insertion is closer to the reference value. Therefore, in the first control cycle, the lower arm submodule will delay for one cycle before acting; while... Figure 4 It can be seen that in the second control cycle, inserting the lower bridge arm submodule first and then delaying it will result in a circulating current value that is closer to the reference value than a direct delay. This is the principle of adding a delay cycle.

[0033] To further implement the above technical solution, in step S2, for non-delay control objects, the number of sub-modules deployed is updated according to the action mode determined in step S1; for delay control objects, the number of sub-modules deployed corresponding to the direct action path is updated according to the action mode determined in S1, while the number of sub-modules deployed corresponding to the delayed action path remains unchanged.

[0034] In this embodiment, based on the principle of adding a delay period, the insertion index of the upper bridge arm is... From the current number of insertions Become This indicates that a submodule is bypassed; the insertion index of the lower bridge arm. From the current insertion number Become Indicates a direct insert operation, or a hold operation. This indicates a delayed insertion operation.

[0035] To further implement the above technical solution, based on the discrete mathematical model of the converter, using the system state variables at the current moment and the number of sub-modules to be put into operation for the direct action and the delayed action determined in step S2, the circulating current prediction values ​​corresponding to the direct action and the delayed action are calculated respectively.

[0036] To further implement the above technical solution, the circulating current prediction values ​​for the two operation modes, direct action and delayed action, in step S3 are:

[0037]

[0038]

[0039]

[0040]

[0041] Where x is the phase of the modular multilevel converter. , For the circulating current prediction value of the delayed insertion operation, For the circulation prediction value of the direct insertion operation, For the current moment circulation, Unit delay period, For bridge arm inductance, This is the DC bus voltage. The number of prediction steps determined based on linear extrapolation (the prediction delay object is determined from the current input quantity). Increase to full capacity The process, therefore ), This is the cumulative sum of the number of submodules used for non-delayed objects within the forecast period. The average voltage of the non-delayed object submodule. The cumulative sum of submodules when a delayed action is performed on a delayed object. The average voltage of the delayed object submodule, The cumulative sum of submodules when a deferred object performs a direct action. This represents the insertion count of the non-delayed object submodule at the current moment. This represents the current insertion count of the delayed object submodule; in this embodiment, the lower bridge arm is the delayed object, and the upper bridge arm is the non-delayed object.

[0042] To further implement the above technical solution, in step S3, the absolute error between the predicted circulation value and the reference value is:

[0043]

[0044] in, Predicted circulating current for delayed actions Compared with reference value The absolute error, Predicted circulation values ​​for direct action Compared with reference value The absolute error; like This indicates that the delayed control object will perform a delayed action; if This indicates that the direct action of the controlled object is delayed.

[0045] To further implement the above technical solutions, circulation for Output current for , For the upper bridge arm current, For the lower arm current, the control objective of commutation is to reduce the circulating current. Guided to the steady-state value corresponding to the target conducting bridge arm.

[0046] To further implement the above technical solution, when the output current... When the target conduction arm is positive and the upper arm is the control arm, Tend to When the output current When the target conduction arm is the lower arm, control Tend to When the output current When the value is negative and the target conducting arm is the lower arm, the control circulation is... Tend to When the output current When the value is negative and the target conducting arm is the upper arm, the control circulation is... Tend to .

[0047] In this embodiment, since the lower bridge arm is explicitly defined as the object of delay control, and the target conducting bridge arm for commutation is the upper bridge arm, the final physical steady-state target for commutation in this stage is: the upper bridge arm is fully conducting and carrying the entire load current, while the lower bridge arm current drops to zero. Therefore, the control... Tend to .

[0048] Due to the discrete physical states of MMC submodules, especially when the number of submodules is small in quasi-two-level mode, the whole-cycle delay method cannot guarantee that the current can accurately track the reference value, resulting in the capacitor being in a continuous charging and discharging state. To solve this problem and ensure that the circulating current eventually reaches the reference value accurately... (Right now This embodiment, based on the coarse adjustment of integer period delay, further introduces a duty cycle-based adjustment. The piecewise linear trajectory synthesis fine-tuning mechanism.

[0049] To further implement the above technical solution, in step S4, rolling optimization determines that delayed actions are superior to direct actions, meaning that the absolute error between the predicted circulation value and the reference value corresponding to the delayed action is small, and the predicted circulation value of the delayed action is... The absolute value exceeds the reference value. When the absolute value of the time is reached (i.e., overshoot occurs), the duty cycle is introduced to finely adjust the timing of the action and determine the specific time of the delayed action.

[0050] To further implement the above technical solution, the duty cycle in step S4... The calculation is based on the principle of linear interpolation, specifically as follows:

[0051] in, This is a reference value for the circulation; the value varies depending on the direction of the circulation. The two can be switched. These are the predicted circulation values ​​under direct action. This represents the predicted circulation value under delayed action.

[0052] To further implement the above technical solutions, such as Figure 5 As shown by the solid line in the middle, the piecewise linear trajectory synthesis fine-tuning mechanism based on duty cycle combines the current change rates of two discrete physical states sequentially in the time domain within one control cycle. Implementation, specifically including: The high-gain acceleration phase, i.e. During the initial phase of the control cycle, the control delay locks the previously controlled switching state and does not activate. (At this time, fewer submodules are connected to the lower bridge arm, and due to the small back electromotive force of the bridge arm, the inductor will experience a large forward voltage difference.) Figure 5 The solid line in the middle represents the actual current trajectory with a high rate of change of current. It utilizes the high gain characteristic to reduce the current difference from the reference value at the fastest physically permissible speed. The low-gain steady-current state stage, i.e. During the phase, at t=k+dT dAt any given moment, the control delay object performs the only switching action within this cycle to complete the switching of the bridge arm submodule. At this time, the circuit topology changes, the delay control object switches to the multi-submodule access state, the bridge arm voltage increases, the voltage difference across the inductor decreases, and the current change rate of the corresponding actual current trajectory decreases. The trajectory direction becomes smoother during this stage, avoiding the situation where the circulating current prediction value exceeds the reference value due to the full cycle delay (i.e., overshoot occurs). This embodiment combines two stages: a high-gain acceleration state and a low-gain steady-state state. It then uses a formula to precisely calculate the optimal balance between delayed and direct actions, ensuring that the actual current... Always stay precisely at the reference value This effectively solves the problem that the current cannot accurately track the reference value due to the whole cycle delay.

[0053] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0054] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A commutation method for a modular multilevel converter in quasi-two-level mode, characterized in that, include: S1. Determine the current stage of the commutation and the polarity of the output current, identify the object to be delayed, and determine the operation mode of the upper arm submodule and the lower arm submodule based on the switching direction of the output phase voltage level of the upper arm or the lower arm, including bypass operation and access operation. S2. Perform the operation directly on the non-delay control object according to the determined action mode, and construct two candidate paths for the delay control object based on the single delay assumption: direct action and delayed action. The delayed action path is to keep the number of sub-modules invested unchanged at the current time and postpone the execution of the access action or bypass action determined in step S1 to the next time. S3. Based on the candidate paths constructed in step S2, calculate the circulating current prediction values ​​corresponding to direct actions and delayed actions, as well as the absolute error between the circulating current prediction values ​​and the reference values. Based on the principle of minimizing the error, determine whether the delayed control object should execute a direct action or a delayed action at the current moment. S4. Introduce a piecewise linear trajectory synthesis fine-tuning mechanism based on duty cycle. Determine the delay time of the bridge arm sub-module by calculating the duty cycle. Repeat steps S3-S4 until the commutation process ends.

2. The commutation method of a modular multilevel converter in quasi-two-level mode as described in claim 1, characterized in that, Step S4: By using rolling optimization, it is determined that delayed action is better than direct action. That is, when the absolute error between the predicted circulation value and the reference value of the delayed action is small, and the absolute value of the predicted circulation value of the delayed action exceeds the absolute value of the reference value, the duty cycle is introduced to finely adjust the timing of the action and determine the specific time of the delayed action.

3. The commutation method of a modular multilevel converter in quasi-two-level mode as described in claim 1, characterized in that, Duty cycle in step S4 The calculation is based on the principle of linear interpolation, specifically: in, This is a reference value for the circulation; the value varies depending on the direction of the circulation. The transformation between the two For phase output current, These represent the three phases of the inverter. These are the predicted circulation values ​​under direct action. This represents the predicted circulation value under delayed action.

4. The commutation method of a modular multilevel converter in quasi-two-level mode as described in claim 1, characterized in that, The duty cycle-based piecewise linear trajectory synthesis fine-tuning mechanism is achieved by sequentially combining the current change rates of two discrete physical states in the time domain within a control cycle, specifically including: The high-gain acceleration phase, i.e. In the initial stage of the control cycle, the control delay locks the switching state of the control object from the previous moment and does not operate. By utilizing the high gain characteristic, the current reduces the difference from the reference value at the fastest physically permissible speed. The low-gain steady-current state stage, i.e. During the phase, at t=k+dT d At any given moment, the control delay object performs the only switching action within this cycle to complete the switching of the bridge arm submodule.

5. The commutation method of a modular multilevel converter in quasi-two-level mode as described in claim 1, characterized in that, In step S1, when the polarity of the output current is positive, the lower bridge arm is the object of delay; when the polarity is negative, the upper bridge arm is the object of delay. When the output phase voltage level of the upper or lower bridge arm shows a downward trend, the corresponding submodule is determined to be in bypass operation mode, while the opposite bridge arm submodule is in access operation mode. When the output phase voltage level of the upper or lower bridge arm shows an upward trend, the corresponding submodule is determined to be in access operation mode, while the opposite bridge arm submodule is in bypass operation mode.

6. The commutation method of a modular multilevel converter in quasi-two-level mode as described in claim 1, characterized in that, In step S2, for non-delay control objects, the number of sub-modules deployed is updated according to the action mode determined in step S1; for delay control objects, the number of sub-modules deployed corresponding to the direct action path is updated according to the action mode determined in S1, while the number of sub-modules deployed corresponding to the delayed action path remains unchanged.

7. The commutation method of a modular multilevel converter in quasi-two-level mode as described in claim 6, characterized in that, In step S3, based on the discrete mathematical model of the converter, the system state variables at the current moment and the number of sub-modules to be deployed corresponding to the direct action and the delayed action determined in step S2 are used to calculate the circulating current prediction values ​​corresponding to the direct action and the delayed action, respectively.

8. The commutation method of a modular multilevel converter in quasi-two-level mode as described in claim 1, characterized in that, In step S3, the absolute error between the predicted circulation value and the reference value is: in, Predicted circulating current for delayed actions Compared with reference value The absolute error, Predicted circulation values ​​for direct action Compared with reference value The absolute error; like This indicates that the delayed control object will perform a delayed action; if This indicates that the direct action of the controlled object is delayed.

9. The commutation method of a modular multilevel converter in quasi-two-level mode as described in claim 8, characterized in that, circulation for Phase output current for , For the upper bridge arm current, For the lower arm current, the control objective of commutation is to reduce the circulating current. Guided to the steady-state value corresponding to the target conducting bridge arm.

10. The commutation method of a modular multilevel converter in quasi-two-level mode as described in claim 9, characterized in that, When the output current When the target conduction arm is positive and the upper arm is the control arm, Tend to When the output current When the target conduction arm is the lower arm, control Tend to When the output current When the value is negative and the target conducting arm is the lower arm, the control circulation is... Tend to When the output current When the value is negative and the target conducting arm is the upper arm, the control circulation is... Tend to .

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