Control method for improving small signal stability of voltage source type current converter

Abstract

The invention discloses a control method for improving small signal stability of a voltage source type current converter. The control method comprises the following steps: respectively establishing small signal models of a current converter, an alternating current line and an infinite power grid; determining a value of an adaptive parameter according to a small signal stability analysis result; acquiring the voltage and current of the grid-connected point of the converter, the actual value and active power of the output active power of the converter, and the reference value of the voltage amplitude of the power grid; calculating a phase reference value of the alternating current side of the converter; calculating a reference value of the current of the grid-connected point of the current converter under the dq rotating coordinate system according to a set voltage amplitude reference value of the grid-connected point of the current converter; and according to the phase reference value of the AC side of the converter and the reference value of the grid-connected point current in the dq rotating coordinate system, calculating the reference value of the modulation voltage at the AC outlet of the converter in the abc static coordinate system. According to the invention, the self-adaptive parameters are self-adaptively adjusted according to the operation condition so as to improve the small signal stability of the voltage source type converter under various power grid intensities.

Description

Technical Field

[0001] The present invention belongs to the technical field of voltage source converter control, and in particular relates to a control method for improving the small signal stability of a voltage source converter. Background Art

[0002] In actual engineering, most new energy units need to pass through voltage source converters (VSC). The control strategies of VSC mainly include grid-following (GFL) and grid-forming (GFM). Grid-following VSC mainly adopts vector current control, which independently controls the active power and reactive power output by the converter by decoupling the variables in the dq coordinate system. The phase-locked loop (PLL) in its control strategy can track the grid connection point voltage to achieve synchronization between the converter and the grid. However, the grid-following VSC is equivalent to a controllable current source, and a synchronous generator or synchronous condenser is required to establish the grid connection point voltage to maintain stable operation. With the increase in the proportion of new energy, the proportion of synchronous generators in the power system has decreased year by year, which has brought challenges to the stable operation of the grid-following VSC. Especially when the power generation units in the future power system are all composed of power electronic converters, the grid-following control cannot build voltage, and the power system will not be able to operate normally.

[0003] The grid-type VSC is essentially a voltage source, which achieves synchronization by controlling the output power or DC voltage of the converter itself rather than through PLL. Because of this characteristic of the grid-type VSC, it can be connected to an extremely weak power grid or even a passive network. However, since the grid-type VSC is controlled as a voltage source, when it is connected to a strong power grid, the grid-type VSC and the strong power grid are equivalent to two voltage sources directly connected in parallel. The stronger the power grid, the smaller the connection impedance between the two voltage sources, which is less conducive to the stable operation of the power system.

[0004] Therefore, neither the grid-following control nor the grid-building control can maintain small-signal stability in strong and weak grids at the same time. Therefore, it is necessary to propose a control method that enables the converter to maintain good small-signal stability under various grid strengths. Summary of the invention

[0005] The main purpose of the present invention is to overcome the shortcomings and deficiencies of the prior art and to provide a control method for improving the small signal stability of a voltage source converter. The control method obtains an adaptive parameter value method through a small signal stability analysis result, and the adaptive parameters can be adaptively changed according to the grid strength so that the voltage source converter always maintains good small signal stability under various grid strengths.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A control method for improving the small signal stability of a voltage source converter, the control method comprising the following steps:

[0008] S1. Use state space equation method to model small signals of voltage source converter, AC line and infinite power grid respectively;

[0009] S2. Performing a small signal stability analysis on the established small signal model, and obtaining a method for determining the value of the adaptive parameter K based on the small signal stability analysis result;

[0010] S3, obtaining the voltage and current at the grid-connected point of the voltage source converter, and performing dq decomposition on the voltage and current at the grid-connected point of the voltage source converter, respectively obtaining the d-axis component and q-axis component of the voltage at the grid-connected point of the voltage source converter in the dq rotating coordinate system, obtaining the active power output by the voltage source converter, and obtaining reference values ​​of the active power output by the voltage source converter and the voltage amplitude at the grid-connected point of the voltage source converter;

[0011] S4. Calculate the phase reference value θ on the AC side of the voltage source converter * ;

[0012] S5. Calculate the d-axis current reference value i of the voltage source converter grid connection point vd_ref , q-axis current reference value i vq_ref ;

[0013] S6. Calculate the d-axis voltage reference value u at the AC outlet of the voltage source converter vd_ref and q-axis voltage reference value u vq_ref ;

[0014] S7. Calculate the a-axis voltage reference value u of the modulation voltage at the AC outlet of the voltage source converter in the abc stationary coordinate system va_ref , b-axis voltage reference value u vb_ref and c-axis voltage reference value u vc_ref ;

[0015] S8. Based on the reference value of the modulation voltage and using pulse width modulation theory, generate corresponding control pulses to control the voltage source converter.

[0016] Furthermore, in step S1, the present invention adopts the state space equation method to establish a small signal model. According to the control equations of each control loop of the converter control system, the small signal state space model of the voltage source converter can be obtained as follows:

[0017]

[0018] In the above formula, K is the proportional coefficient in the hybrid synchronous control loop, which is used to adjust the ratio of the phase-locked loop and the power synchronous loop. f is the filter inductance, J is the moment of inertia of the virtual synchronous generator, D is the damping coefficient, P ref and P are the reference value and active power of the voltage source converter output, ω is the angular frequency, ω 0 is the rated value of angular frequency, m is the proportional coefficient of proportional control, θ is the phase angle value of the hybrid synchronous control loop output, u sq is the q-axis component of the grid-connected point voltage, u sd 、u sq and u sd_ref 、u sq_ref are the d-axis component and q-axis component of the grid-connected point voltage and the d-axis component reference value and q-axis component reference value respectively, i vd_ref and i vq_ref are the d-axis component and q-axis component of the current reference value on the converter side, L v With R v is the coefficient of the first-order inertia link, u vd_ref and u vq_ref are the d-axis component and q-axis component of the voltage reference value on the converter side, respectively. vd with i vq are the d-axis component and q-axis component of the converter side current, respectively, and k p and k i are the proportional gain and integral gain of the current inner loop proportional integral controller, and the current inner loop proportional integral controller is referred to as PI. 1 、x 2 It is the integral value of the input deviation of the current inner loop PI link over time. represents the derivative of the time variable t;

[0019] The small signal model of the AC line and the infinite power grid is as follows:

[0020]

[0021] in

[0022]

[0023] In the above formula, C f is the filter capacitance, L eq is the equivalent inductance of the transformer and the line inductance, T wt is the transformer ratio, u gdq is the voltage dq axis component of the infinite power grid, i sdq is the dq axis component of the current at the grid connection point, i vq0 、i vd0 、u sq0 、u sd0、i sq0 、i sd0

[0024] They are respectively the values ​​of the dq-axis component of the converter side current, the grid-connected point voltage and the dq-axis component of the current at the steady-state operating point.

[0025] Furthermore, in step S2, based on the small signal model established in step S1, the eigenvalue method is used to analyze the small signal stability of the system, and the relationship between the small signal stability, the grid strength and the adaptive parameter K is obtained according to the small signal stability analysis result. In order to enable the converter to maintain good small signal stability under different grid strengths, the adaptive parameter K value selection method is designed as follows:

[0026]

[0027] Where SCR is the system short-circuit ratio. Here, the grid side of the transformer is regarded as an infinite system. The system short-circuit ratio is determined by the line impedance L g The SCR calculation formula is as follows:

[0028]

[0029] Where S base is the reference power, u sbase is the reference voltage, f is the frequency, Z * , Z base , and Z are the per unit value, reference value and actual value of the line impedance respectively.

[0030] Furthermore, in step S4, the hybrid synchronous control loop of the converter is obtained by fusing the power synchronous control loop and the phase-locked loop through the adaptive coefficient K. The control loop can change the adaptive parameter K under different system conditions to enable the converter to have the advantages of both grid-forming control and grid-following control. The phase-locked loop in the hybrid synchronous control loop is proportional control. According to the control equation of the hybrid synchronous control, the phase reference value θ on the AC side of the converter can be obtained. * The calculation formula is as follows:

[0031]

[0032] Among them, s is the Laplace operator, ω 0 is the rated angular frequency.

[0033] Furthermore, in step S5, the voltage outer loop of the converter uses virtual impedance control, the purpose of which is to control the output voltage of the converter to a constant value, so that the converter can autonomously construct frequency and voltage, and the external characteristics are shown as a controllable voltage source. According to the voltage outer loop control equation, the d-axis current reference value i of the converter grid connection point can be obtained vd_ref and q-axis current reference value ivq_ref The calculation formula is as follows:

[0034]

[0035] Furthermore, in step S6, the main function of the current inner loop in the converter is to decouple the voltage and current components and obtain a voltage reference value to obtain a voltage modulation wave. According to the control equation of the current inner loop, the d-axis voltage reference value u of the modulation voltage at the converter outlet can be obtained. vd_ref and q-axis voltage reference value u vq_ref The calculation formula is as follows:

[0036]

[0037] Furthermore, in step S7, the d-axis voltage reference value u of the modulation voltage at the converter outlet obtained by the current inner loop in step S6 is vd_ref and q-axis voltage reference value u vq_ref The Park transformation equation is required to transform the modulation voltage from the dq rotating coordinate system to the abc three-phase stationary coordinate system. The Park transformation equation can be used to obtain the a-axis voltage reference value u of the modulation voltage at the converter outlet in the abc stationary coordinate system. va_ref , b-axis voltage reference value u vb_ref and c-axis voltage reference value u vc_ref The calculation formula is as follows:

[0038]

[0039] Furthermore, after obtaining the a-axis voltage reference value, b-axis voltage reference value and c-axis voltage reference value of the modulation voltage in the abc stationary coordinate system, the control pulses corresponding to each IGBT of the converter can be generated through the commonly used pulse width modulation theory, thereby controlling the voltage source converter.

[0040] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0041] The present invention can be based on the small signal model established by the state space equation in step S1, and the relationship between the small signal stability of the system, the adaptive parameter K and the grid strength (expressed by SCR in the present invention) can be obtained by the eigenvalue analysis method. According to the small signal stability of the system when the adaptive parameter K takes different values ​​under a strong grid and the small signal stability of the system when the adaptive parameter K takes different values ​​under a weak grid, the relationship between the value of the adaptive parameter K and the small signal stability of the system under different grid strengths is obtained. Finally, according to this relationship, a method for taking the value of the adaptive parameter K that enables the system to maintain good small signal stability in both strong and weak grids is proposed. The adaptive parameter K can be adaptively changed according to the grid strength so that the voltage source converter maintains good small signal stability under various grid strengths. The method proposed by the present invention greatly improves the small signal stability of the system, so that the system can stably operate under strong and weak grids at the same time, and greatly improves the short-circuit ratio operating range of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0043] Figure 1 It is a main circuit diagram of the voltage source converter grid-connected system used in the present invention.

[0044] Figure 2 It is a flow chart of a control method for improving small signal stability of a voltage source converter disclosed in the present invention;

[0045] Figure 3 It is a frequency waveform comparison verification diagram of the small signal model established in Simulink of the present invention and the electromagnetic transient model established in PSCAD under the condition of power step.

[0046] Figure 4 It is a motion trajectory diagram of the dominant pole of the system when the system short circuit ratio changes from 1.2 to 12 using the control method for improving the small signal stability of the voltage source converter of the present invention;

[0047] Figure 5 It is a schematic diagram of a simulated waveform of a d-axis component of a converter output current when the system short-circuit ratio changes from 1.2 to 12 using the control method for improving the small-signal stability of a voltage source converter of the present invention. DETAILED DESCRIPTION

[0048] In order to enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with 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 of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present application.

[0049] Reference to "embodiments" in this application means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described in this application may be combined with other embodiments.

[0050] Example 1

[0051] See also Figure 1 , the system circuit diagram is given, the converter passes through the filter inductor L f And the filter capacitor C f connected to the transformer, which is connected to the transformer through the line inductance L g Connected to the infinite grid, the AC side refers to the right side of the converter.

[0052] See also Figure 2 This embodiment discloses a control method for improving the small signal stability of a voltage source converter, comprising the following steps:

[0053] S1. Use the state space equation method to perform small signal modeling on the converter, AC line and infinite power grid respectively;

[0054] Among them, the small signal model of the converter is as follows:

[0055]

[0056]

[0057] In the above formula, K is the proportional coefficient in the hybrid synchronous control loop, which is used to adjust the ratio of the phase-locked loop to the power synchronous loop in the hybrid synchronous control loop. f is the filter inductance, J is the moment of inertia of the virtual synchronous generator, D is the damping coefficient, P ref and P are the reference value and active power of the converter output respectively, ω is the angular frequency, ω 0 is the rated value of angular frequency, m is the proportional coefficient of proportional control, θ is the phase angle value of the hybrid synchronous control loop output, u sq is the q-axis component of the grid-connected point voltage, usd 、u sq and u sd_ref 、u sq_ref are the dq axis components of the grid connection point voltage and their reference values, i vd_ref and i vq_ref are the dq axis components of the converter side current reference value, L v With R v is the coefficient of the first-order inertia link, u vd_ref and u vq_ref are the dq axis components of the voltage reference value on the converter side, i vd with i vq are the dq axis components of the converter side current, k p and k i are the proportional gain and integral gain of the current inner loop PI controller, respectively, 1 、x 2 It is the integral value of the input deviation of the current inner loop PI link over time. represents the derivative with respect to the time variable t.

[0058] The small signal model of the AC line and the infinite power grid is as follows:

[0059]

[0060] in

[0061]

[0062] Where C f is the filter capacitance, L eq is the equivalent inductance of the transformer and the line inductance, T wt is the transformer ratio, u gdq is the voltage dq axis component of the infinite power grid, i sdq is the dq axis component of the current at the grid connection point, i vq0 、i vd0 、u sq0 、u sd0 、i sq0 、i sd0 are their values ​​at the steady-state operating point respectively.

[0063] S2. Perform small signal stability analysis on the established small signal model, and derive a method for selecting values ​​of adaptive parameters based on the analysis results;

[0064] Among them, the value of the controller adaptive parameter K is as follows:

[0065]

[0066] Where SCR is the system short-circuit ratio. Here, the high voltage side of the transformer is regarded as an infinite system. The system short-circuit ratio is determined by the line impedance L g The SCR calculation formula is as follows:

[0067]

[0068] Where S base is the reference power, u sbase is the reference voltage, f is the frequency, Z * , Z base , and Z are the per unit value, reference value and actual value of the line impedance respectively.

[0069] S3, obtaining the grid voltage and current of the converter grid connection point, and performing dq decomposition on the voltage and current of the converter grid connection point, respectively obtaining the d-axis component and q-axis component of the voltage of the converter grid connection point in the dq rotating coordinate system, obtaining the active power output by the converter, and obtaining reference values ​​of the converter output active power and the voltage amplitude of the converter grid connection point;

[0070] S4, calculating the phase reference value of the AC side of the converter;

[0071] Among them, the phase reference value θ on the AC side of the converter * The calculation formula is as follows:

[0072]

[0073] Among them, s is the Laplace operator, ω 0 is the rated angular frequency.

[0074] S5, calculating the d-axis current reference value and the q-axis current reference value of the converter grid connection point;

[0075] Among them, the d-axis current reference value i of the converter grid connection point is vd_ref and q-axis current reference value i vq_ref The calculation formula is as follows:

[0076]

[0077] S6, calculating the d-axis voltage reference value and the q-axis voltage reference value at the AC outlet of the converter;

[0078] Among them, the d-axis voltage reference value u of the modulation voltage at the converter outlet is vd_ref and q-axis voltage reference value u vq_ref The calculation formula is as follows:

[0079]

[0080] S7, calculating the a-axis voltage reference value, the b-axis voltage reference value and the c-axis voltage reference value of the modulation voltage at the AC outlet of the converter in the abc stationary coordinate system;

[0081] S8. Based on the reference value of the modulation voltage and using pulse width modulation theory, a corresponding control pulse is generated to control the converter.

[0082] Among them, the a-axis voltage reference value u of the modulation voltage at the converter outlet in the abc stationary coordinate system is va_ref , b-axis voltage reference value u vb_ref and c-axis voltage reference value u vc_ref The calculation formula is as follows:

[0083]

[0084] According to the converter small signal model and AC side small signal model constructed in step S1, the small signal model of the system can be obtained as follows:

[0085]

[0086] Among them A sys is the system state matrix. The small signal stability of the system is related to A sys The characteristic roots of the matrix.

[0087] x=[θ,i vd_ref ,i vq_ref ,i vd ,i vq ,u sd ,u sq ,i sd ,i sq ,ω,x 1 , x 2 ],u=[u sd_ref ,u sq_ref , P ref ]. sys is the input matrix.

[0088] In order to verify the correctness of the small signal model, this embodiment builds the following Figure 1 The inverter grid-connected system shown in the figure compares the response waveforms of the electromagnetic transient model built in PSCAD and the small signal model built in Simulink when the system experiences a power step. The specific simulation conditions are: K = 0.8, P ref =1p.u, and add an active power reference value step at 40s of simulation (the active power reference value drops from 1p.u to 0.9pu at 40s). Figure 3It can be seen that the power step response waveforms of the small signal model and the electromagnetic transient model have a high degree of consistency, which verifies the correctness and accuracy of the small signal model established in the present invention.

[0089] Example 2

[0090] Based on a control method for improving the small signal stability of a voltage source converter disclosed in Example 1, this embodiment uses the small signal model established in Example 1 to perform small signal stability analysis and verification. Here, by changing the line inductance L g To change the system short circuit ratio SCR, SCR gradually changes from 1.2 to 12, at which time the power grid strength of the converter gradually changes from an extremely weak power grid to a strong power grid. The small signal model adopts the small signal model established by the state space equation method in Example 1, and the converter adopts the voltage source converter control method involved in the present invention, wherein the most critical part is the hybrid synchronous control loop, which is formed by merging the power synchronous loop and the phase-locked loop through the adaptive parameter K. In addition to the synchronous control loop, there is also a voltage outer loop using virtual impedance control and a current inner loop using a PI controller.

[0091] The simulation condition is: by changing the line inductance L g The system short-circuit ratio SCR is 1.2 in the initial stage, then gradually increases from 1.2 and finally rises to 12. Figure 4 It can be seen that when the system short-circuit ratio gradually changes from 1.2 to 12, the system dominant pole first gradually moves to the left half plane, and the system small signal stability is enhanced. However, as the system short-circuit ratio SCR continues to increase, the system dominant pole gradually moves to the right half plane, and the system small signal stability gradually weakens. However, the system dominant pole always remains on the left side of the imaginary axis, indicating that the system using the control method proposed in the present invention can stably operate in the system with SCR1.2 to 12, and can stably operate in extremely weak power grids and strong power grids at the same time, greatly improving the system's short-circuit ratio operating range and small signal stability.

[0092] Example 3

[0093] Based on a control method for improving the small signal stability of a voltage source converter disclosed in Example 1, this embodiment adopts the control strategy proposed in Example 1, builds a corresponding electromagnetic transient simulation model in PSCAD, and the converter adopts the voltage source converter control method involved in the present invention, among which the most critical is the hybrid synchronous control loop, which is formed by the fusion of a power synchronous loop and a phase-locked loop through an adaptive parameter K. In addition to the synchronous control loop, there are also a voltage outer loop and a current inner loop using virtual impedance control.

[0094] The simulation condition is: by changing the line inductance L gThe system short-circuit ratio SCR is 1.2 in the initial stage, then gradually increases from 1.2 and finally rises to 12. Figure 5 It can be seen that when the system short-circuit ratio SCR gradually increases from 1.2 to 12, the steady-state value of the d-axis component of the converter output current gradually increases, but the converter can always maintain stable operation. Figure 5 The simulation results verify that the control method for improving the small signal stability of the voltage source converter proposed in Example 1 can enable the system adopting the control method proposed in the present invention to operate stably in extremely weak power grids and strong power grids, greatly improving the short-circuit ratio operating range of the system.

[0095] It should be noted that, for the sake of convenience, the aforementioned method embodiments are all expressed as a series of action combinations, but those skilled in the art should know that the present invention is not limited to the described order of actions, because according to the present invention, certain steps can be performed in other orders or simultaneously.

[0096] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0097] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A control method for improving the small signal stability of a voltage source converter, characterized in that: The control method comprises the following steps: S1. Use state space equation method to model small signals of voltage source converter, AC line and infinite power grid respectively; S2. Performing a small signal stability analysis on the established small signal model, and obtaining a method for determining the value of the adaptive parameter K based on the small signal stability analysis result; S3, obtaining the voltage and current of the grid-connected point of the voltage source converter, and performing dq decomposition on the voltage and current of the grid-connected point of the voltage source converter, respectively obtaining the d-axis component and q-axis component of the voltage of the grid-connected point of the voltage source converter in the dq rotating coordinate system, obtaining the active power output by the voltage source converter, and obtaining the reference value of the active power output by the voltage source converter and the d-axis component of the voltage of the grid-connected point of the voltage source converter; S4. Calculate the phase reference value θ on the AC side of the voltage source converter * ; S5. Calculate the d-axis current reference value i of the voltage source converter grid connection point vd_ref , q-axis current reference value i vq_ref ; S6. Calculate the d-axis voltage reference value u at the AC outlet of the voltage source converter vd_ref and q-axis voltage reference value u vq_ref ; S7. Calculate the a-axis voltage reference value u of the modulation voltage at the AC outlet of the voltage source converter in the abc stationary coordinate system va_ref , b-axis voltage reference value u vb_ref and c-axis voltage reference value u vc_ref ; S8. Based on the reference value of the modulation voltage and using pulse width modulation theory, generate corresponding control pulses to control the voltage source converter.

2. A control method for improving small signal stability of a voltage source converter according to claim 1, characterized in that: In step S1, the small signal model of the voltage source converter is as follows: In the above formula, K is the proportional coefficient in the hybrid synchronous control loop, which is used to adjust the ratio of the phase-locked loop to the power synchronous loop in the hybrid synchronous control loop. f is the filter inductance, J is the moment of inertia of the virtual synchronous generator, D is the damping coefficient, P ref and P are the reference value and active power of the voltage source converter output respectively, ω is the angular frequency, ω0 is the rated value of the angular frequency, m is the proportional coefficient of proportional control, θ is the phase angle value of the hybrid synchronous control loop output, u sq is the q-axis component of the grid-connected point voltage, u sd 、u sq and u sd_ref 、u sq_ref are the d-axis component and q-axis component of the grid-connected point voltage and the d-axis component reference value and q-axis component reference value respectively, i vd_ref and i vq_ref are the d-axis component and q-axis component of the current reference value on the converter side, L v With R v is the coefficient of the first-order inertia link, u vd_ref and u vq_ref are the d-axis component and q-axis component of the voltage reference value on the converter side, respectively. vd with i vq are the d-axis component and q-axis component of the converter side current, respectively, and k p and k i They are the proportional gain and integral gain of the current inner loop proportional integral controller, and the current inner loop proportional integral controller is referred to as PI. x1 and x2 are the integral values ​​of the input deviation of the current inner loop PI link over time. represents the derivative of the time variable t; The small signal model of the AC line and the infinite power grid is as follows: in In the above formula, C f is the filter capacitance, L eq is the equivalent inductance of the transformer and the line inductance, T wt is the transformer ratio, u gdq is the voltage dq axis component of the infinite power grid, u sdq is the dq axis component of the grid connection point voltage, i sdq is the dq axis component of the current at the grid connection point, i vq0 、i vd0 、u sq0 、u sd0 、i sq0 、i sd0 They are respectively the steady-state values ​​of the dq-axis component of the converter side current, the grid-connected point voltage and the dq-axis component of the current at the steady-state operating point.

3. A control method for improving small signal stability of a voltage source converter according to claim 1, characterized in that: In step S2, the adaptive parameter K is selected as follows: Where SCR is the system short-circuit ratio. Here, the high voltage side of the transformer is regarded as an infinite system. The system short-circuit ratio is determined by the line impedance L g The SCR calculation formula is as follows: Where S base is the reference power, u sbase is the reference voltage, f is the frequency, Z * , Z base , and Z are the per unit value, reference value and actual value of the line impedance respectively.

4. A control method for improving small signal stability of a voltage source converter according to claim 2, characterized in that: In step S4, the phase reference value θ on the AC side of the converter * The calculation formula is as follows: Where s is the Laplace operator, m is the proportional control coefficient of the phase-locked loop in the hybrid synchronous control loop, and u sq is the q-axis component of the grid-connected point voltage, and ω0 is the rated angular frequency.

5. A control method for improving small signal stability of a voltage source converter according to claim 2, characterized in that: In step S5, the d-axis current reference value i of the converter grid connection point is vd_ref and q-axis current reference value i vq_ref The calculation formula is as follows:

6. A control method for improving small signal stability of a voltage source converter according to claim 2, characterized in that: In step S6, the d-axis voltage reference value u of the modulation voltage at the converter outlet vd_ref and q-axis voltage reference value u vq_ref The calculation formula is as follows:

7. A control method for improving small signal stability of a voltage source converter according to claim 1, characterized in that: In step S7, the modulation voltage at the converter outlet is the a-axis voltage reference value u in the abc stationary coordinate system. va_ref , b-axis voltage reference value u vb_ref and c-axis voltage reference value u vc_ref The calculation formula is as follows:

8. A control method for improving small signal stability of a voltage source converter according to claim 1, characterized in that: In step S8, after obtaining the a-axis voltage reference value, b-axis voltage reference value and c-axis voltage reference value of the modulation voltage in the abc stationary coordinate system, the control pulses corresponding to each IGBT of the converter are generated through pulse width modulation theory, thereby controlling the voltage source converter.

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