A stability analysis method for new energy multi-level all-direct-current gathering and sending system

Abstract

This application provides a stability analysis method for a multi-level, all-DC collection and transmission system for new energy sources. The system comprises a new energy power plant, a medium-voltage DC transformer, a medium-voltage DC bus, a medium-voltage energy storage converter, a high-voltage DC transformer, a high-voltage DC transmission line, a receiving-end MMC converter, and a receiving-end three-phase AC power grid. The method includes: obtaining the port admittances of the new energy power plant and the medium-voltage energy storage converter, the DC-side port impedance of the receiving-end MMC converter, the transfer function of the equivalent model of the high-voltage DC transmission line, and the transfer functions of the equivalent models of the medium-voltage and high-voltage DC transformers; constructing a system stability analysis determinant based on the obtained transfer functions; and determining the small-disturbance stability of the system by judging whether the determinant contains right-half-plane zeros. This method effectively avoids the non-minimum phase problem and enables accurate and rapid stability analysis of complex multi-level, all-DC collection and transmission systems for new energy sources.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission technology, specifically to a stability analysis method for a multi-level all-DC collection and transmission system for new energy. Background Technology

[0002] In recent years, the installed capacity and power generation of new energy sources have grown rapidly, driving the application of DC transmission technology in power systems. However, as the scale of DC transmission systems continues to expand, the dynamic interactions between various types of power electronic equipment and the main AC power grid system become more complex. This strong coupling can easily cause oscillations in the DC bus voltage. If this problem cannot be effectively analyzed and suppressed, the safe and reliable operation of the system cannot be guaranteed, greatly restricting the development and application of DC transmission technology in new power systems.

[0003] For small-disturbance stability assessment of multi-level DC-DC collection and transmission systems for new energy sources, existing stability assessment methods have shortcomings in this novel scenario. In particular, existing technologies often fail to fully and accurately analyze the dynamic behavior of the entire DC-DC collection and transmission system and the interactive effects of various power electronic devices. Furthermore, existing stability analysis methods are generally only applicable to minimum-phase systems, and determining system stability requires knowing whether power electronic devices have right-half-plane poles, making it difficult to accurately analyze the instability mechanisms of complex DC systems.

[0004] Therefore, it is urgent to study a stability analysis method that is more suitable for multi-level DC collection and transmission systems of new energy, so as to improve the accuracy of stability analysis of multi-level DC collection and transmission systems of new energy and provide a more solid theoretical basis for their planning, design, parameter tuning and stable operation. Summary of the Invention

[0005] In view of this, the technical problem to be solved by the present invention is to propose a stability analysis method for a multi-level all-DC collection and transmission system of new energy, so as to solve the problem that the existing technology has the problem of modeling accuracy defects in the stability analysis of multi-level all-DC collection and transmission systems of new energy, especially the difficulty in analyzing the dynamic characteristics and interactive effects of systems with non-minimum phase problems.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a stability analysis method for a multi-level DC-DC collection and transmission system for new energy, comprising a multi-level DC-DC collection and transmission system for new energy, wherein the multi-level DC-DC collection and transmission system for new energy includes a new energy power station, a medium-voltage DC transformer, a medium-voltage DC bus, a medium-voltage energy storage converter, a high-voltage DC transformer, a high-voltage DC transmission line, a receiving-end MMC converter, and a receiving-end three-phase AC power grid; the new energy power station and the medium-voltage DC transformer are respectively configured as follows: αThe system comprises multiple new energy power plants whose output terminals are connected in parallel and then sequentially stepped up and collected through medium-voltage DC transformers and high-voltage DC transformers before being transmitted over long distances via high-voltage DC transmission lines. The stability analysis method for this multi-level all-DC collection and transmission system for new energy includes the following steps:

[0007] S1: Obtain the port admittance of the new energy power station and the medium-voltage energy storage converter respectively;

[0008] S2: Obtain the DC-side port impedance of the receiving-end MMC converter;

[0009] S3: Obtain the four transfer functions of the two-port equivalent model of the high-voltage direct current transmission line: input impedance, voltage gain, current gain, and output admittance.

[0010] S4: Obtain the four transfer functions of the two-port equivalent models of the medium-voltage DC transformer and the high-voltage DC transformer respectively: input impedance, voltage gain, current gain and output admittance.

[0011] S5: Substitute all transfer functions obtained from S1 to S4 into the stability analysis determinant and calculate the number of zeros in the right half-plane of the stability analysis determinant. When the number of zeros in the right half-plane of the stability analysis determinant is 0, the new energy multi-level full DC collection and transmission system is determined to be stable; otherwise, the new energy multi-level full DC collection and transmission system is determined to be unstable.

[0012] Preferably, the new energy power station described in S1 is equipped with a new energy converter. All of these converters employ MPPT control and have the same DC voltage level at their output ports. Within the allowable error range, the new energy power station is equivalent to the Norton model. The Norton model consists of an ideal DC current source connected in parallel with an equivalent port admittance. α The port admittance of each new energy power station is Y Lo,α ;

[0013] The medium-voltage energy storage converter adopts a constant power control method. Within the allowable error range, the medium-voltage energy storage converter is equivalent to the Norton model, and its port admittance is... Y Eo .

[0014] Preferably, the receiving-end MMC converter described in S2 adopts a constant high-voltage DC bus voltage control method, equivalent to the Thevenin model on the high-voltage DC side. The Thevenin model consists of an ideal DC voltage source connected in series with an equivalent port impedance. The high-voltage DC side port impedance of the equivalent Thevenin model of the receiving-end MMC converter is... Z Min .

[0015] Preferably, the high-voltage direct current transmission line described in S3 is connected to a high-voltage direct current transformer at its starting end and to a receiving-end MMC converter at its terminating end. The mathematical expression of its two-port equivalent model is as follows:

[0016]

[0017] In the formula, v Hbus,1 This refers to the DC voltage at the starting end of a high-voltage direct current transmission line. i Hbus,1 Input current for high-voltage direct current transmission lines. i Hbus,2 This is the output current for high-voltage direct current transmission lines. v Hbus,2 DC voltage at the termination of a high-voltage direct current transmission line, Z HLin For input impedance, G HLv For voltage gain, G HLi For current gain, Y HLo This is the output admittance.

[0018] Preferably, the medium-voltage DC transformer in S3 includes a low-voltage port and a medium-voltage port, which are respectively connected to the new energy power station and the medium-voltage bus. The medium-voltage DC transformer adopts a low-voltage port DC voltage control method. The two-port model of the medium-voltage DC transformer is composed of the Thevenin model of the low-voltage port and the Norton model of the medium-voltage port. α The mathematical expression for the two-port model of the medium-voltage DC transformer is:

[0019]

[0020] In the formula, v LDin,α Input voltage to the low-voltage port, i LDin,α Input current to the low-voltage port. i MDo,α This is the output current for the medium voltage port. v Mbus This is the medium-voltage DC bus voltage. Z MDin,α For input impedance, G MDv,α For voltage gain, G MDi,α For current gain, Y MDo,α For output admittance;

[0021] The high-voltage direct current transformer includes a medium-voltage port and a high-voltage port, which are respectively connected to the medium-voltage bus and the high-voltage direct current transmission line. The high-voltage direct current transformer adopts a DC voltage control method at the medium-voltage port. The two-port model of the high-voltage direct current transformer is composed of the Thevenin model of the medium-voltage port and the Norton model of the high-voltage port. The mathematical expression of the two-port model of the high-voltage direct current transformer is:

[0022]

[0023] In the formula, i MDin Input current to the medium voltage port. i HDo Output current for the high-voltage port. v Hbus,1 This refers to the DC voltage at the input terminal of the high-voltage direct current transmission line. Z HDin For input impedance, G HDv For voltage gain, G HDi For current gain, Y HDo This is the output admittance.

[0024] As a preferred embodiment, the mathematical expression for the stable analysis determinant described in S5 is:

[0025]

[0026] In the formula, the matrix A, B, C, D The order of all are α+ 3, and their calculation formulas are as follows:

[0027]

[0028]

[0029]

[0030] In the formula, Y Lo,α For the first α Port admittance of a new energy power station. Y Eo The port admittance of the medium-voltage energy storage converter. Z Min The DC-side port impedance of the receiving-end MMC converter. Z HLin The input impedance is the equivalent model of a two-port high-voltage direct current transmission line. G HLvThe voltage gain is represented by the equivalent model of the two-port high-voltage direct current transmission line. G HLi The current gain is the equivalent model current gain of a two-port high-voltage direct current transmission line. Y HLo Output admittance for the two-port equivalent model of a high-voltage direct current transmission line. Z MDin,α For the first α The input impedance of a two-port model of a medium-voltage DC transformer. G MDv,α For the first α Voltage gain of a two-port model of a medium-voltage DC transformer. G MDi,α For the first α The current gain of a two-port model of a medium-voltage DC transformer. Y MDo,α For the first α Output admittance of a two-port model of a medium-voltage DC transformer. Z HDin This represents the input impedance of the two-port model of a high-voltage DC transformer. G HDv The voltage gain of a two-port model of a high-voltage DC transformer. G HDi The current gain of a two-port model of a high-voltage DC transformer. Y HDo This is the output admittance of the two-port model of the high-voltage DC transformer.

[0031] Preferably, if all converters in the new energy multi-level all-DC collection and transmission system can operate independently and stably, meaning that each converter can be guaranteed to be stable when its port is connected to an ideal voltage source or ideal current source during the design phase, then the stability analysis determinant will definitely not have any right-half-plane poles. Therefore, the stability analysis method satisfies the minimum phase criterion, effectively avoiding the non-minimum phase problem existing in the current stability analysis method, and can accurately assess the small-disturbance stability of the new energy multi-level all-DC collection and transmission system.

[0032] Compared with existing technologies, the stability analysis method for a new energy multi-level all-DC collection and transmission system provided by this invention has the following advantages:

[0033] (1) The stability analysis method fills the gap in stability methods for multi-level DC collection and transmission of new energy, and promotes the application of DC transmission technology in the scenario of new energy transmission.

[0034] (2) The stability analysis method satisfies the minimum phase criterion, which can effectively avoid the non-minimum phase problem in the existing stability analysis method, and can simply and accurately evaluate the small disturbance stability of the new energy multi-level full DC collection and transmission system.

[0035] (3) The stability analysis method improves the accuracy of stability analysis of new energy multi-level all-DC collection and transmission system, and provides a more solid theoretical basis for its planning, design, parameter tuning and stable operation. Attached Figure Description

[0036] Figure 1 A flowchart illustrating the stability analysis method for a new energy multi-level all-DC collection and transmission system provided in an embodiment of this application is shown.

[0037] Figure 2 A structural block diagram of a typical structure of a new energy multi-level all-DC collection and transmission system provided according to an embodiment of this application is shown;

[0038] Figure 3 A schematic flowchart illustrating a stability analysis method for a new energy multi-level all-DC collection and transmission system according to an embodiment of this application is shown.

[0039] Figure 4 A schematic diagram of the equivalent form of a stability analysis method for a new energy multi-level all-DC collection and transmission system provided according to an embodiment of this application is shown.

[0040] Figure 5 A schematic diagram of a small disturbance model of a new energy multi-level all-DC collection and transmission system provided according to an embodiment of this application is shown. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. All embodiments and technical features described in the present invention can be arbitrarily combined with each other.

[0042] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0043] As described in the background section, existing technologies suffer from modeling accuracy deficiencies in stability analysis of multi-level all-DC collection and transmission systems for new energy sources, particularly in analyzing the dynamic characteristics and interactive effects of systems with non-minimum phase problems. To overcome this technical obstacle, embodiments of this application provide a method for stability analysis of multi-level all-DC collection and transmission systems for new energy sources.

[0044] To more clearly demonstrate the technical solution of the present invention, a detailed description will be provided below in conjunction with the accompanying drawings and specific embodiments.

[0045] Figure 1 This is a flowchart illustrating the stability analysis method for a new energy multi-level all-DC collection and transmission system according to an embodiment of this application. Figure 2 As shown, the new energy multi-level all-DC collection and transmission system consists of α One new energy power station, α The system consists of a medium-voltage DC transformer, a medium-voltage DC bus, a medium-voltage energy storage converter, a high-voltage DC transformer, a high-voltage DC transmission line, a receiving-end MMC converter, and a receiving-end three-phase AC power grid. The stability analysis method includes:

[0046] Step (S1): Obtain the port admittance of the new energy power station and the medium-voltage energy storage converter respectively;

[0047] Specifically, the new energy power plants all employ MPPT control and have the same DC voltage level at their output ports. The new energy power plants can be considered equivalent to the Norton model. Y Lo,α For the first α Port admittance of a new energy power station;

[0048] Specifically, the medium-voltage energy storage converter adopts a constant power control method, which can be equivalent to the Norton model, and its port admittance is... Y Eo .

[0049] Step (S2): Obtain the DC-side port impedance of the receiving-end MMC converter;

[0050] Specifically, the receiving-end MMC converter adopts a constant high-voltage DC bus voltage control method, which can be equivalent to the Thevenin model of the high-voltage DC side, and its DC side port impedance is... Z Min .

[0051] Step (S3) Obtain the four transfer functions of the two-port equivalent model of the high-voltage direct current transmission line: input impedance, voltage gain, current gain and output admittance.

[0052] Specifically, the starting end of the high-voltage direct current transmission line is connected to a high-voltage direct current transformer, and the terminating end is connected to a receiving-end MMC converter. The mathematical expression of its two-port equivalent model is as follows:

[0053]

[0054] in, v Hbus,1 This refers to the DC voltage at the starting end of a high-voltage direct current transmission line. i Hbus,1 Input current for high-voltage direct current transmission lines. iHbus,2 This is the output current for high-voltage direct current transmission lines. v Hbus,2 DC voltage at the termination of a high-voltage direct current transmission line, Z HLin For input impedance, G HLv For voltage gain, G HLi For current gain, Y HLo This is the output admittance.

[0055] Step (S4) Obtain the four transfer functions of the two-port equivalent models of the medium-voltage DC transformer and the high-voltage DC transformer: input impedance, voltage gain, current gain and output admittance;

[0056] Specifically, the medium-voltage DC transformer includes a low-voltage port and a medium-voltage port, which are respectively connected to the new energy power station and the medium-voltage bus, and adopt a low-voltage port DC voltage control method; the two-port model of the medium-voltage DC transformer is composed of the Thevenin model of the low-voltage port and the Norton model of the medium-voltage port, the second... α The mathematical expression for the two-port model of the medium-voltage DC transformer is:

[0057]

[0058] in, v LDin,α Input voltage to the low-voltage port, i LDin,α Input current to the low-voltage port. i MDo,α This is the output current for the medium voltage port. v Mbus This is the medium-voltage DC bus voltage. Z MDin,α For input impedance, G MDv,α For voltage gain, G MDi,α For current gain, Y MDo,α For output admittance;

[0059] Specifically, the high-voltage direct current transformer includes a medium-voltage port and a high-voltage port, which are respectively connected to the medium-voltage bus and the high-voltage direct current transmission line, and adopt a DC voltage control method at the medium-voltage port; the two-port model of the high-voltage direct current transformer is composed of the Thevenin model of the medium-voltage port and the Norton model of the high-voltage port, and the mathematical expression of the two-port model of the high-voltage direct current transformer is:

[0060]

[0061] in,i MDin Input current to the medium voltage port. i HDo Output current for the high-voltage port. v Hbus,1 This refers to the DC voltage at the input terminal of the high-voltage direct current transmission line. Z HDin For input impedance, G HDv For voltage gain, G HDi For current gain, Y HDo This is the output admittance.

[0062] Step (S5): Substitute all the transfer functions obtained in steps (S4~S5) into the stability analysis determinant and calculate the number of zeros in the right half-plane of the stability analysis determinant. When the number of zeros in the right half-plane of the stability analysis determinant is 0, the new energy multi-level full DC collection and transmission system is determined to be stable; otherwise, the new energy multi-level full DC collection and transmission system is determined to be unstable.

[0063] Specifically, the mathematical expression for the stability analysis determinant is:

[0064]

[0065] Among them, matrix A, B, C, D The order of all are α+ 3, and their calculation formulas are as follows:

[0066]

[0067]

[0068]

[0069]

[0070] In the formula, Y Lo,α For the first α Port admittance of a new energy power station. Y Eo The port admittance of the medium-voltage energy storage converter. Z Min The DC-side port impedance of the receiving-end MMC converter. Z HLin The input impedance is the equivalent model of a two-port high-voltage direct current transmission line. G HLv The voltage gain is represented by the equivalent model of the two-port high-voltage direct current transmission line. G HLiThe current gain is the equivalent model current gain of a two-port high-voltage direct current transmission line. Y HLo Output admittance for the two-port equivalent model of a high-voltage direct current transmission line. Z MDin,α For the first α The input impedance of a two-port model of a medium-voltage DC transformer. G MDv,α For the first α Voltage gain of a two-port model of a medium-voltage DC transformer. G MDi,α For the first α The current gain of a two-port model of a medium-voltage DC transformer. Y MDo,α For the first α Output admittance of a two-port model of a medium-voltage DC transformer. Z HDin This represents the input impedance of the two-port model of a high-voltage DC transformer. G HDv The voltage gain of a two-port model of a high-voltage DC transformer. G HDi The current gain of a two-port model of a high-voltage DC transformer. Y HDo This is the output admittance of the two-port model of the high-voltage DC transformer.

[0071] Specifically, if all converters in the aforementioned multi-level DC-DC collection and transmission system for new energy can operate independently and stably, meaning that each converter can be designed to be stable when its ports are connected to an ideal voltage or current source, then the determinant of the stability analysis will certainly not have any right-half-plane poles. Therefore, the stability analysis method satisfies the minimum phase criterion, effectively avoiding the non-minimum phase problem existing in current stability analysis methods, and can accurately assess the small-disturbance stability of the multi-level DC-DC collection and transmission system for new energy.

[0072] To enable those skilled in the art to gain a deeper understanding of the technical solution of the present invention, the implementation process of the stability analysis method for a new energy multi-level all-DC collection and transmission system of the present invention will be described in detail below with reference to specific embodiments.

[0073] Figure 2 This paper illustrates a typical structure of a multi-level all-DC collection and transmission system for new energy, including: α One new energy power station, α The system consists of a medium-voltage DC transformer, a medium-voltage DC bus, a medium-voltage energy storage converter, a high-voltage DC transformer, a high-voltage DC transmission line, a receiving-end MMC converter, and a receiving-end three-phase AC power grid. Multiple new energy power plants are connected in parallel, and their outputs are sequentially stepped up and collected through medium-voltage DC transformers and high-voltage DC transformers before being transmitted over long distances via high-voltage DC transmission lines. Figure 2 Taking the new energy multi-level all-DC collection and transmission system shown as an example, the embodiment involves a stability analysis method for a new energy multi-level all-DC collection and transmission system, such as... Figure 3 As shown, it includes the following steps:

[0074] Step 1: As Figure 2 As shown, the topology of the new energy multi-level all-DC collection and transmission system is first obtained. The converters are equivalent according to the control method. All new energy converters using MPPT control and medium-voltage energy storage converters using constant output power control inside the new energy power station are equivalent to Norton models. The receiving-end MMC converter using constant high-voltage DC bus voltage control is equivalent to a Thevenin model. The high-voltage DC transmission line is equivalent to a two-port model composed of the Thevenin model of the input port and the Norton model of the output port. The medium-voltage DC transformer using low-voltage port DC voltage control is equivalent to a two-port model composed of the Thevenin model of the low-voltage port and the Norton model of the medium-voltage port. The high-voltage DC transformer using medium-voltage port DC voltage control is equivalent to a two-port model composed of the Thevenin model of the medium-voltage port and the Norton model of the high-voltage port.

[0075] Step 2: Establish a small-disturbance equivalent model for the new energy multi-level all-DC collection and transmission system.

[0076] Step 3: Based on the small disturbance equivalent model established by the new energy multi-level all-DC collection and transmission system, the determinant of the stability analysis is obtained.

[0077] Step 4: Determine whether the determinant of the stability analysis has a right half-plane zero. If yes, the system is unstable; otherwise, the system is stable.

[0078] As can be seen from the above description, the embodiments of this application achieve the following technical effects:

[0079] This application discloses a stability analysis method for a multi-level DC-DC collection and transmission system for new energy. First, the port admittances of the new energy power station and the medium-voltage energy storage converter are obtained. Second, the DC-side port impedance of the receiving-end MMC converter is obtained. Then, the four transfer functions of the two-port equivalent model of the high-voltage DC transmission line are obtained: input impedance, voltage gain, current gain, and output admittance. Next, the four transfer functions of the two-port equivalent models of the medium-voltage DC transformer and the high-voltage DC transformer are obtained: input impedance, voltage gain, current gain, and output admittance. Finally, the above transfer functions are substituted into the stability analysis determinant, and the number of zeros in the right half-plane of the stability analysis determinant is calculated. When the number of zeros in the right half-plane of the stability analysis determinant is 0, the multi-level DC-DC collection and transmission system for new energy is determined to be stable; otherwise, it is determined to be unstable. This method establishes a small-disturbance model for the multi-level DC-DC collection and transmission system for new energy, obtains the stability analysis determinant characterizing the system's stability, and determines the system stability based on whether the stability analysis determinant contains zeros in the right half-plane. This scheme considers renewable energy power plants, medium-voltage DC transformers, medium-voltage DC buses, medium-voltage energy storage converters, high-voltage DC transformers, high-voltage DC transmission lines, receiving-end MMC converters, and receiving-end three-phase AC power grids. Compared to existing analysis methods, this method overcomes the limitations of current technologies in applying to multi-level, all-DC aggregation and transmission scenarios for renewable energy, thus expanding the accuracy of stability assessment. By accurately considering key dynamic factors of the system and successfully avoiding the risk of misjudgment under non-minimum phase systems, the reliability of the assessment results is significantly improved.

[0080] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0081] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A stability analysis method for a multi-level all-DC collection and transmission system for new energy, characterized in that, This includes a multi-level DC collection and transmission system for new energy, comprising a new energy power plant, a medium-voltage DC transformer, a medium-voltage DC bus, a medium-voltage energy storage converter, a high-voltage DC transformer, a high-voltage DC transmission line, a receiving-end MMC converter, and a receiving-end three-phase AC power grid; the new energy power plant and the medium-voltage DC transformer are respectively configured as follows: α The system comprises multiple new energy power plants whose output terminals are connected in parallel and then sequentially stepped up and collected through medium-voltage DC transformers and high-voltage DC transformers before being transmitted over long distances via high-voltage DC transmission lines. The stability analysis method for this multi-level all-DC collection and transmission system includes the following steps: S1: Obtain the port admittance of the new energy power station and the medium-voltage energy storage converter respectively; S2: Obtain the DC-side port impedance of the receiving-end MMC converter; S3: Obtain the four transfer functions of the two-port equivalent model of the high-voltage direct current transmission line: input impedance, voltage gain, current gain, and output admittance. S4: Obtain the four transfer functions of the two-port equivalent models of the medium-voltage DC transformer and the high-voltage DC transformer respectively: input impedance, voltage gain, current gain and output admittance; S5: Substitute all the transfer functions obtained from S1 to S4 into the stability analysis determinant and calculate the number of zeros in the right half-plane of the stability analysis determinant; when the number of zeros in the right half-plane of the stability analysis determinant is 0, the new energy multi-level full DC collection and transmission system is determined to be stable; otherwise, the new energy multi-level full DC collection and transmission system is determined to be unstable. The stability analysis determinant described in S5 is a determinant formed by performing specific mathematical operations on all impedances, voltage gains, current gains, and admittances obtained from S1 to S4. Its mathematical expression is: In the formula, the matrix A, B, C, D The order of all are α+ 3, and their calculation formulas are as follows: In the formula, Y Lo,α For the first α Port admittance of a new energy power station. Y Eo The port admittance of the medium-voltage energy storage converter. Z Min The DC-side port impedance of the receiving-end MMC converter. Z HLin The input impedance is the equivalent model of a two-port high-voltage direct current transmission line. G HLv The voltage gain is represented by the equivalent model of the two-port high-voltage direct current transmission line. G HLi The current gain is the equivalent model current gain of a two-port high-voltage direct current transmission line. Y HLo Output admittance for the two-port equivalent model of a high-voltage direct current transmission line. Z MDin,α For the first α The input impedance of a two-port model of a medium-voltage DC transformer. G MDv,α For the first α Voltage gain of a two-port model of a medium-voltage DC transformer. G MDi,α For the first α The current gain of a two-port model of a medium-voltage DC transformer. Y MDo,α For the first α Output admittance of a two-port model of a medium-voltage DC transformer. Z HDin This represents the input impedance of the two-port model of a high-voltage DC transformer. G HDv The voltage gain of a two-port model of a high-voltage DC transformer. G HDi The current gain of a two-port model of a high-voltage DC transformer. Y HDo This is the output admittance of the two-port model of the high-voltage DC transformer.

2. The stability analysis method for a new energy multi-level all-DC collection and transmission system according to claim 1, characterized in that, The new energy power station described in S1 is equipped with a new energy converter. All of these converters employ MPPT control and have the same DC voltage level at their output ports. Within the allowable error range, the new energy power station is equivalent to the Norton model. The Norton model consists of an ideal DC current source connected in parallel with an equivalent port admittance. α The port admittance of the equivalent Norton model for a new energy power station is Y Lo,α ; The medium-voltage energy storage converter adopts a constant power control method. Within the allowable error range, the medium-voltage energy storage converter is equivalent to the Norton model, and its port admittance is... Y Eo。 3. The stability analysis method for a new energy multi-level all-DC collection and transmission system according to claim 1, characterized in that, The receiving-end MMC converter described in S2 adopts a constant high-voltage DC bus voltage control method, equivalent to the Thevenin model on the high-voltage DC side. The Thevenin model consists of an ideal DC voltage source connected in series with an equivalent port impedance. The high-voltage DC side port impedance of the equivalent Thevenin model of the receiving-end MMC converter is... Z Min .

4. The stability analysis method for a new energy multi-level all-DC collection and transmission system according to claim 1, characterized in that, The high-voltage direct current transmission line described in S3 has a high-voltage direct current transformer at its starting end and a receiving-end MMC converter at its terminating end. The mathematical expression for its two-port equivalent model is as follows: In the formula, v Hbus,1 This refers to the DC voltage at the starting end of a high-voltage direct current transmission line. i Hbus,1 Input current for high-voltage direct current transmission lines. i Hbus,2 This is the output current for high-voltage direct current transmission lines. v Hbus,2 DC voltage at the termination of a high-voltage direct current transmission line, Z HLin For input impedance, G HLv For voltage gain, G HLi For current gain, Y HLo This is the output admittance.

5. The stability analysis method for a new energy multi-level all-DC collection and transmission system according to claim 1, characterized in that, The medium-voltage DC transformer described in S3 includes a low-voltage port and a medium-voltage port, which are respectively connected to the renewable energy power station and the medium-voltage bus. The medium-voltage DC transformer adopts a low-voltage port DC voltage control method. The two-port model of the medium-voltage DC transformer is composed of the Thevenin model of the low-voltage port and the Norton model of the medium-voltage port. α The mathematical expression for the two-port model of the medium-voltage DC transformer is: In the formula, v LDin,α Input voltage to the low-voltage port, i LDin,α Input current to the low-voltage port. i MDo,α Output current for the medium voltage port. v Mbus This is the medium-voltage DC bus voltage. Z MDin,α For input impedance, G MDv,α For voltage gain, G MDi,α For current gain, Y MDo,α For output admittance; The high-voltage direct current transformer includes a medium-voltage port and a high-voltage port, which are respectively connected to the medium-voltage bus and the high-voltage direct current transmission line. The high-voltage direct current transformer adopts a DC voltage control method at the medium-voltage port. The two-port model of the high-voltage direct current transformer is composed of the Thevenin model of the medium-voltage port and the Norton model of the high-voltage port. The mathematical expression of the two-port model of the high-voltage direct current transformer is: In the formula, i MDin Input current to the medium voltage port. i HDo Output current for the high-voltage port. v Hbus,1 This refers to the DC voltage at the input terminal of the high-voltage direct current transmission line. Z HDin For input impedance, G HDv For voltage gain, G HDi For current gain, Y HDo This is the output admittance.

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