An algebraic operation phase locking method, a converter controller and a grid-connected converter feasible region construction method

Abstract

The application relates to an algebraic operation phase-locked loop method, a converter controller and a grid-connected converter feasible region construction method. Compared with a traditional phase-locked loop, the algebraic operation phase-locked loop has better dynamic performance, and the order of the phase-locked loop can be reduced through algebraic operation, so that the stable operation feasible region of a system is conveniently constructed. The construction method is based on the algebraic operation phase-locked loop, a small signal model of a grid-connected converter and a controller thereof is derived, a system characteristic equation is solved based on a multiple-input multiple-output system theory, and a system stability condition is given. Based on the system stability condition, a construction method of a system stable feasible region is proposed. Compared with a traditional synchronous rotating coordinate system phase-locked loop, the algebraic operation phase-locked loop has faster response speed and better dynamic performance. Compared with a traditional parameter enumeration and repeated verification method, the grid-connected converter feasible region construction method based on the algebraic operation phase-locked loop is more simple and intuitive, and repeated analysis when parameters change is avoided.

Description

Technical Field

[0001] This invention relates to an algebraic operation phase-locked loop method, a converter controller, and a method for constructing the feasible region of a grid-connected converter, belonging to the field of power system and new energy power generation system control and stability analysis. Background Technology

[0002] In recent years, with the rapid development of renewable energy, new energy sources such as photovoltaics and wind turbines have been connected to the power grid through power electronic converters, and the power system has gradually transformed into a new type of power system dominated by new energy. Compared with the traditional power system dominated by synchronous machines, the high degree of power electronics has brought a series of stability problems to the power system, especially in weak grid environments, where broadband oscillations and harmonic resonance accidents occur frequently. Phase-locked loops (PLLs) are a key link in the grid connection of power electronic converters and play a crucial role in ensuring the stable operation of new energy grid-connected systems. Currently, phase-locked loops based on synchronous rotating coordinate systems (SRF-PLLs) are widely used, as their control structure is relatively simple and the control algorithm is easy to implement. However, the introduction of SRF-PLLs has brought significant challenges to system stability operation and stability analysis. On the one hand, the stability and dynamic performance of SRF-PLLs impose a dual limitation on their bandwidth. Lower bandwidth is beneficial for maintaining stability, but results in poor dynamic characteristics. Increasing the bandwidth is beneficial for improving dynamic characteristics, but reduces the stability margin, which is not conducive to maintaining system stability. This characteristic brings many inconveniences to the selection of SRF-PLL parameters. On the other hand, the introduction of SRF-PLL increases the system order, and its parameters are deeply coupled with converter controller parameters and weak grid parameters, making stability analysis more cumbersome. Existing methods mostly employ numerical verification techniques, combining specific case parameters, enumerating several parameter changes, and using tools such as Nyquist and Bode plots to conduct stability trend analysis. These methods require repeated analysis and verification and cannot cope with the rapid changes in interface grid characteristics. Especially against the backdrop of rapid development of new energy sources and continuous increase in grid connection scale, the grid strength is constantly weakening, placing higher demands on the simplicity and accuracy of phase-locked loop design and system stability analysis. Summary of the Invention

[0003] This invention proposes an algebraic operation phase-locked loop (PLL) method, a converter controller, and a method for constructing the feasible region of a grid-connected converter. Compared with traditional PLLs, the algebraic operation PLL has better dynamic performance. At the same time, algebraic operations can reduce the PLL order, thereby facilitating the construction of the feasible region for stable system operation and reducing the repetition of stability analysis.

[0004] The present invention adopts the following technical solution:

[0005] A phase-locked method for algebraic operations, specifically:

[0006] The trigonometric function values ​​required for the Park transformation of the three-phase AC voltage at the grid connection point with the direct-axis, quadrature-axis, and zero-axis voltages in a synchronous rotating coordinate system are obtained through algebraic operations, thereby achieving phase-locking. The algebraic operations for the required trigonometric function values ​​are as follows:

[0007]

[0008] The Park transformation relationships between the three-phase AC voltage at the grid connection point and the voltages along the direct axis, quadrature axis, and zero axis in the synchronous rotating coordinate system are as follows:

[0009]

[0010] Among them, v a v b v c This indicates the three-phase AC voltage at the grid connection point, v d v q v0 represents the direct axis, quadrature axis, and zero axis voltages in the synchronous rotating coordinate system (dq), θ represents the voltage phase angle at the grid connection point, and V g This represents the magnitude of the combined voltage vector at the grid connection point.

[0011] A converter controller based on algebraic operation phase-locked loop includes a memory, a processor, and a computer program stored in the memory and executable on the processor. The processor executes the computer program to implement at least the aforementioned algebraic operation phase-locked method.

[0012] A method for constructing the feasible region of a grid-connected converter, wherein the grid-connected converter includes a converter, a three-phase filter, and a converter controller based on an algebraic operation phase-locked loop (PLL), and the output voltage and current of the grid-connected converter are tracked and controlled by the PLL-based PLL controller; the method includes the following steps:

[0013] Step 1: Obtain the main parameters of the grid-connected converter, establish mathematical models of the converter, three-phase filter, and converter controller, and transform them to the dq coordinate system; at the steady-state operating point, linearize the mathematical models of the converter, three-phase filter, and converter controller and perform Laplace transform to obtain the small-signal admittance model Y of the grid-connected converter in the dq coordinate system; Step 2: The grid-connected converter interface grid is represented by the equivalent impedance of an ideal voltage source series network. Obtain the equivalent model of the network impedance at the grid connection point, transform it to the dq coordinate system and linearize it to obtain the small-signal model Z of the network impedance in the dq coordinate system.

[0014] Step 3: Based on the small-signal admittance model Y of the grid-connected converter and the small-signal impedance model Z of the network, obtain the closed-loop characteristic transfer function matrix G(s) = (I-YZ) of the system. -1Y, based on the stability theory of multiple-input multiple-output systems, the determinant of the closed-loop characteristic transfer function matrix of the system, det(I-YZ)=0, is obtained, which is also the characteristic equation of the system.

[0015] Step 4: Based on the system characteristic equation, the necessary and sufficient condition for system stability is that the coefficients of the characteristic equation are positive and the Hurwitz determinant and its principal minors are positive. Since the system characteristic equation formed by the grid-connected converter based on algebraic operation phase-locked loop and the network equivalent impedance is a second-order equation, its stability is necessary and sufficient because the coefficients of the characteristic equation are positive. Based on this stability condition, the system's stable feasible region is constructed with the parameters to be analyzed as the horizontal and vertical axes. If the parameters to be analyzed are within the feasible region, the system is stable; if the parameters to be analyzed are outside the feasible region, the system is unstable.

[0016] Furthermore, step one specifically includes:

[0017] Obtain the main parameters of the grid-connected converter, establish mathematical models of the converter, three-phase filter, and converter controller, and transform them to the dq coordinate system:

[0018]

[0019] Where L is the inductance of the three-phase filter, and the converter controller uses proportional-integral (PI) control with a proportional coefficient of k. p The integral coefficient is k i S d and S q These are the outputs of the d-axis integral controller and the q-axis integral controller, respectively. and For active current and reactive current, and These are active current and reactive current commands, respectively. Due to the dynamic influence of the phase-locked loop (PLL), the grid-connected converter contains two dq coordinate systems: the system dq coordinate system and the controller dq coordinate system. Relevant variables in the system dq coordinate system are denoted by the superscript 's', and relevant variables in the controller dq coordinate system are denoted by the superscript 'c'. In steady state, these two dq coordinate systems coincide. After a disturbance, an angle difference occurs between these two coordinate systems, i.e., a small-signal disturbance in the phase angle, denoted by Δθ. The transformation relationship between the state variables in the system dq coordinate system and the state variables in the controller dq coordinate system is as follows:

[0020]

[0021] Right now,

[0022]

[0023] in, This represents the state variables of the system in the dq coordinate system, which can be current. and It can also be voltage. and This represents the state variable of the controller in the dq coordinate system, which can be current. and It can also be voltage. and

[0024] Linearizing the above system at the equilibrium point and performing a Laplace transform, we have:

[0025]

[0026] Where “Δ” represents the small perturbation of the corresponding state variable, and s is the Laplace operator.

[0027] With small perturbations, Δθ is generally small, therefore the small-signal model of the algebraic operation phase-locked loop is:

[0028]

[0029] Where V g This represents the steady-state amplitude of the grid connection point voltage. For v g The q-axis component.

[0030] In summary, the small-signal admittance model of the grid-connected converter in the dq coordinate system is as follows:

[0031]

[0032]

[0033] Furthermore, step two specifically involves:

[0034] The AC power grid system is represented by the equivalent impedance of an ideal voltage source series network. The equivalent impedance model of the network at the grid connection point is obtained and transformed to the dq coordinate system:

[0035]

[0036] in, and For v g The d-axis and q-axis components, v g Indicates the voltage at the grid connection point. and For v s The d-axis and q-axis components, v s L represents the voltage of a three-phase ideal voltage source. gThe network equivalent inductance is represented by ω = 100π, which is the rated angular frequency of the AC power grid system.

[0037] By linearizing the above model at the equilibrium point and performing a Laplace transform, the small-signal model Z of the network impedance can be obtained.

[0038]

[0039]

[0040] Where “Δ” represents the small perturbation of the corresponding state variable, and s is the Laplace operator.

[0041] Furthermore, in step three, the system characteristic equation is:

[0042] (L-α d L g k p )s 2 +[(1+α q ωL g )k p -α d L g k i ]s+(1+α q ωL g ) = 0

[0043] in, L g ω represents the network equivalent inductance, and ω is the rated angular frequency of the AC power grid system.

[0044] Furthermore, in step four, the necessary and sufficient condition for system stability is:

[0045]

[0046] Typically, the grid connection point voltage is V. g and the voltage of the main power grid v s The phase angle difference between them is less than 90 degrees, therefore we have

[0047]

[0048] Among them, V s The grid connection point voltage v s The amplitude. Therefore That is, 1+α q ωL g The condition > 0 always holds true. Therefore, the necessary and sufficient condition for system stability can be simplified to the following formula.

[0049]

[0050] Furthermore, step four also includes constructing a stable feasible region of the system by combining network impedance and pulse width modulation (PWM) limiting, wherein the network impedance limiting is V. g >0, PWM limit is in V represents the voltage modulation signal along the d and q axes, respectively. dc This indicates the DC side voltage.

[0051] Furthermore, in step four, the parameters that need to be analyzed are the active current and reactive current commands. and

[0052] Compared with the prior art, the advantages of the present invention are:

[0053] (1) Algebraic operation phase-locked loops rely solely on algebraic operations of the port phase voltages to obtain the trigonometric function information required for the Park transform and its inverse transform. They are zero-order systems and do not introduce additional system state variables. The grid-connected converter control method based on algebraic operation phase-locked loops has a faster synchronization process and superior dynamic performance. Simultaneously, it reduces the system order, which simplifies stability analysis.

[0054] (2) The method for constructing the feasible region of grid-connected converter based on algebraic operation phase-locked loop gives the closed analytical solution form of system stability, obtains the necessary and sufficient conditions for system stability, constructs a more intuitive feasible region for stable system operation, and avoids repeated analysis when parameters change. Attached Figure Description

[0055] Figure 1 The topology of the grid-connected converter and its controller structure are described below;

[0056] Figure 2 The system stability region is constructed with active and reactive current commands as the horizontal and vertical axes;

[0057] Figure 3 For simulation verification of the system's stability domain;

[0058] Figure 4 The dynamic performance of the phase-locked loop for algebraic operations was simulated and verified. Detailed Implementation

[0059] In an embodiment of the present invention, a phase-locked method for algebraic operations is provided, specifically as follows:

[0060] The trigonometric function values ​​required for the Park transformation of the three-phase AC voltage at the grid connection point with the direct-axis, quadrature-axis, and zero-axis voltages in a synchronous rotating coordinate system are obtained through algebraic operations, thereby achieving phase-locking. The algebraic operations for the required trigonometric function values ​​are as follows:

[0061]

[0062] The Park transformation relationships between the three-phase AC voltage at the grid connection point and the voltages along the direct axis, quadrature axis, and zero axis in the synchronous rotating coordinate system are as follows:

[0063]

[0064] Among them, v a v b v c This indicates the three-phase AC voltage at the grid connection point, v d v q v0 represents the direct axis, quadrature axis, and zero axis voltages in the synchronous rotating coordinate system (dq), θ represents the voltage phase angle at the grid connection point, and V g This represents the magnitude of the combined voltage vector at the grid connection point.

[0065] In an embodiment of the present invention, a converter controller based on an algebraic operation phase-locked loop is also provided, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it at least implements the aforementioned algebraic operation phase-locked loop method. The algebraic operation phase-locked loop of the present invention can be applied to any device with data processing capabilities, such as a computer or other similar device. The mathematical model of this algebraic operation phase-locked loop is as follows:

[0066]

[0067] In embodiments of the present invention, a method for controlling and constructing the feasible region of a grid-connected converter based on the above-described algebraic operation phase-locked loop is also provided. The grid-connected converter described in this invention is as follows: Figure 1 As shown, the grid-connected converter includes a converter, a filter, and a converter controller based on an algebraic phase-locked loop. The DC-side voltage is V. dc The converter output voltage is u a,b,c Output current i a,b,c The signal is fed into the AC weak current grid system after being filtered by a three-phase filter. The inductance of the three-phase filter is L. The AC weak current grid is approximated by the Thevenin equivalent circuit, where the network equivalent inductance is L. g An ideal voltage source is v sa,b,c The grid-connected converter's grid connection point voltage is V. ga,b,c An algebraic operation phase-locked loop (AO-PLL) provides the converter controller with the trigonometric function values ​​required for the Parker transformation and inverse transformation. The converter controller includes a PI-based current loop and decoupling terms, and the generated pulse-width modulation (PWM) signal is used to control the converter switching transistors. The invention will be further described below with reference to specific embodiments.

[0068] In one embodiment of the present invention, the main parameters of the system are shown in Table 1.

[0069] Table 1 Main System Parameters

[0070]

[0071] The first step of the method of this invention is to obtain the main parameters of the grid-connected converter as shown in Table 1, establish mathematical models of the converter, three-phase filter, and converter controller, and transform them to the dq coordinate system:

[0072]

[0073] Where L is the inductance of the three-phase filter, and the converter controller uses proportional-integral (PI) control with a proportional coefficient of k. p The integral coefficient is k i S d and S q These are the outputs of the d-axis integral controller and the q-axis integral controller, respectively. and For active current and reactive current, and These are active current and reactive current commands, respectively. Due to the dynamic influence of the phase-locked loop (PLL), the grid-connected converter contains two dq coordinate systems: the system dq coordinate system and the controller dq coordinate system. Relevant variables in the system dq coordinate system are denoted by the superscript 's', and relevant variables in the controller dq coordinate system are denoted by the superscript 'c'. In steady state, these two dq coordinate systems coincide. After a disturbance, an angular difference occurs between these two coordinate systems, equal to the small disturbance Δθ of the phase angle. The transformation relationship between the state variables of the two coordinate systems is as follows:

[0074]

[0075] Right now,

[0076]

[0077] in, This represents the state variables in the coordinate system of the grid-connected converter system, which can be current. and It can also be voltage. and This represents the state variables in the grid-connected converter controller coordinate system, which can be current. and It can also be voltage. and

[0078] Linearizing the above system at the equilibrium point and performing a Laplace transform yields the small-signal model as follows:

[0079]

[0080] Where “Δ” represents the small perturbation of the corresponding state variable, and s is the Laplace operator.

[0081] With small perturbations, Δθ is generally small, therefore the small-signal model of the algebraic operation phase-locked loop is:

[0082]

[0083] Where V g The grid connection point voltage v g steady-state amplitude, For v g q-axis component, For v g The d-axis component.

[0084] In summary, the small-signal admittance model Y of the grid-connected converter in the dq coordinate system is shown in the following equation.

[0085]

[0086]

[0087] In an embodiment of the method of the present invention, the second step is to construct an equivalent model of the grid connection network impedance of the AC power grid system represented by the equivalent impedance of the ideal voltage source series network, and transform it to the dq coordinate system for linearization, and construct the small-signal model Z of the network impedance in the dq coordinate system as shown in the following formula.

[0088]

[0089]

[0090] In an embodiment of the method of the present invention, the third step is to obtain the closed-loop characteristic transfer function matrix G(s) = (I-YZ) based on the small-signal admittance model Y of the grid-connected converter and the small-signal impedance model Z of the network. -1 Y, based on the stability theory of multi-input multi-output systems, the determinant of the closed-loop characteristic transfer function matrix of the system, det(I-YZ), is 0, which is also the characteristic equation of the system:

[0091] (L-α d L g k p )s 2 +[(1+α q ωL g )k p -α d Lg k i ]s+(1+α q ωL g ) = 0

[0092] in, ω represents the rated angular frequency of the power grid, which is typically 100π.

[0093] In an embodiment of the method of the present invention, the fourth step, based on the system characteristic equation, ensures that the necessary and sufficient condition for system stability is that the coefficients of each term in the characteristic equation are positive and the Herwitz determinant and its principal minors are positive. Since the system characteristic equation formed by the grid-connected converter based on the algebraic operation phase-locked loop and the network equivalent impedance is a second-order equation, its stability is necessary and sufficient because the coefficients of each term in the characteristic equation are positive. Specifically, the necessary and sufficient condition for system stability is...

[0094]

[0095] Furthermore, the grid connection point voltage V is typically... g and the voltage of the main power grid v s The phase angle difference between the voltages of the ideal voltage sources in the equivalent circuit is less than 90 degrees, therefore...

[0096]

[0097] Among them, V s For the main power grid voltage v s The amplitude. Therefore That is, 1+α q ωL g The condition > 0 always holds true. Therefore, the necessary and sufficient condition for system stability can be simplified to the following equation:

[0098]

[0099] Based on the necessary and sufficient conditions for system stability, respectively using Using the x and y axes, construct the stable and feasible region of the system, such as... Figure 2 As shown in the diagram. The points in the diagram represent different operating states. The system remains stable when the value is within the feasible region. When the value is outside the feasible region, the system becomes unstable. Testing at three operating points, the system is relatively stable when the operating state is (350A, -150A); when the operating state is (350A, -50A), the system remains stable, but the stability margin decreases; when the operating state is (350A, 50A), the stability constraint is not met, and the system becomes unstable. Furthermore, the network impedance (V... g >0) and PWM limiting This also limits the output current, which can be addressed by constructing a stable feasible region for the system, such as... Figure 2 As shown.

[0100] A simulation model of the grid-connected converter was built using MATLAB / Simulink to verify the correctness of the feasible region construction method. The dq-axis current and voltage waveforms of the system when subjected to equilibrium point changes are shown below. Figure 3 As shown, the system's active current output Reactive current remains unchanged at 350A. At t = 0.7s, the reactive current abruptly changes from -150A to -100A. The system remains stable at this point, but experiences slight oscillations due to the reduced stability margin. At t = 0.9s, the reactive current... The system abruptly jumps to 50 Å, at which point it becomes unstable. The simulation results are consistent with the analysis results, verifying the effectiveness of the proposed stability region construction method.

[0101] Verify the dynamic performance of the algebraic operation phase-locked loop as follows: Figure 4 As shown, at t=0.7s and t=0.9s, the grid voltage phase changes abruptly by 90 degrees, and the grid-connected converter maintains its output active current. and inductive reactive current constant. Figure 4 (a) is the dq-axis current and voltage waveforms of the system when the system experiences a sudden phase angle change based on the algebraic operation phase-locked loop; Figure 4 (b) The dq-axis current and voltage waveforms of the system under the same phase angle change based on the SRF-PLL. The comparison shows that the response time of the SRF-PLL to the phase angle change is approximately 0.04 to 0.06 seconds, while the response time of the algebraic operation PLL is approximately 0.01 to 0.02 seconds, which is superior to the SRF-PLL. Furthermore, during grid voltage fluctuations, the algebraic operation PLL can always accurately lock the phase, maintaining the q-axis voltage at 0.

[0102] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A phase-locked loop method for algebraic operations, characterized in that, Specifically: The trigonometric function values ​​required for the Park transformation of the three-phase AC voltage at the grid connection point with the direct-axis, quadrature-axis, and zero-axis voltages in a synchronous rotating coordinate system are obtained through algebraic operations, thereby achieving phase-locking. The algebraic operations for the required trigonometric function values ​​are as follows: The Park transformation relationships between the three-phase AC voltage at the grid connection point and the voltages along the direct axis, quadrature axis, and zero axis in the synchronous rotating coordinate system are as follows: Among them, v a v b v c This indicates the three-phase AC voltage at the grid connection point, v d v q v0 represents the direct axis, quadrature axis, and zero axis voltages in the synchronous rotating coordinate system (dq), and θ represents the voltage phase angle at the grid connection point. This represents the magnitude of the combined voltage vector at the grid connection point.

2. A converter controller based on an algebraic phase-locked loop, characterized in that, The invention includes a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that the processor, when executing the computer program, at least implements the algebraic operation phase-locked loop method as described in claim 1.

3. A method for constructing the feasible region of a grid-connected converter, wherein the grid-connected converter comprises a converter, a three-phase filter, and a converter controller based on an algebraic operation phase-locked loop as described in claim 2, wherein the output voltage and current of the grid-connected converter are tracked and controlled by the converter controller; characterized in that, Includes the following steps: Step 1: Obtain the main parameters of the grid-connected converter, establish mathematical models of the converter, three-phase filter, and converter controller, and transform them to the dq coordinate system; at the steady-state operating point, linearize the mathematical models of the converter, three-phase filter, and converter controller and perform Laplace transform to obtain the small-signal admittance model Y of the grid-connected converter in the dq coordinate system. Step 2: The AC power grid system is represented by the equivalent impedance of the ideal voltage source series network. The equivalent model of the network impedance at the grid connection point is obtained and transformed into the dq coordinate system for linearization to obtain the small-signal model Z of the network impedance in the dq coordinate system. Step 3: Based on the small-signal admittance model Y of the grid-connected converter and the small-signal impedance model Z of the network, obtain the closed-loop characteristic transfer function matrix of the system. Based on the stability theory of multiple-input multiple-output systems, the determinant of the closed-loop characteristic transfer function matrix of the system is obtained. That is, the system characteristic equation; Step 4: Based on the system characteristic equation and the necessary and sufficient conditions for system stability, construct the system stability feasible region with the parameters to be analyzed as the horizontal and vertical axes. If the parameters to be analyzed are located within the feasible region, the system is stable; if the parameters to be analyzed are located outside the feasible region, the system is unstable.

4. The method as described in claim 3, characterized in that, Step one specifically involves: Obtain the main parameters of the grid-connected converter, establish mathematical models of the converter, three-phase filter, and converter controller, and transform them to the dq coordinate system: Where L is the inductance of the three-phase filter, and the converter controller uses proportional-integral (PI) control. , These are the proportional and integral coefficients of the converter controller, respectively. and These are the outputs of the d-axis integral controller and the q-axis integral controller, respectively. and For active current and reactive current, and These are active current and reactive current commands, respectively; the superscript 's' indicates the system dq coordinate system, and the superscript 'c' indicates the converter controller dq coordinate system. The conversion relationship between the state variables in the system dq coordinate system and the state variables in the converter controller dq coordinate system is as follows: in, , The state variable representing the system in the dq coordinate system is the current. and or voltage and ; , The state variable representing the converter controller in the dq coordinate system is the current. and or voltage and ; It is the angular difference between two coordinate systems; At the steady-state operating point, the mathematical models of the converter, three-phase filter, and converter controller are linearized and subjected to Laplace transform: Where "Δ" represents the small perturbation of the corresponding state variable, and s is the Laplace operator; The small-signal model of an algebraic phase-locked loop is as follows: The small-signal admittance model Y of the grid-connected converter in the dq coordinate system is obtained by synthesis: in, These are the grid connection point voltages. The d-axis and q-axis components.

5. The method as described in claim 3, characterized in that, Step two specifically involves: The AC power grid system is represented by the equivalent impedance of an ideal voltage source series network. The equivalent impedance model of the network at the grid connection point is obtained and transformed to the dq coordinate system: in, and for The d-axis and q-axis components, Indicates the voltage at the grid connection point. and for The d-axis and q-axis components, L represents the voltage of an AC power grid system. g Indicates the network equivalent inductance. The rated angular frequency of the AC power grid system; the superscript 's' indicates the system's dq coordinate system. and These are active current and reactive current; The equivalent model of network impedance at the grid connection point is linearized at the equilibrium point and subjected to Laplace transform to obtain the small-signal model Z of network impedance in dq coordinate system. Where "Δ" represents the small perturbation of the corresponding state variable, and s is the Laplace operator.

6. The method as described in claim 4, characterized in that, In step three, the system characteristic equation is: in, , L g Indicates the network equivalent inductance. This is the rated angular frequency of the AC power grid system.

7. The method as described in claim 6, characterized in that, In step four, the necessary and sufficient condition for system stability is: 。 8. The method as described in claim 3, characterized in that, Step four further includes constructing a stable feasible region of the system by combining network impedance and pulse width modulation (PWM) limiting, wherein the network impedance limiting is... PWM limiting is ,in , V represents the voltage modulation signal along the d and q axes, respectively. dc This indicates the DC side voltage.

9. The method as described in claim 3, characterized in that, The parameters that need to be analyzed are active current and reactive current command. and .

Images