Dual-end frequency coordinated control and stability analysis method of flexible DC transmission system

Abstract

This invention discloses a dual-end frequency coordinated control method for flexible DC transmission systems and its stability analysis method. First, it establishes a linear mapping relationship between the DC voltage of the inverter station and the AC system at the receiving end. Then, it establishes a quantitative relationship between the AC frequency at the receiving end and the DC voltage of the rectifier station. Next, it establishes the dual-end AC frequency system control relationship of the flexible DC transmission system. Then, it establishes a linearized model of the system after the dual-end frequency coordinated control is implemented. Finally, it performs system stability analysis under frequency coordinated control. This invention's frequency coupling control method can effectively improve the overall frequency stability of the system through power mutual assistance between the sending and receiving end systems.

Description

Technical Field

[0001] This invention belongs to the field of power system technology, specifically relating to a dual-end frequency coordinated control method for a flexible DC transmission system and its stability analysis method. Background Technology

[0002] In the developed eastern regions of my country, high load density and dense AC grid structures have led to prominent issues such as excessive short-circuit currents and difficulties in power flow control. The scale effect of AC grids is approaching saturation, and the marginal benefits of AC grid investment are significantly decreasing. Using flexible DC systems to partition the AC grid can effectively address these issues in high-load-density areas, but it reduces the size of synchronous AC systems and degrades frequency stability. Flexible DC systems offer flexible and varied control methods, simulating different characteristics. Therefore, if the flexible control and power regulation capabilities of flexible DC systems can be fully utilized, maintaining frequency coupling after AC grid partitioning becomes possible, thus providing a feasible solution for grid structure designs that adapt to the trend of high-density load development.

[0003] Existing frequency regulation methods for flexible DC systems, such as those in the literature [Liu Yingpei, Xie Qian, Liang Haiping. Adaptive Virtual Inertial Frequency Regulation Control Strategy for Flexible DC Transmission Systems [J]. Automation of Electric Power Systems, 2021, 45(05): 129-136], typically provide frequency support in a single direction to either the sending or receiving end of the power grid, failing to achieve the frequency coupling effect similar to that of AC lines. This is because flexible DC systems have one active power control degree of freedom at both the sending and receiving inverter stations, and one of these degrees must be used to control the DC voltage. For AC systems, flexible DC systems only have one available active power degree of freedom, increasing the difficulty of bidirectional frequency regulation for flexible DC systems.

[0004] The literature [Hong Chun, Shao Zongxue, Zhao Wei, et al. Research on emergency frequency control of AC / DC hybrid power systems with flexible DC [J]. Journal of Electrical Engineering, 2023, 38(20): 5590-5604] has proven that frequency regulation through emergency power support is a feasible method that can take into account both sending and receiving end frequency regulation. However, the realization of bidirectional emergency power support requires the use of communication means, and the action threshold and support power size need to be determined offline through multiple simulations. This will cause an excessive burden of simulation calculation in new energy power systems with variable operating modes. Frequency support for back-to-back flexible DC system regional interconnection systems and long-line flexible DC systems has also received widespread attention, but there are corresponding unresolved issues. Therefore, the bidirectional 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 for a flexible DC 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 dual-end frequency coordinated control and stability analysis method for a flexible DC transmission system includes the following steps:

[0007] (1) Based on the control mode of the flexible DC transmission system, establish a linear mapping relationship between the DC voltage of the inverter station and the AC system at the receiving end;

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

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

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

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

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

[0013] ΔU dciref =K uf Δf re

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

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

[0016]

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

[0018] Furthermore, in step (3), the control relationship of the AC frequency between the sending and receiving ends is established through 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 K represents the change in the rectifier station power reference value. Pf and K If These are the proportional coefficient and the integral coefficient, respectively, K LB For the power feedback coefficient, Δf se This refers to the frequency deviation of the sending-end AC system.

[0021] Furthermore, the frequency control relationship can be used to achieve frequency coupling between the AC systems at both ends of the power transmission system via a flexible DC transmission system; if the power feedback coefficient K is... LB If the value is set to 0, the control objective of the flexible DC transmission system is to ensure that the AC systems at both the sending and receiving ends are at the same frequency, thus exhibiting a similar function to an AC line.

[0022] Furthermore, the linearized model of the system 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 The state matrix, H is the first derivative of x. se and H re P represents the inertia of the AC systems at the sending and receiving ends, respectively. mr and P mi P represents the mechanical power of the AC systems at the sending and receiving ends, respectively. erand P ei The electromagnetic power of the AC system at the sending and receiving ends, D, are respectively. se and D re These are the damping coefficients of the AC systems at the sending and receiving ends, C and C, respectively. se and C re These are the equivalent MMC capacitors of the AC systems at the sending and receiving ends, respectively, P conr and P coni These represent the sending-end power and receiving-end power of a DC line, i sdr K is the active current of the sending-end MMC. dqr and K dqi U is the integral coefficient. sr U is the converter bus voltage of the sending-end MMC. si i is the converter bus voltage of the receiving-end MMC. sdi U is the active current of the receiving-end MMC. dci P is the DC voltage of the inverter station. in For the input power of the flexible DC transmission system, ΔU dcr Let Δi be the change in DC voltage at the rectifier station. sdi Δi represents the change in active current at the receiving end of the MMC. sdr This represents the change in active current at the sending-end MMC. T This indicates transpose.

[0027] Furthermore, in step (5), by changing K... uf K LB H se H re The state matrix A is plotted based on the values ​​of the control parameters, including those included. sys The variation of eigenvalues ​​in the complex plane is used to perform stability analysis on the system.

[0028] A computer device includes a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the above-described method for dual-end frequency coordinated control and stability analysis of a flexible DC transmission system.

[0029] A computer-readable storage medium storing a computer program, which, when executed by a processor, implements the above-described method for dual-end frequency coordinated control and stability analysis of a flexible DC transmission system.

[0030] This invention addresses the frequency coupling issue at the sending and receiving ends of flexible DC transmission systems, proposing a dual-end frequency coordinated control method and its stability analysis. The frequency coupling control method of this invention can effectively improve the overall frequency stability of the system through power mutual assistance between the sending and receiving ends. Furthermore, in practical applications, the DC voltage-receiving end frequency droop coefficient K can be appropriately increased. ufPower feedback coefficient K in frequency coupling control of sending-end converter station LB This ensures system stability. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the equivalent model structure of a flexible DC transmission system at both ends.

[0032] Figure 2 The eigenvalues ​​of the state matrix are proportional coefficients K uf A diagram illustrating the changes.

[0033] Figure 3 The eigenvalues ​​of the state matrix follow the feedback coefficients K LB A diagram illustrating the changes.

[0034] Figure 4 The eigenvalues ​​of the state matrix follow the inertia H of the receiving system. re A diagram illustrating the changes.

[0035] Figure 5 The eigenvalues ​​of the state matrix follow the inertia H of the sending system. re A diagram illustrating the changes.

[0036] Figure 6 This is a schematic diagram of the modified IEEE 11-node system topology.

[0037] Figure 7 This is a schematic diagram of frequency changes under disturbances in the sending-end system.

[0038] Figure 8 This is a schematic diagram of DC voltage changes under disturbances in the sending-end system.

[0039] Figure 9 This is a schematic diagram of DC power changes under disturbances in the sending-end system.

[0040] Figure 10 This is a schematic diagram of frequency changes under disturbances in the receiving system.

[0041] Figure 11 This is a schematic diagram of DC voltage changes under disturbances in the receiving-end system.

[0042] Figure 12 This is a schematic diagram of DC power changes under disturbances in the receiving-end system.

[0043] Figure 13 For different K uf The simulation results of small perturbations under different sizes are shown in the figure.

[0044] Figure 14 For different K LB The simulation results of small perturbations under different sizes are shown in the figure. Detailed Implementation

[0045] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0046] The present invention relates to a dual-end frequency coordinated control and stability analysis method for a flexible DC transmission system, comprising the following steps:

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

[0048] ΔU dciref =K uf Δf re

[0049] In the formula: ΔU dciref K represents the change in the DC voltage reference value of the inverter station. uf Δf is the proportionality constant. re This represents 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 as follows:

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

[0053] In the formula: U dcr U is the DC voltage of the rectifier station. dci I is the DC voltage of the inverter station. dc For direct current, R dc It is a DC resistance.

[0054] Direct current can also be expressed as:

[0055]

[0056] In the formula: P dcr This refers to the active power of the rectifier station.

[0057] The relationship between the DC voltage of the inverter station and the DC voltage and power of the rectifier station can then be obtained as follows:

[0058]

[0059] Since the frequency response time scale is on the order of seconds, while the response time scale of the converter station power or DC voltage command value is on the order of 10 milliseconds, when considering the frequency characteristics of the AC system, the electrical quantities can be considered to follow the command value in real time.

[0060] U dci =U dciref

[0061] P dcr =P dcrref

[0062] In the formula: P dcrref This is the power command value for the flexible DC rectifier station.

[0063] The quantitative relationship between the AC frequency at the receiving end and the DC voltage at the rectifier station can then be obtained as follows:

[0064]

[0065] In the formula: U dci0 This represents the initial value of the DC voltage at 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 and receiving frequencies without loss of generality, the rectifier power command value can be set in the following form:

[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 These are the proportional and integral coefficients, respectively, K LB For the feedback coefficient, Δf se For the frequency deviation of the sending system, ΔP dcrref This represents the change in the power reference value.

[0070] The above formula can be used to achieve frequency coupling between the sending and receiving AC systems through power transmission in a flexible DC system; if the feedback coefficient K is included in the above formula... LB If set to 0, the control objective of the flexible DC system is to make the sending and receiving ends operate at the same frequency, thus exhibiting a similar function to an AC line.

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

[0072] First, establish such as Figure 1 The system equivalent model shown in the figure, P inFor the input power of the flexible DC system, P con P represents the line transmission power. out For the output power of the flexible DC system, C se and C re The equivalent capacitances R of the MMC at the sending and receiving ends are respectively. dc This refers to the resistance of a DC line.

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

[0074]

[0075]

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

[0077] The differential equation for the equivalent capacitance of the sending and receiving end MMC converter stations can be written as:

[0078]

[0079]

[0080] In the formula: C se and C re These are the equivalent capacitances of the MMC at the sending and receiving ends, respectively, P conr and P coni These represent the sending and receiving power 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 element, the change of AC side active current can be approximated as shown in the following formula:

[0082]

[0083] In the formula: i sdr K is the active current of the sending-end MMC. dqr is the integral coefficient.

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

[0085]

[0086] The relationship between current and power is as follows:

[0087] P in =i sdr U sr

[0088] In the formula: U sr This refers to the voltage of the MMC converter bus at the sending end.

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

[0090]

[0091] In the formula: i sdr K represents the active current of the receiving-end MMC. dqr U is the integral coefficient. si This is the voltage of the receiving-end MMC converter bus.

[0092] The circuit equation for the flexible DC system is:

[0093]

[0094] In the formula: I dc It is direct current.

[0095] Combining all the above formulas, the linearized model of the system after the frequency coupling control at the sending 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) Stability analysis of the system under frequency coupling control.

[0100] Typical parameters are selected as shown in Table 1. To analyze the impact of frequency coupling control on system stability, matrix A is plotted by changing the control parameters. sys The variation of the characteristic roots in the complex plane. First, change the droop ratio K between the DC voltage and AC frequency of the receiving-end flexible DC converter station. uf , obtain K ufThe changes in the eigenvalues ​​of matrix Asys during the increase are as follows: Figure 2 As shown, from Figure 2 As can be seen from K uf As the value of increases, the system's eigenvalues ​​gradually move away from the imaginary axis, indicating an improvement in system stability. According to K... uf From the physical meaning, K uf The larger the value, the greater the amplitude of the change in DC voltage of the flexible DC system with the AC frequency of the receiving end system. The DC voltage can better reflect the frequency change information, which is beneficial to the stability of the system during frequency coupling control.

[0101] Table 1

[0102]

[0103] The receiving-end flexible DC converter station employs 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 employs sending-receiving-end frequency deviation integral control. Without loss of generality, a DC power feedback signal is added to the sending-receiving-end frequency deviation. If the feedback coefficient KLB is changed, the changes in the system characteristic roots under frequency-coupled control are obtained as follows: Figure 3 As shown. From Figure 3 As can be seen, with the feedback coefficient K LB As the feedback coefficient K increases, the system's eigenvalues ​​shift away from the imaginary axis. The imaginary part of the eigenvalues ​​remains unchanged; only the real part decreases. This indicates that increasing the feedback coefficient K... LB This can increase the stability of frequency-coupled control. However, it should be noted that as the feedback coefficient K... LB Increasing the feedback coefficient K will decrease the frequency coupling between the sending and receiving ends; LB When K is 0, the goal of frequency coupling control is to make the sending and receiving systems operate at the same frequency. LB When the value is greater than 0, the frequency of the sending and receiving ends will maintain a certain deviation.

[0104] The above analysis examined the impact of two key control parameters in frequency-coupled control on system stability. Besides these parameters, the magnitude of the inertia of the sending and receiving systems is also a crucial factor affecting stability. Generally, if the difference in inertia between the sending and receiving systems is too large, it becomes difficult for the flexible DC system to maintain synchronized frequencies. Conversely, if the inertia of the sending and receiving systems are similar, frequency-coupled control is more likely to achieve better control performance. First, by increasing the inertia of the receiving system, the change in the system's characteristic roots is obtained as follows: Figure 4 As shown, from Figure 4 As the inertia of the receiving system increases, the system's characteristic roots shift toward the imaginary axis, indicating that the system's stability deteriorates.

[0105] Furthermore, by changing the magnitude of the inertia of the sending-end system, the changes in the system's characteristic roots are obtained as follows: Figure 5 As shown, from Figure 5 As can be seen, an increase in the inertia of the sending-end system causes the eigenvalues ​​on the real axis to shift towards the imaginary part; however, the conjugate eigenvalues ​​closest to the imaginary axis hardly shift with the increase in the inertia of the sending-end system. This phenomenon indicates that an increase in the inertia of the sending-end system also has a negative impact on the stability of frequency-coupled control, but the mechanism differs from that of changes in the inertia of the receiving-end system.

[0106] As can be seen from the above analysis, the larger the inertia of the sending and receiving end system, the greater the negative impact of frequency coupling control on system stability. The stability of frequency coupling control can be improved by increasing the DC voltage-frequency droop coefficient or increasing the power feedback coefficient.

[0107] (6) Case analysis.

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

[0109] Table 2

[0110]

[0111] To verify the effectiveness of frequency coupling control, the frequency coupling control parameters were set as shown in Table 3:

[0112] Table 3

[0113]

[0114] Assume that the active power step disturbance of 3 pu occurs in the load L1 of the sending-end system. After the disturbance occurs, the frequency change of the sending-end and receiving-end systems is as follows: Figure 7 As shown. From Figure 7 As can be seen, after a power disturbance occurs in the sending-end system, the frequency of the sending-end system first drops. Through the frequency coupling control of the flexible DC system, the transmitted power is reduced. The receiving-end system also senses the power deficit, and its frequency drops, sharing the power disturbance with the sending-end system. Figure 7 The document also shows the estimated frequency at the receiving end, which is the frequency change of the receiving end system estimated based on the change in DC voltage. As can be seen, the estimation result can basically follow the frequency change at the receiving end with a small error.

[0115] Under the power disturbance of the sending-end system, the changes in DC voltage and power of the flexible DC system are as follows: Figure 7 and Figure 8 As shown. From Figure 8 and Figure 9 As can be seen, due to the decrease in the frequency of the sending-end system, frequency coupling control reduces the transmission power of the flexible DC system, helping the sending-end system to cover the power deficit. Because of the decrease in the transmission power of the flexible DC system, the receiving-end system experiences a power deficit, which further leads to a decrease in the frequency of the receiving-end system. When the flexible DC system senses the decrease in the frequency of the receiving-end system, the receiving-end converter station will actively lower its own DC voltage reference value, thus using the DC voltage as an information carrier of the AC frequency change in the receiving-end system. The sending-end converter station estimates the change in the AC frequency of the receiving-end 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] The results above show 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-end system. This is due to the power feedback coefficient K... LB The existence of frequency coupling control means that the goal is not to guarantee that the frequencies of the sending and receiving ends are completely consistent; there will be some deviation. To further verify the effect of frequency coupling control, a simulation calculation is performed below under power disturbance in the receiving end system. The active power of the load L2 in the receiving end system is increased by 3pu, and the frequency change of the sending and receiving end systems is obtained as follows: Figure 10 As shown. From Figure 10 As can be seen, after a power disturbance occurs in the receiving-end system, its frequency initially drops. Through frequency coupling control of the flexible DC system, the transmitted power is increased. The sending-end system also senses the power deficit, causing its frequency to drop as well, thus sharing the power disturbance with the receiving-end system. Similarly, the estimated frequency of the receiving-end system is also shown in... Figure 10 As can be seen, 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 end system, the changes in DC voltage and power of the flexible DC system are as follows: Figure 11 and Figure 12 As shown. From Figure 11 and Figure 12 As can be seen, when the AC frequency of the receiving-end system decreases, the DC voltage decreases. The converter station at the flexible DC-DC transmission end senses the frequency change of the receiving-end grid through the change in DC voltage, and thus actively increases the reference value of the power transmission of the flexible DC-DC system in order to provide power support for the receiving-end system.

[0118] The simulation results show that, regardless of whether power disturbances occur 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 earlier, the setting of frequency coupling control parameters has a significant impact on system stability. Below, we will verify the influence of key frequency coupling control parameters on system stability through simulation. First, we will change the DC voltage-frequency droop coefficient K. uf The size of K is used to obtain different values. uf Simulation results of small perturbations under different sizes are as follows: Figure 13 As shown, from Figure 13 As can be seen from this, when K uf After the value is reduced, the system stability under frequency coupling control deteriorates significantly. Under small disturbances, the DC power exhibits obvious oscillations, indicating that the reduction is detrimental to system stability, thus verifying the theoretical analysis conclusions above.

[0120] Then change the power feedback coefficient K LB The size of the disturbance is used to obtain simulation results for small disturbances, such as... Figure 14 As shown, from Figure 14 As can be seen from this, when K LB After the value was reduced, the DC power exhibited significant oscillations under small disturbances, indicating a deterioration in system frequency stability. Therefore, in practical applications, the DC voltage-frequency droop coefficient K should be appropriately increased in frequency-coupled control. uf and power feedback coefficient K LB The value is set to ensure system stability.

[0121] The above description of the embodiments is provided to enable those skilled in the art to understand and apply the present invention. Those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A dual-end frequency coordinated control and stability analysis method for a flexible DC transmission system, comprising the following steps: (1) Based on the control mode of the flexible DC transmission system, establish a linear mapping relationship between the DC voltage of the inverter station and the AC system at the receiving end; (2) Based on the above linear mapping relationship, establish a quantitative relationship between the frequency of the receiving-end AC system and the DC voltage of the rectifier station; (3) Based on the above quantitative relationship, the frequency control relationship of the AC systems at both ends of the sending and receiving ends 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 ) in: ΔP dcrref K represents the change in the rectifier station power reference value. Pf and K If These are the proportional coefficient and the integral coefficient, respectively, K LB For the power feedback coefficient, Δf se For the frequency deviation of the sending-end AC system, Δf re This refers to the frequency variation of the receiving-end AC system. (4) Based on the above frequency control relationship, establish a linearized model of the system after the implementation of dual-end frequency coordinated control. The specific expression is 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 The state matrix, H is the first derivative of x. se and H re P represents the inertia of the AC systems at the sending and receiving ends, respectively. mr and P mi P represents the mechanical power of the AC systems at the sending and receiving ends, respectively. er and P ei The electromagnetic power of the AC system at the sending and receiving ends, D, are respectively. se and D re These are the damping coefficients of the AC systems at the sending and receiving ends, C and C, respectively. se and C re These are the equivalent MMC capacitors of the AC systems at the sending and receiving ends, respectively, P conr and P coni These represent the sending-end power and receiving-end power of a DC line, i sdr K is the active current of the sending-end MMC. uf K is the frequency droop coefficient. dpr and K dpi Let ΔU be the integral coefficient. dci U represents the change in DC voltage at the inverter station. dcr R is the DC voltage of the rectifier station. dc U is the DC resistance. sr U is the converter bus voltage of the sending-end MMC. si i is the converter bus voltage of the receiving-end MMC. sdi U is the active current of the receiving-end MMC. dci P is the DC voltage of the inverter station. in For the input power of the flexible DC transmission system, ΔU dcr Let Δi be the change in DC voltage at the rectifier station. sdi Δi represents the change in active current at the receiving end of the MMC. sdr The change in active current at the sending end MMC is represented by T, where T denotes transpose. (5) Use the system linearization model to perform stability analysis on the system under frequency cooperative control.

2. The dual-end frequency cooperative control and its stability analysis method according to claim 1, characterized in that: In step (1), under the constant DC voltage control mode of the inverter station and the constant active power control mode of the rectifier station, the linear mapping relationship between the DC voltage of the inverter station and the AC system at the receiving end is established as follows: ΔU dciref =K uf Δf re Where: ΔU dciref This represents the change in the DC voltage reference value of the inverter station.

3. The dual-end frequency cooperative control and its stability analysis method according to claim 2, characterized in that: In step (2), the quantitative relationship between the frequency of the AC system at the receiving end and the DC voltage of the rectifier station is expressed by the following expression; Where: P dcr U represents the active power of the rectifier station. dci0 This represents the initial value of the DC voltage at the inverter station.

4. The dual-end frequency cooperative control and its stability analysis method according to claim 1, characterized in that: The frequency control relationship described above enables frequency coupling between the AC systems at both ends of a flexible DC transmission system for power transmission; if the power feedback coefficient K is... LB If the value is set to 0, the control objective of the flexible DC transmission system is to ensure that the AC systems at both the sending and receiving ends are at the same frequency, thus exhibiting a similar function to an AC line.

5. The dual-end frequency coordinated control and its stability analysis method according to claim 1, characterized in that: In step (5), by changing K uf K LB H se H re The state matrix A is plotted based on the values ​​of the control parameters, including those included. sys The variation of eigenvalues ​​in the complex plane is used to perform stability analysis on the system.

6. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: The processor is used to execute the computer program to implement the dual-end frequency cooperative control and its stability analysis method as described in any one of claims 1 to 5.

7. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by the processor, it implements the dual-end frequency cooperative control and its stability analysis method as described in any one of claims 1 to 5.

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