Hybrid cascaded dc power transmission power transfer capability optimization control method and system

Abstract

The application relates to a hybrid cascade HVDC power transmission project power transfer capacity optimization control method and system, which comprises the following steps: calculating a steady-state modulation ratio, judging whether the operation mode of a flexible DC converter is single-pole half-voltage operation according to the steady-state modulation ratio, and selecting the maximum modulation ratio according to the judgment result; judging whether the steady-state modulation ratio is greater than the maximum modulation ratio, raising the converter gear position if yes, and maintaining the original gear position if no; calculating the modulation ratio of a DC control system, judging whether to inject the third harmonic according to the modulation ratio of the DC control system; when a fault occurs, the maximum power under the voltage reduction capability limitation of the flexible DC converter is obtained, and the power of the DC control system after the fault is calculated according to the maximum power; the power instruction value is updated according to the power of the DC control system after the fault, the flexible DC converter adjusts the DC voltage according to the power instruction value, and the adjustment result is fed back to an automatic control device to execute generator tripping. The application can improve the power transfer capacity of the hybrid cascade HVDC power transmission project, and improve the reliability of system operation.

Description

TECHNICAL FIELD

[0001] The application relates to a hybrid cascaded DC power transmission project power transfer capacity optimization control method and system, and belongs to the field of DC power transmission, in particular to the optimization technical field of a hybrid UHV DC power transmission system. BACKGROUND

[0002] In China, energy resources and load centers are inversely distributed, and the construction of a UHV backbone power grid needs to be steadily promoted to exert the transmission capacity of the UHV power grid. In some load centers in China, the demand for DC power feeding is increasing, but the dense feeding of external DC power will cause the electrical distance between the converter stations to decrease, the multi-feeding short-circuit ratio to decrease, and the risk of simultaneous commutation failure of multiple DC power to gradually increase, so that the power grid faces serious security and stability problems. In order to realize long-distance and large-capacity power transmission and multi-drop power supply and solve the problem of the decrease of the multi-feeding short-circuit ratio at the receiving end, a hybrid cascaded UHV DC power transmission technology can be used, that is, a technical scheme in which a conventional DC converter and multiple flexible DC converters are connected in cascade. The technology combines the advantages of the conventional DC and the flexible DC converter, can effectively improve the stability of the receiving end AC power grid, has high reliability, flexible operation mode, and wide application prospect, and is a key technology for building a future energy internet.

[0003] When a single pole or a valve group fault occurs in a UHV DC project, in order to reduce the power loss as much as possible, the power of the healthy pole and the healthy valve group needs to be rapidly increased as much as possible to transfer the power of the fault pole or the fault valve group. At this time, the DC current needs to be rapidly increased, so that the line voltage drop will rapidly increase, and the voltage rising capacity of the sending end is very limited, so the power transfer mainly depends on the rapid reduction of the voltage of the receiving end valve group. Since the low-voltage end of the hybrid is a half-bridge type modular multilevel converter, when the converter rapidly reduces the voltage, the original AC voltage output needs to be maintained by rapidly increasing the modulation ratio. If the rapid reduction amplitude is small, the modulation ratio will reach the maximum adjustment range, and a series of consequences such as output voltage "top cutting" distortion, controller saturation, converter out of control and the like will be caused, and the rapid reduction capacity of the half-bridge type modular multilevel converter is limited. Therefore, for a hybrid cascaded UHV DC power transmission project, if special design and control measures are not taken, the power transfer capacity of the healthy pole and the healthy valve group when a single pole or a single valve group fault occurs will be severely limited, and even the healthy pole or the healthy valve group may not only not be able to transfer power, but also need to reduce its own power, which will seriously reduce the reliability of the hybrid cascaded UHV DC power transmission project. SUMMARY

[0004] In view of the above problems, the application aims to provide a hybrid cascaded DC power transmission project power transfer capacity optimization control method and system, which can improve the power transfer capacity of the hybrid cascaded UHV DC power transmission project, thereby reducing the power loss of the system when a pole or a valve group fails and improving the reliability of the system operation.

[0005] In order to achieve the above object, the present application provides the following technical scheme: a hybrid cascade HVDC power transmission project power transfer capacity optimization control method, comprising: calculating a steady-state modulation ratio, judging whether the operation mode of the flexible DC converter is single-pole half-voltage operation according to the steady-state modulation ratio, and selecting the maximum modulation ratio according to the judgment result; judging whether the steady-state modulation ratio is greater than the maximum modulation ratio, and if so, increasing the converter transformer gear, and if not, maintaining the original gear unchanged; the DC control system calculates the modulation ratio in real time, judges whether to inject the third harmonic according to the modulation ratio of the DC control system; when a fault occurs, the maximum power under the voltage reduction capability limitation of the flexible DC converter is obtained, and the power of the DC control system after the fault is calculated according to the maximum power; the power instruction value is updated according to the power of the DC control system after the fault, the flexible DC converter adjusts the DC voltage according to the power instruction value, and feeds back to the protection and control device to execute generator tripping.

[0006] Further, the calculation formula of the modulation ratio is:

[0007]

[0008] Wherein, u 1p is the fundamental wave modulation peak value generated by the converter; U dc is the DC rated DC voltage, and U dc .

[0009] Further, if the operation mode of the flexible DC converter is not single-pole half-voltage operation, the first maximum modulation ratio is selected as the maximum modulation ratio, indicating that the rapid voltage reduction capability of the flexible DC converter needs to be considered; if the operation mode of the flexible DC converter is single-pole half-voltage operation, the second maximum modulation ratio is selected as the maximum modulation ratio, indicating that the rapid voltage reduction capability of the flexible DC converter does not need to be considered.

[0010] Further, the method for judging whether to inject the third harmonic according to the modulation ratio of the DC control system is: judging whether the modulation ratio of the DC control system is greater than the third maximum modulation ratio, if so, injecting the third harmonic, and if not, not injecting the third harmonic.

[0011] Further, the calculation method of the power of the DC control system after the fault is: after the fault occurs, judging whether the to-be-calculated pole contains the flexible DC converter; if so, determining the calculation parameters, calculating the first maximum power under the voltage reduction capability limitation of the flexible DC converter, comparing the first maximum power and the second maximum power, and the power of the DC control system after the fault is the smaller one of the two; updating the power instruction value, the flexible DC converter adjusts the DC voltage according to the power instruction value, and feeds back to the protection and control device to execute generator tripping; if not, the power of the DC control system after the fault is the second maximum power, which is directly fed back to the protection and control device to execute generator tripping.

[0012] Further, the calculation formula of the first maximum power of bipolar symmetric, unipolar ground return and unipolar metal return after fault is:

[0013]

[0014] Wherein, P max1 is the first maximum power; U dcrec is the DC voltage of the sending end of the pole to be calculated after fault; k is the operation mode coefficient, k is 2 when the pole is full voltage operation, and k is 1 when the pole is half voltage operation; R p is the loop resistance of the pole to be calculated; U dcvsc0 is the DC voltage of the flexible DC converter before fault; M0 is the current modulation ratio; M maxabs is the maximum modulation ratio to ensure that the flexible DC converter does not overmodulate.

[0015] Further, the calculation formula of bipolar asymmetric operation after fault is:

[0016]

[0017] Wherein, R e is the sum of the sending and receiving end grounding pole line resistance and grounding pole resistance; R b is the resistance of a single pole line; I dcop is the operating current of the other pole except the pole to be calculated when bipolar asymmetric operation.

[0018] Further, the loop resistance R p of the pole to be calculated is R b when bipolar symmetric operation; the loop resistance R p of the pole to be calculated is the sum of the pole line resistance, the sending and receiving end grounding pole line and the grounding pole resistance when unipolar ground return operation; the loop resistance R p of the pole to be calculated is equal to 2R b when unipolar metal return operation.

[0019] The application discloses a kind of hybrid cascade DC power transmission engineering power transfer belt capacity optimization control systems, comprising: steady-state modulation ratio calculation module, for calculating steady-state modulation ratio, according to steady-state modulation ratio whether the operation mode of flexible DC converter is single pole half voltage operation is judged, according to the maximum modulation ratio of the selection result;Modulation ratio adjustment module, for judging whether steady-state modulation ratio is greater than the maximum modulation ratio, if yes, raise converter gear, if no, maintain original gear unchanged;Third harmonic injection module, for calculating the modulation ratio of DC control system, according to the modulation ratio of DC control system whether to inject third harmonic is judged;Maximum power calculation module, for obtaining the maximum power under the voltage reduction capability limit of flexible DC converter when fault occurs, and calculating the power of DC control system after fault according to the maximum power;Update execution module, for updating power instruction value according to the power of DC control system after fault, and the DC voltage is adjusted according to the power instruction value, and feedback to the safety control device to execute machine.

[0020] Further, the calculation method of the power of DC control system after fault is: after fault occurs, whether the to-be-calculated pole contains flexible DC converter is judged;If yes, the first maximum power is obtained according to the maximum power table under different faults preset in DC control system, the first maximum power and the second maximum power are compared, and the power of DC control system after fault is the smaller one of the two;The power instruction value is updated, and the DC voltage is adjusted according to the power instruction value, and feedback to the safety control device to execute machine;If no, the power of DC control system after fault is the second maximum power, and directly feedback to the safety control device to execute machine.

[0021] The application has the following advantages due to the above technical scheme:

[0022] 1、The scheme of the application can improve the power transfer belt capacity of hybrid cascade UHV DC power transmission engineering by flexibly adjusting the modulation ratio according to the operation mode and calculating the maximum power under the modulation ratio limit in real time, thereby reducing the power loss of the system when the pole or valve group exits and improving the reliability of system operation.

[0023] 2、The scheme of the application can realize full utilization of sub-module level and full improvement of DC voltage utilization rate by flexibly selecting whether to inject third harmonic according to the real-time calculation result of modulation ratio, and avoid over-modulation of flexible DC converter. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is a hybrid cascade UHV control system topology structure in an embodiment of the application;

[0025] Figure 2 It is a modulation ratio control strategy flow chart when steady-state running in an embodiment of the application;

[0026] Figure 3 is a flow chart of the third harmonic injection dynamic switching strategy in an embodiment of the present application;

[0027] Figure 4 is a flow chart of the online calculation of the power transfer control when a transient occurs in an embodiment of the present application;

[0028] Figure 5 is a flow chart of the offline calculation of the power transfer control when a transient occurs in an embodiment of the present application. DETAILED DESCRIPTION

[0029] For the skilled in the art to better understand the technical direction of the present application, the present application is described in detail through specific embodiments. However, it should be understood that the specific embodiments are provided only for better understanding of the present application, and they should not be understood as limiting the present application. In the description of the present application, it is understood that the terms used are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0030] Based on the existing hybrid cascade UHV DC transmission project, the ability of the healthy pole and the healthy valve group to transfer power when a single pole or a single valve group exits will be severely limited, which seriously reduces the reliability of the hybrid cascade UHV DC transmission project. The present application provides a hybrid cascade DC transmission project power transfer capacity optimization control method and system, which controls the steady-state modulation ratio range through the operation mode of the flexible DC converter needing to be quickly stepped down after a fault; the DC control system sets the dynamic switching strategy of the third harmonic injection according to the size of the modulation ratio; after a fault occurs, the transfer power capacity limit P max1 is calculated according to the step-down capacity limit of the flexible DC converter; the DC control system determines the power of the DC control system after a fault in combination with the maximum power P max2 under steady-state conditions; finally, the power instruction of the DC control system is adjusted, and the DC voltage reference value of the flexible DC converter after a fault is adjusted according to the power instruction, and the maximum operating power of the system at this time is fed back to the protection control system to execute generator tripping. This scheme can improve the power transfer capacity of the hybrid cascade UHV DC transmission project, thereby reducing the power loss of the system when a pole or a valve group exits, and improving the reliability of the system operation. The scheme of the present application will be described in detail through embodiments in combination with the accompanying drawings.

[0031] Embodiment one

[0032] Figure 1 is a hybrid cascade UHV control system topology structure diagram in this embodiment, as shown in Figure 1As shown, the sending end of the hybrid cascaded UHVDC power transmission system adopts a UHVDC topology structure, including a plurality of poles, each of which is formed by cascading two twelve-pulse conventional DC converters; the receiving end adopts a hybrid cascaded UHVDC topology structure, each pole being formed by cascading a twelve-pulse DC converter at a high-voltage end and a plurality of parallel flexible DC converters at a low-voltage end, the flexible DC converters adopting half-bridge modular multilevel converters, and the conventional DC converters at the receiving end and each flexible DC converter being fed into different AC buses.

[0033] The power transfer capability optimization control method of the hybrid cascaded DC power transmission project in the embodiment is used for optimizing the power transfer capability of the hybrid cascaded UHVDC control system. Figure 1 The power transfer capability optimization control method of the hybrid cascaded DC power transmission project in the embodiment is used for optimizing the power transfer capability of the hybrid cascaded UHVDC control system.

[0034] S1, a steady-state modulation ratio is calculated, whether the operation mode of the flexible DC converter is single-pole half-voltage operation is judged according to the steady-state modulation ratio, and the largest modulation ratio is selected according to the judgment result.

[0035] The calculation formula of the modulation ratio in the embodiment is as follows:

[0036]

[0037] Wherein, u 1p is the peak value of the fundamental modulation wave generated by the converter; U dc is the DC rated DC voltage, and U dc = 800 kV. It should be noted that the steady-state modulation ratio in this step and the modulation ratio of the DC control system in step S3 are not the same in value, that is, they are different modulation ratios, but the calculation formulas of the two are the same, only the specific parameter values in the formulas are different.

[0038] Figure 2 is the flow chart of the modulation ratio control strategy in the steady-state operation in the embodiment, and the corresponding steps are step S1 and step S2, as shown in Figure 2 Whether the operation mode of the flexible DC converter is single-pole half-voltage operation is judged according to the steady-state modulation ratio, single-pole half-voltage operation includes single-pole half-voltage ground return and single-pole half-voltage metal return, if the operation mode of the flexible DC converter is not single-pole half-voltage operation, the current operation mode is five types of double-pole full-voltage operation, double-pole half-voltage operation, double-pole mixed-voltage operation, single-pole full-voltage ground return operation, and single-pole full-voltage metal return operation, if a fault occurs, the flexible DC converter is quickly depressurized to perform power transfer, at this time, the first maximum modulation ratio M max1 is selected as the largest modulation ratio. Since compared with a pure flexible DC system, the first maximum modulation ratio M max1 should be lower, in the embodiment, the first maximum modulation ratio M max1 is preferably 0.9.

[0039] If the operation mode of the flexible DC converter is single-pole half-voltage operation, the reactive power transfer after the fault can not consider the rapid voltage reduction capability of the flexible DC, and the second maximum modulation M max2 is selected The second maximum modulation ratio M max2 is preferably 1.05, but the values of the first maximum modulation ratio M max1 and the second maximum modulation ratio M max2 need to be confirmed according to the actual situation, and are not limited to the values given in the embodiment.

[0040] S2 determines whether the steady-state modulation ratio is greater than the maximum modulation ratio. If yes, the converter transformer gear is raised, and if no, the original gear is maintained unchanged.

[0041] For the operation mode of the flexible DC converter being bipolar full-voltage operation, bipolar half-voltage operation, bipolar mixed-voltage operation, single-pole full-voltage ground return operation, and single-pole full-voltage metal return operation, if the steady-state modulation ratio is greater than the first maximum modulation M max1 , the converter transformer gear is raised until the steady-state modulation ratio is reduced to the first maximum modulation M max1 .

[0042] For the operation mode of the flexible DC converter being single-pole half-voltage operation, if the steady-state modulation ratio is greater than the second maximum modulation M max2 , the converter transformer gear is raised until the steady-state modulation ratio is reduced to the second maximum modulation M max2 .

[0043] The fundamental modulation wave peak value is u 1p , the DC rated DC voltage is U dc , the modulation ratio M = 2u 1p / U dc ; the DC control system identifies the system operation mode. For the five types of bipolar full-voltage operation, bipolar half-voltage operation, bipolar mixed-voltage operation, single-pole full-voltage ground return operation, and single-pole full-voltage metal return operation, when the modulation ratio > M max1 , the converter transformer gear is raised until the modulation ratio is limited to M max1 , and compared with the pure flexible DC project, M max1 needs to be reduced to a lower level, and 0.9 is recommended; for the two types of single-pole half-voltage ground return and single-pole half-voltage metal return, when the modulation ratio > M max2 , the converter transformer gear is raised until the modulation ratio is limited to M max2 , and M max2 is recommended to be 1.05.

[0044] S3 calculates the modulation ratio of the DC control system, and determines whether to inject the third harmonic according to the modulation ratio of the DC control system.

[0045] Figure 3 This is a flowchart of the third harmonic injection dynamic switching strategy in this embodiment, as follows: Figure 3 As shown, the method for determining whether to inject a third harmonic based on the modulation ratio of the DC control system is as follows: determine whether the modulation ratio of the DC control system is greater than the third maximum modulation ratio M. max3 If so, the third harmonic is injected; otherwise, it is not. Injecting the third harmonic allows for full utilization of the submodule's voltage levels and a significant improvement in DC voltage utilization, while preventing overmodulation of the flexible DC converter. The third maximum modulation ratio M... max3 The maximum modulation ratio of the flexible DC converter without the addition of third harmonics needs to be considered, and a certain margin is required. In this embodiment, it is preferably 0.95, but it is not limited to this.

[0046] When a fault occurs, S4 obtains the maximum power under the step-down capability limit of the flexible DC converter and calculates the power of the DC control system after the fault based on the maximum power.

[0047] The power of the DC control system after a fault can be obtained through either online calculation or offline calculation. The two methods are explained below.

[0048] Figure 4 This is a flowchart of the online calculation of power transfer control during transient events in this embodiment, as shown below. Figure 4 As shown, the method for calculating the power of the DC control system after a fault is as follows:

[0049] After a fault occurs, determine whether the pole to be calculated includes a flexible DC-DC converter; if so, determine the calculation parameters and calculate the first maximum power P under the voltage reduction capability limitation of the flexible DC-DC converter. max1 First maximum power P max1 The calculation method is as follows: First, set the parameters, P max1 The first maximum power; U dcrec The DC voltage at the electrode sending end to be calculated after the fault; k is the operating mode coefficient, which is 2 for full-voltage operation and 1 for half-voltage operation; R p U is the loop resistance of the pole to be calculated; dcvsc0 M0 represents the DC voltage of the flexible DC converter before the fault; M0 represents the current modulation ratio; M maxabs To ensure the maximum modulation ratio for over-modulation operation of the flexible DC converter, in this embodiment, M maxabs The preferred value is 1.05. During bipolar symmetrical operation, the loop resistance R of the pole to be calculated... p The resistance R of a single pole b During single-polarity return operation, the loop resistance R of the pole to be calculated pR is the sum of the polar resistance, the sending and receiving end grounding polar line resistance and the grounding polar resistance; when in single-pole metal return operation, R is the loop resistance of the pole to be calculated p equal to 2R b R e is the sum of the sending and receiving end grounding polar line resistance and the grounding polar resistance; I dcop is the operating current of the other pole except the pole to be calculated when in bipolar asymmetric operation, I is zero when in monopole operation. dcop

[0050] Secondly, the fast voltage reduction capability of the flexible DC converter is calculated according to the current modulation ratio M0, that is, the new sending end DC voltage of the pole to be calculated after the fault is calculated, and the calculation method is as follows:

[0051] U dcvsc_new = kU dcvsc0 M0 / M maxabs

[0052] Finally, the first maximum power P max1 limited by the modulation ratio after the fault is calculated according to the new sending end DC voltage of the pole to be calculated after the fault, and the calculation formula of the first maximum power in bipolar symmetric, monopole ground return and monopole metal return after the fault is as follows:

[0053]

[0054] wherein, P max1 is the first maximum power; U dcrec is the sending end DC voltage of the pole to be calculated after the fault; k is the operation mode coefficient, k is 2 when the pole is in full voltage operation, and k is 1 when the pole is in half voltage operation; R p is the loop resistance of the pole to be calculated; U dcvsc0 is the DC voltage of the flexible DC converter before the fault; M0 is the current modulation ratio; M maxabs is the maximum modulation ratio to ensure that the flexible DC converter does not overmodulate.

[0055] The calculation formula of the bipolar asymmetric operation after the fault is as follows:

[0056]

[0057] wherein, R e is the sum of the sending and receiving end grounding polar line resistance and the grounding polar resistance; R b is the resistance of a single polar line; I dcop is the operating current of the other pole except the pole to be calculated when in bipolar asymmetric operation.

[0058] The first maximum power P max1 and the second maximum power P max2 ​, the power of the DC control system after the fault is the smaller one of the two, i.e.

[0059] P new = min(P max1 , P max2 )

[0060] The second maximum power is the maximum power under the steady state condition, which is determined by the DC control system according to the new operation mode formed after the fault.

[0061] The power instruction value is updated, the flexible DC converter adjusts the DC voltage according to the power instruction value, and is fed back to the protection device to execute the generator tripping;

[0062] If not, the power of the DC control system after the fault is the second maximum power, which is directly fed back to the protection device to execute the generator tripping.

[0063] Figure 5 is the off-line calculation flow chart of the power transfer control when the transient state occurs, as shown in Figure 5 The calculation method of the power of the DC control system after the fault is:

[0064] After the fault occurs, it is judged whether the to-be-calculated pole contains the flexible DC converter;

[0065] If yes, the first maximum power is obtained by looking up the table of the maximum power under different faults preset in the DC control system, the first maximum power and the second maximum power are compared, and the power of the DC control system after the fault is the smaller one of the two, i.e. new = min(P max1 , P max2 ). When the preset value is calculated, the most severe working condition needs to be considered, and it is assumed that each operation mode is operated at the maximum modulation ratio under the steady state, and the line resistance takes the maximum value. According to the off-line calculation result, the power before the fault and the maximum power after the fault under each operation mode are plotted. After the fault occurs, the off-line calculation is automatically called and the maximum power curve set in the control system is determined to determine the first maximum power P max1 . In addition, if the limited operation condition is less, the limited operation condition can be uniformly limited to a single power instruction value to reduce the complexity of the control design.

[0066] The power instruction value is updated, the flexible DC converter adjusts the DC voltage according to the power instruction value, and is fed back to the protection device to execute the generator tripping;

[0067] If not, the power of the DC control system after the fault is the second maximum power, which is directly fed back to the protection device to execute the generator tripping.

[0068] S5 updates the power instruction value according to the power of the post-fault DC control system, and the flexible DC converter adjusts the DC voltage according to the power instruction value,

[0069] The power instruction value is updated according to the power of the post-fault DC control system, and the flexible DC converter adjusts the DC voltage reference value according to the power instruction value and the loop resistance. Under the action of the constant DC voltage controller, the phase difference angle between the converter output voltage and the system voltage is adjusted, the real-time DC voltage tracks the DC voltage reference value without deviation, and is fed back to the protection device to execute generator tripping.

[0070] Embodiment two

[0071] Based on the same inventive concept, the embodiment discloses a hybrid cascade DC power transmission project power transfer capacity optimization control system, comprising:

[0072] A steady-state modulation ratio calculation module is configured to calculate a steady-state modulation ratio, determine whether the operation mode of the flexible DC converter is a single-pole half-voltage operation according to the steady-state modulation ratio, and select the largest modulation ratio according to the determination result.

[0073] A modulation ratio adjustment module is configured to determine whether the steady-state modulation ratio is greater than the largest modulation ratio, and if so, increase the converter transformer gear position, and if not, maintain the original gear position unchanged.

[0074] A third harmonic injection module is configured to calculate the modulation ratio of the DC control system, and determine whether to inject a third harmonic according to the modulation ratio of the DC control system.

[0075] A maximum power calculation module is configured to obtain the maximum power under the voltage reduction capability limitation of the flexible DC converter when a fault occurs, and calculate the power of the post-fault DC control system according to the maximum power.

[0076] An update execution module is configured to update the power instruction value according to the power of the post-fault DC control system, and the flexible DC converter adjusts the DC voltage according to the power instruction value, and feeds back to the protection device to execute generator tripping.

[0077] The calculation method of the power of the post-fault DC control system is as follows:

[0078] After the fault occurs, it is determined whether the to-be-calculated pole contains the flexible DC converter.

[0079] If so, the first maximum power is obtained by referring to the maximum power table under different faults in the DC control system, the first maximum power and the second maximum power are compared, and the power of the post-fault DC control system is the smaller one of the two.

[0080] The power instruction value is updated, the flexible DC converter adjusts the DC voltage according to the power instruction value, and is fed back to the protection device to execute generator tripping;

[0081] If not, the power of the DC control system after the fault is the second maximum power, which is directly fed back to the protection device to execute generator tripping.

[0082] Those skilled in the art will understand that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROMs, optical storage, etc.) containing computer usable program code.

[0083] The present application is described with reference to flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of flows and / or blocks in the flowcharts and / or block diagrams can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing apparatus to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing apparatus produce a device that implements the functions specified in the flowcharts and / or block diagrams. Figure 1 The functions specified in one or more flows and / or blocks. Figure 1 The devices that implement the functions specified in one or more flows and / or blocks.

[0084] These computer program instructions can also be stored in a computer-readable memory that can direct the computer or other programmable data processing apparatus to work in a specific manner, so that the instructions stored in the computer-readable memory produce a manufactured product including instruction devices that implement the functions specified in the flowcharts and / or block diagrams. Figure 1 The functions specified in one or more flows and / or blocks. Figure 1 The devices that implement the functions specified in one or more flows and / or blocks.

[0085] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus, so that a series of operation steps are performed on the computer or other programmable data processing apparatus to produce a computer-implemented process, so that the instructions executed on the computer or other programmable data processing apparatus provide a process for implementing the functions specified in the flowcharts and / or block diagrams. Figure 1 The functions specified in one or more flows and / or blocks. Figure 1 The devices that implement the functions specified in one or more flows and / or blocks.

[0086] It should be pointed out finally that the above embodiments are only used to illustrate the technical solutions of the present application but not to limit it. Although the present application has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that the specific embodiments of the present application can be modified or replaced equivalently without departing from the spirit and scope of the present application, and any modification or equivalent replacement should be covered in the protection scope of the claims of the present application. The above content is only a specific embodiment of the present application, but the protection scope of the present application is not limited to this. Any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for optimizing the power transfer capability of a hybrid cascaded HVDC power transmission project, characterized in that, The method comprises the following steps: calculating a steady-state modulation ratio, judging whether the operation mode of the flexible DC converter is single-pole half-voltage operation according to the steady-state modulation ratio, and selecting the maximum modulation ratio according to the judgment result; judging whether the steady-state modulation ratio is greater than the maximum modulation ratio, and if yes, increasing the converter transformer tap position, and if not, maintaining the original tap position unchanged; the DC control system calculates the modulation ratio in real time, and judges whether to inject the third harmonic according to the modulation ratio of the DC control system; when a fault occurs, the maximum power under the voltage reduction capability limitation of the flexible DC converter is obtained, and the power of the post-fault DC control system is calculated according to the maximum power; the power instruction value is updated according to the power of the post-fault DC control system, the flexible DC converter adjusts the DC voltage according to the power instruction value, and the adjustment result is fed back to the safety control device to execute generator tripping; the calculation method of the power of the post-fault DC control system is as follows: after the fault occurs, it is judged whether the to-be-calculated pole contains the flexible DC converter; if yes, the calculation parameters are determined, the first maximum power under the voltage reduction capability limitation of the flexible DC converter is calculated, the first maximum power and the second maximum power are compared, and the power of the post-fault DC control system is the smaller one of the two; the power instruction value is updated, the flexible DC converter adjusts the DC voltage according to the power instruction value, and the adjustment result is fed back to the safety control device to execute generator tripping; if not, the power of the post-fault DC control system is the second maximum power, which is directly fed back to the safety control device to execute generator tripping; the second maximum power is the maximum power under the steady-state condition, which is determined by the DC control system according to the new operation mode formed after the fault.

2. The method of claim 1, wherein the power transfer capability of the hybrid cascaded HVDC power transmission project is optimized by, The calculation formula of the steady-state modulation ratio is as follows: wherein a peak value of the fundamental modulation wave generated for the inverter; U dc is a DC rated DC voltage.

3. The method of claim 1 or 2, wherein the power transfer capability of the hybrid cascaded HVDC power transmission project is optimized by, if the operation mode of the flexible DC converter is not single-pole half-voltage operation, the first maximum modulation ratio is selected as the maximum modulation ratio, indicating that the rapid voltage reduction capability of the flexible DC converter needs to be considered; if the operation mode of the flexible DC converter is single-pole half-voltage operation, the second maximum modulation ratio is selected as the maximum modulation ratio, indicating that the rapid voltage reduction capability of the flexible DC converter does not need to be considered.

4. The method of claim 1 or 2, wherein the power transfer capability of the hybrid cascaded HVDC power transmission project is optimized by, The method for judging whether to inject the third harmonic according to the modulation ratio of the DC control system is as follows: judging whether the modulation ratio of the DC control system is greater than the third maximum modulation ratio, if yes, injecting the third harmonic, and if not, not injecting the third harmonic.

5. The method of claim 4, wherein the power transfer capability of the hybrid cascaded HVDC power transmission project is optimized by, The calculation formula of the first maximum power under the post-fault bipolar symmetric operation, single-pole ground return and single-pole metal return is as follows: wherein, P max1 is the first maximum power; U dcrec is the post-fault to-be-calculated pole sending-end DC voltage; k is an operation mode coefficient, k is 2 when the pole is in full voltage operation, and k is 1 when the pole is in half voltage operation; R p is the loop resistance of the to-be-calculated pole; U dcvsc0 is the DC voltage of the flexible DC converter before the fault; M 0 is the current modulation ratio; M maxabs is the maximum modulation ratio to ensure that the flexible DC converter does not operate in over-modulation.

6. The method of claim 5, wherein the power transfer capability of the hybrid cascaded HVDC power transmission project is optimized by, The calculation formula of the first maximum power under the post-fault bipolar asymmetric operation is as follows: wherein R e is the sum of the ground electrode resistance and the ground electrode line resistance of the sending or receiving end; R b is the resistance of the single pole line; I dcop is the operating current of the other pole than the pole to be calculated in the case of asymmetric operation of the bipolar.

7. The method of claim 6, wherein the power transfer capability of the hybrid cascaded HVDC power transmission project is optimized by, In bipolar symmetric operation, the loop resistance of the pole to be calculated R p is the resistance of the single pole line R b In monopole earth return operation, the loop resistance of the pole to be calculated R p is the sum of the pole line resistance, the sending and receiving earth pole line and the earth pole resistance; in monopole metal return operation, the loop resistance of the pole to be calculated R p is equal to 2 R b .

8. A hybrid cascaded HVDC power transmission project power transfer capability optimization control system, characterized in that, The method comprises the following steps: a steady-state modulation ratio calculation module is configured to calculate a steady-state modulation ratio, judge whether the operation mode of the flexible DC converter is single-pole half-voltage operation according to the steady-state modulation ratio, and select the maximum modulation ratio according to the judgment result; a modulation ratio adjustment module is configured to judge whether the steady-state modulation ratio is greater than the maximum modulation ratio, and if yes, increase the converter transformer tap position, and if not, maintain the original tap position unchanged; a third harmonic injection module is configured to calculate the modulation ratio of the DC control system, and judge whether to inject the third harmonic according to the modulation ratio of the DC control system. The maximum power calculation module is configured to obtain a maximum power under a voltage reduction capability limit of the flexible HVDC converter when the fault occurs, and calculate a power of the DC control system after the fault according to the maximum power. The update execution module is configured to update a power instruction value according to the power of the DC control system after the fault, and adjust a DC voltage of the flexible HVDC converter according to the power instruction value and feed back to the protection and control device to execute generator tripping. The method for calculating the power of the DC control system after the fault comprises the following steps: After the fault occurs, it is determined whether the to-be-calculated pole contains the flexible HVDC converter. If yes, the calculation parameters are determined, the first maximum power under the voltage reduction capability limit of the flexible HVDC converter is calculated, the first maximum power and the second maximum power are compared, and the power of the DC control system after the fault is the smaller one of the two. The power instruction value is updated, the DC voltage of the flexible HVDC converter is adjusted according to the power instruction value, and the adjustment result is fed back to the protection and control device to execute generator tripping. If no, the power of the DC control system after the fault is the second maximum power, and the power is directly fed back to the protection and control device to execute generator tripping. The second maximum power is a maximum power under a steady-state condition, which is determined by the DC control system according to a new operation mode formed after the fault.

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