Power grid impedance measurement method and apparatus

Abstract

Disclosed in the present invention are a power grid impedance measurement method and apparatus. The method comprises: generating a characteristic harmonic current instruction on the basis of a harmonic-signal generation algorithm, and inputting the characteristic harmonic current instruction into a direct-current voltage feedback control loop; receiving the characteristic harmonic current instruction to generate a control signal, and performing current tracking control to obtain a harmonic current to be injected; converting an information electronic signal of a main circuit into a driving signal in a device control loop, and selecting a preset harmonic injection mode in real time on the basis of a harmonic injection target frequency; when active harmonic injection is selected, controlling the turning-on and turning-off of a switch tube, and injecting said harmonic current into a power grid; when passive harmonic injection is selected, controlling the turning-on and turning-off of an IGBT, controlling a time and a current flow direction of when a capacitor is connected to the power grid for operation, and injecting said harmonic current into the power grid; and selecting voltage and current signals to perform signal decomposition, processing and analysis, so as to obtain a power grid impedance. By means of the present embodiment, a power grid impedance is effectively measured, the disturbance to a power grid during impedance measurement is reduced, and the measurement precision is improved.

Description

A method and device for measuring power grid impedance Technical Field

[0001] The present invention relates to the field of electronic measurement technology, and in particular to a method and device for measuring power grid impedance. Background Art

[0002] The widespread use of power electronics in power grids, particularly the integration of renewable energy grid-connected power generation devices, has made grid impedance characteristics more complex than ever before. To ensure power system stability, various grid-connected devices must adjust control parameters based on grid impedance characteristics, making online measurement of grid impedance particularly important. In power systems, the performance and resonance suppression of grid-connected inverters are closely related to grid impedance characteristics, which vary over time and with grid operating conditions. Therefore, accurate grid impedance measurement is a key technology for achieving high-performance adaptive control of grid-connected inverters in weak grid conditions.

[0003] Existing grid impedance measurement methods fall into two categories: passive and active. Due to the high computational complexity and low accuracy of passive methods, active methods are more commonly used in practical grid impedance measurement. Active methods inject characteristic harmonics into the grid and then measure the characteristic harmonic voltage and current at the common coupling point between the grid and the converter to analyze the grid impedance. Common active grid impedance measurement methods include the capacitor switching method, the thyristor branch switching method, and the harmonic current source injection method. Each method has its own unique characteristics, advantages, and disadvantages.

[0004] The capacitor switching method, which uses capacitor switching for harmonic impedance measurement, utilizes the harmonic current injected into the grid when capacitors are switched at substations and transformer substations. This method eliminates the need for specialized harmonic current injection equipment; the measurement process simply records the harmonic current and voltage waveforms before and after the capacitors are switched. Since capacitor switching is a routine grid operation, it minimally disrupts the grid. However, since the substation's shunt capacitors are used, whose capacity is set based on reactive power compensation parameters, insufficient capacitor capacity may result in insufficient harmonic current injection. This insufficient harmonic current injection reduces the accuracy of the harmonic impedance measurement. The thyristor branch switching method uses thyristor-controlled branches, allowing for convenient control of the on-time. This allows for the magnitude of the injected harmonic current, and therefore the impact on the grid, to be controlled. This achieves the optimal balance between ensuring harmonic impedance measurement accuracy and minimizing grid disturbance. Furthermore, measurement conditions can be adjusted at any time by varying the thyristor on-time as network conditions change. Both the capacitor switching method and the thyristor branch switching method use the harmonic current injected when the capacitor branch or thyristor branch is switched as the basic measurement condition. Both methods have a disadvantage, that is, the harmonic content is incomplete and uncontrollable.

[0005] The harmonic current source injection method allows the harmonic current source to be controlled at all harmonic frequencies and amplitudes. This prevents inaccurate harmonic measurements caused by under-inflated harmonic currents within certain harmonic ranges, resulting in extremely high measurement accuracy. However, this method requires a specific harmonic current source, and in large, operating power systems, it is difficult to allow large amounts of harmonic current injection to disrupt stable operation. Furthermore, the harmonic current content of passive harmonic injection varies significantly across frequency ranges, affecting measurement accuracy. Active harmonic injection, on the other hand, can significantly interfere with the power grid and increase voltage distortion.

[0006] Summary of the Invention

[0007] The present invention provides a method and device for measuring power grid impedance, which can effectively measure power grid impedance, reduce disturbance of the impedance measurement process to the power grid, and improve measurement accuracy.

[0008] In order to solve the above technical problems, an embodiment of the present invention provides a grid impedance measurement method, comprising:

[0009] When the grid impedance measurement device is connected to the grid and operates stably, a characteristic harmonic current command is generated according to the harmonic signal generation algorithm, and the characteristic harmonic current command is input into the DC voltage feedback control loop; wherein, the DC voltage feedback control loop detects the DC voltage in real time to maintain voltage stability;

[0010] A characteristic harmonic current command is received in a DC voltage feedback control loop, a control signal is generated, and current tracking control is performed on the DC voltage feedback control loop based on a voltage-current dual closed-loop structure according to the characteristic harmonic current command and the control signal to obtain a harmonic current to be injected; wherein the harmonic current to be injected is a current of a specific amplitude and a specific frequency;

[0011] According to the received control signal, the information electronic signal of the main circuit is converted into the driving signal of the device control loop, and according to the harmonic injection target frequency, the preset harmonic injection method is selected in real time; wherein, the harmonic injection method includes passive harmonic injection method and active harmonic injection method;

[0012] When the active harmonic injection mode is selected, the switch is controlled to be turned on and off according to the drive signal and the first specific sequence, and the harmonic current to be injected is injected into the grid at the common coupling point;

[0013] When the passive harmonic injection mode is selected, the IGBT is controlled to be turned on and off according to the driving signal and the second specific sequence, and the time and current flow direction of the capacitor being put into the grid are controlled, and the harmonic current to be injected is injected into the grid at the common coupling point;

[0014] After injecting the harmonic current to be injected into the power grid, the voltage and current signals of the port of the branch to be measured in the multi-point voltage and current signals of the power grid are selected for signal decomposition processing and analysis to obtain the power grid impedance of the port of the branch to be measured; wherein, the multi-point voltage and current signals of the power grid are obtained by real-time acquisition of voltage and current at different locations in the power grid.

[0015] In the implementation of the embodiment of the present invention, when the grid impedance measuring device is connected to the grid for stable operation, a characteristic harmonic current instruction is generated according to the harmonic signal generation algorithm, and the characteristic harmonic current instruction is input into the DC voltage feedback control loop; wherein, the DC voltage feedback control loop detects the DC voltage in real time to keep the voltage stable; the characteristic harmonic current instruction is received in the DC voltage feedback control loop, and a control signal is generated; according to the characteristic harmonic current instruction and the control signal, the DC voltage feedback control loop is subjected to current tracking control based on a voltage-current dual closed-loop structure to obtain the harmonic current to be injected; wherein, the harmonic current to be injected is a current of a specific amplitude and a specific frequency; according to the received control signal, the information electronic signal of the main circuit is converted into a drive signal of the device control loop, and according to the harmonic injection target frequency, the preset harmonic injection mode is selected in real time. ; Among them, the harmonic injection method includes a passive harmonic injection method and an active harmonic injection method; when the active harmonic injection method is selected, the opening and closing of the switch tube is controlled according to the driving signal and the first specific sequence, and the harmonic current to be injected is injected into the power grid at the common coupling point; when the passive harmonic injection method is selected, the opening and closing of the IGBT is controlled according to the driving signal and the second specific sequence, and the time and current flow direction of the capacitor being put into the power grid for operation are controlled, and the harmonic current to be injected is injected into the power grid at the common coupling point; after the harmonic current to be injected is injected into the power grid, the voltage and current signals of the port of the branch to be measured in the multi-point voltage and current signals of the power grid are selected for signal decomposition processing and analysis to obtain the power grid impedance of the port of the branch to be measured; wherein, the multi-point voltage and current signals of the power grid are obtained by real-time acquisition of voltages and currents at different positions in the power grid. The grid impedance measurement device implements a signal generation-reception-analysis process, generating a specific harmonic signal based on a harmonic signal injection algorithm. The harmonic signal is then collected and separated at other locations on the grid. Based on the separated specific harmonic voltage and current signals, the grid impedance at the port of the branch to be measured is analyzed and calculated, thereby achieving grid impedance measurement. By selecting preset passive and active harmonic injection methods in real time, this grid impedance measurement method combines the complementary advantages of active and passive distribution network impedance measurement, achieving an organic combination of passive and active harmonic injection. This significantly reduces disturbances to the grid during the impedance measurement process, avoids grid voltage distortion, and improves measurement accuracy across the entire frequency range.

[0016] As a preferred solution, a preset harmonic injection method is selected according to the harmonic injection target frequency, specifically:

[0017] When the harmonic injection target frequency is lower than the first frequency, the current target frequency band is determined to be a low frequency band, and the active harmonic injection mode is selected;

[0018] When the harmonic injection target frequency is higher than the first frequency and lower than the second frequency, the current target frequency band is determined to be the mid-frequency band, and the passive harmonic injection method is selected;

[0019] When the harmonic injection target frequency is higher than the second frequency, the current target frequency band is determined to be a high frequency band, and the active harmonic injection mode is selected.

[0020] As a preferred solution, according to the driving signal and the first specific sequence, the switching tube is controlled to be turned on and off, specifically:

[0021] Performing voltage space vector pulse width modulation on the driving signal to determine a time-varying on / off sequence of the switch tube to obtain a first specific sequence; wherein the voltage space vector pulse width modulation includes coordinate transformation, sector and region determination, basic voltage space vector action time calculation, and basic voltage space vector action sequence planning;

[0022] The switching tube is controlled to be turned on and off based on the first specific sequence.

[0023] As a preferred solution, according to the driving signal and the second specific sequence, the IGBT is controlled to be turned on and off, and the time when the capacitor is put into operation in the grid and the direction of current flow are controlled, specifically:

[0024] Performing voltage space vector pulse width modulation on the driving signal to determine the on-off sequence of the IGBT that changes with time, thereby obtaining a second specific sequence;

[0025] controlling the turning on and off of the IGBT based on a second specific sequence;

[0026] According to the on-time of the IGBT, the time for the capacitor to be put into operation in the grid is controlled, and the amplitude of the harmonic current to be injected is controlled;

[0027] According to the changes in the on-off state of the IGBT, the direction of the current flow of the capacitor running in the power grid is controlled.

[0028] As a preferred solution, a characteristic harmonic current instruction is generated according to a harmonic signal generation algorithm, specifically:

[0029] In the inverter SVPWM modulation, the frequency of the harmonic signal is changed according to the harmonic injection target frequency. By setting the change rule of the modulation wave, a modulation wave frequency spectrum is generated in which the harmonic amplitude gradually increases from low frequency to the harmonic injection target frequency, and the injected harmonic signal is obtained. Based on the injected harmonic signal, the characteristic harmonic current instruction is generated.

[0030] As a preferred solution, the voltage and current signals of the port of the branch to be measured in the multi-point voltage and current signals of the power grid are selected for signal decomposition processing and analysis to obtain the power grid impedance of the port of the branch to be measured, specifically:

[0031] Obtaining the node data to be measured according to the voltage and current signals of the port of the branch to be measured in the multi-point voltage and current signals of the power grid;

[0032] Perform fast Fourier transform on the node data to be tested to obtain frequency domain data;

[0033] Extract the amplitude and phase information of the frequency domain data, and convert the three-phase voltage and current into positive and negative sequence voltage and current according to the amplitude and phase information of the frequency domain data;

[0034] The positive and negative sequence voltages and currents are used to calculate the grid impedance to obtain the grid impedance of the port of the branch to be measured. The grid impedance includes positive sequence impedance and negative sequence impedance. The formula for calculating the grid impedance is:

[0035] Among them, Z pos is the positive sequence impedance, Z neg is the negative sequence impedance, U pos is the positive sequence harmonic voltage, U neg is the negative sequence harmonic voltage, I pos is the positive sequence harmonic current, I neg is the negative sequence harmonic current, f k is the harmonic frequency.

[0036] As a preferred solution, current tracking control is specifically as follows:

[0037] The output of the fuzzy PI parameter self-tuning controller is multiplied by the three-phase voltage, and the multiplication result is added to the three-phase voltage control target to perform closed-loop control on the DC voltage and track the control current;

[0038] Among them, the fuzzy PI parameter self-tuning controller is constructed by online optimizing and adjusting the control parameters of the PI regulator through the fuzzy control algorithm; the three-phase voltage control target is obtained by coordinate transformation of the output result of the fuzzy PI parameter self-tuning controller.

[0039] As a preferred solution, the fuzzy PI parameter self-tuning controller is constructed by online optimizing and adjusting the control parameters of the PI regulator through the fuzzy control algorithm, specifically:

[0040] By calculating the current error and the current error change rate, fuzzy reasoning is performed using fuzzy rules to output the change in the control parameters of the PI regulator;

[0041] In the process of fuzzy reasoning, the current error, current error change rate and parameter change of the fuzzy PI parameter self-tuning controller are divided into several fuzzy subsets respectively, and the membership function adopts the triangular function.

[0042] The control parameters of the PI regulator are corrected, and the control parameter changes are superimposed on the initial control parameters to obtain the control parameters of the current working conditions.

[0043] In order to solve the same technical problem, an embodiment of the present invention further provides a grid impedance measurement device, comprising: a voltage and current acquisition module, a harmonic signal injection module, a DC voltage control module, a current tracking control module, a drive module, a power electronic conversion module, a passive harmonic injection module and a grid impedance calculation module;

[0044] The connections of each module are as follows: the voltage and current acquisition module is connected to the current tracking control module, the grid impedance calculation module and the grid respectively; the current tracking control module is connected to the harmonic signal injection module, the drive module and the DC voltage control module respectively; the drive module is connected to the power electronic conversion module and the passive harmonic injection module respectively; the output end of the power electronic conversion module is connected to the grid through the three-phase LCL passive filter;

[0045] The harmonic signal injection module is used to generate characteristic harmonic current instructions according to the harmonic signal generation algorithm when the grid impedance measurement device is connected to the grid and operates stably, and input the characteristic harmonic current instructions into the DC voltage feedback control loop;

[0046] The DC voltage control module is used to build a DC voltage feedback control loop, which detects the DC voltage in real time to keep the voltage stable.

[0047] The current tracking control module is used to receive a characteristic harmonic current command in a DC voltage feedback control loop, generate a control signal, and perform current tracking control on the DC voltage feedback control loop based on a voltage-current dual closed-loop structure according to the characteristic harmonic current command and the control signal to obtain a harmonic current to be injected; wherein the harmonic current to be injected is a current of a specific amplitude and a specific frequency;

[0048] The driving module is used to convert the information electronic signal of the main circuit into the driving signal of the device control loop according to the received control signal, and select the preset harmonic injection mode in real time according to the harmonic injection target frequency; wherein the harmonic injection mode includes passive harmonic injection mode and active harmonic injection mode;

[0049] The power electronic conversion module is used to control the on and off of the switch tube according to the drive signal and the first specific sequence when the active harmonic injection mode is selected, so as to inject the harmonic current to be injected into the power grid at the common coupling point;

[0050] The passive harmonic injection module is used to control the on and off of the IGBT according to the drive signal and the second specific sequence when the passive harmonic injection mode is selected, control the time and current flow direction of the capacitor when it is put into the power grid, and inject the harmonic current to be injected into the power grid at the common coupling point;

[0051] The grid impedance calculation module is used to inject the harmonic current to be injected into the grid, select the voltage and current signals of the port of the branch to be measured from the multi-point voltage and current signals of the grid, perform signal decomposition processing and analysis, and obtain the grid impedance of the port of the branch to be measured;

[0052] The voltage and current acquisition module is used to obtain multi-point voltage and current signals of the power grid by real-time acquisition of voltage and current at different locations in the power grid.

[0053] As a preferred solution, the voltage and current acquisition module includes a voltage transformer and a current transformer;

[0054] The harmonic signal injection module is constructed and generated by the harmonic signal generation algorithm;

[0055] The DC voltage control module is generated by constructing a DC voltage feedback control loop;

[0056] The current tracking control module consists of a voltage and current double closed loop structure, and the controller adopts a fuzzy PI parameter self-tuning controller;

[0057] The driving module is composed of a driving circuit;

[0058] The power electronic conversion module consists of two parts: the DC side energy storage capacitor and the voltage type inverter. The voltage type inverter adopts a three-phase H-bridge structure.

[0059] The passive harmonic injection module consists of an IGBT and a capacitor connected;

[0060] The grid impedance calculation module includes FFT calculation link, positive and negative sequence conversion link and positive and negative sequence impedance calculation link. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] FIG1 is a flow chart of an embodiment of a method for measuring grid impedance provided by the present invention;

[0062] FIG2 is a diagram showing a specific implementation scheme of an impedance measuring device according to an embodiment of a method for measuring grid impedance provided by the present invention;

[0063] FIG3 is a diagram showing the connection between an impedance measuring device and a power grid according to an embodiment of a power grid impedance measurement method provided by the present invention;

[0064] FIG4 is a schematic diagram of a fuzzy PI parameter self-tuning controller according to an embodiment of a grid impedance measurement method provided by the present invention;

[0065] FIG5 is a schematic diagram showing the connection structure of an impedance measuring device according to an embodiment of the present invention. DETAILED DESCRIPTION

[0066] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0067] Example 1

[0068] Please refer to Figure 1, which is a flow chart of a grid impedance measurement method provided in an embodiment of the present invention. The grid impedance measurement method of this embodiment is applicable to a grid impedance measurement device. This embodiment combines active harmonic injection and passive harmonic injection methods to complement the advantages of active and passive distribution network impedance measurement, effectively measuring grid impedance, reducing the disturbance to the grid during the impedance measurement process, and improving measurement accuracy. The grid impedance measurement method includes steps 101 to 106, each of which is specifically as follows:

[0069] Step 101: When the grid impedance measurement device is connected to the grid and operates stably, a characteristic harmonic current instruction is generated according to a harmonic signal generation algorithm, and the characteristic harmonic current instruction is input into a DC voltage feedback control loop; wherein the DC voltage feedback control loop detects the DC voltage in real time to maintain voltage stability.

[0070] In this embodiment, a grid impedance measurement device implements a grid impedance measurement method. A specific implementation diagram of the impedance measurement device is shown in FIG2 . The grid impedance measurement device is connected in parallel between the grid and the load and includes a voltage and current acquisition module, a harmonic signal injection module, a DC voltage control module, a current tracking control module, a drive module, a power electronics conversion module, a passive harmonic injection module, and a grid impedance calculation module. Each module collaborates to complete the grid impedance measurement. The distribution network impedance measurement device, which complements the active and passive grids, is equivalent to a signal generation, reception, and analysis device. It can generate characteristic harmonic signals based on a harmonic signal generation algorithm and inject the characteristic harmonic signals into the grid at a common coupling point. The harmonic signals are then collected, processed, and decomposed at multiple locations in the grid where impedance characteristics are to be measured. Based on the decomposed characteristic harmonic voltage and current signals, the impedance characteristics of the node are analyzed and calculated, thereby enabling impedance characteristic measurements of different branches in the grid. For ease of control, the entire control system is implemented in a two-phase synchronously rotating coordinate system, requiring coordinate transformation. The modules and types of coordinate transformation are shown in FIG2 . The angle information required for coordinate transformation is calculated by a phase-locked loop (PLL). A three-phase LCL passive filter is connected between the output end of the power electronic conversion module and the power grid to filter out useless harmonic signals.

[0071] It should be noted that the grid impedance measurement device achieves stable grid-connected operation by completing grid voltage phase locking, charging the DC-side energy storage unit, and dynamically balancing energy exchange between the grid and DC sides. After stable grid-connected operation, it completes current tracking control and DC voltage control. Figure 3 shows the connection diagram of the impedance measurement device and the grid, illustrating the topology of the grid and impedance measurement device, as well as the direction of current flow. FPGAs are superior to single-chip microcontrollers in terms of chip capacity, combinational logic, operating speed, and design flexibility, so the impedance measurement device's control process is implemented using FPGAs. The impedance measurement device consists of two main components: an active impedance meter and a passive impedance meter. The device is connected to the grid using a parallel topology, with the device connected as a whole in parallel between the grid and the load. The grid and impedance measurement device each generate grid current and harmonic injection current. At the common coupling point between the grid and the impedance measurement device, the grid current and harmonic injection current converge to form the load current.

[0072] In this embodiment, the DC voltage control module continuously monitors the DC voltage of the power electronics conversion module and ensures voltage stability by constructing a DC voltage control loop. During the design of the DC voltage control module, an optimized DC voltage control method was developed based on the concept of inverter modulation. By performing closed-loop control of the DC voltage, the output of the fuzzy PI regulator is multiplied by the three-phase voltage and the result is superimposed on the three-phase voltage obtained in the previous step. This indirectly changes the modulation of the inverter process in the power electronics conversion module, thereby simplifying the DC voltage control process and improving the DC voltage control effect. A harmonic signal injection instruction is also superimposed on the output of the DC voltage control module and the coordinate transformation module.

[0073] Optionally, a characteristic harmonic current instruction is generated according to a harmonic signal generation algorithm, specifically:

[0074] In the inverter SVPWM modulation, the frequency of the harmonic signal is changed according to the harmonic injection target frequency. By setting the change rule of the modulation wave, a modulation wave frequency spectrum is generated in which the harmonic amplitude gradually increases from low frequency to the harmonic injection target frequency, and the injected harmonic signal is obtained. Based on the injected harmonic signal, the characteristic harmonic current instruction is generated.

[0075] In this embodiment, the harmonic signal injection module generates a characteristic harmonic current command with a specific waveform and frequency based on a harmonic signal generation algorithm and inputs this command into the DC voltage feedback control loop. As an example of this embodiment, the harmonic signal injection module is implemented using a DDS algorithm within an FPGA controller. Based on the harmonic signal injection algorithm, a harmonic signal with a specific waveform and frequency can be generated. By outputting this command signal to the current tracking control module, the harmonic signal is injected into the power grid.

[0076] It should be noted that the frequency of the harmonic injection signal is usually fixed. This technical solution injects a variable-frequency harmonic signal into the power grid by changing the frequency of the harmonic injection signal as the current tracking control target, and then obtains the power grid impedance by collecting and calculating these harmonic signals in the power grid voltage and current. Existing power grid impedance measurement methods are more about analyzing the power grid impedance by injecting characteristic harmonics into the power grid and measuring the voltage and current of the characteristic harmonics at both ends of the power grid. The injected characteristic harmonics include single harmonic injection, several harmonic injections, and broadband harmonic injection. In the single harmonic injection method, a typical example is to select 75Hz harmonic current injection. This harmonic is close to the power frequency 50Hz, and there is almost no interference signal of this harmonic in the power grid, so the detection accuracy of the power frequency power grid impedance is high. However, the power grid impedance is often not a first-order system or even nonlinear. Therefore, the use of a harmonic injection method with multiple frequencies can more accurately obtain the relationship between the power grid impedance and frequency. The injected harmonic signal has a variable frequency. In the inverter's SVPWM modulation, the harmonic signal frequency is varied to fluctuate around a set frequency. By setting the modulation wave's variation pattern, a modulation wave frequency spectrum is obtained, in which the harmonic amplitude gradually increases from low frequency to the set frequency. This frequency-varying harmonic signal serves as the modulation wave for SVPWM modulation. It is modulated with the carrier generated by the inverter controller to generate a pulse signal to control the inverter's switches. The inverter's output side, thus controlled, also contains harmonic signals of various frequencies. The inverter's output voltage and current signals are sampled and analyzed to determine the grid impedance at various frequencies.

[0077] Step 102: Receive a characteristic harmonic current command in a DC voltage feedback control loop, generate a control signal, and perform current tracking control on the DC voltage feedback control loop based on a voltage-current dual closed-loop structure according to the characteristic harmonic current command and the control signal to obtain a harmonic current to be injected; wherein the harmonic current to be injected is a current of a specific amplitude and a specific frequency.

[0078] In this embodiment, the current tracking control module receives the characteristic harmonic current command from the harmonic signal injection module, generates a control signal for the power electronics conversion module, completes the tracking control process of the command current, and ultimately controls the power electronics conversion module to inject the characteristic harmonic current into the grid's common coupling point. The characteristic harmonic current command (harmonic signal command) serves as the target of current tracking control, used to control the power electronics conversion module to inject harmonic currents of specific amplitude and frequency into the grid.

[0079] It should be noted that open-loop control suffers from low control accuracy due to the lack of observation and feedback of control results. To improve control accuracy and response speed, closed-loop control is adopted in the current tracking control module. To enhance the system's anti-interference capability, the PI regulator is introduced into conventional control systems. However, since constant-coefficient PI regulation is inherently a linear control, it does not achieve good results when used to control complex, nonlinear power systems. The fuzzy PI parameter self-tuning controller consists of two modules: a fuzzy inference module and a PI regulator. It combines the fuzzy control algorithm with the PI regulator and uses the fuzzy control algorithm to online optimize and adjust the control parameters of the PI regulator, enabling the control system to better resist disturbances and comply with given conditions. The fuzzy inference module calculates the current error and its rate of change, performs fuzzy reasoning using fuzzy rules, and outputs the change in the control parameters of the PI regulator. The principle of the fuzzy PI parameter self-tuning controller is shown in Figure 4. The fuzzy inference module calculates the current error and its rate of change, performs fuzzy reasoning using fuzzy rules, and outputs the change in the control parameters of the PI regulator.

[0080] Optional current tracking control, specifically: multiplying the output of the fuzzy PI parameter self-tuning controller with the three-phase voltage, and adding the multiplication result to the three-phase voltage control target, performing closed-loop control on the DC voltage, and tracking the control current;

[0081] Among them, the fuzzy PI parameter self-tuning controller is constructed by online optimizing and adjusting the control parameters of the PI regulator through the fuzzy control algorithm; the three-phase voltage control target is obtained by coordinate transformation of the output result of the fuzzy PI parameter self-tuning controller.

[0082] In this embodiment, based on the concept of inverter modulation, a DC voltage control method is optimized. By performing closed-loop control on the DC voltage, the output of a fuzzy PI parameter self-tuning controller (fuzzy PI regulator) is multiplied by the three-phase voltages, and the result is superimposed on the three-phase voltage control target obtained in the previous step. The output of the fuzzy PI parameter self-tuning controller is transformed to obtain the three-phase voltage control target in a three-phase stationary coordinate system. Based on this, the DC voltage control signal and the harmonic injection signal are superimposed and output to the voltage space vector pulse width modulation stage to generate the drive signal.

[0083] By implementing the embodiment of the present invention, the above-mentioned fuzzy PI parameter self-tuning control process is performed, which in disguise changes the modulation degree of the inversion process in the power electronic conversion module, thereby simplifying the DC voltage control process and improving the control effect of the DC voltage. The fuzzy control algorithm is combined with the PI regulator, and the control parameters of the PI regulator are optimized and adjusted online through the fuzzy control algorithm.

[0084] Optionally, a fuzzy PI parameter self-tuning controller is constructed by online optimizing and adjusting the control parameters of the PI regulator through a fuzzy control algorithm, specifically:

[0085] By calculating the current error and the current error change rate, fuzzy reasoning is performed using fuzzy rules to output the change in the control parameters of the PI regulator. During the fuzzy reasoning process, the current error, current error change rate and parameter change of the fuzzy PI parameter self-tuning controller are divided into several fuzzy subsets, and the membership function adopts a triangular function. The control parameters of the PI regulator are corrected, and the control parameter changes are superimposed on the initial control parameters to obtain the control parameters of the current working conditions.

[0086] In this embodiment, the fuzzy reasoning module calculates the current error and its rate of change, performs fuzzy reasoning using fuzzy rules, and then outputs the control parameter change of the PI regulator; in this process, the input error E and error change rate EC of the fuzzy PI parameter self-tuning controller and the output PI parameter change are divided into 7 fuzzy subsets {NB, NM, NS, Z, PS, PM, PB}, and the membership function adopts a triangular function, which can be a simple triangular function; the PI regulator corrects the control parameters, superimposes the control parameter change on the initial control parameters, and then obtains the control parameters suitable for the current working conditions.

[0087] Step 103: Based on the received control signal, the information electronic signal of the main circuit is converted into a drive signal of the device control loop, and a preset harmonic injection method is selected in real time according to the harmonic injection target frequency; wherein the harmonic injection method includes a passive harmonic injection method and an active harmonic injection method.

[0088] In this embodiment, the driver module receives a control signal from the current tracking control module and converts the signal from the information electronic circuit (the information electronic signal of the main circuit) into a drive signal applied to the device control loop. Based on the harmonic frequency suitable for injection by the passive harmonic injection module and the target frequency of the harmonic injection, the driver module determines whether to use passive harmonic injection or active harmonic injection. When a frequency sweep is selected to inject harmonics into the power grid from a low frequency band to a high frequency band, the active harmonic injection method is used in the low frequency band, the passive harmonic injection method is switched to in the intermediate frequency band, and the active harmonic injection method is switched back to in the high frequency band.

[0089] It should be noted that the drive module is composed of a drive circuit, which serves as an interface circuit between the main circuit and the control circuit. It converts the signal from the information electronic circuit into a drive signal added to the device control loop. The drive module receives the signal-level power electronic converter drive signal output by the current tracking control module, and uses the drive circuit to complete the signal conversion to obtain a drive-level drive signal that can drive the power electronic converter.

[0090] By implementing the embodiments of the present invention, the harmonic current content injected by the passive harmonic injection method varies greatly in different frequency ranges and affects the measurement accuracy, and the active harmonic injection method causes significant interference to the power grid and increases the power grid voltage distortion rate. The present invention adopts a power grid impedance measurement method that complements the advantages of active and passive distribution network impedance measurement. By organically combining the two methods, the disturbance of the impedance measurement process to the power grid is greatly reduced and the measurement accuracy is improved within the entire frequency band.

[0091] Optionally, select a preset harmonic injection method based on the harmonic injection target frequency, specifically:

[0092] When the harmonic injection target frequency is lower than the first frequency, the current target frequency band is determined to be a low frequency band, and the active harmonic injection mode is selected;

[0093] When the harmonic injection target frequency is higher than the first frequency and lower than the second frequency, the current target frequency band is determined to be the mid-frequency band, and the passive harmonic injection method is selected;

[0094] When the harmonic injection target frequency is higher than the second frequency, the current target frequency band is determined to be a high frequency band, and the active harmonic injection mode is selected.

[0095] In this embodiment, when harmonics are injected into the power grid in a sweeping manner from a low frequency (e.g., 10 Hz) to a high frequency (e.g., 2000 Hz), an active harmonic injection method is used in the low frequency band, a passive harmonic injection method is switched to in the intermediate frequency band, and the active harmonic injection method is switched back to in the high frequency band. Due to differences in power grid impedance characteristics in different regions, the frequency bands can be divided into low, intermediate, and high frequency bands by comprehensively considering the disturbances to the power grid caused by different harmonic injection methods and the accuracy of impedance measurement. With the goal of minimizing overall disturbance and maximizing measurement accuracy, the first and second frequencies are determined, thereby dividing the low, intermediate, and high frequency bands.

[0096] By implementing the embodiments of the present invention, the branch controlled by the switch tube can conveniently control the on-time according to the control algorithm and drive signal. This allows the magnitude of the injected harmonic current to be controlled, that is, the magnitude of the impact on the network to be controlled. Generally speaking, the larger the injected harmonic current, the higher the measurement accuracy, and the greater the impact on the network. By gradually increasing the harmonic current, when the measurement accuracy reaches the target value, the harmonic current at this time is the minimum current that impacts the network. This achieves the best balance between the two aspects, ensuring the accuracy of harmonic impedance measurement while causing the smallest possible disturbance to the power grid. Furthermore, when network conditions change, the measurement conditions can be changed at any time by changing the on-time of the switch tube.

[0097] Step 104: When the active harmonic injection mode is selected, the switching tube is controlled to be turned on and off according to the driving signal and the first specific sequence, and the harmonic current to be injected is injected into the power grid at the common coupling point.

[0098] In this embodiment, when operating in active harmonic injection mode (active harmonic injection method), the power electronic conversion module turns on and off the switches in a specific sequence according to the drive signal, injecting characteristic harmonic currents of a specific waveform and frequency into the common coupling point between the grid and the device. The power electronic conversion module consists of two parts: a DC-side energy storage capacitor and a voltage-source inverter. The voltage-source inverter adopts a three-phase H-bridge structure.

[0099] It should be noted that when active harmonic injection is required, the distribution network impedance measurement device switches to active harmonic injection mode during operation. The device first completes the grid-connected inversion process, which specifically includes locking the grid voltage phase, charging the DC-side energy storage unit, and dynamically balancing the energy exchange between the grid and DC sides. Furthermore, the current tracking control module receives grid operating parameters, control commands, and harmonic signals from the voltage and current acquisition module, the DC voltage control module, and the harmonic signal injection module. By inputting harmonic commands into the control loop, the power electronics conversion module completes the harmonic signal injection process. The driver module serves as an interface circuit, connecting the main circuit and the control circuit. Finally, the grid impedance calculation module performs fast Fourier transformation (FFT) analysis and impedance characteristic analysis and calculation on the characteristic harmonic voltages and currents at the common coupling point between the grid and the converter, as collected by the voltage and current acquisition module, thereby achieving the distribution network impedance measurement objective.

[0100] Optionally, the switching on and off of the switch tube is controlled according to the driving signal and the first specific sequence, specifically: the driving signal is subjected to voltage space vector pulse width modulation, the switching on and off sequence of the switch tube that changes with time is determined, and the first specific sequence is obtained; wherein, the voltage space vector pulse width modulation includes coordinate transformation, sector and area judgment, basic voltage space vector action time calculation and basic voltage space vector action order planning; the switching on and off of the switch tube is controlled based on the first specific sequence.

[0101] In this embodiment, the inverter SVPWM modulation process utilizes an SVPWM control algorithm. This SVPWM control algorithm (voltage space vector pulse width modulation) includes four steps: coordinate transformation, sector and region determination, basic voltage space vector action time calculation, and basic voltage space vector action sequence planning. The voltage space vector pulse width modulation process generates a time-varying on / off sequence for the switches. This on / off sequence is at the signal level, and lacks sufficient energy to effectively control the on / off switching of the switches.

[0102] It should be noted that, among inverter modulation strategies, voltage space vector pulse width modulation (SVPWM) improves voltage utilization by 15% compared to sinusoidal pulse width modulation (SPWM), resulting in superior modulation effects. The inverter modulation strategy of this method and device selects SVPWM.

[0103] Step 105: When the passive harmonic injection mode is selected, the IGBT is controlled to be turned on and off according to the drive signal and the second specific sequence, and the time and current flow direction of the capacitor being put into the grid are controlled, and the harmonic current to be injected is injected into the grid at the common coupling point.

[0104] Optionally, according to the driving signal and the second specific sequence, the IGBT is controlled to be turned on and off, and the time when the capacitor is put into operation in the power grid and the direction of current flow are controlled, specifically:

[0105] The drive signal is subjected to voltage space vector pulse width modulation to determine the on-off sequence of the IGBT that varies with time, thereby obtaining a second specific sequence; the on-off and off-off of the IGBT are controlled based on the second specific sequence; the time during which the capacitor is put into operation in the power grid is controlled according to the on-time of the IGBT, and the amplitude of the harmonic current to be injected is controlled; and the direction of current flow in the capacitor when it is put into operation in the power grid is controlled according to the change in the on-off state of the IGBT.

[0106] In this embodiment, when operating in the passive harmonic injection mode (passive harmonic injection method), the passive harmonic injection module completes the opening and closing process of the switch tube according to the driving signal in a specific sequence, controls the time when the capacitor is put into grid operation and the direction of current flow, and injects a characteristic harmonic current of a specific frequency into the common coupling point between the grid and the device.

[0107] It should be noted that when passive harmonic injection is required, the device switches to passive harmonic injection mode during operation, causing the control loop to operate according to the passive harmonic injection algorithm. During operation, by controlling the IGBT on-time, the duration of capacitor connection to the grid can be controlled, thereby controlling the amplitude of the injected harmonic current and, therefore, the impact on the grid. This achieves the optimal balance between ensuring harmonic impedance measurement accuracy while minimizing grid disturbance. Furthermore, as network conditions change, measurement conditions can be adjusted at any time by varying the switch on-time. The injected current can be adjusted based on grid noise, achieving a balance between harmonic impedance measurement accuracy and grid-injected interference. Finally, the grid impedance calculation module performs fast Fourier transformation (FFT) analysis and impedance characteristic calculation on the characteristic harmonic voltages and currents at the common coupling point between the grid and the converter, as collected by the voltage and current acquisition module, thereby achieving distribution network impedance measurement.

[0108] It should be noted that the passive harmonic injection module consists of a push-pull IGBT inverter circuit connected in series with a capacitor. By controlling the IGBT's conduction time and sequence, this indirectly controls the timing and direction of the capacitor's connection to the grid and the output of harmonic current, thereby controlling the amplitude of the harmonic current injected into the grid by the impedance measurement device. The IGBT drive signal is provided by the driver module and corresponds to the IGBT drive signal. The control loop is designed based on the IGBT drive target. During the control loop design process, compared to the active harmonic injection module control loop, the basic control loop architecture remains unchanged, requiring only adjustments to the loop parameters.

[0109] Step 106: After injecting the harmonic current to be injected into the power grid, the voltage and current signals of the port of the branch to be measured in the multi-point voltage and current signals of the power grid are selected for signal decomposition processing and analysis to obtain the power grid impedance of the port of the branch to be measured; wherein the multi-point voltage and current signals of the power grid are obtained by real-time acquisition of voltages and currents at different locations in the power grid.

[0110] In this embodiment, the voltage and current acquisition module performs real-time monitoring of the grid voltage and current. Using voltage and current sensors, the acquisition module collects three-phase voltage and current signals from the grid, generating multi-point grid voltage and current signals. The grid impedance calculation module selects the voltage and current signals collected by the multiple voltage and current acquisition modules to obtain the port voltage and current signals of the branch to be measured. It then processes and decomposes the signals to analyze and calculate the impedance characteristics of that branch in the grid.

[0111] Optionally, the voltage and current signals of the port of the branch to be measured are selected from the multi-point voltage and current signals of the power grid for signal decomposition processing and analysis to obtain the power grid impedance of the port of the branch to be measured, specifically:

[0112] Obtaining the node data to be measured according to the voltage and current signals of the port of the branch to be measured in the multi-point voltage and current signals of the power grid;

[0113] Perform fast Fourier transform on the node data to be tested to obtain frequency domain data;

[0114] Extract the amplitude and phase information of the frequency domain data, and convert the three-phase voltage and current into positive and negative sequence voltage and current according to the amplitude and phase information of the frequency domain data;

[0115] The positive and negative sequence voltages and currents are used to calculate the grid impedance to obtain the grid impedance of the port of the branch to be measured. The grid impedance includes positive sequence impedance and negative sequence impedance. The formula for calculating the grid impedance is:

[0116] Among them, Z pos is the positive sequence impedance, Z neg is the negative sequence impedance, U pos is the positive sequence harmonic voltage, U neg is the negative sequence harmonic voltage, I pos is the positive sequence harmonic current, I neg is the negative sequence harmonic current, f k is the harmonic frequency, ∠* represents the phase angle, and |*| represents the amplitude.

[0117] In this embodiment, the grid impedance calculation module receives the output signals from the voltage and current signal acquisition module and performs data selection, decomposition, and processing. Its implementation includes three steps: a fast Fourier transform (FFT), a positive-negative sequence conversion, and a positive-negative sequence impedance calculation. After acquiring the three-phase voltage and current responses at the measurement point during the pulse injection period, the grid impedance calculation module performs an FFT (Fast Fourier Transform) on the measurement point data (the node data to be measured), extracting its amplitude and phase information in the frequency domain. It then converts the three-phase voltage and current into positive-negative sequence voltage and current, thereby calculating the positive-negative sequence grid impedance.

[0118] The implementation of the embodiments of the present invention, based on the grid impedance measurement technology of the power electronic converter, has made innovations and improvements in DC voltage control, power electronic conversion, and current tracking control. Based on the characteristics of passive harmonic injection and active harmonic injection methods, especially the problems that the harmonic current content injected by the passive harmonic injection method varies greatly in different frequency ranges and affects measurement accuracy, and the active harmonic injection method can cause significant interference to the power grid and increase the grid voltage distortion rate, a grid impedance measurement method is designed that complements the advantages of active and passive distribution network impedance measurement. By organically combining the active and passive methods, the disturbance of the impedance measurement process to the power grid is greatly reduced and the measurement accuracy is improved across the entire frequency range.

[0119] In addition, the present invention has the following advantages: (1) The active grid impedance measurement method uses the controller of the inverter itself to generate disturbances and actively applies the disturbances to the grid. Then, by collecting the corresponding responses and performing signal processing, the required information is extracted to calculate the grid impedance. (2) A method and device for measuring the impedance of the distribution network that complements the advantages of active and passive distribution networks are designed. By organically combining the two methods, the disturbance of the impedance measurement process to the grid is greatly reduced and the measurement accuracy is improved within the full frequency range. (3) The control loop regulator adopts a fuzzy PI regulator, which can optimize the loop control parameters online for different working conditions and disturbances, realizes the self-adaptation of the control parameters, and greatly improves the dynamic performance and anti-disturbance capability of the system. (4) The connection combination of the push-pull IGBT inverter circuit and the capacitor makes the operating time of the capacitor in the grid and the flow direction of the current controllable, thereby controlling the magnitude of the injected harmonic current. (5) Based on the concept of inverter modulation, the DC voltage control method is optimized, the DC voltage control process is simplified, and the control effect of the DC voltage is improved.

[0120] Example 2

[0121] Accordingly, see Figure 5, which is a schematic diagram of the connection structure of an impedance measurement device according to a second embodiment of the present invention. As shown in Figure 5, the grid impedance measurement device includes: a voltage and current acquisition module, a harmonic signal injection module, a DC voltage control module, a current tracking control module, a drive module, a power electronics conversion module, a passive harmonic injection module, and a grid impedance calculation module.

[0122] The connections of each module are as follows: the voltage and current acquisition module is connected to the current tracking control module, the grid impedance calculation module and the grid respectively; the current tracking control module is connected to the harmonic signal injection module, the drive module and the DC voltage control module respectively; the drive module is connected to the power electronic conversion module and the passive harmonic injection module respectively; the output end of the power electronic conversion module is connected to the grid through the three-phase LCL passive filter;

[0123] In this embodiment, as shown in Figure 5, the voltage and current acquisition module is connected to the current tracking control module and the grid impedance calculation module. The harmonic signal injection module and the DC voltage control module are also connected to the current tracking control module. The current tracking control module is connected to the drive module, and the drive module is also connected to the power electronic conversion module and the passive harmonic injection module. A three-phase LCL passive filter is also connected between the output end of the power electronic conversion module and the grid.

[0124] The harmonic signal injection module is used to generate characteristic harmonic current instructions according to the harmonic signal generation algorithm when the grid impedance measurement device is connected to the grid and operates stably, and input the characteristic harmonic current instructions into the DC voltage feedback control loop;

[0125] The DC voltage control module is used to build a DC voltage feedback control loop, which detects the DC voltage in real time to keep the voltage stable.

[0126] The current tracking control module is used to receive a characteristic harmonic current command in a DC voltage feedback control loop, generate a control signal, and perform current tracking control on the DC voltage feedback control loop based on a voltage-current dual closed-loop structure according to the characteristic harmonic current command and the control signal to obtain a harmonic current to be injected; wherein the harmonic current to be injected is a current of a specific amplitude and a specific frequency;

[0127] The driving module is used to convert the information electronic signal of the main circuit into the driving signal of the device control loop according to the received control signal, and select the preset harmonic injection mode in real time according to the harmonic injection target frequency; wherein the harmonic injection mode includes passive harmonic injection mode and active harmonic injection mode;

[0128] The power electronic conversion module is used to control the on and off of the switch tube according to the drive signal and the first specific sequence when the active harmonic injection mode is selected, so as to inject the harmonic current to be injected into the power grid at the common coupling point;

[0129] The passive harmonic injection module is used to control the on and off of the IGBT according to the drive signal and the second specific sequence when the passive harmonic injection mode is selected, control the time and current flow direction of the capacitor when it is put into the power grid, and inject the harmonic current to be injected into the power grid at the common coupling point;

[0130] The grid impedance calculation module is used to inject the harmonic current to be injected into the grid, select the voltage and current signals of the port of the branch to be measured from the multi-point voltage and current signals of the grid, perform signal decomposition processing and analysis, and obtain the grid impedance of the port of the branch to be measured;

[0131] The voltage and current acquisition module is used to obtain multi-point voltage and current signals of the power grid by real-time acquisition of voltage and current at different locations in the power grid.

[0132] Optionally, the voltage and current acquisition module includes a voltage transformer and a current transformer, which can detect the voltage and current of the power grid and acquire the three-phase voltage and current of the power grid.

[0133] The harmonic signal injection module is constructed and generated by the harmonic signal generation algorithm. The harmonic signal injection module is a software module implemented by an algorithm program. It can generate a harmonic current command signal of a specific waveform and frequency according to the harmonic signal injection algorithm.

[0134] The DC voltage control module is constructed by a DC voltage feedback control loop, which constantly detects the DC voltage of the power electronic conversion module and ensures voltage stability.

[0135] The current tracking control module adopts a voltage and current dual closed-loop structure, and the controller adopts a fuzzy PI parameter self-tuning controller. The current tracking control module is a software module implemented by an algorithm program. It can receive current tracking control instructions and complete the tracking control process of the instruction current.

[0136] The drive module is composed of a drive circuit, which serves as an interface circuit between the main circuit and the control circuit, converting the signal from the information electronic circuit into a drive signal added to the device control loop.

[0137] The power electronic conversion module consists of two parts: the DC side energy storage capacitor and the voltage type inverter. The voltage type inverter adopts a three-phase H-bridge structure.

[0138] The passive harmonic injection module is composed of an IGBT and a capacitor connected together, and can control the operating time of the capacitor in the power grid and the direction of current flow. Preferably, the passive harmonic injection module is composed of three groups of push-pull IGBT inverter circuits connected in series with a capacitor. By controlling the conduction sequence and conduction time of the two IGBTs, the time and direction of the capacitor connecting to the power grid and outputting the harmonic current are controlled in disguise, thereby controlling the amplitude of the harmonic current injected into the power grid by the impedance measuring device.

[0139] The grid impedance calculation module includes the FFT calculation phase, the positive-negative sequence conversion phase, and the positive-negative sequence impedance calculation phase. The voltage and current collected by the voltage and current acquisition module are decomposed and processed, and then the impedance characteristics of the grid are calculated and analyzed.

[0140] In the implementation of the embodiment of the present invention, the impedance measurement device is equivalent to a signal generation-reception-analysis device, which can generate a specific harmonic signal according to the harmonic signal injection algorithm, and then collect and separate the harmonic signal at other locations in the power grid. According to the separated specific harmonic voltage and current signals, the power grid impedance of the node is analyzed and calculated, thereby realizing the power grid impedance measurement. Based on the characteristics of the passive harmonic injection and active harmonic injection methods, especially the fact that the harmonic current content injected by the passive harmonic injection method has a large difference in different frequency ranges and affects the measurement accuracy, and the active harmonic injection method will cause greater interference to the power grid and cause the power grid voltage distortion rate to increase. The present invention designs a method and device for measuring the impedance of the active and passive distribution network that complement each other's advantages. By organically combining the two methods, the disturbance of the impedance measurement process to the power grid is greatly reduced and the measurement accuracy is improved within the full frequency band.

[0141] The aforementioned power grid impedance measurement device can implement a power grid impedance measurement method of the aforementioned method embodiment. The optional options in the aforementioned method embodiment also apply to this embodiment and are not described in detail here. The remaining contents of the embodiments of this application can be referenced to the contents of the aforementioned method embodiment and are not further described in this embodiment.

[0142] The above specific embodiments further illustrate the objectives, technical solutions, and beneficial effects of the present invention. It should be understood that the above are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention for those skilled in the art.

Claims

1. A method for measuring power grid impedance, characterized in that: include: When the grid impedance measuring device is connected to the grid and operates stably, a characteristic harmonic current instruction is generated according to the harmonic signal generation algorithm, and the characteristic harmonic current instruction is input into a DC voltage feedback control loop; wherein the DC voltage feedback control loop detects the DC voltage in real time to keep the voltage stable; The characteristic harmonic current instruction is received in the DC voltage feedback control loop, a control signal is generated, and according to the characteristic harmonic current instruction and the control signal, current tracking control is performed on the DC voltage feedback control loop based on a voltage-current double closed-loop structure to obtain a harmonic current to be injected; wherein the harmonic current to be injected is a current with a specific amplitude and a specific frequency; According to the received control signal, the information electronic signal of the main circuit is converted into a driving signal of the device control loop, and according to the harmonic injection target frequency, a preset harmonic injection method is selected in real time; wherein the harmonic injection method includes a passive harmonic injection method and an active harmonic injection method; When the active harmonic injection mode is selected, the switching tube is controlled to be turned on and off according to the driving signal and the first specific sequence, and the harmonic current to be injected is injected into the power grid at the common coupling point; When the passive harmonic injection mode is selected, the IGBT is turned on and off according to the drive signal and the second specific sequence, the time when the capacitor is put into operation in the power grid and the current flow direction are controlled, and the harmonic current to be injected is injected into the power grid at the common coupling point; After the harmonic current to be injected is injected into the power grid, the voltage and current signals of the port of the branch to be measured in the multi-point voltage and current signals of the power grid are selected for signal decomposition processing and analysis to obtain the power grid impedance of the port of the branch to be measured; wherein the multi-point voltage and current signals of the power grid are obtained by real-time collection of voltages and currents at different positions in the power grid.

2. The grid impedance measurement method according to claim 1, characterized in that: According to the harmonic injection target frequency, a preset harmonic injection method is selected, specifically: When the harmonic injection target frequency is lower than the first frequency, the current target frequency band is determined to be a low frequency band, and the active harmonic injection mode is selected; When the harmonic injection target frequency is higher than the first frequency and lower than the second frequency, the current target frequency band is determined to be a mid-frequency band, and the passive harmonic injection method is selected; When the harmonic injection target frequency is higher than the second frequency, the current target frequency band is determined to be a high frequency band, and the active harmonic injection mode is selected.

3. The grid impedance measurement method according to claim 1, characterized in that: According to the driving signal and the first specific sequence, the switching tube is controlled to be turned on and off, specifically: The driving signal is subjected to voltage space vector pulse width modulation, and the on-off sequence of the switch tube that changes with time is determined to obtain the first specific sequence; wherein the voltage space vector pulse width modulation includes coordinate transformation, sector and region judgment, basic voltage space vector action time calculation and basic voltage space vector action order planning; The switching tube is controlled to be turned on and off based on the first specific sequence.

4. The grid impedance measurement method according to claim 3, characterized in that: According to the driving signal and the second specific sequence, the IGBT is controlled to be turned on and off, and the time and current flow direction of the capacitor being put into operation in the power grid are controlled, specifically: Performing voltage space vector pulse width modulation on the driving signal to determine the on-off sequence of the IGBT that varies with time, and obtaining the second specific sequence; Controlling the turning on and off of the IGBT based on the second specific sequence; According to the on-time of the IGBT, controlling the time for which the capacitor is put into operation in the power grid and controlling the amplitude of the harmonic current to be injected; According to the change of the on-off state of the IGBT, the flow direction of the current of the capacitor put into operation in the power grid is controlled.

5. The grid impedance measurement method according to claim 1, characterized in that: The characteristic harmonic current instruction is generated according to the harmonic signal generation algorithm, specifically: In the inverter SVPWM modulation, the frequency of the harmonic signal is changed according to the harmonic injection target frequency, and by setting the change law of the modulation wave, a modulation wave frequency spectrum with harmonic amplitude gradually increasing from low frequency to the harmonic injection target frequency is generated to obtain an injected harmonic signal, and the characteristic harmonic current instruction is generated according to the injected harmonic signal.

6. The grid impedance measurement method according to claim 1, characterized in that: The voltage and current signals of the ports of the branch to be measured are selected from the multi-point voltage and current signals of the power grid for signal decomposition processing and analysis to obtain the power grid impedance of the ports of the branch to be measured, specifically: According to the voltage and current signals of the ports of the branches to be measured in the multi-point voltage and current signals of the power grid, the node data to be measured are obtained; Performing fast Fourier transform on the node data to be tested to obtain frequency domain data; Extracting the amplitude and phase information of the frequency domain data, and converting the three-phase voltage and current into positive and negative sequence voltage and current according to the amplitude and phase information of the frequency domain data; The positive and negative sequence voltage and current are subjected to grid impedance calculation to obtain the grid impedance of the port of the branch to be measured; wherein the grid impedance includes positive sequence impedance and negative sequence impedance; the formula for calculating the grid impedance is: Among them, Z pos is the positive sequence impedance, Z neg is the negative sequence impedance, U pos is the positive sequence harmonic voltage, U neg is the negative sequence harmonic voltage, I pos is the positive sequence harmonic current, I neg is the negative sequence harmonic current, f k is the harmonic frequency.

7. The grid impedance measurement method according to claim 1, characterized in that: Also includes: The current tracking control is specifically as follows: The output result of the fuzzy PI parameter self-tuning controller is multiplied by the three-phase voltage, and the multiplication result is added to the three-phase voltage control target to perform closed-loop control on the DC voltage and track the control current; The fuzzy PI parameter self-tuning controller is constructed by optimizing and adjusting the control parameters of the PI regulator online through a fuzzy control algorithm; The three-phase voltage control target is obtained by performing coordinate transformation on the output result of the fuzzy PI parameter self-tuning controller.

8. The grid impedance measurement method according to claim 7, characterized in that: The fuzzy PI parameter self-tuning controller is constructed by online optimizing and adjusting the control parameters of the PI regulator through a fuzzy control algorithm, specifically: By calculating the current error and the current error change rate, fuzzy reasoning is performed using fuzzy rules to output the control parameter change of the PI regulator; In the process of fuzzy reasoning, the current error, the current error change rate and the parameter change amount of the fuzzy PI parameter self-tuning controller are divided into several fuzzy subsets respectively, and the membership function adopts a triangular function; The control parameters of the PI regulator are corrected, and the control parameter changes are superimposed on the initial control parameters to obtain the control parameters of the current working condition.

9. A power grid impedance measuring device, characterized in that: include: Voltage and current acquisition module, harmonic signal injection module, DC voltage control module, current tracking control module, drive module, power electronic conversion module, passive harmonic injection module and grid impedance calculation module; The connections of the modules are as follows: the voltage and current acquisition module is respectively connected to the current tracking control module, the grid impedance calculation module and the grid; the current tracking control module is respectively connected to the harmonic signal injection module, the drive module and the DC voltage control module; the drive module is respectively connected to the power electronic conversion module and the passive harmonic injection module; the output end of the power electronic conversion module is connected to the grid through a three-phase LCL passive filter; The harmonic signal injection module is used to generate a characteristic harmonic current instruction according to a harmonic signal generation algorithm when the grid impedance measurement device is connected to the grid and operates stably, and input the characteristic harmonic current instruction into a DC voltage feedback control loop; The DC voltage control module is used to construct the DC voltage feedback control loop, and the DC voltage feedback control loop detects the DC voltage in real time to keep the voltage stable; The current tracking control module is used to receive the characteristic harmonic current instruction in the DC voltage feedback control loop, generate a control signal, and perform current tracking control on the DC voltage feedback control loop based on the voltage-current double closed-loop structure according to the characteristic harmonic current instruction and the control signal to obtain the harmonic current to be injected; wherein the harmonic current to be injected is a current with a specific amplitude and a specific frequency; The driving module is used to convert the information electronic signal of the main circuit into a driving signal of the device control loop according to the received control signal, and select a preset harmonic injection mode in real time according to the harmonic injection target frequency; wherein the harmonic injection mode includes a passive harmonic injection mode and an active harmonic injection mode; The power electronic conversion module is used to control the switching on and off of the switch tube according to the drive signal and the first specific sequence when the active harmonic injection mode is selected, so as to inject the harmonic current to be injected into the power grid at the common coupling point; The passive harmonic injection module is used to control the on and off of the IGBT, control the time and current flow direction of the capacitor in the power grid when the passive harmonic injection mode is selected, and inject the harmonic current to be injected into the power grid at the common coupling point according to the drive signal and the second specific sequence; The grid impedance calculation module is used to select the voltage and current signals of the port of the branch to be measured from the grid multi-point voltage and current signals after injecting the harmonic current to be injected into the grid, perform signal decomposition processing and analysis, and obtain the grid impedance of the port of the branch to be measured; The voltage and current acquisition module is used to acquire the voltage and current at multiple points of the power grid by real-time acquisition of the voltage and current at different locations in the power grid. Signal.

10. The grid impedance measuring device according to claim 9, characterized in that: The voltage and current acquisition module includes a voltage transformer and a current transformer; The harmonic signal injection module is constructed and generated by the harmonic signal generation algorithm; The DC voltage control module is generated by the DC voltage feedback control loop; The current tracking control module is composed of a voltage and current double closed-loop structure, and the controller adopts a fuzzy PI parameter self-tuning controller; The driving module is composed of a driving circuit; The power electronic conversion module consists of two parts: a DC side energy storage capacitor and a voltage-type inverter, wherein the voltage-type inverter adopts a three-phase H-bridge structure; The passive harmonic injection module is composed of the IGBT and the capacitor connected; The grid impedance calculation module includes an FFT calculation link, a positive-negative sequence conversion link, and a positive-negative sequence impedance calculation link.

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