A wide short-circuit ratio grid impedance simulation method for grid-type converter testing

Abstract

This invention discloses a wide short-circuit ratio grid impedance simulation method for testing grid-connected converters, relating to the field of power electronics technology. It addresses the problem of low accuracy in existing grid impedance simulations. The method includes the following steps: determining the target grid impedance based on a preset short-circuit ratio; obtaining the inherent impedance of the output filter of the test platform; subtracting the filter inductance from the equivalent grid inductance to obtain a virtual inductance, and subtracting the equivalent series resistance from the equivalent grid resistance to obtain a virtual resistance, thus obtaining a virtual impedance; obtaining the current component of the output current through coordinate transformation; generating a virtual impedance voltage drop; superimposing the virtual impedance voltage drop onto the voltage command; performing delay compensation; and adjusting the switching signal of the output power switch via PWM. This invention achieves accurate simulation of grid impedance over a wide short-circuit ratio range by treating the virtual impedance element as an equivalent circuit impedance element and performing low-pass filtering, voltage drop superposition, and delay compensation on the output current.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and more particularly to a method for simulating grid impedance with a wide short-circuit ratio for testing grid-type converters. Background Technology

[0002] The proportion of wind and solar renewable energy sources in the installed capacity and power generation of the power system is constantly increasing. These renewable energy sources need to be connected to the grid through converters, resulting in the power system exhibiting characteristics such as low inertia and weak support. Grid-type converters, with their control flexibility, have the ability to simulate the mechanical inertia of synchronous generator sets, provide damping, participate in primary frequency regulation and voltage support, and are key equipment for building new power systems.

[0003] The grid strength adaptability of grid-connected converters is a key indicator for supporting the stable operation of the power grid, requiring them to operate stably within a wide range of AC port short-circuit ratio (SCR). Therefore, conducting grid strength adaptability tests on grid-connected converters can effectively evaluate their performance and help power grid operators determine whether they meet grid access standards and technical specifications.

[0004] Currently, there are two main methods for simulating power grid impedance: Method 1: Using passive impedance components. Different short-circuit ratios are simulated by switching passive components. The disadvantages of this method are: it requires a large number of passive components when the short-circuit ratio varies over a wide range, resulting in high cost and large size of the device; the test conditions are limited, making it difficult to simulate complex power grid conditions; and losses are significant in high-power converter test scenarios.

[0005] Method 2: Employing a converter-based test platform. This method dynamically simulates grid impedance by controlling the converter. Specifically, the resistance and reactance are calculated based on the required short-circuit ratio. Currently, there are two implementation methods: First, the active and reactive power outputs of the converter under test are collected, and the voltage reference value of the converter within the test platform is calculated based on the resistance and reactance components of the required impedance and written into the control command. Second, the output current of the test platform is collected and fed forward through the resistance and reactance of the required impedance to the modulation voltage command of the converter within the test platform.

[0006] However, neither of the above two converter-based schemes takes into account the output impedance effects of the converter's own output filter and the converter control circuit within the test platform, resulting in low accuracy of the simulated grid impedance and making it difficult to achieve accurate grid impedance simulation over a wide frequency range.

[0007] Therefore, in order to meet the testing requirements for the grid strength adaptability of grid-type converters, achieving accurate simulation of grid impedance over a wide range of short-circuit ratio variations (such as SCR 1.1~10) has become a key technical challenge for evaluating the performance of grid-type converters. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, the present invention aims to provide a method for simulating grid impedance over a wide short-circuit ratio for testing grid-type converters. This method achieves accurate simulation of grid impedance over a wide short-circuit ratio range by treating the virtual impedance link as an equivalent circuit impedance element and performing low-pass filtering, voltage drop superposition, and delay compensation on the output current.

[0009] One of the objectives of this invention is achieved through the following technical solution: A method for simulating grid impedance over a wide short-circuit ratio for testing grid-type converters includes the following steps: The target grid impedance is determined based on a preset short-circuit ratio, and the target grid impedance includes the equivalent grid inductance and the equivalent grid resistance. Obtain the inherent impedance of the converter output filter of the test platform, wherein the inherent impedance includes the filter inductance and the equivalent series resistance; The virtual inductance is obtained by subtracting the filter inductance from the equivalent grid inductance, and the virtual resistance is obtained by subtracting the equivalent series resistance from the equivalent grid resistance. The virtual impedance is obtained based on the virtual inductance and the virtual resistance. The virtual impedance effect is achieved by controlling the converter, including the following steps: The output current of the converter on the test platform is collected, and the current component of the output current in the dq coordinate system is obtained through coordinate transformation. The current component is low-pass filtered and then input into the virtual impedance to generate a virtual impedance voltage drop. The virtual impedance voltage drop is superimposed on the voltage command to generate a modulated voltage command; Delay compensation is performed according to the modulation voltage command, and the switching signal of the output power switch is adjusted by PWM to control the converter bridge arm switching transistor.

[0010] To make this method quantifiable, the target grid impedance is determined based on a preset short-circuit ratio, including: based on the system rated voltage. System rated capacity System resistance-to-inductance ratio The target grid impedance is calculated, the short-circuit ratio is used to characterize the grid strength, and the system impedance-to-inductance ratio is used to characterize the resistance-to-inductance ratio of the grid impedance. The calculation satisfies the following: , , in, For the equivalent grid inductance, For the equivalent grid resistance, This indicates the preset short-circuit ratio.

[0011] Furthermore, the transfer function of the low-pass filter The calculation satisfies: ,in, The cutoff angular frequency, For Laplace variables. First-order low-pass filters have a simple structure and low computational cost, making them suitable for real-time digital control. By setting the cutoff angular frequency, high-frequency sampling noise can be filtered out, avoiding spurious voltage drops across the virtual impedance caused by high-frequency components.

[0012] To fully describe the physical model of the virtual impedance, the current component is low-pass filtered and then input into the virtual impedance to generate a virtual impedance voltage drop, including: The current component after low-pass filtering is input to the virtual impedance to obtain the virtual impedance voltage drop, which is calculated to satisfy: , The calculation of virtual impedance satisfies: , in, Represents virtual impedance. and This represents the d-axis and q-axis components of the current after low-pass filtering. and These represent the d-axis pressure drop and the q-axis pressure drop, respectively. Represents virtual inductance. Indicates virtual resistance. For Laplace variables, It represents the imaginary unit.

[0013] To ensure the converter port exhibits the expected positive impedance characteristics and avoid control loop stability issues, the virtual impedance voltage drop is superimposed on the voltage command to generate a modulated voltage command. This includes: superimposing the virtual impedance voltage drop on the d-axis component of the voltage input command. and q-axis components Above, generate modulation voltage command and ,satisfy: , ,in, and These represent the d-axis pressure drop and the q-axis pressure drop, respectively.

[0014] The output current component in the dq coordinate system is obtained through coordinate transformation, wherein the coordinate transformation is a Park transformation, which uses a transformation matrix to transform the current component in the abc coordinate system to the current component in the dq coordinate system. satisfy: , in, This indicates a phase angle command, which is expressed via the fundamental angular frequency. The calculations show that the following conditions are met: ,in, It is a time variable.

[0015] The standard Park transformation is used to convert the AC quantity in the abc coordinate system into the DC quantity in the dq coordinate system, which facilitates linear control. The phase angle is obtained by integrating the constant angular frequency, eliminating the need for a phase-locked loop (PLL) and avoiding the problem of reduced accuracy of the PLL under weak power grid conditions.

[0016] To ensure a stable fundamental voltage output from the test platform converter and simulate the characteristics of an ideal voltage source, the virtual impedance voltage drop is superimposed on the voltage command. The method further includes: obtaining the output voltage level of the test platform to obtain a constant voltage command, including a d-axis component for setting the amplitude of the test platform converter output voltage, and a q-axis component that is always zero.

[0017] Delay compensation based on the modulation voltage command includes: generating the d-axis component of the control voltage command through delay compensation. and q-axis components ,satisfy: , ,in, and This indicates the modulation voltage command for the d-axis and q-axis. The transfer function representing delay compensation is calculated to satisfy: ,in, Indicates a sampling period. For Laplace variables.

[0018] Pre-compensation significantly improves the phase accuracy of impedance simulation in the high-frequency band.

[0019] To form a more complete control closed-loop link, the method further includes: converting the d-axis and q-axis components of the control voltage command obtained after delay compensation into a three-phase voltage command in the abc coordinate system through dq-abc coordinate transformation, and inputting the three-phase voltage command into PWM modulation to generate switching signals for controlling the switching transistors of the converter bridge arm.

[0020] The main circuit of the converter in the test platform includes 6 power switching transistors, 1 three-phase L filter and 1 three-phase AC contactor. The three-phase L filter includes 3 filter inductors and 3 filter inductor equivalent series resistances.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention discloses a high-precision simulation method for grid impedance under wide-range short-circuit ratio variations. By equating the virtual impedance element to a circuit impedance element and taking into account the converter's filter inductance, accurate simulation of grid impedance can be achieved over a wide frequency range. It enables continuous adjustment of the SCR from 1.1 to 10 without the need for additional resistors or inductors, significantly reducing the size and losses of the test platform. Furthermore, this invention requires no additional passive components; simultaneously, the control structure is simple and highly versatile, solving the problems of low simulation accuracy and high-frequency distortion caused by neglecting the output impedance and control delay in existing converter-based solutions. This provides a reliable and efficient solution for grid strength adaptability testing of grid-connected converters. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the converter circuit structure of the test platform in this embodiment; Figure 2 This is a schematic diagram of the converter control structure of the test platform in this embodiment; Figure 3 This is a complex vector diagram of the converter and control system in the embodiment; Figure 4 This is a complex vector diagram in the embodiment after the control part of the converter is equivalent to the circuit part; Figure 5 This is a flowchart of a wide short-circuit ratio grid impedance simulation method for testing grid-type converters, as described in this embodiment. Figure 6 This is a frequency domain curve comparing the theoretical grid impedance and the impedance simulated by the converter in the embodiment. Detailed Implementation

[0023] The present invention will now be described in more detail with reference to the accompanying drawings. It should be noted that the following description of the present invention with reference to the accompanying drawings is merely illustrative and not restrictive. Various embodiments can be combined with each other to form other embodiments not shown in the following description.

[0024] Example 1 Example 1 provides a wide short-circuit ratio grid impedance simulation method for grid-type converter testing, aiming to achieve high-precision grid impedance simulation over a wide range of SCR 1.1~10 by using a control algorithm.

[0025] Please refer to Figure 1 The test platform used in the method of this embodiment shown includes a converter main circuit structure with 6 power switches and 1 three-phase L-filter (composed of 3 filter inductors). L ti The equivalent series resistance of the three filter inductors R ti Composition), 1 three-phase AC contactor. U dc DC side voltage i tiabc This is the output current of the converter.

[0026] Please refer to Figure 2 The control structure diagram shown includes a voltage command stage, a phase angle generation stage, a Park transformation stage, a virtual impedance stage, a delay stage, and a PWM modulation stage. The converter's control and main circuit are represented in complex vector form as shown in the diagram below. Figure 3 As shown, the control section of the converter is equivalent to the main circuit section. The schematic diagram of the equivalent complex vector form is shown below. Figure 4 As shown. Please refer to the work process of each step. Figure 5 The flowchart shown illustrates a method for simulating grid impedance with a wide short-circuit ratio for testing grid-type converters, comprising the following steps: S1. Determine the target grid impedance according to the preset short-circuit ratio. The target grid impedance includes the equivalent grid inductance and the equivalent grid resistance. S1 includes: based on the system rated voltage System rated capacity System resistance-to-inductance ratio The target grid impedance is calculated, the short-circuit ratio is used to characterize the grid strength, and the system impedance-to-inductance ratio is used to characterize the resistance-to-inductance ratio of the grid impedance. The calculation satisfies the following: , , in, For the equivalent grid inductance, For the equivalent grid resistance, This indicates the preset short-circuit ratio. The system impedance-inductance ratio can be determined based on the grid strength to be simulated. The preset SCR is determined based on the grid strength to be simulated.

[0027] Step S1 provides a basis for achieving continuous adjustment of a wide short-circuit ratio range (such as SCR 1.1~10), meeting the full-condition testing requirements from extremely weak power grids to strong power grids.

[0028] S2. Obtain the inherent impedance of the converter output filter of the test platform, wherein the inherent impedance includes the filter inductance and the equivalent series resistance; S3. Subtract the filter inductance from the equivalent grid inductance to obtain the virtual inductance, subtract the equivalent series resistance from the equivalent grid resistance to obtain the virtual resistance, and obtain the virtual impedance based on the virtual inductance and virtual resistance; Specifically, the equivalent grid inductance and equivalent grid resistance Subtract the filter inductance of the test platform respectively and the equivalent series resistance of the filter inductor To obtain virtual inductance = - and virtual resistance = - .

[0029] Step S3 eliminates the influence of the test platform's own hardware on the accuracy of impedance simulation, solving the long-standing problem of "low accuracy due to failure to consider its own output impedance" in existing technologies.

[0030] S4. Collect the output current of the converter on the test platform, and obtain the current component of the output current in the dq coordinate system through coordinate transformation; In this embodiment, the coordinate transformation of S4 uses the Park transformation to transform the current component in the abc coordinate system to the current component in the dq coordinate system through a transformation matrix. satisfy: , in, This indicates a phase angle command, which is expressed via the fundamental angular frequency. The calculations show that the following conditions are met: ,in, For time variables. Phase angle instruction. The phase angle required for system coordinate transformation.

[0031] The standard Park transformation is used to convert the AC quantities in the abc coordinate system into DC quantities in the dq coordinate system, which facilitates linear control; the phase angle command avoids the problem of reduced accuracy of the phase-locked loop under weak power grid conditions.

[0032] S5. The current component is low-pass filtered and then input into the virtual impedance to generate a virtual impedance voltage drop; The transfer function of the low-pass filter of S5 The calculation satisfies: ,in, The cutoff angular frequency, For Laplace variables. First-order low-pass filters have a simple structure and low computational cost, making them suitable for real-time digital control. By setting the cutoff angular frequency, high-frequency sampling noise can be filtered out, avoiding spurious voltage drops across the virtual impedance caused by high-frequency components.

[0033] d-axis component of converter output current and q-axis components Each through a low-pass filter The filtered current component was then obtained. and Its calculation satisfies: , , Specifically, the current component after low-pass filtering is input to the virtual impedance to obtain the virtual impedance voltage drop, the calculation of which satisfies: , The complex vector expression for virtual impedance satisfies: , in, Represents virtual impedance. and This represents the d-axis and q-axis components of the current after low-pass filtering. and These represent the d-axis pressure drop and the q-axis pressure drop, respectively. Represents virtual inductance. Indicates virtual resistance. For Laplace variables, It represents the imaginary unit.

[0034] S6. The virtual impedance voltage drop is superimposed on the voltage command to generate a modulation voltage command; By using negative feedback to ensure that the equivalent impedance of the converter port is precisely equal to the target grid impedance, the virtual impedance voltage drop is superimposed on the voltage command to generate a modulated voltage command. This includes: superimposing the virtual impedance voltage drop on the d-axis component of the voltage input command. and q-axis components Above, generate modulation voltage command and ,satisfy: , ,in, and These represent the d-axis pressure drop and the q-axis pressure drop, respectively.

[0035] The voltage command stage determines the output voltage level of the test platform. Step S6, which involves superimposing the virtual impedance voltage drop onto the voltage command, further includes: obtaining the output voltage level of the test platform to obtain a constant voltage command, including the d-axis component used to set the amplitude of the converter output voltage of the test platform. v tgd_ref and the q-axis component that is always zero v tgq_ref . v tgd_ref The amplitude of the output voltage level is determined in this embodiment. v tgq_ref Set to 0.

[0036] S7. Perform delay compensation according to the modulation voltage command, and adjust the switching signal of the output power switch through PWM to control the converter bridge arm switching transistor.

[0037] In this embodiment, the S7 modulation voltage command generates the d-axis component of the control voltage command through a delay circuit. and q-axis components ,satisfy: , ,in, and This indicates the modulation voltage command for the d-axis and q-axis. The transfer function represents the delay compensation, where the delay element mainly consists of a sampling period. The computation delay and half a sampling period are 0.5. The delay caused by pulse width modulation is constituted, and its calculation satisfies: ,in, Indicates a sampling period. For Laplace variables.

[0038] It also includes: converting the d-axis and q-axis components of the control voltage command obtained after delay compensation into a three-phase voltage command in the abc coordinate system through dq-abc coordinate transformation, and inputting the three-phase voltage command into PWM modulation to generate switching signals for controlling the switching transistors of the converter bridge arm. Transformation matrix satisfy: .

[0039] The component of the control voltage command in the abc coordinate system is input to the pulse width modulation stage, and the switching signal of the output power switch is used to control the switching transistors of the converter bridge arm.

[0040] S7 completes the inverse transformation from the dq coordinate system back to the abc coordinate system, generating three-phase voltage commands that can be directly used for PWM modulation, forming a complete control closed-loop link, which has high engineering practicality.

[0041] This embodiment introduces a virtual impedance. Dynamic part sL v To improve the accuracy of impedance simulation over a wide frequency range. To address the Laplace variable. s To address the influence of differential effects on harmonic amplification in practical circuits, this invention utilizes a low-pass filter. G LPF After preprocessing the converter output current and then comparing it with the virtual impedance Z v Cascading. Virtual impedance part equivalent impedance G LPF Z v G d like Figure 4 As shown, the grid impedance required to simulate and test the grid adaptability of a grid-type converter can be achieved by combining the impedance of the converter filter main circuit. Figure 6 This is a simulation diagram of the grid impedance. The theoretical impedance represents the frequency domain model of the grid impedance to be simulated, and the simulated impedance represents the measured output impedance of the converter. It can be seen that the output impedance of the converter can simulate the grid impedance over a wide frequency range.

[0042] Taking the main circuit topology of the three-phase two-level converter of this invention as an example, it should be noted that for high-voltage and high-capacity application scenarios, power modules can be connected in series and parallel or a modular multi-level main circuit structure can be used, but the control method proposed in this invention is still effective.

[0043] For those skilled in the art, various other corresponding changes and modifications can be made based on the technical solutions and concepts described above, and all such changes and modifications should fall within the protection scope of the claims of this invention.

Claims

1. A method for simulating grid impedance over a wide short-circuit ratio for testing grid-type converters, characterized in that, Includes the following steps: The target grid impedance is determined based on a preset short-circuit ratio, and the target grid impedance includes the equivalent grid inductance and the equivalent grid resistance. Obtain the inherent impedance of the converter output filter of the test platform, wherein the inherent impedance includes the filter inductance and the equivalent series resistance; The virtual inductance is obtained by subtracting the filter inductance from the equivalent grid inductance, and the virtual resistance is obtained by subtracting the equivalent series resistance from the equivalent grid resistance. The virtual impedance is obtained based on the virtual inductance and the virtual resistance. The output current of the converter on the test platform is collected, and the current component of the output current in the dq coordinate system is obtained through coordinate transformation. The current component is low-pass filtered and then input into the virtual impedance to generate a virtual impedance voltage drop. The virtual impedance voltage drop is superimposed on the voltage command to generate a modulated voltage command; Delay compensation is performed according to the modulation voltage command, and the switching signal of the output power switch is adjusted by PWM to control the converter bridge arm switching transistor.

2. The wide short-circuit ratio grid impedance simulation method for grid-type converter testing as described in claim 1, characterized in that, The target grid impedance is determined based on the preset short-circuit ratio, including: based on the system rated voltage. System rated capacity System resistance-to-inductance ratio The target grid impedance is calculated, the short-circuit ratio is used to characterize the grid strength, and the system impedance-to-inductance ratio is used to characterize the resistance-to-inductance ratio of the grid impedance. The calculation satisfies the following: ， ， in, For the equivalent grid inductance, For the equivalent grid resistance, This indicates the preset short-circuit ratio.

3. The wide short-circuit ratio grid impedance simulation method for grid-type converter testing as described in claim 1, characterized in that, The transfer function of the low-pass filter The calculation satisfies: ,in, The cutoff angular frequency, For Laplace variables.

4. The wide short-circuit ratio grid impedance simulation method for grid-type converter testing as described in claim 1 or 3, characterized in that, The current component is low-pass filtered and then input into the virtual impedance to generate a virtual impedance voltage drop, including: The current component after low-pass filtering is input to the virtual impedance to obtain the virtual impedance voltage drop, which is calculated to satisfy: ， The calculation of virtual impedance satisfies: ， in, Represents virtual impedance. and This represents the d-axis and q-axis components of the current after low-pass filtering. and These represent the d-axis pressure drop and the q-axis pressure drop, respectively. Represents virtual inductance. Indicates virtual resistance. For Laplace variables, It represents the imaginary unit.

5. The wide short-circuit ratio grid impedance simulation method for grid-type converter testing as described in claim 1, characterized in that, The virtual impedance voltage drop is superimposed on the voltage command to generate a modulated voltage command, including: superimposing the virtual impedance voltage drop on the d-axis component of the voltage input command. and q-axis components Above, generate modulation voltage command and ,satisfy: , ,in, and These represent the d-axis pressure drop and the q-axis pressure drop, respectively.

6. The method for simulating wide short-circuit ratio grid impedance for testing grid-type converters as described in claim 1, characterized in that, The output current component in the dq coordinate system is obtained through coordinate transformation, wherein the coordinate transformation is a Park transformation, which uses a transformation matrix to transform the current component in the abc coordinate system to the current component in the dq coordinate system. satisfy: ， in, This indicates a phase angle command, which is expressed via the fundamental angular frequency. The calculations show that the following conditions are met: ,in, It is a time variable.

7. The wide short-circuit ratio grid impedance simulation method for grid-type converter testing as described in claim 1, characterized in that, The method of superimposing the virtual impedance voltage drop onto the voltage command further includes: obtaining the output voltage level of the test platform to obtain a constant voltage command, including a d-axis component for setting the amplitude of the converter output voltage of the test platform, and a q-axis component that is always zero.

8. The wide short-circuit ratio grid impedance simulation method for grid-type converter testing as described in claim 1, characterized in that, Delay compensation based on the modulation voltage command includes: generating the d-axis component of the control voltage command through delay compensation. and q-axis components ,satisfy: ， ,in, and This indicates the modulation voltage command for the d-axis and q-axis. The transfer function representing delay compensation is calculated to satisfy: ,in, Indicates a sampling period. For Laplace variables.

9. The wide short-circuit ratio grid impedance simulation method for grid-type converter testing as described in claim 8, characterized in that, Also includes: The d-axis and q-axis components of the control voltage command obtained after delay compensation are converted into a three-phase voltage command in the abc coordinate system through dq-abc coordinate transformation, and the three-phase voltage command is input to PWM modulation to generate switching signals for controlling the switching transistors of the converter bridge arm.

10. The method for simulating wide short-circuit ratio grid impedance for testing grid-type converters as described in claim 1, characterized in that, The main circuit of the converter in the test platform includes 6 power switching transistors, 1 three-phase L filter and 1 three-phase AC contactor. The three-phase L filter includes 3 filter inductors and 3 filter inductor equivalent series resistances.

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