Impedance estimation method and system suitable for grid- and grid-forming converters

Abstract

The application discloses an impedance estimation method and system suitable for grid-following and grid-forming converters, comprising: monitoring whether grid-connected impedance changes based on the fluctuation of the output voltage amplitude of the converter; when detecting that the grid-connected impedance changes, delaying for a set time, sampling the voltage, current effective value, active power and reactive power output by the converter at the current time, and the voltage instantaneous value signal in a set time period; adjusting the reactive power reference value of the converter; delaying for a set time, and sampling again; and calculating the grid-connected resistance and reactance of the converter based on the sampling results of the two times. The method and system provided by the application do not need strict decoupling control of the active and reactive power output by the converter, and thus are suitable for grid-following and grid-forming converters and power systems with various impedance ratios.

Description

Technical Field

[0001] This invention relates to the field of converter impedance estimation technology, and in particular to an impedance estimation method and system applicable to grid-connected and grid-connected converters. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] As the proportion of renewable energy generation such as wind and solar power in the power system continues to increase, the structure of the power system is undergoing rapid changes. Currently, most renewable energy sources are connected to the grid through grid-connected converters to ensure maximum active power delivery. However, grid-connected converters typically function as current sources, and related research indicates that their increasing penetration rate can negatively impact grid security and stability. Conversely, grid-connected converters typically function as voltage sources and provide strong support for the safe and stable operation of the grid.

[0004] The control of grid-connected and grid-connected converters plays a crucial role in the safe and stable operation of power systems. The realization of functions such as voltage control, stability analysis, and fault detection of converters requires accurate estimation of their grid-connected impedance parameters.

[0005] The inventors discovered that most existing methods for estimating converter grid-connected impedance only apply to grid-connected converters. These methods estimate grid resistance and reactance by changing the active and reactive power outputs of the converter, respectively. This approach relies on the decoupling control of the active and reactive power outputs of the converter, which is impractical for grid-connected converters. Furthermore, in medium- and low-voltage distribution systems, excessively low system impedance ratios can lead to active and reactive power coupling, thus affecting the estimation accuracy of this method. Summary of the Invention

[0006] To address the aforementioned issues, this invention proposes an impedance estimation method and system applicable to grid-connected and grid-connected converters. By changing the reactive power output of the converter and sampling the effective values ​​of the converter's output voltage and current before and after the power change, as well as the phase angle difference of the voltage signal before and after the power change, the grid-connected resistance and reactance of the converter are calculated. This method eliminates the need for decoupling control of the active and reactive power outputs of the converter and is applicable to both grid-connected and grid-connected converters, as well as circuits with any impedance ratio.

[0007] In some implementations, the following technical solutions are adopted:

[0008] An impedance estimation method applicable to grid-connected and grid-connected converters includes:

[0009] Monitor whether the grid connection impedance changes based on the fluctuation of the converter output voltage amplitude;

[0010] When a change in grid impedance is detected, after a set delay, the effective values ​​of voltage and current, active power and reactive power of the converter output at the current time t1, as well as the instantaneous voltage signal for the set time period are sampled and obtained.

[0011] Maintain the active power reference value of the converter unchanged, and adjust the reactive power reference value of the converter; after a set delay, sample again to obtain the effective values ​​of voltage and current, active power and reactive power output by the converter at the current time t2, as well as the instantaneous voltage signal for the set time period.

[0012] Based on the sampled values ​​at times t1 and t2, the power factor angle and the phase angle difference of the voltage signal before and after reactive power adjustment at the two sampling times are calculated respectively; and the grid-connected resistance and reactance of the converter are calculated based on the power factor angle and the phase angle difference.

[0013] In other embodiments, the following technical solutions are adopted:

[0014] An impedance estimation system applicable to grid-connected and grid-connected converters includes:

[0015] The grid-connected impedance monitoring module is configured to monitor whether the grid-connected impedance changes based on fluctuations in the amplitude of the converter output voltage.

[0016] The sampling module is configured to, when a change in grid-connected impedance is detected, sample and acquire the effective values ​​of voltage and current, active power and reactive power output by the converter at the current time t1, as well as the instantaneous voltage value signal for the set time period, after a delay of a set time; maintain the active power reference value of the converter unchanged, and adjust the reactive power reference value of the converter; after a delay of a set time, sample and acquire the effective values ​​of voltage and current, active power and reactive power output by the converter at the current time t2, as well as the instantaneous voltage value signal for the set time period again;

[0017] The grid-connected impedance estimation module is used to calculate the power factor angle and the phase angle difference of the voltage signal before and after reactive power adjustment at the two sampling times t1 and t2, respectively, based on the sampled values ​​at the two sampling times; and to calculate the grid-connected resistance and reactance of the converter based on the power factor angle and the phase angle difference.

[0018] In other embodiments, the following technical solutions are adopted:

[0019] A terminal device includes a processor and a memory, the processor being used to implement instructions; the memory being used to store multiple instructions adapted to be loaded and executed by the processor, the above-described impedance estimation method applicable to grid-connected and grid-connected converters.

[0020] In other embodiments, the following technical solutions are adopted:

[0021] A computer-readable storage medium storing a plurality of instructions adapted for loading and execution by a processor of a terminal device of the above-described impedance estimation method applicable to grid-connected and grid-connected converters.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] (1) The grid-connected impedance estimation method of the present invention does not require decoupling control of the active and reactive power of the converter, so it is applicable to both grid-connected and grid-connected converters.

[0024] (2) This invention eliminates the need for global phase angle measurement during the estimation process, and is therefore more suitable for distribution networks that are not equipped with phase angle measurement equipment. At the same time, since only the reactive power output of the converter is changed during the estimation process, the converter can maintain its active power output unchanged, thus causing less disturbance to the system.

[0025] Other features and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0026] Figure 1 This is a flowchart of the impedance estimation method applicable to grid-connected and grid-connected converters in this embodiment of the invention;

[0027] Figure 2 This is a schematic diagram of the converter grid connection structure in an embodiment of the present invention;

[0028] Figure 3 This is a schematic diagram of signal triggering in an embodiment of the present invention;

[0029] Figure 4 This is a block diagram of the grid-type converter control in an embodiment of the present invention;

[0030] Figure 5 This is a block diagram of the grid-type converter control in an embodiment of the present invention;

[0031] Figure 6 These are the resistance estimation results for grid-connected and grid-connected converters in the embodiments of the present invention;

[0032] Figure 7 The inductance estimation results for grid-connected and grid-connected converters in this embodiment of the invention are shown. Detailed Implementation

[0033] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0034] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0035] Example 1

[0036] As the proportion of new energy sources connected to the power system through grid-connected and grid-built converters continues to increase, the control of various types of converters plays a crucial role in the safe and stable operation of the power system. The realization of converter functions such as voltage control, stability analysis, and fault detection requires accurate estimation of their grid-connected impedance parameters.

[0037] As mentioned in the background section, most existing converter grid-connected impedance estimation methods are only applicable to grid-connected converters. These methods estimate grid resistance and reactance by changing the active and reactive power outputs of the converter, respectively. This approach relies on the decoupling control of the active and reactive power outputs of the converter, which is impractical for grid-connected converters. Furthermore, in medium- and low-voltage distribution systems, excessively low system impedance ratios can lead to active and reactive power coupling, thus affecting the estimation accuracy of this method.

[0038] Based on this, in one or more embodiments, an impedance estimation method applicable to grid-connected and grid-connected converters is disclosed, combined with Figure 1 Specifically, it includes the following process:

[0039] (1) Monitor whether the grid connection impedance changes based on the fluctuation of the converter output voltage amplitude;

[0040] In this embodiment, for Figure 2 The converter grid-connected system shown in the figure and These are the voltage and current vectors output by the converter, respectively, Z. l Let V be the line impedance from the converter to the grid connection point. The external power grid is modeled as a voltage source V using Thevenin's equivalent model. g With impedance Z g The grid-connected impedance of the converter is Z. eq-inv That is, Z l +Z g .

[0041] Considering that impedance estimation procedures can cause some disturbance to the system, they should only be executed after an impedance change is detected. Changes in grid-connected impedance can be caused by load changes, changes in system operating mode, the connection or disconnection of other converters, etc., and these events can cause voltage fluctuations at various nodes in the system. Therefore, in this invention, the change in grid-connected impedance is determined by monitoring the voltage amplitude fluctuations at the converter output. The criteria for determination are:

[0042]

[0043] In the formula: V s V is the voltage sensitivity coefficient, used to determine whether the system impedance has changed. * V is the rated RMS voltage of the converter, and V is the RMS voltage at the converter outlet. In actual operation, the RMS voltage V at the converter outlet is sampled in real time, and the voltage sensitivity coefficient V0 is calculated. s When the value exceeds a certain threshold of 5%, it can be determined that the grid connection impedance has changed, thereby triggering the generation of the next grid connection impedance estimation procedure.

[0044] (2) When a change in grid impedance is detected, after a set delay, the effective values ​​of voltage and current, active power and reactive power of the converter output at the current time t1, as well as the instantaneous voltage signal of the set time period are sampled and obtained.

[0045] Specifically, since the change in the grid-connected impedance of the converter is usually caused by a series of disturbances, a certain delay should be set before executing the impedance estimation program to ensure that the system recovers to a steady state before executing the impedance estimation program, so as to avoid estimation errors caused by system disturbances.

[0046] The set delay is as follows Figure 3 As shown, after detecting a change in grid impedance at time t0, the system first delays for a period of τ1 to time t1 to ensure that the system recovers to a steady state before sampling at the first time. The sampled values ​​are the effective values ​​of the current output voltage and current V1 and I1, the active power P1 and reactive power Q1 output by the converter, and the instantaneous voltage signal u1 at a certain time interval.

[0047] (3) Keep the active power reference value of the converter unchanged and adjust the reactive power reference value of the converter; after delaying the set time, sample again to obtain the effective values ​​of voltage and current, active power and reactive power output by the converter at the current time t2, as well as the instantaneous voltage value signal of the set time period.

[0048] After sampling, adjust the reactive power reference value Q of the converter. ref This is to increase its reactive power output and maintain the active power reference value P. ref constant.

[0049] Because the control architectures of grid-connected and grid-connected converters differ significantly, the methods for changing their reactive power reference values ​​also differ. For grid-connected converters, the control architecture is as follows: Figure 4 As shown, its power reference value and its inner loop current reference value I are changed. d ref I q ref The relationship between them is:

[0050]

[0051]

[0052] In the formula, V d This represents the d-axis component of the voltage vector. The converter controller calculates the corresponding inner loop current reference value in real time based on the power reference value, and uses this reference value as a reference to generate a PWM wave to control the converter through inner loop current control.

[0053] For grid-type converters, the control architecture is as follows: Figure 5 As shown, it uses the Pf and QV droop control method to generate the voltage amplitude reference value V. ref With frequency reference value ω ref The relationship between its power reference value and voltage and frequency reference values ​​is as follows:

[0054] ω ref =ω * -m p (PP ref (4)

[0055] V ref =V * -m q (QQ ref (5)

[0056] In the formula, ω * With V * For the rated frequency and voltage, m p With m q Here, P and Q are the frequency and voltage droop control coefficients, respectively, and the active and reactive power outputs of the converter are P and Q, respectively. They can be calculated using the following formula:

[0057]

[0058]

[0059] In the formula, V d With V q I represents the voltage components along the d and q axes. d with I qThese are the current components along the d and q axes.

[0060] exist Figure 3 As shown, after issuing the converter reactive power adjustment command at time t1, and then after a delay of time τ2, waiting for the system to recover from the disturbance caused by the reactive power change to steady state at time t2, a second sampling is performed. The sampled values ​​are the effective values ​​of the current output voltage and current V2 and I2, the active power P2 and reactive power Q2 output by the converter, and the instantaneous voltage signal u2 at a certain time interval. After sampling, the reactive power output of the converter is adjusted to the initial value.

[0061] (4) Based on the sampled values ​​at time t1 and time t2, calculate the power factor angle and the phase angle difference of the voltage signal before and after reactive power adjustment at the two sampling times respectively; and calculate the grid-connected resistance and reactance of the converter according to the power factor angle and the phase angle difference.

[0062] In this embodiment, the power factor angles φ1 and φ2 at two sampling times are first calculated using the sampled output power signal:

[0063]

[0064] Then, by comparing the zero-crossing points of the two recorded instantaneous voltage signals u1 and u2, the phase angle difference of the voltage signals before and after reactive power adjustment is obtained. At this point, the grid-connected resistance and reactance of the converter can be calculated as follows:

[0065]

[0066]

[0067] In the formula, R eq-inv With L eq-inv V1, I1, and φ1 are the grid-connected resistance and inductance of the converter, respectively, and ω is the system frequency. V1, I1, and φ1 are the RMS values ​​of the converter output voltage and current and the power factor angle sampled at time t1, respectively; V2, I2, and φ2 are the RMS values ​​of the converter output voltage and current and the power factor angle sampled at time t2, respectively. This represents the phase angle difference of the voltage signal before and after reactive power adjustment.

[0068] based on Figure 2 The effectiveness of the proposed method is verified by the given converter grid-connected system. It is assumed that the grid-connected resistance of the converter decreases from 1.5Ω to 1Ω in 1.5s, and the grid-connected inductance decreases from 3mH to 2mH. Figure 6 and Figure 7The resistance estimation results for grid-connected and grid-connected converters are presented separately. Simulation results show that the proposed method has good estimation performance for the grid-connected impedance of both grid-connected and grid-connected converters. This is because the proposed method achieves this by sampling the effective values ​​of voltage and current instead of the d-axis and q-axis components, thereby eliminating the requirement for decoupling control of the active and reactive power outputs of the converter. Furthermore, this improvement makes the proposed method applicable to circuits with any impedance ratio, as shown in Table 1.

[0069] Table 1 shows the estimation performance of the proposed method under different impedance ratios.

[0070]

[0071] As can be seen from Table 1, the proposed method has good estimation performance in systems with different impedance ratios.

[0072] Example 2

[0073] In one or more embodiments, an impedance estimation system applicable to grid-connected and grid-connected converters is disclosed, comprising:

[0074] The grid-connected impedance monitoring module is configured to monitor whether the grid-connected impedance changes based on fluctuations in the amplitude of the converter output voltage.

[0075] The sampling module is configured to, when a change in grid-connected impedance is detected, sample and acquire the effective values ​​of voltage and current, active power and reactive power output by the converter at the current time t1, as well as the instantaneous voltage value signal for the set time period, after a delay of a set time; maintain the active power reference value of the converter unchanged, and adjust the reactive power reference value of the converter; after a delay of a set time, sample and acquire the effective values ​​of voltage and current, active power and reactive power output by the converter at the current time t2, as well as the instantaneous voltage value signal for the set time period again;

[0076] The grid-connected impedance estimation module is used to calculate the power factor angle and the phase angle difference of the voltage signal before and after reactive power adjustment at the two sampling times t1 and t2, respectively, based on the sampled values ​​at the two sampling times; and to calculate the grid-connected resistance and reactance of the converter based on the power factor angle and the phase angle difference.

[0077] Specifically, the grid-connected impedance is monitored in real time based on the fluctuation of the converter output voltage amplitude. This involves calculating the voltage sensitivity coefficient based on the effective voltage value at the converter outlet and the effective voltage value of the converter's rated voltage. When the voltage sensitivity coefficient exceeds a set threshold, the grid-connected impedance is considered to have changed.

[0078] The specific implementation methods of the above modules are the same as those in Example 1, and will not be described in detail again.

[0079] In one or more embodiments, a terminal device is disclosed, including a server. The server includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, it implements the impedance estimation method applicable to grid-connected and grid-connected converters as described in Embodiment 1. For simplicity, further details are omitted here.

[0080] It should be understood that in this embodiment, the processor can be a central processing unit (CPU), or it can be other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or any conventional processor, etc.

[0081] Memory may include read-only memory and random access memory, and provides instructions and data to the processor. A portion of memory may also include non-volatile random access memory. For example, memory may also store information about the device type.

[0082] In the implementation process, each step of the above method can be completed by the integrated logic circuits in the processor hardware or by software instructions.

[0083] Example 4

[0084] In one or more embodiments, a computer-readable storage medium is disclosed, wherein a plurality of instructions are stored, the instructions being adapted to be loaded by a processor of a terminal device and executed by the impedance estimation method applicable to grid-connected and grid-connected converters as described in Embodiment 1.

[0085] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. An impedance estimation method applicable to grid-connected and grid-connected converters, characterized in that, include: Monitor whether the grid connection impedance changes based on the fluctuation of the converter output voltage amplitude; When a change in grid impedance is detected, after a set delay, the effective values ​​of voltage and current, active power and reactive power of the converter output at the current time t1, as well as the instantaneous voltage signal for the set time period are sampled and obtained. Maintain the active power reference value of the converter unchanged, and adjust the reactive power reference value of the converter; after a set delay, sample again to obtain the effective values ​​of voltage and current, active power and reactive power output by the converter at the current time t2, as well as the instantaneous voltage signal for the set time period. Based on the sampled values ​​at times t1 and t2, the power factor angle and the phase angle difference of the voltage signal before and after reactive power adjustment are calculated at the two sampling times respectively; and the grid-connected resistance and reactance of the converter are calculated based on the power factor angle and the phase angle difference.

2. The impedance estimation method applicable to grid-connected and grid-connected converters as described in claim 1, characterized in that, The grid-connected impedance is monitored in real time based on the fluctuation of the converter output voltage amplitude. Specifically, the voltage sensitivity coefficient is calculated based on the effective voltage value at the converter outlet and the effective voltage value of the converter rated voltage. When the voltage sensitivity coefficient exceeds the set threshold, the grid-connected impedance is considered to have changed.

3. The impedance estimation method applicable to grid-connected and grid-connected converters as described in claim 1, characterized in that, The power factor angle at two sampling times is calculated as follows: Where Q is the reactive power at the sampling time, and P is the active power at the sampling time.

4. The impedance estimation method applicable to grid-connected and grid-connected converters as described in claim 1, characterized in that, By comparing the zero-crossing points of the two recorded instantaneous voltage signals u1 and u2, the phase angle difference of the voltage signals before and after reactive power adjustment is obtained.

5. The impedance estimation method applicable to grid-connected and grid-connected converters as described in claim 1, characterized in that, The grid-connected resistance R of the computing current transformer is calculated eq-inv , specifically: Where V1, I1, and φ1 are the effective values ​​of voltage and current and the power factor angle of the converter output sampled at time t1, respectively; V2, I2, and φ2 are the effective values ​​of voltage and current and the power factor angle of the converter output sampled at time t2, respectively. This represents the phase angle difference of the voltage signal before and after reactive power adjustment.

6. The impedance estimation method applicable to grid-connected and grid-connected converters as described in claim 1, characterized in that, The specific calculation of the converter's reactance is as follows: Where V1, I1, and φ1 are the effective values ​​of voltage and current and the power factor angle of the converter output sampled at time t1, respectively; V2, I2, and φ2 are the effective values ​​of voltage and current and the power factor angle of the converter output sampled at time t2, respectively. This represents the phase angle difference of the voltage signal before and after reactive power adjustment.

7. An impedance estimation system applicable to grid-connected and grid-connected converters, characterized in that, include: The grid-connected impedance monitoring module is configured to monitor whether the grid-connected impedance changes based on fluctuations in the amplitude of the converter output voltage. The sampling module is configured to, when a change in grid-connected impedance is detected, sample and acquire the effective values ​​of voltage and current, active power and reactive power output by the converter at the current time t1, as well as the instantaneous voltage value signal for the set time period, after a delay of a set time; maintain the active power reference value of the converter unchanged, and adjust the reactive power reference value of the converter; after a delay of a set time, sample and acquire the effective values ​​of voltage and current, active power and reactive power output by the converter at the current time t2, as well as the instantaneous voltage value signal for the set time period again; The grid-connected impedance estimation module is used to calculate the power factor angle and the phase angle difference of the voltage signal before and after reactive power adjustment at the two sampling times t1 and t2, respectively, based on the sampled values ​​at the two sampling times; and to calculate the grid-connected resistance and reactance of the converter based on the power factor angle and the phase angle difference.

8. The impedance estimation system applicable to grid-connected and grid-connected converters as described in claim 7, characterized in that, The grid-connected impedance is monitored in real time based on the fluctuation of the converter output voltage amplitude. Specifically, the voltage sensitivity coefficient is calculated based on the effective voltage value at the converter outlet and the effective voltage value of the converter rated voltage. When the voltage sensitivity coefficient exceeds the set threshold, the grid-connected impedance is considered to have changed.

9. A terminal device comprising a processor and a memory, the processor for implementing instructions; the memory for storing multiple instructions, characterized in that, The instructions are adapted to be loaded by a processor and executed by the impedance estimation method applicable to grid-connected and grid-connected converters as described in any one of claims 1-6.

10. A computer-readable storage medium storing a plurality of instructions, characterized in that, The instructions are adapted to be loaded by the processor of the terminal device and executed by the impedance estimation method applicable to grid-connected and grid-connected converters as described in any one of claims 1-6.

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