A fast charging method, system and medium for modular multi-level converter

Abstract

The invention discloses a fast charging method, system and medium for a modular multi-level converter; it relates to the technical field of flexible direct current transmission; the whole charging process of the modular multi-level converter is divided into a first charging process and a second charging process; in the first charging process, a current threshold is set, and whether to cut off a submodule is determined according to the relationship between the charging current and the current threshold; in the second charging process, the time and selection criteria for cutting off or putting into use a submodule are determined according to the charging current amplitude and the submodule input optimization method; the scheme improves the method on the basis of the traditional charging method, determines whether to cut off or put into use the number of submodules according to the charging current and the threshold current, and avoids the process in the late stage of charging due to the reduction of the charging current and the slow rise of the submodule voltage; under the premise of keeping the charging current safe, the charging rate of all submodules to be charged is within a certain range.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible direct current power transmission, and in particular to a fast charging method, system and medium for a modular multi-level converter. Background Art

[0002] Since modular multilevel converters usually contain dozens to thousands of submodules (mainly composed of power capacitors, diodes, and IGBTs), if the charging rate is too slow, the voltage of the submodules that have been charged will decay. Pre-charging is performed based on the basic principle that the sum of the capacitor voltages of each phase bridge arm is the line voltage peak, the number of capacitors gradually decreases, and the voltage of the remaining submodules gradually increases. The charging process is mainly divided into two stages. The first charging process is that after all submodules are put into use, the submodules are gradually removed until the voltage of a submodule in the submodules reaches its rated value; the second charging process is that during the first charging process, when the voltage of a submodule reaches the rated value, it is removed and the remaining submodules that have not been fully charged are put into use, and this process is repeated until the charging is completed.

[0003] Based on the basic principle that the sum of the capacitor voltages of each phase bridge arm is the line voltage peak value, the number of capacitors gradually decreases, and the voltage of the remaining sub-modules gradually increases, there are many pre-charging methods, among which the representative ones are: (1) the charging method of sub-modules being put into use in batches and being removed at a constant voltage. However, this method assumes that the sub-module parameters are completely consistent and the charging rates are exactly the same, which is less practical; (2) in the first charging process, the sub-modules are removed one by one at a fixed time; in the second charging process, after the charged sub-modules are removed, the sub-modules with the lowest voltage among the remaining sub-modules to be charged are put into use first. In this method, if the time interval is not properly obtained during the first charging process, the charging current caused by the removal of the previous submodule has not decayed to a certain extent, and a submodule with a higher voltage is removed, causing a larger charging current to be superimposed, resulting in a charging current that continues to increase and oscillate, and the charging current decay time constant becomes larger, causing serious heat generation, which is easy to damage the circuit or converter, and at the same time, the charging current will be too large during the second charging process; (3) In the first charging process, the voltage is constant and removed one by one; in the second charging process, after the submodule that has been charged is removed, the submodule with the lowest voltage among the remaining submodules to be charged is first put into use. In the first charging process, each time a submodule is removed, the voltage of the remaining submodule rises to the peak value of the line voltage as the practice basis for removal, and then removed one by one. Since the charging current drops rapidly in the late stage of the first charging process, the charging rate of the capacitor also decreases rapidly. This method consumes a lot of time each time a submodule is removed. At the same time, there is also the problem of excessive charging current in the second charging process. (4) In the first charging process, the submodules with the highest voltage are removed one by one at a certain frequency. In the second charging process, after the submodules that have been charged are removed, the submodules with the lowest voltage among the remaining submodules to be charged are first put into use. This method is due to the decrease in the number of submodules, the time constant gradually increases, and the circuit parameters are constantly changing. The frequency needs to be calculated continuously in real time, which is complex to calculate. There is also the problem of excessive charging current in the second charging process.

[0004] In each previous method, at the beginning of the second charging process, after the fully charged submodule is removed, the submodule with the lowest voltage among the remaining submodules to be charged is put into use first; since the parameters of all submodules are basically the same, in the early stage of the second charging process, the original submodule (that is, among all submodules, the submodule whose voltage reaches its rated value and is not removed and is being charged) reaches the submodule rated voltage value almost at the same time and is removed. If the submodule with the lowest voltage among the remaining submodules to be charged is put into use first at this time, the voltage in the charging circuit will drop greatly, the charging current will be very large, and the current decay will be very slow, which may easily cause abnormal heating and malfunction of the equipment. Summary of the invention

[0005] The technical problem to be solved by the present invention is that the traditional charging method has the problems of large voltage drop, large charging current and slow current decay in the charging circuit, which easily leads to abnormal heating and fault of the equipment. The purpose of the present invention is to provide a fast charging method, system and medium of a modular multi-level converter, and to improve the method on the basis of the traditional charging method. In the first charging process, whether to cut off the submodule is determined based on the charging current and the threshold current, thereby avoiding the process in the early stage of charging due to the reduction of the charging current and the slow rise of the submodule voltage; in the second charging process, the time and selection criteria for cutting off or putting into use the submodule are determined in combination with the charging current amplitude control method and the submodule input optimization method, so that the charging rate of all submodules to be charged is within a certain range.

[0006] The present invention is achieved through the following technical solutions:

[0007] This solution provides a fast charging method for a modular multi-level converter, including the following method:

[0008] The entire charging process of the modular multilevel converter is divided into a first charging process and a second charging process; the modular multilevel converter includes m submodules;

[0009] In the first charging process, a current threshold is set, and whether to remove the submodule is determined based on the relationship between the charging current and the current threshold; in the second charging process, the time and selection criteria for removing or putting the submodule into use are determined based on the charging current amplitude and the submodule input optimization method.

[0010] Working principle of this scheme: The purpose of the present invention is to provide a fast charging method, system and medium for a modular multi-level converter, which improves the method on the basis of the traditional charging method. In the first charging process, whether to cut off the submodule is determined based on the charging current and the threshold current, thereby avoiding the process in the later stage of charging where the charging current is reduced and the submodule voltage rises slowly; in the second charging process, the time and selection criteria for cutting off or putting into use the submodule are determined in combination with the charging current amplitude control method and the submodule input optimization method, so that the charging rate of all submodules to be charged is within a certain range.

[0011] A further optimized solution is that the first charging process includes: all submodules start to charge until the first submodule is fully charged;

[0012] The second charging process includes: charging of one submodule is completed until charging of all submodules is completed.

[0013] A further optimization scheme is that in the first charging process, a current threshold is set, and whether to remove the submodule is determined according to the relationship between the charging current and the current threshold, including the method:

[0014] S1, charging all submodules in the modular multilevel converter;

[0015] S2, obtain the current charging current I S , the current charging current I S With the current threshold I ZD Compare, if I S >I ZD , no action is taken; otherwise, n submodules are removed; n≤m;

[0016] S3, obtaining the voltage of the charging submodules that have been put into operation, and comparing the voltage of each charging submodule that has been put into operation with the corresponding rated voltage. If the voltage of each charging submodule that has been put into operation is less than the corresponding rated voltage, returning to step S2;

[0017] S4, for the submodules that have been put into charging and whose voltage amplitude is equal to the corresponding rated voltage, the submodules that are currently put into charging are removed; the voltages of the submodules that are not put into charging are obtained, and the submodules that are not put into charging and whose charging has not been completed and have the highest voltage are put into charging.

[0018] A further optimization scheme is to determine the time and selection criteria for removing or putting into use the submodules in the second charging process according to the charging current amplitude and the submodule input optimization method, including the following methods:

[0019] G1, obtain the voltage of the charged submodules, and determine whether the charged submodules have been fully charged based on the voltage; if none of the charged submodules have been fully charged, no action is taken and the process proceeds to step G2; otherwise, remove the fully charged submodules from the charged submodules and proceed to step G3;

[0020] G2, obtain the current charging current I S , detect the current charging current I S Is it in I ZD with I MAX Between, if I S <I ZD , then find the charging submodule A with the lowest voltage and cut off the charging submodule A, thereby increasing the charging current and improving the charging rate; if I S >I MAX , then find the submodule B with the highest voltage that is not put into charging, and put it into charging submodule B, thereby reducing the charging current and ensuring safety; if I S in I ZD with I MAX If the time is between , no action is taken and the process returns to step G1;

[0021] G3, check whether all the submodules not put into charging have been charged. If so, do not take any action and go to step G4; otherwise, obtain the voltage of the submodules not put into charging and not completed charging, put the submodule not put into charging and not completed charging with the highest voltage into charging, and return to step G2;

[0022] G4, repeat step G3 until all sub-modules are removed, and then lock all sub-modules.

[0023] A further optimization scheme is that the method for setting the current threshold includes:

[0024] The current threshold I is calculated according to the following formula ZD :

[0025] I ZD = k * I MAX

[0026] Among them, k is the reliability coefficient, I MAX It is the maximum current allowed for safe operation of the charging bridge arm.

[0027] A further optimization scheme is that, in the first charging process, n is also determined according to the relationship between the charging current and the current threshold.

[0028] A further optimization scheme is that the method for determining n includes:

[0029]

[0030] Among them, round() is the rounding function. is the voltage vector at the time of cutting off the preset value, The current vector at the time of cutting off the preset time, is the average voltage of the submodule charged at the time of removal, ω is the electrical angle of the power grid, and C is the capacitance value of a single submodule; is the current threshold vector.

[0031] A further optimization scheme is that the method for determining n also includes:

[0032]

[0033] In the formula, round() is the rounding function, U CN is the rated voltage of the submodule, The average voltage of the submodule charged at the time of pre-cut-off.

[0034] In the first charging process, the above method is used to ensure that no overcurrent fault occurs when a submodule is removed, and to ensure that after each submodule is removed, the submodule put into use gets enough charging time to achieve an ideal charging speed; at the same time, while ensuring that the charging current is controllable, the number of times the submodule is removed is greatly reduced, thereby improving the charging rate.

[0035] At the same time, an improvement is proposed for the method of putting the electronic modules to be charged in the second charging process. After the voltage of the sub-module put in is reaches its rated voltage and it is cut off, the sub-module with the highest voltage among the remaining sub-modules to be charged is put in first. At the same time, combined with the charging current amplitude control method, the sub-modules are switched on and off to make the charging current within the specified range, so that the entire second charging process can be charged quickly within a reasonable charging rate.

[0036] The present solution also provides a fast charging system for a modular multi-level converter, which is used to implement the fast charging method for a modular multi-level converter. The system includes:

[0037] A division module, used for dividing the entire charging process of the modular multilevel converter into a first charging process and a second charging process; the modular multilevel converter includes m submodules;

[0038] A first operating module, used for setting a current threshold in a first charging process, and determining whether to cut off a submodule according to a relationship between the charging current and the current threshold;

[0039] The second operating module is used to determine the time and selection criteria for removing or putting into use a submodule according to the charging current amplitude and the submodule putting into use optimization method in the second charging process.

[0040] The present solution also provides a computer-readable medium having a computer program stored thereon, and the computer program is executed by a processor to implement the above-mentioned method for fast charging of a modular multi-level converter.

[0041] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0042] The present invention provides a fast charging method, system and medium for a modular multi-level converter, which improves the traditional charging method. In the first charging process, whether to cut off the submodule is determined based on the charging current and the threshold current, thereby avoiding the process in which the submodule voltage rises slowly in the late charging period due to the reduction of the charging current. In the second charging process, the charging current amplitude control method and the submodule input optimization method are combined. In the second charging process, the time and selection criteria for cutting off or putting into use the submodule are determined based on the charging current amplitude and the submodule input optimization method, so that the charging rate of all submodules to be charged is within a certain range. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

[0044] Figure 1 A schematic flow chart of a fast charging method for a modular multi-level converter;

[0045] Figure 2 Schematic diagram of the fast charging process of the modular multi-level converter;

[0046] Figure 3 It is a schematic diagram of the topological structure of a modular multilevel converter;

[0047] Figure 4 It is a schematic diagram of the circuit structure topology of the submodule;

[0048] Figure 5 is the equivalent circuit diagram of the submodule;

[0049] Figure 6 It is a schematic diagram of the relationship between charging current, submodule voltage and time;

[0050] Figure 7 A schematic diagram comparing the charging rates of the conventional method and the present invention;

[0051] Figure 8 Schematic diagram of submodule charging time and voltage under the method of the present invention. DETAILED DESCRIPTION

[0052] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0053] Embodiment 1: This embodiment provides a fast charging method for a modular multi-level converter, such as Figure 1 As shown, including methods:

[0054] Step 1: Divide the entire charging process of the modular multilevel converter into a first charging process and a second charging process; the first charging process includes: all submodules have been charged until the first submodule has been charged;

[0055] The second charging process includes: one submodule is fully charged and then removed until all submodules are fully charged. The modular multilevel converter includes m submodules;

[0056] Step 2: In the first charging process, a current threshold is set, and whether to remove the submodule is determined according to the relationship between the charging current and the current threshold; the specific method includes:

[0057] S1, charging all submodules in the modular multilevel converter;

[0058] S2, obtain the current charging current I S , the current charging current I S With the current threshold I ZD Compare, if I S >I ZD , no action is taken; otherwise, n submodules are removed; n≤m;

[0059] The equivalent circuit diagram of the charging current is as follows: Figure 5 As shown, the method for setting the current threshold includes:

[0060] The current threshold I is calculated according to the following formula ZD :

[0061] I ZD = k * I MAX

[0062] Among them, k is the reliability coefficient, I MAX It is the maximum current allowed for safe operation of the charging bridge arm.

[0063] S3, obtaining the voltage of the charging submodules that have been put into operation, and comparing the voltage of each charging submodule that has been put into operation with the corresponding rated voltage. If the voltage of each charging submodule that has been put into operation is less than the corresponding rated voltage, returning to step S2;

[0064] S4, for the charged submodules whose voltage amplitude is equal to the corresponding rated voltage, the currently charged submodules are removed; the voltages of the non-charged submodules are obtained, and the non-charged submodules with the highest voltage are charged.

[0065] Step 3: In the second charging process, the time and selection criteria for removing or putting the submodule into operation are determined according to the charging current amplitude and the submodule input optimization method. This step specifically includes the following methods:

[0066] G1, obtain the voltage of the charged submodules, and determine whether the charged submodules have been fully charged based on the voltage; if none of the charged submodules have been fully charged, no action is taken and the process proceeds to step G2; otherwise, remove the fully charged submodules from the charged submodules and proceed to step G3;

[0067] G2, obtain the current charging current I S , detect the current charging current IS Is it in I ZD with I MAX Between, if I S <I ZD , then find the charging submodule A with the lowest voltage and cut off the charging submodule A, thereby increasing the charging current and improving the charging rate; if I S >I MAX , then find the submodule B with the highest voltage that is not put into charging, and put it into charging submodule B, thereby reducing the charging current and ensuring safety; if I S in I ZD with I MAX If the time is between , no action is taken and the process returns to step G1;

[0068] G3, check whether all the submodules not put into charging have been charged. If so, do not take any action and go to step G4; otherwise, obtain the voltage of the submodules not put into charging and not completed charging, put the submodule not put into charging and not completed charging with the highest voltage into charging, and return to step G2;

[0069] G4, repeat step G3 until all sub-modules are removed, and then lock all sub-modules.

[0070] Methods for determining n include:

[0071]

[0072] Among them, round() is the rounding function. is the voltage vector at the time of cutting off the preset value, The current vector at the time of cutting off the preset time, is the average voltage of the submodule charged at the time of removal, ω is the electrical angle of the power grid, and C is the capacitance value of a single submodule; is the current threshold vector. The above formula is used to calculate the number of submodules to be removed each time, ensuring that no overcurrent fault occurs when n submodules are removed, and ensuring that after each submodule is removed, the submodule put into use reaches the ideal charging speed; this greatly reduces the number of times the submodule is removed and improves the charging rate.

[0073] Methods for determining n also include:

[0074]

[0075] In the formula, round() is the rounding function, U CN is the rated voltage of the submodule, The average voltage of the submodule charged at the time of pre-cut-off.

[0076] The number of submodules removed each time is calculated by the above formula to ensure that no overcurrent fault occurs when n submodules are removed, and that the submodule put into use reaches the ideal charging speed after each submodule is removed; this greatly reduces the number of times the submodule is removed and improves the charging rate.

[0077] The topology diagram of modular multilevel converter is as follows: Figure 3 As shown, the three-phase circuit U A , U B , U C After connecting an inductor L0 in series, the switches S, K and the current limiting resistor are connected, and finally the multiple sub-modules SM1 to SMn are connected through the inductor L1; the switch K is connected in parallel on both sides of the current limiting resistor. The output voltage of the modular multilevel converter is U dc ; The circuit structure topology of each submodule is as follows Figure 4 As shown, the submodule includes a transistor T1, a transistor T2, a diode D1, a diode D2 and a capacitor C0; one end of the capacitor C0 is connected to the collector of the transistor T1, and the other end is connected to the emitter of the transistor T2; the emitter of the transistor T1 is connected to the collector of the transistor T2; the cathode of the diode D1 is connected to the collector of the transistor T1, and the anode of the diode D1 is connected to the emitter of the transistor T1; the cathode of the diode D2 is connected to the collector of the transistor T2, and the anode of the diode D2 is connected to the emitter of the transistor T2; the voltage across the capacitor C0 is U C0 ; The input voltage of the modular multilevel converter is u SM ; The input current of the modular multilevel converter is i SM The equivalent circuit diagram of the submodule charging process is as follows: Figure 5 As shown, R eq , L eq , C eq and U eq They are the equivalent resistance, equivalent inductance, equivalent capacitance and equivalent voltage when the submodule is charged.

[0078] like Figure 2 As shown, during the first charging process:

[0079] (1) Open switch K, insert a current limiting resistor, and then close switch S to apply a low level to transistors T1 and T2 of each submodule, so that they are in a fully locked state for charging.

[0080] (2) Detect the current charging current and current threshold I ZD If the former is greater than the latter, no action is taken and the process returns to step (1); otherwise, n sub-modules are removed, that is, a low level is applied to the transistor T1 of the sub-module to be removed and a high level is applied to the transistor T2 to remove it.

[0081] (3) Detect the voltage amplitude of the sub-modules that have been put into operation. If the voltage of the sub-modules that have been put into operation is less than their rated voltage, return to step (2); if the voltage of a sub-module that has been put into operation reaches its rated voltage, apply a low level to the transistor T1 of the sub-module that has been put into operation, apply a high level to the transistor T2 to cut off, apply a low level to the transistor T2 of the sub-module with the highest voltage among the remaining sub-modules to be charged, and enter step (4).

[0082] In the second charging process:

[0083] (4) Check whether the voltage of the submodule that has been put into operation has reached the rated value. If not, no operation is performed on it and the process goes to step (5); if so, a high level is applied to the transistor T2 of the submodule that has been put into operation to cut off the submodule and the process goes to step (6).

[0084] (5) Check whether the current charging current is within I ZD with I MAX If the lower limit I ZD , a high level is applied to the transistor T2 of the lowest submodule voltage, so that the charging current increases; if it exceeds the upper limit I MAX , a low level is applied to the transistor T2 with the highest voltage of the remaining submodules that have not been put into use and have not been fully charged, so that the charging current is reduced; if it does not exceed the upper limit or the lower limit, no action is taken. Return to step (4).

[0085] (6) Check whether there are any sub-modules that have not been fully charged and are waiting to be put into operation. If so, apply a low level to the transistor T2 of the sub-module with the highest voltage among the remaining sub-modules to be charged and return to step (5); if not, do nothing and return to step (6). When all sub-modules are removed, apply a high level to the transistors T2 of all sub-modules and wait for unlocking and operation.

[0086] This scheme is based on the charging current and I ZD The time to remove the submodule group and the number of submodules in the submodule group are determined based on the relationship between the charging current and the number of submodules in the submodule group, avoiding the process in the later stage of charging where the charging current decreases and the submodule voltage rises slowly; in the second charging process, the charging current amplitude control method and the submodule input optimization method are combined to make the charging rate of all submodules to be charged within a certain range. ZD When the charging current is less than I ZD When the charging current is greater than I ZD When the charging current is reduced, a certain number of sub-modules are put into use;

[0087] In the first charging process, the above method is used to ensure that no overcurrent fault occurs when a submodule is removed, and to ensure that after each submodule is removed, the submodule put into use gets enough charging time to achieve an ideal charging speed; at the same time, while ensuring that the charging current is controllable, the number of times the submodule is removed is greatly reduced, thereby improving the charging rate.

[0088] At the same time, an improvement is proposed for the method of putting the sub-module to be charged into operation during the second charging process. After the voltage of the sub-module that has been put into operation reaches its rated voltage and it is cut off, the sub-module with the highest voltage among the remaining sub-modules to be charged is put into operation first. At the same time, combined with the charging current amplitude control method, the sub-modules are switched on and off to make the charging current within the specified range, so that the entire second charging process can be charged quickly within a reasonable charging rate.

[0089] Embodiment 2: This embodiment provides a fast charging system of a modular multi-level converter, which is used to implement the above-mentioned fast charging method of a modular multi-level converter. The system includes:

[0090] A division module, used for dividing the entire charging process of the modular multilevel converter into a first charging process and a second charging process; the modular multilevel converter includes m submodules;

[0091] A first operating module, used for setting a current threshold in a first charging process, and determining whether to cut off a submodule according to a relationship between the charging current and the current threshold;

[0092] The second operating module is used to determine the time and selection criteria for removing or putting into use a submodule according to the charging current amplitude and the submodule putting into use optimization method in the second charging process.

[0093] Embodiment 3: This embodiment provides a computer-readable medium on which a computer program is stored. The computer program is executed by a processor to implement a fast charging method for a modular multi-level converter as described in Embodiment 1. Specifically, the following steps are performed:

[0094] Step 1: dividing the entire charging process of the modular multilevel converter into a first charging process and a second charging process; the modular multilevel converter includes m submodules;

[0095] Step 2: In the first charging process, a current threshold is set, and whether to remove the submodule is determined according to the relationship between the charging current and the current threshold;

[0096] Step 3: During the second charging process, the time and selection criteria for removing or putting the submodule into operation are determined according to the charging current amplitude and the submodule input optimization method.

[0097] During the submodule charging process, the submodule voltage and current change curves with charging time are as follows: Figure 6 As shown, the charging current corresponding to time t1 is A1, and the submodule voltage is U1; this embodiment charges the modular multilevel converter using the conventional method and the method of the above-mentioned embodiment 1, and the charging rate comparison diagram is shown in Figure 7 As shown, according to Figure 7 It can be seen that the charging rate of the method in Example 1 is significantly higher; the schematic diagram of the charging time and voltage change of different submodules under the method of the present invention is shown in FIG. Figure 8 As shown, Figure 8 The voltage of submodules SM1 to SM7 varies with charging time. Figure 8 It can be seen that each submodule is charged at a faster rate.

[0098] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A fast charging method for a modular multi-level converter, characterized in that: Included methods: The entire charging process of the modular multilevel converter is divided into a first charging process and a second charging process; the modular multilevel converter includes m submodules; In the first charging process, a current threshold is set, and whether to cut off the submodule is determined according to the relationship between the charging current and the current threshold; In the second charging process, the time and selection criteria for removing or putting the submodule into operation are determined according to the charging current amplitude and the submodule input optimization method; The first charging process includes: all submodules start to charge until the first submodule is fully charged; The second charging process includes: charging of one submodule is completed until charging of all submodules is completed; In the second charging process, the time and selection criteria for removing or putting the submodule into operation are determined according to the charging current amplitude and the submodule input optimization method, including the following method: G1, obtain the voltage of the charged submodules, and determine whether the charged submodules have been fully charged based on the voltage; if none of the charged submodules have been fully charged, no action is taken and the process proceeds to step G2; otherwise, remove the fully charged submodules from the charged submodules and proceed to step G3; G2, obtain the current charging current I S , detect the current charging current I S Is it within the current threshold? I ZD and I MAX Between, if I S < I ZD , then find the charging submodule A with the lowest voltage and cut off the charging submodule A; if I S > I MAX , then find the submodule B with the highest voltage that is not put into charging, and put it into charging submodule B; if I S exist I ZD and I MAX If the time is between , no action is taken and the process returns to step G1; I MAX The maximum current allowed for safe operation of the charging bridge arm; G3, check whether all the submodules not put into charging have been charged. If so, do not take any action and go to step G4; otherwise, obtain the voltage of the submodules not put into charging and not completed charging, put the submodule not put into charging and not completed charging with the highest voltage into charging, and return to step G2; G4, repeat step G3 until all sub-modules are removed, and then lock all sub-modules.

2. A fast charging method for a modular multi-level converter according to claim 1, characterized in that: In the first charging process, a current threshold is set, and whether to remove the submodule is determined according to the relationship between the charging current and the current threshold, including the following method: S1, charging all submodules in the modular multilevel converter; S2, obtain the current charging current I S , the current charging current I S With current threshold I ZD Compare, if I S ＞ I ZD , then no action; otherwise, cut off submodules; ≤m; S3, obtaining the voltage of the charging submodules that have been put into operation, and comparing the voltage of each charging submodule that has been put into operation with the corresponding rated voltage. If the voltage of all the charging submodules that have been put into operation is less than the corresponding rated voltage, returning to step S2; S4, the submodules that have been put into charging and whose voltage amplitude is equal to the corresponding rated voltage are cut off; the voltages of the submodules that have not been put into charging and whose charging has not been completed are obtained, and the submodules that have not been put into charging and whose charging has not been completed and whose voltage is the highest are put into charging.

3. The fast charging method of a modular multi-level converter according to claim 1, characterized in that: Methods for setting the current threshold include: The current threshold is calculated according to the following formula I ZD : I ZD = k * I MAX in, k is the reliability coefficient, I MAX It is the maximum current allowed for safe operation of the charging bridge arm.

4. A fast charging method for a modular multi-level converter according to claim 2, characterized in that: In the first charging process, the current threshold is determined based on the relationship between the charging current and the current threshold. .

5. A fast charging method for a modular multi-level converter according to claim 4, characterized in that: Methods for determining include: ； in, is the floor function, is the voltage vector at the time of cutting off the preset value, The current vector at the time of cutting off the preset time, The average voltage of the submodule charged at the time of removal. is the electrical angle of the grid, C is the capacitance value of a single submodule; is the current threshold vector.

6. A fast charging method for a modular multi-level converter according to claim 1, characterized in that: Methods for determining include: ； In the formula, is the floor function, is the rated voltage of the submodule, The average voltage of the submodule charged at the time of pre-cut-off.

7. A fast charging system of a modular multi-level converter, characterized in that: A method for fast charging a modular multi-level converter according to any one of claims 1 to 6, the system comprising: A division module, used for dividing the entire charging process of the modular multilevel converter into a first charging process and a second charging process; the modular multilevel converter includes m submodules; A first operating module, used for setting a current threshold in a first charging process, and determining whether to cut off a submodule according to a relationship between the charging current and the current threshold; The second operating module is used to determine the time and selection criteria for removing or putting into use a submodule according to the charging current amplitude and the submodule putting into use optimization method in the second charging process.

8. A computer readable medium having a computer program stored thereon, characterized in that: The computer program is executed by a processor to implement a fast charging method for a modular multi-level converter as claimed in any one of claims 1 to 6.

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