DC overvoltage suppression method, device, computer equipment and storage medium

Abstract

The embodiment of the present invention discloses a method, device, computer equipment and storage medium for suppressing DC overvoltage. The method includes: by real-time inputting reactive power compensation control strategy of faulty converter station MMC, the active transmission capacity of faulty converter station MMC is greatly improved, thereby assisting in reducing surplus power value to suppress the rise of DC voltage. In addition, if the actual AC voltage effective value after compensation at the grid coupling point (PCC) of faulty converter station MMC is less than the preset threshold value, the faulty converter station MMC adaptively negatively inputs a certain number of full-bridge submodules to compensate for the rise of inter-station DC voltage, and at the same time, the rectifier station LCC quantitatively adjusts the command value of the fixed DC current mode after a short delay to reduce the unbalanced charging power of the submodules of the faulty converter station MMC, so as to achieve DC overvoltage suppression, and the non-faulty converter station MMC switches the original control mode to the DC voltage control mode, so as to achieve the DC voltage of the hybrid multi-terminal DC transmission system to be stable at the rated value level during the fault.

Description

Technical Field

[0001] The present invention relates to the technical field of power grid transmission systems, and in particular to a direct current overvoltage suppression method, device, computer equipment and storage medium. Background Art

[0002] When an AC fault occurs at the receiving-end master station MMC of the LCC-MMC hybrid multi-terminal DC transmission system, the active power output capacity of the receiving-end master station MMC where the AC fault occurs will be reduced accordingly with the amplitude of the AC voltage drop, and the upper limit of the active power output capacity depends on the limit setting value of the output current. Due to the communication delay between stations and other reasons, the sending-end rectifier station LCC still maintains the original current setting value for a short period of time and sends all the active power into the DC system, resulting in power surplus at the sending and receiving ends. This surplus power cannot flow into the AC power grid, but can only charge the capacitor of the MMC bridge arm submodule, further raising the DC voltage of the entire transmission system, and the DC voltage rises rapidly to the DC voltage protection setting value within tens of milliseconds. Ultimately, the occurrence of derivative faults such as overvoltage lockout, equipment damage, and even shutdown of the entire network at the receiving-end master station MMC will seriously affect the safe and stable operation of the transmission system. Therefore, it is necessary to reduce the surplus power to suppress the rise of the DC voltage. Summary of the invention

[0003] The main purpose of the present invention is to provide a DC overvoltage suppression method, device, computer equipment and storage medium, which can solve the DC overvoltage problem of the hybrid multi-terminal DC transmission system when an AC fault occurs in the receiving end master station MMC of the hybrid multi-terminal DC transmission system in the prior art.

[0004] To achieve the above-mentioned object, the first aspect of the present invention provides a method for suppressing direct current overvoltage, which is applied to a hybrid multi-terminal direct current transmission system, wherein the hybrid multi-terminal direct current transmission system includes a multi-terminal converter station, that is, includes a rectifier station LCC and at least two inverter stations MMC, one of the at least two inverter stations MMC is used as a master station MMC, and the remaining inverter stations MMCs except the master station MMC of the at least two inverter stations MMC are used as slave stations MMC, the rectifier station LCC is a sending end of the hybrid multi-terminal direct current transmission system, and the at least two inverter stations MMC are receiving ends of the hybrid multi-terminal direct current transmission system, and the method includes:

[0005] When a hybrid multi-terminal DC transmission system has a faulty converter station MMC due to an AC fault at the receiving end, the faulty converter station MMC is put into reactive power compensation control strategy in real time to compensate for the drop in the effective value of the AC voltage at the grid-connected coupling point of the faulty converter station MMC; wherein the faulty converter station MMC is the main station MMC where the AC fault occurs;

[0006] The faulty converter station MMC obtains the actual AC voltage effective value after the grid-connected coupling point compensation of the faulty converter station MMC, and compares the actual AC voltage effective value after the grid-connected coupling point compensation with a preset threshold value;

[0007] If the effective value of the actual AC voltage after compensation at the grid coupling point is not less than the preset threshold, the hybrid multi-terminal DC transmission system maintains the original control mode and operation state to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault period;

[0008] If the effective value of the actual AC voltage after compensation at the grid-connected coupling point is less than a preset threshold, the faulty converter station MMC generates a negative level to compensate for the rise in the DC voltage between the converter stations MMC by putting the full-bridge submodule into operation. At the same time, the rectifier station LCC quantitatively adjusts the current command value, and the non-faulty converter station MMC switches the original control mode to the DC voltage control mode to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault; wherein the non-faulty converter station MMC is a slave station MMC where no AC fault occurs, and the converter station MMC refers to the inverter station MMC.

[0009] In combination with the first aspect, in a possible implementation method, the above-mentioned faulty converter station MMC implements a reactive compensation control strategy in real time to compensate for the drop in the effective value of the AC voltage at the grid coupling point of the faulty converter station MMC, including: the faulty converter station MMC obtains the actual effective value of the AC voltage at the grid coupling point in real time, and calculates the difference between the actual effective value of the AC voltage at the grid coupling point and the AC voltage reference value; wherein the AC voltage reference value is the steady-state rated value of the AC voltage at the grid coupling point; the faulty converter station MMC calculates a reactive reference value based on the difference, and compensates for the drop in the effective value of the AC voltage at the grid coupling point of the faulty converter station MMC based on the reactive reference value.

[0010] In combination with the first aspect, in a possible implementation method, the above-mentioned faulty converter station MMC generates a negative level to compensate for the increase in the DC voltage between the converter stations MMC by investing in full-bridge sub-modules, including: the faulty converter station MMC calculates the number of full-bridge sub-modules to be invested according to the unbalanced power value of the converter station MMC and the target DC voltage value to be achieved during the fault; wherein the target DC voltage value is taken as the rated value; the faulty converter station MMC invests in full-bridge sub-modules according to the number of full-bridge sub-modules to generate a negative level to compensate for the increase in the DC voltage between the converter stations MMC.

[0011] In combination with the first aspect, in a possible implementation method, the above-mentioned faulty converter station MMC calculates the number of full-bridge sub-modules put into operation according to the unbalanced power value of the converter station MMC and the target DC voltage value to be achieved during the fault, including: the faulty converter station MMC calculates the total number of sub-modules put into operation that can generate a positive level equivalently per phase of the converter station MMC according to the unbalanced power value of the converter station MMC and the target DC voltage value to be achieved during the fault; the faulty converter station MMC calculates the number of full-bridge sub-modules put into operation according to the total number of sub-modules put into operation that can generate a positive level equivalently per phase of the converter station MMC, and the total number of sub-modules put into operation per phase of the converter station MMC in steady state.

[0012] In combination with the first aspect, in a possible implementation, the above-mentioned faulty converter station MMC calculates the total number of submodules that are equivalent to generating a positive level per phase of the converter station MMC as follows:

[0013]

[0014] Where N′(t) is the total number of submodules that generate positive voltage per phase in the MMC converter station; round is the rounding function; U c (0) is the initial capacitance value of the MMC submodule of the converter station; U dcref is the target DC voltage value; ΔP is the unbalanced power value in the converter station MMC; C0 is the capacitance value of a single submodule of the converter station MMC in steady state.

[0015] In combination with the first aspect, in a possible implementation method, the above-mentioned rectifier station LCC quantitatively adjusts the current command value, including: the faulty converter station MMC obtains the actual active transmission capacity value of the faulty MMC converter station during the fault period according to the actual AC voltage effective value of the grid coupling point; the rectifier station LCC calculates the total active power that can be transmitted by the faulty converter station MMC and the non-fault converter station MMC within a preset time length according to the active transmission capacity value and the active transmission capacity of the non-fault converter station MMC; wherein the preset time length is the communication and control delay between the converter stations MMC; the rectifier station LCC calculates the current command value of the rectifier station LCC according to the total active power and the target DC voltage value, and quantitatively adjusts the current command value according to the calculated current command value.

[0016] To achieve the above-mentioned object, the second aspect of the present invention provides a DC overvoltage suppression device, which is applied to a hybrid multi-terminal DC power transmission system, wherein the hybrid multi-terminal DC power transmission system includes a multi-terminal converter station, that is, includes a rectifier station LCC and at least two inverter stations MMC, one of the at least two inverter stations MMC is used as a master station MMC, and the remaining inverter stations MMCs except the master station MMC in the at least two inverter stations MMC are used as slave stations MMC, the rectifier station LCC is the sending end of the hybrid multi-terminal DC power transmission system, and the at least two inverter stations MMC are the receiving ends of the hybrid multi-terminal DC power transmission system, and the device includes:

[0017] The input module is used for, when the hybrid multi-terminal DC transmission system has a faulty converter station MMC due to an AC fault at the receiving end, the faulty converter station MMC is used to input the reactive power compensation control strategy in real time to compensate for the drop in the effective value of the AC voltage at the grid-connected coupling point of the faulty converter station MMC; wherein the faulty converter station MMC is the main station MMC where the AC fault occurs;

[0018] Comparison module: used for the fault converter station MMC to obtain the actual AC voltage effective value after the grid coupling point compensation of the fault converter station MMC, and compare the actual AC voltage effective value after the grid coupling point compensation with the preset threshold value;

[0019] The first maintaining module is used for maintaining the original control mode and operation state of the hybrid multi-terminal DC transmission system if the effective value of the actual AC voltage after compensation at the grid-connected coupling point is not less than a preset threshold value, so as to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at a rated value during the fault period;

[0020] The second maintaining module is used for, if the effective value of the actual AC voltage after the grid-connected coupling point compensation is less than the preset threshold value, the faulty converter station MMC generates a negative level by putting into operation the full-bridge submodule to compensate for the rise of the DC voltage between the converter stations MMC; at the same time, the rectifier station LCC quantitatively adjusts the current command value, and the non-faulty converter station MMC switches the original control mode to the DC voltage control mode, so as to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault; wherein, the non-faulty converter station MMC is a slave station MMC where no AC fault occurs, and the converter station MMC refers to the inverter station MMC.

[0021] In combination with the second aspect, in a possible implementation, the above-mentioned input module includes a calculation module: used for the faulty converter station MMC to obtain the actual effective value of the AC voltage at the grid coupling point in real time, and calculate the difference between the actual effective value of the AC voltage at the grid coupling point and the AC voltage reference value; wherein the AC voltage reference value is the steady-state rated value of the AC voltage at the grid coupling point; the faulty converter station MMC calculates a reactive reference value based on the difference, and compensates for the drop in the effective value of the AC voltage at the grid coupling point of the faulty converter station MMC based on the reactive reference value.

[0022] To achieve the above object, the third aspect of the present invention provides a computer-readable storage medium storing a computer program, wherein when the computer program is executed by a processor, the processor performs the following steps:

[0023] When a hybrid multi-terminal DC transmission system has a faulty converter station MMC due to an AC fault at the receiving end, the faulty converter station MMC is put into reactive power compensation control strategy in real time to compensate for the drop in the effective value of the AC voltage at the grid-connected coupling point of the faulty converter station MMC; wherein the faulty converter station MMC is the main station MMC where the AC fault occurs;

[0024] The faulty converter station MMC obtains the actual AC voltage effective value after the grid-connected coupling point compensation of the faulty converter station MMC, and compares the actual AC voltage effective value after the grid-connected coupling point compensation with a preset threshold value;

[0025] If the effective value of the actual AC voltage after compensation at the grid coupling point is not less than the preset threshold, the hybrid multi-terminal DC transmission system maintains the original control mode and operation state to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault period;

[0026] If the effective value of the actual AC voltage after compensation at the grid-connected coupling point is less than a preset threshold, the faulty converter station MMC generates a negative level to compensate for the rise in the DC voltage between the converter stations MMC by putting the full-bridge submodule into operation. At the same time, the rectifier station LCC quantitatively adjusts the current command value, and the non-faulty converter station MMC switches the original control mode to the DC voltage control mode to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault; wherein the non-faulty converter station MMC is a slave station MMC where no AC fault occurs, and the converter station MMC refers to the inverter station MMC.

[0027] To achieve the above object, a fourth aspect of the present invention provides a computer device, comprising a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor performs the following steps:

[0028] When a hybrid multi-terminal DC transmission system has a faulty converter station MMC due to an AC fault at the receiving end, the faulty converter station MMC is put into reactive power compensation control strategy in real time to compensate for the drop in the effective value of the AC voltage at the grid-connected coupling point of the faulty converter station MMC; wherein the faulty converter station MMC is the main station MMC where the AC fault occurs;

[0029] The faulty converter station MMC obtains the actual AC voltage effective value after the grid-connected coupling point compensation of the faulty converter station MMC, and compares the actual AC voltage effective value after the grid-connected coupling point compensation with a preset threshold value;

[0030] If the effective value of the actual AC voltage after compensation at the grid coupling point is not less than the preset threshold, the hybrid multi-terminal DC transmission system maintains the original control mode and operation state to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault period;

[0031] If the effective value of the actual AC voltage after compensation at the grid-connected coupling point is less than a preset threshold, the faulty converter station MMC generates a negative level to compensate for the rise in the DC voltage between the converter stations MMC by putting the full-bridge submodule into operation. At the same time, the rectifier station LCC quantitatively adjusts the current command value, and the non-faulty converter station MMC switches the original control mode to the DC voltage control mode to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault; wherein the non-faulty converter station MMC is a slave station MMC where no AC fault occurs, and the converter station MMC refers to the inverter station MMC.

[0032] The embodiments of the present invention have the following beneficial effects:

[0033] 1) When an AC fault occurs on the grid side of the master station MMC, the faulty converter station MMC will be put into reactive power compensation control strategy in real time. This reactive power compensation control strategy uses the reactive power limit issued by the faulty converter station MMC to compensate for the drop in the effective value of the AC voltage at the grid coupling point (PCC), and cooperates with the active output current limit value to maximize the active power transmission capacity of the converter station MMC during the fault period, thereby assisting in reducing the surplus power value to suppress the rise of the DC voltage;

[0034] 2) By comparing the effective value of the actual AC voltage after compensation at the grid coupling point (PCC) of the faulty converter station MMC with the preset threshold, it is equivalent to determining the deviation between the actual active transmission capacity of the faulty converter station MMC and the steady-state value. If the effective value of the actual AC voltage after compensation at the grid coupling point (PCC) of the faulty converter station MMC is less than the preset threshold, the faulty converter station MMC, in the modulation link, adaptively invests a certain number of full-bridge sub-modules according to the unbalanced power value and the target DC voltage value required to be achieved during the fault period, so as to generate a negative level compensation DC voltage rise. At the same time, after a short delay such as inter-station communication, the rectifier station LCC quantitatively adjusts the command value of the fixed DC current mode according to the actual active transmission capacity of the faulty converter station MMC and the non-faulty converter station MMC during the fault period, reduces the unbalanced charging power of the converter station MMC sub-module, thereby quickly eliminating the surplus power, maintaining the transmission energy balance, and realizing DC overvoltage suppression. The non-faulty converter station MMC switches the original control mode to the DC voltage control mode, so as to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0036] in:

[0037] Figure 1 A topological structure diagram of a hybrid multi-terminal direct current transmission system in an embodiment of the present invention;

[0038] Figure 2 A schematic diagram of a flow chart of a DC overvoltage suppression method according to an embodiment of the present invention;

[0039] Figure 3 A schematic diagram of a reactive power compensation control strategy in an embodiment of the present invention;

[0040] Figure 4 This is a single-pole equivalent circuit diagram of an MMC fault converter station in an embodiment of the present invention;

[0041] Figure 5 A single-pole DC equivalent circuit of a converter station MMC in an embodiment of the present invention;

[0042] Figure 6 A strategy for quantitatively adjusting the current command value after a short time delay in the LCC of the sending-end converter station in an embodiment of the present invention;

[0043] Figure 7 It is a structural block diagram of a DC overvoltage suppression device in an embodiment of the present invention;

[0044] Figure 8 4 is a structural block diagram of a computer device in an embodiment of the present invention. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work are within the scope of protection of the present invention.

[0046] The hybrid multi-terminal DC transmission system is a type of DC transmission system. "Hybrid" means that different converters are used at the sending and receiving ends of the transmission system. The sending end refers to the sending end of the DC transmission system, that is, the power generation end. "Multi-terminal" means that the DC transmission system includes 3 or more converter stations. The converter station refers to a site established in the high-voltage DC transmission system to complete the conversion of AC into DC or DC into AC, and to meet the power system's requirements for safety, stability and power quality.

[0047] The present invention provides a DC overvoltage suppression method, which is used to suppress the rise of the DC voltage of a hybrid multi-terminal DC power transmission system when an AC fault occurs in a receiving-end converter station of the hybrid multi-terminal DC power transmission system.

[0048] In order to better illustrate the method, an embodiment of the present invention provides a topological structure diagram of a hybrid multi-terminal direct current transmission system, referring to Figure 1 , Figure 1 A topological structure diagram of a hybrid multi-terminal direct current transmission system provided by an embodiment of the present invention, such as Figure 1 As shown in the figure, the hybrid multi-terminal DC transmission system includes a three-terminal converter station, that is, a rectifier station as the sending end and two inverter stations as the receiving end. The sending end adopts the grid-commutated converter LCC, and the two receiving ends adopt the modular multi-level converter MMC, and the MMC is a hybrid form of full-bridge + half-bridge submodules. Since each port has positive and negative poles, each port is composed of 2 converters, 4 MMCs correspond to 2 transmission system receiving ends, and 2 LCCs correspond to 1 transmission system sending end, that is Figure 1As shown in , two MMC1s form an inverter station corresponding to one receiving end of a power transmission system, two MMC2s form an inverter station corresponding to one receiving end of a power transmission system, and two LCCs form a rectifier station corresponding to one sending end of a power transmission system. It should be noted that the present method embodiment is not limited to a hybrid multi-terminal DC power transmission system applied to a three-terminal converter station, but can be applied to a hybrid multi-terminal DC power transmission system with three-terminal converter stations and above, that is, including a rectifier station LCC and at least two inverter stations MMC.

[0049] The overall control strategy of the hybrid multi-terminal DC transmission system is divided into two parts. One is a steady-state basic control strategy. The steady-state basic control strategy is to equip the sending-end rectifier station LCC with basic constant DC current control to maintain the power stability of the hybrid multi-terminal DC transmission system. In this embodiment, the receiving-end inverter station MMC with the largest capacity during normal operation is used as the master station MMC, and the remaining receiving-end inverter stations MMC are used as slave stations MMC. The master station MMC with a larger capacity during normal operation is used as a power balancing station, and an outer loop control method of constant DC voltage and constant reactive power is adopted, while the slave station MMC is an outer loop control method of constant active power and constant reactive power. The other is a DC overvoltage suppression strategy when an AC fault occurs in the receiving-end master station MMC of the hybrid multi-terminal DC transmission system, that is, a DC overvoltage suppression method proposed in an embodiment of the present invention.

[0050] The following is a detailed description of the DC overvoltage suppression strategy. Figure 2 , Figure 2 A flow chart of a DC overvoltage suppression method provided by an embodiment of the present invention is shown in FIG. Figure 2 As shown, the method includes:

[0051] Step S101: When a faulty converter station MMC exists in the hybrid MTDC transmission system due to an AC fault at the receiving end, the faulty converter station MMC implements a reactive power compensation control strategy in real time to compensate for a drop in the effective value of the AC voltage at the grid-connected coupling point of the faulty converter station MMC.

[0052] The faulty converter station MMC is the main station MMC where the AC fault occurs.

[0053] When an AC fault occurs on the grid side of the receiving master station MMC, for the convenience of description, in this embodiment, the receiving master station MMC where the AC fault occurs is recorded as the faulty converter station MMC, and the faulty converter station MMC is put into reactive power compensation control strategy in real time. The reactive power compensation control strategy uses the reactive power limit issued by the faulty converter station MMC to compensate for the drop in the effective value of the AC voltage at the grid coupling point (PCC) of the faulty converter station MMC, so that it can cooperate with the active output current limit value to maximize the active transmission capacity of the converter station MMC during the AC fault of the receiving master station MMC of the hybrid multi-terminal DC transmission system, thereby assisting in reducing the surplus power value. Among them, the converter station MMC refers to the inverter station MMC.

[0054] The reactive power compensation control strategy is specifically to take the AC voltage value of the grid coupling point (PCC) of the faulty converter station MMC as the control target, and to obtain the reactive power reference value of the outer loop reactive power compensation control by controlling the actual value of the AC voltage of the grid coupling point (PCC) of the faulty converter station MMC to follow its AC voltage reference value, wherein the AC voltage reference value can be taken as the steady-state rated value of the AC voltage.

[0055] The following is a detailed introduction to the reactive power compensation control strategy:

[0056] Step S1011: the faulty converter station MMC obtains the actual effective value of the AC voltage at the grid coupling point in real time, and calculates the difference between the actual effective value of the AC voltage at the grid coupling point and the AC voltage reference value.

[0057] The AC voltage reference value is the steady-state rated value of the AC voltage at the grid coupling point.

[0058] Step S1012: the faulty converter station MMC calculates a reactive power reference value according to the difference, and compensates for a drop in the effective value of the AC voltage at the grid coupling point of the faulty converter station MMC according to the reactive power reference value.

[0059] Reference Figure 3 , Figure 3 A schematic diagram of a reactive power compensation control strategy provided by an embodiment of the present invention, such as Figure 3 As shown, Figure 3 Middle U PCC-actual Indicates the actual AC voltage RMS value at the grid coupling point (PCC), U PCC-ref Indicates the AC voltage reference value, Q r ' ef It represents the result of the reactive reference theoretical value calculation formula output, Q ref Represents reactive power reference value, Q refn is the steady-state reactive power reference value, Q max , -Q maxIt is the maximum reactive power that the MMC can generate or absorb. The positive and negative signs represent the generation or absorption, that is, the reactive power constraint condition.

[0060] based on Figure 3 The specific reactive power compensation control strategy principle is as follows: the fault converter station MMC adaptively switches into the reactive power compensation control strategy, and the fault converter station MMC obtains the actual AC voltage effective value U of the grid coupling point (PCC) in real time. PCC_actual , and calculate the actual AC voltage effective value U PCC_actual With AC voltage reference value U PCC_ref The deviation is input into the PI control algorithm, and the fault converter station MMC equivalent AC output voltage and AC voltage reference value U PCC-ref Substitute the theoretical value of reactive power reference into the calculation formula, and compare the output of the deviation obtained by PI control algorithm with the equivalent AC output voltage of the faulty converter station MMC and the AC voltage reference value U PCC-ref Substitute the results obtained by the reactive reference value theoretical value calculation formula and sum them to obtain the reactive reference value Q ref , according to the reactive power reference value, compensate the drop of the AC voltage effective value at the grid coupling point of the faulty converter station MMC at the receiving end. max and-Q max The reactive power reference value does not exceed the system reactive power constraint condition.

[0061] Reference Figure 4 , Figure 4 A single-pole equivalent circuit diagram of a faulty converter station MMC provided by an embodiment of the present invention is shown in FIG. Figure 4 As shown, Figure 4 Middle U PCC_ref is the AC voltage reference value of the fault converter station grid coupling point (PCC) ( Figure 4 PCC), U diff is the effective value of the equivalent output AC voltage of the faulty converter station ( Figure 4 F in the middle), L1 and L2 are transformer 1 and transformer 2 respectively, U dc is the input voltage of the faulty converter station.

[0062] based on Figure 4 , the theoretical value calculation formula of reactive power reference value can be deduced as:

[0063]

[0064] Where: U PCC_ref is the AC voltage reference value of the faulty converter station MMC grid coupling point (PCC); U diff X is the effective value of the equivalent output AC voltage of the faulty converter station MMC; T is the AC voltage reference value U PCC_refThe equivalent output AC voltage RMS value U diff X0 is the equivalent reactance value of the transformer between the two, and X1 is the equivalent reactance value of the MMC bridge arm of the fault converter station.

[0065] Through the adaptive reactive compensation control strategy of the faulty converter station MMC, it can be achieved that when the reactive support capacity of the faulty converter station MMC is greatly reduced due to a serious AC fault, and the faulty converter station MMC cannot maintain the PCC AC voltage at the steady-state rated value level, the faulty converter station MMC can also emit reactive power to compensate the drop of AC voltage to the maximum extent.

[0066] Step S102: the faulty converter station MMC obtains the effective value of the actual AC voltage after the grid-connected coupling point compensation of the faulty converter station MMC, and compares the effective value of the actual AC voltage after the grid-connected coupling point compensation with a preset threshold.

[0067] After the faulty converter station MMC adaptively switches into the reactive power compensation control strategy, further, according to the different drop degrees of the AC voltage of the faulty converter station MMC, the DC voltage suppression strategy is determined. In this embodiment, the relationship between the effective value of the actual AC voltage after compensation at the grid coupling point (PCC) of the faulty converter station MMC and the preset threshold value can be used as the basis for judging the degree of AC voltage drop. Specifically, the faulty converter station MMC obtains the effective value of the actual AC voltage after compensation at the grid coupling point of the faulty converter station MMC, and compares the effective value of the actual AC voltage after compensation at the grid coupling point with the preset threshold value. The preset threshold value U th The choice is mainly related to the active current limit value of the converter MMC.

[0068] By comparing the actual AC voltage RMS value after compensation at the fault converter station MMC grid coupling point (PCC) with the preset threshold U th The size of is equivalent to determining the deviation between the actual active power transmission capacity of the faulty converter station MMC and the active power steady-state rated value.

[0069] Step S103: If the effective value of the actual AC voltage after compensation at the grid coupling point is not less than the preset threshold, the hybrid multi-terminal DC transmission system maintains the original control mode and operation state to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault.

[0070] If the actual AC voltage effective value after compensation at the grid-connected coupling point is not less than the preset threshold value U th When , it indicates that the AC voltage drop caused by the AC fault is relatively slight. The hybrid multi-terminal transmission system can eliminate the surplus power by adjusting its own steady-state power margin. Therefore, in this case, the hybrid multi-terminal transmission system maintains the original control mode and operating state, and can still maintain the stability of the DC voltage according to the steady-state power transfer value.

[0071] Step S104: If the effective value of the actual AC voltage after compensation at the grid-connected coupling point is less than the preset threshold, the faulty converter station MMC generates a negative level to compensate for the rise of the DC voltage between the converter stations MMC by putting the full-bridge submodule into operation. At the same time, the rectifier station LCC quantitatively adjusts the current command value, and the non-faulty converter station MMC switches the original control mode to the DC voltage control mode to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault.

[0072] The non-fault converter station MMC is a slave station MMC where no AC fault occurs, and the converter station MMC refers to the inverter station MMC.

[0073] If the actual AC voltage RMS value after compensation at the faulty converter station MMC grid coupling point (PCC) is less than the preset threshold value U th , the faulty converter station MMC adaptively puts into use a certain number of full-bridge sub-modules to generate a negative level to compensate for the rise of the DC voltage. At the same time, the sending-end rectifier station LCC quantitatively adjusts the command value of the fixed DC current mode to reduce the unbalanced charging power of the converter station MMC sub-module, thereby quickly eliminating the surplus power and maintaining the transmission energy balance. The non-faulty converter station MMC switches the original control mode to the DC voltage control mode to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value.

[0074] The following specifically introduces how the faulty converter station MMC adaptively puts into use a certain number of full-bridge sub-modules.

[0075] Step S1041: The faulty converter station MMC calculates the number of full-bridge sub-modules to be put into operation according to the unbalanced power value of the converter station MMC and the target DC voltage value to be achieved during the fault period.

[0076] The target DC voltage value is the rated value.

[0077] Step S1042: the faulty converter station MMC puts in full-bridge sub-modules according to the number of the full-bridge sub-modules to generate a negative level to compensate for the increase of the DC voltage between the converter stations MMCs.

[0078] Among them, the fault converter station MMC calculates the number of full-bridge submodules to be put into operation according to the unbalanced power value of the converter station MMC and the target DC voltage value to be achieved during the fault period, specifically:

[0079] Step S10411: The faulty converter station MMC calculates the total number of submodules that are put into use and that can generate a positive voltage equivalently for each phase of the converter station MMC according to the unbalanced power value of the converter station MMC and the target DC voltage value that needs to be achieved during the fault period.

[0080] Step S10412: the faulty converter station MMC calculates the number of full-bridge submodules put into operation according to the total number of submodules put into operation that equivalently generate a positive level in each phase of the converter station MMC and the total number of submodules put into operation in each phase of the converter station MMC in a steady state.

[0081] Reference Figure 5 , Figure 5 A converter station MMC unipolar DC equivalent circuit is provided in an embodiment of the present invention, wherein the converter station MMC refers to an inverter station MMC, such as Figure 5 As shown, L eq , R eq , C eq are the equivalent reactance, equivalent resistance and equivalent capacitance of the single pole of the converter station MMC, ΔP is the unbalanced power value (surplus power) in the converter station MMC during the fault period, i arm is the bridge arm current, U dc is the input voltage of the converter station MMC.

[0082] During steady state, the equivalent capacitance of the MMC single pole of the converter station is as follows:

[0083]

[0084] Where N and C0 are the total number of sub-modules per phase in the converter station MMC and the capacitance value of a single sub-module in steady state, respectively.

[0085] When analyzing the charging process of the unbalanced power on the MMC bridge arm of the converter station, the influence of the equivalent resistance and inductance can be ignored. Assuming that the charging time of the unbalanced power is Δt and the initial charging time is t=0, the unbalanced energy of the unbalanced power charging the equivalent capacitor can be expressed as follows:

[0086]

[0087] Where U dc (0) is the input voltage of the converter station MMC at the initial moment; U dc (Δt) is the input voltage of the converter station MMC under the charging time of Δt.

[0088] During the charging process, assuming that the unbalanced power ΔP is a constant, the above formula can be rewritten as:

[0089]

[0090] At the same time, the above formula shows that during the period of unbalanced power, the DC voltage expression between the MMCs of the converter station is:

[0091]

[0092] Where U dc(t) is the input voltage of the converter station at time t, C eq (t) represents the equivalent capacitance at time t, C eq (0) represents the equivalent capacitance at the initial moment.

[0093] As the unbalanced power charges the MMC bridge arm submodule capacitor, the DC voltage between the MMCs in the converter station will rise rapidly. Considering that the MMC converter unit used in the hybrid multi-terminal transmission system is a hybrid submodule structure of "full bridge + half bridge", the negative level output characteristics of the full-bridge submodule can be fully utilized. During the period when the submodule capacitor voltage rises to the protection rated value, a certain number of full-bridge submodules that generate negative levels are invested in the MMC modulation link of the converter station in the form of an algorithm, so as to achieve the control effect of suppressing the DC overvoltage of the converter station MMC in the early stage of the fault.

[0094] The specific implementation plan for the number of full-bridge sub-modules is as follows:

[0095] If the DC voltage between the MMCs in the converter station increases, after a certain number of full-bridge submodules are put into use, the single-pole equivalent capacitance C of the MMC in the converter station e ' q Is the following formula:

[0096]

[0097] In the formula, C e ' q is the single-pole equivalent capacitance of the converter station MMC, C0 is the capacitance value of a single submodule of the converter station MMC in steady state, and N′ is the total number of submodules that generate positive level equivalently per phase of the converter station MMC.

[0098] In summary, in order to keep the DC voltage stable during the fault period, the DC voltage change rate can be set to 0, that is,

[0099]

[0100] Thus, the total number of submodules N′(t) that can generate positive level equivalently per phase in the converter station MMC that can satisfy the DC overvoltage suppression effect is obtained, as shown in the following formula:

[0101]

[0102] Where: round is the rounding function; U c (0) is the initial capacitance value of the MMC submodule of the converter station, i.e., the steady-state value; U dcref is the target DC voltage value desired during a fault, wherein the target DC voltage value is taken as the rated value.

[0103] The number of full-bridge submodules that generate negative voltage per phase in the converter station MMC changes with the unbalanced power adaptive change. During the fault period, the number of full-bridge negative voltages required to maintain the DC voltage of the converter station MMC at the rated value is N. full for:

[0104] N full =NN′(t)

[0105] Where N is the total number of submodules put into operation per phase of the converter station MMC in steady state, and N′(t) is the total number of submodules put into operation per phase of the converter station MMC that can generate an equivalent positive level.

[0106] The above method can be used to calculate the total number of full-bridge sub-modules that can generate a negative level in each phase of the faulty converter station MMC, and full-bridge sub-modules are put into use according to the calculated number of full-bridge sub-modules to generate a negative level to compensate for the increase in the DC voltage between the faulty converter station MMCs.

[0107] The above describes how to calculate the total number of full-bridge submodules put into operation by the faulty converter station MMC. The following describes how to quantitatively adjust the current command value of the rectifier station LCC:

[0108] Step S1043: The faulty converter station MMC obtains the actual active transmission capacity value of the faulty MMC converter station during the fault period according to the actual effective value of the AC voltage at the grid coupling point.

[0109] Step S1044: The rectifier station LCC calculates the total active power that can be transmitted by the faulty converter station MMC and the non-faulty converter station MMC within a preset time period according to the active transmission capacity value and the active transmission capacity of the non-faulty converter station MMC.

[0110] The preset duration is the communication and control delay between the converter station MMCs.

[0111] Step S1045: The rectifier station LCC calculates a current command value of the rectifier station LCC according to the total active power and the target DC voltage value, and quantitatively adjusts the current command value according to the calculated current command value.

[0112] Reference Figure 6 , Figure 6 A strategy for quantitatively adjusting the current command value of the LCC at the sending-end converter station after a short time delay is provided in an embodiment of the present invention, such as Figure 6 As shown, Figure 6 Middle U PCC_actual is the actual AC voltage effective value at the fault converter station MMC grid coupling point (PCC), P M ' MC1 is the actual active power transmission capability value of the faulty converter station MMC, P MMC2maxis the upper limit of the active transmission capacity of the non-fault converter station MMC, N / D represents the division operation, two numbers are divided, U dcn is the steady-state DC voltage rating, I dcrefN is the steady-state DC current rating, I dcref It is the current reference value of the constant DC current control mode.

[0113] The sending-end rectifier station LCC adjusts the current command value quantitatively after a short delay. Specifically, the fault converter station MMC obtains the actual active transmission capacity value P of the fault converter station MMC during the fault period according to the actual AC voltage effective value of the grid coupling point (PCC) through the fault converter station MMC active transmission capacity calculation module. M ' MC1 Among them, the active transmission capacity calculation module is mainly composed of coordinate transformation and instantaneous power theory. At the same time, considering that the non-fault converter station MMC switches to a constant DC voltage mode, and each converter station MMC has an upper limit value of active transmission, therefore, in this embodiment, the active transmission capacity of the non-fault converter station MMC is selected as the maximum value of active transmission P MMC2max , the purpose is to transmit as much system active power as possible during the fault period. Further, after a short delay (communication and control delay between converter stations), the sum of the active power that can be transmitted by the two receiving converter stations MMC is divided by the target DC voltage value, that is, the sum of the active power that can be transmitted by the faulty converter station MMC and the non-faulty converter station MMC is calculated, and the sum of the active power is divided by the target DC voltage value to obtain the current command value of the sending rectifier station LCC. During the fault period, the current command value is adaptively reduced following the active transmission capacity of the receiving faulty converter station MMC, which can quickly reduce the unbalanced charging power (surplus power) of the sub-module capacitor of the receiving faulty converter station MMC, maintain the transmission energy balance between the sending and receiving faulty converter stations MMC, and avoid damage to the sub-module capacitor due to overvoltage.

[0114] Through the above method, when an AC fault occurs on the grid side of the master station MMC, the faulty converter station MMC is put into reactive power compensation control strategy in real time, and the reactive power limit issued by the faulty converter station MMC is used to compensate for the drop in the effective value of the AC voltage at the grid coupling point (PCC), and the active output current limit value is combined to maximize the active transmission capacity of the converter station MMC during the transmission system fault, thereby assisting in reducing the surplus power value to suppress the increase in DC voltage. In addition, on the basis of the reactive power compensation control strategy, the actual effective value of the AC voltage after compensation at the grid coupling point (PCC) of the faulty converter station MMC is compared with the preset threshold value. If the actual effective value of the AC voltage after compensation at the grid coupling point (PCC) of the faulty converter station MMC is less than the preset threshold value, the fault is detected. In the modulation link, the converter station MMC adaptively invests a certain number of full-bridge sub-modules according to the unbalanced power value and the target DC voltage value to be achieved during the fault period, so as to generate a negative level to compensate for the rise of the DC voltage between stations. At the same time, after a short delay such as inter-station communication, the rectifier station LCC quantitatively adjusts the command value of the fixed DC current mode according to the actual active transmission capacity of the faulty converter station MMC and the slave station MMC during the fault period, reduces the unbalanced charging power of the sub-modules of the converter station MMC, thereby quickly eliminating the surplus power, maintaining the transmission energy balance, and realizing DC overvoltage suppression. The non-fault converter station MMC switches the original control mode to the DC voltage control mode, so as to achieve the DC voltage of the hybrid multi-terminal DC transmission system to be stable at the rated value level during the fault period.

[0115] In order to better implement the above method, the embodiment of the present invention provides a DC overvoltage suppression device, referring to Figure 7 , Figure 7 A DC overvoltage suppression device is provided in an embodiment of the present invention, such as Figure 6 As shown, the device 70 includes:

[0116] The input module 701 is used for, when the hybrid multi-terminal direct current transmission system has a faulty converter station MMC due to an AC fault at the receiving end, the faulty converter station MMC is used to input the reactive power compensation control strategy in real time to compensate for the drop of the effective value of the AC voltage at the grid-connected coupling point of the faulty converter station MMC; wherein the faulty converter station MMC is the main station MMC where the AC fault occurs;

[0117] Comparison module 702: used for the faulty converter station MMC to obtain the effective value of the actual AC voltage after the grid-connected coupling point compensation of the faulty converter station MMC, and compare the effective value of the actual AC voltage after the grid-connected coupling point compensation with a preset threshold value;

[0118] The first maintaining module 703 is used for maintaining the original control mode and operation state of the hybrid multi-terminal direct current transmission system if the effective value of the actual AC voltage after compensation at the grid coupling point is not less than a preset threshold value, so as to maintain the DC voltage of the hybrid multi-terminal direct current transmission system stable at a rated value during the fault period;

[0119] The second maintaining module 704 is used for, if the effective value of the actual AC voltage after the grid-connected coupling point compensation is less than the preset threshold value, the faulty converter station MMC generates a negative level by putting into operation the full-bridge submodule to compensate for the rise of the DC voltage between the converter stations MMC, and at the same time, the rectifier station LCC quantitatively adjusts the current command value, and the non-faulty converter station MMC switches the original control mode to the DC voltage control mode, so as to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault period; wherein, the non-faulty converter station MMC is a slave station MMC where no AC fault occurs, and the converter station MMC refers to the inverter station MMC.

[0120] Among them, the input module 701 includes a calculation module, which is used for the faulty converter station MMC to obtain the actual AC voltage effective value of the grid coupling point in real time, and calculate the difference between the actual AC voltage effective value of the grid coupling point and the AC voltage reference value; wherein the AC voltage reference value is the AC voltage steady-state rated value of the grid coupling point; the faulty converter station MMC calculates the reactive power reference value according to the difference, and compensates for the drop in the AC voltage effective value of the grid coupling point of the faulty converter station MMC according to the reactive power reference value.

[0121] In one possible design, the second maintaining module 704 is specifically used for: the faulty converter station MMC calculates the number of full-bridge sub-modules to be put into use according to the unbalanced power value of the converter station MMC and the target DC voltage value to be achieved during the fault; wherein the target DC voltage value is a rated value; the faulty converter station MMC puts into use full-bridge sub-modules according to the number of full-bridge sub-modules to generate a negative level to compensate for the rise in DC voltage between the converter stations MMCs.

[0122] In one possible design, the second maintaining module 704 is specifically used for: the faulty converter station MMC calculates the total number of sub-modules that are put into operation to generate a positive level per phase of the converter station MMC according to the unbalanced power value of the converter station MMC and the target DC voltage value that needs to be achieved during the fault; the faulty converter station MMC calculates the number of full-bridge sub-modules put into operation according to the total number of sub-modules that are put into operation to generate a positive level per phase of the converter station MMC and the total number of sub-modules put into operation per phase of the converter station MMC in a steady state.

[0123] In a possible design, the second maintaining module 704 is specifically used for: the faulty converter station MMC calculates the total number of submodules that are equivalent to generating a positive level per phase of the converter station MMC using the following calculation formula:

[0124]

[0125] Where N′(t) is the total number of submodules that generate positive voltage per phase in the MMC converter station; round is the rounding function; U c (0) is the initial capacitance value of the MMC submodule of the converter station; U dcref is the target DC voltage value; ΔP is the unbalanced power value in the converter station MMC; C0 is the capacitance value of a single submodule of the converter station MMC in steady state.

[0126] In one possible design, the second maintaining module 704 is specifically used for: the faulty converter station MMC obtains the actual active transmission capacity value of the faulty MMC converter station during the fault period according to the actual effective value of the AC voltage at the grid coupling point; the rectifier station LCC calculates the total active power that can be transmitted by the faulty converter station MMC and the non-faulty converter station MMC within a preset time length according to the active transmission capacity value and the active transmission capacity of the non-faulty converter station MMC; wherein the preset time length is the communication and control delay between the converter stations MMC; the rectifier station LCC calculates the current command value of the rectifier station LCC according to the total active power and the target DC voltage value, and quantitatively adjusts the current command value according to the calculated current command value.

[0127] Based on the above device, when an AC fault occurs on the grid side of the master station MMC, the faulty converter station MMC will put into real-time reactive power compensation control strategy, and use the reactive power limit issued by the faulty converter station MMC to compensate for the drop in the effective value of the AC voltage at the grid coupling point (PCC), in conjunction with the active output current limit value, to maximize the active power transmission capacity of the converter station MMC during the transmission system fault, thereby assisting in reducing the surplus power value to suppress the rise in DC voltage. In addition, on the basis of the reactive power compensation control strategy, the actual effective value of the AC voltage after compensation at the grid coupling point (PCC) of the faulty converter station MMC is compared with the preset threshold value. If the actual effective value of the AC voltage after compensation at the grid coupling point (PCC) of the faulty converter station MMC is less than the preset threshold value, then the faulty converter station MMC is faulted. In the modulation link, the fault converter station MMC adaptively invests a certain number of full-bridge sub-modules according to the unbalanced power value and the target DC voltage value to be achieved during the fault period, so as to generate a negative level to compensate for the rise of the DC voltage. At the same time, after a short delay such as inter-station communication, the rectifier station LCC quantitatively adjusts the command value of the fixed DC current mode according to the actual active transmission capacity of the fault converter station MMC and the slave station MMC during the fault period, reduces the unbalanced charging power of the sub-modules of the converter station MMC, thereby quickly eliminating the surplus power, maintaining the transmission energy balance, and realizing DC overvoltage suppression. The non-fault converter station MMC switches the original control mode to the DC voltage control mode, so as to achieve the DC voltage of the hybrid multi-terminal DC transmission system to be stable at the rated value level during the fault period.

[0128] Figure 8 FIG. 1 shows an internal structure diagram of a computer device in an embodiment. The computer device may be a terminal or a server. Figure 8 As shown, the computer device includes a processor, a memory and a network interface connected via a system bus. The memory includes a non-volatile storage medium and an internal memory. The non-volatile storage medium of the computer device stores an operating system and may also store a computer program. When the computer program is executed by the processor, the processor can implement all the steps of the above method. The internal memory may also store a computer program. When the computer program is executed by the processor, the processor can implement all the steps of the above method. Those skilled in the art will understand that Figure 8 The structure shown in the figure is only a block diagram of a part of the structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0129] In one embodiment, a computer device is provided, including a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor executes each step of the aforementioned method.

[0130] In one embodiment, a computer-readable storage medium is provided, which stores a computer program. When the computer program is executed by a processor, the processor executes each step of the aforementioned method.

[0131] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program, and the program can be stored in a non-volatile computer-readable storage medium. When the program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to memory, storage, database or other media used in the embodiments provided in this application can include non-volatile and / or volatile memory. Non-volatile memory may include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) or flash memory. Volatile memory may include random access memory (RAM) or external cache memory. As an illustration and not limitation, RAM is available in many forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM).

[0132] The technical features of the above embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0133] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A method for suppressing direct current overvoltage, the method being applied to a hybrid multi-terminal direct current transmission system, wherein: The hybrid multi-terminal direct current power transmission system comprises a multi-terminal converter station, that is, a rectifier station LCC and at least two inverter stations MMC, one of the at least two inverter stations MMC is used as a master station MMC, and the remaining inverter stations MMCs except the master station MMC are used as slave stations MMC, the rectifier station LCC is a sending end of the hybrid multi-terminal direct current power transmission system, and the at least two inverter stations MMC are receiving ends of the hybrid multi-terminal direct current power transmission system, characterized in that the method comprises: When a hybrid multi-terminal DC transmission system has a faulty converter station MMC due to an AC fault at the receiving end, the faulty converter station MMC is put into reactive power compensation control strategy in real time to compensate for the drop in the effective value of the AC voltage at the grid-connected coupling point of the faulty converter station MMC; wherein the faulty converter station MMC is the main station MMC where the AC fault occurs; The faulty converter station MMC obtains the actual AC voltage effective value after the grid-connected coupling point compensation of the faulty converter station MMC, and compares the actual AC voltage effective value after the grid-connected coupling point compensation with a preset threshold value; If the effective value of the actual AC voltage after compensation at the grid coupling point is not less than the preset threshold, the hybrid multi-terminal DC transmission system maintains the original control mode and operation state to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault period; If the effective value of the actual AC voltage after compensation at the grid-connected coupling point is less than the preset threshold, the faulty converter station MMC generates a negative level to compensate for the rise of the DC voltage between the converter station MMCs by putting the full-bridge submodule into operation. At the same time, the rectifier station LCC quantitatively adjusts the current command value, and the non-faulty converter station MMC switches the original control mode to the DC voltage control mode to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault period; wherein the non-faulty converter station MMC is a slave station MMC without an AC fault, and the converter station MMC refers to the inverter station MMC; The rectifier station LCC quantitatively adjusts the current command value, including: The faulty converter station MMC obtains the actual active transmission capacity value of the faulty MMC converter station during the fault period according to the actual AC voltage effective value of the grid coupling point; The rectifier station LCC calculates the sum of active power that can be transmitted by the faulty converter station MMC and the non-faulty converter station MMC within a preset time length according to the active transmission capacity value and the active transmission capacity of the non-faulty converter station MMC; wherein the preset time length is the communication and control delay between the converter stations MMC; The rectifier station LCC calculates a current command value of the rectifier station LCC according to the total active power and the target DC voltage value, and quantitatively adjusts the current command value according to the calculated current command value.

2. The method according to claim 1, characterized in that The faulty converter station MMC uses a reactive power compensation control strategy in real time to compensate for the drop in the effective value of the AC voltage at the grid-connected coupling point of the faulty converter station MMC, including: The fault converter station MMC obtains the actual AC voltage effective value of the grid coupling point in real time, and calculates the difference between the actual AC voltage effective value of the grid coupling point and the AC voltage reference value; wherein the AC voltage reference value is the AC voltage steady-state rated value of the grid coupling point; The faulty converter station MMC calculates a reactive power reference value according to the difference, and compensates for a drop in the effective value of the AC voltage at the grid coupling point of the faulty converter station MMC according to the reactive power reference value.

3. The method according to claim 1, characterized in that The faulty converter station MMC generates a negative level to compensate for the increase of the DC voltage between the converter stations MMCs by putting the full bridge submodule into operation, including: The fault converter station MMC calculates the number of full-bridge submodules to be put into operation according to the unbalanced power value of the converter station MMC and the target DC voltage value to be achieved during the fault period; wherein the target DC voltage value is a rated value; The faulty converter station MMC puts in full-bridge sub-modules according to the number of the full-bridge sub-modules to generate a negative level to compensate for the rise of the DC voltage between the converter stations MMCs.

4. The method according to claim 3, characterized in that The fault converter station MMC calculates the number of full-bridge submodules to be put into operation according to the unbalanced power value of the converter station MMC and the target DC voltage value to be achieved during the fault period, including: The faulty converter station MMC calculates the total number of submodules that are put into operation to generate a positive voltage equivalently for each phase of the converter station MMC according to the unbalanced power value of the converter station MMC and the target DC voltage value that needs to be achieved during the fault period; The faulty converter station MMC calculates the number of full-bridge submodules put into operation according to the total number of submodules put into operation that equivalently generate a positive level in each phase of the converter station MMC and the total number of submodules put into operation in each phase of the converter station MMC in a steady state.

5. The method according to claim 4, characterized in that: The calculation formula for calculating the total number of submodules that generate positive level equivalently per phase of the faulty converter station MMC is: in, is the total number of submodules that generate positive voltage equivalently per phase in the converter station MMC; round is the rounding function; is the initial capacitance value of the MMC submodule of the converter station; is the target DC voltage value; is the unbalanced power value in the converter station MMC; are the capacitance values ​​of individual submodules of the converter station MMC in steady state; N is the total number of submodules per phase put into operation in the converter station MMC in steady state; is the charging time of unbalanced power.

6. A DC overvoltage suppression device, the device being applied to a hybrid multi-terminal DC transmission system, wherein: The hybrid multi-terminal DC power transmission system includes a multi-terminal converter station, that is, a rectifier station LCC and at least two inverter stations MMC, one of the at least two inverter stations MMC is used as a master station MMC, and the remaining inverter stations MMCs except the master station MMC are used as slave stations MMC. The rectifier station LCC is a sending end of the hybrid multi-terminal DC power transmission system, and the at least two inverter stations MMC are receiving ends of the hybrid multi-terminal DC power transmission system. The device includes: The input module is used for, when the hybrid multi-terminal DC transmission system has a faulty converter station MMC due to an AC fault at the receiving end, the faulty converter station MMC is used to input the reactive power compensation control strategy in real time to compensate for the drop in the effective value of the AC voltage at the grid-connected coupling point of the faulty converter station MMC; wherein the faulty converter station MMC is the main station MMC where the AC fault occurs; Comparison module: used for the fault converter station MMC to obtain the actual AC voltage effective value after the grid coupling point compensation of the fault converter station MMC, and compare the actual AC voltage effective value after the grid coupling point compensation with the preset threshold value; The first maintaining module is used for maintaining the original control mode and operation state of the hybrid multi-terminal DC transmission system if the effective value of the actual AC voltage after compensation at the grid-connected coupling point is not less than a preset threshold value, so as to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at a rated value during the fault period; The second maintenance module is used for, if the effective value of the actual AC voltage after the grid-connected coupling point compensation is less than the preset threshold value, the faulty converter station MMC generates a negative level by putting into use the full-bridge submodule to compensate for the rise of the DC voltage between the converter stations MMC, and at the same time, the rectifier station LCC quantitatively adjusts the current command value, and the non-faulty converter station MMC switches the original control mode to the DC voltage control mode, so as to maintain the DC voltage of the hybrid multi-terminal DC transmission system stable at the rated value during the fault period; wherein, the non-faulty converter station MMC is a slave station MMC without AC fault, and the converter station MMC refers to the inverter station MMC; The second maintaining module is further used for the faulty converter station MMC to obtain the actual active transmission capacity value of the faulty MMC converter station during the fault period according to the actual AC voltage effective value of the grid coupling point; The rectifier station LCC calculates the sum of active power that can be transmitted by the faulty converter station MMC and the non-faulty converter station MMC within a preset time length according to the active transmission capacity value and the active transmission capacity of the non-faulty converter station MMC; wherein the preset time length is the communication and control delay between the converter stations MMC; The rectifier station LCC calculates a current command value of the rectifier station LCC according to the total active power and the target DC voltage value, and quantitatively adjusts the current command value according to the calculated current command value.

7. The device according to claim 6, characterized in that The input module includes: Calculation module: used for the faulty converter station MMC to obtain the actual effective value of the AC voltage at the grid coupling point in real time, and calculate the difference between the actual effective value of the AC voltage at the grid coupling point and the AC voltage reference value; wherein the AC voltage reference value is the steady-state rated value of the AC voltage at the grid coupling point; the faulty converter station MMC calculates the reactive power reference value according to the difference, and compensates for the drop in the effective value of the AC voltage at the grid coupling point of the faulty converter station MMC according to the reactive power reference value.

8. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the processor is caused to perform the steps of the method according to any one of claims 1 to 5.

9. A computer device comprising a memory and a processor, characterized in that: The memory stores a computer program, and when the computer program is executed by the processor, the processor is caused to perform the steps of the method according to any one of claims 1 to 5.

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