LCC-MMC hybrid HVDC system control method and system

Abstract

The present invention discloses a control method and system for an LCC-MMC hybrid high-voltage direct current transmission system. The method comprises controlling a sending-end LCC converter to operate in a constant direct current voltage mode; determining a theoretical value of a trigger angle of the sending-end LCC converter based on a system direct current voltage control expected value, a system direct current current control expected value, and an effective value of an alternating current bus line voltage; determining, in a closed-loop control link, a compensation amount for the theoretical trigger angle value based on the system direct current voltage control expected value and an actual sampled value of the direct current voltage; determining an actual control instruction for the trigger angle of the LCC converter based on the compensation amount and the theoretical trigger angle value; and controlling the sending-end LCC converter using the actual control instruction, thereby introducing a theoretical calculated value of the trigger angle to compensate for the closed-loop control link. This method can effectively improve the influence of the closed-loop controller bandwidth on the dynamic characteristics of the system direct current voltage, achieve precise and rapid regulation of the direct current voltage, and simultaneously maintain the stability of the direct current voltage, thereby having high robustness.

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 an LCC-MMC hybrid high-voltage direct current (HVDC) power transmission system. Background Art

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

[0003] The LCC-MMC hybrid HVDC system is a new HVDC transmission system that combines traditional LCC-based HVDC transmission technology with MMC-based flexible HVDC transmission technology. The operating principle of the LCC-MMC hybrid HVDC system combines the characteristics of both LCC and MMC. Under normal operating conditions, the LCC converter controls the DC current, while the MMC converter controls the DC voltage. The receiving-end MMC converter typically adopts a constant DC voltage control mode. This control method leverages the respective advantages of LCC and MMC, achieving more efficient DC transmission.

[0004] In the existing technology, due to the influence of the LCC's own control period and control bandwidth, the traditional single-loop voltage control strategy is difficult to achieve rapid and stable control of the system DC voltage, which easily leads to DC side voltage and current oscillations, and is not conducive to its application in ultra-long distances where the DC line equivalent inductance is large. Summary of the Invention

[0005] The present invention provides a control method and system for an LCC-MMC hybrid high-voltage direct current power 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 applied to a sending-end LCC converter in the LCC-MMC hybrid HVDC transmission system, comprising:

[0007] Controlling the sending-end LCC converter to operate in a constant DC voltage mode;

[0008] Determining a theoretical value of a trigger angle of the sending-end LCC converter based on a preset system DC voltage control expected value, a system DC current control expected value, and the acquired AC bus line voltage effective value;

[0009] In the closed-loop control link, the compensation amount of the trigger angle theoretical value is determined based on the preset system DC voltage control expected value and the acquired DC voltage actual sampling value;

[0010] Determining an actual control instruction for the trigger angle of the LCC converter according to the compensation amount and the theoretical value of the trigger angle;

[0011] The sending-end LCC converter is controlled using the actual control instruction.

[0012] As one preferred solution, determining the theoretical value of the trigger angle of the sending-end LCC converter based on a preset system DC voltage control expected value, a system DC current control expected value, and the acquired AC bus line voltage effective value includes:

[0013] Determining a first intermediate variable based on the system DC current control expected value, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and an equivalent commutation reactance of the sending-end LCC converter;

[0014] Determining a second intermediate variable based on the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer of the sending-end LCC converter;

[0015] The trigger angle theoretical value is determined based on the system DC voltage control expected value, the first intermediate variable, and the second intermediate variable.

[0016] As one preferred solution, determining the first intermediate variable based on the expected value of the system DC current control, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes:

[0017] Calculating the product of the system DC current control expected value, the ripple coefficient, and the equivalent commutation reactance as a first product;

[0018] The first product is multiplied by a preset first coefficient to obtain the first intermediate variable.

[0019] As one preferred solution, determining the second intermediate variable based on the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer of the sending-end LCC converter includes:

[0020] Calculating the product of the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer as a second product;

[0021] The second product is multiplied by a preset second coefficient to obtain the second intermediate variable.

[0022] As one preferred solution, determining the compensation amount of the theoretical value of the trigger angle based on a preset system DC voltage control expected value and an acquired DC voltage actual sampling value includes:

[0023] In the closed-loop control link, the system DC voltage control expected value and the actual DC voltage sampling value are calculated based on the proportional-integral control strategy to obtain the error caused by the device 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.

[0024] As one preferred solution, determining the compensation amount of the theoretical value of the trigger angle based on a preset system DC voltage control expected value and an acquired DC voltage actual sampling value includes:

[0025] Acquiring model input data, wherein the model input data includes: the expected control value of the system DC voltage and the actual sampled value of the DC voltage, and the model input data also 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;

[0026] The model input data is input into a preset neural network to obtain a compensation amount of the firing angle theoretical value output by the neural network.

[0027] As one preferred solution, determining 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 includes:

[0028] The compensation amount and the theoretical value of the trigger angle are added to obtain the actual control instruction.

[0029] Another embodiment of the present invention provides an LCC-MMC hybrid HVDC transmission system control system, which is used for a sending-end LCC converter in the LCC-MMC hybrid HVDC transmission system, including:

[0030] An operation module, configured to control the sending-end LCC converter to operate in a constant DC voltage mode;

[0031] A theoretical value module, configured to determine a theoretical value of a trigger angle of the sending-end LCC converter based on a preset system DC voltage control expected value, a system DC current control expected value, and the acquired AC bus line voltage effective value;

[0032] A compensation module is used to determine the compensation amount of the trigger angle theoretical value based on a preset system DC voltage control expected value and an acquired DC voltage actual sampling value in a closed-loop control link;

[0033] An instruction module, configured to determine an actual control instruction for the trigger angle of the LCC converter based on the compensation amount and the theoretical value of the trigger angle;

[0034] A control module is used to control the sending-end LCC converter using the actual control instruction.

[0035] As one preferred solution, determining the theoretical value of the trigger angle of the sending-end LCC converter based on a preset system DC voltage control expected value, a system DC current control expected value, and the acquired AC bus line voltage effective value includes:

[0036] Determining a first intermediate variable based on the system DC current control expected value, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and an equivalent commutation reactance of the sending-end LCC converter;

[0037] Determining a second intermediate variable based on the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer of the sending-end LCC converter;

[0038] The trigger angle theoretical value is determined based on the system DC voltage control expected value, the first intermediate variable, and the second intermediate variable.

[0039] As one preferred solution, determining the first intermediate variable based on the expected value of the system DC current control, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes:

[0040] Calculating the product of the system DC current control expected value, the ripple coefficient, and the equivalent commutation reactance as a first product;

[0041] The first product is multiplied by a preset first coefficient to obtain the first intermediate variable.

[0042] Compared with the prior art, the embodiments of the present invention have the following advantages:

[0043] The sending-end LCC converter is controlled to operate in a constant DC voltage mode; based on a preset system DC voltage control expected value, a system DC current control expected value and the obtained AC bus line voltage effective value, the trigger angle theoretical value of the sending-end LCC converter is determined; in a closed-loop control link, based on a preset system DC voltage control expected value and the obtained DC voltage actual sampling value, a compensation amount for the trigger angle theoretical value is determined; according to the compensation amount and the trigger angle theoretical value, an actual control instruction for the trigger angle of the LCC converter is determined; and the sending-end LCC converter is controlled by the actual control instruction. Thus, on the basis of not changing the original control architecture of the sending-end LCC converter, the trigger angle theoretical calculation value is introduced to compensate for the closed-loop control link, 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, and maintain the stability of the DC voltage at the same time, with high robustness. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a schematic diagram of the system structure in which both the sending and receiving ends adopt the LCC type conventional high-voltage direct current transmission technology;

[0045] Figure 2 1 is a schematic diagram of the system structure of an LCC-MMC hybrid high-voltage direct current transmission system constructed in one embodiment of the present invention;

[0046] Figure 3 This is a schematic diagram of the traditional voltage single closed-loop control strategy;

[0047] Figure 4 1 is a flow chart of a control method for an LCC-MMC hybrid HVDC power transmission system in one embodiment of the present invention;

[0048] Figure 5 It is a schematic diagram of the two links of theoretical value calculation and closed-loop regulation in one embodiment of the present invention;

[0049] Figure 6 It is a structural block diagram of an LCC-MMC hybrid HVDC transmission system control system in one embodiment of the present invention. DETAILED DESCRIPTION

[0050] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. The purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0051] In the description of this application, the terms "first," "second," "third," etc. are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, a feature specified as "first," "second," "third," etc. may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0052] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installed", "connected" and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be a communication between the two components. The terms "vertical", "horizontal", "left", "right", "up", "down" and similar expressions used herein are for illustrative purposes only, and do not indicate or imply that the device or component referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0053] In the description of this application, it should be noted that, unless otherwise defined, all technical and scientific terms used in this application have the same meanings as those commonly understood by those skilled in the art. The terms used in this specification are only for the purpose of describing specific embodiments and are not intended to limit the present invention. Those skilled in the art will understand the specific meanings of the above terms in this application in specific circumstances.

[0054] It should be noted that in order to keep up with the trend of energy technology advancement and promote the transformation of clean and low-carbon energy, the power industry must build a new power system with new energy as the main body and promote the high-quality development of clean energy. Figure 1 , Figure 1 The figure shows the system structure diagram of the existing conventional high-voltage direct current transmission technology using LCC at both the sending and receiving ends. 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.

[0055] However, LCC uses thyristors without self-shutoff capability as commutation devices, which places certain demands on the strength of the receiving AC grid. In addition, commutation failure is prone to occur when a fault occurs in the receiving AC system, posing a threat to the safe and stable operation of the receiving AC grid. To improve the safety and stability of the receiving grid, the receiving LCC can be replaced with an MMC. For details, see Figure 2 , Figure 2 The figure shows a schematic diagram of the system structure of an LCC-MMC hybrid HVDC transmission system constructed in one embodiment of the present invention, which fully utilizes the many advantages of MMC, such as no commutation failure, power and reactive power decoupling control, and no need for filters and reactive power compensation devices.

[0056] For a hybrid LCC-MMC HVDC transmission system, this embodiment controls the sending-end LCC converter using a constant DC voltage control mode, while the receiving-end MMC converter can also use a constant DC voltage control mode. This control method can help maintain normal system operation. However, if a fault occurs in the receiving-end AC system and the DC system's transmitted power cannot be absorbed, it can easily cause a power surplus within the DC system, leading to severe overvoltage. This is primarily due to the fact that the sending-end converter station cannot autonomously sense the receiving-end AC fault information and proactively reduce the transmitted power. Instead, it relies on inter-station communication to obtain fault information. During the fault information transmission process, the sending-end LCC still transmits the preset power to the DC system, resulting in a power surplus within the DC system and an overvoltage. This situation is even more serious in ultra-long-distance transmission scenarios (where inter-station communication time is long).

[0057] To solve the above problem, the control mode of the sending-end LCC converter station and the receiving-end MMC converter station can be adjusted to improve the situation. That is, LCC adopts a constant DC voltage control mode, and MMC adopts a constant DC current / power control mode. When the DC voltage rises due to a fault in the receiving-end AC system, the LCC will actively reduce its transmission power without communication to maintain DC voltage stability, effectively reducing the surplus power in the system and suppressing system overvoltage. However, the control cycle of LCC itself is slow, and the use of Figure 3 The traditional voltage single closed-loop control strategy shown in the figure is to control the expected value U of the system DC voltage. dc_ref , the actual measured DC voltage actual sampling value U dc The input signal is fed into a subtractor (differentiator) to calculate the difference between the two, i.e., the error signal. This signal is then fed into a PI (proportional-integral) controller. The output of the PI controller is then fed into a gain stage (the gain is -1, meaning the signal is inverted). Finally, the gained signal is fed into an adder and superimposed with another input signal, π, to obtain the final output. However, due to the controller's bandwidth, rapid and stable control of the system's DC voltage is difficult to achieve, which can easily lead to DC side voltage and current oscillations. This phenomenon is more pronounced in ultra-long-distance applications where the DC line has a large equivalent inductance.

[0058] In view of this, an embodiment of the present invention provides a control method for an LCC-MMC hybrid HVDC transmission system. For details, see Figure 4 , Figure 4 FIG2 is a flow chart of a control method for an LCC-MMC hybrid HVDC transmission system according to one embodiment of the present invention, which is used for a sending-end LCC converter in the LCC-MMC hybrid HVDC transmission system and includes S401 to S405 .

[0059] S401, controlling the sending-end LCC converter to operate in a constant DC voltage mode.

[0060] S402 : Determine a theoretical value of a trigger angle of the sending-end LCC converter based on a preset system DC voltage control expected value, a system DC current control expected value, and the acquired AC bus line voltage effective value.

[0061] S403 , in a closed-loop control link, determining a compensation amount for the theoretical value of the trigger angle based on a preset system DC voltage control expected value and the acquired DC voltage actual sampling value.

[0062] S404: Determine an actual control instruction for the trigger angle of the LCC converter according to the compensation amount and the theoretical value of the trigger angle.

[0063] S405: Control the sending-end LCC converter using the actual control instruction.

[0064] For ease of explanation, the control method provided in the embodiment of the present invention can be understood as including two steps: theoretical value calculation and closed-loop control. The theoretical value calculation step calculates the theoretical value of the LCC converter trigger angle based on the parameters collected by the system (RMS value of the AC bus line voltage), the first control target expected value (the system DC current control expected value), and the second control target expected value (the system DC voltage control expected value). The closed-loop control step calculates the adjustment amount of the theoretical value of the LCC converter trigger angle based on the second control target expected value (the system DC voltage control expected value) and the actual value (the actual DC voltage sampling value) for compensation, thereby obtaining the actual control instruction of the LCC converter trigger angle.

[0065] In an embodiment of the present invention, a control method is applicable to a sending-end LCC converter station of a high-voltage direct current system, wherein the sending-end LCC converter in the converter station operates in a constant direct current voltage mode. Furthermore, the control method includes two steps: theoretical value calculation and closed-loop control. The theoretical calculation step obtains a theoretical value of the trigger angle of the LCC converter for achieving control of the expected direct current voltage value. Based on this value, the closed-loop control step obtains a compensation amount for the theoretical value of the trigger angle of the LCC converter, thereby obtaining an actual control instruction for the trigger angle of the LCC converter. This method is described in detail below.

[0066] In one embodiment, determining the theoretical value of the trigger angle of the sending-end LCC converter based on a preset system DC voltage control expected value, a system DC current control expected value, and the acquired AC bus line voltage effective value includes:

[0067] Determining a first intermediate variable based on the system DC current control expected value, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and an equivalent commutation reactance of the sending-end LCC converter;

[0068] Determining a second intermediate variable based on the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer of the sending-end LCC converter;

[0069] The trigger angle theoretical value is determined based on the system DC voltage control expected value, the first intermediate variable, and the second intermediate variable.

[0070] In one embodiment, determining the first intermediate variable based on the expected value of the system DC current control, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes:

[0071] Calculating the product of the system DC current control expected value, the ripple coefficient, and the equivalent commutation reactance as a first product;

[0072] The first product is multiplied by a preset first coefficient to obtain the first intermediate variable.

[0073] In one embodiment, determining the second intermediate variable based on the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer of the sending-end LCC converter includes:

[0074] Calculating the product of the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer as a second product;

[0075] The second product is multiplied by a preset second coefficient to obtain the second intermediate variable.

[0076] In one embodiment, determining 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 includes:

[0077] The compensation amount and the theoretical value of the trigger angle are added to obtain the actual control instruction.

[0078] In one embodiment, determining the compensation amount of the theoretical value of the trigger angle based on a preset system DC voltage control expected value and the acquired DC voltage actual sampling value includes:

[0079] In the closed-loop control link, the system DC voltage control expected value and the actual DC voltage sampling value are calculated based on the proportional-integral control strategy to obtain the error caused by the device 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.

[0080] Specifically, the equivalent mathematical model of the sending-end LCC converter can be transformed to obtain corresponding intermediate variables, and the theoretical value of the trigger angle can be calculated using the intermediate variables. The mathematical model of the sending-end LCC converter is:

[0081]

[0082] Where U dc is the DC side voltage of the LCC converter (i.e. the actual sampling value of the DC voltage), U ac is the effective value of the AC bus voltage, I dc is the DC side current of the LCC converter (i.e., the actual value of the DC current), k is the primary-to-secondary transformation ratio of the converter transformer, X is the equivalent commutation reactance, α is the control command value of the LCC converter trigger angle, and m is the ripple 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.

[0083] The sending-end LCC converter operates stably, and the AC voltage on the valve side of the converter transformer can be stabilized by controlling the converter transformer tap (equivalent to U ac Stable), I can be achieved through the control of the receiving end converter station dc Stable, according to the above formula, the U of LCC converter is dc It can be directly controlled by its α, so the schematic diagram of the traditional control method of the DC voltage of the sending-end LCC converter is as shown above Figure 3 As shown in the figure, when its DC voltage is less than the expected value, α will decrease through the control of the closed-loop control link, and the DC voltage of the LCC converter will increase. When its DC voltage is greater than the expected value, α will increase through the control of the closed-loop control link, and the DC voltage of the LCC converter will decrease. It will eventually stabilize at the expected value, realizing the control of the DC voltage.

[0084] However, the control cycle of LCC converter power devices is slower than that of fully controlled power devices. The control cycle of 6-pulse converter is 3.33ms, and the control cycle of 12-pulse converter is 1.67ms. At the same time, when performing DC voltage control, the adjustment of α is mainly achieved by the PI controller in the closed-loop link. For the sake of system stability, the control bandwidth of the closed-loop controller is generally narrow. The combined influence of these two factors makes it difficult for traditional DC voltage control methods to achieve rapid and stable control of the system DC voltage.

[0085] The converter control cycle cannot be optimized due to the influence of the device operating characteristics. Therefore, we can only 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 calculation without the influence of the controller bandwidth, and the DC voltage can be quickly controlled. At the same time, considering the device 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 cause its DC voltage to deviate from the expected value. Therefore, an offset can be added to the theoretical value as the α of the LCC converter. The offset can be used to fine-tune the theoretical value of the trigger angle according to the actual operating conditions to achieve precise control of the DC voltage.

[0086] For details, see Figure 5 , Figure 5 The figure 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 DC voltage expected value control is obtained through the theoretical calculation link. cal On this basis, the compensation value α of the theoretical value of the LCC converter trigger angle is obtained through the closed-loop control link pi , the two are superimposed as the actual control instruction value α of the LCC converter trigger angle;

[0087] The input quantity of the theoretical calculation value link is the system DC voltage control expected value U dc_ref , the system DC current control expected value I dc_ref , and the effective value of the AC bus voltage U ac The output of the theoretical calculation value link is the theoretical value of the LCC converter trigger angle α cal First, calculate the first intermediate variable temp1, the calculation formula is:

[0088]

[0089] Then, the second intermediate variable temp2 is calculated using the following formula:

[0090]

[0091] Finally, U dc_ref Add it to temp1, divide the result of addition by temp2, and then take the arc cosine (ARCCOS, i.e. arc cosine function) of the result to get the theoretical value of the trigger angle α cal ; Compensation value α of the theoretical value of the LCC converter firing angle pi Through U dc_ref with U dcThe difference between them is controlled by proportional integral and then negative; finally, the actual control command value α of the trigger angle of the LCC converter is obtained by the theoretical value α of the trigger angle. cal and compensation α pi The specific calculation method is as follows:

[0092] α=α cal +α pi

[0093] Without changing the original control architecture of the sending-end LCC converter, the embodiment of the present invention adaptively introduces the theoretical calculated value of the trigger angle to compensate for the closed-loop control link. This allows the calculated actual trigger angle of the LCC converter to be based primarily on the theoretical value, supplemented by the closed-loop adjustment value. This effectively improves the impact of the closed-loop controller bandwidth on the dynamic characteristics of the system's DC voltage, achieves precise and rapid regulation of the DC voltage, and maintains DC voltage stability, thus exhibiting high robustness.

[0094] Furthermore, when a fault occurs on the DC side, embodiments of the present invention employ a fast and effective fault-restart strategy to clear the fault current and restore system power. For hybrid LCC-MMC DC transmission systems, the rapid response capabilities of the MMC can be leveraged to block the fault current and quickly restore the system.

[0095] In one embodiment, determining the compensation amount of the theoretical value of the trigger angle based on a preset system DC voltage control expected value and the acquired DC voltage actual sampling value includes:

[0096] Acquiring model input data, wherein the model input data includes: the expected control value of the system DC voltage and the actual sampled value of the DC voltage, and the model input data also 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;

[0097] The model input data is input into a preset neural network to obtain a compensation amount of the firing angle theoretical value output by the neural network.

[0098] In this embodiment, the neural network may be pre-trained and capable of taking model input data as input and outputting a deviation control coefficient. During training, sample model input data (the specific data composition of which may be determined based on the data included in the model input data) and a label corresponding to the sample model input data may be obtained, with the label being suitable for representing an ideal compensation amount. The sample model input data is then input into the neural network to be trained, obtaining the output of the neural network to be trained. Based on the output of the neural network to be trained and the ideal compensation amount represented by the label, a backpropagation algorithm is used to minimize a loss function (e.g., mean squared error), thereby training the neural network to be trained. The neural network to be trained may include a fully connected deep neural network, a convolutional neural network, or a recurrent neural network.

[0099] Another embodiment of the present invention provides a LCC-MMC hybrid HVDC transmission system control system. Figure 6 , Figure 6 The figure shows a block diagram of a control system for an LCC-MMC hybrid HVDC transmission system according to one embodiment of the present invention, which is used for a sending-end LCC converter in the LCC-MMC hybrid HVDC transmission system and includes:

[0100] An operation module 601 is used to control the sending-end LCC converter to operate in a constant DC voltage mode;

[0101] Theoretical value module 602 is used to determine the theoretical value of the trigger angle of the sending-end LCC converter based on the preset system DC voltage control expected value, the system DC current control expected value and the acquired AC bus line voltage effective value;

[0102] The compensation module 603 is used to determine the compensation amount of the trigger angle theoretical value based on the preset system DC voltage control expected value and the acquired DC voltage actual sampling value in the closed-loop control link;

[0103] An instruction module 604 is configured to determine an actual control instruction for the trigger angle of the LCC converter based on the compensation amount and the theoretical value of the trigger angle;

[0104] The control module 605 is configured to control the sending-end LCC converter using the actual control instruction.

[0105] As one preferred solution, determining the theoretical value of the trigger angle of the sending-end LCC converter based on a preset system DC voltage control expected value, a system DC current control expected value, and the acquired AC bus line voltage effective value includes:

[0106] Determining a first intermediate variable based on the system DC current control expected value, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and an equivalent commutation reactance of the sending-end LCC converter;

[0107] Determining a second intermediate variable based on the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer of the sending-end LCC converter;

[0108] The trigger angle theoretical value is determined based on the system DC voltage control expected value, the first intermediate variable, and the second intermediate variable.

[0109] As one preferred solution, determining the first intermediate variable based on the expected value of the system DC current control, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes:

[0110] Calculating the product of the system DC current control expected value, the ripple coefficient, and the equivalent commutation reactance as a first product;

[0111] The first product is multiplied by a preset first coefficient to obtain the first intermediate variable.

[0112] As one preferred solution, determining the second intermediate variable based on the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer of the sending-end LCC converter includes:

[0113] Calculating the product of the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer as a second product;

[0114] The second product is multiplied by a preset second coefficient to obtain the second intermediate variable.

[0115] As one preferred solution, determining the compensation amount of the theoretical value of the trigger angle based on a preset system DC voltage control expected value and an acquired DC voltage actual sampling value includes:

[0116] In the closed-loop control link, the system DC voltage control expected value and the actual DC voltage sampling value are calculated based on the proportional-integral control strategy to obtain the error caused by the device 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.

[0117] As one preferred solution, determining the compensation amount of the theoretical value of the trigger angle based on a preset system DC voltage control expected value and an acquired DC voltage actual sampling value includes:

[0118] Acquiring model input data, wherein the model input data includes: the expected control value of the system DC voltage and the actual sampled value of the DC voltage, and the model input data also 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;

[0119] The model input data is input into a preset neural network to obtain a compensation amount of the firing angle theoretical value output by the neural network.

[0120] As one preferred solution, determining 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 includes:

[0121] The compensation amount and the theoretical value of the trigger angle are added to obtain the actual control instruction.

[0122] In combination with the above-mentioned related embodiments, the embodiments of the present invention provide a method and system for controlling an LCC-MMC hybrid HVDC power transmission system, which has the beneficial effects of at least one of the following:

[0123] (1) There is no need to change the original control architecture of the sending-end LCC converter, and no large-scale transformation or upgrade of the original control system is required. This helps maintain its compatibility with the existing power grid and other power equipment, ensuring the smooth operation of the system, thereby avoiding the instability factors that may be caused by technology updates or upgrades. In addition, during the system debugging and testing process, the re-verification and debugging steps of the control system can be omitted, thereby simplifying the entire debugging process.

[0124] (2) The embodiment of the present invention adaptively introduces the theoretical calculated value of the trigger angle and compensates the closed-loop control link, so that the calculated actual trigger angle of the LCC converter is mainly based on the theoretical value and supplemented by the closed-loop adjustment amount. This can effectively improve the influence of the closed-loop controller bandwidth on the dynamic characteristics of the system DC voltage, achieve accurate and rapid regulation of the DC voltage, and maintain the stability of the DC voltage at the same time, with high robustness.

[0125] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A control method for an LCC-MMC hybrid high-voltage direct current transmission system, characterized in that: For a sending-end LCC converter in an LCC-MMC hybrid HVDC transmission system, wherein the receiving-end MMC converter station in the LCC-MMC hybrid HVDC transmission system adopts a constant DC current / power control mode, the method comprising: Controlling the sending-end LCC converter to operate in a constant DC voltage mode; Determining a theoretical value of a trigger angle of the sending-end LCC converter based on a preset system DC voltage control expected value, a system DC current control expected value, and the acquired AC bus line voltage effective value; In the closed-loop control link, the compensation amount of the trigger angle theoretical value is determined based on the preset system DC voltage control expected value and the acquired DC voltage actual sampling value; Determining an actual control instruction for 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 using the actual control instruction; The method of determining the theoretical value of the trigger angle of the sending-end LCC converter based on the preset system DC voltage control expected value, the system DC current control expected value, and the acquired AC bus line voltage effective value includes: Determining a first intermediate variable based on the system DC current control expected value, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and an 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 primary-to-secondary transformation ratio of the converter transformer of the sending-end LCC converter; The trigger angle theoretical value is determined based on the system DC voltage control expected value, the first intermediate variable, and the second intermediate variable.

2. The LCC-MMC hybrid HVDC system control method according to claim 1, wherein: The determining of the first intermediate variable based on the expected value of the system DC current control, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes: Calculating the product of the system DC current control expected value, the ripple coefficient, and the equivalent commutation reactance as a first product; The first product is multiplied by a preset first coefficient to obtain the first intermediate variable.

3. The LCC-MMC hybrid HVDC system control method according to claim 1, wherein: The determining of the second intermediate variable based on the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer of the sending-end LCC converter includes: Calculating the product of the effective value of the AC bus line voltage, the ripple coefficient, and the primary-to-secondary transformation ratio of the converter transformer as a second product; The second product is multiplied by a preset second coefficient to obtain the second intermediate variable.

4. The LCC-MMC hybrid HVDC system control method according to claim 1, wherein: The determining of the compensation amount of the trigger angle theoretical value based on the preset system DC voltage control expected value and the acquired DC voltage actual sampling value includes: In the closed-loop control link, the system DC voltage control expected value and the actual DC voltage sampling value are calculated based on the proportional-integral control strategy to obtain the error caused by the device 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.

5. The LCC-MMC hybrid HVDC system control method according to claim 1, wherein: The determining of the compensation amount of the trigger angle theoretical value based on the preset system DC voltage control expected value and the acquired DC voltage actual sampling value includes: Acquiring model input data, wherein the model input data includes: the expected control value of the system DC voltage and the actual sampled value of the DC voltage, and the model input data also 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; The model input data is input into a preset neural network to obtain a compensation amount of the firing angle theoretical value output by the neural network.

6. The LCC-MMC hybrid HVDC system control method according to claim 1, wherein: The determining, based on the compensation amount and the theoretical value of the trigger angle, an actual control instruction of the trigger angle of the LCC converter includes: The compensation amount and the theoretical value of the trigger angle are added to obtain the actual control instruction.

7. An LCC-MMC hybrid high-voltage direct current transmission system control system, characterized in that: Used in a sending-end LCC converter in an LCC-MMC hybrid HVDC transmission system, where the receiving-end MMC converter station in the LCC-MMC hybrid HVDC transmission system adopts a constant DC current / power control mode. The control system includes: An operation module, configured to control the sending-end LCC converter to operate in a constant DC voltage mode; A theoretical value module, configured to determine a theoretical value of a trigger angle of the sending-end LCC converter based on a preset system DC voltage control expected value, a system DC current control expected value, and the acquired AC bus line voltage effective value; A compensation module is used to determine the compensation amount of the trigger angle theoretical value based on a preset system DC voltage control expected value and an acquired DC voltage actual sampling value in a closed-loop control link; An instruction module, configured to determine an actual control instruction for the trigger angle of the LCC converter based on the compensation amount and the theoretical value of the trigger angle; A control module, configured to control the sending-end LCC converter using the actual control instruction; The method of determining the theoretical value of the trigger angle of the sending-end LCC converter based on the preset system DC voltage control expected value, the system DC current control expected value, and the acquired AC bus line voltage effective value includes: Determining a first intermediate variable based on the system DC current control expected value, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and an 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 primary-to-secondary transformation ratio of the converter transformer of the sending-end LCC converter; The trigger angle theoretical value is determined based on the system DC voltage control expected value, the first intermediate variable, and the second intermediate variable.

8. The LCC-MMC hybrid HVDC power transmission system control system according to claim 7, characterized in that: The determining of the first intermediate variable based on the expected value of the system DC current control, a pulsation coefficient matching the pulsation number of the sending-end LCC converter, and the equivalent commutation reactance of the sending-end LCC converter includes: Calculating the product of the system DC current control expected value, the ripple coefficient, and the equivalent commutation reactance as a first product; The first product is multiplied by a preset first coefficient to obtain the first intermediate variable.

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