Method and device for optimizing dead zone time of flexible direct-current power transmission system

Abstract

The embodiment of the invention provides a flexible direct current transmission system dead time optimization method and device, and the method comprises the steps: building a time domain harmonic analysis model of a modular multilevel converter, and building a direct current side equivalent harmonic impedance model of a flexible direct current transmission system; based on a time domain harmonic analysis model and a direct current side equivalent harmonic impedance model, harmonic interaction influence between alternating current and direct current in the flexible direct current power transmission system is analyzed, and a harmonic component in neutral line voltage is obtained; dynamically adjusting the dead time based on the change of the load condition, and determining the initial dead time; and optimizing the initial dead time through model prediction control based on the harmonic component, and determining the target dead time. According to the method, the adaptability of the dead time is improved, the harmonic waves in the neutral line voltage are effectively suppressed, and the system stability and the power quality are remarkably improved. The power system can operate more stably and efficiently, the influence of harmonic waves on power transmission is reduced, and the reliability and efficiency of the system are improved.

Description

Technical Field

[0001] The embodiments of this specification relate to the technical field of flexible DC transmission, and particularly to a method for optimizing the dead time of a flexible DC transmission system. Background Art

[0002] Currently, with the growth of power demand and the large-scale access of renewable energy, flexible DC transmission technology has become an important power transmission method in the power system, especially suitable for long-distance and large-scale power transmission.

[0003] The neutral line voltage harmonics are mainly caused by factors such as the switching actions of converters, the nonlinear characteristics of the system, and load fluctuations. In a flexible DC transmission system, especially when technologies such as modular multilevel converters (MMCs) are adopted, the design of the dead time directly affects the power quality and system stability. The dead time is the time interval set to avoid short-circuit phenomena when power electronic switching devices change states. If the dead time is too long, it will lead to slow system response, increased voltage fluctuations, and may exacerbate harmonic generation; if the dead time is too short, it may cause system instability and increased losses. Therefore, reasonably adjusting the dead time becomes the key to solving the problem of neutral line voltage harmonics.

[0004] In the prior art, although some methods suppress harmonics by adding filters and improving control strategies, these methods often require additional hardware support, increasing the system complexity and cost. Traditional control methods fail to fully consider the influence of load changes and system dynamic characteristics on the dead time, resulting in inflexible dead time settings and difficulty in optimizing system performance under different operating conditions. Therefore, how to optimize the dead time, especially dynamically adjust the dead time according to the real-time changes of load and harmonics, suppress the neutral line voltage harmonics of the flexible DC transmission system, and ensure the stability and efficient operation of the system has become an urgent problem to be solved.

[0005] Therefore, there is an urgent need for a better solution. Summary of the Invention

[0006] In view of this, the embodiments of this specification provide a method for optimizing the dead time of a flexible DC transmission system. One or more embodiments of this specification simultaneously relate to a device for optimizing the dead time of a flexible DC transmission system, a computing device, a computer-readable storage medium, and a computer program to solve the technical defects existing in the prior art.

[0007] According to the first aspect of the embodiments of this specification, a method for optimizing the dead time of a flexible DC transmission system is provided, including: Establish a time-domain harmonic analysis model of a modular multilevel converter and establish a DC-side equivalent harmonic impedance model of a flexible DC transmission system; Based on the time-domain harmonic analysis model and the DC-side equivalent harmonic impedance model, analyze the harmonic interaction between the AC and DC sides in the flexible DC transmission system to obtain the harmonic components in the neutral line voltage; Dynamically adjust the dead time based on the change of the load condition to determine the initial dead time; Optimize the initial dead time through model predictive control based on the harmonic components to determine the target dead time.

[0008] In a possible implementation, establish a time-domain harmonic analysis model of the modular multilevel converter, including: Determine the loop equations of the upper and lower arms of the modular multilevel converter; Based on the loop equations, determine the expressions of the AC-DC side dynamic characteristics; Based on the expressions of the AC-DC side dynamic characteristics, determine the expressions of the capacitor dynamic characteristics; Based on the expressions of the capacitor dynamic characteristics, determine the time-domain harmonic analysis model.

[0009] In a possible implementation, establish a DC-side equivalent harmonic impedance model of the flexible DC transmission system, including: Determine the common-mode voltage expression and the relationship between the capacitor voltage of the sub-module and the DC-side current; Based on the common-mode voltage expression and the relationship between the capacitor voltage of the sub-module and the DC-side current, determine the DC-side equivalent harmonic impedance model.

[0010] In a possible implementation, based on the time-domain harmonic analysis model and the DC-side equivalent harmonic impedance model, analyze the harmonic interaction between the AC and DC sides in the flexible DC transmission system to obtain the harmonic components in the neutral line voltage, including: Convert the time-domain harmonic analysis model by moving the Fourier coefficients to determine the conversion model; Separate the positive and negative sequences of the conversion model by the symmetrical component method to determine the sequence-moving Fourier coefficient model; Simplify based on the sequence-moving Fourier coefficient model to determine the simplified model; Based on the simplified model and the DC-side equivalent harmonic impedance model, analyze to obtain the harmonic components in the neutral line voltage.

[0011] In a possible implementation, dynamically adjust the dead time based on the change of the load condition to determine the initial dead time, including: Determine the load power based on the change of the load condition; When the load power is less than the first rated power, shorten the dead time; When the load power is less than the second rated power and greater than or equal to the first rated power, keep the dead time; When the load power is greater than the second rated power, extend the dead time; wherein, the second rated power is greater than the first rated power.

[0012] In a possible implementation, based on the harmonic components, optimize the initial dead time through model predictive control to determine the target dead time, including: When the harmonic components are greater than the harmonic threshold, optimize the initial dead time through model predictive control to determine the target dead time; When the harmonic components are less than or equal to the harmonic threshold, use the initial dead time as the target dead time.

[0013] In a possible implementation, when the harmonic components are greater than the harmonic threshold, optimize the initial dead time through model predictive control to determine the target dead time, including: Determine the cost function of the initial dead time; Based on the cost function, perform linear programming to determine the linearized cost function; Based on the linearized cost function, perform a solution to determine the optimization result; Select the dead time at the current moment from the optimization result as the target dead time.

[0014] According to the second aspect of the embodiments of the present specification, a device for optimizing the dead time of a flexible DC transmission system is provided, including: A model establishment module, configured to establish a time-domain harmonic analysis model of a modular multilevel converter and establish a DC-side equivalent harmonic impedance model of the flexible DC transmission system; A harmonic component module, configured to analyze the harmonic interaction between AC and DC in the flexible DC transmission system based on the time-domain harmonic analysis model and the DC-side equivalent harmonic impedance model to obtain the harmonic components in the neutral line voltage; A dead time adjustment module, configured to dynamically adjust the dead time based on the change of the load condition to determine the initial dead time; A dead time optimization module, configured to optimize the initial dead time through model predictive control based on the harmonic components to determine the target dead time.

[0015] According to the third aspect of the embodiments of the present specification, a computing device is provided, including: A memory and a processor; The memory is used to store computer-executable instructions, and the processor is used to execute the computer-executable instructions. When the computer-executable instructions are executed by the processor, the steps of the above-mentioned method for optimizing the dead time of the flexible DC transmission system are implemented.

[0016] According to the fourth aspect of the embodiments of the present specification, a computer-readable storage medium is provided, which stores computer-executable instructions that, when executed by a processor, implement the steps of the above-mentioned dead-time optimization method for a flexible DC transmission system.

[0017] According to the fifth aspect of the embodiments of the present specification, a computer program is provided, wherein when the computer program is executed on a computer, the computer is made to execute the steps of the above-mentioned dead-time optimization method for a flexible DC transmission system.

[0018] The embodiments of the present specification provide a dead-time optimization method and device for a flexible DC transmission system, including: establishing a time-domain harmonic analysis model of a modular multilevel converter and establishing a DC-side equivalent harmonic impedance model of the flexible DC transmission system; based on the time-domain harmonic analysis model and the DC-side equivalent harmonic impedance model, analyzing the harmonic interaction between AC and DC in the flexible DC transmission system to obtain the harmonic components in the neutral line voltage; dynamically adjusting the dead time based on the change of the load condition to determine the initial dead time; optimizing the initial dead time through model predictive control based on the harmonic components to determine the target dead time. This method effectively suppresses the harmonics in the neutral line voltage by improving the adaptability of the dead time, significantly improves the system stability and power quality. It enables the power system to operate more stably and efficiently, reduces the impact of harmonics on power transmission, and improves the reliability and efficiency of the system. Description of the Drawings

[0019] Figure 1 is a flowchart of a dead-time optimization method for a flexible DC transmission system provided by an embodiment of the present specification; Figure 2 is an engineering topology diagram of a dead-time optimization method for a flexible DC transmission system provided by an embodiment of the present specification; Figure 3 is an overall flowchart of a dead-time optimization method for a flexible DC transmission system provided by an embodiment of the present specification; Figure 4 is an MMC control block diagram of a dead-time optimization method for a flexible DC transmission system provided by an embodiment of the present specification; Figure 5 is a schematic diagram of the harmonic transfer law between the AC and DC sides of an MMC of a dead-time optimization method for a flexible DC transmission system provided by an embodiment of the present specification; Figure 6 is a basic principle structure diagram of model predictive control of a dead-time optimization method for a flexible DC transmission system provided by an embodiment of the present specification; Figure 7 is a schematic structural diagram of a dead-time optimization device for a flexible DC transmission system provided by an embodiment of the present specification; Figure 8 It is a structural block diagram of a computing device provided by an embodiment of this specification. Specific embodiments

[0020] Numerous specific details are set forth in the following description in order to provide a thorough understanding of this specification. However, this specification can be implemented in many other ways different from those described herein, and those skilled in the art can make similar generalizations without departing from the connotation of this specification. Therefore, this specification is not limited by the specific embodiments disclosed below.

[0021] The terms used in one or more embodiments of this specification are for the purpose of describing specific embodiments only and are not intended to limit one or more embodiments of this specification. The singular forms "a" and "the" used in one or more embodiments of this specification and the appended claims are also intended to include the plural forms unless the context clearly dictates otherwise. It should also be understood that the term "and / or" used in one or more embodiments of this specification refers to and encompasses any and all possible combinations of one or more of the associated listed items.

[0022] It should be understood that although the terms first, second, etc. may be used in one or more embodiments of this specification to describe various information, such information should not be limited to these terms. These terms are only used to distinguish the same type of information from each other. For example, without departing from the scope of one or more embodiments of this specification, the first may also be referred to as the second, and similarly, the second may also be referred to as the first. Depending on the context, the word "if" as used herein can be interpreted as "when" or "upon" or "in response to determining".

[0023] First, the noun terms related to one or more embodiments of this specification are explained.

[0024] Deadband, sometimes also called neutral zone or no-action zone, refers to the range of input signals corresponding to zero output in the transfer function of a control system.

[0025] In this specification, a method for optimizing the dead-time of a flexible DC transmission system is provided. This specification also relates to a device for optimizing the dead-time of a flexible DC transmission system, a computing device, and a computer-readable storage medium, which will be described in detail one by one in the following embodiments.

[0026] See Figure 1 , Figure 1 shows a flowchart of a method for optimizing the dead-time of a flexible DC transmission system provided by an embodiment of this specification, which specifically includes the following steps.

[0027] Step 101: Establish a time-domain harmonic analysis model of the modular multilevel converter and establish an equivalent harmonic impedance model on the DC side of the flexible DC transmission system.

[0028] As Figure 2 shown, the system includes a DC voltage control module, a reactive power regulation module, an angular velocity integration module, and a current loop controller module. Electrical energy is derived from the three-phase AC power grid, converted into DC at the converter station, transmitted through the overhead line to the receiving point, and then converted back into AC at the other converter station and fed into the receiving-side AC power grid.

[0029] In a possible implementation, establishing a time-domain harmonic analysis model of the modular multilevel converter includes: determining the loop equations of the upper and lower bridge arms of the modular multilevel converter; determining the expressions of the AC-DC side dynamic characteristics based on the loop equations; determining the expressions of the capacitor dynamic characteristics based on the expressions of the AC-DC side dynamic characteristics; and determining the time-domain harmonic analysis model based on the expressions of the capacitor dynamic characteristics.

[0030] In practical applications, referring to Figure 3 , Step S1: Establish a time-domain harmonic analysis model of the modular multilevel converter (MMC).

[0031] Specifically, as Figure 4 shown in the MMC control block diagram implemented in the present invention, where u sa , u sb , u sc are the voltage sources of the a, b, and c phases of the AC system; u ca , u cb , u cc are the voltages of the a, b, and c phases on the AC side of the MMC; i a , i b , i c are the currents of the a, b, and c phases on the AC side; u pa , u pb , u pc and u na , u nb , u nc are respectively the sums of the voltages of the a, b, and c phases of the upper and lower arm sub-modules; i pa ,i pb , i pc and i na , i nb , i nc are the phase a, b, and c currents of the upper and lower bridge arms respectively; u dc and i dc are the voltage and current on the DC side respectively; L 0 and R 0 are the MMC bridge arm inductance and the bridge arm loss resistance respectively; R s is the equivalent resistance of the AC system; L s is the equivalent inductance of the AC system; R dc is the MMC DC side load resistance.

[0032] j Phase AC side current i j and the upper and lower bridge arm currents i pj , i nj satisfy the equation:

[0033] Writing the voltage loop equations for the upper and lower bridge arms of the MMC respectively, we have:

[0034] In the formula u o'o is the potential difference between the AC and DC neutral points; define u diffj and u comj are the differential mode voltage and the common mode voltage of the upper and lower bridge arms respectively; i dcj is the circulating current; their expressions are:

[0035] Obtain the expressions representing the dynamic characteristics of the MMC AC and DC sides:

[0036] Define the sum of the capacitor voltages of the upper and lower bridge arm sub-modules as u cpj , ucnj , an expression for characterizing the dynamic characteristics of the sub-module capacitor can be obtained:

[0037] In the formula, C is the sub-module capacitor; N is the number of sub-modules in the upper / lower arm; S j is j the average switching function of the upper arm of phase

[0038] In the formula, S jn is j the switching function of the n th sub-module in the upper arm of phase u nj , u pj and u cnj , u cpj The relationship between

[0039] The relationship between the DC current and the arm current is:

[0040] The time-domain state-space model of the MMC can be obtained: .

[0041] In a possible implementation, an equivalent harmonic impedance model of the DC side of the flexible DC transmission system is established, including: determining the common-mode voltage expression, and determining the relationship between the sub-module capacitor voltage and the DC side current; based on the common-mode voltage expression and the relationship between the sub-module capacitor voltage and the DC side current, determining the equivalent harmonic impedance model of the DC side.

[0042] In practical applications, refer to Figure 3 , step S2, establishing an equivalent harmonic impedance model of the DC side of the flexible DC transmission system.

[0043] Specifically, the common-mode voltage expression is:

[0044] The relationship between the sub-module capacitor voltage and the DC side current is:

[0045] Let the switching function , then Equation (7) can be written as:

[0046] Then the common-mode voltage can be expressed in terms of the sub-module capacitor voltages as:

[0047] Considering that the upper and lower bridge arm voltages are equal during steady-state operation, we have:

[0048] The differential equation of the DC-side voltage and current is obtained: .

[0049] Step 102: Based on the time-domain harmonic analysis model and the DC-side equivalent harmonic impedance model, analyze the harmonic interaction between the AC and DC in the flexible DC transmission system to obtain the harmonic components in the neutral line voltage.

[0050] In a possible implementation, based on the time-domain harmonic analysis model and the DC-side equivalent harmonic impedance model, analyze the harmonic interaction between the AC and DC in the flexible DC transmission system to obtain the harmonic components in the neutral line voltage, including: converting the time-domain harmonic analysis model by shifting the Fourier coefficients to determine the conversion model; separating the positive and negative sequences of the conversion model by the symmetrical component method to determine the sequence-shifted Fourier coefficient model; simplifying based on the sequence-shifted Fourier coefficient model to determine the simplified model; and analyzing based on the simplified model and the DC-side equivalent harmonic impedance model to obtain the harmonic components in the neutral line voltage.

[0051] In practical applications, referring to Figure 3 , Step S3: Combine the MMC time-domain harmonic analysis model with the DC-side equivalent harmonic impedance model to analyze the harmonic interaction between the AC and DC in the flexible DC transmission system to obtain the harmonic components in the neutral line voltage.

[0052] Specifically, convert the time-domain model of the MMC by shifting the Fourier coefficients, where R = R s + R 0 / 2, L = L s + L 0 / 2. We get:

[0053] Separate the positive and negative sequences of the above equation by the symmetrical component method to obtain the sequence-shifted Fourier coefficient model of the MMC:

[0054] wherein, is the m-th order Fourier coefficient of the DC interface voltage; is the m-th order Fourier coefficient of the DC interface current, and are the positive and negative sequence m-th order Fourier coefficients of the switching average function, and represent the positive and negative sequence m-th order Fourier coefficients of the AC interface voltage; and represent the positive and negative sequence m-th order Fourier coefficients of the AC interface current; Considering that the MMC is composed of a large number of sub-modules, and the switching frequency of each sub-module is relatively low, the harmonic components in the switching function are ignored, and a simplified sequence-shifting Fourier coefficient model of the MMC is obtained:

[0055] As Figure 5 shown by the harmonic transfer law of the AC and DC sides of the MMC in the embodiments of the present invention, the m-th harmonic on the AC side induces the (m + 1)-th and (m - 1)-th harmonics through the converter and conducts them to the DC side.

[0056] Step 103: Dynamically adjust the dead time based on the change of the load condition to determine the initial dead time.

[0057] In a possible implementation manner, dynamically adjusting the dead time based on the change of the load condition to determine the initial dead time includes: determining the load power based on the change of the load condition; shortening the dead time when the load power is less than the first rated power; keeping the dead time when the load power is less than the second rated power and greater than or equal to the first rated power; extending the dead time when the load power is greater than the second rated power; wherein, the second rated power is greater than the first rated power.

[0058] In practical applications, referring to Figure 3 , step S4, dynamically adjust the dead time according to the change of the load condition, shorten the dead time appropriately when the load is light; extend the dead time appropriately when the load is heavy, and set the dead time within a reasonable range.

[0059] Specifically, according to the changes in real-time load and harmonic suppression requirements, the dead time is dynamically adjusted. For the case of a lighter load, the dead time can be shortened to reduce the neutral line voltage harmonics caused by too long dead time; for a heavier load, the dead time is appropriately increased to ensure the stability of switch operation and prevent excessive switch losses or damage to switch devices. According to different ranges of the load, several intervals are preset, and each interval corresponds to a reasonable dead time. For specific embodiments, see Table 1: Table 1

[0060] Step 104: Optimize the initial dead time based on the harmonic components through model predictive control to determine the target dead time.

[0061] In a possible implementation manner, optimizing the initial dead time based on the harmonic components through model predictive control to determine the target dead time includes: in the case where the harmonic components are greater than the harmonic threshold, optimizing the initial dead time through model predictive control to determine the target dead time; in the case where the harmonic components are less than or equal to the harmonic threshold, taking the initial dead time as the target dead time.

[0062] Further, in the case where the harmonic components are greater than the harmonic threshold, optimizing the initial dead time through model predictive control to determine the target dead time includes: determining the cost function of the initial dead time; performing linear programming based on the cost function to determine the linearized cost function; solving based on the linearized cost function to determine the optimization result; selecting the dead time at the current moment from the optimization result as the target dead time.

[0063] In practical applications, see Figure 3 , Step S5: If the harmonic content of the system neutral line voltage satisfies less than or equal to 3%, it is determined that the system meets the requirements, and there is no need to adjust the dead time, and enter Step S7; if the harmonic content of the system neutral line voltage satisfies greater than 3%, it is determined that the system does not meet the requirements, and enter Step S6; Step S6: According to the harmonic content of the neutral line voltage, combined with model predictive control, further optimize the dead time through linear programming, set it to the optimal value suitable for harmonic suppression, and adjust the dead time threshold based on the system model prediction of future errors and their impacts; Step S7: The system uses the dead time obtained in Step S4 for control.

[0064] Specifically, such as Figure 6As shown in the basic principle structure diagram of the model predictive control implemented in the present invention, the dead time is optimized by predicting the harmonic content of the neutral line voltage in the future state of the flexible DC system, and the cost function takes into account the suppression of the harmonic of the neutral line voltage. Under the framework of model predictive control, the objective function should include the following factors: Harmonic suppression: Minimize the harmonic content caused by the dead time; Dead time control: Adjust the dead time to reduce the generation of high-frequency harmonics; Power quality: Ensure that the voltage and current waveforms of the system are as close as possible to the ideal sine wave. This control sequence will determine the dead time at the future moment, thereby optimizing the dynamic response and harmonic suppression of the system. At each moment k , calculate the harmonic content of the current voltage and make predictions. Construct an optimization problem to minimize the objective function J :

[0065] where is the harmonic content at the k th moment, usually by extracting the harmonic components of the neutral line voltage. is the change in the dead time, representing the deviation between the dead time and the current optimal value. , , is the weight coefficient in the objective function and can be adjusted according to actual needs. is the power error in the flexible DC system, representing the deviation from the reference power waveform.

[0066] The purpose of this objective function is to minimize the harmonic components in the system, while ensuring that the impact of the dead time on power quality is minimized and the system can operate stably.

[0067] In step 6, the linear programming specifically refers to: In each control period, the model predictive control will calculate an optimal control sequence based on the current system state and future prediction information.

[0068] Set the cost function of the model predictive control J as the weighted sum of multiple indicators including harmonic content, dead time, and power quality. Minimize the harmonics caused by the dead time. For linearization, it can be set that the harmonic amplitude is linearly related to the dead time. The linear relationship can be approximated as:

[0069] where a 1 and b 1 are constants, representing the impact of the dead time on the harmonics. To control the fluctuation of the dead time, the change in the dead time can be used as part of the objective, using a linear expression:

[0070] Among them a 2 and b 2 are constants, representing the influence of power fluctuations on harmonics. Assume that the power error is linearly related to the dead time:

[0071] Taking these three parts into comprehensive consideration, the objective function J can be linearized as:

[0072] From the results of the optimization solution, select the dead time at the current moment and apply it to the system. After that, the system will continue to execute the optimization process based on the new state feedback. After each control cycle ends, update the objective function and the optimization problem according to the feedback information of the change in the harmonic content of the system. In this way, the system will readjust the dead time in each cycle to ensure that it can always adapt to the current system state and effectively suppress harmonics.

[0073] The embodiments of this specification provide a method and device for optimizing the dead time of a flexible DC transmission system, including: establishing a time-domain harmonic analysis model of a modular multilevel converter, and establishing an equivalent harmonic impedance model of the DC side of the flexible DC transmission system; based on the time-domain harmonic analysis model and the equivalent harmonic impedance model of the DC side, analyzing the harmonic interaction between the AC and DC in the flexible DC transmission system to obtain the harmonic components in the neutral line voltage; dynamically adjusting the dead time based on the change of the load condition to determine the initial dead time; optimizing the initial dead time through model predictive control based on the harmonic components to determine the target dead time. By real-time monitoring the load and harmonic changes, dynamically adjusting the dead time according to the load change, and combining model predictive control and linear programming to optimize the system performance. This method effectively suppresses the harmonics in the neutral line voltage by improving the adaptability of the dead time, significantly improves the system stability and power quality. It can enable the power system to operate more stably and efficiently, reduce the influence of harmonics on power transmission, and improve the reliability and efficiency of the system.

[0074] Corresponding to the above method embodiments, this specification also provides embodiments of a device for optimizing the dead time of a flexible DC transmission system, Figure 7 showing a schematic structural diagram of a device for optimizing the dead time of a flexible DC transmission system provided by an embodiment of this specification. As Figure 7 shown, this device includes: A model establishment module 701, configured to establish a time-domain harmonic analysis model of a modular multilevel converter and establish an equivalent harmonic impedance model of the DC side of the flexible DC transmission system; The harmonic component module 702 is configured to analyze the harmonic interaction between the AC and DC sides in the flexible DC transmission system based on the time-domain harmonic analysis model and the DC-side equivalent harmonic impedance model, and obtain the harmonic components in the neutral line voltage; The dead-time adjustment module 703 is configured to dynamically adjust the dead time based on the change of the load condition and determine the initial dead time; The dead-time optimization module 704 is configured to optimize the initial dead time through model predictive control based on the harmonic components and determine the target dead time.

[0075] In a possible implementation, establishing a time-domain harmonic analysis model of the modular multilevel converter includes: Determining the loop equations of the upper and lower bridge arms of the modular multilevel converter; Based on the loop equations, determining the expressions of the AC-DC side dynamic characteristics; Based on the expressions of the AC-DC side dynamic characteristics, determining the expressions of the capacitor dynamic characteristics; Based on the expressions of the capacitor dynamic characteristics, determining the time-domain harmonic analysis model.

[0076] In a possible implementation, establishing a DC-side equivalent harmonic impedance model of the flexible DC transmission system includes: Determining the common-mode voltage expression and the relationship between the capacitor voltage of the sub-module and the DC-side current; Based on the common-mode voltage expression and the relationship between the capacitor voltage of the sub-module and the DC-side current, determining the DC-side equivalent harmonic impedance model.

[0077] In a possible implementation, analyzing the harmonic interaction between the AC and DC sides in the flexible DC transmission system based on the time-domain harmonic analysis model and the DC-side equivalent harmonic impedance model, and obtaining the harmonic components in the neutral line voltage, includes: Converting the time-domain harmonic analysis model by moving the Fourier coefficients to determine the conversion model; Separating the positive and negative sequences of the conversion model by the symmetrical component method to determine the sequence-moving Fourier coefficient model; Simplifying based on the sequence-moving Fourier coefficient model to determine the simplified model; Analyzing based on the simplified model and the DC-side equivalent harmonic impedance model to obtain the harmonic components in the neutral line voltage.

[0078] In a possible implementation, dynamically adjusting the dead time based on the change of the load condition and determining the initial dead time includes: Determining the load power based on the change of the load condition; When the load power is less than the first rated power, shortening the dead time; When the load power is less than the second rated power and greater than or equal to the first rated power, the dead time is maintained; When the load power is greater than the second rated power, the dead time is extended; Among them, the second rated power is greater than the first rated power.

[0079] In a possible implementation manner, based on the harmonic components, the initial dead time is optimized through model predictive control to determine the target dead time, including: When the harmonic components are greater than the harmonic threshold, the initial dead time is optimized through model predictive control to determine the target dead time; When the harmonic components are less than or equal to the harmonic threshold, the initial dead time is used as the target dead time.

[0080] In a possible implementation manner, when the harmonic components are greater than the harmonic threshold, the initial dead time is optimized through model predictive control to determine the target dead time, including: Determine the cost function of the initial dead time; Based on the cost function, perform linear programming to determine the linearized cost function; Based on the linearized cost function, perform a solution to determine the optimization result; Select the dead time at the current moment from the optimization result as the target dead time.

[0081] The embodiments of this specification provide a method and device for optimizing the dead time of a flexible DC transmission system, including: establishing a time-domain harmonic analysis model of a modular multilevel converter, and establishing an equivalent harmonic impedance model of the DC side of the flexible DC transmission system; based on the time-domain harmonic analysis model and the equivalent harmonic impedance model of the DC side, analyze the harmonic interaction between the AC and DC in the flexible DC transmission system to obtain the harmonic components in the neutral line voltage; dynamically adjust the dead time based on the change of the load condition to determine the initial dead time; based on the harmonic components, optimize the initial dead time through model predictive control to determine the target dead time. This method effectively suppresses the harmonics in the neutral line voltage by improving the adaptability of the dead time, significantly improves the system stability and power quality. It enables the power system to operate more stably and efficiently, reduces the impact of harmonics on power transmission, and improves the reliability and efficiency of the system.

[0082] The above is a schematic solution of a dead-time optimization device for a flexible DC transmission system according to this embodiment. It should be noted that the technical solution of the dead-time optimization device for the flexible DC transmission system and the technical solution of the above-mentioned dead-time optimization method for the flexible DC transmission system belong to the same concept. For the details not described in the technical solution of the dead-time optimization device for the flexible DC transmission system, reference can be made to the description of the technical solution of the dead-time optimization method for the flexible DC transmission system.

[0083] Figure 8 FIG. shows a structural block diagram of a computing device 800 according to an embodiment of the present specification. The components of the computing device 800 include, but are not limited to, a memory 810 and a processor 820. The processor 820 is connected to the memory 810 through a bus 830, and a database 850 is used to store data.

[0084] The computing device 800 further includes an access device 840, which enables the computing device 800 to communicate via one or more networks 860. Examples of these networks include the Public Switched Telephone Network (PSTN), Local Area Network (LAN), Wide Area Network (WAN), Personal Area Network (PAN), or a combination of communication networks such as the Internet. The access device 840 may include one or more of any type of wired or wireless network interfaces (e.g., a network interface card (NIC)), such as an IEEE 802.11 Wireless Local Area Network (WLAN) wireless interface, a Worldwide Interoperability for Microwave Access (Wi-MAX) interface, an Ethernet interface, a Universal Serial Bus (USB) interface, a cellular network interface, a Bluetooth interface, a Near Field Communication (NFC).

[0085] In an embodiment of the present specification, the above components of the computing device 800 and Figure 8 other components not shown in the figure may also be connected to each other, for example, through a bus. It should be understood that Figure 8 the shown structural block diagram of the computing device is only for illustrative purposes and is not a limitation on the scope of the present specification. Those skilled in the art can add or replace other components as needed.

[0086] The computing device 800 can be any type of stationary or mobile computing device, including a mobile computer or mobile computing device (e.g., a tablet computer, personal digital assistant, laptop computer, notebook computer, netbook, etc.), a mobile phone (e.g., a smartphone), a wearable computing device (e.g., a smartwatch, smart glasses, etc.) or other types of mobile devices, or a stationary computing device such as a desktop computer or personal computer (PC). The computing device 800 can also be a mobile or stationary server.

[0087] Among them, the processor 820 is used to execute the following computer-executable instructions, and when the computer-executable instructions are executed by the processor, the steps of the above-mentioned flexible DC transmission system dead-time optimization method are implemented. The above is a schematic solution of a computing device in this embodiment. It should be noted that the technical solution of this computing device and the technical solution of the above-mentioned flexible DC transmission system dead-time optimization method belong to the same concept. For the details not described in detail in the technical solution of the computing device, reference can be made to the description of the technical solution of the above-mentioned flexible DC transmission system dead-time optimization method.

[0088] An embodiment of this specification also provides a computer-readable storage medium, which stores computer-executable instructions, and when the computer-executable instructions are executed by a processor, the steps of the above-mentioned flexible DC transmission system dead-time optimization method are implemented.

[0089] The above is a schematic solution of a computer-readable storage medium in this embodiment. It should be noted that the technical solution of this storage medium and the technical solution of the above-mentioned flexible DC transmission system dead-time optimization method belong to the same concept. For the details not described in detail in the technical solution of the storage medium, reference can be made to the description of the technical solution of the above-mentioned flexible DC transmission system dead-time optimization method.

[0090] An embodiment of this specification also provides a computer program, wherein when the computer program is executed on a computer, the computer is made to execute the steps of the above-mentioned flexible DC transmission system dead-time optimization method.

[0091] The above is a schematic solution of a computer program in this embodiment. It should be noted that the technical solution of this computer program and the technical solution of the above-mentioned flexible DC transmission system dead-time optimization method belong to the same concept. For the details not described in detail in the technical solution of the computer program, reference can be made to the description of the technical solution of the above-mentioned flexible DC transmission system dead-time optimization method.

[0092] The above describes specific embodiments of this specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recited in the claims may be performed in a different order than in the embodiments and still achieve the desired result. Additionally, the processes depicted in the drawings do not necessarily require the particular order or sequential order shown to achieve the desired result. In certain embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0093] The computer instructions include computer program code, which may be in the form of source code, object code, executable files, or some intermediate form, etc. The computer-readable medium may include: any entity or device capable of carrying the computer program code, recording medium, USB flash drive, mobile hard disk, magnetic disk, optical disc, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signals, telecommunication signals, and software distribution media, etc. It should be noted that the content included in the computer-readable medium can be appropriately increased or decreased according to the requirements of legislation and patent practice in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practice, the computer-readable medium does not include electrical carrier signals and telecommunication signals.

[0094] It should be noted that for the foregoing method embodiments, for the sake of simplicity of description, they are all expressed as a series of action combinations. However, those skilled in the art should know that the embodiments of this specification are not limited by the described order of actions, because according to the embodiments of this specification, certain steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also know that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily essential to the embodiments of this specification.

[0095] In the above embodiments, the descriptions of each embodiment have their own focuses. For the parts not detailed in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0096] The preferred embodiments of this specification disclosed above are only used to help explain this specification. The alternative embodiments do not elaborate on all the details and do not limit the invention to only the specific embodiments described. Obviously, many modifications and variations can be made according to the content of the embodiments of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the embodiments of this specification, so that those skilled in the art can understand and utilize this specification well. This specification is only limited by the claims and their full scope and equivalents.

Claims

1. A method for optimizing dead time of a flexible direct current transmission system, characterized in that: include: Establish a time-domain harmonic analysis model for modular multilevel converters and an equivalent harmonic impedance model for the DC side of a flexible DC transmission system; Based on the time domain harmonic analysis model and the DC side equivalent harmonic impedance model, the harmonic interaction between AC and DC in the flexible DC power transmission system is analyzed to obtain the harmonic component in the neutral line voltage; Dynamically adjust the dead time based on changes in load conditions and determine the initial dead time; The initial dead time is optimized by model predictive control based on the harmonic component to determine a target dead time.

2. The method according to claim 1, characterized in that The method of establishing a time domain harmonic analysis model of a modular multilevel converter comprises: Determining loop equations of an upper bridge arm and a lower bridge arm of the modular multilevel converter; Determine expressions for dynamic characteristics of the AC and DC sides based on the loop equations; Determining an expression of capacitor dynamic characteristics based on the expression of AC and DC side dynamic characteristics; A time domain harmonic analysis model is determined based on the expression of the dynamic characteristics of the capacitor.

3. The method according to claim 1, characterized in that The method of establishing a DC side equivalent harmonic impedance model of the flexible DC power transmission system includes: Determine the common mode voltage expression and the relationship between the submodule capacitor voltage and the DC side current; The DC side equivalent harmonic impedance model is determined based on the common mode voltage expression and the relationship between the submodule capacitor voltage and the DC side current.

4. The method according to claim 1, characterized in that: The harmonic interaction between AC and DC in the flexible DC power transmission system is analyzed based on the time domain harmonic analysis model and the DC side equivalent harmonic impedance model to obtain the harmonic components in the neutral line voltage, including: The time domain harmonic analysis model is transformed by shifting Fourier coefficients to determine a transformation model; The conversion model is separated into positive and negative sequences by a symmetric component method to determine a sequence-shifted Fourier coefficient model; Simplify based on the sequential moving Fourier coefficient model to determine a simplified model; An analysis is performed based on the simplified model and the DC side equivalent harmonic impedance model to obtain the harmonic components in the neutral line voltage.

5. The method according to claim 1, characterized in that The dynamically adjusting the dead time based on the change of the load condition and determining the initial dead time includes: determining load power based on changes in load conditions; When the load power is less than the first rated power, shortening the dead time; When the load power is less than the second rated power and greater than or equal to the first rated power, maintaining the dead time; When the load power is greater than the second rated power, extending the dead time; Wherein, the second rated power is greater than the first rated power.

6. The method according to claim 1, characterized in that The step of optimizing the initial dead time by model predictive control based on the harmonic component to determine the target dead time includes: When the harmonic component is greater than the harmonic threshold, optimizing the initial dead time through model predictive control to determine a target dead time; When the harmonic component is less than or equal to the harmonic threshold, the initial dead time is used as the target dead time.

7. The method according to claim 6, characterized in that When the harmonic component is greater than the harmonic threshold, optimizing the initial dead time by model predictive control to determine the target dead time includes: Determining a cost function of the initial dead time; Perform linear programming based on the cost function to determine a linearized cost function; Solving based on the linearized cost function to determine an optimization result; The dead time at the current moment is selected from the optimization results as the target dead time.

8. A device for optimizing dead time of a flexible direct current transmission system, characterized in that: include: A model building module is configured to build a time domain harmonic analysis model of a modular multilevel converter and a DC side equivalent harmonic impedance model of a flexible DC transmission system; A harmonic component module is configured to analyze the harmonic interaction between AC and DC in the flexible DC power transmission system based on the time domain harmonic analysis model and the DC side equivalent harmonic impedance model to obtain the harmonic component in the neutral line voltage; A dead time adjustment module is configured to dynamically adjust the dead time based on changes in load conditions and determine an initial dead time; The dead time optimization module is configured to optimize the initial dead time through model predictive control based on the harmonic component to determine a target dead time.

9. A computing device, characterized in that include: Memory and processor; The memory is used to store computer executable instructions, and the processor is used to execute the computer executable instructions. When the computer executable instructions are executed by the processor, the steps of the method for optimizing the dead time of a flexible direct current transmission system according to any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium storing computer-executable instructions, which, when executed by a processor, implement the steps of the method for optimizing the dead time of a flexible direct current transmission system according to any one of claims 1 to 7.

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