Improved small-signal model establishment method for photovoltaic system under unbalanced grid voltage

Abstract

The application discloses a method for establishing a small signal model of an improved photovoltaic system under unbalanced grid voltage, constructs an improved photovoltaic model under unbalanced grid voltage, and comprises a main circuit module, a phase-locked loop module and a control module, wherein the phase-locked loop module adopts a second-order generalized integrator phase-locked loop considering a time delay control link, and the control module is additionally provided with a damping control; a nonlinear state space model of a voltage source converter main circuit, a nonlinear state space model of a phase-locked loop, and nonlinear state space models of a filter module, an outer loop control module, an additional damping control module and an inner loop control module are constructed, and a complete small signal model of the improved photovoltaic system under unbalanced grid voltage is obtained by simultaneous equations. The application provides a theoretical basis for designing control system parameters of the improved photovoltaic system and improving system stability.

Description

Technical Field

[0001] This invention relates to improved phase-locked loop modeling, additional damping control modeling, and small-signal modeling techniques. Specifically, it relates to an improved method for establishing a small-signal model of a photovoltaic system under unbalanced grid voltage. Background Technology

[0002] Voltage imbalance in the power grid is a major reason why phase-locked loops (PLLs) cannot obtain grid phase information. It also leads to problems such as grid current imbalance and low-frequency pulsations in DC-side voltage. Faults are the most common form of grid imbalance, and reducing the impact of faults on PLLs and DQ decoupling control is beneficial for better photovoltaic (PV) operation. PLLs are widely used in PV systems, especially in DQ coordinate system control, where accurate grid phase information is crucial. The locked phase contains grid phase information and is the foundation of the system loop control; accurate PLL results lead to accurate loop control results. PV systems obtain grid phase information through PLLs and control the switching transistors through DQ decoupling control. However, when the grid voltage is unbalanced, PV systems cannot obtain accurate grid information, resulting in control fluctuations. Analysis of domestic and international PV research shows that there is more research and control under balanced conditions, but less research on reducing the impact of three-phase voltage imbalance and grounding faults on PV systems. Therefore, reducing the impact of three-phase voltage imbalance on PV systems is extremely important. Summary of the Invention

[0003] The purpose of this invention is to provide a method for establishing a small-signal model of an improved photovoltaic system under unbalanced grid voltage, providing a theoretical basis for designing control parameters of the improved photovoltaic system and improving system stability.

[0004] The technical solution to achieve the purpose of this invention is: a method for establishing an improved small-signal model of a photovoltaic system under unbalanced grid voltage, specifically including the following steps:

[0005] Step 1: Construct an improved photovoltaic model under unbalanced grid voltage, including a main circuit module, a phase-locked loop (PLL) module, and a control module. The PLL module adopts a second-order generalized integrator PLL that takes into account the delay control loop. The control module adds damping control. The PLL module obtains grid information from the main circuit module and then sends the grid information to the control module. The control module uses the grid information to control the voltage source converter.

[0006] Step 2: Based on the voltage source converter system topology, establish the nonlinear state-space model of the main circuit of the voltage source converter, the nonlinear state-space model of the phase-locked loop, and the nonlinear state-space models of the filter module, outer loop control module, additional damping control module, and inner loop control module.

[0007] Step 3: Combine the nonlinear state-space model of the main circuit, the nonlinear state-space model of the phase-locked loop, and the nonlinear state-space models of the filter module, the outer loop control module, the additional damping control module, and the inner loop control module to obtain the complete small-signal model of the improved photovoltaic system under unbalanced grid voltage.

[0008] A method for establishing a small-signal model of an improved photovoltaic system under unbalanced grid voltage is provided. The method, implemented to model the small-signal system under unbalanced grid voltage, includes:

[0009] An improved photovoltaic model construction module under unbalanced grid voltage is constructed, which includes a main circuit module, a phase-locked loop module, and a control module. The phase-locked loop module obtains grid information from the main circuit module and then sends the grid information to the control module. The control module uses the grid information to control the voltage source converter.

[0010] The nonlinear state-space model construction module establishes nonlinear state-space models of the main circuit, phase-locked loop, filter module, outer loop control module, additional damping control module, and inner loop control module based on the topology of the voltage source converter system.

[0011] The small-signal model construction module combines the nonlinear state-space model of the main circuit, the nonlinear state-space model of the phase-locked loop, and the nonlinear state-space models of the filtering module, the outer loop control module, the additional damping control module, and the inner loop control module to obtain a complete small-signal model of the improved photovoltaic system under unbalanced grid voltage.

[0012] A computer device includes 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 implements the method for establishing a small-signal model of an improved photovoltaic system under unbalanced grid voltage, thereby realizing the modeling of a small-signal model of an improved photovoltaic system under unbalanced grid voltage.

[0013] A computer-readable storage medium storing a computer program thereon, wherein when the computer program is executed by a processor, the method for establishing a small-signal model of an improved photovoltaic system under unbalanced grid voltage is implemented, thereby realizing the modeling of a small-signal model of an improved photovoltaic system under unbalanced grid voltage.

[0014] Compared with the prior art, the significant advantages of this invention are: 1) It uses a second-order generalized integrator with added delay element to eliminate DC in the sampled quantity through the delay element, thereby enhancing the stability of the phase-locked loop under unbalanced conditions; 2) It adds additional damping control on the basis of the original decoupling control, thereby improving the stability of the system and reducing the impact of faults. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the improved photovoltaic model topology under unbalanced grid voltage.

[0016] Figure 2 This is a schematic diagram of the phase-locked loop topology of the improved photovoltaic model under unbalanced grid voltage.

[0017] Figure 3 This is a schematic diagram of the main circuit topology of the improved photovoltaic model under unbalanced grid voltage.

[0018] Figure 4 This is a schematic diagram of the topology for the additional damping control of the improved photovoltaic model under unbalanced grid voltage.

[0019] Figure 5 The waveforms are of a second-order generalized integrator phase-locked loop (PLL). (a) is a traditional PLL, and (b) is an improved PLL.

[0020] Figure 6 The active power waveforms of photovoltaic undamped control are shown in (a) for single-phase short circuit and (b) for two-phase short circuit.

[0021] Figure 7 The reactive power waveforms of photovoltaic undamped control are shown in (a) for single-phase short circuit and (b) for two-phase short circuit.

[0022] Figure 8 The active power waveforms of the photovoltaic system with additional damping control are shown in (a) for a single-phase short circuit and (b) for a two-phase short circuit.

[0023] Figure 9 The active power waveforms of the photovoltaic system with additional damping control are shown in (a) for a single-phase short circuit and (b) for a two-phase short circuit.

[0024] Figure 10 The waveforms of the photovoltaic small-signal model and simulation model are compared: (a) represents active power, and (b) represents reactive power. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0026] A method for establishing an improved small-signal model of a photovoltaic system under unbalanced grid voltage, the specific steps of which are as follows:

[0027] Step 1: Construct an improved photovoltaic model under unbalanced grid voltage, including a main circuit module, a phase-locked loop module, and a control module. The phase-locked loop module obtains grid information from the main circuit module and then sends the grid information to the control module. The control module uses the grid information to control the voltage source converter.

[0028] The main circuit module includes an infinite power supply and a line resistor R. s Line inductance L s Filter resistor R, filter capacitor C f The system includes a filter inductor L, a voltage source converter, and a DC power supply; the phase-locked loop (PLL) module includes a delay module and a second-order generalized integrator PLL module; the control module includes a filter module, an outer loop control module, an inner loop control module, and an additional damping control module, employing constant active and reactive power control for the photovoltaic system. Among these:

[0029] In the main circuit module, the infinite power supply connection line resistance R s Line resistance R s Connection line inductance L s Line inductance L s The filter resistor R is connected to the photovoltaic grid connection point (Point of common coupling, PCC), and the filter capacitor C... f Connected at point PCC, the filter resistor R is connected to the filter inductor L, and the filter inductor L is connected to the voltage source converter and the DC power supply.

[0030] In the phase-locked loop module, the voltage at point PCC is acquired, zero-sequence is eliminated through a delay circuit, and then the output is obtained through Clark transformation. tα and u tβ u tα and u tβ The second-order generalized integrator (SOGI) is used for calculation, and the component u is obtained after calculation. tα1 qu tα1 u tβ1 qu tβ1 , will (u tα1 -qu tβ1 ) / 2 and (qu tα1 +u tβ1 Transforming from αβ to dq axis, the dq axis output is u. td and u tq u tq The power grid information θ is obtained after passing through a PI controller and integrator. + Adding a delay module to the phase-locked loop (PLL) module can eliminate DC current in the PLL. The expression for the delay is as follows:

[0031]

[0032] In the formula, u ta The voltage of phase A at the photovoltaic grid connection point (PCC), u tb Let u be the phase B voltage at point PCC. tc The voltage of phase C at point PCC, u ta 'The voltage of phase A at point PCC after the delay circuit, u' tb 'The voltage of phase B at point PCC after the delay stage, u' tc ' is the voltage of phase C at point PCC after the delay stage, and T is the delay constant.

[0033] In the control module, the outer loop control module collects grid voltage and current information to calculate active power P and reactive power Q, and sets an active power reference value P. ref and active power reference value Q ref The voltage at point PCC is collected, subjected to dq transformation, and filtered. The filtered voltage parameters at point PCC are v. tdm v tqm The current at the PCC point is collected, subjected to dq transformation, and filtered. The filtered current parameters at the PCC point are i. tdm i tqm Compare P and Q with the reference value P respectively. ref and Q ref The subtraction is performed, and the resulting values ​​are then output as inner-loop control reference values ​​i through the outer-loop PI controller. * d i * q The PI controller parameter is Kp. p Ki p Kp Q Ki Q In the additional damping control module, P and Q respectively obtain the additional damping control output signal P through the additional damping controller. 1zu and Q 1zu The additional damping control parameter is T. d1 T d2 T q1 T q2 In the inner loop control module, the inner loop control input value is calculated from the inner loop control reference value, the parameters after the PCC point current dq transformation and filtering, and the additional damping control output value. The inner loop control input value is passed through the inner loop PI controller to obtain the inner loop control parameters. The inner loop control parameters are decoupled and calculated to obtain the voltage source converter control signal, where the PI controller parameters are Kp1, Ki1, Kp2, and Ki2.

[0034] An additional damping control component is added to the original decoupling control. The additional damping control can provide positive damping when a photovoltaic system fails, reducing the impact of the failure on the active power and shortening the failure recovery time.

[0035] Step 2: Based on the voltage source converter system topology, establish the nonlinear state-space model of the main circuit of the voltage source converter, the nonlinear state-space model of the phase-locked loop, and the nonlinear state-space models of the filter module, outer loop control module, additional damping control module, and inner loop control module.

[0036] Nonlinear state-space model of the main circuit:

[0037]

[0038] In the formula, C f For the filter capacitor, L s L is the line inductance, R is the filter inductance, and R is the line inductance. s R is the line resistance, R is the filter resistance, ω is the photovoltaic system frequency, and v is the voltage. yd and v yq For an infinite power supply, the d-axis and q-axis voltages are v. td and v tq V represents the d-axis and q-axis voltages at point PCC. cd and v cq For the d-axis and q-axis voltages at the connection point of the voltage source converter, i 2d and i 2q Let i be the d-axis and q-axis current between the power grid and the PCC point. 1d and i 1q These represent the d-axis and q-axis currents between the PCC point and the voltage source converter access point.

[0039] Nonlinear state-space model of phase-locked loop module:

[0040]

[0041]

[0042] In the formula, u tα u tβ The parameter u is the voltage at point PCC after the delay element and dq transformation. tα1 qu tα1 u tβ1 qu tβ1 For u tα u tβ After passing through the parameters of the second-order generalized integrator, u tq The q-axis components are obtained by dq transformation of the parameters of the second-order generalized integrator, ω0 is the angular frequency of the grid locked by the phase-locked loop, and θ + To lock the power grid information using a phase-locked loop, Kppll K ipll Here are the phase-locked loop (PLL) PI parameters: ea1, ea2, eb1, eb2, xq0, θ + These are the state variables during the phase-locked loop (PLL) calculation process.

[0043] Nonlinear state-space model of the filtering module:

[0044]

[0045]

[0046]

[0047]

[0048] In the formula, v td v tq i 1d i 1q For the voltage and current at point PCC, v tdm v tqm i 1dm i 1qm The parameters T are the filtered values ​​of the voltage and current dq at point PCC. mvd T mvq T mid T miq For filter parameters, x 01 x 02 x 03 x 04 These are the state variables during the filtering process.

[0049] Nonlinear state-space model of the outer loop control module:

[0050]

[0051]

[0052] In the formula, P ref Q is the active power reference value. ref This is the active power reference value, v tdm v tqm i 1dm i 1qm Kp represents the filtered parameters of the voltage and current dq at point PCC. p Ki p Kp V Ki V For the outer loop PI controller parameters, e P e V x1 and x2 are intermediate variables in the outer loop control process, and x1 and x2 are state variables in the outer loop control process.

[0053] Nonlinear state-space model of the additional damping control module:

[0054]

[0055] In the formula, v tdm v tqm i 1dm i 1qm P represents the filtered parameters of the voltage and current dq transformation at point PCC. 1zu Q is the output value of active power after additional damping control. 1zu T represents the output value of reactive power after additional damping control. d1 T d2 T q1 T q2 For damping controller parameters, P 1zu Q 1zu For the state variables of the additional damping control process.

[0056] Nonlinear state-space model of the inner loop control module:

[0057]

[0058]

[0059] In the formula, v tdm v tqm i 1dm i 1qm The parameters T are the filtered values ​​of the voltage and current dq at point PCC. d1 T d2 T q1 T q2 Here are the damping controller parameters, and Kp1, Ki1, Kp2, and Ki2 are the inner-loop PI controller parameters. * d i * q e is the reference value for inner loop control. id e iq a zu b zu x3 and x4 are intermediate variables in the inner loop control process, while x4 and x5 are state variables in the outer loop control process.

[0060] Step 3: Simultaneously establish the nonlinear state-space model of the main circuit, and the nonlinear state-space models of the filter module, outer loop control module, additional damping control module, and inner loop control module to obtain the complete small-signal model of the improved photovoltaic system under unbalanced grid voltage, wherein:

[0061] The small-signal model expression is as follows:

[0062]

[0063] In the formula, X is a column vector consisting of state variables of the improved photovoltaic system small-signal model under unbalanced grid voltage, and U is a column vector consisting of input variables of the improved photovoltaic system small-signal model under unbalanced grid voltage.

[0064]

[0065] In the formula, P ref Q ref These are reference values ​​for photovoltaic active power and reactive power.

[0066] Step 4: Perform step changes on the electromagnetic transient model and small-signal model command values ​​of the improved photovoltaic system under unbalanced grid voltage, and analyze the change process of both to verify the effectiveness of the model.

[0067] In PSCAD / EMTDC, the command values ​​of the improved electromagnetic transient model of the photovoltaic system under unbalanced grid voltage were stepped to obtain the dynamic response of the system. Then, the same step was applied to the small-signal model established in Matlab to obtain the dynamic response of the small-signal model. The changes of the two were compared to verify the accuracy of the small-signal model.

[0068] This invention also proposes a modeling and verification system for an improved small-signal model of a photovoltaic system under unbalanced grid voltage. By implementing the method for establishing the improved small-signal model of a photovoltaic system under unbalanced grid voltage, the modeling and verification of the improved small-signal model of the photovoltaic system under unbalanced grid voltage can be achieved.

[0069] A computer device includes 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 implements the method for establishing a small-signal model of an improved photovoltaic system under unbalanced grid voltage, thereby realizing the modeling and verification of the small-signal model of the improved photovoltaic system under unbalanced grid voltage.

[0070] A computer-readable storage medium storing a computer program thereon, wherein when the computer program is executed by a processor, it implements the method for establishing a small-signal model of an improved photovoltaic system under unbalanced grid voltage, thereby realizing the modeling and verification of the small-signal model of the improved photovoltaic system under unbalanced grid voltage.

[0071] Example

[0072] To verify the effectiveness of the present invention, the following experiment was conducted.

[0073] An improved PSCAD / EMTDC electromagnetic transient model of a photovoltaic system under unbalanced grid voltage is established. When the grid contains DC current, the waveform of the traditional second-order generalized integrator phase-locked loop is as follows: Figure 5 As shown in (a), it can be seen that the phase-locked loop (PLL) locks onto the grid frequency, which fluctuates around 50Hz. Under the same conditions, considering the delay control loop, the second-order generalized integrator PLL waveform is as follows: Figure 5 As shown in (b), it can be seen that the phase-locked loop locks the grid frequency at 50Hz.

[0074] When a single-phase ground fault occurs in the system without additional damping control within 10 seconds, and the fault lasts for 2 seconds, the waveform is as follows: Figure 6 As shown in (a), P decreases to 0.75 pu, and the fault recovery time is 1.6 s; a two-phase ground fault occurs at 10 s, and the fault lasts for 2 s, with the waveform as shown. Figure 6 As shown in (b), P decreases to 0.4 pu, and the fault recovery time is 1.6 s; when the system has additional damping control, a single-phase ground fault occurs at 10 s, and the fault lasts for 2 s, with the waveform as shown. Figure 7 As shown in (a), P fluctuates and then stabilizes at 1 pu, with a fault recovery time of 0.3 s; a two-phase ground fault occurs at 10 s, lasting for 2 s, with the waveform as shown. Figure 7 As shown in (b), when P is reduced to 0.5 pu, the fault recovery time is 1.2 s. A comparison with and without additional damping control clearly shows that additional damping control reduces the impact of faults on the photovoltaic system and reduces the fault recovery time.

[0075] Step values ​​were applied to the PSCAD / EMTDC electromagnetic transient model and Matlab small-signal model command values ​​of an improved photovoltaic system under unbalanced grid voltage. For example... Figure 10 As shown in (a), the active power command value P jumps from 1 to 0.5 at 12s; Figure 10 As shown in (b), the reactive power command value Q jumps from 0 to -0.2 in 10 seconds. Figure 10 It can be seen that during the step operation of the command value, the dynamic responses of the electromagnetic transient model and the small-signal model control the system parameters are basically consistent, thus effectively verifying the accuracy of the small-signal model.

[0076] The technical features of the above embodiments can be combined in any way. For the sake of brevity, 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.

[0077] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these modifications and improvements all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A method for establishing an improved small-signal model of a photovoltaic system under unbalanced grid voltage, characterized in that, Includes the following steps: Step 1: Construct an improved photovoltaic model under unbalanced grid voltage, including a main circuit module, a phase-locked loop (PLL) module, and a control module. The PLL module adopts a second-order generalized integrator PLL that takes into account the delay control loop. The control module adds damping control. The PLL module obtains grid information from the main circuit module and then sends the grid information to the control module. The control module uses the grid information to control the voltage source converter. Step 2: Based on the voltage source converter system topology, establish the nonlinear state-space model of the main circuit of the voltage source converter, the nonlinear state-space model of the phase-locked loop, and the nonlinear state-space models of the filter module, outer loop control module, additional damping control module, and inner loop control module. Step 3: Combine the nonlinear state-space model of the main circuit, the nonlinear state-space model of the phase-locked loop, and the nonlinear state-space models of the filter module, the outer loop control module, the additional damping control module, and the inner loop control module to obtain the complete small-signal model of the improved photovoltaic system under unbalanced grid voltage. In the main circuit module, the infinite power supply connection line resistance R s Line resistance R s Connection line inductance L s Line inductance L s The filter resistor R is connected to the photovoltaic grid connection point, and the filter capacitor C... f Connected at point PCC, filter resistor R is connected to filter inductor L, and filter inductor L is connected to voltage source converter and DC power supply; In the phase-locked loop module, the delay module performs zero-sequence elimination on the voltage at the PCC point, and then outputs u after Clark transformation. tα and u tβ u tα and u tβ The input is a second-order generalized integrator for calculation, and the component u is obtained after calculation. tα1 qu tα1 u tβ1 qu tβ1 , will (u tα1 -qu tβ1 ) / 2 and (qu tα1 +u tβ1 Transforming from αβ to dq axis, the dq axis output is u. td and u tq u tq The power grid information θ is obtained through a PI controller and integrator. + Add a delay module to the phase-locked loop module. The delay expression is as follows: ； In the formula, u ta The voltage of phase A at the photovoltaic grid connection point, u tb Let u be the phase B voltage at point PCC. tc The voltage of phase C at point PCC, u ta 'The voltage of phase A at point PCC after the delay circuit, u' tb 'The voltage of phase B at point PCC after the delay stage, u' tc ' is the voltage of phase C at point PCC after the delay stage, and T is the delay constant; In the control module, the outer loop control module collects grid voltage and current information to calculate active power P and reactive power Q, and sets an active power reference value P. ref and active power reference value Q ref The voltage at point PCC is collected, subjected to dq transformation, and filtered. The filtered voltage parameters at point PCC are v. tdm v tqm The current at the PCC point is collected, subjected to dq transformation, and filtered. The filtered current parameters at the PCC point are i. tdm i tqm Compare P and Q with the reference value P respectively. ref and Q ref Subtract the values, and then output the inner loop control reference value i through the PI controller. * d i * q The PI controller parameter is Kp. p Ki p Kp Q Ki Q In the additional damping control module, P and Q respectively obtain the additional damping control output signal P through the additional damping controller. 1zu and Q 1zu The additional damping control parameter is T. d1 T d2 T q1 T q2 In the inner loop control module, the inner loop control input value is calculated from the inner loop control reference value, the parameters after the PCC point current dq transformation and filtering, and the additional damping control output value. The inner loop control input value is passed through the PI controller to obtain the inner loop control parameters. The inner loop control parameters are decoupled and calculated to obtain the voltage source converter control signal, where the PI controller parameters are Kp1, Ki1, Kp2, and Ki2.

2. The method for establishing an improved small-signal model of a photovoltaic system under unbalanced grid voltage as described in claim 1, characterized in that, Step 2: Based on the voltage source converter system topology, establish the nonlinear state-space model of the main circuit of the voltage source converter, the nonlinear state-space model of the phase-locked loop, and the nonlinear state-space models of the filter module, outer loop control module, additional damping control module, and inner loop control module. Nonlinear state-space model of the main circuit: ； In the formula, C f For the filter capacitor, L s L is the line inductance, R is the filter inductance, and R is the line inductance. s R is the line resistance, R is the filter resistance, ω is the photovoltaic system frequency, and v is the voltage. yd and v yq For an infinite power supply, the d-axis and q-axis voltages are v. td and v tq V represents the d-axis and q-axis voltages at point PCC. cd and v cq For the d-axis and q-axis voltages at the connection point of the voltage source converter, i 2d and i 2q Let i be the d-axis and q-axis current between the power grid and the PCC point. 1d and i 1q For the d-axis and q-axis currents between the PCC point and the voltage source converter access point; Nonlinear state-space model of phase-locked loop module: ； ； In the formula, u tα u tβ The parameters of the PCC point voltage after the delay element and dq transformation are given by u. tα1 qu tα1 u tβ1 qu tβ1 For u tα u tβ After passing through the parameters of the second-order generalized integrator, u tq The q-axis components are obtained by dq transformation of the parameters of the second-order generalized integrator, ω0 is the angular frequency of the grid locked by the phase-locked loop, and θ + To lock the power grid information using a phase-locked loop, K ppll K ipll Here are the phase-locked loop (PLL) PI parameters: ea1, ea2, eb1, eb2, xq0, θ + These are the state variables during the phase-locked loop (PLL) calculation process; Nonlinear state-space model of the filtering module: ； ； ； ； In the formula, v td v tq i 1d i 1q For the voltage and current at point PCC, v tdm v tqm i 1dm i 1qm The parameters T are the filtered values ​​of the voltage and current dq at point PCC. mvd T mvq T mid T miq For filter parameters, x 01 x 02 x 03 x 04 These are state variables during the filtering process; Nonlinear state-space model of the outer loop control module: ； ； In the formula, P ref Q is the active power reference value. ref This is the active power reference value, v tdm v tqm i 1dm i 1qm Kp represents the filtered parameters of the voltage and current dq at point PCC. p Ki p Kp V Ki V For the outer loop PI controller parameters, e P e V x1 and x2 are intermediate variables in the outer loop control process, and x1 and x2 are state variables in the outer loop control process. Nonlinear state-space model of the additional damping control module: ； In the formula, v tdm v tqm i 1dm i 1qm P represents the filtered parameters of the voltage and current dq transformation at point PCC. 1zu Q is the output value of the active power after additional damping control. 1zu T represents the output value of reactive power after additional damping control. d1 T d2 T q1 T q2 For damping controller parameters, P 1zu Q 1zu For the state variables of the additional damping control process; Nonlinear state-space model of the inner loop control module: ； ； In the formula, v tdm v tqm i 1dm i 1qm The parameters T are the filtered values ​​of the voltage and current dq at point PCC. d1 T d2 T q1 T q2 Here are the damping controller parameters, Kp1, Ki1, Kp2, and Ki2 are the inner-loop PI controller parameters, and i * d i * q e is the reference value for inner loop control. id e iq a zu b zu x3 and x4 are intermediate variables in the inner loop control process, while x4 and x5 are state variables in the outer loop control process.

3. The method for establishing an improved small-signal model of a photovoltaic system under unbalanced grid voltage as described in claim 2, characterized in that, Step 3: Simultaneously establish the nonlinear state-space model of the main circuit, the nonlinear state-space model of the phase-locked loop module, and the nonlinear state-space models of the filter module, outer loop control module, additional damping control module, and inner loop control module to obtain the complete small-signal model of the improved photovoltaic system under unbalanced grid voltage, wherein: The small-signal model expression is as follows: ； In the formula, X is a column vector composed of state variables of the improved photovoltaic system small-signal model under unbalanced grid voltage, and U is a column vector composed of input variables of the improved photovoltaic system small-signal model under unbalanced grid voltage. ； ； In the formula, P ref Q ref These are reference values ​​for photovoltaic active and reactive power.

4. The method for establishing an improved small-signal model of a photovoltaic system under unbalanced grid voltage as described in claim 1, characterized in that, The process also includes step 4, which involves stepping the command values ​​of the electromagnetic transient model and the small-signal model of the improved photovoltaic system under unbalanced grid voltage, analyzing the changes in both to verify the effectiveness of the model. The specific method is as follows: In PSCAD and EMTDC, the command values ​​of the improved electromagnetic transient model of the photovoltaic system under unbalanced grid voltage were stepped to obtain the dynamic response of the system. Then, the same step was applied to the small-signal model established in Matlab to obtain the dynamic response of the small-signal model. The changes of the two were compared to verify the accuracy of the small-signal model.

5. A method for establishing an improved small-signal model of a photovoltaic system under unbalanced grid voltage, characterized in that, The method for establishing a small-signal model of an improved photovoltaic system under unbalanced grid voltage as described in any one of claims 1-4, comprising the following steps: An improved photovoltaic model construction module under unbalanced grid voltage is constructed, which includes a main circuit module, a phase-locked loop module, and a control module. The phase-locked loop module obtains grid information from the main circuit module and then sends the grid information to the control module. The control module uses the grid information to control the voltage source converter. The nonlinear state-space model construction module establishes nonlinear state-space models of the main circuit, phase-locked loop, filter module, outer loop control module, additional damping control module, and inner loop control module based on the topology of the voltage source converter system. The small-signal model construction module combines the nonlinear state-space model of the main circuit, the nonlinear state-space model of the phase-locked loop, and the nonlinear state-space models of the filtering module, the outer loop control module, the additional damping control module, and the inner loop control module to obtain a complete small-signal model of the improved photovoltaic system under unbalanced grid voltage.

6. A computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the computer program, it implements the method for establishing a small-signal model of an improved photovoltaic system under unbalanced grid voltage as described in any one of claims 1-4, thereby realizing the modeling of a small-signal model of an improved photovoltaic system under unbalanced grid voltage.

7. A computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed by a processor, it implements the method for establishing a small-signal model of an improved photovoltaic system under unbalanced grid voltage as described in any one of claims 1-4, thereby realizing the modeling of a small-signal model of an improved photovoltaic system under unbalanced grid voltage.

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