Control method and system for LCC-MMC hybrid high-voltage direct-current power transmission system

Abstract

The invention discloses an LCC-MMC hybrid high-voltage direct-current power transmission system control method and system. The method comprises the following steps: controlling a sending-end LCC converter to operate in a constant direct-current voltage mode; based on the system DC voltage control expected value, the system DC current control expected value and the AC bus line voltage effective value, determining a trigger angle theoretical value of the sending end LCC; in the closed-loop control link, the compensation amount of the theoretical value of the trigger angle is determined based on a system direct-current voltage control expected value and a direct-current voltage actual sampling value; determining an actual control instruction of the trigger angle of the LCC according to the compensation amount and the theoretical value of the trigger angle; the actual control instruction is used for controlling the transmitting-end LCC converter, so that the theoretical calculation value of the trigger angle can be introduced to compensate the closed-loop control link, the influence of the bandwidth of the closed-loop controller on the dynamic characteristics of the direct-current voltage of the system can be effectively improved, the direct-current voltage is accurately and rapidly regulated and controlled, meanwhile, the stability of the direct-current voltage is maintained, and the stability of the system is improved. And the method has relatively high robustness.

Description

Technical Field

[0001] The present invention relates to the technical field of converter station control, and particularly to a control method and system for an LCC-MMC hybrid high-voltage direct current (HVDC) transmission system. Background Art

[0002] With the continuous development of power electronics technology and the wide application of new energy, the LCC (Line Commutated Converter)-MMC (Modular Multilevel Converter) hybrid HVDC transmission system has become an important development direction of current HVDC transmission technology.

[0003] The LCC-MMC hybrid HVDC transmission system is a new type of HVDC transmission system that combines the traditional HVDC transmission technology based on LCC and the flexible HVDC transmission technology based on MMC. The working principle of the LCC-MMC hybrid HVDC transmission system combines the characteristics of both LCC and MMC. Under normal operating conditions, the LCC converter is responsible for controlling the DC current, while the MMC converter is responsible for controlling the DC voltage, and the receiving-end MMC converter usually adopts a constant DC voltage control mode. This control method can give full play to the respective advantages of LCC and MMC and achieve more efficient DC power transmission.

[0004] In the prior art, due to the influence of the control period and control bandwidth of LCC itself, the traditional single closed-loop voltage control strategy is difficult to achieve fast and stable control of the system DC voltage, which easily leads to oscillations of the DC-side voltage and current, and thus is not conducive to applications in ultra-long distances with a large equivalent inductance of the DC line. Summary of the Invention

[0005] The present invention provides a control method and system for an LCC-MMC hybrid HVDC transmission system.

[0006] To solve the above technical problems, an embodiment of the present invention provides a control method for an LCC-MMC hybrid HVDC transmission system, which is used for a sending-end LCC converter in the LCC-MMC hybrid HVDC transmission system, and includes: Controlling the sending-end LCC converter to operate in a constant DC voltage mode; Based on a preset expected value of system DC voltage control, an expected value of system DC current control, and the obtained effective value of the AC bus line voltage, determining a theoretical value of the trigger angle of the sending-end LCC converter; In a closed-loop control link, based on a preset expected value of system DC voltage control and the obtained actual sampled value of the DC voltage, determining a compensation amount of the theoretical value of the trigger angle; Determine the actual control command of the triggering angle of the LCC converter according to the compensation amount and the theoretical value of the triggering angle; Control the sending-end LCC converter with the actual control command.

[0007] As one of the preferred solutions, the determination of the theoretical value of the triggering angle of the sending-end LCC converter based on the preset expected value of the system DC voltage control, the expected value of the system DC current control, and the obtained effective value of the AC bus line voltage includes: Determine a first intermediate variable based on the expected value of the system DC current control, the pulsation coefficient matching the number of pulsations of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter; Determine a second intermediate variable based on the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer of the sending-end LCC converter; Determine the theoretical value of the triggering angle based on the expected value of the system DC voltage control, the first intermediate variable, and the second intermediate variable.

[0008] As one of the preferred solutions, the determination of the first intermediate variable based on the expected value of the system DC current control, the pulsation coefficient matching the number of pulsations of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes: Calculate the product of the expected value of the system DC current control, the pulsation coefficient, and the equivalent commutation reactance as a first product; Multiply the first product by a preset first coefficient to obtain the first intermediate variable.

[0009] As one of the preferred solutions, the determination of the second intermediate variable based on the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer of the sending-end LCC converter includes: Calculate the product of the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer as a second product; Multiply the second product by a preset second coefficient to obtain the second intermediate variable.

[0010] As one of the preferred solutions, the determination of the compensation amount of the theoretical value of the triggering angle based on the preset expected value of the system DC voltage control and the obtained actual sampling value of the DC voltage includes: In the closed-loop control link, calculate the expected value of the system DC voltage control and the actual sampling value of the DC voltage based on the proportional-integral control strategy to obtain the error caused by the equipment parameter deviation and control deviation during the actual operation of the sending-end LCC converter, so as to determine the compensation amount of the theoretical value of the triggering angle.

[0011] As one of the preferred solutions, determining the compensation amount of the theoretical trigger angle based on the preset expected value of the system DC voltage control and the actually sampled value of the DC voltage includes: Obtain model input data, where the model input data includes: the expected value of the system DC voltage control and the actually sampled value of the DC voltage, and the model input data further includes at least one of the following parameters of the sending-end LCC converter: the actual value of the DC current, the theoretical trigger angle, and the current actual trigger angle; Input the model input data into a preset neural network to obtain the compensation amount of the theoretical trigger angle output by the neural network.

[0012] As one of the preferred solutions, determining the actual control command of the LCC converter trigger angle according to the compensation amount and the theoretical trigger angle includes: Add the compensation amount and the theoretical trigger angle to obtain the actual control command.

[0013] Another embodiment of the present invention provides a control system for an LCC-MMC hybrid HVDC transmission system, which is used for the sending-end LCC converter in the LCC-MMC hybrid HVDC transmission system, including: An operation module for controlling the sending-end LCC converter to operate in a constant DC voltage mode; A theoretical value module for determining the theoretical trigger angle of the sending-end LCC converter based on the preset expected value of the system DC voltage control, the expected value of the system DC current control, and the actually obtained effective value of the AC bus line voltage; A compensation amount module for determining the compensation amount of the theoretical trigger angle based on the preset expected value of the system DC voltage control and the actually sampled value of the DC voltage in the closed-loop control link; An instruction module for determining the actual control command of the LCC converter trigger angle according to the compensation amount and the theoretical trigger angle; A control module for controlling the sending-end LCC converter with the actual control command.

[0014] As one of the preferred solutions, determining the theoretical trigger angle of the sending-end LCC converter based on the preset expected value of the system DC voltage control, the expected value of the system DC current control, and the actually obtained effective value of the AC bus line voltage includes: Determine a first intermediate variable based on the expected value of the system DC current control, the pulsation coefficient matching the number of pulsations of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter; Determine a second intermediate variable based on the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer of the sending-end LCC converter; Determine the theoretical value of the trigger angle based on the expected value of the DC voltage control of the system, the first intermediate variable, and the second intermediate variable.

[0015] As one of the preferred solutions, the determining of the first intermediate variable based on the expected value of the DC current control of the system, the pulsation coefficient matching the pulsation number of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes: Calculate the product of the expected value of the DC current control of the system, the pulsation coefficient, and the equivalent commutation reactance as the first product; Multiply the first product by a preset first coefficient to obtain the first intermediate variable.

[0016] Compared with the prior art, the beneficial effects of the embodiments of the present invention are as follows: By controlling the sending-end LCC converter to operate in a constant DC voltage mode; determining the theoretical value of the trigger angle of the sending-end LCC converter based on the preset expected value of the DC voltage control of the system, the expected value of the DC current control of the system, and the obtained effective value of the AC bus line voltage; in the closed-loop control link, determining the compensation amount of the theoretical value of the trigger angle based on the preset expected value of the DC voltage control of the system and the obtained actual sampling value of the DC voltage; determining the actual control command of the trigger angle of the LCC converter according to the compensation amount and the theoretical value of the trigger angle; controlling the sending-end LCC converter with the actual control command, so that on the basis of not changing the original control architecture of the sending-end LCC converter, the theoretical calculation value of the trigger angle can be introduced to compensate the closed-loop control link, which can effectively improve the influence of the closed-loop controller bandwidth on the dynamic characteristics of the DC voltage of the system, realize the accurate and rapid regulation of the DC voltage, and maintain the stability of the DC voltage at the same time, and has high robustness. Description of the Drawings

[0017] Figure 1 is a schematic diagram of the system structure of the conventional high-voltage DC transmission technology with LCC type at both the sending end and the receiving end; Figure 2 is a schematic diagram of the system structure of the LCC-MMC hybrid high-voltage DC transmission system constructed in one of the embodiments of the present invention; Figure 3 is a schematic diagram of the traditional voltage single closed-loop control strategy; Figure 4 is a schematic diagram of the flow of the control method of the LCC-MMC hybrid high-voltage DC transmission system in one of the embodiments of the present invention; Figure 5It is a schematic diagram of the two links of theoretical value calculation and closed-loop regulation in one embodiment of the present invention; Figure 6 It is a structural block diagram of the control system of the LCC-MMC hybrid high-voltage DC transmission system in one embodiment of the present invention. Specific embodiments

[0018] 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. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0019] 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, the 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 specified, the meaning of "a plurality" is two or more.

[0020] In the description of this application, it should be noted that unless otherwise clearly specified and limited, the terms "installed", "connected", "connected" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected, or indirectly connected through an intermediate medium, and it may be the communication inside 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 element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of 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.

[0021] 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 specification of the present invention 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.

[0022] It should be noted in advance that in order for the power industry to keep up with the trend of energy technology progress and promote the clean and low-carbon transformation of energy, it is crucial to build a new power system with new energy as the main body and promote the high-quality development of clean energy. Currently, please refer to Figure 1 , Figure 1 which shows a schematic diagram of the system structure of the conventional high-voltage direct current (HVDC) transmission technology where both the sending end and the receiving end adopt the line-commutated converter (LCC) type. It has the advantages of long transmission distance, large transmission capacity, low power loss, and low project cost, and is the main technical solution for large-scale new energy transmission.

[0023] However, the LCC uses thyristors without self-turn-off ability as commutation devices, which have certain requirements for the strength of the receiving-end AC grid. And when a fault occurs in the receiving-end AC system, commutation failure is likely to occur, threatening the safe and stable operation of the receiving-end AC grid. To improve the safety and stability level of the receiving-end grid, the receiving-end LCC can be replaced by an MMC. Specifically, please refer to Figure 2 , Figure 2 which shows a schematic diagram of the system structure of the LCC-MMC hybrid HVDC transmission system constructed in one embodiment of the present invention. It fully exploits the advantages of the MMC, such as no commutation failure, power and reactive power decoupling control, and no need for filters and reactive power compensation devices.

[0024] For the LCC-MMC hybrid HVDC transmission system, in this embodiment, the sending-end LCC converter is controlled in a constant DC voltage control mode, while the receiving-end MMC converter can adopt a constant DC voltage control mode. This control method can help maintain the normal operation of the system. However, when a fault occurs in the receiving-end AC system and it cannot absorb the DC system transmission power, it is easy to cause a power surplus inside the DC system, leading to serious overvoltage. The main reason is that the sending-end converter station cannot independently sense the receiving-end AC fault information and actively reduce the transmission power. It needs to rely on inter-station communication to obtain the fault information. During the transmission of the fault information, the sending-end LCC still transmits the preset power to the inside of the DC system, resulting in a power surplus inside the DC system and causing overvoltage. This situation is more serious in the application scenario of ultra-long-distance power transmission (long inter-station communication time).

[0025] To solve the above problems, it can be improved by adjusting the control modes of the sending-end LCC converter station and the receiving-end MMC converter station, that is, the LCC adopts a constant DC voltage control mode, and the MMC adopts a constant DC current / power control mode. When the receiving-end AC system fault causes the DC voltage to rise, the LCC will actively reduce its transmission power without communication to maintain the DC voltage stability, effectively reducing the surplus power inside the system and suppressing the system overvoltage. However, the LCC itself has a slow control cycle and adopts a traditional single-closed-loop voltage control strategy as shown in Figure 3 , that is, the expected value U of the system DC voltage control dc_ref, the actual sampled value U of the DC voltage actually measured dc As the input signal, it is input into a subtractor (differentiator) to calculate the difference between the two, that is, the error signal, and then input into a PI (proportional-integral) controller. Then, the output of the PI controller is continuously input into a gain link (the gain is -1, that is, the signal is inverted). Finally, the signal after gain is input into an adder to be superimposed with another input signal π to obtain the final output. Here, due to the influence of the controller control bandwidth, it is difficult to achieve fast and stable control of the system DC voltage, which easily leads to oscillations of the DC-side voltage and current. This phenomenon is more obvious in the ultra-long-distance application scenario where the equivalent inductance of the DC line is large.

[0026] In view of this, an embodiment of the present invention provides a control method for an LCC-MMC hybrid high-voltage DC transmission system. Specifically, please refer to Figure 4 , Figure 4 which shows a schematic flow chart of a control method for an LCC-MMC hybrid high-voltage DC transmission system in one of the embodiments of the present invention. It is used for the sending-end LCC converter in the LCC-MMC hybrid high-voltage DC transmission system and includes S401~S405.

[0027] S401, control the sending-end LCC converter to operate in a constant DC voltage mode.

[0028] S402, based on the preset expected value of system DC voltage control, the expected value of system DC current control, and the obtained effective value of the AC bus line voltage, determine the theoretical value of the trigger angle of the sending-end LCC converter.

[0029] S403, in the closed-loop control link, based on the preset expected value of system DC voltage control and the actually sampled value of the obtained DC voltage, determine the compensation amount of the theoretical value of the trigger angle.

[0030] S404, according to the compensation amount and the theoretical value of the trigger angle, determine the actual control instruction of the trigger angle of the LCC converter.

[0031] S405, control the sending-end LCC converter with the actual control instruction.

[0032] For ease of explanation, the control method provided by the embodiments of the present invention can be understood as including two links: theoretical value calculation and closed-loop control. The theoretical value calculation link calculates the theoretical value of the trigger angle of the LCC converter based on the parameters collected by the system (the effective value of the AC bus line voltage), the expected value of the first control target (the expected value of the system DC current control), and the expected value of the second control target (the expected value of the system DC voltage control). The closed-loop control link calculates the adjustment amount of the theoretical value of the trigger angle of the LCC converter according to the expected value of the second control target (the expected value of the system DC voltage control) and the actual value (the actual sampled value of the DC voltage) for compensation, and then obtains the actual control command of the trigger angle of the LCC converter.

[0033] In the embodiments of the present invention, the control method is applicable to the sending-end LCC converter station of the HVDC system, and the sending-end LCC converter in the converter station operates in a constant DC voltage mode; further, the control method includes two links: theoretical value calculation and closed-loop control. The theoretical calculation link obtains the theoretical value of the trigger angle for the LCC converter to achieve the expected DC voltage value control. On this basis, the closed-loop control link obtains the compensation amount of the theoretical value of the trigger angle of the LCC converter, and then obtains the actual control command of the trigger angle of the LCC converter. The following is a detailed description.

[0034] In one embodiment, determining the theoretical value of the trigger angle of the sending-end LCC converter based on the preset expected value of the system DC voltage control, the expected value of the system DC current control, and the obtained effective value of the AC bus line voltage includes: Determining a first intermediate variable based on the expected value of the system DC current control, the ripple coefficient matching the number of ripples of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter; Determining a second intermediate variable based on the effective value of the AC bus line voltage, the ripple coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer of the sending-end LCC converter; Determining the theoretical value of the trigger angle based on the expected value of the system DC voltage control, the first intermediate variable, and the second intermediate variable.

[0035] In one embodiment, determining a first intermediate variable based on the expected value of the system DC current control, the ripple coefficient matching the number of ripples of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes: Calculating the product of the expected value of the system DC current control, the ripple coefficient, and the equivalent commutation reactance as a first product; Multiplying the first product by a preset first coefficient to obtain the first intermediate variable.

[0036] In one embodiment, determining the second intermediate variable based on the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer of the sending-end LCC converter includes: Calculating the product of the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer as the second product; Multiplying the second product by a preset second coefficient to obtain the second intermediate variable.

[0037] In one embodiment, determining the actual control command for the triggering angle of the LCC converter according to the compensation amount and the theoretical triggering angle value includes: Adding the compensation amount and the theoretical triggering angle value to obtain the actual control command.

[0038] In one embodiment, determining the compensation amount for the theoretical triggering angle value based on a preset expected value of the system DC voltage control and the actually sampled value of the DC voltage includes: In a closed-loop control link, calculating the preset expected value of the system DC voltage control and the actually sampled value of the DC voltage based on a proportional-integral control strategy to obtain the error caused by the equipment parameter deviation and control deviation during the actual operation of the sending-end LCC converter, so as to determine the compensation amount for the theoretical triggering angle value.

[0039] Specifically, the equivalent mathematical model of the sending-end LCC converter can be transformed to obtain the corresponding intermediate variable, and the theoretical triggering angle value is calculated with the intermediate variable; the mathematical model of the sending-end LCC converter is: In the formula, U dc is the DC-side voltage of the LCC converter (i.e., the actually sampled value of the DC voltage), U ac is the effective value of the AC bus line voltage, I dc is the DC-side current of the LCC converter (i.e., the actual value of the DC current), k is the turns ratio of the primary and secondary sides of the commutation transformer, X is the equivalent commutation reactance, α is the control command value of the triggering angle of the LCC converter, m is the pulsation coefficient. If the LCC converter is a 6-pulse converter, the value of m is 1; if the LCC converter is a 12-pulse converter, the value of m is 2.

[0040] When the sending-end LCC converter operates stably, the AC voltage on the valve side of the commutation transformer can be stabilized (equivalent to U ac stable) through the control of the commutation transformer tap, and I dc can be stabilized through the control of the receiving-end converter station. According to the above formula, it can be known that the U dc of the LCC converter can be directly controlled by its α. Therefore, the schematic diagram of the traditional control method for the DC voltage of the sending-end LCC converter is as aboveFigure 3 As shown, when its DC voltage is less than the expected value, through the control of the closed-loop control link, α will decrease. At this time, the DC voltage of the LCC converter will rise. When its DC voltage is greater than the expected value, through the control of the closed-loop control link, α will increase. At this time, the DC voltage of the LCC converter will decrease and finally stabilize at the expected value, realizing the control of the DC voltage.

[0041] However, the control period of the power devices of the LCC converter is slower than that of fully controlled power devices. The control period of a 6-pulse converter is 3.33 ms, and the control period of a 12-pulse converter is 1.67 ms. At the same time, when controlling the DC voltage, the adjustment of α is mainly realized by the PI controller of the closed-loop link. For the stability of the system, the control bandwidth of the closed-loop controller is generally narrow. The superposition of these two factors makes it difficult for the traditional DC voltage control method to achieve fast and stable control of the system DC voltage.

[0042] Regarding the converter control period, it is affected by the device operating characteristics and cannot be optimized. Therefore, it is only possible to start from the DC voltage control method. Considering that when the system parameters and operating conditions are determined, the theoretical value of the trigger angle can be calculated according to the mathematical model of the LCC converter. The theoretical value is directly obtained through mathematical calculations without the influence of the controller bandwidth, and fast DC voltage control can be achieved. At the same time, considering the equipment parameter deviation and control deviation during the actual operation of the LCC, controlling the LCC converter according to the theoretical value of the trigger angle may also cause a deviation between its DC voltage and the expected value. Therefore, a deviation amount can be added to the theoretical value as the α of the LCC converter, and the deviation amount can fine-tune the theoretical value of the trigger angle according to the actual operating conditions to achieve precise control of the DC voltage.

[0043] Specifically, please refer to Figure 5 , Figure 5 which shows a schematic diagram of the two links of theoretical value calculation and closed-loop regulation in one embodiment of the present invention. The theoretical value α of the trigger angle for the LCC converter to achieve the expected DC voltage control is obtained through the theoretical calculation link. cal On this basis, the compensation amount α of the theoretical value of the trigger angle of the LCC converter is obtained through the closed-loop control link. pi The two are superimposed as the actual control command value α of the trigger angle of the LCC converter; The input quantities of the theoretical calculation value link are the expected value U of the system DC voltage control dc_ref ,the expected value I of the system DC current control dc_ref ,and the effective value U of the AC bus line voltage ac ,and the output quantity of the theoretical calculation value link is the theoretical value α of the trigger angle of the LCC converter. cal First, calculate the first intermediate variable temp1, and the calculation formula is: Then, calculate the second intermediate variable temp2, and the calculation formula is: Finally, add U dc_ref to temp1, divide the sum by temp2, and then take the arccosine (ARCCOS, i.e., the inverse cosine function) of the division result to obtain the theoretical value α of the firing angle cal ; The compensation amount α of the theoretical value of the LCC converter firing angle pi is obtained by performing proportional-integral control on the difference between U dc_ref and U dc and then taking the negative value; Finally, the actual control command value α of the LCC converter firing angle is obtained by superimposing the theoretical value α cal of the firing angle and the compensation amount α pi . The specific calculation method is as follows: α = α cal + α pi Based on the original control architecture of the sending-end LCC converter without change, the embodiment of the present invention adaptively introduces the theoretically calculated value of the firing angle to compensate the closed-loop control link, so that the actually calculated firing angle of the LCC converter is mainly based on the theoretical value and supplemented by the closed-loop adjustment amount, which can effectively improve the influence of the closed-loop controller bandwidth on the dynamic characteristics of the system DC voltage, realize the accurate and rapid regulation of the DC voltage, maintain the stability of the DC voltage at the same time, and has high robustness.

[0044] In addition, when a fault occurs on the DC side, the embodiment of the present invention adopts a fast and effective fault restart strategy to clear the fault current and restore the system power supply. For the LCC-MMC hybrid DC transmission system, the fast response ability of the MMC can be utilized to block the fault current and quickly restore the system.

[0045] In an embodiment, determining the compensation amount of the theoretical value of the firing angle based on the preset expected value of the system DC voltage control and the actually sampled value of the DC voltage includes: Obtain model input data, where the model input data includes: the expected value of the system DC voltage control and the actually sampled value of the DC voltage, and the model input data further includes at least one of the following parameters of the sending-end LCC converter: the actual value of the DC current, the theoretical value of the firing angle, and the current actual firing angle; Input the model input data into a preset neural network to obtain the compensation amount of the theoretical value of the firing angle output by the neural network.

[0046] In this embodiment, the above neural network can be pre-trained and has the ability to take the model input data as input and the deviation control coefficient as output. During specific training, sample model input data (the 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 ideal compensation amount. Then, the sample model input data is input into the neural network to be trained to obtain the output of the neural network to be trained. Based on the output of the neural network to be trained and the ideal compensation amount characterized by the label, the backpropagation algorithm is used to minimize the loss function (such as the mean square error), so as to train 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.

[0047] Another embodiment of the present invention provides a control system for an LCC-MMC hybrid high-voltage DC transmission system. Specifically, please refer to Figure 6 , Figure 6 which is shown as the structural block diagram of the control system for the LCC-MMC hybrid high-voltage DC transmission system in one of the embodiments of the present invention. It is used for the sending-end LCC converter in the LCC-MMC hybrid high-voltage DC transmission system and includes: An operation module 601, configured to control the sending-end LCC converter to operate in a constant DC voltage mode; A theoretical value module 602, configured to determine the theoretical value of the trigger angle of the sending-end LCC converter based on a preset expected value of system DC voltage control, an expected value of system DC current control, and the effective value of the AC bus line voltage obtained; A compensation amount module 603, configured to determine the compensation amount of the theoretical value of the trigger angle based on a preset expected value of system DC voltage control and the actually sampled value of the DC voltage obtained in the closed-loop control link; An instruction module 604, configured to determine the actual control instruction of the trigger angle of the LCC converter according to the compensation amount and the theoretical value of the trigger angle; A control module 605, configured to control the sending-end LCC converter with the actual control instruction.

[0048] As one of the preferred solutions, the determining the theoretical value of the trigger angle of the sending-end LCC converter based on a preset expected value of system DC voltage control, an expected value of system DC current control, and the effective value of the AC bus line voltage obtained includes: Determining a first intermediate variable based on the expected value of the system DC current control, a pulsation coefficient matching the number of pulsations of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter; Determine a second intermediate variable based on the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer of the sending-end LCC converter; Determine the theoretical value of the trigger angle based on the expected value of the system DC voltage control, the first intermediate variable, and the second intermediate variable.

[0049] As one of the preferred solutions, the determining of the first intermediate variable based on the expected value of the system DC current control, the pulsation coefficient matching the pulsation number of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes: Calculate the product of the expected value of the system DC current control, the pulsation coefficient, and the equivalent commutation reactance as the first product; Multiply the first product by a preset first coefficient to obtain the first intermediate variable.

[0050] As one of the preferred solutions, the determining of the second intermediate variable based on the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer of the sending-end LCC converter includes: Calculate the product of the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer as the second product; Multiply the second product by a preset second coefficient to obtain the second intermediate variable.

[0051] As one of the preferred solutions, the determining of the compensation amount of the theoretical value of the trigger angle based on the preset expected value of the system DC voltage control and the actually sampled value of the DC voltage obtained includes: In the closed-loop control link, calculate the expected value of the system DC voltage control and the actually sampled value of the DC voltage based on the proportional-integral control strategy to obtain the error caused by the equipment parameter deviation and control deviation during the actual operation of the sending-end LCC converter, so as to determine the compensation amount of the theoretical value of the trigger angle.

[0052] As one of the preferred solutions, the determining of the compensation amount of the theoretical value of the trigger angle based on the preset expected value of the system DC voltage control and the actually sampled value of the DC voltage obtained includes: Obtain model input data, where the model input data includes: the expected value of the system DC voltage control and the actually sampled value of the DC voltage, and the model input data further includes at least one of the following parameters of the sending-end LCC converter: the actual value of the DC current, the theoretical value of the trigger angle, and the current actual trigger angle; Input the model input data into a preset neural network to obtain the compensation amount of the theoretical value of the trigger angle output by the neural network.

[0053] As one of the preferred solutions, determining the actual control command for the trigger angle of the LCC converter according to the compensation amount and the theoretical value of the trigger angle includes: Adding the compensation amount and the theoretical value of the trigger angle to obtain the actual control command.

[0054] Combined with the above related embodiments, the control method and system for an LCC-MMC hybrid high-voltage DC transmission system provided by the embodiments of the present invention have beneficial effects in at least one of the following aspects: (1) There is no need to change the original control architecture of the sending-end LCC converter, and there is no need to conduct large-scale transformation or upgrade of the original control system, which helps to maintain its compatibility with the existing power grid and other power equipment, ensures the stable operation of the system, and thus avoids the unstable factors that may be brought about by technology update or upgrade. In addition, during the system commissioning and testing process, the steps of re-verifying and debugging the control system can be omitted, thus simplifying the entire commissioning process.

[0055] (2) The embodiments of the present invention adaptively introduce the theoretical calculation value of the trigger angle and compensate the closed-loop control link, so that the actual trigger angle of the LCC converter obtained by calculation is mainly based on the theoretical value and supplemented by the closed-loop regulation amount, which can effectively improve the influence of the closed-loop controller bandwidth on the dynamic characteristics of the system DC voltage, realize accurate and rapid regulation of the DC voltage, maintain the stability of the DC voltage at the same time, and have high robustness.

[0056] The above embodiments only represent several implementation manners of the present invention, and their descriptions are relatively specific and detailed, but should not be construed as limiting 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 still 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 should be subject to the appended claims.

Claims

1. A control method for an LCC-MMC hybrid high-voltage DC transmission system, characterized in that The sending - end LCC converter in the LCC - MMC hybrid HVDC transmission system includes: Controlling the sending - end LCC converter to operate in a constant DC voltage mode; Based on the preset expected values of system DC voltage control, system DC current control, and the obtained effective value of the AC bus line voltage, determining the theoretical value of the trigger angle of the sending - end LCC converter; In the closed - loop control link, based on the preset expected value of system DC voltage control and the obtained actual sampled value of DC voltage, determining the compensation amount of the theoretical trigger - angle value; According to the compensation amount and the theoretical trigger - angle value, determining the actual control command of the trigger angle of the LCC converter; Controlling the sending - end LCC converter with the actual control command.

2. The control method of the LCC-MMC hybrid high-voltage DC power transmission system according to claim 1, wherein The determining of the theoretical trigger - angle value of the sending - end LCC converter based on the preset expected values of system DC voltage control, system DC current control, and the obtained effective value of the AC bus line voltage includes: Based on the expected value of system DC current control, the pulsation coefficient matching the number of pulsations of the sending - end LCC converter, and the equivalent commutation reactance of the sending - end LCC converter, determining a first intermediate variable; Based on the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer of the sending - end LCC converter, determining a second intermediate variable; Based on the preset expected value of system DC voltage control, the first intermediate variable, and the second intermediate variable, determining the theoretical trigger - angle value.

3. The control method of the LCC-MMC hybrid HVDC transmission system according to claim 2, wherein, The determining of the first intermediate variable based on the expected value of system DC current control, the pulsation coefficient matching the number of pulsations of the sending - end LCC converter, and the equivalent commutation reactance of the sending - end LCC converter includes: Calculating the product of the expected value of system DC current control, the pulsation coefficient, and the equivalent commutation reactance as a first product; Multiplying the first product by a preset first coefficient to obtain the first intermediate variable.

4. The control method of the LCC-MMC hybrid HVDC transmission system according to claim 2, characterized in that, The determining of the second intermediate variable based on the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer of the sending - end LCC converter includes: Calculating the product of the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer as a second product; Multiplying the second product by a preset second coefficient to obtain the second intermediate variable.

5. The control method of the LCC-MMC hybrid high-voltage DC power transmission system according to claim 1, wherein The determining of the compensation amount of the theoretical trigger - angle value based on the preset expected value of system DC voltage control and the obtained actual sampled value of DC voltage includes: In the closed - loop control link, calculating the preset expected value of system DC voltage control and the actual sampled value of DC voltage based on the proportional - integral control strategy to obtain the error caused by the equipment parameter deviation and control deviation during the actual operation of the sending - end LCC converter, so as to determine the compensation amount of the theoretical trigger - angle value.

6. The control method of the LCC-MMC hybrid high-voltage DC transmission system according to claim 1, characterized in that, The determining of the compensation amount of the theoretical trigger - angle value based on the preset expected value of system DC voltage control and the obtained actual sampled value of DC voltage includes: Obtain model input data, where the model input data includes: the expected value of the system DC voltage control and the actual sampled value of the DC voltage. The model input data further includes at least one of the following parameters of the sending-end LCC converter: the actual value of the DC current, the theoretical value of the firing angle, and the current actual firing angle; Input the model input data into a preset neural network to obtain the compensation amount of the theoretical value of the firing angle output by the neural network.

7. The control method of the LCC-MMC hybrid HVDC transmission system according to claim 1, wherein Determine the actual control command for the firing angle of the LCC converter according to the compensation amount and the theoretical value of the firing angle, including: Add the compensation amount and the theoretical value of the firing angle to obtain the actual control command.

8. A control system for an LCC-MMC hybrid high-voltage DC transmission system, characterized in that, For the sending-end LCC converter in the LCC-MMC hybrid HVDC transmission system, including: An operation module for controlling the sending-end LCC converter to operate in a constant DC voltage mode; A theoretical value module for determining the theoretical value of the firing angle of the sending-end LCC converter based on the preset expected value of the system DC voltage control, the expected value of the system DC current control, and the obtained effective value of the AC bus line voltage; A compensation amount module for determining the compensation amount of the theoretical value of the firing angle based on the preset expected value of the system DC voltage control and the obtained actual sampled value of the DC voltage in the closed-loop control link; An instruction module for determining the actual control command for the firing angle of the LCC converter according to the compensation amount and the theoretical value of the firing angle; A control module for controlling the sending-end LCC converter with the actual control command.

9. The control system of the LCC-MMC hybrid HVDC transmission system according to claim 8, characterized in that, The determining the theoretical value of the firing angle of the sending-end LCC converter based on the preset expected value of the system DC voltage control, the expected value of the system DC current control, and the obtained effective value of the AC bus line voltage includes: Determine a first intermediate variable based on the expected value of the system DC current control, the pulsation coefficient matching the number of pulsations of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter; Determine a second intermediate variable based on the effective value of the AC bus line voltage, the pulsation coefficient, and the turns ratio of the primary and secondary sides of the commutation transformer of the sending-end LCC converter; Determine the theoretical value of the firing angle based on the expected value of the system DC voltage control, the first intermediate variable, and the second intermediate variable.

10. The control system of the LCC-MMC hybrid HVDC transmission system according to claim 9, wherein, The determining a first intermediate variable based on the expected value of the system DC current control, the pulsation coefficient matching the number of pulsations of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes: Calculate the product of the expected value of the system DC current control, the pulsation coefficient, and the equivalent commutation reactance as the first product; Multiply the first product by a preset first coefficient to obtain the first intermediate variable.

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