Transient stability analysis method for hybrid double-end feed-in direct current transmission receiving end and control strategy thereof

Abstract

The invention discloses a hybrid double-end feed-in direct current transmission receiving end transient stability analysis method and a control strategy thereof, and relates to the field of high-voltage direct current transmission transient stability evaluation. An equivalent network port model and an LCC-HVDC large signal model in an MMC-LCC hybrid double-end feed-in high-voltage direct current transmission system are established; in combination with the established equivalent network port model and the nonlinear transient large signal model, the dynamic characteristics of the extinction angle in the transient process are analyzed by applying a phase diagram method, and the occurrence mechanism of the continuous commutation failure is clarified; the transient evolution law of the system under different controller parameters is analyzed, and the influence of the controller parameters on the transient stability of the LCC is disclosed; according to the LCC-HVDC receiving end transient state stabilization control strategy provided by the invention, normal operation under a steady state working condition can be realized, and meanwhile, the stable operation capability of the hybrid double-infeed high-voltage direct-current power transmission system under various power grid voltage sag degrees is improved.

Description

Technical Field

[0001] The present invention relates to the field of transient stability assessment of high voltage direct current transmission, and in particular to a method for analyzing transient stability of a hybrid double-end-fed direct current transmission receiving end.

[0002] The present invention also relates to a DC voltage control strategy based on transient stability analysis of a hybrid double-end-fed DC power transmission receiving end. Background Art

[0003] When the infeed point of flexible DC projects represented by modular multilevel converter high voltage DC transmission (MMC-HVDC) is close to the receiving converter station of conventional grid phase-commutated converter high voltage DC transmission (LCC-HVDC) that has been put into operation, a hybrid multi-infeed DC transmission system is formed. The hybrid multi-infeed DC transmission system has various converter types, variable operating conditions, and strong coupling characteristics between different components, which poses new challenges to the safe and stable operation and transient stability of the power grid.

[0004] Taking a hybrid double-feed DC transmission system composed of a LCC-HVDC and an MMC-HVDC as an example, a hybrid multi-feed DC transmission system is a special DC transmission topology that combines the technical characteristics of LCC-HVDC and MMC-HVDC. In its basic component units, the current source and the current source converter are closely coupled in terms of electrical and control; and the operating conditions of the LCC-HVDC subsystem and the MMC-HVDC subsystem are changeable and the control combination modes are diverse, making the interaction between the two subsystems more complicated.

[0005] To study the transient stability of the hybrid double-end-fed DC transmission system, it is necessary to analyze the changing rules of electrical parameters in the transient process, deeply explore the instability mechanism and propose corresponding optimization schemes based on it. This is of great theoretical and practical significance for ensuring that the high-voltage DC transmission system maintains safe and stable operation in the face of large disturbances. Therefore, it is necessary to analyze the transient stability of the hybrid double-end-fed DC transmission receiving system under the temporary voltage drop of the power grid. Summary of the invention

[0006] The purpose of the present invention is to: in view of the above-mentioned problems, the present invention provides a method for analyzing the transient stability of a hybrid double-end-fed HVDC receiving end, by establishing an equivalent network port model of an MMC-LCC hybrid double-fed HVDC transmission system and a large signal model of an LCC-HVDC system therein, and combining the above-mentioned equivalent port model and the nonlinear transient large signal model to apply a phase diagram method to analyze the dynamic characteristics of the arc extinction angle in the transient process, clarify the occurrence mechanism of continuous commutation failure, and analyze the transient evolution law of the system under different controller parameters and reveal the influence of the controller parameters on the transient stability of the MMC-LCC system; a transient stabilization control strategy for an LCC-HVDC receiving end is proposed, which can achieve normal operation under steady-state conditions while improving the ability of the hybrid double-fed HVDC transmission system to operate stably under the degree of grid voltage sag.

[0007] The technical solution adopted by the present invention is as follows:

[0008] The transient stability analysis method of hybrid double-end infeed DC transmission receiving end is implemented in the following steps:

[0009] Step 1: construct an MMC-LCC hybrid high-voltage direct current transmission receiving-end circuit model, and simultaneously construct an LCC-HVDC receiving-end control model and an MMC-HVDC receiving-end control model;

[0010] Step 2: Analyze the output characteristics of the MMC and LCC through the LCC-HVDC receiving end control model and the MMC-HVDC receiving end control model constructed in step 1, describe the output characteristics of the MMC and LCC receiving ends in the form of equivalent current sources, and establish an equivalent network port model of the MMC-LCC system. The equivalent network port is combined with the high-voltage direct current transmission receiving end circuit model to form a simplified circuit model of the system, and establish node current equations and voltage equations in the simplified circuit model;

[0011] Step 3, respectively analyzing the coupling characteristics of the common bus voltage of the MMC-LCC control system through the node current equation and voltage equation established in step 2, and solving the LCC bus voltage amplitude after the fault occurs;

[0012] Step 4, obtaining the initial state of the LCC at the time of the fault through the coupled bus voltage amplitude obtained in step 3 and the LCC control system, and establishing a dynamic nonlinear mathematical equation for the arc extinction angle of the LCC-HVDC under a symmetrical grid drop fault;

[0013] Step 5: Use the dynamic nonlinear mathematical equation of the arc extinction angle obtained in step 4 to quantitatively analyze the evolution process and transient stability of the system during the transient process, solve the boundary control parameters that maintain the stability of the system, and optimize the control parameters of the LCC-HVDC.

[0014] Furthermore, the high-voltage direct current transmission receiving end circuit model specifically includes:

[0015] MMC and LCC are connected in parallel to form a hybrid high-voltage direct current transmission receiving-end circuit model. The inverter side includes MMC-HVDC and LCC-HVDC dual subsystems. The dual subsystems are connected to two sections of AC busbars through converter transformers. The LCC-HVDC subsystem is connected to the reactive compensation device. The two sections of AC busbars are connected through a tie line; the two sections of AC busbars are connected to two groups of AC power grids through line impedance.

[0016] Furthermore, the LCC-HVDC receiving-end control model is specifically that the inverter side of the circuit model adopts constant DC voltage control, and its DC current remains unchanged under the control of the rectifier side; the MMC-HVDC receiving-end control model is specifically that the inverter side of the circuit model adopts constant active power and reactive power control.

[0017] Furthermore, the output port network of the LCC is specifically as follows:

[0018]

[0019] In the formula, v dc 、v dcref Respectively represent DC voltage and DC voltage reference value; u t1 is the AC bus voltage amplitude connected to the LCC; β, γ are the trigger advance angle and arc extinction angle; k pLCC and k iLCC are the coefficients of the PI controller in the constant DC voltage control of the LCC inverter; k is the transformer ratio, R c is the equivalent commutation resistance of LCC; i dc is direct current;

[0020] The output port network of the MMC is as follows:

[0021]

[0022] Where P ref , Q ref , P and Q represent reference active power, reference reactive power, MMC output active power and output reactive power respectively; k p , k i and k pPLL ,ki PLL Respectively represent the proportional coefficient and integral coefficient of the current inner loop PI controller; θ PLL and ω n Respectively represent the phase angle of the phase-locked loop output and the rated frequency; i dref 、i qref and v qThey represent the d-axis reference current, q-axis reference current and the AC bus voltage on the q-axis respectively.

[0023] Furthermore, the node current equation and voltage equation obtained by simplifying the circuit model in step 2 through Kirchhoff's theorem are as follows:

[0024]

[0025] In the formula, u t2 is the AC bus voltage amplitude connected to the MMC; θ t1 ,θ t2 are the phase angles of the two AC bus voltages respectively; 11 ,y 12 ,y 21 ,y 22 They are the four impedance matrix coefficients of the simplified circuit model, θ 11 ,θ 12 ,θ 21 ,θ 22 are the impedance angles of each impedance coefficient; i1 and i2 are the output current amplitudes of LCC and MMC respectively; are the power factor angles of LCC and MMC respectively; x1 and x2 are the amplitudes of line impedances X1 and X2 respectively, v g1 and v g2 is the amplitude of the connected AC power grid; θ x1 and θ x2 are the phases of the line impedances X1 and X2 respectively, and j represents the 90° rotation factor in the phasor.

[0026] Further, according to the characteristics of the MMC-LCC control system in step 2, the initial trigger lead angle is specifically obtained by the following formula:

[0027] β or =β nop -k pLCC ·v dcref (4)

[0028] In the formula, β or , β nop are the initial trigger advance angle of the fault and the trigger advance angle during normal operation before the fault, respectively; the node current equation and voltage equation in step 2 are combined with the initial trigger advance angle and the initial phase-locked loop output phase angle when the grid symmetrical sag occurs to solve the voltage amplitude of the MMC and LCC coupled buses at the moment of the fault, and the initial trigger advance angle and the initial phase-locked loop output phase angle are substituted into the node current and voltage equations to solve the line voltage amplitude of the common AC bus of the MMC-LCC system at the moment of the fault.

[0029] Furthermore, the dynamic nonlinear mathematical equation of the arc extinction angle of LCC-HVDC under the symmetrical grid drop fault in step 4 is specifically as follows:

[0030]

[0031] The DC voltage control strategy based on the transient stability analysis of the hybrid double-end-fed DC transmission receiving end adopts the hybrid double-end-fed DC transmission receiving end transient stability analysis method, which is specifically implemented in the following steps:

[0032] Step 1, draw the phase diagram curve under different DC voltage reference values ​​in combination with the arc extinction angle nonlinear dynamic equation, and analyze the influence of the DC voltage reference value in the constant DC voltage control of LCC-HVDC on the transient stability of LCC;

[0033] Step 2: Based on the influence of the DC voltage reference value on the transient stability of LCC in step 1, a maximum coefficient boundary is obtained to maintain the LCC-HVDC in the MMC-LCC system from continuous commutation failure under a grid voltage sag. The DC voltage reference value below the coefficient boundary ensures that the LCC-HVDC does not suffer from continuous commutation failure.

[0034] Furthermore, the influence of the DC voltage reference value in the constant DC voltage control of LCC-HVDC on the transient stability of LCC is analyzed in step 1 as follows: from multiple sets of phase diagram curves, it can be obtained that when the DC voltage reference value is reduced, the phase diagram curve of the arc extinction angle gradually moves up, and the arc extinction angle gradually avoids decreasing to the minimum value γ0 that ensures that no commutation failure occurs, that is, the controller reduces the DC voltage reference value to enhance the transient stability of the LCC.

[0035] Furthermore, in step 2, the maximum coefficient boundary for maintaining the LCC-HVDC without continuous commutation failure is the DC voltage reference value obtained when the derivative of the arc extinction angle is equal to zero when the arc extinction angle takes its minimum value γ0, which is specifically as follows:

[0036]

[0037] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0038] 1. The transient stability analysis method proposed in the present invention for the hybrid double-end-fed DC transmission receiving-end system under symmetrical grid voltage sag fills the gap in the transient stability analysis of LCC-HVDC in the hybrid feeding system under symmetrical grid voltage sag. The established dynamic nonlinear differential equation of the arc extinction angle can quantitatively analyze the action characteristics and evolution law of the hybrid double-end-fed DC transmission receiving-end in the transient process under large disturbances and deduce the LCC continuous commutation failure mechanism.

[0039] 2. The LCC-HVDC stabilization control strategy for the receiving end of the hybrid double-end fed DC transmission system proposed in the present invention is intended to ensure that the LCC inverter operates stably when a symmetrical drop in the grid voltage occurs, and that continuous commutation failure does not occur; the correction of the DC voltage reference value ensures the adaptive adjustment of the system when a fault occurs; the stabilization control strategy significantly improves the stability and operational reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 A receiving-end circuit model of a hybrid double-end-fed DC transmission system provided in an embodiment of the present invention;

[0041] Figure 2 The LCC-HVDC control model and the MMC-HVDC control model provided by the embodiments of the present invention;

[0042] Figure 3 An equivalent simplified circuit model of a hybrid double-end-fed DC transmission receiving end provided in an embodiment of the present invention;

[0043] Figure 4 A phase diagram curve of the arc extinction angle under different DC voltage controller parameters provided by an embodiment of the present invention;

[0044] Figure 5 A process for solving a critical DC voltage reference coefficient in a transient stabilization control strategy provided by an embodiment of the present invention;

[0045] FIG6 is a diagram showing the LCC-HVDC arc extinction angle and DC voltage simulation waveforms under different DC voltage reference values ​​provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The present invention will be described in detail below in conjunction with the accompanying drawings.

[0047] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0048] Example 1

[0049] This embodiment discloses a method for transient stability analysis of a hybrid double-end-fed DC transmission receiving-end system, which is specifically implemented in the following steps:

[0050] Step 1: construct a high-voltage direct current transmission receiving-end circuit model consisting of an MMC and an LCC in parallel; construct an LCC-HVDC receiving-end control model based on the DC voltage control strategy on the inverter side and the DC current control strategy on the rectifier side; construct an MMC-HVDC receiving-end control model based on the constant power control strategy on the inverter side;

[0051] like Figure 1 As shown in the figure, in the MMC and LCC hybrid double-feed HVDC transmission receiving end circuit model, the two parallel MMC-HVDC and LCC-HVDC subsystems on the inverter side are core components, which are connected to the two AC busbars through converter transformers. The ratio of the converter transformer is k:1, and the amplitude and phase angle of the two AC busbars are u t1 、u t2 ,θ t1 ,θ t2 , where the LCC-HVDC subsystem needs to be connected to the reactive power compensation device B c , the inductance of the connecting line X passes through the two AC busbars tie Connection; the two sections of AC busbars are connected to two groups of AC power grids (equivalent voltage sources) through line impedances X1 and X2 respectively;

[0052] Step 2: Based on the MMC control model and the LCC control model in step 2, the output characteristics of the two are analyzed and the two are equivalent to an AC current source, and an equivalent output port network of the MMC-LCC system is established. The output port network is combined with the circuit model in step 1 to generate a simplified circuit model of the system. In the simplified circuit model, Kirchhoff's theorem is used to write node current equations and voltage equations;

[0053] Step 3: Based on the control characteristics of the MMC-LCC system in step 2, the initial values ​​of the MMC phase-locked loop phase angle and the LCC trigger lead angle when the grid symmetrical sag occurs are obtained, and the voltage amplitudes of the MMC and LCC coupled buses at the fault moment are solved by combining the established node current equation and voltage equation;

[0054] Step 4: Based on the coupled bus voltage amplitude obtained in step 3 and in combination with the structure of the DC voltage controller and the DC current control model, a dynamic nonlinear mathematical equation for the arc extinction angle of the LCC-HVDC under a symmetrical grid drop fault is established.

[0055] Step 5: Based on the dynamic nonlinear mathematical equation of the arc extinction angle established in step 4, the evolution law of the arc extinction angle of LCC-HVDC during a symmetrical grid voltage sag fault is analyzed, and the continuous commutation failure mechanism is revealed.

[0056] like Figure 2 As shown, the DC voltage of the LCC inverter at the receiving end of the hybrid dual-feed DC transmission in this embodiment is respectively composed of Figure 2 The DC voltage controller in the middle (a) is used for control. The DC voltage controller collects the DC voltage of the LCC inverter and passes it through a first-order filter, then makes a difference with the reference voltage and passes the error through a PI controller to generate a trigger lead angle β, and then a trigger angle α. The MMC inverter adopts constant power control. Figure 2Part (b) shows that the MMC inverter adopts constant power control. The difference between the reference power and the actual power is obtained by the PI controller to obtain the dq axis current reference value. The dq axis current reference value is input into the current loop to generate a drive signal to control the switching tube to open or close.

[0057] Example 2

[0058] Embodiment 2 is a further explanation of Embodiment 1, and the same components are not repeated here. In this embodiment, the output port network of the LCC inverter in step 1 of the hybrid double-end-fed direct current transmission receiving end transient stability analysis method is as follows:

[0059]

[0060] In the formula, v dc 、v dcref Respectively represent DC voltage and DC voltage reference value; u t1 is the AC bus voltage amplitude connected to the LCC; β, γ are the trigger advance angle and arc extinction angle; k pLCC and k iLCC are the coefficients of the PI controller in the constant DC voltage control of the LCC inverter; k is the transformer ratio, R c is the equivalent commutation resistance of LCC; i dc is a DC current; the DC side current of the LCC is kept constant under the action of the constant current controller on the rectifier side, and the converter valve feeds current into the AC system, so the LCC can be equivalent to a controlled current source.

[0061] The output port network of MMC can be specifically expressed as follows according to the control strategy:

[0062]

[0063] Where P ref , Q ref , P and Q represent reference active power, reference reactive power, MMC output active power and output reactive power respectively; k p , k i and k pPLL ,ki PLL Respectively represent the proportional coefficient and integral coefficient of the current inner loop PI controller; θ PLL and ω n Respectively represent the phase angle of the phase-locked loop output and the rated frequency; i dref 、i qref and v q They represent the d-axis reference current, q-axis reference current and the AC bus voltage on the q-axis respectively. Under the action of the control strategy, the MMC is equivalent to a controlled current source.

[0064] In this embodiment, the output port network is combined with the circuit model in step 1 to generate a simplified circuit model of the system, such as Figure 3 As shown, at the same time, the node current equation and voltage equation obtained by using Kirchhoff's theorem in the simplified circuit model are:

[0065]

[0066] In the formula, u t2 is the AC bus voltage amplitude connected to the MMC; θ t1 ,θ t2 are the phase angles of the two AC bus voltages respectively; 11 ,y 12 ,y 21 ,y 22 They are the four impedance matrix coefficients of the simplified circuit model, θ 11 ,θ 12 ,θ 21 ,θ 22 are the impedance angles of each impedance coefficient; i1 and i2 are the output current amplitudes of LCC and MMC respectively; and are the power factor angles of LCC and MMC respectively; x1 and x2 are the amplitudes of line impedances X1 and X2 respectively, v g1 and v g2 is the amplitude of the connected AC power grid; θ x1 and θ x2 The phases of the line impedances X1 and X2 respectively.

[0067] In this embodiment, the node current equation and the voltage equation are combined with the initial trigger lead angle and the initial phase-locked loop output phase angle when the grid symmetrical sag occurs to respectively solve the voltage amplitude of the MMC and LCC coupled buses at the time of the fault. When the grid symmetrical sag fault occurs, the phase-locked loop output phase angle will not change suddenly due to the effect of the integral link, so the initial phase angle is the same as the normal operating phase angle before the fault. The initial trigger lead angle of the LCC-HVDC can be solved by the LCC-HVDC output port network in step S1:

[0068] β or =β nop -k pLCC ·v dcref (4)

[0069] In the formula, β or , β nop They are the initial trigger advance angle of the fault and the trigger advance angle during normal operation before the fault; the initial trigger advance angle and the initial phase-locked loop output phase angle are substituted into the node current and voltage equations to solve the common AC bus line voltage amplitude of the MMC-LCC system during the fault.

[0070] In this embodiment, the dynamic nonlinear mathematical equation of the LCC-HVDC arc extinction angle under the grid symmetrical drop fault in step 4 can be obtained by substituting the AC bus voltage amplitude in step 3 into the LCC-HVDC output port network equation group in step 2 and solving the equation group in parallel:

[0071]

[0072] In this embodiment, the arc extinction angle evolution law of LCC-HVDC in step 5 when the grid voltage is symmetrically temporarily depressed is as follows: the arc extinction angle moves along the phase diagram curve from the initial point, and the arc extinction angle will continue to decrease with its differential value until it enters the commutation failure area. When the arc extinction angle is less than its minimum limit value γ0, the LCC will fail to commutate continuously and lose stability.

[0073] The specific mechanism of continuous commutation failure of LCC in the MMC-LCC control system when the grid voltage drops symmetrically is as follows: when the grid voltage drops symmetrically, the constant DC voltage controller keeps operating, causing the trigger advance angle to decrease continuously, and then causing the arc extinction angle to decrease continuously. When the arc extinction angle is less than the minimum arc extinction angle γ0 (usually 10°, the arc extinction angle mentioned in this article adopts radian system rad, angle value = radian system * 180 / π, the minimum arc extinction angle is 10°, and the corresponding radian system is 0.174rad), it will cause the LCC inverter to fail to commutate continuously during the transient process, and the MMC-LCC control system will lose stability.

[0074] Example 3

[0075] Embodiment 3 further supplements and elaborates on the technical solution of the present invention in detail. In this embodiment, the phase diagram curve of the arc extinction angle is drawn by taking the arc extinction angle and the arc extinction angle differential of the LCC inverter as the independent variable and the dependent variable, and combining the arc extinction angle dynamic nonlinear mathematical equation obtained in step 4 to draw the arc extinction angle phase diagram curve under different controller parameters. The specific phase diagram is as follows: Figure 4 As shown; it can be found from multiple sets of phase diagram curves that when the DC voltage reference value decreases, the phase diagram curve of LCC gradually moves up, which is conducive to the arc extinction angle away from its minimum value γ0, that is, reducing the DC voltage reference value can enhance the transient stability of LCC.

[0076] Example 4

[0077] In combination with Example 3, the hybrid double-end fed DC transmission receiving end transient stability analysis method of this embodiment provides a DC voltage controller control method, that is, a DC voltage control strategy based on the hybrid double-end fed DC transmission receiving end transient stability analysis, which is specifically implemented in the following steps:

[0078] Step 1, combining the dynamic nonlinear mathematical equation of the arc extinction angle to draw the phase diagram curve under different DC voltage reference values, and analyzing the influence of the DC voltage reference value on the transient stability of LCC in the constant DC voltage control of LCC-HVDC;

[0079] Step 2: Based on the influence of the above controller parameters on the arc extinction angle, the maximum coefficient boundary for maintaining the LCC-HVDC without commutation failure can be obtained. That is, after calculating the AC voltage at the initial moment of the fault, the DC voltage reference value obtained when the arc extinction angle derivative is equal to zero when the arc extinction angle takes the minimum value γ0 is the maximum coefficient boundary:

[0080]

[0081] like Figure 5 As shown in the figure, the process of designing the DC voltage reference coefficient in the LCC DC voltage control is as follows: determine the electrical parameters such as the LCC transformer ratio, equivalent commutation resistance and LCC DC current, determine the depth of the grid voltage sag and then solve the bus voltage amplitude; set the minimum arc extinction angle to solve the maximum coefficient boundary of the DC voltage reference; use the obtained maximum coefficient boundary to draw a phase diagram to determine whether instability will occur, and when continuous commutation failure occurs, reduce the DC voltage reference value with a fixed step length until continuous commutation failure does not occur, and then the DC voltage reference value is determined as the maximum coefficient under this working condition.

[0082] Example 5

[0083] Verification Example

[0084] As shown in Figure 6 (a), when the grid voltage temporarily drops to 0.5pu, the arc extinction angle and DC voltage simulation waveform of LCC-HVDC under multiple sets of DC voltage reference values. Figure 4 The phase diagram analysis was experimentally verified, and the DC voltage reference value was set to 0.7pu after the fault. It can be seen from the figure that the change trend of LCC-HVDC is consistent with the phase diagram analysis. At the same time, the arc extinction angle after reaching the new stable point is basically consistent with the result predicted by the phase diagram, which verifies the correctness of the transient stability analysis method.

[0085] As shown in Figure 6 (b), the experimental waveforms of the system correspond to the DC voltage reference value switching to less than the maximum boundary coefficient and equal to or greater than the maximum boundary coefficient when the grid voltage drops from 1.0pu to 0.5pu respectively. The critical value of the calculated DC voltage reference value is 0.764pu. It can be seen that only when it is less than the maximum boundary coefficient can the system maintain stable operation under the condition of grid voltage sag, while when it is greater than the maximum boundary coefficient, a commutation failure occurs; the effectiveness of the transient stabilization control strategy is verified.

[0086] The principles and implementation methods of the present invention are described in this article using specific embodiments. The description of the above embodiments is only used to help understand the method and core ideas of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. A method for transient stability analysis of a hybrid double-end-fed DC transmission receiving end, characterized in that: Please follow the steps below to implement: Step 1: construct an MMC-LCC hybrid high-voltage direct current transmission receiving-end circuit model, and simultaneously construct an LCC-HVDC receiving-end control model and an MMC-HVDC receiving-end control model; Step 2: Analyze the output characteristics of the MMC and LCC through the LCC-HVDC receiving end control model and the MMC-HVDC receiving end control model constructed in step 1, describe the output characteristics of the MMC and LCC receiving ends in the form of equivalent current sources, and establish an equivalent network port model of the MMC-LCC system. The equivalent network port is combined with the high-voltage direct current transmission receiving end circuit model to form a simplified circuit model of the system, and establish node current equations and voltage equations in the simplified circuit model; Step 3, respectively analyzing the coupling characteristics of the common bus voltage of the MMC-LCC control system through the node current equation and voltage equation established in step 2, and solving the LCC bus voltage amplitude after the fault occurs; Step 4, obtaining the initial state of the LCC at the time of the fault through the coupled bus voltage amplitude obtained in step 3 and the LCC control system, and establishing a dynamic nonlinear mathematical equation for the arc extinction angle of the LCC-HVDC under a symmetrical grid drop fault; Step 5: Use the dynamic nonlinear mathematical equation of the arc extinction angle obtained in step 4 to quantitatively analyze the evolution process and transient stability of the system during the transient process, solve the boundary control parameters that maintain the stability of the system, and optimize the control parameters of the LCC-HVDC.

2. The method for analyzing transient stability of a hybrid double-ended direct current transmission receiving end according to claim 1 is characterized in that: The high-voltage direct current transmission receiving end circuit model specifically includes: MMC and LCC are connected in parallel to form a hybrid high-voltage direct current transmission receiving-end circuit model. The inverter side includes MMC-HVDC and LCC-HVDC dual subsystems. The dual subsystems are connected to two sections of AC busbars through converter transformers. The LCC-HVDC subsystem is connected to the reactive compensation device. The two sections of AC busbars are connected through a tie line; the two sections of AC busbars are connected to two groups of AC power grids through line impedance.

3. The method for transient stability analysis of hybrid double-ended feed-in direct current transmission receiving end according to claim 1 is characterized in that: The LCC-HVDC receiving-end control model is specifically that the inverter side of the circuit model adopts constant DC voltage control, and its DC current remains unchanged under the control of the rectifier side; the MMC-HVDC receiving-end control model is specifically that the inverter side of the circuit model adopts constant active power and reactive power control.

4. The method for analyzing transient stability of a hybrid double-ended direct current transmission receiving end according to claim 2 or 3, characterized in that: The output port network of the LCC is specifically as follows: In the formula, v dc 、v dcref Respectively represent DC voltage and DC voltage reference value; u t1 is the AC bus voltage amplitude connected to the LCC; β, γ are the trigger advance angle and arc extinction angle; k pLCC and k iLCC are the coefficients of the PI controller in the constant DC voltage control of the LCC inverter; k is the transformer ratio, R c is the equivalent commutation resistance of LCC; i dc is direct current; The output port network of the MMC is as follows: Where P ref , Q ref , P and Q represent reference active power, reference reactive power, MMC output active power and output reactive power respectively; k p , k i and k pPLL ,ki PLL Respectively represent the proportional coefficient and integral coefficient of the current inner loop PI controller; θ PLL and ω n Respectively represent the phase-locked loop output phase angle and rated frequency; i dref 、i qref and v q They represent the d-axis reference current, q-axis reference current and the AC bus voltage on the q-axis respectively.

5. The method for analyzing transient stability of a hybrid double-ended direct current transmission receiving end according to claim 4 is characterized in that: The node current equation and voltage equation obtained by simplifying the circuit model in step 2 through Kirchhoff's theorem are as follows: In the formula, u t2 is the AC bus voltage amplitude connected to the MMC; θ t1 ,θ t2 are the phase angles of the two AC bus voltages respectively; 11 ,y 12 ,y 21 ,y 22 They are the four impedance matrix coefficients of the simplified circuit model, θ 11 ,θ 12 ,θ 21 ,θ 22 are the impedance angles of each impedance coefficient; i1 and i2 are the output current amplitudes of LCC and MMC respectively; and are the power factor angles of LCC and MMC respectively; x1 and x2 are the amplitudes of line impedances X1 and X2 respectively, v g1 and v g2 is the amplitude of the connected AC power grid; θ x1 and θ x2 are the phases of the line impedances X1 and X2 respectively, and j represents the 90° rotation factor in the phasor.

6. The method for analyzing transient stability of a hybrid double-ended direct current transmission receiving end according to claim 5 is characterized in that: According to the characteristics of the MMC-LCC control system in step 2, the initial trigger lead angle is specifically obtained by the following formula: b or =b nop -k pLCC ·v dcref (4) In the formula, β or , β nop are the initial trigger advance angle of the fault and the trigger advance angle during normal operation before the fault, respectively; the node current equation and voltage equation in step 2 are combined with the initial trigger advance angle and the initial phase-locked loop output phase angle when the grid symmetrical sag occurs to solve the voltage amplitude of the MMC and LCC coupled buses at the moment of the fault, and the initial trigger advance angle and the initial phase-locked loop output phase angle are substituted into the node current and voltage equations to solve the line voltage amplitude of the common AC bus of the MMC-LCC system at the moment of the fault.

7. The method for analyzing transient stability of a hybrid double-ended direct current transmission receiving end according to claim 6 is characterized in that: The dynamic nonlinear mathematical equation of the arc extinction angle of LCC-HVDC under the symmetrical grid drop fault in step 4 is as follows:

8. A DC voltage control strategy based on transient stability analysis of a hybrid double-ended direct current transmission receiving end, using a hybrid double-ended direct current transmission receiving end transient stability analysis method as claimed in any one of claims 1 to 7, characterized in that: Please follow the steps below to implement: Step 1, draw the phase diagram curve under different DC voltage reference values ​​in combination with the arc extinction angle nonlinear dynamic equation, and analyze the influence of the DC voltage reference value in the constant DC voltage control of LCC-HVDC on the transient stability of LCC; Step 2: Based on the influence of the DC voltage reference value on the transient stability of LCC in step 1, a maximum coefficient boundary is obtained to maintain the LCC-HVDC in the MMC-LCC system from continuous commutation failure under a grid voltage sag. The DC voltage reference value below the coefficient boundary ensures that the LCC-HVDC does not suffer from continuous commutation failure.

9. The DC voltage control strategy based on transient stability analysis of hybrid double-end fed DC transmission receiving end according to claim 8, characterized in that: The influence of the DC voltage reference value in the constant DC voltage control of LCC-HVDC on the transient stability of LCC is analyzed in step 1 as follows: from multiple sets of phase diagram curves, when the DC voltage reference value is reduced, the phase diagram curve of the arc extinction angle gradually moves up, and the arc extinction angle gradually avoids being reduced to the minimum value γ0 that ensures that no commutation failure occurs, that is, the controller reduces the DC voltage reference value to enhance the transient stability of the LCC.

10. The DC voltage control strategy based on transient stability analysis of hybrid double-end fed DC transmission receiving end according to claim 9, characterized in that: In step 2, the maximum coefficient boundary for maintaining the LCC-HVDC without continuous commutation failure is the DC voltage reference value obtained when the derivative of the arc extinction angle is equal to zero when the arc extinction angle takes its minimum value γ0, which is specifically as follows:

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