A verification method and system for flexible DC transmission projects

Abstract

This invention discloses a verification method and system for flexible DC transmission projects, comprising: obtaining multiple harmonic voltages generated by the AC system and the flexible DC converter station at the common access point (PCC) when the AC system is connected to the flexible DC converter station in the flexible DC transmission project, based on the operating parameters at the PCC under different operating conditions; superimposing the same harmonic voltages generated by the AC system and the flexible DC converter station at the PCC under the same operating condition to obtain the superimposed harmonic voltage and extracting the maximum harmonic voltage at the PCC under all operating conditions; and verifying the performance results of the flexible DC transmission project based on the relationship between the maximum harmonic voltage at the PCC under all operating conditions and a preset standard. This invention provides a method for verifying whether the performance of a designed flexible DC transmission project meets the requirements.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of flexible DC power transmission engineering, in particular to a flexible DC power transmission engineering verification method and system. BACKGROUND

[0002] With the construction of global energy internet, the existing power transmission and transformation technology and equipment have been unable to adapt to the needs of intercontinental power transmission. Flexible DC power transmission has the characteristics of flexible and rapid power regulation, no need for reactive power compensation, long transmission distance, etc. It has significant advantages in renewable energy grid connection, DC network construction, asynchronous grid interconnection, large city power supply, etc. With the further improvement of voltage level and capacity in the future, flexible DC power transmission technology will play an important role in promoting intercontinental efficient power transmission, large-scale renewable energy access, and large grid interconnection, and will be one of the important technical means to realize the global energy internet strategy.

[0003] With the continuous development and increasing maturity of flexible DC power transmission technology in various countries, it has become an important trend to expand the application of flexible DC power transmission system analysis and design technology to the global market. Compared with the standards set by various countries, the standards for harmonic content of flexible DC power transmission system in other countries have different provisions and requirements. Therefore, it is urgent to carry out corresponding analysis and calculation of flexible DC power transmission engineering in other countries to verify the performance of the flexible DC power transmission engineering designed by the country. SUMMARY

[0004] In order to solve the problem of no method for verifying the performance of flexible DC system in the prior art, the present application provides a flexible DC power transmission engineering verification method, which comprises:

[0005] Based on the operating parameters at the common access point PCC when the AC system accesses the flexible DC converter station in the flexible DC power transmission engineering, the multiple harmonic voltages generated by the AC system and the flexible DC converter station at the PCC point under different operating conditions are obtained.

[0006] The same harmonic voltage generated by the AC system and the flexible DC converter station at the PCC point under the same operating condition is superimposed to obtain the superimposed harmonic voltage and extract the maximum harmonic voltage at the PCC point under all operating conditions.

[0007] Based on the relationship between the maximum harmonic voltage at the PCC point under all operating conditions and the preset standard, the performance of the flexible DC power transmission engineering is verified.

[0008] Preferably, based on the operating parameters at the common access point PCC when the AC system accesses the flexible DC converter station in the flexible DC power transmission engineering, the multiple harmonic voltages generated by the AC system and the flexible DC converter station at the PCC point under different operating conditions are obtained, which comprises:

[0009] Based on the harmonic impedance of the AC system, the harmonic voltage of the AC system and the harmonic impedance of each flexible DC converter station, the multiple harmonic voltages generated at the PCC point of the AC system under different operating conditions are obtained.

[0010] Based on the harmonic impedance of the AC system, the harmonic impedance of each flexible DC converter station, and the output harmonic voltage of the flexible DC converter station, the multiple harmonic voltages generated at the PCC point of the flexible DC converter station under different operating conditions are obtained.

[0011] The operating parameters include: AC system harmonic impedance and AC system harmonic voltage, as well as the harmonic impedance and output harmonic voltage of multiple flexible DC converter stations under the same operating conditions.

[0012] Preferably, the harmonic impedance of the flexible DC converter station is calculated using the following formula:

[0013]

[0014] In the formula: Z_converter(s): harmonic impedance of the flexible DC converter station in the s-domain; R line : Equivalent resistance of AC line; L line : Equivalent inductance of AC line; L t K: The inductance value corresponding to the short-circuit impedance of the transformer. t : The turns ratio on the YY side of the transformer; L arm : Equivalent inductance of each bridge arm.

[0015] Preferably, the acquisition of the output harmonic voltage of the flexible DC converter station includes:

[0016] At least two operating conditions should be selected from the system's active and reactive power curves for simulation.

[0017] The output harmonic voltage of the flexible DC converter station under different operating conditions was calculated through simulation.

[0018] Preferably, after calculating the output harmonic voltage of the flexible DC converter station under different operating conditions through simulation, the method further includes:

[0019] The output harmonic voltage of the flexible DC converter station under different operating conditions was verified based on the established harmonic voltage amplitude expression.

[0020] Preferably, the expression for the harmonic voltage amplitude is as follows:

[0021]

[0022] Where: M harm (n): Amplitude of the output harmonic voltage of the flexible DC converter station; U dcN : Rated DC voltage; N: Number of submodules without considering redundancy; θi The angle at which the stepped wave level changes from the (i-1)th to the ith level within the first 1 / 4 of the cycle; i: level number; n: harmonic order; M: maximum output level;

[0023] Specifically, the angle θ from the (i-1)th to the ith step wave level within the first 1 / 4 of the cycle. i Calculate using the following formula:

[0024] θ i =arcsin[(2i-1) / mN]

[0025] In the formula: m: adjustment system.

[0026] Preferably, the multiple harmonic voltages generated by the AC system at the PCC point are calculated using the following formula:

[0027]

[0028] In the formula, The kth harmonic voltage generated by the AC system at point PCC; Z w _converter: The w-th harmonic impedance of the flexible DC converter station under the k-th harmonic; Z_sys: The harmonic impedance of the AC system; V background Harmonic voltage in AC system.

[0029] Preferably, the multiple harmonic voltages generated at the PCC point by the flexible DC converter station are calculated using the following formula:

[0030]

[0031] In the formula: The j-th harmonic voltage generated at the PCC point by the flexible DC converter station; Z w _converter: The w-th harmonic impedance of the flexible DC converter station under the i-th harmonic; Z_sys: The harmonic impedance of the AC system; V converter : Output harmonic voltage of flexible DC converter station.

[0032] Preferably, the same harmonic voltages generated at the PCC point by the AC system and the flexible DC converter station under the same operating condition are superimposed according to the following formula:

[0033]

[0034] In the formula: The kth harmonic voltage generated by the AC system at point PCC; The j-th harmonic voltage generated at the PCC point by the flexible DC converter station; U h: The h-th harmonic voltage resulting from the superposition of the same harmonic voltages generated at the PCC point by the AC system and the flexible DC converter station; α: Process parameter.

[0035] Preferably, the verification of the performance of the flexible DC transmission project based on the relationship between the maximum harmonic voltage of the PCC point under all operating conditions and a preset standard includes:

[0036] Based on the maximum harmonic voltage at the PCC point under all operating conditions, the impact of the converter output voltage on the AC system is verified.

[0037] If the maximum harmonic voltage at the PCC point is greater than the preset standard under all operating conditions, then the performance of the designed flexible DC transmission project is qualified; otherwise, the performance of the flexible DC transmission project is unqualified.

[0038] Preferably, the verification of the performance of the flexible DC transmission project based on the relationship between the maximum harmonic voltage of the PCC point under all operating conditions and a preset standard further includes:

[0039] When the performance of the flexible DC transmission project is unqualified, the parameters of the AC filter are designed based on the maximum harmonic voltage of the PCC point under all operating conditions.

[0040] Based on the same inventive concept, the present invention also provides a verification system for flexible DC transmission projects, comprising:

[0041] The acquisition module is used to obtain multiple harmonic voltages generated by the AC system and the flexible DC converter station at the PCC point under different operating conditions based on the operating parameters at the common access point (PCC) when the AC system is connected to the flexible DC converter station in the flexible DC transmission project.

[0042] The superposition module is used to superimpose the same harmonic voltages generated at the PCC point of the AC system and the flexible DC converter station under the same operating conditions, obtain the superimposed harmonic voltages, and extract the maximum harmonic voltage at the PCC point under all operating conditions.

[0043] The verification module is used to verify the performance of flexible DC transmission projects based on the relationship between the maximum harmonic voltage of the PCC point and the preset standard under all operating conditions.

[0044] Preferably, the acquisition module includes:

[0045] The AC system parameter acquisition unit is used to obtain multiple harmonic voltages generated at the PCC point of the AC system under different operating conditions based on the AC system harmonic impedance, AC system harmonic voltage and harmonic impedance of each flexible DC converter station.

[0046] The parameter acquisition unit is used to obtain multiple harmonic voltages generated at the PCC point of the flexible DC converter station under different operating conditions based on the harmonic impedance of the AC system, the harmonic impedance of each flexible DC converter station, and the output harmonic voltage of the flexible DC converter station.

[0047] The operating parameters include: AC system harmonic impedance and AC system harmonic voltage, as well as the harmonic impedance and output harmonic voltage of multiple flexible DC converter stations under the same operating conditions.

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] The technical solution provided by this invention obtains multiple harmonic voltages generated by the AC system and the flexible DC converter station at the common access point (PCC) when the AC system is connected to the flexible DC converter station in a flexible DC transmission project, based on the operating parameters at the PCC. It then superimposes the same harmonic voltages generated by the AC system and the flexible DC converter station at the PCC under the same operating condition to obtain the superimposed harmonic voltage and extracts the maximum harmonic voltage at the PCC under all operating conditions. Based on the relationship between the maximum harmonic voltage at the PCC under all operating conditions and a preset standard, it verifies the performance results of the flexible DC transmission project, providing a method for verifying whether the performance of the designed flexible DC transmission project meets the preset standard.

[0050] The technical solution provided by this invention clarifies whether an AC filter needs to be installed based on the maximum harmonic voltage of the PCC point under all operating conditions, and provides input for the parameter design and equipment selection of the AC filter, thereby improving the system performance of the flexible DC converter station.

[0051] The technical solution provided by this invention proposes the calculation principle of harmonic content at PCC point in a system containing a flexible DC converter station and the calculation method of harmonic impedance of the flexible DC converter station, and clarifies the calculation method of harmonic content at PCC point in an AC system.

[0052] The technical solution provided by this invention can guide the design and selection of parameters for equipment such as AC filters in flexible DC transmission projects by calculating the AC harmonic content of the designed flexible DC transmission projects, and propose comprehensive optimization measures. It can not only provide technical support for improving the performance of flexible DC engineering systems, but also lay a theoretical and practical foundation for the reliability assessment of China's AC / DC hybrid power grid. Attached Figure Description

[0053] Figure 1-a This is a schematic diagram of a single-phase model of MMC;

[0054] Figure 1-b This is a schematic diagram of the equivalent model of MMC;

[0055] Figure 2 This is a schematic diagram of the harmonic equivalent model at the PCC point according to the present invention;

[0056] Figure 3 This is a schematic diagram of the equivalent simplified model of the flexible DC converter station of the present invention;

[0057] Figure 4 This is a flowchart of the verification method of the present invention;

[0058] Figure 5 This is a detailed flowchart of the verification method in this embodiment. Detailed Implementation

[0059] To better understand this invention, the following description, in conjunction with the accompanying drawings and examples, will further illustrate the invention.

[0060] Example 1

[0061] This invention proposes a theoretical analytical method for converter valve output harmonic voltage based on an MMC equivalent mathematical model, including:

[0062] (1) Single-phase equivalent circuit model of converter valve

[0063] ① Construct a single-phase circuit model for the converter valve

[0064] First, under the condition that the capacitor voltage does not fluctuate, a theoretical analytical method for the output voltage of a flexible DC converter valve is established based on the MMC equivalent mathematical model, which can be used to verify the simulation calculation results.

[0065] Figure 1-a This is a single-phase circuit diagram for a converter valve, where L... arm The equivalent inductance for each bridge arm, neglecting the equivalent resistance of the bridge arm. p Indicates the output voltage of the upper bridge arm, u n The lower bridge arm output voltage is indicated by "A", which indicates the AC side point of the MMC. The upper bridge arm reactor is connected between point "A" and point "O", and the lower bridge arm reactor is connected between point "A" and point "O'".

[0066] ② Equivalent circuit diagram of the converter valve

[0067] Based on the basic operating principles of MMC Figure 1-a It can be equivalent to Figure 1-b .

[0068] The output voltage of each phase inside the MMC is:

[0069]

[0070] In the formula: u mmc This represents the output voltage of each phase.

[0071] (2) Single-phase output harmonic voltage of converter valve

[0072] Assuming the submodule capacitor voltage does not fluctuate, the ideal module capacitance value U dcN Calculate using the following formula:

[0073]

[0074] The MMC bridge arm employs a closest-level approximation modulation strategy, resulting in a stepped output voltage for each phase within the MMC. Therefore, the desired output voltage for each phase of the MMC is:

[0075]

[0076] Considering only the MMC output level, the ideal and actual output voltage levels of equation (3) are as follows:

[0077]

[0078] Among them U dcN The value represents the rated DC voltage, N represents the number of submodules without considering redundancy, and m represents the modulation index.

[0079] Since the waveform is symmetrical, the output voltage has no even harmonics under ideal conditions. Let the maximum output level be M = round(mN / 2) (5)

[0080] Let i = 1, 2, ..., M represent the step wave level changing from the (i-1)th angle to the ith angle within the first 1 / 4 of a cycle.

[0081] θ i =arcsin[(2i-1) / mN] (6)

[0082] The theoretical expression for the amplitude of each harmonic voltage (n = {3, 5, 7…}) can be calculated as follows:

[0083]

[0084] Where: M harm (n): Amplitude of the output harmonic voltage of the flexible DC converter station; U dcN : Rated DC voltage; N: Number of submodules without considering redundancy; θ i The angle at which the stepped wave level changes from the (i-1)th to the ith level within the first 1 / 4 of the cycle; i: level number; n: harmonic order; M: maximum output level;

[0085] Furthermore, this invention proposes a method for calculating the harmonic content at the PCC point of an AC system containing a flexible DC converter station, including:

[0086] (1) Calculation principle of harmonic content of AC system at PCC point

[0087] First, the principle for calculating the harmonic content at the point of common coupling (PCC) of the AC system after it is connected to the flexible DC converter station in the design of overseas flexible DC transmission projects is proposed:

[0088] like Figure 2 As shown, both background harmonics and converter valve output harmonics will generate harmonic voltages at the PCC point of the system. The harmonic content at the PCC point of the system can be calculated based on the superposition principle. Figure 2 China V background Represents the system harmonic voltage, V converter Z represents the harmonic voltage of the flexible DC converter station. sys Z represents the system harmonic impedance. converter This represents the harmonic impedance of the flexible DC converter station.

[0089] (2) Calculation model of harmonic impedance of flexible DC converter station

[0090] Without considering the influence of frequency, the simplified equivalent model of the flexible DC converter station is as follows: Figure 3 As shown. Due to the short length of the AC line, it can be equivalent to an RL-type model, as shown in the figure. line and L line Let L be the equivalent resistance and inductance of the AC line. Also, assume that the transformer and bridge arm reactors are linear, L... t K represents the inductance value corresponding to the short-circuit impedance of the transformer. t U represents the turns ratio on the YY side of the transformer. mmc This is the output voltage of the converter valve.

[0091] The harmonic impedance of a flexible DC converter station in the s-domain is expressed as:

[0092]

[0093] like Figure 4 As shown, this invention provides a verification method for flexible DC transmission projects, comprising:

[0094] S1. Based on the operating parameters at the common access point PCC when the AC system is connected to the flexible DC converter station in the flexible DC transmission project, obtain the multiple harmonic voltages generated by the AC system and the flexible DC converter station at the PCC point under different operating conditions;

[0095] S2. Superimpose the harmonic voltages of the same order generated at the PCC point of the AC system and the flexible DC converter station under the same operating condition to obtain the superimposed harmonic voltage and extract the maximum harmonic voltage at the PCC point under all operating conditions.

[0096] S3. Based on the relationship between the maximum harmonic voltage of the PCC point and the preset standard under all operating conditions, verify the performance results of the flexible DC transmission project.

[0097] like Figure 5 As shown, the verification method provided by this invention will be described in detail below:

[0098] S1. Based on the operating parameters at the common access point (PCC) when the AC system is connected to the flexible DC converter station in a flexible DC transmission project, obtain the multiple harmonic voltages generated by the AC system and the flexible DC converter station at the PCC point under different operating conditions, including:

[0099] 1) Calculate the harmonic impedance Z of the flexible DC converter station based on the established formula (8). converter When the converter valve operates under the same operating conditions, each harmonic has multiple impedance values;

[0100] 2) Based on the obtained background harmonic voltage and harmonic impedance of the AC system, calculate the nth harmonic voltage generated at the PCC point under different harmonic impedances using the following formula:

[0101]

[0102] In the formula, The kth harmonic voltage generated by the AC system at point PCC; Z w _converter: The w-th harmonic impedance of the flexible DC converter station under the k-th harmonic; Z_sys: The harmonic impedance of the AC system; V background Harmonic voltage in AC system.

[0103] 3) Establish an MMC simulation model, simulate and calculate the harmonic characteristics of the converter valve output voltage under different operating conditions, and verify the accuracy of the results obtained by the simulation model by using a theoretical analytical method for converter valve output harmonic voltage based on the MMC equivalent mathematical model proposed in this invention.

[0104] Several typical operating conditions are selected from the system's active and reactive power curves, and the influence of negative sequence voltage and system background harmonics on the output voltage of the converter valve is considered.

[0105] Based on the harmonic voltage output of the converter valve obtained from simulation, the nth harmonic voltage generated by the converter at the PCC point under different operating conditions is calculated using the following formula:

[0106]

[0107] In the formula: The j-th harmonic voltage generated at the PCC point by the flexible DC converter station; Z w _converter: The w-th harmonic impedance of the flexible DC converter station under the i-th harmonic; Z_sys: The harmonic impedance of the AC system; V converter : Output harmonic voltage of flexible DC converter station.

[0108] S2. Superimpose the harmonic voltages of the same order generated at the PCC point of the AC system and the flexible DC converter station under the same operating condition to obtain the superimposed harmonic voltage and extract the maximum harmonic voltage at the PCC point under all operating conditions. Specifically, this includes:

[0109] 4) Based on the superposition principle, the nth harmonic voltage is superimposed, that is, multiple values ​​under the same harmonic voltage are superimposed, and finally the maximum value is selected.

[0110] When the phase is unknown, the superposition can be performed according to the following formula:

[0111]

[0112] in,

[0113] In the formula: h refers to the harmonic order, U h The h-th harmonic voltage refers to the superposition of the same harmonic voltages generated at the PCC point by the AC system and the flexible DC converter station; α refers to the process parameter.

[0114] S3. Based on the relationship between the maximum harmonic voltage at the PCC point and the preset standard under all operating conditions, verify the performance of the flexible DC transmission project, including:

[0115] 5) Extract and save the maximum values ​​of the harmonic voltage at the PCC point under all operating conditions to verify the impact of the converter output voltage on the AC system under the worst-case scenario;

[0116] 6) Obtain the maximum value of each harmonic content and calculate the harmonic voltage THD, compare it with the preset standard to verify whether the performance of the designed flexible DC converter station meets the requirements. Input can be provided for AC filter parameter design.

[0117] The preset standards in this embodiment are set according to overseas IEC, GRID CODE and other standards. It should be noted that: ① Different countries may need to follow different standards, such as engineering recommendation G5 / 4-1 and grid code, etc.; ② Each harmonic voltage has a corresponding maximum limit; ③ For DC systems greater than 20kV, the voltage THD range is less than 3%.

[0118] Example 2

[0119] Based on the same inventive concept, this invention provides a verification system for flexible DC transmission projects, comprising:

[0120] The acquisition module is used to obtain multiple harmonic voltages generated by the AC system and the flexible DC converter station at the PCC point under different operating conditions based on the operating parameters at the common access point (PCC) when the AC system is connected to the flexible DC converter station in the flexible DC transmission project.

[0121] The superposition module is used to superimpose the same harmonic voltages generated at the PCC point of the AC system and the flexible DC converter station under the same operating conditions, obtain the superimposed harmonic voltages, and extract the maximum harmonic voltage at the PCC point under all operating conditions.

[0122] The verification module is used to verify the performance of flexible DC transmission projects based on the relationship between the maximum harmonic voltage of the PCC point and the preset standard under all operating conditions.

[0123] In this embodiment, the acquisition module includes:

[0124] The AC system parameter acquisition unit is used to obtain multiple harmonic voltages generated at the PCC point of the AC system under different operating conditions based on the AC system harmonic impedance, AC system harmonic voltage and harmonic impedance of each flexible DC converter station.

[0125] The parameter acquisition unit is used to obtain multiple harmonic voltages generated at the PCC point of the flexible DC converter station under different operating conditions based on the harmonic impedance of the AC system, the harmonic impedance of each flexible DC converter station, and the output harmonic voltage of the flexible DC converter station.

[0126] The operating parameters include: AC system harmonic impedance and AC system harmonic voltage, as well as the harmonic impedance and output harmonic voltage of multiple flexible DC converter stations under the same operating conditions.

[0127] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0128] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations 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, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in one or more flowchart illustrations and / or one or more block diagrams.

[0129] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means that implement the functions specified in one or more flowcharts and / or one or more block diagrams.

[0130] These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, such that the instructions, which execute on the computer or other programmable apparatus, provide steps for implementing the functions specified in one or more flowcharts and / or one or more block diagrams.

[0131] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of the claims of the present invention pending approval.

Claims

1. A method of validating a flexible direct current power transmission project, characterized in that, The method comprises the following steps: Based on the operating parameters at the PCC point when the AC system accesses the flexible DC converter station in the flexible DC power transmission project, the multiple harmonic voltages generated by the AC system and the flexible DC converter station at the PCC point under different working conditions are obtained; The same-order harmonic voltages generated by the AC system and the flexible DC converter station at the PCC point under the same working condition are superimposed to obtain the superimposed harmonic voltage and extract the maximum harmonic voltage at the PCC point under all working conditions; Based on the relationship between the maximum harmonic voltage at the PCC point under all working conditions and the preset standard, the performance of the flexible DC power transmission project is verified; The method comprises the following steps: Based on the AC system harmonic impedance, the AC system harmonic voltage and the harmonic impedance of each flexible DC converter station, the multiple harmonic voltages generated by the AC system at the PCC point under different working conditions are obtained; Based on the AC system harmonic impedance, the harmonic impedance of each flexible DC converter station and the harmonic voltage output by the flexible DC converter station, the multiple harmonic voltages generated by the flexible DC converter station at the PCC point under different working conditions are obtained; The operating parameters include the AC system harmonic impedance and the AC system harmonic voltage, and the harmonic impedance of multiple flexible DC converter stations and the harmonic voltage output by the flexible DC converter station under the same working condition; The multiple harmonic voltages generated by the AC system at the PCC point are calculated according to the following formula: wherein the kth harmonic voltage generated at the PCC point by the ac system; Z w converter: the wth harmonic impedance of the flexible dc converter station at the kth harmonic; Z_sys: the harmonic impedance of the ac system; V background : the harmonic voltage of the ac system; The multiple harmonic voltages generated by the flexible DC converter station at the PCC point are calculated according to the following formula: In the formula: Vj: the jth harmonic voltage generated by the flexible DC converter station at the PCC point; Z w Zwi: the wth harmonic impedance of the flexible DC converter station at the ith harmonic; Z_sys: the harmonic impedance of the AC system; V converter : the harmonic voltage output by the flexible DC converter station; The same-order harmonic voltages generated by the AC system and the flexible DC converter station at the PCC point under the same working condition are superimposed according to the following formula: wherein: the kth harmonic voltage generated at the PCC point by the ac system; the jth harmonic voltage generated at the PCC point by the flexible HVDC converter station; U h the hth harmonic voltage generated at the PCC point by the superposition of the same harmonic voltages generated by the ac system and the flexible HVDC converter station; a: process parameter.

2. The method of claim 1, wherein, The harmonic impedance of the flexible DC converter station is calculated according to the following formula: wherein: Z converter(s) : Harmonic impedance of the flexible HVDC converter station in the s-domain; R line : Equivalent resistance of the AC line; L line : Equivalent inductance of the AC line; L t : Inductance value corresponding to the short circuit impedance of the transformer; K t : Transformer ratio on the Y-Y side; L arm : Equivalent inductance of each bridge arm.

3. The method of claim 1, wherein, The acquisition of the harmonic voltage output by the flexible DC converter station comprises the following steps: At least two groups of working conditions are selected from the system active power and reactive power curve for simulation; The harmonic voltage output by the flexible DC converter station under different working conditions is calculated through simulation.

4. The method of claim 3, wherein, After the harmonic voltage output by the flexible DC converter station under different working conditions is calculated through simulation, the following step is further included: The harmonic voltage output by the flexible DC converter station under different working conditions is verified based on the set harmonic voltage amplitude expression.

5. The method of claim 4, wherein, The harmonic voltage amplitude expression is shown in the following formula: In the formula, M harm (n): amplitude of output harmonic voltage of flexible HVDC converter station; U dcN : rated DC voltage; N: number of sub-modules not considering redundancy state; θ i : angle of step wave level from i-1th jump to i th in the first 1 / 4 cycle i: level number; n: harmonic order; M: maximum output level; wherein the angle θ of the step wave level from the i-1th jump to the i th in the first 1 / 4 cycle i is calculated by the following equation: θ i = arcsin[(2i - 1) / mN] In the formula: m: modulation factor.

6. The method of claim 1, wherein, The performance of the flexible DC power transmission project is verified based on the relationship between the maximum harmonic voltage at the PCC point under all working conditions and the preset standard, which comprises the following steps: Based on the maximum harmonic voltage at the PCC point under all working conditions, the influence of the converter output voltage on the AC system is verified; When the maximum harmonic voltage at the PCC point under all working conditions is greater than the preset standard, the performance of the designed flexible DC power transmission project is qualified, otherwise the performance of the flexible DC power transmission project is unqualified.

7. The method of claim 6, wherein, The performance of the flexible DC power transmission project is verified based on the relationship between the maximum harmonic voltage at the PCC point under all working conditions and the preset standard, which further comprises the following steps: When the performance of the flexible HVDC project is unqualified, parameters of the AC filter are designed based on the maximum harmonic voltage at the PCC point under all operating conditions.

8. A validation system for a flexible DC power transmission project for the method of claim 1, characterized by The method comprises the steps of: an acquisition module, configured to acquire, based on operating parameters at a public access point (PCC) when an AC system accesses a flexible HVDC converter station in the flexible HVDC project, harmonic voltages generated by the AC system and the flexible HVDC converter station at the PCC under different operating conditions; a superposition module, configured to superimpose harmonic voltages of the same order generated by the AC system and the flexible HVDC converter station at the PCC under the same operating condition, to obtain superimposed harmonic voltages and extract the maximum harmonic voltage at the PCC under all operating conditions; a verification module, configured to verify the performance of the flexible HVDC project based on a relationship between the maximum harmonic voltage at the PCC under all operating conditions and a preset standard.

9. The system of claim 8, wherein, The acquisition module comprises: an acquisition unit for AC system parameters, configured to acquire, based on AC system harmonic impedance, AC system harmonic voltage, and harmonic impedance of each flexible HVDC converter station, harmonic voltages generated by the AC system at the PCC under different operating conditions; an acquisition unit for parameters, configured to acquire, based on AC system harmonic impedance, harmonic impedance of each flexible HVDC converter station, and output harmonic voltage of the flexible HVDC converter station, harmonic voltages generated by the flexible HVDC converter station at the PCC under different operating conditions; wherein the operating parameters comprise AC system harmonic impedance and AC system harmonic voltage, and harmonic impedance and output harmonic voltage of the flexible HVDC converter station under the same operating condition.

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