Double-end frequency cooperative control and stability analysis method of flexible direct current power transmission system

Abstract

The invention discloses a double-end frequency cooperative control and stability analysis method for a flexible direct-current power transmission system, and the method comprises the steps: firstly building a linear mapping relation between the direct-current voltage of an inverter station and a receiving-end alternating-current system, and then building a quantitative relation between the direct-current voltage of a receiving-end alternating-current frequency and the direct-current voltage of a rectifier station; and then establishing a flexible DC system double-end AC frequency system control relationship, further establishing a system linearization model after double-end frequency cooperative control is input, and finally performing system stability analysis under frequency cooperative control. According to the frequency coupling control method, the overall frequency stability of the system can be effectively improved through power mutual assistance between the sending end system and the receiving end system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power systems, and in particular relates to a method for double-end frequency coordinated control and stability analysis of a flexible direct current transmission system. Background Art

[0002] In the developed eastern regions of my country, the load density is high and the AC grid structure is dense. Under this power grid structure, problems such as excessive short-circuit current and difficulty in power flow control are prominent, the scale effect of the AC power grid tends to be saturated, and the marginal benefits of AC grid investment have significantly decreased. Using flexible DC systems to partition the AC power grid can effectively solve problems such as excessive short-circuit current and difficulty in power flow control in high-load density areas, but it will cause the size of the synchronous AC system to decrease and the frequency stability to deteriorate. The control mode of the flexible DC system is flexible and can simulate different characteristics; therefore, if the flexible control and power regulation capabilities of the flexible DC system can be fully utilized, it will be possible to maintain frequency coupling after the AC power grid is partitioned, thereby providing a feasible solution for the grid structure form that adapts to the development trend of high-density loads.

[0003] Existing frequency regulation methods for flexible DC systems, such as the literature [Liu Yingpei, Xie Qian, Liang Haiping. Adaptive virtual inertia frequency regulation control strategy for flexible DC transmission system [J]. Automation of Electric Power Systems, 2021, 45(05): 129-136], usually provide frequency support in a single direction to the sending or receiving power grid, and fail to achieve frequency coupling similar to AC lines. The reason is that each inverter station in the flexible DC system has one active control degree of freedom, and one of them must be used to control the DC voltage. For AC systems, the flexible DC system has only one active degree of freedom, which increases the difficulty of the flexible DC system to participate in frequency regulation in both directions.

[0004] Reference [Hong Chun, Shao Zongxue, Zhao Wei, et al. Research on emergency frequency control of AC / DC hybrid power system with flexible DC [J]. Transactions of China Electrotechnical Society, 2023, 38(20): 5590-5604] Frequency regulation through emergency power support has been proven to be a feasible way to take into account both the frequency regulation of the transmitter and the receiver; however, the implementation of bidirectional emergency power support requires the use of communication means, and the action threshold and the size of the support power need to be determined offline through multiple simulation verifications, which will cause excessive simulation calculation burden in new energy power systems with variable operating modes. The frequency support of back-to-back flexible DC system partition interconnection system and long-line flexible DC system has also received widespread attention, but there are corresponding problems to be solved. Therefore, the dual-end frequency coordinated control and stability analysis of flexible DC systems need further research. Summary of the invention

[0005] In view of the above, the present invention provides a dual-end frequency coordinated control and stability analysis method of a flexible direct current transmission system, which can effectively improve the overall frequency stability of the system through power mutual assistance between the sending and receiving end systems.

[0006] A method for double-terminal frequency coordinated control and stability analysis of a flexible direct current transmission system comprises the following steps:

[0007] (1) According to the control method of the flexible DC transmission system, a linear mapping relationship between the DC voltage of the inverter station and the AC system at the receiving end is established;

[0008] (2) Based on the above linear mapping relationship, a quantitative relationship between the receiving-end AC system frequency and the rectifier station DC voltage is established;

[0009] (3) Based on the above quantitative relationship, establish the frequency control relationship of the AC system at both ends of the transmitter and the receiver;

[0010] (4) Based on the above frequency control relationship, a system linearization model after the dual-end frequency coordinated control is established;

[0011] (5) Use the system linearization model to perform stability analysis on the system under frequency coordinated control.

[0012] Furthermore, in the step (1), under the inverter station fixed DC voltage control mode and the rectifier station fixed active power control mode, a linear mapping relationship between the inverter station DC voltage and the receiving end AC system is established as follows:

[0013] ΔU dciref =K uf Δf re

[0014] Where: ΔU dciref K is the change of the DC voltage reference value of the inverter station, uf is the frequency droop coefficient, Δf re is the frequency change of the receiving AC system.

[0015] Furthermore, in step (2), the quantitative relationship between the AC system frequency at the receiving end and the DC voltage at the rectifier station is expressed by the following expression:

[0016]

[0017] Where: ΔU dci is the DC voltage change of the inverter station, U dcr is the DC voltage of the rectifier station, P dcr is the active power of the rectifier station, R dc is the DC resistance, U dci0 is the initial value of the DC voltage of the inverter station.

[0018] Furthermore, in step (3), the control relationship of the AC frequency between the transmitting and receiving ends is established by the following expression:

[0019] ΔP dcrref =K If ∫(Δf se -Δf re -K LB ΔP dcrref )dt+K Pf (Δf se -Δf re -K LB ΔP dcrref )

[0020] Where: ΔP dcrref is the change in the power reference value of the rectifier station, K Pf and K If are proportional coefficient and integral coefficient respectively, K LB is the power feedback coefficient, Δf se is the frequency deviation of the sending-end AC system.

[0021] Furthermore, the frequency control relationship can be used to realize the frequency coupling of the AC systems at both ends of the transmitter and the receiver through the power transmission of the flexible DC transmission system; if the power feedback coefficient K LB If it is set to 0, the control goal of the flexible DC transmission system is to make the AC systems at both ends of the transmission and the reception to be of the same frequency, thus showing a similar effect to that of an AC line.

[0022] Furthermore, the system linearization model in step (4) is expressed as follows:

[0023]

[0024] x=[Δf re ,Δf se ,ΔU dci ,ΔU dcr ,Δi sdi ,Δi sdr ,ΔP dcrref ] T

[0025]

[0026] Where: x is the state vector, A sys is the state matrix, is the first-order derivative of x, H se and H re are the inertia of the AC system at both ends, P mr and P mi are the mechanical power of the AC system at both ends, P erand P ei are the electromagnetic power of the AC system at both ends, D se and D re are the damping coefficients of the AC systems at both ends, C se and C re are the MMC equivalent capacitance of the AC system at both ends, P conr and P coni are the sending end power and receiving end power of the DC line respectively, i sdr is the active current of the sending-end MMC, K dqr and K dqi is the integral coefficient, U sr is the commutation bus voltage of the sending-end MMC, U si is the commutation bus voltage of the receiving MMC, i sdi is the active current of the receiving MMC, U dci is the DC voltage of the inverter station, P in is the input power of the flexible DC transmission system, ΔU dcr is the DC voltage change of the rectifier station, Δi sdi is the active current change of the receiving MMC, Δi sdr is the active current change of the sending-end MMC, T Indicates transpose.

[0027] Furthermore, in step (5), by changing K uf , K LB , H se , H re The control parameter size including the state matrix A is drawn sys The change of characteristic roots in the complex plane can be used to analyze the stability of the system.

[0028] A computer device comprises a memory and a processor, wherein the memory stores a computer program, and the processor is used to execute the computer program to implement the above-mentioned two-terminal frequency coordinated control and stability analysis method of the flexible direct current transmission system.

[0029] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the two-terminal frequency coordinated control and stability analysis method of the flexible direct current transmission system.

[0030] The present invention takes into account the frequency coupling problem of the flexible DC transmission and receiving ends, and proposes a dual-end frequency coordinated control and stability analysis method of the flexible DC transmission system. The frequency coupling control method of the present invention can effectively improve the overall frequency stability of the system through power mutual assistance between the transmission and receiving end systems. In addition, in practical applications, the DC voltage-receiving end frequency droop coefficient K is appropriately increased. ufThe power feedback coefficient K in the frequency coupling control of the sending-end converter station LB , which can ensure system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the equivalent model structure of the two-terminal flexible DC transmission system.

[0032] Figure 2 is the state matrix characteristic root with the proportional coefficient K uf Schematic diagram of the changes.

[0033] Figure 3 is the characteristic root of the state matrix with the feedback coefficient K LB Schematic diagram of the changes.

[0034] Figure 4 is the characteristic root of the state matrix with the receiving end system inertia H re Schematic diagram of the changes.

[0035] Figure 5 is the characteristic root of the state matrix with the inertia of the sending end system H re Schematic diagram of the changes.

[0036] Figure 6 Schematic diagram of the modified IEEE 11-node system topology.

[0037] Figure 7 Schematic diagram of frequency change under disturbance of the sending-end system.

[0038] Figure 8 Schematic diagram of DC voltage change under sending-end system disturbance.

[0039] Fig. 9 Schematic diagram of DC power change under sending-end system disturbance.

[0040] Fig.10 Schematic diagram of frequency change under the disturbance of the receiving system.

[0041] Fig.11 Schematic diagram of DC voltage change under receiving system disturbance.

[0042] Fig.12 Schematic diagram of DC power change under receiving system disturbance.

[0043] Fig.13 For different K uf Graph of small perturbation simulation calculation results under different sizes.

[0044] Fig.14 For different K LB Graph of small perturbation simulation calculation results under different sizes. DETAILED DESCRIPTION

[0045] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0046] The method for double-terminal frequency coordinated control and stability analysis of a flexible direct current transmission system of the present invention comprises the following steps:

[0047] (1) Under the control mode of fixed DC voltage at inverter station and fixed active power at rectifier station in flexible DC system, a linear mapping relationship between DC voltage at inverter station and AC system at receiving end is established:

[0048] ΔU dciref =K uf Δf re

[0049] Where: ΔU dciref K is the change of the DC voltage reference value of the inverter station, uf is the proportionality coefficient, Δf re is the frequency change of the receiving system.

[0050] (2) Establish a quantitative relationship between the AC frequency at the receiving end and the DC voltage at the rectifier station.

[0051] The relationship between the DC voltage at the sending end and the DC voltage at the receiving end is:

[0052] U dcr =U dci +I dc R dc

[0053] Where: U dcr is the DC voltage of the rectifier station, U dci is the DC voltage of the inverter station, I dc is the DC current, R dc is the DC resistance.

[0054] The DC current can be expressed as:

[0055]

[0056] Where: P dcr is the active power of the rectifier station.

[0057] Then we can get the relationship between the DC voltage of the inverter station and the DC voltage and power of the rectifier station as follows:

[0058]

[0059] Since the frequency response time scale is in the second level, and the response time scale of the converter station power or DC voltage command value is in the 10 millisecond level, when considering the frequency characteristics of the AC system, it can be considered that the electrical quantity follows the command value in real time:

[0060] U dci =U dciref

[0061] P dcr =P dcrref

[0062] Where: P dcrref is the power command value of the flexible DC rectifier station.

[0063] Then we can get the quantitative relationship between the AC frequency at the receiving end and the DC voltage at the rectifier station:

[0064]

[0065] Where: U dci0 is the initial value of the DC voltage of the inverter station.

[0066] (3) Establish the AC frequency coupling relationship between the sending and receiving ends of the flexible DC system.

[0067] In order to couple the sending end frequency and the receiving end frequency, without loss of generality, the rectifier station power command value can be set as follows:

[0068] ΔP dcrref =K If ∫(Δf se -Δf re -K LB ΔP dcrref )dt+K Pf (Δf se -Δf re -K LB ΔP dcrref )

[0069] Where: K Pf and K If are the control proportional and integral coefficients, K LB is the feedback coefficient, Δf se is the frequency deviation of the sending end system, ΔP dcrref is the change in power reference value.

[0070] The above formula can be used to achieve frequency coupling of the AC system at the sending and receiving ends through flexible DC system power transmission; if the feedback coefficient K is LB If it is set to 0, the control goal of the flexible DC system is to make the transmitting and receiving ends have the same frequency, thus showing a similar effect to an AC line.

[0071] (4) Establish a linear model of the system after the frequency coupling control of the transmitting and receiving ends is put into operation.

[0072] First, establish Figure 1 The system equivalent model shown in the figure is P inis the input power of the flexible DC system, P con is the line transmission power, P out is the output power of the flexible DC system, C se and C re are the equivalent capacitances of the MMC at the sending and receiving ends, R dc is the DC line resistance.

[0073] For the AC system at the transmitting and receiving ends, the first-order frequency fluctuation equation is used to describe the frequency dynamic characteristics:

[0074]

[0075]

[0076] Where: H se and H re are the inertia of the power grid at the sending and receiving ends, P mr and P mi are the mechanical power of the power grid at the sending and receiving ends, P er and P ei are the electromagnetic power of the transmitting and receiving end grids, D se and D re are the damping coefficients of the transmitting and receiving power grids respectively.

[0077] The equivalent capacitance differential equation of the MMC converter station at the transmitting and receiving ends can be written as:

[0078]

[0079]

[0080] Where: C se and C re are the MMC equivalent capacitances at the sending and receiving ends, P conr and P coni are the power at the sending and receiving ends of the DC line respectively.

[0081] For the sending-end converter station, ignoring the inner loop control, it is approximately assumed that the inner loop output can follow the control target without delay, and ignoring the outer loop proportional link, it can be approximately assumed that the active current change on the AC side is as follows:

[0082]

[0083] Where: i sdr is the active current of the MMC at the sending end, K dqr is the integration coefficient.

[0084] Under the frequency coupling control of the transmitting and receiving ends, ignoring the proportional coefficient in step (3), the change of the power reference value can be written as:

[0085]

[0086] The relationship between current and power is:

[0087] P in =i sdr U sr

[0088] Where: U sr is the MMC commutation bus voltage at the sending end.

[0089] Similarly, for the receiving MMC, the differential algebraic equation can be written as:

[0090]

[0091] Where: i sdr is the receiving end MMC active current, K dqr is the integral coefficient, U si is the MMC commutation bus voltage at the receiving end.

[0092] The line equation of the flexible DC system is:

[0093]

[0094] Where: I dc is a direct current.

[0095] Combining all the above formulas, the linearized model of the system after the frequency coupling control of the transmitting and receiving ends is derived as follows:

[0096]

[0097] x=[Δf re ,Δf se ,ΔU dci ,ΔU dcr ,Δi sdi ,Δi sdr ,ΔP dcrref ] T

[0098]

[0099] (5) System stability analysis under frequency coupling control.

[0100] Typical parameters are selected as shown in Table 1. In order to analyze the impact of frequency coupling control input on system stability, the matrix A is plotted by changing the control parameter size. sys The change of characteristic roots in the complex plane. First, change the droop proportional coefficient K between the DC voltage and AC frequency of the receiving flexible DC converter station. uf , get K ufThe changes of the characteristic roots of the matrix Asys during the enlargement process are as follows Figure 2 As shown, from Figure 2 As can be seen in uf As K increases, the system characteristic root gradually moves away from the imaginary axis, which means that the system stability is improved. uf From the physical meaning of K uf The larger it is, the greater the amplitude of the change of the DC voltage of the flexible DC system with the AC frequency of the receiving system, and the more the DC voltage can reflect the frequency change information, which is beneficial to the stability of the system during the frequency coupling control process.

[0101] Table 1

[0102]

[0103] The receiving-end flexible DC converter station adopts DC voltage-frequency droop control, while the sending-end flexible DC converter station estimates the receiving-end AC frequency based on the DC voltage and adopts the sending-end and receiving-end frequency deviation integral control. Without loss of generality, the DC power feedback signal is added to the sending-end and receiving-end frequency deviation. If the feedback coefficient KLB is changed, the change of the system characteristic root under frequency coupling control is obtained as follows: Figure 3 As shown. Figure 3 It can be seen that as the feedback coefficient K LB As the feedback coefficient K increases, the system characteristic root moves away from the imaginary axis. The imaginary part of the characteristic root does not change, only the real part decreases, indicating that increasing the feedback coefficient K LB It can increase the stability of frequency coupling control. However, it should be noted that as the feedback coefficient K LB As the feedback coefficient K increases, the frequency coupling degree between the sending and receiving ends will decrease. LB When K is 0, the goal of frequency coupling control is to make the transmitting and receiving systems have the same frequency. LB When it is greater than 0, the frequencies of the sending and receiving ends will maintain a certain deviation.

[0104] The above content analyzes the impact of two key control parameters in frequency coupling control on system stability. In addition to the control parameters, the inertia of the sending and receiving systems is also a key factor affecting stability. Generally speaking, if the inertia difference between the sending and receiving systems is too large, it is difficult for the flexible DC system to try to maintain the same frequency of the sending and receiving systems. If the inertia of the sending and receiving systems is similar, frequency coupling control is easier to obtain better control effects. First, increase the inertia of the receiving system to obtain the change of the system characteristic root as shown in the figure below. Figure 4 As shown, from Figure 4 It can be seen that as the inertia of the receiving system increases, the system characteristic roots move toward the direction close to the imaginary axis, indicating that the system stability deteriorates.

[0105] Furthermore, by changing the inertia of the sending end system, the change of the system characteristic root is obtained as follows: Figure 5 As shown, from Figure 5 It can be seen that the increase in the inertia of the sending system will cause the characteristic root on the real axis to move toward the imaginary part; however, the conjugate characteristic root closest to the imaginary axis hardly moves with the increase in the inertia of the sending system. The above phenomenon shows that the increase in the inertia of the sending system also has a negative impact on the stability of the frequency coupling control, but the mechanism of action is different from that of the change in the inertia of the receiving end.

[0106] According to the above analysis, the greater the inertia of the sending and receiving systems, the greater the negative impact on system stability after the frequency coupling control is put into use. The stability of the frequency coupling control can be improved by increasing the DC voltage-frequency droop coefficient or increasing the power feedback coefficient.

[0107] (6) Example analysis

[0108] The frequency coupling control method of the present invention is simulated and verified in a modified IEEE 11-node system. The modified IEEE 11-node system topology is as follows: Figure 6 As shown in Figure 2, the AC tie line between area 1 and area 2 is replaced by a flexible DC system at both ends. The key parameters of the flexible DC system are shown in Table 2. Under normal conditions, the transmission power is 500MW, area 1 is the sending end system, area 2 is the receiving end system, the load and generator parameters are consistent with the original IEEE 11-node system, and the system benchmark capacity is 100MVA.

[0109] Table 2

[0110]

[0111] In order to verify the effect of frequency coupling control, the frequency coupling control parameters are set as shown in Table 3:

[0112] Table 3

[0113]

[0114] Assume that a 3pu active power step disturbance occurs in the load L1 of the sending end system. After the disturbance occurs, the frequency of the sending and receiving end systems changes as follows: Figure 7 As shown. Figure 7 It can be seen that after the power disturbance occurs in the sending system, the sending system frequency drops first, and the transmission power is reduced through the frequency coupling control of the flexible DC system. The receiving system also senses the power shortage and the frequency drops, and it shares the power disturbance with the sending system. Figure 7 The estimated frequency of the receiving end is also shown. This frequency is the change of the receiving end system frequency estimated based on the change of the DC voltage. It can be seen that the estimated result can basically follow the change of the receiving end frequency with a small error.

[0115] Under the power disturbance of the sending end system, the changes of DC voltage and power of the flexible DC system are as follows: Figure 7 and Figure 8 As shown. Figure 8 and Fig. 9 As can be seen in the figure, due to the decrease in the frequency of the sending system, the frequency coupling control will reduce the transmission power of the flexible DC system, helping the sending system to bear the power shortage; due to the decrease in the transmission power of the flexible DC system, the receiving system will have a power shortage, which will further cause the receiving system frequency to decrease. When the flexible DC system senses that the frequency of the receiving system has decreased, the receiving converter station will actively reduce its own DC voltage reference value, thereby using the DC voltage as an information carrier for the change in the AC frequency of the receiving system. The sending converter station estimates the change in the AC frequency of the receiving system through the change in DC voltage, and adjusts its own transmission power according to the frequency difference between the sending and receiving systems.

[0116] From the above results, we can see that frequency coupling control can effectively achieve frequency coupling between the sending and receiving systems by changing the operating state of the flexible DC system after a power disturbance occurs in the sending system. LB The goal of frequency coupling control is not to ensure that the frequencies of the transmitting and receiving ends are completely consistent, and there will be a certain deviation. In order to further verify the effect of frequency coupling control, the following simulation calculation is performed under the power disturbance of the receiving system, and the active power of the receiving system load L2 is set to have a 3pu step, and the frequency change of the transmitting and receiving systems is obtained as follows Fig.10 As shown. Fig.10 It can be seen that after the power disturbance occurs in the receiving system, the frequency of the receiving system decreases first. Through the frequency coupling control of the flexible DC system, the transmission power is increased. The sending system also senses the power shortage and the frequency decreases, and it shares the power disturbance with the receiving system. Similarly, the estimated frequency of the receiving system is also shown in Fig.10 It can be seen that the error between the estimated value and the actual value is small, thus ensuring the accuracy of frequency coupling control.

[0117] Under the power disturbance of the receiving system, the DC voltage and power changes of the flexible DC system are as follows: Fig.11 and Fig.12 As shown. Fig.11 and Fig.12 It can be seen that after the AC frequency of the receiving system drops, the DC voltage drops. The flexible DC sending-end converter station senses the change in the receiving-end power grid frequency through the change in DC voltage, and actively increases the reference value of the flexible DC system's transmission power, in order to provide power support for the receiving-end system.

[0118] The above simulation results show that no matter whether power disturbance occurs in the sending or receiving system, the frequency coupling control of the flexible DC system can effectively change the transmission power of the flexible DC system, thereby achieving power mutual assistance between the sending and receiving systems and improving the frequency stability of the sending and receiving systems.

[0119] As mentioned above, the setting of frequency coupling control parameters has a great influence on system stability. Now we verify the influence of key parameters of frequency coupling control on system stability through simulation. First, change the DC voltage-frequency droop coefficient K uf The size of K uf The simulation results of small disturbances are as follows: Fig.13 As shown, from Fig.13 As can be seen in uf After the value decreases, the system stability under frequency coupling control deteriorates significantly, and the DC power shows obvious oscillation under small disturbances, indicating that the reduction of is not conducive to system stability, thus verifying the conclusions of the previous theoretical analysis.

[0120] Then change the power feedback coefficient K LB The size of the small disturbance simulation result is as follows Fig.14 As shown, from Fig.14 As can be seen in LB After the value is reduced, the DC power shows obvious oscillation under small disturbances, indicating that the system frequency stability has deteriorated. Therefore, when frequency coupling control is used in practice, the DC voltage-frequency droop coefficient K should be appropriately increased. uf and power feedback coefficient K LB to ensure system stability.

[0121] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art to the present invention based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A method for coordinated frequency control of a flexible direct current transmission system and its stability analysis, comprising the following steps: (1) According to the control method of the flexible DC transmission system, a linear mapping relationship between the DC voltage of the inverter station and the AC system at the receiving end is established; (2) Based on the above linear mapping relationship, a quantitative relationship between the receiving end AC system frequency and the rectifier station DC voltage is established; (3) Based on the above quantitative relationship, establish the frequency control relationship of the AC system at both ends of the transmitter and the receiver; (4) Based on the above frequency control relationship, a system linearization model after the dual-end frequency coordinated control is established; (5) Use the system linearization model to perform stability analysis on the system under frequency coordinated control.

2. The method for double-end frequency coordinated control and stability analysis thereof according to claim 1 is characterized in that: In the step (1), under the inverter station fixed DC voltage control mode and the rectifier station fixed active power control mode, a linear mapping relationship between the inverter station DC voltage and the receiving end AC system is established as follows: ΔU dciref =K uf Δf re Where: ΔU dciref K is the change of the DC voltage reference value of the inverter station, uf is the frequency droop coefficient, Δf re is the frequency change of the receiving AC system.

3. The method for double-end frequency coordinated control and stability analysis thereof according to claim 2 is characterized in that: In the step (2), the quantitative relationship between the AC system frequency at the receiving end and the DC voltage at the rectifier station is expressed by the following expression: Where: ΔU dci is the DC voltage change of the inverter station, U dcr is the DC voltage of the rectifier station, P dcr is the active power of the rectifier station, R dc is the DC resistance, U dci0 is the initial value of the DC voltage of the inverter station.

4. The method for double-end frequency coordinated control and stability analysis thereof according to claim 3 is characterized in that: In the step (3), the control relationship of the AC frequency at both ends of the transmitter and the receiver is established by the following expression: ΔP dcrref =K If ∫(Δf se -Δf re -K LB ΔP dcrref )dt+K Pf (Δf se -Δf re -K LB ΔP dcrref ) Where: ΔP dcrref is the change in the power reference value of the rectifier station, K Pf and K If are proportional coefficient and integral coefficient respectively, K LB is the power feedback coefficient, Δf se is the frequency deviation of the sending-end AC system.

5. The method for double-end frequency coordinated control and stability analysis thereof according to claim 4 is characterized in that: The frequency control relationship can be used to realize the frequency coupling of the AC systems at both ends of the transmitter and the receiver through the power transmission of the flexible DC transmission system. LB If it is set to 0, the control target of the flexible DC transmission system is to make the AC systems at both ends of the transmission and reception have the same frequency, thus showing a similar effect to that of an AC line.

6. The method for double-end frequency coordinated control and stability analysis thereof according to claim 4 is characterized in that: The system linearization model in step (4) is expressed as follows: x=[Δf re ,Δf se ,ΔU dci ,ΔU dcr ,Δi sdi ,Δi sdr ,ΔP dcrref ] T Where: x is the state vector, A sys is the state matrix, is the first-order derivative of x, H se and H re are the inertia of the AC system at both ends, P mr and P mi are the mechanical power of the AC system at both ends, P er and P ei are the electromagnetic power of the AC system at both ends, D se and D re are the damping coefficients of the AC systems at both ends, C se and C re are the MMC equivalent capacitance of the AC system at both ends, P conr and P coni are the sending end power and receiving end power of the DC line respectively, i sdr is the active current of the sending-end MMC, K dqr and K dqi is the integration coefficient, U sr is the commutation bus voltage of the sending-end MMC, U si is the commutation bus voltage of the receiving MMC, i sdi is the active current of the receiving MMC, U dci is the DC voltage of the inverter station, P in is the input power of the flexible DC transmission system, ΔU dcr is the DC voltage change of the rectifier station, Δi sdi is the active current change of the receiving MMC, Δi sdr is the active current change of the sending-end MMC, T Indicates transpose.

7. The method for double-end frequency coordinated control and stability analysis thereof according to claim 6 is characterized in that: In step (5), by changing K uf , K LB , H se , H re The control parameter size including the state matrix A is drawn sys The changes of characteristic roots in the complex plane are used to analyze the stability of the system.

8. A computer device comprising a memory and a processor, wherein a computer program is stored in the memory, wherein: The processor is used to execute the computer program to implement the two-terminal frequency cooperative control and stability analysis method as described in any one of claims 1 to 7.

9. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the two-terminal frequency cooperative control and stability analysis method thereof as described in any one of claims 1 to 7 are implemented.

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