A method, system and device for hybrid control of dc side voltage of a grid-forming converter

Abstract

The application discloses a kind of network type converter DC side voltage hybrid control method, system and device, belong to electric power field, including proposing network type converter system DC side voltage hybrid control strategy, under synchronous stability scale, the network type converter transient mathematical model of establishing containing DC side voltage hybrid control ring;According to the mathematical model established, the stability mechanism is analyzed using equal-area rule, and the influence law of main parameters is qualitatively analyzed;Using numerical integration principle to solve system critical fault clearing time (CCT), quantitative analysis of the influence of different parameters;According to the parameter analysis result, the parameter optimization design is carried out on the DC side voltage hybrid control ring of network type converter;It can accurately describe the transient synchronization mechanism of network type converter under DC side voltage disturbance, and provides a feasible control strategy and optimization scheme for realizing the transient stability improvement of network type converter.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electric power, in particular to a grid-forming converter DC side voltage hybrid control method, system and device. BACKGROUND

[0002] A large number of renewable energy sources are connected to the grid through converters, which greatly reduces the inertia and disturbance resistance of the grid, bringing challenges to large-scale new energy consumption and safe and stable operation of the power system. Power electronic converters have the characteristics of flexible control, and by controlling the converter, it can actively support and stabilize the voltage and frequency when the grid is disturbed, which is called grid-forming converter. Through the large-scale access of grid-forming converters, on the one hand, it can solve the problem of weak disturbance resistance of low-inertia power systems, and on the other hand, it can greatly reduce the demand for rotating reserve capacity of thermal and hydro power systems.

[0003] However, power electronic converters have weak overcurrent capability and poor robustness in adapting to complex grid conditions. Due to the limited capacity of the DC side of the grid-forming converter, it is difficult to achieve fast power support. When the grid experiences extreme conditions such as short-circuit faults, the DC side voltage fluctuation leads to a decrease in the stability domain of the grid-forming converter, making it prone to transient step-out and other stability problems, and there is an urgent need for a DC side voltage control strategy with strong stability and optimization design method research to provide theoretical guidance for engineering design. SUMMARY

[0004] In view of the deficiencies of the prior art, the present application proposes a grid-forming converter DC side voltage hybrid control method, system and device.

[0005] The purpose of the present application can be achieved by the following technical solutions:

[0006] A grid-forming converter DC side voltage hybrid control method, characterized in that it comprises the following steps:

[0007] Based on the topology and system parameters of the grid-forming converter, a DC side voltage hybrid control strategy is established;

[0008] Based on the dynamic characteristics of the DC side capacitor circuit, an equivalent dynamic equation of the DC side of the grid-forming converter is constructed; based on the angular frequency of each output in the topology structure, a power control loop of the grid-forming converter is constructed; based on the transmission characteristics of the two-port network, an active and reactive power equation of the converter output is established; combined with the equivalent dynamic equation of the DC side, the power control loop of the grid-forming converter and the active and reactive power equation of the converter output, a transient mathematical model of the grid-forming converter under the synchronous stability scale is constructed;

[0009] The change curve of the reference active power is determined through the transient mathematical model of the grid-forming converter, and the influence of the parameters on the transient stability of the grid-forming converter is determined based on the equal-area rule analysis of the change curve.

[0010] The critical fault clearing time of the converter under different parameter perturbations is calculated by a numerical integration method;

[0011] To improve the transient stability of the converter, a DC side voltage hybrid control strategy is adjusted based on the influence of each parameter on the transient stability of the grid-connected converter and the critical fault clearing time of the converter.

[0012] Further, the DC side voltage hybrid control strategy adopts zero-error control for the DC side voltage deviation value, and the output values are respectively fed back to the reference angular frequency value and the reference active power value. The reference angular frequency value is the output value multiplied by the proportional coefficient 1-K (0≤K≤1), and the reference active power value is the output value multiplied by the proportional coefficient K and the reference DC side voltage value.

[0013] Further, the construction of the equivalent dynamic equation of the DC side of the grid-connected converter includes the following steps:

[0014] The mathematical model of the DC side voltage hybrid control link is as follows:

[0015]

[0016] Wherein, and U dc are the reference DC side voltage and the measured voltage, respectively, K p1 and K i1 represent the proportional and integral coefficients of the DC side PI control loop, K is the hybrid coefficient, and P0 is the reference active power output by the DC side voltage control loop.

[0017] Considering the dynamic characteristics of the DC side capacitor circuit, its mathematical model can be established as follows:

[0018]

[0019] Wherein i c is the DC side capacitor current, i in and i out represent the input and output currents of the DC side, respectively; i d is the current flowing through the DC side crowbar circuit, C dc is the DC side capacitor value.

[0020] Based on formula (2), the equivalent dynamic equation of the DC side of the grid-connected converter can be obtained:

[0021]

[0022] Wherein, P in and P out represent the input and output active powers, respectively, and P dis the prying rod power loss, generally, the prying rod power loss is not considered, that is, P d = 0; J DC is defined as the equivalent inertia coefficient of the converter, and the value is equal to C dc U2 dc / P rated , and the rated capacity P dc is related; ignoring the power loss of the switch tube of the converter, the active power of the converter DC side input is equal to the active power of the AC side output, that is, P rated = P out = P em = T em ω0.

[0023] Further, the power control loop of the grid-forming converter can be represented as:

[0024]

[0025] Where ω0is the reference angular frequency value, ω1is the angular frequency output by the VSG power loop, ω2is the angular frequency output by the VSG DC side voltage hybrid control, and ω II is the total angular frequency of the VSG, and θ represents the phase angle, T0is the reference torque T em is the electromagnetic torque, and J and D p represent the inertia and damping coefficient respectively.

[0026] Further, the construction of the active and reactive power equation of the converter includes the following steps:

[0027] Taking the grid phase angle as the reference coordinate system, define δ as the phase angle difference between the grid-forming converter and the grid

[0028]

[0029] Considering that the voltage and current inner loop response speed is fast, it can be assumed that the dynamic process of the voltage and current inner loop can be ignored, and the relationship between the grid-forming converter port voltage and the output current is as shown in the following formula

[0030] V abc = G(s)E abc -Z(s)i abc (6)

[0031] Where V abc is the voltage after filtering, G(s)E abc is the equivalent internal potential of the converter, i abc is the output current value of the converter, and Z(s) is the equivalent impedance considering the inner loop control and line impedance;

[0032] According to the two-port network transmission characteristics, the active and reactive power output by the converter is

[0033]

[0034] wherein G eq = R eq / (R2 eq+X2 eq), B eq = -X eq / (R2 eq+X2 eq); X eq and R eq are conductance and susceptance of the equivalent impedance Z eq .

[0035] Further, the reference active power can be represented as:

[0036]

[0037] Further, the calculation of the critical fault clearing time of the converter under different parameter perturbations by the numerical integration method comprises the following steps:

[0038] The condition for the calculation of the critical fault clearing time is

[0039]

[0040] Substituting equation (9) into equation (10) can obtain

[0041]

[0042] The above equation can be solved by a numerical calculation method; when δ max = δ u , the obtained δ c of the converter is the critical fault clearing angle δ cr ; the critical fault clearing angle is substituted into equation (4) to obtain the critical fault clearing time t cr , as shown in the following equation:

[0043]

[0044] The application also proposes a hybrid control system for the DC side voltage of a grid-forming converter, comprising the following modules:

[0045] A control module: a hybrid control strategy for the DC side voltage is established based on the topology structure and system parameters of the grid-forming converter;

[0046] The transient mathematical model of the grid-forming converter is established by the following steps: based on the dynamic characteristics of the capacitor circuit on the DC side, an equivalent dynamic equation of the DC side of the grid-forming converter is constructed; based on the angular frequency of each output item in the topology structure, a power control loop of the grid-forming converter is constructed; based on the transmission characteristics of the two-port network, an active and reactive power equation of the output of the converter is established; and the transient mathematical model of the grid-forming converter under the synchronous stability scale is constructed by combining the equivalent dynamic equation of the DC side, the power control loop of the grid-forming converter and the active and reactive power equation of the output of the converter.

[0047] The parameter analysis module is configured to determine a change curve of the reference active power through the transient mathematical model of the grid-forming converter, and analyze the influence of the parameters on the transient stability of the grid-forming converter based on the equal-area rule.

[0048] The fault analysis module is configured to calculate the critical fault clearing time of the converter under different parameter perturbations by using a numerical integration method.

[0049] The feedback module is configured to adjust the DC side voltage hybrid control strategy in the control module based on the parameter analysis results of the parameter analysis module and the fault analysis module.

[0050] The application also provides a storage medium having a computer executable program stored therein, and the computer executable program is executed by a processor to implement the grid-forming converter DC side voltage hybrid control method according to any one of the above.

[0051] The application also provides a control device, which comprises:

[0052] at least one memory for storing a program;

[0053] at least one processor for loading the program to execute the grid-forming converter DC side voltage hybrid control method according to any one of the above.

[0054] The application has the following beneficial effects:

[0055] The application feeds back the DC side voltage deviation to the reference active power, establishes the transient mathematical model of the grid-forming converter under the synchronous stability scale, qualitatively analyzes the influence mechanism of the DC side voltage control on the stability of the converter, quantitatively analyzes the influence law of different parameters by using the critical fault clearing time, and provides a reference for the optimal design of the DC side voltage control parameters. The grid-forming converter after the optimal design has strong stability and ensures the safe and stable operation of the converter. BRIEF DESCRIPTION OF DRAWINGS

[0056] The application will be further described below with reference to the drawings.

[0057] Figure 1 The figure is a topology structure diagram of the DC side control of the grid-forming converter.

[0058] Figure 2 DC side voltage hybrid control block diagram for grid-forming converter

[0059] Figure 3 Mechanism analysis diagram for the influence of DC side voltage hybrid control based on the equal-area criterion on the transient stability of the grid-forming converter

[0060] Figure 4 Quantitative analysis results of the influence of key parameter perturbation of the grid-forming converter

[0061] Figure 5 Flowchart of the implementation steps of the method of the present disclosure

[0062] Figure 6 Comparison chart of simulation results of the grid-forming converter before and after the optimization design of the control parameters DETAILED DESCRIPTION

[0063] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0064] In the description of the present specification, the description of the terms "one embodiment", "example", "specific example" and the like means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are contained in at least one embodiment or example of the present application. In the present specification, the illustrative representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0065] In order to facilitate understanding, the present disclosure establishes the following power electronic converter control structure to explain the concept of the present disclosure, and the converter hardware structure and control structure involved are as follows Figure 1The converter hardware structure includes a direct current micro source and a DC / AC converter for power conversion. The direct current micro source includes, but is not limited to, an energy storage battery, a photovoltaic panel and other direct current power sources. The direct current input is converted into alternating current by the DC / AC converter and connected to a common bus. The output end of the DC / AC converter is connected to an LC filter inductor and capacitor; the power electronic converter control structure mainly includes a direct current side voltage control module, a power control module, a voltage control module, a current control module and a pulse width modulation / driving module. The direct current side voltage control is realized by collecting the input voltage control ring of the converter direct current side voltage; the input active and reactive power is calculated by collecting the converter port voltage and output current, and the control phase angle and reference voltage amplitude are obtained through the active power ring and the reactive control ring respectively; the reference current value is output by the voltage control module through the PI control ring for the reference voltage; the output current amplitude of the converter is controlled by the current control module, and the given current reference value is realized through the PI control ring for the difference tracking; the pulse width modulation and / or driving module generates a modulation signal to drive the IGBT switch device.

[0066] Step 1: The topological structure and system parameters of the grid-forming converter are first acquired in the embodiment of the present disclosure, and the control structure of the grid-forming converter is as shown in Figure 1 For the problem of direct current side voltage dynamic control, a direct current side voltage hybrid control strategy is proposed, and the control block diagram and the topological structure of the power control ring are as shown in Figure 2 The direct current side voltage hybrid control strategy includes the following steps:

[0067] The direct current side voltage deviation value is controlled through the PI control to realize the difference control, and the output values are respectively fed back to the reference angle frequency value and the reference active power value. The reference angle frequency value is the output value multiplied by the proportional coefficient 1-K (0≤K≤1), and the reference active power value is the output value multiplied by the proportional coefficient K and the reference direct current side voltage value, so as to realize the direct current side voltage hybrid control.

[0068] Step 2: Before establishing the grid-forming converter transient model under the synchronous stability dimension, it is considered that the transient response of the grid-forming converter is mainly determined by the power control ring dynamic, and the voltage and current control ring can quickly reach the stable state, that is, the amplitude of the converter port voltage V abc is equal to the reference voltage value E q , and the output current I dq is equal to the reference current value I dq of the converter. The mathematical model of the direct current side voltage hybrid control link is as shown in the following formula:

[0069]

[0070] P0 is the reference active power output by the direct current side voltage control ring; and U dcare the reference and measured DC side voltages, respectively, K p1 and K i1 represent the PI control loop proportional and integral coefficients of the DC side, and K is the mixing coefficient. Considering the dynamic characteristics of the DC side capacitor circuit, its mathematical model can be established as follows:

[0071]

[0072] where i c is the DC side capacitor current, i in and i out represent the input and output currents of the DC side, respectively, C dc is the DC side capacitor value. i d is the current flowing through the DC side crowbar circuit. Based on equation (2), the equivalent dynamic equation of the grid-forming converter on the DC side can be obtained:

[0073]

[0074] where P in and P out represent the input and output active powers, respectively, P d is the crowbar power loss, which is generally not considered, i.e., P d = 0. J DC is defined as the equivalent inertia coefficient of the converter, whose value is equal to C dc U2 dc / P rated , which is related to the DC side capacitor U dc and its rated capacity P rated . Ignoring the power loss of the converter switching tube, the input active power on the DC side of the converter is equal to the output active power on the AC side, i.e., P out = P em = T em ω0.

[0075] The power control loop of the grid-forming converter can be represented as:

[0076]

[0077] where ω0is the reference angular frequency value, ω1is the angular frequency output by the VSG power loop, ω2is the angular frequency output by the VSG DC side voltage mixing control, ω II is the total angular frequency of the VSG, θ represents the phase angle, T em is the electromagnetic torque, and T0is the reference torque. J and D p represent the inertia and damping coefficients, respectively. Taking the grid phase angle as the reference coordinate system, δ is defined as the phase angle difference between the grid-forming converter and the grid

[0078]

[0079] Considering the fast response of the inner voltage and current loop, the dynamic process of the inner voltage and current loop can be ignored, and the relationship between the grid-connected converter port voltage and the output current is shown in the following equation

[0080] V abc = G(s)E abc - Z(s)i abc (6)

[0081] where V abc is the voltage after filtering, G(s)E abc is the equivalent internal voltage of the converter, i abc is the output current value of the converter, and Z(s) is the equivalent impedance considering the inner loop control and the line impedance. According to the two-port network transmission characteristics, the active and reactive power output by the converter is

[0082]

[0083] where G eq = R eq / (R2 eq+X2 eq), B eq = -X eq / (R2 eq+X2 eq). E is the effective value of the converter port voltage, V g is the equivalent voltage value of the grid, X eq and R eq are the conductance and susceptance of the equivalent impedance Z eq . According to equations (3), (4) and (7), the transient mathematical model of the grid-connected converter under the synchronous stability scale can be obtained

[0084]

[0085] Step 3: According to the transient mathematical model of the grid-connected converter established in step 2, the transient synchronous stability of the converter is mainly determined by the DC side voltage control loop and the power control loop. According to equation (8), the DC side voltage control loop will output the reference active power P0. Therefore, the influence of the DC side voltage control loop on the grid-connected converter mainly reflects in the change of the reference active power, and the mathematical relationship satisfies equation (9).

[0086]

[0087] When the grid fails, the active power curve of the converter changes from curve I to curve II, as shown in Figure 3 . The decrease of the active power P em will cause the imbalance of the input and output power of the DC side capacitor, and further cause the increase of the reference active power P0, as shown in Figure 3The increase of reference active power will result in the acceleration area increasing from S1 to S1+S3 compared with constant reference active power. When the grid fault is cleared, the active power output curve of the converter changes from curve II to curve III. At this time, the active power output of the converter is greater than the rated active power, and the converter enters the deceleration state, and the deceleration area is S2+S4. Through the above analysis, it can be known that the DC side voltage control will result in the acceleration area of the grid-connected converter increasing when the grid fault occurs, thereby deteriorating the stability of the converter, and the deterioration degree is mainly determined by formula (9). The influence of the key parameters C dc , K p1 and K i1 in formula (9) can be analyzed by the numerical integration method, and the analysis results are shown in Table 1.

[0088] Table 1 Influence of key parameters on transient stability of grid-connected converter

[0089]

[0090] Step 4: According to the conclusion obtained in step 2, the critical fault clearance time of the converter under the perturbation of different parameters is calculated by using the numerical integration method. The calculation condition of the critical fault clearance time is

[0091]

[0092] Substituting formula (9) into formula (10) can obtain

[0093]

[0094] The above equation can be solved by numerical calculation. When δ max = δ u , the δ c of the converter obtained is the critical fault clearance angle δ cr . The critical fault clearance angle is substituted into formula (4) to obtain the critical fault clearance time t cr , as shown in the following formula.

[0095]

[0096] Step 5: According to the quantitative analysis result of step 3 based on the critical fault clearance time, it can be known that the increase of K p and K i parameters will deteriorate the system stability, and increasing the size of the DC side capacitor is beneficial to the stability of the system. According to the above analysis result, the DC side voltage control parameters of the grid-connected converter are optimized and designed, and the transient stability ability of the converter is enhanced.

[0097] Figure 6The comparative diagram of the phase plane simulation results of the grid-forming converter under different control parameters of the grid-forming converter when the grid voltage drops to 30% at 1s and the fault is removed at 1.3s is given. It can be observed from the diagram that the grid-forming converter after parameter optimization can return to the stable operation state after the fault is removed, and the converter under the control of the active power feedback of the DC side voltage or the feedback reference angle frequency will appear the transient synchronous instability problem, and the results verify the effectiveness and accuracy of the proposed DC side voltage hybrid control strategy and the optimization design method.

[0098] The embodiment of the present application also discloses a grid-forming converter DC side voltage hybrid control system, comprising the following modules:

[0099] The control module: the DC side voltage hybrid control strategy is established based on the topological structure and system parameters of the grid-forming converter;

[0100] The grid-forming converter transient mathematical model establishment module: the DC side equivalent dynamic equation of the grid-forming converter is constructed based on the dynamic characteristics of the DC side capacitor circuit; the power control loop of the grid-forming converter is constructed based on the output angle frequency of each item in the topological structure; the active and reactive power equations of the converter are established based on the transmission characteristics of the two-port network; the grid-forming converter transient mathematical model under the synchronous stability scale is constructed by combining the DC side equivalent dynamic equation, the power control loop of the grid-forming converter and the active and reactive power equations of the converter;

[0101] The parameter analysis module: the change curve of the reference active power is determined through the grid-forming converter transient mathematical model, and the influence of the parameters on the transient stability of the grid-forming converter is determined based on the equal-area rule analysis of the change curve;

[0102] The fault analysis module: the critical fault removal time of the converter under different parameter perturbations is calculated by the numerical integral method;

[0103] The feedback module: the DC side voltage hybrid control strategy in the control module is adjusted based on the parameter analysis results of the parameter analysis module and the fault analysis module.

[0104] The embodiment of the present application also discloses a control device for running a database stored procedure, wherein the database stored procedure is executed as the grid-forming converter DC side voltage hybrid control method disclosed above.

[0105] The embodiment of the present application also discloses a computer storage medium, and the storage medium comprises a database stored procedure, wherein the device where the storage medium is located is controlled to execute the grid-forming converter DC side voltage hybrid control method disclosed above when the database stored procedure is run.

[0106] In the context of this disclosure, a computer storage medium can be a tangible medium that can contain or store a program for use by or in connection with an instruction execution system, apparatus, or device. Machine-readable media can be machine-readable signal media or machine-readable storage media. Machine-readable media can include, but are not limited to, electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of machine-readable storage media can include one or more lines of electrical connections, portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or Flash memory), optical fibers, portable compact disc read-only memory (CD-ROM), optical storage devices, magnetic storage devices, or any suitable combination of the foregoing.

[0107] The foregoing is considered as illustrative only of the principles of the application. Those skilled in the art will appreciate that the application is capable of being practiced with various modifications and changes without departing from the spirit and scope of the application. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature and not as restrictive.

Claims

1. A hybrid control method for a DC side voltage of a meshed network converter, characterized in that, The method comprises the following steps: a DC side voltage hybrid control strategy is established based on a topology structure and system parameters of the grid-connected converter; an equivalent dynamic equation of the DC side of the grid-connected converter is constructed based on dynamic characteristics of a DC side capacitor circuit; a power control loop of the grid-connected converter is constructed based on each angular frequency output in the topology structure; an active and reactive power equation of the converter output is established based on two-port network transmission characteristics; a transient mathematical model of the grid-connected converter under a synchronous stability scale is constructed by combining the equivalent dynamic equation of the DC side, the power control loop of the grid-connected converter and the active and reactive power equation of the converter output; 2. The hybrid DC voltage control method for network-forming converters according to claim 1, characterized in that, a change curve of a reference active power is determined through the transient mathematical model of the grid-connected converter, and an influence of parameters on transient stability of the grid-connected converter is determined based on the change curve and the equal-area rule; 3. The hybrid DC voltage control method for network-forming converters according to claim 1, characterized in that, a critical fault clearing time of the converter under different parameter perturbations is calculated through a numerical integration method; the DC side voltage hybrid control strategy is adjusted based on the influence of the parameters on the transient stability of the grid-connected converter and the critical fault clearing time of the converter, with the aim of improving transient stability of the converter. wherein, and U dc are the reference and measured DC side voltages, respectively, K p1 and K i1 represent the proportional and integral coefficients of the DC side PI control loop, K is a mixing coefficient, and P0 is the reference active power output of the DC side voltage control loop. The DC side voltage hybrid control strategy adopts zero-error control on a DC side voltage deviation value, and output values are respectively fed back to a reference angular frequency value and a reference active power value; the reference angular frequency value is the output value multiplied by a proportional coefficient 1-K (0≤K≤1), and the reference active power value is the output value multiplied by a proportional coefficient K and a reference DC side voltage value. where i c is the DC side capacitor current, i in and i out represent the DC side input and output currents, respectively; i d is the current flowing through the DC side crowbar circuit, C dc is the DC side capacitance value; The construction of the equivalent dynamic equation of the DC side of the grid-connected converter comprises the following steps: wherein P in and P out represent the input and output active power, P d is the crowbar power loss, which is generally not considered, i.e. P d = 0; J DC is defined as the equivalent inertia coefficient of the converter, which has a value equal to C dc U2 dc / P rated , related to the DC side capacitor U dc and its rated capacity P rated ; ignoring the power loss of the converter switching tube, the input active power of the converter DC side is equal to the output active power of the AC side, i.e. P out = P em = T em ω0.

4. The hybrid DC voltage control method for network-forming converters according to claim 3, characterized in that, A mathematical model of the DC side voltage hybrid control loop is as follows: where ω0is the reference angular frequency value, ω1is the angular frequency of the VSG power loop output, ω2is the angular frequency of the VSG DC side voltage hybrid control output, ω II is the total angular frequency of the VSG, θ represents the phase angle, T0is the reference torque T em is the electromagnetic torque, J and D p represent the inertia and damping coefficient, respectively.

5. The hybrid DC voltage control method for grid-forming converters according to claim 4, characterized in that, Considering the dynamic characteristics of the DC side capacitor circuit, a mathematical model thereof can be established as follows: Based on formula (2), an equivalent dynamic equation of the DC side of the grid-connected converter can be obtained as follows: The power control loop of the grid-connected converter can be expressed as: V abc = G(s)E abc - Z(s)i abc (6) where V abc is the voltage after the filter, G(s)E abc is the equivalent internal voltage of the converter, i abc is the output current value of the converter, and Z(s) is the equivalent impedance considering the inner loop control and line impedance. The construction of the active and reactive power equation of the converter output comprises the following steps: where G eq = R eq / (R2 eq+X2 eq), B eq = -X eq / (R2 eq+X2 eq); X eq and R eq are the conductance and susceptance, respectively, of an equivalent impedance Z eq .

6. The hybrid DC voltage control method for grid-forming converters according to claim 5, characterized in that, Taking a grid phase angle as a reference coordinate system, δ is defined as a phase angle difference between the grid-connected converter and the grid 7. The hybrid DC voltage control method for grid-forming converters according to claim 6, characterized in that, Considering that a voltage and current inner loop has a relatively fast response speed, it can be assumed that a dynamic process of the voltage and current inner loop can be ignored, and a relationship between a port voltage and an output current of the grid-connected converter is as follows According to two-port network transmission characteristics, active and reactive power output by the converter is The reference active power can be expressed as: The above equation can be solved by numerical calculation method; when δ max = δ u , the δ c of the obtained converter is the critical fault removal angle δ cr ; the critical fault removal angle is substituted into equation (4) to obtain the critical fault removal time t cr , as shown in the following equation:

8. A hybrid control system for a network-forming converter DC side voltage, characterized by The calculation of the critical fault clearing time of the converter under different parameter perturbations through the numerical integration method comprises the following steps: A condition for the calculation of the critical fault clearing time is By substituting formula (9) into formula (10), the following formula is obtained The method comprises the following modules: a control module: a DC side voltage hybrid control strategy is established based on a topology structure and system parameters of the grid-connected converter; The grid-forming converter transient mathematical model establishment module: based on the dynamic characteristics of the DC side capacitor circuit, the equivalent dynamic equation of the DC side of the grid-forming converter is constructed; based on the output of each item angular frequency in the topology structure, the power control loop of the grid-forming converter is constructed; based on the transmission characteristics of the two-port network, the active and reactive power equations of the converter output are established; the grid-forming converter transient mathematical model under the synchronous stability scale is constructed by combining the equivalent dynamic equation of the DC side, the power control loop of the grid-forming converter and the active and reactive power equations of the converter output; The parameter analysis module: determine the change curve of the reference active power through the grid-forming converter transient mathematical model, and analyze the influence of the parameters on the transient stability of the grid-forming converter based on the equal-area rule. The fault analysis module: calculate the critical fault clearing time of the converter under different parameter perturbations through the numerical integration method; The feedback module: adjust the DC side voltage hybrid control strategy in the control module based on the parameter analysis results of the parameter analysis module and the fault analysis module.

9. A storage medium, characterized by The computer executable program is stored in the computer readable storage medium, and the computer executable program is executed by the processor to implement the grid-forming converter DC side voltage hybrid control method of any one of claims 1-7.

10. A control device characterized by , comprising: At least one memory for storing programs; At least one processor for loading the program to execute the grid-forming converter DC side voltage hybrid control method of any one of claims 1-7.

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