AC fault ride-through control method and system for ultra-high voltage flexible direct current transmission system

Abstract

The present invention discloses a method and system for controlling AC fault ride-through of an ultra-high voltage flexible direct current transmission system. The receiving-end MMC converter adopts a semi-full-bridge sub-module hybrid topology structure as the converter sub-module. The method comprises: controlling the sending-end MMC converter in a constant DC current control mode during the fault and recovery process of the receiving-end AC system; controlling the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the sub-module capacitor voltages of the converter sub-modules, performing double frequency circulating current control on the receiving-end MMC converter according to the total average value of the sub-module capacitor voltages, and performing active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system. During the fault occurrence and recovery process, the dynamic energy balance and high power transmission capacity of the entire system can be maintained without additionally configuring a DC energy-consuming device, eliminating the influence of surplus power during the fault ride-through period, and realizing uninterrupted operation of the entire system.

Description

Technical Field

[0001] The present invention relates to the technical field of converter station control, and in particular to an AC fault ride-through control method and system for an ultra-high voltage flexible direct current transmission system. Background Art

[0002] The HVDC transmission system based on modular multi-level converter is a new generation of DC transmission technology, also known as flexible DC transmission technology. In the UHV application scenario, it can be called UHV flexible DC transmission system. The system structure diagram is as follows: Figure 1 The Modular Multilevel Converter (MMC) used at both the receiving and sending ends of UHV Flexible DC transmission systems has been widely recognized for its advantages, including the absence of commutation failure, active and reactive power decoupling control, and the elimination of filters and reactive power compensation devices. Its transmission capacity has reached UHV levels, comparable to conventional thyristor-based DC transmission.

[0003] Due to the long transmission distance of the UHV flexible DC transmission system, overhead line transmission lines must be used for economic reasons. However, overhead line transmission lines have a high probability of temporary failure due to reasons such as lightning strikes.

[0004] Although UHVDC flexible transmission systems do not experience commutation failure after an AC fault occurs on the receiving end, the AC system's inability to absorb the power transmitted by the DC system can easily lead to excess power within the DC system, causing overvoltage and threatening equipment safety. Excessive excess energy can also cause fault ride-through failure on the AC side of the entire DC system.

[0005] To address this issue, a DC energy dissipation device can be deployed within the DC system to dissipate excess power during a fault in the receiving AC system, maintaining a dynamic balance of energy within the entire system. However, this approach significantly increases investment costs and is less economical. Therefore, maintaining a dynamic balance of energy across the entire system and maximizing power transmission capacity during a fault has become a pressing technical challenge for those skilled in the art. Summary of the Invention

[0006] The present invention provides an AC fault ride-through control method and system for an ultra-high voltage flexible direct current transmission system.

[0007] To solve the above technical problems, an embodiment of the present invention provides an AC fault ride-through control method for a UHV flexible DC transmission system. The receiving-end MMC converter adopts a hybrid topology structure of a semi-full-bridge submodule as a converter submodule. The method includes:

[0008] During the receiving-end AC system failure and recovery process, the sending-end MMC converter is controlled by a constant DC current control method; and

[0009] The receiving-end MMC converter is controlled to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules, and the receiving-end MMC converter is subjected to double frequency circulating current control according to the total average value of the submodule capacitor voltages. In addition, the receiving-end MMC converter is subjected to active power outer loop control according to the AC voltage of the receiving-end AC system.

[0010] As one preferred solution, controlling the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules includes:

[0011] respectively setting a submodule capacitor voltage threshold value and a deviation control coefficient of the converter submodule, and calculating a total average value of the submodule capacitor voltage of the converter submodule;

[0012] Determining a DC component value according to the submodule capacitor voltage threshold value, the deviation control coefficient, the total average value of the submodule capacitor voltage and the converter DC voltage control instruction value;

[0013] The DC port voltage of the receiving-end MMC converter is adjusted according to the DC component value.

[0014] As one preferred solution, the submodule capacitor voltage threshold value is higher than the submodule capacitor voltage rated value of the converter submodule.

[0015] As one preferred solution, the double frequency circulating current control of the receiving-end MMC converter is performed according to the total average value of the submodule capacitor voltage, including:

[0016] Calculating the total average value of the submodule capacitor voltage;

[0017] determining a second signal assignment value according to a submodule capacitor voltage threshold value and a submodule capacitor voltage rated value of the converter submodule, and a total average value of the submodule capacitor voltage;

[0018] According to the second signal assignment, the double frequency component in the converter arm voltage reference value of the receiving-end MMC converter is adjusted, so as to perform double frequency circulating current control according to the adjusted double frequency component.

[0019] As one preferred solution, performing active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system includes:

[0020] Calculating in real time the effective value of the positive sequence component of the sampled value of the AC voltage of the receiving-end AC system, and determining the rated effective value of the AC voltage of the receiving-end AC system;

[0021] determining a first signal assignment according to the effective value and the rated effective value;

[0022] According to the first signal assignment, the current command value of the d-axis of the current inner loop of the receiving-end MMC converter is adjusted to achieve active power outer loop control.

[0023] As one preferred solution, the method for determining the deviation control coefficient includes:

[0024] Obtaining model input data, wherein the model input data includes: an average value of the bridge arm submodule voltage of the converter submodule, a bridge arm modulation wave of the converter submodule, a bridge arm current of the converter submodule, a current actual value of the DC port voltage, a command value of the reactive power of the receiving-end MMC converter, an actual value of the reactive power of the receiving-end MMC converter, and a current actual value of the circulating current of an output of a double frequency circulating current control link;

[0025] The model input data is input into a preset neural network to obtain the deviation control coefficient output by the neural network.

[0026] Another embodiment of the present invention provides an AC fault ride-through control system for a UHVDC flexible power transmission system, wherein a receiving-end MMC converter adopts a semi-full-bridge submodule hybrid topology structure as a converter submodule. The AC fault ride-through control system for the UHVDC flexible power transmission system includes:

[0027] The sending-end control module is used to control the sending-end MMC converter in a constant DC current control mode during the failure and recovery process of the receiving-end AC system; and

[0028] The receiving-end control module is used to control the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules, perform double frequency circulating current control on the receiving-end MMC converter according to the total average value of the submodule capacitor voltages, and perform active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system.

[0029] As one preferred solution, controlling the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules includes:

[0030] respectively setting a submodule capacitor voltage threshold value and a deviation control coefficient of the converter submodule, and calculating a total average value of the submodule capacitor voltage of the converter submodule;

[0031] Determining a DC component value according to the submodule capacitor voltage threshold value, the deviation control coefficient, the total average value of the submodule capacitor voltage and the converter DC voltage control instruction value;

[0032] The DC port voltage of the receiving-end MMC converter is adjusted according to the DC component value.

[0033] As one preferred solution, the submodule capacitor voltage threshold value is higher than the submodule capacitor voltage rated value of the converter submodule.

[0034] As one preferred solution, the double frequency circulating current control of the receiving-end MMC converter is performed according to the total average value of the submodule capacitor voltage, including:

[0035] Calculating the total average value of the submodule capacitor voltage;

[0036] determining a second signal assignment value according to a submodule capacitor voltage threshold value and a submodule capacitor voltage rated value of the converter submodule, and a total average value of the submodule capacitor voltage;

[0037] According to the second signal assignment, the double frequency component in the converter arm voltage reference value of the receiving-end MMC converter is adjusted, so as to perform double frequency circulating current control according to the adjusted double frequency component.

[0038] Compared with the prior art, the embodiments of the present invention have the following advantages:

[0039] By controlling the sending-end MMC converter in a constant DC current control mode during the fault and recovery process of the receiving-end AC system; and controlling the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules, performing double frequency circulating current control on the receiving-end MMC converter according to the total average value of the submodule capacitor voltages, and performing active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system, the dynamic energy balance and high power transmission capacity of the entire system can be maintained as much as possible during the fault occurrence and recovery process without the need for additional configuration of DC energy consumption devices, eliminating the impact of surplus power during fault ride-through, and achieving uninterrupted operation of the entire DC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 1. It is a schematic diagram of the system structure of the prior art ultra-high voltage flexible direct current transmission system;

[0041] Figure 2 1 is a topological diagram of a half-full bridge submodule hybrid topology in one embodiment of the present invention;

[0042] Figure 3 This is a control structure block diagram of a receiving-end MMC converter in one embodiment of the present invention;

[0043] Figure 4 It is a flow chart of an AC fault ride-through control method for a UHVDC flexible power transmission system in one embodiment of the present invention;

[0044] Figure 5 This is a block diagram of the MMC converter control structure in one embodiment of the present invention;

[0045] Figure 6 It is a structural block diagram of an AC fault ride-through control system of an ultra-high voltage flexible direct current transmission system in one embodiment of the present invention. DETAILED DESCRIPTION

[0046] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. The purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0047] In the description of this application, the terms "first," "second," "third," etc. are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, a feature specified as "first," "second," "third," etc. may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0048] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installed", "connected" and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be a communication between the two components. The terms "vertical", "horizontal", "left", "right", "up", "down" and similar expressions used herein are for illustrative purposes only, and do not indicate or imply that the device or component referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0049] In the description of this application, it should be noted that, unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by those skilled in the art. The terms used in this specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. Those skilled in the art will understand the specific meanings of the above terms in this application in specific circumstances.

[0050] See Figure 2 , Figure 2 The topology diagram of the hybrid topology structure of half-bridge and full-bridge sub-modules in an embodiment of the present invention is shown. HBSM refers to (Half-Bridge Sub-Module, half-bridge sub-module) and FBSM refers to (Full-Bridge Sub-Module, full-bridge sub-module). arm_au , I arm_bu , I arm_cu Represent the current in the upper three phase arms, I arm_ad , I arm_bd , I arm_cd Represents the current in the lower three phase arms, u a 、u b 、u c The three phase voltages in the three-phase AC voltage respectively.

[0051] It should be noted in advance that in order to better maintain the stable operation of the UHV Flexible DC transmission system, under normal circumstances, the sending-end MMC converter can adopt a constant DC current control mode and a constant reactive power control mode, and the receiving-end MMC converter adopts a constant DC voltage control mode and a constant reactive power control mode. The traditional control structure block diagram of the receiving-end MMC converter is as follows: Figure 3As shown, it includes an active power outer loop (constant DC voltage control), a reactive power outer loop (reactive power control), a current inner loop, a double frequency circulating current control link, and a bridge arm voltage reference value calculation link, wherein the DC voltage control outputs a first reference current i dref , reactive power control outputs a second reference current i qref The current is converted into the actual current command in the current inner loop, and the dq / abc transformation is performed (used to convert the dq coordinate system to the abc coordinate system) to obtain e abc , and then e abc Perform the calculation: ,in is the reference value of DC side voltage, is the modulation index, It is an error signal; during the occurrence and recovery of the receiving-end AC system fault, the AC system's ability to absorb the DC system's transmission power is limited. During this period, if the DC system can autonomously adjust its transmission power to achieve matching between power transmission and absorption, it can maintain the dynamic energy balance of the entire system and avoid overvoltage problems caused by internal energy surplus in the system; considering that the receiving-end MMC converter in the UHV flexible DC transmission system has flexible DC voltage regulation capabilities and can even control the DC voltage to zero, therefore, after the receiving-end AC fault occurs and the DC current of the DC system is stable, the receiving-end MMC converter adjusts its DC side voltage to achieve DC system transmission power control.

[0052] In view of this, an embodiment of the present invention provides an AC fault ride-through control method for a UHVDC flexible power transmission system. For details, see Figure 4~Figure 5 , Figure 4 FIG2 is a flow chart showing a method for controlling AC fault ride-through of a UHVDC flexible power transmission system in one embodiment of the present invention. Figure 5 The figure shows a block diagram of an MMC converter control structure in one embodiment of the present invention, wherein the receiving-end MMC converter adopts a semi-full-bridge submodule hybrid topology structure as a converter submodule. The AC fault ride-through control method of the ultra-high voltage flexible direct current transmission system includes steps S401-S402:

[0053] S401, during the failure and recovery process of the receiving-end AC system, controlling the sending-end MMC converter in a constant DC current control mode; and

[0054] S402: Control the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules, perform double frequency circulating current control on the receiving-end MMC converter according to the total average value of the submodule capacitor voltages, and perform active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system.

[0055] In the embodiment of the present invention, considering that the receiving-end MMC converter has a strong DC voltage control capability, when a fault occurs and during recovery of the receiving-end AC system, if the receiving-end MMC converter can autonomously adjust its DC port voltage to adapt to changes in the total average value of the sub-module capacitor voltage, the DC system transmission power can be adjusted so that the entire DC system can maintain energy dynamic balance and maintain a high power transmission capacity as much as possible, thereby achieving AC fault ride-through. This is described in detail below.

[0056] The control method of the embodiment of the present invention is applicable to the ultra-high voltage flexible direct current transmission system, and the receiving-end MMC adopts a semi-full bridge sub-module hybrid topology structure and has the ability to operate at zero DC voltage; during the fault and recovery process of the receiving-end AC system, the sending-end MMC converter maintains the constant DC current control mode unchanged, and the receiving-end MMC converter adjusts its DC port voltage according to the total average value of the sub-module capacitor voltage, and is assisted by adaptive switching control of the output of the active power outer loop and the double frequency circulating current control link.

[0057] In one example, the DC port voltage can be adjusted only according to the change of the total average value of the sub-module capacitor voltage so that the DC port voltage adapts to the change. Similarly, the double frequency circulation control can be performed only according to the change of the total average value of the sub-module capacitor voltage, for example, the output of the double frequency circulation control link is adjusted so that the output of the double frequency circulation control link adapts to the change.

[0058] In one example, the active power outer loop control of the receiving-end MMC converter can be performed only based on the AC voltage of the receiving-end AC system, for example, the output of the active power outer loop of the receiving-end MMC converter can be adjusted to ensure that the converter can dynamically adjust the converter active power output according to the voltage condition of the receiving-end AC system to maintain stable operation of the system.

[0059] In one embodiment, controlling the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules includes:

[0060] respectively setting a submodule capacitor voltage threshold value and a deviation control coefficient of the converter submodule, and calculating a total average value of the submodule capacitor voltage of the converter submodule;

[0061] Determining a DC component value according to the submodule capacitor voltage threshold value, the deviation control coefficient, the total average value of the submodule capacitor voltage and the converter DC voltage control instruction value;

[0062] The DC port voltage of the receiving-end MMC converter is adjusted according to the DC component value.

[0063] Specifically, first, set the submodule capacitor voltage threshold value U for adjusting the DC voltage sm_th , and the deviation control coefficient k, calculate the total average value of the submodule capacitor voltage U sm_ave ;

[0064] Then, you can sm_ave with U sm_th After the difference, multiply it by k to get the first intermediate variable temp1;

[0065] Next, temp1 is clipped to obtain the second intermediate variable temp2. ​​Specifically, when temp1 ≥ 1, temp2 = 1; when temp1 ≤ 0, temp2 = 0; when 0 < temp1 < 1, temp2 = temp1.

[0066] Finally, subtract 1 from temp2 and multiply it by U dc_ref / 2, get the DC component value, U dc_ref is the converter DC voltage control instruction value, and the DC component (DC port voltage) in the converter bridge arm voltage reference value of the receiving-end MMC converter is set to the obtained DC component value.

[0067] Furthermore, the submodule capacitor voltage threshold value U sm_th The submodule capacitor voltage threshold value U sm_th , the deviation control coefficient k can be adaptively adjusted according to the parameters of the DC transmission system.

[0068] Based on the above control method, when the submodule capacitor voltage rises, it means that there is surplus energy in the system. The receiving-end MMC converter will actively reduce the DC voltage and reduce the DC system transmission power until it reaches a stable state during the fault period. When the fault is cleared, the receiving-end AC system resumes its normal absorption capacity. At this time, power loss will occur and the submodule capacitor voltage will decrease. The receiving-end MMC converter will actively increase the DC voltage and increase the DC system transmission power, eventually returning to the stable operating state before the fault.

[0069] During fault ride-through, under extreme fault conditions, the AC system voltage drops significantly, almost completely losing its power absorption capacity. At this point, the active power outer loop will become saturated, causing the inner current loop command value to be excessively large. This leads to high AC-side currents, and the arm current of the receiving-end MMC converter risks triggering protection and causing shutdown. Under these conditions, the arm current needs to be limited. The specific method is described in detail below.

[0070] In one embodiment, performing active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system includes:

[0071] Calculating in real time the effective value of the positive sequence component of the sampled value of the AC voltage of the receiving-end AC system, and determining the rated effective value of the AC voltage of the receiving-end AC system;

[0072] determining a first signal assignment according to the effective value and the rated effective value;

[0073] According to the first signal assignment, the current command value of the d-axis of the current inner loop of the receiving-end MMC converter is adjusted to achieve active power outer loop control.

[0074] Specifically, first, the first signal tri1 is assigned a value. When U ac_rms ≤0.1×U ac_rated When tri1=0, when U ac_rms >0.1×U ac_rated When tri1=1, in the inequality, U ac_rms is the effective value of the positive sequence component of the AC voltage sampled value of the receiving AC system, U ac_rated is the rated effective value of the AC voltage of the receiving-end AC system; then, when tri1=1, the d-axis current command value of the inner loop of the MMC converter current is assigned to 0; when tri1=0, the d-axis current command value of the inner loop of the MMC converter current is assigned to the output value of the active power outer loop, thereby converting the output value of the active power outer loop into a d-axis current command value that can be directly used by the inner loop, thereby realizing the active power outer loop control of the receiving-end MMC converter.

[0075] In the above process, it is necessary to calculate U in real time ac_rms , and set U ac_rated . When U ac_rms ≤0.1×U ac_rated When U ac_rms >0.1×U ac_ratedWhen tri1 = 1, the receiving-end AC system voltage is within the normal or high range, maintaining normal operation of the MMC converter. In this case, tri1 = 1. When tri1 = 1, the d-axis current command value of the MMC converter's inner current loop is forcibly assigned to 0 to prevent excessive current under low voltage conditions and protect the converter from damage. When tri1 = 0, the d-axis current command value of the MMC converter's inner current loop is assigned to the output value of the active power outer loop to better maintain or adjust active power transmission and ensure stable operation of the power system.

[0076] In addition, during the fault ride-through process, the receiving-end MMC converter is in an abnormal operating state. At this time, the double-frequency circulating current control link may affect the sub-module voltage balance between each bridge arm of the converter. To avoid this impact, special processing is required. In an embodiment of the present invention, the double-frequency circulating current control of the receiving-end MMC converter based on the total average value of the sub-module capacitor voltage includes:

[0077] Calculating the total average value of the submodule capacitor voltage;

[0078] determining a second signal assignment value according to a submodule capacitor voltage threshold value and a submodule capacitor voltage rated value of the converter submodule, and a total average value of the submodule capacitor voltage;

[0079] According to the second signal assignment, the double frequency component in the converter arm voltage reference value of the receiving-end MMC converter is adjusted, so as to perform double frequency circulating current control according to the adjusted double frequency component.

[0080] Specifically, first, the second signal tri2 is assigned a value. When U sm_ave >U sm_th When tri2=1, when U sm_ave <U sm_rated When tri2=0, when U sm_rated ≤U sm_ave ≤U sm_th , let tri2 maintain its original value unchanged, in the inequality, U sm_rated is the submodule capacitor voltage rating, U sm_th is the submodule capacitor voltage threshold, U sm_ave Then, when tri2=1, the double frequency component in the converter bridge arm voltage reference value of the receiving-end MMC converter is assigned to 0; when tri2=0, the double frequency component in the converter bridge arm voltage reference value of the receiving-end MMC converter is assigned to the output of the double frequency circulating current control link.

[0081] In the above process, the average value U of the submodule capacitor voltage is first calculated in real time.sm_ave , and set two thresholds: U sm_rated and U sm_th , where U sm_th >U sm_rated . When U sm_ave >U sm_th When , it means that the submodule capacitor voltage is too high and measures need to be taken to reduce the voltage. In this case, let tri2 = 1. sm_ave sm_rated When , it means that the submodule capacitor voltage is too low and needs to be maintained or increased. In this case, tri2 = 0. sm_rated ≤U sm_ave ≤U sm_th When tri2 = 1, the double frequency component in the voltage reference value of the converter bridge arm of the receiving MMC converter is forcibly assigned to 0 to suppress possible circulating current and voltage fluctuations. When tri2 = 0, the double frequency component in the voltage reference value of the converter bridge arm of the receiving MMC converter is assigned to the output of the double frequency circulating current control link to maintain or adjust the circulating current and ensure the stable operation of the power system. Of course, in practical applications, it is necessary to ensure that U sm_rated and U sm_th The setting value of is reasonable to avoid the impact of frequent tri2 value switching on system stability, which will not be described in detail in the embodiments of the present invention.

[0082] In one embodiment, as one preferred solution, the method for determining the deviation control coefficient includes:

[0083] Obtaining model input data, wherein the model input data includes: an average value of the bridge arm submodule voltage of the converter submodule, a bridge arm modulation wave of the converter submodule, a bridge arm current of the converter submodule, a current actual value of the DC port voltage, a command value of the reactive power of the receiving-end MMC converter, an actual value of the reactive power of the receiving-end MMC converter, and a current actual value of the circulating current of an output of a double frequency circulating current control link;

[0084] The model input data is input into a preset neural network to obtain the deviation control coefficient output by the neural network.

[0085] ​In this embodiment, the neural network may be pre-trained and capable of taking model input data as input and outputting a deviation control coefficient. During training, sample model input data (the specific data composition of which may be determined based on the data included in the model input data) and a label corresponding to the sample model input data may be obtained, with the label being suitable for representing the expected deviation control coefficient. The sample model input data is then input into the neural network to be trained, obtaining the output of the neural network to be trained. Based on the output of the neural network to be trained and the expected deviation control coefficient represented by the label, a backpropagation algorithm is used to minimize a loss function (e.g., mean squared error), thereby training the neural network to be trained. The neural network to be trained may include a fully connected deep neural network, a convolutional neural network, or a recurrent neural network.

[0086] For details, see Figure 6 , Figure 6 The figure shows a structural block diagram of an AC fault ride-through control system for a UHVDC flexible power transmission system in one embodiment of the present invention. The receiving-end MMC converter adopts a semi-full-bridge submodule hybrid topology structure as a converter submodule. The AC fault ride-through control system for the UHVDC flexible power transmission system includes:

[0087] The sending-end control module 601 is used to control the sending-end MMC converter in a constant DC current control mode during the receiving-end AC system failure and recovery process; and

[0088] The receiving-end control module 602 is used to control the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules, perform double frequency circulating current control on the receiving-end MMC converter according to the total average value of the submodule capacitor voltages, and perform active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system.

[0089] As one preferred solution, controlling the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules includes:

[0090] respectively setting a submodule capacitor voltage threshold value and a deviation control coefficient of the converter submodule, and calculating a total average value of the submodule capacitor voltage of the converter submodule;

[0091] Determining a DC component value according to the submodule capacitor voltage threshold value, the deviation control coefficient, the total average value of the submodule capacitor voltage and the converter DC voltage control instruction value;

[0092] The DC port voltage of the receiving-end MMC converter is adjusted according to the DC component value.

[0093] As one preferred solution, the submodule capacitor voltage threshold value is higher than the submodule capacitor voltage rated value of the converter submodule.

[0094] As one preferred solution, the double frequency circulating current control of the receiving-end MMC converter is performed according to the total average value of the submodule capacitor voltage, including:

[0095] Calculating the total average value of the submodule capacitor voltage;

[0096] determining a second signal assignment value according to a submodule capacitor voltage threshold value and a submodule capacitor voltage rated value of the converter submodule, and a total average value of the submodule capacitor voltage;

[0097] According to the second signal assignment, the double frequency component in the converter arm voltage reference value of the receiving-end MMC converter is adjusted, so as to perform double frequency circulating current control according to the adjusted double frequency component.

[0098] As one preferred solution, performing active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system includes:

[0099] Calculating in real time the effective value of the positive sequence component of the sampled value of the AC voltage of the receiving-end AC system, and determining the rated effective value of the AC voltage of the receiving-end AC system;

[0100] determining a first signal assignment according to the effective value and the rated effective value;

[0101] According to the first signal assignment, the current command value of the d-axis of the current inner loop of the receiving-end MMC converter is adjusted to achieve active power outer loop control.

[0102] As one preferred solution, the method for determining the deviation control coefficient includes:

[0103] Obtaining model input data, wherein the model input data includes: an average value of the bridge arm submodule voltage of the converter submodule, a bridge arm modulation wave of the converter submodule, a bridge arm current of the converter submodule, a current actual value of the DC port voltage, a command value of the reactive power of the receiving-end MMC converter, an actual value of the reactive power of the receiving-end MMC converter, and a current actual value of the circulating current of an output of a double frequency circulating current control link;

[0104] The model input data is input into a preset neural network to obtain the deviation control coefficient output by the neural network.

[0105] In combination with the above-mentioned related embodiments, the embodiments of the present invention provide a method and system for controlling AC fault ride-through of a UHVDC flexible power transmission system, which has the beneficial effects of at least one of the following:

[0106] (1) The embodiments of the present invention do not require changing the original control architecture of the receiving-end MMC converter, and can ensure that the system continues to maintain good control performance, such as reactive power compensation and low harmonic distortion, which helps to ensure the stable operation of the system, improve the reliability and availability of the system, and reduce the modification cost and maintenance cost of the system.

[0107] (2) Design a specific AC fault ride-through control strategy, which can maintain the dynamic energy balance and high power transmission capacity of the entire system during the fault occurrence and recovery process without the need for additional DC energy consumption devices, eliminate the impact of surplus power during fault ride-through, and effectively reduce the current stress level of the receiving end MMC converter bridge arm and the voltage imbalance between sub-modules in different bridge arms, ultimately achieving uninterrupted operation of the entire DC system.

[0108] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for controlling AC fault ride-through in a UHVDC flexible power transmission system, characterized in that: The receiving-end MMC converter adopts a semi-full-bridge submodule hybrid topology structure as a converter submodule, and the method includes: During the receiving-end AC system failure and recovery process, the sending-end MMC converter is controlled by a constant DC current control method; and Controlling the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules to adapt to changes in the total average value of the submodule capacitor voltages, performing double frequency circulating current control on the receiving-end MMC converter according to the total average value of the submodule capacitor voltages, and performing active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system; The performing of active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system includes: Calculating in real time the effective value of the positive sequence component of the sampled value of the AC voltage of the receiving-end AC system, and determining the rated effective value of the AC voltage of the receiving-end AC system; determining a first signal assignment according to the effective value and the rated effective value; According to the first signal assignment, the current command value of the d-axis of the current inner loop of the receiving-end MMC converter is adjusted to implement active power outer loop control, wherein the current command value is used directly by the inner loop.

2. The AC fault ride-through control method for a UHVDC flexible power transmission system according to claim 1, wherein: The controlling the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules includes: respectively setting a submodule capacitor voltage threshold value and a deviation control coefficient of the converter submodule, and calculating a total average value of the submodule capacitor voltage of the converter submodule; Determining a DC component value according to the submodule capacitor voltage threshold value, the deviation control coefficient, the total average value of the submodule capacitor voltage and the converter DC voltage control instruction value; The DC port voltage of the receiving-end MMC converter is adjusted according to the DC component value.

3. The AC fault ride-through control method for a UHVDC flexible power transmission system according to claim 2, characterized in that: The submodule capacitor voltage threshold value is higher than the submodule capacitor voltage rating value of the converter submodule.

4. The AC fault ride-through control method for a UHVDC flexible power transmission system according to claim 1, wherein: The performing double frequency circulating current control on the receiving-end MMC converter according to the total average value of the submodule capacitor voltages includes: Calculating the total average value of the submodule capacitor voltage; determining a second signal assignment value according to a submodule capacitor voltage threshold value and a submodule capacitor voltage rated value of the converter submodule, and a total average value of the submodule capacitor voltage; According to the second signal assignment, the double frequency component in the converter arm voltage reference value of the receiving-end MMC converter is adjusted, so as to perform double frequency circulating current control according to the adjusted double frequency component.

5. The AC fault ride-through control method for a UHVDC flexible power transmission system according to claim 2, wherein: The method for determining the deviation control coefficient includes: Obtaining model input data, wherein the model input data includes: an average value of the bridge arm submodule voltage of the converter submodule, a bridge arm modulation wave of the converter submodule, a bridge arm current of the converter submodule, a current actual value of the DC port voltage, a command value of the reactive power of the receiving-end MMC converter, an actual value of the reactive power of the receiving-end MMC converter, and a current actual value of the circulating current of an output of a double frequency circulating current control link; The model input data is input into a preset neural network to obtain the deviation control coefficient output by the neural network.

6. An AC fault ride-through control system for a UHVDC flexible power transmission system, characterized in that: The receiving-end MMC converter adopts a semi-full-bridge submodule hybrid topology structure as a converter submodule. The AC fault ride-through control system of the ultra-high voltage flexible direct current transmission system includes: The sending-end control module is used to control the sending-end MMC converter in a constant DC current control mode during the failure and recovery process of the receiving-end AC system; and a receiving-end control module, configured to control the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules to adapt to changes in the total average value of the submodule capacitor voltages, perform double frequency circulating current control on the receiving-end MMC converter according to the total average value of the submodule capacitor voltages, and perform active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system; The performing of active power outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system includes: Calculating in real time the effective value of the positive sequence component of the sampled value of the AC voltage of the receiving-end AC system, and determining the rated effective value of the AC voltage of the receiving-end AC system; determining a first signal assignment according to the effective value and the rated effective value; According to the first signal assignment, the current command value of the d-axis of the current inner loop of the receiving-end MMC converter is adjusted to implement active power outer loop control, wherein the current command value is used directly by the inner loop.

7. The AC fault ride-through control system for a UHVDC flexible power transmission system according to claim 6, characterized in that: The controlling the receiving-end MMC converter to adjust the DC port voltage of the receiving-end MMC converter according to the total average value of the submodule capacitor voltages of the converter submodules includes: respectively setting a submodule capacitor voltage threshold value and a deviation control coefficient of the converter submodule, and calculating a total average value of the submodule capacitor voltage of the converter submodule; Determining a converter DC voltage control instruction value according to the submodule capacitor voltage threshold value, the deviation control coefficient, and the total average value of the submodule capacitor voltage; The DC port voltage of the receiving-end MMC converter is adjusted according to the converter DC voltage control instruction value.

8. The AC fault ride-through control system for a UHVDC flexible power transmission system according to claim 7, characterized in that: The submodule capacitor voltage threshold value is higher than the submodule capacitor voltage rating value of the converter submodule.

9. The AC fault ride-through control system for a UHVDC flexible power transmission system according to claim 6, characterized in that: The performing double frequency circulating current control on the receiving-end MMC converter according to the total average value of the submodule capacitor voltages includes: Calculating the total average value of the submodule capacitor voltage; determining a second signal assignment value according to a submodule capacitor voltage threshold value and a submodule capacitor voltage rated value of the converter submodule, and a total average value of the submodule capacitor voltage; According to the second signal assignment, the double frequency component in the converter arm voltage reference value of the receiving-end MMC converter is adjusted, so as to perform double frequency circulating current control according to the adjusted double frequency component.

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