Alternating current fault ride-through control method and system for extra-high voltage flexible direct current transmission system

Abstract

The invention discloses an AC fault ride-through control method and system for an extra-high voltage flexible DC power transmission system, a receiving-end MMC converter adopts a half-full bridge sub-module hybrid topology structure as a converter sub-module, and the method comprises the following steps: controlling a sending-end MMC converter in a constant DC current control mode in a receiving-end AC system fault and recovery process; controlling the receiving-end MMC to adjust the direct-current port voltage of the receiving-end MMC according to the total average value of the sub-module capacitor voltage of the converter sub-module, performing frequency doubling circulation control on the receiving-end MMC according to the total average value of the sub-module capacitor voltage, and performing active power class outer loop control on the receiving-end MMC according to the alternating-current voltage of the receiving-end alternating-current system. In the fault occurrence and recovery process, the energy dynamic balance and the large power transmission capability of the whole system can be maintained without additionally configuring a direct current energy consumption device, the surplus power influence in the fault ride-through period is eliminated, and the uninterrupted operation of the whole system is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of converter station control, and in particular to a control method and system for AC fault ride-through of a UHV flexible DC transmission system. Background Art

[0002] As a new generation of DC transmission technology, the high-voltage DC transmission system based on modular multilevel converters, also known as flexible DC transmission technology, can be called a UHV flexible DC transmission system in UHV application scenarios. Its system structure schematic diagram is as Figure 1 shown. In the UHV flexible DC transmission system, the MMC (Modular Multilevel Converter) adopted at the receiving end and the sending end has been fully recognized in aspects such as large-scale clean energy consumption and improving the safety and stability of the power grid due to its many advantages such as no commutation failure, active and reactive decoupled control, and no need for filters and reactive power compensation devices. Its transmission capacity has reached the UHV level and is comparable to that of conventional thyristor-based DC transmission.

[0003] Since the transmission distance of the UHV flexible DC transmission system is relatively long, overhead transmission lines are required for economic considerations. However, the probability of temporary faults occurring in overhead transmission lines due to lightning strikes and other reasons is relatively high.

[0004] Although there is no commutation failure problem in the UHV flexible DC transmission system after an AC fault occurs at the receiving end, when the receiving-end AC system cannot absorb the power transmitted by the DC system after the fault, it is easy to cause power surplus inside the DC system, resulting in overvoltage problems inside the DC system and threatening the safety of equipment. If the surplus energy is too much, it will also lead to the problem of failure of the AC side of the entire DC system to ride through the fault.

[0005] To solve the above problems, a DC energy dissipation device can be configured inside the DC system to dissipate the surplus power during the AC system fault at the receiving end and maintain the dynamic balance of the energy inside the entire system. However, this approach greatly increases the investment cost and has poor economy. Therefore, how to maintain the dynamic balance of the energy of the entire system and the maximum power transmission capacity during the fault has become a technical problem that needs to be urgently solved by those skilled in the art. Summary of the Invention

[0006] The present invention provides a control method and system for AC fault ride-through of a UHV flexible DC transmission system.

[0007] To solve the above technical problems, an embodiment of the present invention provides a control method for AC fault ride-through of a UHV flexible DC transmission system. The receiving-end MMC converter adopts a half-full bridge sub-module hybrid topology as the converter sub-module. The method includes: During the fault and recovery process of the receiving-end AC system, the sending-end MMC converter is controlled in a constant DC current control mode; and, 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 sub-module capacitor voltages of the converter sub-modules, perform double-frequency circulating current control on the receiving-end MMC converter according to the total average value of the sub-module capacitor voltages, and perform active power type outer-loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system.

[0008] As one of the preferred solutions, the control of 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 includes: Set the threshold value of the sub-module capacitor voltage and the deviation control coefficient of the converter sub-modules respectively, and calculate the total average value of the sub-module capacitor voltages of the converter sub-modules; Determine the DC component value according to the sub-module capacitor voltage threshold value, the deviation control coefficient, the total average value of the sub-module capacitor voltages and the converter DC voltage control command value; Adjust the DC port voltage of the receiving-end MMC converter according to the DC component value.

[0009] As one of the preferred solutions, the threshold value of the sub-module capacitor voltage is higher than the rated value of the sub-module capacitor voltage of the converter sub-module.

[0010] As one of the preferred solutions, the double-frequency circulating current control of the receiving-end MMC converter according to the total average value of the sub-module capacitor voltages includes: Calculate the total average value of the sub-module capacitor voltages; Determine the second signal assignment according to the threshold value of the sub-module capacitor voltage and the rated value of the sub-module capacitor voltage of the converter sub-module, and the total average value of the sub-module capacitor voltages; Adjust the double-frequency frequency component in the converter arm voltage reference value of the receiving-end MMC converter according to the second signal assignment, so as to perform double-frequency circulating current control according to the adjusted double-frequency frequency component.

[0011] As one of the preferred solutions, the active power type outer-loop control of the receiving-end MMC converter according to the AC voltage of the receiving-end AC system includes: Calculate the effective value of the positive sequence component of the sampled value of the AC voltage of the receiving-end AC system in real time, and determine the rated effective value of the AC voltage of the receiving-end AC system; Determine the first signal assignment according to the effective value and the rated effective value; According to the assignment of the first signal, adjust the current command value of the d-axis of the current inner loop of the receiving-end MMC converter to achieve the outer loop control of the active power class.

[0012] As one of the preferred solutions, the method for determining the deviation control coefficient includes: Obtain model input data, where the model input data includes: the average voltage of the arm sub-module of the converter sub-module, the arm modulation wave of the converter sub-module, the arm current of the converter sub-module, the current actual value of the DC port voltage, the command value of the reactive power of the receiving-end MMC converter, the actual value of the reactive power of the receiving-end MMC converter, and the current actual value of the circulating current output by the second-harmonic circulating current control link; Input the model input data into a preset neural network to obtain the deviation control coefficient output by the neural network.

[0013] Another embodiment of the present invention provides an AC fault ride-through control system for a UHV flexible DC transmission system. The receiving-end MMC converter adopts a half-full-bridge sub-module hybrid topology as the converter sub-module. The AC fault ride-through control system for the UHV flexible DC transmission system includes: A sending-end control module for 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, A receiving-end control module for 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-module, performing second-harmonic circulating current control on the receiving-end MMC converter according to the total average value of the sub-module capacitor voltages, and performing outer loop control of the active power class on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system.

[0014] As one of the preferred solutions, 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 sub-module capacitor voltages of the converter sub-module includes: Respectively set the sub-module capacitor voltage threshold value and the deviation control coefficient of the converter sub-module, and calculate the total average value of the sub-module capacitor voltages of the converter sub-module; Determine the DC component value according to the sub-module capacitor voltage threshold value, the deviation control coefficient, the total average value of the sub-module capacitor voltages, and the converter DC voltage control command value; Adjust the DC port voltage of the receiving-end MMC converter according to the DC component value.

[0015] As one of the preferred solutions, the sub-module capacitor voltage threshold is higher than the rated value of the sub-module capacitor voltage of the converter sub-module.

[0016] As one of the preferred solutions, the double-frequency circulating current control of the receiving-end MMC converter according to the total average value of the sub-module capacitor voltage includes: Calculate the total average value of the sub-module capacitor voltage; Determine the second signal assignment according to the sub-module capacitor voltage threshold and the rated value of the sub-module capacitor voltage of the converter sub-module, and the total average value of the sub-module capacitor voltage; Adjust the double-frequency frequency component in the converter arm voltage reference value of the receiving-end MMC converter according to the second signal assignment, so as to perform double-frequency circulating current control according to the adjusted double-frequency frequency component.

[0017] Compared with the prior art, the beneficial effects of the embodiments of the present invention are as follows: By controlling the sending-end MMC converter in a constant DC current control mode during the AC system fault and recovery process at the receiving end; 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 sub-module capacitor voltage of the converter sub-module, performing double-frequency circulating current control on the receiving-end MMC converter according to the total average value of the sub-module capacitor voltage, and, performing active power outer-loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system, it is possible to maintain the energy dynamic balance of the entire system and a large power transmission capacity as much as possible without additional DC energy-consuming devices during the fault occurrence and recovery process, eliminate the influence of surplus power during the fault ride-through period, and realize the uninterrupted operation of the entire DC system. Description of the Drawings

[0018] Figure 1 is a schematic diagram of the system structure of the prior art UHV flexible DC transmission system; Figure 2 is a schematic diagram of the topology of the half-full-bridge sub-module hybrid topology in one of the embodiments of the present invention; Figure 3 is a control structure block diagram of the receiving-end MMC converter in one of the embodiments of the present invention; Figure 4 is a schematic diagram of the flow of the AC fault ride-through control method for the UHV flexible DC transmission system in one of the embodiments of the present invention; Figure 5 is a control structure block diagram of the MMC converter in one of the embodiments of the present invention; Figure 6It is a structural block diagram of an AC fault ride-through control system for a UHV flexible DC transmission system in one embodiment of the present invention. Detailed implementation manners

[0019] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. The purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0020] In the description of this application, the terms "first", "second", "third", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, features defined with "first", "second", "third", etc. may explicitly or implicitly include one or more of such features. In the description of this application, unless otherwise stated, the meaning of "a plurality" is two or more.

[0021] In the description of this application, it should be noted that unless otherwise clearly defined and limited, the terms "installed", "connected", "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 directly connected, or indirectly connected through an intermediate medium, and it can be the internal communication of two components. The terms "vertical", "horizontal", "left", "right", "up", "down" and similar expressions used herein are only for the purpose of illustration 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 to the present invention. The term "and / or" used herein includes any and all combinations of one or more of the 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.

[0022] In the description of this application, it should be noted that unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which this technology belongs. The terms used in the description of the present invention in the specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. 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.

[0023] Please refer to Figure 2 , Figure 2The topological schematic diagram of the half-full bridge sub-module hybrid topology according to the embodiment of the present invention is shown. Among them, HBSM refers to (Half-Bridge Sub-Module), and FBSM refers to (Full-Bridge Sub-Module). Among them, I arm_au 、I arm_bu 、I arm_cu respectively represent the currents in the upper three phase arms, and I arm_ad 、I arm_bd 、I arm_cd respectively represent the currents in the lower three phase arms, and u a 、u b 、u c respectively represent the three phase voltages in the three-phase AC voltage.

[0024] 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 the constant DC current control mode and the constant reactive power control mode, and the receiving-end MMC converter adopts the constant DC voltage control and the constant reactive power control mode. The traditional control block diagram of the receiving-end MMC converter is as shown in Figure 3 , which includes an outer loop of active power (constant DC voltage control), an outer loop of reactive power (reactive power control), a current inner loop, a double-frequency circulating current control link, and a link for calculating the reference value of the arm voltage. Among them, the DC voltage control outputs a first reference current i dref , and the reactive power control outputs a second reference current i qref to the current inner loop to be converted into the actually required current command, and performs dq / abc transformation (used to convert the dq coordinate system to the abc coordinate system) to obtain e abc , and then calculates e abc : , where is the reference value of the DC-side voltage, is the modulation index, is the error signal; during the occurrence and recovery of the receiving-end AC system fault, the ability of the AC system to absorb the transmission power of the DC system is limited. During this period, if the DC system can autonomously adjust its transmission power to achieve the matching of power transmission and consumption, the energy dynamic balance of the entire system can be maintained, and the overvoltage problem caused by the energy surplus inside the system can be avoided; considering that the receiving-end MMC converter in the UHV flexible DC transmission system has the flexible DC voltage regulation ability and can even control the DC voltage to zero, therefore, on the premise that the DC current of the DC system is stable after the occurrence of the receiving-end AC fault, the receiving-end MMC converter can adjust its DC-side voltage to achieve the control of the transmission power of the DC system.

[0025] In view of this, an embodiment of the present invention provides an AC fault ride-through control method for a UHV flexible DC transmission system. For details, see Figures 4 - 5 , Figure 4 The figure shows a flow chart of an AC fault ride-through control method for a UHV flexible DC power transmission system in one embodiment of the present invention. Figure 5 The figure shows a control structure block diagram of an MMC converter 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, and the AC fault ride-through control method of the ultra-high voltage flexible direct current transmission system includes steps S401-S402: 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, 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 type outer loop control on the receiving-end MMC converter according to the AC voltage of the receiving-end AC system.

[0026] 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 the recovery of the receiving-end AC system, if the receiving-end MMC converter can autonomously adjust its DC port voltage to adapt to the change 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 as much as possible and maintain a large power transmission capacity, thereby achieving AC fault riding, which is described in detail below.

[0027] The control method of the embodiment of the present invention is applicable to an 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 with zero DC voltage; during the failure and recovery of the receiving-end AC system, the sending-end MMC converter maintains a 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.

[0028] 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 circulating current 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 circulating current control link is adjusted so that the output of the double frequency circulating current control link adapts to the change.

[0029] 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 is adjusted to ensure that the converter can dynamically adjust the active power output according to the voltage condition of the receiving - end AC system to maintain the stable operation of the system.

[0030] 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 sub - module capacitor voltages of the converter sub - modules includes: Respectively set the sub - module capacitor voltage threshold value and the deviation control coefficient of the converter sub - modules, and calculate the total average value of the sub - module capacitor voltages of the converter sub - modules; Determine the DC component value according to the sub - module capacitor voltage threshold value, the deviation control coefficient, the total average value of the sub - module capacitor voltages, and the converter DC voltage control command value; Adjust the DC port voltage of the receiving - end MMC converter according to the DC component value.

[0031] Specifically, first, set the sub - module capacitor voltage threshold value U sm_th , and the deviation control coefficient k, and calculate the total average value of the sub - module capacitor voltages U sm_ave ; Then, after taking the difference between U sm_ave and U sm_th and multiplying by k, the first intermediate variable temp1 is obtained; Next, after performing a clipping process on temp1, the second intermediate variable temp2 is obtained. Specifically, when temp1≥1, let temp2 = 1; when temp1≤0, let temp2 = 0; when 0<temp1<1, let temp2 = temp1; Finally, after taking the difference between 1 and temp2 and then multiplying by U dc_ref / 2, the DC component value is obtained, where U dc_ref is the converter DC voltage control command value, and the DC component (DC port voltage) in the converter arm voltage reference value of the receiving - end MMC converter is set to the obtained DC component value.

[0032] Furthermore, the sub - module capacitor voltage threshold value U sm_th should be higher than the rated value of the sub - module capacitor voltage. At the same time, the sub - module capacitor voltage threshold value U sm_th , and the deviation control coefficient k can be adaptively adjusted according to the DC transmission system parameters.

[0033] Based on the above control method, when the capacitor voltage of the sub-module rises, it means that there is surplus energy inside the system. The receiving-end MMC converter will actively reduce the DC voltage and the transmission power of the DC system until it reaches a stable state during the fault. When the fault is cleared and the receiving-end AC system resumes its normal power consumption capacity, there will be a power shortage at this time, and the capacitor voltage of the sub-module will decrease. The receiving-end MMC converter will actively increase the DC voltage and the transmission power of the DC system, and finally resume to the stable operating state before the fault.

[0034] During the fault ride-through process, in extreme fault conditions, the AC system voltage drops deeply, and almost completely loses its power consumption capacity. At this time, the outer loop of the active power class will saturate, resulting in a relatively large command value of the current inner loop and a relatively high AC side current. There is a risk that the arm current of the receiving-end MMC converter will trigger protection and lock out. In this condition, it is necessary to limit the arm current, and the specific method will be described in detail below.

[0035] In one embodiment, the active power class outer loop control of the receiving-end MMC converter according to the AC voltage of the receiving-end AC system includes: Real-time calculate the effective value of the positive sequence component of the sampled value of the AC voltage of the receiving-end AC system, and determine the rated effective value of the AC voltage of the receiving-end AC system; Determine the first signal assignment according to the effective value and the rated effective value; Adjust the current command value of the d-axis of the current inner loop of the receiving-end MMC converter according to the first signal assignment to achieve the active power class outer loop control.

[0036] Specifically, first, assign a value to the first signal tri1. When U ac_rms ≤0.1×U ac_rated , let tri1 = 0. When U ac_rms >0.1×U ac_rated , let tri1 = 1. In the inequality, U ac_rms is the effective value of the positive sequence component of the sampled value of the AC voltage of the receiving-end AC system, and U ac_rated is the rated effective value of the AC voltage of the receiving-end AC system. Then, when tri1 = 1, assign the current command value of the d-axis of the MMC converter current inner loop to 0. When tri1 = 0, assign the current command value of the d-axis of the MMC converter current inner loop to the output value of the active power class outer loop. Thus, the output value of the active power class outer loop is converted into the d-axis current command value that can be directly used by the inner loop, realizing the active power class outer loop control of the receiving-end MMC converter.

[0037] In the above process, it is necessary to first calculate U ac_rms in real time and set U ac_rated . When Uac_rms ≤0.1×U ac_rated When it is the case, it indicates that the voltage of the receiving-end AC system is too low and may not be able to maintain the normal operation of the MMC converter. At this time, let tri1 = 0. When U ac_rms >0.1×U ac_rated When it is the case, it indicates that the voltage of the receiving-end AC system is within the normal or higher range and can maintain the normal operation of the MMC converter. At this time, let tri1 = 1. When tri1 = 1, the d-axis current command value of the current inner loop of the MMC converter is forced to be assigned 0 to avoid excessive current under low voltage conditions and protect the converter from damage. When tri1 = 0, the d-axis current command value of the current inner loop of the MMC converter is assigned the output value of the outer loop of the active power type to better maintain or adjust the transmission of active power and ensure the stable operation of the power system.

[0038] 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 voltage balance among the sub-modules of each bridge arm of the converter. In order to avoid the influence, special treatment is required. In the embodiment of the present invention, the double-frequency circulating current control of the receiving-end MMC converter according to the total average value of the sub-module capacitor voltages includes: Calculating the total average value of the sub-module capacitor voltages; Determining the second signal assignment according to the sub-module capacitor voltage threshold value and the sub-module capacitor voltage rated value of the converter sub-module, and the total average value of the sub-module capacitor voltages; Adjusting the double-frequency frequency component in the converter arm voltage reference value of the receiving-end MMC converter according to the second signal assignment to perform double-frequency circulating current control according to the adjusted double-frequency frequency component.

[0039] Specifically, first, assign a value to the second signal tri2. When U sm_ave >U sm_th When it is the case, let tri2 = 1. When U sm_ave <U sm_rated When it is the case, let 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 sub-module capacitor voltage rated value, U sm_th is the sub-module capacitor voltage threshold value, U sm_aveis the total average value of the capacitor voltages of the sub-modules. Then, when tri2 = 1, the double-frequency component in the converter arm voltage reference value of the receiving-end MMC converter is assigned a value of 0. When tri2 = 0, the double-frequency component in the converter arm voltage reference value of the receiving-end MMC converter is assigned the output value of the double-frequency circulating current control link.

[0040] In the above process, first, the average value U of the capacitor voltages of the sub-modules is calculated in real time sm_ave , and two thresholds are set: U sm_rated and U sm_th , where U sm_th >U sm_rated . When U sm_ave >U sm_th , it indicates that the capacitor voltage of the sub-module is too high and measures need to be taken to reduce the voltage. At this time, tri2 = 1 is set. When U sm_ave <U sm_rated , it indicates that the capacitor voltage of the sub-module is too low and the voltage needs to be maintained or increased. At this time, tri2 = 0 is set. When U sm_rated ≤U sm_ave ≤U sm_th , it indicates that the capacitor voltage of the sub-module is within the normal range. At this time, the current value of tri2 is kept unchanged. When tri2 = 1, the double-frequency component in the converter arm voltage reference value of the receiving-end MMC converter is forcibly assigned a value of 0 to suppress the possible circulating current and voltage fluctuations. When tri2 = 0, the double-frequency component in the converter arm voltage reference value of the receiving-end MMC converter is assigned the output value 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 the set values of U sm_rated and U sm_th are reasonable to avoid the influence of frequent switching of tri2 values on the system stability, which is not elaborated in detail in the embodiments of the present invention.

[0041] In one embodiment, as one of the preferred solutions, the determination method of the deviation control coefficient includes: Obtain model input data, where the model input data includes: the average value of the arm sub-module voltages of the converter sub-module, the arm modulation wave of the converter sub-module, the arm current of the converter sub-module, the current actual value of the DC port voltage, the command value of the reactive power of the receiving-end MMC converter, the actual value of the reactive power of the receiving-end MMC converter, and the current actual value of the circulating current of the output of the double-frequency circulating current control link; Input the model input data into a preset neural network to obtain the deviation control coefficient output by the neural network.

[0042] In this embodiment, the above neural network can be pre-trained and has the ability to take model input data as input and output a deviation control coefficient. During specific training, sample model input data (whose specific data composition can be set accordingly based on the data included in the model input data) and the label corresponding to the sample model input data can be obtained first. The label is suitable for characterizing the expected deviation control coefficient. Then, the sample model input data is input into the neural network to be trained, and the output of the neural network to be trained is obtained. Based on the output of the neural network to be trained and the expected deviation control coefficient characterized by the label, the backpropagation algorithm is used to minimize the loss function (such as mean square error), thereby training the neural network to be trained. Among them, the neural network to be trained can include a fully connected deep neural network, a convolutional neural network, or a recurrent neural network.

[0043] Specifically, please refer to Figure 6 , Figure 6 which shows the structural block diagram of the AC fault ride-through control system of the UHV flexible DC transmission system in one embodiment of the present invention. The receiving-end MMC converter adopts a semi-full bridge sub-module hybrid topology as the converter sub-module. The AC fault ride-through control system of the UHV flexible DC transmission system includes: A sending-end control module 601, configured to control 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, A receiving-end control module 602, 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 sub-module capacitor voltages of the converter sub-module, perform double-frequency circulating current control on the receiving-end MMC converter according to the total average value of the sub-module 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.

[0044] As one preferred solution, 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 sub-module capacitor voltages of the converter sub-module includes: Respectively set the sub-module capacitor voltage threshold value and the deviation control coefficient of the converter sub-module, and calculate the total average value of the sub-module capacitor voltages of the converter sub-module; Determine the DC component value according to the sub-module capacitor voltage threshold value, the deviation control coefficient, the total average value of the sub-module capacitor voltages, and the converter DC voltage control command value; Adjust the DC port voltage of the receiving-end MMC converter according to the DC component value.

[0045] As one of the preferred solutions, the sub-module capacitor voltage threshold is higher than the rated value of the sub-module capacitor voltage of the converter sub-module.

[0046] As one of the preferred solutions, the double-frequency circulating current control of the receiving-end MMC converter according to the total average value of the sub-module capacitor voltages includes: Calculating the total average value of the sub-module capacitor voltages; Determining a second signal assignment according to the sub-module capacitor voltage threshold and the rated value of the sub-module capacitor voltage of the converter sub-module, and the total average value of the sub-module capacitor voltages; Adjusting the double-frequency frequency component in the converter arm voltage reference value of the receiving-end MMC converter according to the second signal assignment, so as to perform double-frequency circulating current control according to the adjusted double-frequency frequency component.

[0047] As one of the preferred solutions, the active power type outer-loop control of the receiving-end MMC converter according to the AC voltage of the receiving-end AC system includes: Real-time calculating 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; Adjusting the current command value of the d-axis of the current inner loop of the receiving-end MMC converter according to the first signal assignment, so as to achieve active power type outer-loop control.

[0048] As one of the preferred solutions, the method for determining the deviation control coefficient includes: Obtaining model input data, where the model input data includes: the average value of the arm sub-module voltage of the converter sub-module, the arm modulation wave of the converter sub-module, the arm current of the converter sub-module, the current actual value of the DC port voltage, the command value of the reactive power of the receiving-end MMC converter, the actual value of the reactive power of the receiving-end MMC converter, and the current circulating current actual value of the output of the double-frequency circulating current control link; Inputting the model input data into a preset neural network to obtain the deviation control coefficient output by the neural network.

[0049] Combined with the above related embodiments, the AC fault ride-through control method and system for a UHV flexible DC transmission system provided by the embodiments of the present invention have at least one of the following beneficial effects: (1) The embodiments of the present invention do not need to change the original control architecture of the receiving - end MMC converter, which can ensure that the system continues to maintain good control performance, such as reactive power compensation, low harmonic distortion, etc., helps to ensure the stable operation of the system, improve the reliability and availability of the system, and reduce the transformation cost and maintenance cost of the system.

[0050] (2) By designing a specific AC fault - ride - through control strategy, during the fault occurrence and recovery process, it is possible to maintain the energy dynamic balance of the entire system and a large - power transmission capacity without additional configuration of DC energy - consuming devices, eliminate the influence of surplus power during fault ride - through, and effectively reduce the arm - current stress level of the receiving - end MMC converter and the voltage unbalance degree between sub - modules of different arms, and finally achieve the uninterrupted operation of the entire DC system.

[0051] The above - described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation to the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the appended claims.

Claims

1. A control method for AC fault ride-through of a UHV flexible DC transmission system, characterized in that The receiving - end MMC converter uses a hybrid topology of half - full - bridge sub - modules as the converter sub - module, and the method includes: During the fault and recovery process of the receiving - end AC system, controlling the sending - end MMC converter in a constant DC current control mode; 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 sub - module capacitor voltages of the converter sub - module, 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 an outer - loop control of active power type on the receiving - end MMC converter according to the AC voltage of the receiving - end AC system.

2. The AC fault ride-through control method for the UHV flexible DC transmission system according to claim 1, 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 sub - module capacitor voltages of the converter sub - module includes: Respectively setting the threshold value of the sub - module capacitor voltage and the deviation control coefficient of the converter sub - module, and calculating the total average value of the sub - module capacitor voltages of the converter sub - module; Determining the DC component value according to the sub - module capacitor voltage threshold value, the deviation control coefficient, the total average value of the sub - module capacitor voltages, and the DC voltage control command value of the converter; Adjusting the DC port voltage of the receiving - end MMC converter according to the DC component value.

3. The AC fault ride-through control method for the UHV flexible DC transmission system according to claim 2, wherein, The threshold value of the sub - module capacitor voltage is higher than the rated value of the sub - module capacitor voltage of the converter sub - module.

4. The AC fault ride-through control method for the UHV flexible DC transmission system according to claim 1, characterized in that, The performing double - frequency circulating current control on the receiving - end MMC converter according to the total average value of the sub - module capacitor voltages includes: Calculating the total average value of the sub - module capacitor voltages; Determining the second signal assignment according to the threshold value of the sub - module capacitor voltage and the rated value of the sub - module capacitor voltage of the converter sub - module, and the total average value of the sub - module capacitor voltages; Adjusting the double - frequency frequency component in the converter arm voltage reference value of the receiving - end MMC converter according to the second signal assignment, so as to perform double - frequency circulating current control according to the adjusted double - frequency frequency component.

5. The AC fault ride-through control method for the UHV flexible DC transmission system according to claim 1, characterized in that, The performing an outer - loop control of active power type on the receiving - end MMC converter according to the AC voltage of the receiving - end AC system includes: Real - time calculating 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 the first signal assignment according to the effective value and the rated effective value; Adjusting the current command value of the d - axis of the current inner - loop of the receiving - end MMC converter according to the first signal assignment to achieve the outer - loop control of active power type.

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

7. A control system for AC fault ride-through of a UHV flexible DC transmission system, characterized in that, The receiving - end MMC converter uses a semi - full - bridge sub - module hybrid topology as the converter sub - module. The AC fault - ride - through control system of the UHV flexible DC transmission system includes: A sending - end control module for 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, A receiving - end control module for 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 an outer - loop active - power control on the receiving - end MMC converter according to the AC voltage of the receiving - end AC system.

8. The AC fault ride-through control system of the UHV flexible DC transmission system according to claim 7, characterized in that The step of 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 includes: Respectively set the threshold value of the sub - module capacitor voltage and the deviation control coefficient of the converter sub - module, and calculate the total average value of the sub - module capacitor voltages of the converter sub - module; Determine the DC voltage control command value of the converter according to the threshold value of the sub - module capacitor voltage, the deviation control coefficient, and the total average value of the sub - module capacitor voltages; Adjust the DC port voltage of the receiving - end MMC converter with the DC voltage control command value of the converter.

9. The AC fault ride-through control system of the UHV flexible DC transmission system according to claim 8, wherein The threshold value of the sub - module capacitor voltage is higher than the rated value of the sub - module capacitor voltage of the converter sub - module.

10. The AC fault ride-through control system of the UHV flexible DC transmission system according to claim 7, characterized in that, The step of performing double - frequency circulating current control on the receiving - end MMC converter according to the total average value of the sub - module capacitor voltages includes: Calculate the total average value of the sub - module capacitor voltages; Determine the second signal assignment according to the threshold value of the sub - module capacitor voltage and the rated value of the sub - module capacitor voltage of the converter sub - module, and the total average value of the sub - module capacitor voltages; Adjust the double - frequency frequency component in the converter arm voltage reference value of the receiving - end MMC converter according to the second signal assignment to perform double - frequency circulating current control according to the adjusted double - frequency frequency component.

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