Multi-infeed interaction factor evaluation method for AC / DC hybrid power system containing MMC-HVDC line

Abstract

The invention relates to the field of power systems, and provides a multi-infeed interaction factor calculation method for an AC / DC hybrid power system containing an MMC-HVDC line, and the method comprises the steps: obtaining a simplified model; and according to the simplified topology model, establishing a quasi-steady-state model of the receiving end alternating current system before direct current access. And deriving an analytical expression of the multi-infeed interaction factor in the AC / DC hybrid power system containing the MMC-HVDC line. And deducing the sensitivity of the active power and the reactive power of the two types of direct current systems relative to the voltage amplitude under different control modes. And giving a modified analytic function based on an AC / DC power flow Jacobian matrix element J. According to the method, comprehensive analysis and coupling degree analysis are carried out on the interaction degree of different direct currents in the system, the dynamic coupling strength between the alternating current and direct current hybrid power system containing the MMC-HVDC line is quantitatively evaluated, an effective scheme is provided for system operation planning, and the safety and reliability of the system are guaranteed.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power systems, and relates to a multi-infeed interaction factor evaluation method for an AC / DC hybrid power system containing an MMC-HVDC line, which is suitable for voltage stability evaluation of an AC / DC hybrid power system containing an MMC-HVDC line, so as to ensure the stability and reliability of system operation. BACKGROUND

[0002] AC / DC hybrid power systems containing modular multilevel converter high-voltage direct current (MMC-HVDC) have become an important form of modern power grids. MMC-HVDC has become a core technical solution for new energy grid connection and multi-DC infeed systems due to its flexible power regulation capability (±0.1 second level response), independent reactive power control characteristics (supporting passive network access), and damping characteristics suitable for weak AC power grids. However, when multiple MMC-HVDC lines are interconnected through a common AC node, the system exhibits strong nonlinear coupling characteristics, and the power interaction (such as voltage fluctuation coupling, circulating current resonance, etc.) between different DC systems significantly affects system stability, and it is necessary to establish an accurate interaction quantitative model.

[0003] DC transmission systems are particularly suitable for long-distance power transmission because they can significantly reduce energy loss during transmission. By combining DC transmission systems with AC systems, more efficient power exchange can be achieved between long-distance transmission and local power grids. DC transmission can be operated in parallel with AC power grids, and by adjusting the power flow of DC transmission, power fluctuations in the AC system can be effectively alleviated, thereby enhancing system stability. In particular, in the integration of renewable energy sources such as wind and solar power, DC transmission can smooth the unstable output of renewable energy sources, improving system stability and reliability. DC transmission systems have significant advantages in the integration of renewable energy sources such as wind and solar power. Renewable energy sources are usually generated in DC form, and through DC transmission systems, these energy sources can be directly connected to the main grid, reducing energy conversion losses and improving overall system energy efficiency. MMC-HVDC (Modular Multilevel Converter High Voltage Direct Current Transmission) is an advanced transmission technology that combines modular design and voltage source converter characteristics, and its core feature is to achieve multi-level voltage output through cascaded sub-modules, significantly reducing harmonic distortion (THD < 3%), and without the need for large-capacity AC filters to meet power quality requirements. Although AC / DC hybrid power systems have the above-mentioned significant advantages, the strong coupling of LCC and MMC has also brought new challenges in voltage interaction evaluation.

[0004] LCC-HVDC cannot achieve reliable control of the converter bus voltage, and the interaction between multi-infeed DC systems will affect the safe and stable operation of the system. Therefore, the CIGRE multi-infeed DC system research group proposed a multi-infeed interaction factor (MIIF) to evaluate the dynamic coupling strength between multi-circuit DC transmission systems. Its definition is based on the ratio of the response of the converter bus voltage disturbance. By injecting a 1% voltage step disturbance into a certain converter bus, the voltage change of other converter buses is measured, and then the MIIF matrix is constructed. This method directly represents the interaction strength between multi-DC in the form of simple voltage deviation ratio. CIGRE does not provide a theoretical calculation formula, so it cannot reflect the quantitative influence of grid topology changes and DC operating state (such as DC landing point migration: tie-line impedance adjustment) on the MIIF value. In view of the problems in the voltage deviation ratio calculation method, a large number of scholars have calculated the MIIF value through the impedance method and the analytical method based on the reduced Jacobian matrix. In most of the research on the interaction factor, while considering the AC receiving end load characteristics, system equivalent impedance, reactive power compensation device capacity and DC control mode, the power exchange on the tie line between systems is often ignored. The impedance method has small calculation amount and is suitable for fast scanning of large-scale power grids, but it simplifies the DC system as a static impedance network and ignores the dynamic adjustment effect of the converter control strategy, which cannot reflect the actual change of power-voltage sensitivity. At the same time, this method is difficult to quantify the influence of grid structure changes, and has poor predictability. The analytical method establishes a differential-algebraic equation, expresses MIIF as a function of control parameters and network parameters, and explicitly introduces the influence of DC control mode on power-voltage sensitivity, which is more consistent with the actual operation mechanism. Since the AC-DC hybrid power system with MMC-HVDC line has MMC converter to provide reactive power support, the reactive power increase and the change of LCC converter bus are coupled, therefore, it is of great theoretical and engineering significance to further study and establish the evaluation method of voltage interaction between different types of DC in the receiving end of the AC-DC hybrid power system with MMC-HVDC line. SUMMARY

[0005] 1. Technical problems to be solved:

[0006] How to ensure that the multi-infeed short-circuit ratio of the system can fully reflect the influence of the MMC-HVDC line on the voltage of the inverter side bus of the AC-DC hybrid power system.

[0007] 2. Technical solutions:

[0008] In order to solve the above problems, the application provides a multi-infeed interaction factor calculation method for an AC-DC hybrid power system with MMC-HVDC line, comprising the following steps:

[0009] Step 1: According to the topology structure of the AC-DC hybrid power system containing the MMC-HVDC line, the transmission lines of different DC types are numbered to obtain a simplified model of the topology structure of the AC-DC hybrid power system containing the MMC-HVDC line.

[0010] Step 2: According to the simplified topology model, a quasi-steady state model of the receiving end AC system before the DC is connected is established, and according to the different characteristics of the converter connected to the DC line of the LCC-HVDC line and the MMC-HVDC line, a quasi-steady state model of the AC-DC hybrid power system containing the MMC-HVDC line after the DC is connected is established.

[0011] Step 3: The quasi-steady state model of the AC-DC hybrid power system containing the MMC-HVDC line after the DC is connected is linearized, and the analytical expression of the multi-infeed interaction factor in the AC-DC hybrid power system containing the MMC-HVDC line is derived.

[0012] Step 4: Based on the LCC-HVDC and MMC-HVDC quasi-steady state models, the sensitivity of the active power and the reactive power of the two types of DC systems under different control modes with respect to the voltage amplitude is derived.

[0013] Step 5: Combined with the characteristics of strong coupling of the amplitude of the voltage and the reactive power and weak coupling of the active power in the AC-DC hybrid power system, the reactive power exchange on the tie line between the multi-infeed AC-DC transmission system is given based on the modified analytical function of the AC-DC power flow Jacobian matrix element J.

[0014] 3. Advantageous effects:

[0015] The application provides a multi-infeed interaction factor evaluation method for an AC-DC hybrid power system containing an MMC-HVDC line, which is used for analyzing and evaluating the dynamic coupling degree in the AC-DC hybrid power system, can accurately analyze the influence degree of the commutation bus voltage disturbance on other DC systems, quantifies the dynamic coupling relationship between multiple DCs, and predicts the probability of simultaneous commutation failure by simulating the influence of AC fault on the commutation voltage of multiple DCs. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 The topology structure of the AC-DC hybrid power system containing the MMC-HVDC line.

[0017] Figure 2 The simplified model of the AC-DC hybrid power system containing the MMC-HVDC line. DETAILED DESCRIPTION

[0018] The technical solutions of the application will be described in detail below with reference to the drawings:

[0019] This invention relates to the field of power systems, specifically a method for calculating the multi-infeed interaction factor in AC / DC hybrid power systems containing MMC-HVDC lines. This method is applicable to voltage stability assessment of AC / DC hybrid power systems containing MMC-HVDC lines, ensuring system stability and reliability. It obtains the sensitivity of the LCC-HVDC inverter-side bus voltage to the reactive power increase of the MMC on the MMC-HVDC inverter side in the AC / DC hybrid power system. Then, it adjusts the control mode and parameters of the LCC-HVDC to obtain the sensitivity of the reactive power relative to the voltage amplitude under different control modes. Finally, based on the multi-infeed interaction factor calculation results, it selects the control mode and parameters with the lowest risk of commutation failure. The basic process is as follows:

[0020] Step 1: Based on the topology of the AC / DC hybrid power system containing MMC-HVDC lines, the transmission lines of different DC types are numbered. Only the coupling effect between the MMC and LCC on the inverter side is considered. The rectifier-side model is equivalent to the active and reactive power injection of DC nodes, and the reactive power compensation device is simplified to the reactive power injection of AC nodes, whose value does not change with time. The focus is on reactive power exchange on the tie lines between AC buses, thus obtaining a simplified topology model of the AC / DC hybrid power system containing MMC-HVDC lines. The specific steps are as follows:

[0021] Step 1-1: [The text appears to be incomplete and contains several grammatical errors. A more accurate translation would require the full context.] Figure 1 The complex topology of the AC / DC hybrid power system containing MMC-HVDC lines is simplified as follows: Figure 2 The simplified topology model is shown.

[0022] Steps 1-2: Assume that the AC / DC hybrid power system containing MMC-HVDC lines has a total of DC line, assuming the first One DC system is an MMC-HVDC line, the rest The DC system is an LCC-HVDC line, therefore the system has a total of N nodes, denoted as _____. .in For LCC-HVDC converter bus, This is the AC bus of the MMC-HVDC converter. and This constitutes the equivalent system at the receiving end.

[0023] Steps 1-3: Figure 2 China's communication system has There are nodes, among which nodes for LCC AC node on the inverter side of a high-voltage direct current system .node for Any AC node at the inverter side of the DC system, . Node LCC AC node at the inverter side of the MMC-HVDC line, .

[0024] Step 2: According to the simplified topology model, a quasi-steady state model of the AC system before the DC is connected is established, and according to the different characteristics of the converter connected by the DC line of the LCC-HVDC line and the MMC-HVDC line, a quasi-steady state model of the AC / DC hybrid power system containing the MMC-HVDC line after the DC is connected is established. The specific steps are as follows:

[0025] Step 2-1: Figure 2 The AC system has nodes, of which the first nodes are the LCC-HVDC AC side nodes of the inverter station of the high-voltage direct-current system, and the node is the MMC-HVDC AC side node, which can be described by the power flow distribution in polar coordinates as formula (1):

[0026] (1)

[0027] In the formula: : and are the voltage amplitudes of nodes : node : node . : are the voltage phase angle differences between nodes : node and node . and are the conductance and susceptance between nodes and nodes . and : and are the active and reactive power injected into nodes : node .

[0028] Formula (1) is written in the form of a correction equation, as shown in formula (2):

[0029] (2)

[0030] In the formula: and : and : and They are nodes :node :node Corrections for injected active and reactive power.

[0031] Step 2-2: When the DC system is connected, the steady-state model of the AC system above is modified. Therefore, equation (2) is modified after the DC connection as follows (power direction is positive with the injected AC node):

[0032] (3)

[0033] In the formula: and : and The first One LCC-HVDC line: The active and reactive power injected into the AC system by the MMC-HVDC line.

[0034] Step 3: Linearize the quasi-steady-state model of the AC / DC hybrid power system with MMC-HVDC lines after DC access, and derive the analytical expression for the multi-infeed interaction factor in the AC / DC hybrid power system with MMC-HVDC lines; the specific steps are as follows:

[0035] Step 3-1: Solve the above power flow equations using the Newton-Raphson method. Equation (2) can be written in the form of equation (4) containing the Jacobian matrix:

[0036] (4)

[0037] In the formula: : : and It is a block matrix of the Jacobian matrix; This is the active power correction matrix for the AC bus. This is the reactive power correction matrix for the AC bus. This is the phase angle correction matrix for the AC bus. This is the AC bus voltage amplitude correction matrix.

[0038] Equation (4) yields the block matrix in the Jacobian matrix of the communication system. : The formulas for calculating the elements are expressed as equation (5) and equation (6) respectively:

[0039] (5) (6)

[0040] Step 3-2: Considering the DC connection, the steady-state power flow equations of the system become:

[0041] (7)

[0042] The power injected into the AC system by the converter station is related to the bus voltage amplitude. The impact of DC connection on the Jacobian matrix is ​​manifested in: : On the diagonal elements and some off-diagonal elements. and diagonal elements and Revised to:

[0043] (8)

[0044] (9)

[0045] MIIF is expressed as equation (10):

[0046] (10)

[0047] In the formula: For the busbar before the reactor is put into operation The voltage; busbar The voltage change. When At this time, it indicates that the two converter stations are in a completely decoupled state, and voltage fluctuations at one node have no observable impact on the other node; while as The coupling characteristics of the system are significantly enhanced, which means that the change in reactive power of one converter station will produce a strong chain effect through the AC network impedance, directly causing the synchronous deviation of the AC bus voltage of another converter station.

[0048] Step 4: Based on the quasi-steady-state models of LCC-HVDC and MMC-HVDC, the sensitivity of active and reactive power to voltage amplitude for the two types of DC systems under different control methods was derived. The specific steps are as follows:

[0049] Step 4-1: The steady-state models of the DC system under three control modes are mainly considered: constant current / constant turn-off angle, constant power / constant turn-off angle, and constant current / constant DC voltage. Under the same system parameters, the control models of the linearized models are grouped for power flow calculation.

[0050] When the DC system is under constant current control on the rectifier side and constant turn-off angle control on the inverter side, the external characteristic equation of the inverter station LCC connected to the AC system can be expressed as:

[0051] (11)

[0052] wherein: is the no-load voltage of the DC side of the inverter LCC; is the AC bus voltage of the inverter side LCC, is the DC current; is the DC voltage of the inverter LCC; is the turn-off angle of the inverter LCC; : is the active power and reactive power injected by the inverter LCC into the AC system, the direction from the converter to the AC system is positive.

[0053] Step 4-2: According to the different external characteristic equations of the three groups of control modes, the sensitivities of the active power and reactive power under different control modes to the AC bus voltage are calculated.

[0054] When the DC system is in the rectifier side constant current / inverter side constant turn-off angle control, according to formula (11), the sensitivities of the active power and reactive power of the inverter LCC to the AC bus voltage are respectively:

[0055] (12)

[0056] (13)

[0057] Step 4-3: Based on the sensitivities of the reactive power to the AC bus voltage under different control modes, the sub-matrix in the Jacobian matrix is modified.

[0058] Substitute formula (12) and formula (13) into the sub-matrix : in the Jacobian matrix.

[0059] Step 5-1: The reactive power correction term on the tie line after the DC is connected is determined by the sensitivity of the reactive power on the tie line to the AC bus voltage amplitude, so from the inverter side LCC converter bus, the reactive power injected into the AC system is expressed as:

[0060] (14)

[0061] wherein: is the reactive power output by the reactive power compensation device; is the reactive power output by the MMC-HVDC inverter side MMC to the LCC-HVDC inverter side LCC through the tie line; is the DC voltage of the LCC-HVDC inverter side LCC under different control strategies, is the DC current, is the number of poles of the DC transmission system, and is the no-load voltage of the inverter side converter bus co-determination.

[0062] Step 5-2: Assuming that the reactive power flow on the tie line between the inverter side LCCs of each DC system is For example, the reactive power on the tie line is expressed as:

[0063] (15)

[0064] In the formula: is the equivalent impedance modulus of the tie line; is the impedance angle of the equivalent impedance of the tie line; is the converter bus voltage phase angle difference of the subsystem : is the converter bus voltage phase angle difference of the subsystem : is the converter bus voltage phase angle difference of the subsystem : corresponding to the converter bus voltage.

[0065] Step 5-3: Considering the steady-state power flow equation of the system after DC access:

[0066] (16).

[0067] According to the expression of the reactive power on the tie line, the reactive power injected into the AC system is modified and expressed as:

[0068] (17)

[0069] In the formula: is the reactive power on the tie line between the inverter side LCCs, is the reactive power on the tie line between the inverter side LCC and the MMC.

[0070] Assuming that the active power on the tie line does not change after DC access, the reactive power sensitivity of the tie line after DC access is increased: the sensitivity of the reactive power support provided by the MMC and the sensitivity of different control modes, with the flow into the converter bus being positive, the reactive power injected into the AC network by each DC system can be obtained as:

[0071] (18).

[0072] The reactive power on the tie line is expressed as:

[0073] (19).

[0074] Step 5-4: The power injected into the AC system by the converter station is related to the amplitude of the bus voltage, and the influence of DC access on the Jacobian matrix is reflected in : diagonal elements and partial off-diagonal elements. Wherein and diagonal elements and are modified as:

[0075] (20)

[0076] (21).

[0077] diagonal elements corresponding to the inverter side of LCC-HVDC : and off-diagonal elements : are modified as:

[0078] (22)

[0079] (23)

[0080] (24).

[0081] elements corresponding to the inverter side of MMC-HVDC line and are modified as:

[0082] (25)

[0083] (26)

[0084] (27)

[0085] (28).

[0086] According to the definition of MIIF, only the reactive power of busbar is changed, so that the voltage changes, and the reactive power on other busbars is basically unchanged, that is . The multi-infeed interaction factor calculation method based on the degradation of Jacobian matrix is expressed as:

[0087] (29).

[0088] Simplifying can get:

[0089] (30).

[0090] Based on the calculation result of the multi-infeed interaction factor, the coupling characteristics between different DC systems can be comprehensively considered, the influence degree of the commutation bus voltage disturbance on other DC systems is comprehensively analyzed, and then the probability of simultaneous commutation failure is predicted by simulating the influence of AC fault on multi-circuit DC commutation voltage. According to the calculation result of the multi-infeed interaction factor, the influence of different DC access positions and capacity configurations on the stability of the system is analyzed, and after identifying the strongly coupled DC system, the coordinated control strategy can be designed accordingly to ensure the stability and efficiency of the system.

Claims

1. A method for calculating the multi-infeed interaction factor in an AC / DC hybrid power system containing MMC-HVDC lines, characterized in that: The method comprises the following steps: Step 1: according to the topological structure of the AC / DC hybrid power system containing the MMC-HVDC line, different DC type transmission lines are numbered to obtain a simplified model of the topological structure of the AC / DC hybrid power system containing the MMC-HVDC line; Step 2: according to the simplified topological model, a quasi-steady state model of the receiving end AC system before the DC is connected is established, and according to the different characteristics of the DC line connected converter of the LCC-HVDC line and the MMC-HVDC line, a quasi-steady state model of the AC / DC hybrid power system containing the MMC-HVDC line after the DC is connected is established; Step 3: the quasi-steady state model of the AC / DC hybrid power system containing the MMC-HVDC line after the DC is connected is linearized to derive an analytical expression of the multi-infeed interaction factor in the AC / DC hybrid power system containing the MMC-HVDC line; Step 4: based on the LCC-HVDC and MMC-HVDC quasi-steady state models, the sensitivity of the active power and the reactive power to the voltage amplitude under different control modes of the two types of DC systems is derived; Step 5: combined with the characteristics that the amplitude of the voltage of the AC / DC hybrid power system is strongly coupled with the reactive power and weakly coupled with the active power, the reactive power exchange on the tie line between the multi-infeed AC / DC transmission systems, a modified analytical function based on the AC / DC power flow Jacobian matrix element J is given. 2.The MMC-HVDC line contained AC / DC hybrid power system multi-infeed interaction factor calculation method of claim 1, characterized in that: In step 1, based on the coupling effect of the inverter side MMC and the LCC, the rectifier side model is equivalent to the active and reactive power injection of the DC node, the reactive power compensation device is simplified as the reactive power injection of the AC node, the value of which does not change with time, and the reactive power exchange on the tie line between the buses is obtained to obtain a simplified model of the topological structure of the AC / DC hybrid power system containing the MMC-HVDC line.

3. The MMC-HVDC line contained AC / DC hybrid power system multi-infeed interaction factor calculation method of claim 2, wherein: Assuming a hybrid AC / DC power system containing MMC-HVDC lines has a total of DC line, assuming the first One DC system is an MMC-HVDC line, the rest... The DC system is an LCC-HVDC line with N nodes, denoted as _____. , For LCC-HVDC converter bus, For the AC bus of the MMC-HVDC converter, and Constructing an equivalent system at the receiving end, where nodes for LCC AC node on the inverter side of a high-voltage direct current system ,node for Any AC node on the inverter side of a DC system, ,node This is the LCC AC node on the inverter side of the MMC-HVDC line. ​ 4.The MMC-HVDC line contained AC / DC hybrid power system multi-infeed interaction factor calculation method of claim 3, characterized in that: The specific steps of step 2 are as follows: Step 2-1: AC system has nodes, where the first node is the LCC-HVDC AC side node and the second node is the MMC-HVDC AC side node, the power flow distribution in polar coordinates is described by equation (1): （1） wherein: , and are the voltage magnitudes of the nodes , the node , the node , respectively; , are the voltage phase angle differences between the nodes , the node and the node , respectively; and are the conductance and susceptance between the nodes and the node , respectively; and , and are the active and reactive power injected into the nodes , the node , respectively, Equation (1) is written in the form of a correction equation as shown in equation (2): （2） wherein: and , and , and are the nodes , the nodes , the nodes injected with the corrected amount of active power and reactive power; Step 2-2: when the DC system is connected, the above steady state model of the AC system is modified, the injection of the AC node is positive in the power direction, and equation (2) is modified as: （3） wherein: and , and are the active and reactive power injected by the LCC-HVDC line, MMC-HVDC line into the AC system, respectively.

5. The MMC-HVDC line contained AC / DC hybrid power system multi-infeed interaction factor calculation method of claim 4, wherein: The specific steps of step 3 are as follows: Step 3-1: the Newton-Raphson method is used to solve the above power flow equation, equation (2) is written in the form of equation (4) containing the Jacobian matrix: （4） wherein: , , and are block matrices of the Jacobian matrix; is the active power correction matrix for the AC bus, is the reactive power correction matrix for the AC bus; is the phase angle correction matrix for the AC bus, is the voltage magnitude correction matrix for the AC bus, The block matrix in the AC system Jacobian matrix is obtained from formula (4) 、 The calculation formulas of the elements are respectively represented as formula (5) and formula (6): （5） （6）； Step 3-2: the steady state power flow equation of the system after the DC is connected becomes: （7） The power injected by the converter station into the AC system is related to the amplitude of the bus voltage. The influence of DC on the Jacobian matrix is reflected in the diagonal elements and some off-diagonal elements of , , where the diagonal elements of and are modified to and . （8） （9） MIIF is represented as equation (10): （10） In the formula: For the busbar before the reactor is put into operation The voltage; busbar Voltage changes. 6.The MMC-HVDC line contained AC / DC hybrid power system multi-infeed interaction factor calculation method of claim 5, characterized in that: The specific steps of step 4 are as follows: Step 4-1: when the DC system is in the rectifier side constant current / inverter side constant turn-off angle control, the LCC external characteristic equation of the inverter station connected to the AC system at this time is represented as: （11） wherein: V0is the no-load voltage of the DC side of the inverter LCC; V is the AC bus voltage of the inverter side LCC, I is the DC current; V0is the DC voltage of the inverter station LCC; is the turn-off angle of the inverter station; , P and Q are the active and reactive power injected by the inverter station LCC into the AC system, the direction from the inverter to the AC system is positive. Step 4-2: according to the different control modes of the three groups of control modes, the sensitivities of the active power and the reactive power to the AC bus voltage under different control modes are obtained, when the DC system is in the rectifier side constant current / inverter side constant turn-off angle control, according to equation (11), the sensitivities of the active power and the reactive power of the inverter station LCC to the converter bus voltage are obtained respectively as: （12） （13） Step 4-3: Substitute formula (12) and formula (13) into the sub-matrix in the Jacobian matrix , . 7.The MMC-HVDC line contained AC / DC hybrid power system multi-infeed interaction factor calculation method of claim 6, characterized in that: The specific steps of step 5 are as follows: Step 5-1: Injecting reactive power into the AC system from the inverter side LCC converter bus is represented as: （14） In the formula: is the reactive power sent by the reactive power compensation device; is the reactive power sent by the MMC-HVDC inverter-side MMC to the LCC-HVDC inverter-side LCC through the tie line; is the DC voltage of the LCC-HVDC inverter-side LCC under different control strategies is the DC current is the number of poles of the DC power transmission system and the no-load voltage of the inverter-side converter bus are jointly determined; Step 5-2: Assume the reactive power flow between the LCCs on the tie-line of each DC system is For example, the reactive power on the tie-line is expressed as: （15） In the formula: is the equivalent impedance modulus of the tie line; is the impedance angle of the equivalent impedance of the tie line; is the subsystem , is the phase angle difference of the converter bus voltage; , is the corresponding converter bus voltage of the subsystem , is the corresponding converter bus voltage of the subsystem Step 5-3: considering the steady state power flow equation of the system after the DC is connected as: （16） According to the expression of the reactive power on the tie line, the reactive power injected into the AC system is modified and expressed as: （17） In the formula: is the reactive power on the tie line between the inverter-side LCCs, is the reactive power on the tie line between the inverter-side LCC and the MMC. 8.The MMC-HVDC line contained AC / DC hybrid power system multi-infeed interaction factor calculation method of claim 7, wherein: Assuming that the active power on the tie-line is constant after the DC access, the reactive power sensitivity of the tie-line after the DC access, the reactive power support sensitivity provided by the MMC, and the sensitivity of different control modes are considered, and the reactive power injected into the AC network by each DC system is obtained as positive, that is: （18） The reactive power on the tie-line is expressed as: （19）。 9.The MMC-HVDC line contained AC / DC hybrid power system multi-infeed interaction factor calculation method of claim 7, wherein: The power injected by the converter station into the AC system is related to the amplitude of the bus voltage, and the influence of the DC on the Jacobian matrix is reflected in the diagonal elements and some off-diagonal elements of the Jacobian matrix, wherein the diagonal elements of the Jacobian matrix are modified as follows: , the diagonal elements of the Jacobian matrix are modified as follows: and the diagonal elements of the Jacobian matrix are modified as follows: and ​ （20） （21） The diagonal element corresponding to the LCC-HVDC inverter side , off-diagonal elements , Revised to: （22） （23） （24）， MMC-HVDC line inverter side MMC corresponding elements and modified to read: （25） （26） （27） （28）， According to the definition of MIIF, only the reactive power of bus is changed, and the voltage of bus changes, the reactive power of other buses is basically unchanged, and the multi-infeed interaction factor calculation method based on the degradation of the Jacobian matrix is represented as: （29） Simplifying it gives: （30）。

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