Method and device for determining grid impedance under grid connection and charging and discharging system

Abstract

The application provides a method and device for determining grid impedance under grid connection and a charging and discharging system, and relates to the technical field of power electronics. The method comprises the following steps: obtaining a port voltage at a steady-state working point before disturbance of an output port of a target phase in a power conversion module when the power conversion module is in a grid-connected operation state; superimposing a first disturbance amount on a given reactive power of a reactive-voltage droop loop in the power conversion module, and / or superimposing a second disturbance amount on a reference voltage, and determining a reactive power change amount of the target phase after disturbance according to a superposition result; if the reactive power change amount after disturbance is greater than a minimum resolution power, inputting at least one of the port voltage, the reactive power change amount after disturbance, and the first and second disturbance amounts into a grid impedance identification model to obtain an equivalent grid inductance corresponding to the target phase, thereby realizing online identification of the equivalent grid inductance, and the method is suitable for various grid operating conditions and has better robustness and universality.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and more specifically, to a method, apparatus, and charging / discharging system for determining grid impedance under grid connection. Background Technology

[0002] In grid-connected inverter systems controlled by grid-connected virtual synchronous generators (VSGs), the equivalent grid impedance (especially inductive impedance) is a key parameter affecting the dynamic response, steady-state accuracy, and system stability of droop control. Therefore, achieving high-precision, low-disturbance, and highly adaptable online identification of grid impedance has become a fundamental and urgent technical requirement for improving the adaptive control capabilities of grid-connected VSGs.

[0003] In existing technologies, grid impedance calculation schemes are mainly based on the voltage and current continuously monitored during inverter operation, and the analysis yields an impedance that only reflects the cable's intrinsic impedance. However, the accuracy of this impedance calculation method is highly dependent on the fluctuation level and signal-to-noise ratio of the grid background harmonics, and reliable results may be difficult to obtain when the grid is relatively stable and the background noise is low. Summary of the Invention

[0004] The purpose of this application is to provide a method, apparatus, and charging / discharging system for determining grid impedance under grid connection, in order to address the shortcomings of the prior art and solve the technical problems existing in the prior art.

[0005] To achieve the above objectives, the technical solutions adopted in the embodiments of this application are as follows: In a first aspect, embodiments of this application provide a method for determining grid impedance under grid connection, applied to a power conversion module, the power conversion module including: a three-phase output port, the method including: When the power conversion module is in grid-connected operation, the port voltage at the steady-state operating point before disturbance of the output port of the target phase in the power conversion module is obtained; A first disturbance is superimposed on the given reactive power of the reactive-voltage droop loop in the power conversion module, and / or a second disturbance is superimposed on the reference voltage of the reactive-voltage droop loop, and the disturbance-induced reactive power change corresponding to the target is determined based on the superposition result. If the change in reactive power after the disturbance is greater than the preset minimum resolvable power, then at least one of the port voltage, the change in reactive power after the disturbance, and the first disturbance and the second disturbance is input to the pre-built grid impedance identification model, and the grid impedance identification model determines the grid equivalent inductive reactance corresponding to the target.

[0006] Optionally, if the power grid system connected to the power conversion module is a high-voltage power grid, then the power grid impedance identification model includes: ; in, For the equivalent inductive reactance of the power grid, The preset rated voltage, This is the first disturbance. This is the second disturbance quantity. This refers to the change in reactive power after the disturbance. It is the droop coefficient in the reactive power-voltage droop loop.

[0007] Optionally, if the power grid system connected to the power conversion module is a weak power grid, then the power grid impedance identification model includes: ; in, For the equivalent inductive reactance of the power grid, The port voltage. The preset rated voltage, This is the first disturbance. This is the second disturbance quantity. This refers to the change in reactive power after the disturbance. It is the droop coefficient in the reactive power-voltage droop loop.

[0008] Optionally, the superposition of a first disturbance on the given reactive power of the reactive-voltage droop loop in the power conversion module, and / or the superposition of a second disturbance on the reference voltage of the reactive-voltage droop loop, includes: A first power adjustment amount is superimposed on the given reactive power of the reactive-voltage droop loop, and the reactive power change after the current disturbance corresponding to the target is determined. If the reactive power change after the current disturbance is less than the minimum resolvable power, then the first power adjustment amount is increased and / or the composite disturbance mode is switched until the reactive power change after multiple disturbances corresponding to the target is greater than the minimum resolvable power. The composite disturbance mode is to add a first disturbance amount to the given reactive power of the reactive-voltage droop loop and a second disturbance amount to the reference voltage.

[0009] Optionally, determining the change in reactive power after disturbance corresponding to the target includes: Determine the reactive power before the disturbance based on the port voltage; Based on the superposition results, determine the reactive power after the disturbance; The difference between the reactive power after the disturbance and the reactive power before the disturbance is taken as the change in reactive power after the disturbance.

[0010] Optionally, the method further includes: After N consecutive disturbances, determine the equivalent inductive reactance of the power grid after each disturbance, where N is an integer greater than 1; If the deviation between the equivalent inductive reactance of the power grid after two adjacent disturbances is less than a set threshold, then the mean value of the equivalent inductive reactance of the power grid after N disturbances is determined, and the mean value is used as the target equivalent inductive reactance of the power grid corresponding to the target.

[0011] Optionally, the method further includes: Based on the target power grid equivalent inductive reactance corresponding to the target, determine the power grid impedance state corresponding to the target; Based on the grid impedance state corresponding to the target, the control parameters in the reactive power-voltage droop loop are adjusted. The control parameters include at least: droop coefficient, voltage loop PI parameter, or virtual impedance value.

[0012] Secondly, embodiments of this application provide a device for determining grid impedance under grid connection, applied to a power conversion module, the power conversion module including: a three-phase output port, and the device including: The acquisition module is used to acquire the port voltage at the pre-disturbance steady-state operating point of the output port of the target phase in the power conversion module when the power conversion module is in grid-connected operation. The superposition module is used to superimpose a first disturbance on the given reactive power of the reactive-voltage droop loop in the power conversion module, and / or superimpose a second disturbance on the reference voltage of the reactive-voltage droop loop. The determination module is used to determine the reactive power change after disturbance corresponding to the target based on the superposition result; if the reactive power change after disturbance is greater than the preset minimum resolution power, then the port voltage, the reactive power change after disturbance, and at least one of the first disturbance and the second disturbance are input to the pre-constructed grid impedance identification model, and the grid impedance identification model determines the grid equivalent inductive reactance corresponding to the target.

[0013] Optionally, the overlay module is specifically used for: A first power adjustment amount is superimposed on the given reactive power of the reactive-voltage droop loop, and the reactive power change after the current disturbance corresponding to the target is determined. If the reactive power change after the current disturbance is less than the minimum resolvable power, then the first power adjustment amount is increased and / or the composite disturbance mode is switched until the reactive power change after multiple disturbances corresponding to the target is greater than the minimum resolvable power. The composite disturbance mode is to add a first disturbance amount to the given reactive power of the reactive-voltage droop loop and a second disturbance amount to the reference voltage.

[0014] Thirdly, embodiments of this application provide a charging and discharging system, the system comprising: at least one power conversion module, the power conversion module comprising: a controller; The input terminal of the controller is connected to the output port of the power conversion module and the output port of the power grid system, respectively, and the output terminal of the controller is connected to the control terminal of the switching unit of each phase in the power conversion module. The controller is used to execute the method for determining the grid impedance under grid connection described above.

[0015] The beneficial effects of this application are: This application provides a method, apparatus, and charging / discharging system for determining grid impedance under grid-connected conditions. In this application, when the power conversion module is in grid-connected operation, the port voltage at the target phase's output port before disturbance at the steady-state operating point is obtained. A first disturbance is superimposed on the given reactive power of the reactive-voltage droop loop, and / or a second disturbance is superimposed on the reference voltage of the reactive-voltage droop loop. The change in reactive power after disturbance corresponding to the target is determined based on the superposition result. This method eliminates the need for open-loop signal injection, avoids additional harmonic injection, and prevents impact. The power grid can be subjected to bivariate disturbances under closed-loop control at any steady-state operating point, or a single variable disturbance can be performed. Based on the superposition result, the reactive power change after the disturbance corresponding to the target can be determined. When the reactive power change after the disturbance is determined to be greater than the minimum resolvable power, the equivalent inductive reactance of the power grid corresponding to the target can be calculated using the port voltage, the reactive power change after the disturbance, the first disturbance, the second disturbance, and the power grid impedance identification model. This enables online identification of the equivalent inductive reactance of the power grid, which is suitable for various power grid operating conditions and has better robustness and versatility.

[0016] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 The main power circuit diagram for a three-phase four-wire topology grid-connected inverter; Figure 2 This is a schematic diagram of a charging and discharging system provided in an embodiment of this application; Figure 3 A flowchart illustrating a method for determining grid impedance under grid connection, provided in an embodiment of this application; Figure 4 A schematic diagram of an equivalent circuit for a reactive-active droop loop provided in an embodiment of this application; Figure 5 Equivalent circuit diagram of inverter voltage and output reactive power based on inductive principle provided in the embodiments of this application; Figure 6 A flowchart illustrating another method for determining grid impedance under grid connection provided in this application embodiment; Figure 7 A flowchart illustrating another method for determining grid impedance under grid connection provided in this application embodiment; Figure 8 A flowchart illustrating another method for determining grid impedance under grid connection provided in this application embodiment; Figure 9 A flowchart illustrating another method for determining grid impedance under grid connection provided in this application embodiment; Figure 10 This is a schematic diagram of a device for determining the impedance of a grid-connected power grid, provided in an embodiment of this application. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0020] Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0021] First, the background technology involved in this application will be introduced.

[0022] Currently, the calculation methods for power grid impedance include the following: The first method, offline modeling, analyzes the voltage and current continuously monitored during inverter operation, revealing only the cable's intrinsic impedance. However, the accuracy of this impedance calculation method is highly dependent on the fluctuation level and signal-to-noise ratio of the power grid's background harmonics, and it cannot calculate the impedance changes caused by intermediate loads or parallel connections.

[0023] The second method is the state observation method: constructing a state-space model of the power grid impedance (such as RL series), designing a Kalman filter, and inferring the impedance state from the voltage / current output. However, this impedance calculation method has high computational complexity and is difficult to meet the requirements of millisecond-level real-time identification.

[0024] The third method is the transient fault calculation method: it uses the voltage / current changes at the moment of a fault such as a short circuit or open circuit to infer the impedance through transient energy or characteristic frequency attenuation rate. However, this impedance calculation method depends on the actual fault and cannot be used for daily adaptive adjustment.

[0025] To address the aforementioned issues, this application provides a method for determining grid impedance under grid connection. This method does not require open-loop signal injection, does not inject additional harmonics, and does not impact the grid. It utilizes closed-loop control at any steady-state operating point to perform bivariate disturbances, calculates the equivalent inductive reactance of the grid corresponding to a single variable, and realizes online calculation for identifying grid impedance under grid connection. This method is suitable for various grid operating conditions.

[0026] Optionally, refer to Figure 1 The diagram shown is the main power circuit diagram of a three-phase four-wire topology grid inverter. Figure 1 As shown, the power conversion module 1 provided in this embodiment can convert the DC voltage Vdc output from the energy storage battery into three-phase AC voltages Va, Vb, and Vc, with R1, R2, and R3 serving as external loads. When the power conversion module 1 operates in grid-connected mode, R1, R2, and R3 are powered by Va, Vb, and Vc output from the power conversion module 1, while the power conversion module 1 simultaneously charges or discharges the entire power grid.

[0027] It is worth noting that, see Figure 1 The power conversion module 1 consists of capacitors Cdc1~Cdc2, N-type metal oxide semiconductor transistors Q1~Q6, inductors L1~L3, and an electromagnetic compatibility module (EMC). The gates of the N-type metal oxide semiconductor transistors Q1~Q6 are connected to the controller in the power conversion module 1, and N is the neutral line.

[0028] When the power conversion module 1 is operating in off-grid mode, only the port voltages Va, Vb, and Vc output by the power conversion module 1 supply power to the loads R1, R2, and R3. At this time, the phase switching units S1, S2, and S3 are all open, and the external public power grid does not need to provide grid voltage. When the controller in the power conversion module 1 receives the off-grid to grid-connected command from the host computer, the port voltages Va, Vb, and Vc output by the power conversion module 1 are clamped by the grid voltages Ua, Ub, and Uc provided by the external public power grid, so that when entering the grid-connected operation mode, the power conversion module 1 operates in constant current or constant power mode and provides stable operating power to the loads R1, R2, and R3. At this time, the phase switching units S1, S2, and S3 are all closed.

[0029] When switching is required, such as from off-grid to grid-connected, to ensure a seamless switch, the off-grid circuit generally needs to be adjusted to be completely consistent with the grid waveform (amplitude, frequency, and phase). If the three phases are different, each phase is adjusted independently. When they are completely consistent, synchronization is considered complete, and the three-phase switching unit can be closed, resulting in a smaller inrush current. Then, the power conversion module 1 starts the grid-connected mode.

[0030] In order to take full advantage of the more flexible and reliable single-phase topology with N-line, the original three-phase symmetrical detection and loop VSG architecture based on three-phase current balance or three-phase voltage balance is no longer applicable. Although the symmetrical loop control has added features such as three-phase voltage imbalance negative sequence detection control, it is only a patchwork and cannot achieve the goal of free adjustment of single phase.

[0031] Therefore, this application proposes a single-phase VSG network system that uses port voltage, reactive power change after disturbance, first disturbance, second disturbance, and grid impedance identification model to calculate the grid equivalent inductive reactance corresponding to the target, thereby realizing online identification of the grid equivalent inductive reactance. This system is suitable for various grid operating conditions and has better robustness and versatility.

[0032] Optionally, refer to Figure 2 The diagram shown is a structural schematic of a charging and discharging system provided in this application. The system includes at least one power conversion module 1, and the power conversion module includes a controller 2. The input terminal of controller 2 is connected to the DC bus port and the output port of the power conversion module, respectively, and the output terminal of controller is connected to the control terminal of the switching unit of each phase in the power conversion module. The controller is used to execute the grid impedance determination method provided in the following embodiments to meet the grid impedance identification requirements of a three-phase four-wire energy storage network system under grid connection.

[0033] The following embodiments will explain in detail the specific implementation process and beneficial effects of the method for determining grid impedance under grid connection proposed in this application.

[0034] Optionally, refer to Figure 3 The diagram shown is a flowchart illustrating a method for determining grid impedance under grid connection provided in this application, applicable to the above-mentioned... Figure 2 The power conversion module shown includes: a three-phase output port; as shown in the figure. Figure 3 As shown, the method includes: S101. When the power conversion module is in grid-connected operation, obtain the port voltage at the steady-state operating point of the target phase output port before disturbance in the power conversion module.

[0035] Before any perturbation (ΔQref / ΔVref) is applied, the power conversion module is at the static operating point required by the small-signal linearization theory.

[0036] In one feasible approach, when the power conversion module is in grid-connected operation and online calculation of the grid impedance corresponding to phase A is required, the port voltage at the pre-disturbance steady-state operating point of phase A's output port can be obtained. .

[0037] S102. A first disturbance is superimposed on the given reactive power of the reactive-voltage droop loop in the power conversion module, and / or a second disturbance is superimposed on the reference voltage of the reactive-voltage droop loop, and the change in reactive power after disturbance corresponding to the target is determined according to the superposition result.

[0038] In one feasible approach, refer to Figure 4 As shown, this is the single-phase reactive power-voltage droop loop of the power conversion module under grid-connected conditions. The reactive power-voltage droop coefficient Kv is superimposed with the reference voltage Vref to obtain the final voltage reference value. It is applied to the power inverter through the dq voltage loop (d-axis voltage loop and q-axis voltage loop) to obtain the output reactive power component regulated voltage V, which is then connected to the grid (voltage E) through an approximately purely inductive impedance value X.

[0039] The above Figure 4 Since only reactive power regulation is considered, V represents a component that is in the same or opposite direction to E, and is not the actual inverter voltage. The actual inverter voltage decomposed into the component in the E direction is V. This component is approximately equal to the given amplitude of the dq voltage loop, and the low-frequency gain is approximately 1. Since the transfer function of the dq voltage loop equivalent circuit is approximated as a low-pass filter with a ratio of 1, considering only the gain, the ratio can be considered to be 1, meaning the given voltage and the actual voltage amplitude are the same.

[0040] In this embodiment, it is proposed that the given reactive power of the reactive-voltage droop loop in the power conversion module is... The first disturbance quantity is superimposed. And / or, the reference voltage in the reactive-voltage droop loop. Superimposed second disturbance That is, it can handle both bivariate and single-variable disturbances; responding to the above superposition results, it acquires the corresponding reactive power after the disturbance within one power frequency cycle (20ms) after the disturbance injection, and calculates the change in reactive power after the disturbance based on the reactive power before and after the disturbance. .

[0041] For example, the first disturbance amount It can be ±3%~5% of the rated reactive power; the second disturbance amount It can be ±0.5% to 1.5% of the rated voltage.

[0042] Optionally, the above superposition operation is completed in the instruction generation layer of the reactive power-voltage droop loop without modifying the underlying PWM modulation or current loop parameters, ensuring that the disturbance only affects the voltage source amplitude regulation behavior.

[0043] S103. If the change in reactive power after the disturbance is greater than the preset minimum resolvable power, then input at least one of the port voltage, the change in reactive power after the disturbance, and the first disturbance and the second disturbance into the pre-built grid impedance identification model, and determine the grid equivalent inductive reactance corresponding to the target by the grid impedance identification model.

[0044] Among them, the power grid impedance identification model is an analytical mathematical model derived from physical laws, which can be applied to the online identification of power grid impedance under both weak and strong grids.

[0045] The minimum resolution power is used to distinguish between the true response and noise in order to identify invalid disturbances.

[0046] In one feasible approach, if If the power exceeds the preset minimum resolution, the disturbance is deemed valid, and the port voltage is adjusted accordingly. Change in reactive power after disturbance and the first disturbance quantity and the second disturbance quantity At least one of the inputs is fed into a pre-built grid impedance identification model to calculate the grid equivalent inductive reactance corresponding to A.

[0047] Optionally, if If the power is less than or equal to the preset minimum resolution, the disturbance amount can be increased. / The perturbation is repeatedly applied, and the equivalent inductive reactance X of the power grid is calculated based on the results of multiple perturbations.

[0048] In summary, this application provides a method for determining grid impedance under grid connection. In this application, when the power conversion module is in grid-connected operation, the port voltage at the steady-state operating point of the target phase's output port before disturbance is obtained. A first disturbance is superimposed on the given reactive power of the reactive-voltage droop loop, and / or a second disturbance is superimposed on the reference voltage of the reactive-voltage droop loop. The change in reactive power after disturbance corresponding to the target is determined based on the superposition result. That is, no open-loop signal injection is required, no additional harmonics are injected, and no voltage surge occurs. The system utilizes closed-loop control at any steady-state operating point to perform bivariate disturbances, or it can disturb a single variable. Based on the superposition results, it determines the reactive power change corresponding to the target after the disturbance. When the reactive power change after the disturbance is determined to be greater than the minimum resolvable power, it calculates the equivalent inductive reactance of the grid corresponding to the target using the port voltage, the reactive power change after the disturbance, the first disturbance, the second disturbance, and the grid impedance identification model. This enables online identification of the equivalent inductive reactance of the grid, making it suitable for various grid operating conditions and exhibiting better robustness and versatility.

[0049] Optionally, if the power grid system to which the power conversion module is connected is a strong grid, then the grid impedance identification model is as shown in the following formula (1): (1) in, For the equivalent inductive reactance of the power grid, The preset rated voltage, This is the first disturbance quantity. This is the second disturbance quantity. This represents the change in reactive power after the disturbance. This is the droop coefficient in the reactive power-voltage droop loop.

[0050] It should be noted that the reference Figure 5 As shown, the voltage difference between the output voltage V of the power conversion module and the grid-side voltage E under grid connection (i.e., The current acts on the line impedance (assuming it is mainly inductive), forming the output current and related active and reactive power. Assume the equivalent component of E in phase is V, and the output current is I and the reactive power is Q.

[0051] In this embodiment, according to the above Figure 4 The control relationship of the reactive power-voltage droop control loop shown can be obtained as shown in the following formula (2): (2) The reactive power expression on the power conversion module side is: In a weak network, there is a certain gap between V and E. As can be seen from the reactive power expression on the power conversion module side, Q and V have a quadratic function relationship, which is quite complicated.

[0052] If V is close to E, replacing the first term with E yields an approximate expression for reactive power on the grid side, i.e. Under strong grid conditions, the error is small when using this expression for power. However, while the error is small under strong grid conditions, the difference between V and E is large under weak grid conditions, making it impossible to calculate Q accurately.

[0053] If the reactive power expression from the grid side is used, i.e. We can obtain a linear function, as shown in formula (3) below: (3) After combining and deriving, we obtain the following formula (4): (4) Based on the above linear expression, when , When the current reference value changes , The following formula (5) can be obtained: (5) From the above formula (5), it can be deduced that the equivalent inductive reactance X of the power grid is as shown in the above formula (1), that is... .

[0054] Formula (1) has little error under strong grid conditions and is an expression that takes into account both reference voltage and reference reactive power under dual disturbance conditions.

[0055] Optionally, if the power grid system to which the power conversion module is connected is a weak grid, then the grid impedance identification model is as shown in the following formula (6): (6) in, For the equivalent inductive reactance of the power grid, The port voltage. The preset rated voltage, This is the first disturbance. This is the second disturbance quantity. This represents the change in reactive power after the disturbance. This is the droop coefficient in the reactive power-voltage droop loop.

[0056] Optionally, the reactive power output by the power conversion module under weak grid conditions will differ significantly from the reactive power obtained by the grid. Therefore, the above formula (1) cannot accurately reflect the precise error of the disturbance.

[0057] Therefore, in this embodiment, if the reactive power expression on the power converter side is used... The quadratic functions of Q and V in the above equation under weak network conditions require new derivation under small signal disturbance. According to the disturbance under the current state, the reactive power and port voltage after the disturbance can be expressed as shown in the following formulas (7)-(8): (7) (8) Therefore, Substituting formulas (7) and (8), we obtain the following formula (9): (9) Ignore higher-order terms DC steady-state point removed The small signal retains only one term, resulting in the following formula (10): (10) That is, The above formula can also be obtained by taking the derivative of Q with respect to V around V0.

[0058] Alternatively, consider the reactive power-voltage droop relationship expression: It can also be written as a small-signal expression, taking into account: , , ; Substituting into the above equation, we obtain the small-signal expression as shown in formula (11): (11) Combine the above formula (10) with the above After combining, we can obtain the following formula (12): (12) According to the above formula (12), the equivalent inductive reactance X of the power grid can be solved as shown in the above formula (6), that is... .

[0059] when At that time, the above formula (6) is the same as the previous approximate formula (1) under strong net, and formula (6) is more general.

[0060] Therefore, the above formula (12) is based on the basic algorithm of reactive power-voltage droop control under weak grid and the formula for calculating the reactive power output of the inverter. It derives the unified small-signal calculation formula under the dual disturbances of reactive power and voltage, and calculates the grid impedance under the dual disturbances.

[0061] In formulas (5) and (12) above, E adopts the standard rated voltage, such as the effective value of single-phase 220V; Kv is determined according to the strong grid parameters and is relatively small, serving as a benchmark.

[0062] Optionally, all of the above are effective values, and each value should be divided by 1.414 when calculating power using amplitude.

[0063] Optionally, refer to Figure 6 As shown, step S103 above includes: S201. Add a first power adjustment amount to the given reactive power of the reactive-voltage droop loop, and determine the reactive power change amount corresponding to the target after the current disturbance.

[0064] For example, the first power regulation amount can be 5% of the rated reactive power, and the direction can be positive or negative, preferably positive, to avoid triggering the reactive power reverse protection.

[0065] S202. If the reactive power change after the current disturbance is less than the minimum resolvable power, increase the first power adjustment amount and / or switch to the composite disturbance mode until the reactive power change after multiple disturbances corresponding to the target is greater than the minimum resolvable power.

[0066] The composite disturbance mode involves superimposing a first disturbance on the given reactive power of the reactive-voltage droop loop and a second disturbance on the reference voltage.

[0067] In one feasible approach, whether grid-connected or islanded, the Q_ref adjustment variable is preferentially used, adjusted to approximately 5% of the rated power. This approach does not affect the operating point and improves identifiability. Specifically, during the initial disturbance injection, after grid-connected steady state, the first power adjustment amount is... The given reactive power superimposed on the current reactive-voltage droop control loop And within 20ms (1 power frequency cycle) after the disturbance injection, calculate the reactive power change corresponding to A after the current disturbance. ,like If the power is less than the minimum resolvable power, it is considered an invalid disturbance, the current disturbance is discarded, and the first power adjustment is increased. For example, the first power adjustment is... Increment by a step size of 1.5, that is =1.5× And repeatedly calculate the reactive power change corresponding to A after the current disturbance. ,like If the power is still less than the minimum resolvable power, the single-variable perturbation mode is immediately terminated, and the system switches to the composite perturbation mode, while simultaneously generating the first perturbation quantity. Second disturbance quantity Furthermore, a first disturbance is superimposed on the given reactive power of the reactive-voltage droop loop, and a second disturbance is superimposed on the reference voltage, i.e., a bivariate disturbance is injected simultaneously, and the data under the combined disturbance are collected. If under composite disturbance If the power is greater than the minimum resolution power, it is considered a valid disturbance, and three independent composite disturbance cycles are executed (e.g., the disturbance amplitude is finely adjusted by ±5% each time). The median filter is applied to the three calculated X values, and the final identification result is output.

[0068] Therefore, in this embodiment, during the initial disturbance stage, a single-variable initial disturbance is first performed to achieve a lightweight and fast probe. If the change in reactive power after a disturbance is less than the minimum resolvable power, a composite disturbance mode is activated. That is, when the single-variable excitation is insufficient, a stronger linear response is stimulated through bivariate synergy to ensure the accuracy of the finally identified grid impedance.

[0069] Optionally, refer to Figure 7 As shown, determining the change in reactive power after disturbance corresponding to the target in step S102 above includes: S301. Determine the reactive power before the disturbance based on the port voltage; and determine the reactive power after the disturbance based on the superposition result.

[0070] S302. The difference between the reactive power after the disturbance and the reactive power before the disturbance shall be taken as the change in reactive power after the disturbance.

[0071] In one feasible approach, the output port voltage of the target phase (e.g., phase A) at its steady-state operating point before the disturbance is obtained, along with the grid phase angle output by the phase-locked loop (PLL) at that moment. The port voltage is then subjected to a dq transformation based on the grid phase angle to obtain the active and reactive voltages. Simultaneously, the output current of the target phase (e.g., phase A) is obtained, and a dq transformation is performed on the output current to obtain the active and reactive currents. Based on the instantaneous power formula, active voltage, reactive voltage, active current, and reactive current, the reactive power corresponding to phase A before the disturbance is calculated. In the reactive power-voltage droop control loop, the first and / or second disturbance quantities are superimposed onto the original command. After the system re-enters a new steady state (non-transient transition process), the reactive power after the disturbance is calculated using the same method described above. The difference between the reactive power after the disturbance and the reactive power before the disturbance is calculated, and this difference is used as the change in reactive power after the disturbance.

[0072] Optionally, refer to Figure 8 As shown, the method also includes: S401. After N consecutive disturbances, determine the equivalent inductive reactance of the power grid after each disturbance.

[0073] Where N is an integer greater than 1; S402. If the deviation between the equivalent inductive reactance of the power grid after two adjacent disturbances is less than the set threshold, then the mean value of the equivalent inductive reactance of the power grid after N disturbances is determined, and the mean value is used as the target equivalent inductive reactance of the power grid corresponding to the target.

[0074] In one feasible approach, to improve the accuracy of the grid equivalent reactance, the following method is proposed: After 10 consecutive disturbances, the grid equivalent reactance corresponding to each disturbance is determined. If the deviation between the grid equivalent reactances corresponding to two adjacent disturbances is less than a set threshold (e.g., the relative deviation of the calculation results of two adjacent X values ​​is less than 2.0%), it is determined that the grid equivalent reactance after multiple disturbances is gradually converging. The mean value of the grid equivalent reactances corresponding to the 10 disturbances is then calculated, and this mean value is used as the target grid equivalent reactance, thereby improving the reliability of the target grid equivalent reactance.

[0075] Alternatively, the median of the equivalent inductive reactance of the power grid after 10 disturbances can be used as the target equivalent inductive reactance of the power grid to eliminate intermittent fault interference.

[0076] Optionally, refer to Figure 9 As shown, the method also includes: S501. Determine the power grid impedance state corresponding to the target based on the equivalent inductive reactance of the target power grid.

[0077] For example, the grid impedance states include: strong grid state, medium grid state and weak grid state, and each grid impedance state belongs to a different resistance value range.

[0078] S502. Adjust the control parameters in the reactive power-voltage droop loop according to the grid impedance state corresponding to the target.

[0079] The control parameters include at least: droop coefficient, voltage loop PI parameter, or virtual impedance value.

[0080] In one feasible approach, based on the target grid equivalent inductive reactance X corresponding to phase A and multiple pre-defined resistance ranges, the resistance range into which the target grid equivalent inductive reactance falls can be determined. The impedance state of the resistance range into which it falls is taken as the grid impedance state corresponding to phase A. For example, the grid impedance state corresponding to phase A is a weak grid state. Based on the grid impedance state corresponding to the target, the control parameters in the reactive power-voltage droop loop are adjusted. For example, if the grid impedance state corresponding to phase A is a weak grid state, the droop system Kv in the reactive power-voltage droop loop is increased to improve the reactive power regulation sensitivity, compensate for line voltage drop, realize dynamic adjustment of the control parameters in the reactive power-voltage droop loop, and improve system stability.

[0081] Therefore, in this embodiment, the control parameters in the reactive power-voltage droop loop can be adaptively adjusted based on the equivalent inductive reactance of the target power grid corresponding to the target, thereby automatically adjusting and reducing errors and avoiding the impact of online impedance changes on control accuracy.

[0082] Optionally, such as Figure 10The diagram shown is a structural schematic of a grid impedance determination device provided in this application, applied to a power conversion module. The power conversion module includes: a three-phase output port, such as... Figure 10 As shown, the device includes: The acquisition module 1001 is used to acquire the port voltage at the steady-state operating point before disturbance of the output port of the target phase in the power conversion module when the power conversion module is in grid-connected operation. The superposition module 1002 is used to superimpose a first disturbance on the given reactive power of the reactive-voltage droop loop in the power conversion module, and / or superimpose a second disturbance on the reference voltage of the reactive-voltage droop loop. The determination module 1003 is used to determine the reactive power change after disturbance corresponding to the target based on the superposition result; if the reactive power change after disturbance is greater than the preset minimum resolution power, then the port voltage, the reactive power change after disturbance, and at least one of the first disturbance and the second disturbance are input to the pre-constructed grid impedance identification model, and the grid impedance identification model determines the grid equivalent inductive reactance corresponding to the target.

[0083] Optionally, if the power grid system connected to the power conversion module is a high-voltage power grid, then the power grid impedance identification model includes: ; in, For the equivalent inductive reactance of the power grid, The preset rated voltage, This is the first disturbance. This is the second disturbance quantity. This refers to the change in reactive power after the disturbance. It is the droop coefficient in the reactive power-voltage droop loop.

[0084] Optionally, if the power grid system connected to the power conversion module is a weak power grid, then the power grid impedance identification model includes: ; in, For the equivalent inductive reactance of the power grid, The port voltage. The preset rated voltage, This is the first disturbance. This is the second disturbance quantity. This refers to the change in reactive power after the disturbance. It is the droop coefficient in the reactive power-voltage droop loop.

[0085] Optionally, the superposition module 1003 is specifically used for: A first power adjustment amount is superimposed on the given reactive power of the reactive-voltage droop loop, and the reactive power change after the current disturbance corresponding to the target is determined. If the reactive power change after the current disturbance is less than the minimum resolvable power, then the first power adjustment amount is increased and / or the composite disturbance mode is switched until the reactive power change after multiple disturbances corresponding to the target is greater than the minimum resolvable power. The composite disturbance mode is to add a first disturbance amount to the given reactive power of the reactive-voltage droop loop and a second disturbance amount to the reference voltage.

[0086] Optionally, the determining module 1003 is specifically used for: Determine the reactive power before the disturbance based on the port voltage; Based on the superposition results, determine the reactive power after the disturbance; The difference between the reactive power after the disturbance and the reactive power before the disturbance is taken as the change in reactive power after the disturbance.

[0087] Optionally, the determining module 1003 is further configured to: After N consecutive disturbances, the equivalent inductive reactance of the power grid after each disturbance is determined, where N is an integer greater than 1. If the deviation between the equivalent inductive reactance of the power grid after two adjacent disturbances is less than a set threshold, the mean value of the equivalent inductive reactance of the power grid after N disturbances is determined, and the mean value is used as the target equivalent inductive reactance of the power grid corresponding to the target.

[0088] Optionally, the determining module 1003 is further configured to: Based on the target power grid equivalent inductive reactance corresponding to the target, determine the power grid impedance state corresponding to the target; Based on the grid impedance state corresponding to the target, the control parameters in the reactive power-voltage droop loop are adjusted. The control parameters include at least: droop coefficient, voltage loop PI parameter, or virtual impedance value.

[0089] The above-described device is used to execute the method provided in the foregoing embodiments, and its implementation principle and technical effect are similar, so they will not be described again here.

[0090] These modules can be one or more integrated circuits configured to implement the above methods, such as one or more Application Specific Integrated Circuits (ASICs), one or more digital signal processors (DSPs), or one or more Field Programmable Gate Arrays (FPGAs). Alternatively, when a module is implemented using processing element scheduler code, the processing element can be a general-purpose processor, such as a Central Processing Unit (CPU) or other processor capable of calling program code. Furthermore, these modules can be integrated together as a system-on-a-chip (SOC).

[0091] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

[0092] It will be apparent to those skilled in the art that this application is not limited to the details of the exemplary embodiments described above, and that this application can be implemented in other specific forms without departing from the spirit or essential characteristics of this application. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of this application is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within this application. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A method for determining the impedance of a grid under grid connection, characterized in that, Applied to a power conversion module, the power conversion module including: a three-phase output port, the method including: When the power conversion module is in grid-connected operation, the port voltage at the steady-state operating point before disturbance of the output port of the target phase in the power conversion module is obtained; A first disturbance is superimposed on the given reactive power of the reactive-voltage droop loop in the power conversion module, and / or a second disturbance is superimposed on the reference voltage of the reactive-voltage droop loop, and the disturbance-induced reactive power change corresponding to the target is determined based on the superposition result. If the change in reactive power after the disturbance is greater than the preset minimum resolvable power, then at least one of the port voltage, the change in reactive power after the disturbance, and the first disturbance and the second disturbance is input to the pre-built grid impedance identification model, and the grid impedance identification model determines the grid equivalent inductive reactance corresponding to the target.

2. The method according to claim 1, characterized in that, If the power grid system connected to the power conversion module is a high-voltage grid, then the grid impedance identification model includes: ； in, For the equivalent inductive reactance of the power grid, The preset rated voltage, This is the first disturbance. This is the second disturbance quantity. This refers to the change in reactive power after the disturbance. It is the droop coefficient in the reactive power-voltage droop loop.

3. The method according to claim 1, characterized in that, If the power grid system connected to the power conversion module is a weak power grid, then the power grid impedance identification model includes: ； in, For the equivalent inductive reactance of the power grid, The port voltage. The preset rated voltage, This is the first disturbance. This is the second disturbance quantity. This refers to the change in reactive power after the disturbance. It is the droop coefficient in the reactive power-voltage droop loop.

4. The method according to claim 1, characterized in that, The superimposition of a first disturbance on the given reactive power of the reactive-voltage droop loop in the power conversion module, and / or the superimposition of a second disturbance on the reference voltage of the reactive-voltage droop loop, includes: A first power adjustment amount is superimposed on the given reactive power of the reactive-voltage droop loop, and the reactive power change after the current disturbance corresponding to the target is determined. If the reactive power change after the current disturbance is less than the minimum resolvable power, then the first power adjustment amount is increased and / or the composite disturbance mode is switched until the reactive power change after multiple disturbances corresponding to the target is greater than the minimum resolvable power. The composite disturbance mode is to add a first disturbance amount to the given reactive power of the reactive-voltage droop loop and a second disturbance amount to the reference voltage.

5. The method according to claim 1, characterized in that, Determining the change in reactive power after disturbance corresponding to the target includes: Determine the reactive power before the disturbance based on the port voltage; Based on the superposition results, determine the reactive power after the disturbance; The difference between the reactive power after the disturbance and the reactive power before the disturbance is taken as the change in reactive power after the disturbance.

6. The method according to claim 1, characterized in that, The method further includes: After N consecutive disturbances, determine the equivalent inductive reactance of the power grid after each disturbance, where N is an integer greater than 1; If the deviation between the equivalent inductive reactance of the power grid after two adjacent disturbances is less than a set threshold, then the mean value of the equivalent inductive reactance of the power grid after N disturbances is determined, and the mean value is used as the target equivalent inductive reactance of the power grid corresponding to the target.

7. The method according to claim 6, characterized in that, The method further includes: Based on the target power grid equivalent inductive reactance corresponding to the target, determine the power grid impedance state corresponding to the target; Based on the grid impedance state corresponding to the target, the control parameters in the reactive power-voltage droop loop are adjusted. The control parameters include at least: droop coefficient, voltage loop PI parameter, or virtual impedance value.

8. A device for determining the impedance of a grid under grid connection, characterized in that, Applied to a power conversion module, the power conversion module includes: a three-phase output port, and the device includes: The acquisition module is used to acquire the port voltage at the pre-disturbance steady-state operating point of the output port of the target phase in the power conversion module when the power conversion module is in grid-connected operation. The superposition module is used to superimpose a first disturbance on the given reactive power of the reactive-voltage droop loop in the power conversion module, and / or superimpose a second disturbance on the reference voltage of the reactive-voltage droop loop. The determination module is used to determine the reactive power change after disturbance corresponding to the target based on the superposition result; if the reactive power change after disturbance is greater than the preset minimum resolution power, then the port voltage, the reactive power change after disturbance, and at least one of the first disturbance and the second disturbance are input to the pre-constructed grid impedance identification model, and the grid impedance identification model determines the grid equivalent inductive reactance corresponding to the target.

9. The apparatus according to claim 8, characterized in that, The overlay module is specifically used for: A first power adjustment amount is superimposed on the given reactive power of the reactive-voltage droop loop, and the reactive power change after the current disturbance corresponding to the target is determined. If the reactive power change after the current disturbance is less than the minimum resolvable power, then the first power adjustment amount is increased and / or the composite disturbance mode is switched until the reactive power change after multiple disturbances corresponding to the target is greater than the minimum resolvable power. The composite disturbance mode is to add a first disturbance amount to the given reactive power of the reactive-voltage droop loop and a second disturbance amount to the reference voltage.

10. A charging and discharging system, characterized in that, The system includes: at least one power conversion module, the power conversion module including: a controller; The input terminal of the controller is connected to the output port of the power conversion module and the output port of the power grid system, respectively, and the output terminal of the controller is connected to the control terminal of the switching unit of each phase in the power conversion module. The controller is used to perform the method for determining the grid impedance under grid connection as described in any one of claims 1-7.

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