Impedance scanning device based on grid-side voltage dynamic compensation under three-phase voltage unbalance working condition

Abstract

The invention provides an impedance scanning device based on network side voltage dynamic compensation under a three-phase voltage imbalance working condition, and relates to the technical field of new energy station power electronic converter topology and control, the device comprises a three-phase rectifier, a direct current bus capacitor, an inverter, a voltage compensation device, a frequency sweeping device, a circuit breaker and a series transformer part, the three-phase rectifier is connected in series with inverters of the voltage compensation device and the frequency sweep device, a secondary side of a series transformer of the voltage compensation device is connected in series with a network side, a secondary side of a series transformer of the frequency sweep device is connected in series with a grid-connected point, and a primary side of the series transformer is connected to the inverters and is coupled to a secondary side of the series transformer; according to the device, the power grid side voltage is rectified through the voltage compensation unit and is compensated to the grid side after being subjected to single-stage inversion transformation, voltage compensation is achieved through the energy of a power grid when the voltage is unbalanced, and an external energy source or an interference source does not need to be introduced. After compensation, the accuracy of the frequency sweeping result is obviously improved.

Description

Technical Field

[0001] This invention relates to the field of power electronic converter topology and control technology in new energy power plants, and in particular to an impedance scanning device based on grid-side voltage dynamic compensation under three-phase voltage imbalance conditions. Background Technology

[0002] The problem of three-phase voltage imbalance in renewable energy power grids is becoming increasingly prominent, mainly due to the access methods and operating characteristics of renewable energy generation, especially distributed photovoltaic (PV) and wind power. Its causes and hazards are more complex and significant than those of traditional power grids. Many residential PV systems are typically connected to a single phase (e.g., phase L1) of the low-voltage distribution network based on roof orientation, convenience, or insufficient initial planning. When sunlight is abundant, this phase receives a large amount of active power, while other phases receive less or no power, resulting in a severe power injection imbalance, which in turn leads to voltage imbalance, significantly raising the voltage of the connected phase. Although large wind farms / PV power plants are usually connected to medium- and high-voltage grids in a three-phase configuration, if multiple renewable energy power plants in a region are connected to the same or two phases of the transmission network, regional three-phase imbalance can also occur. The output of wind and PV power is greatly and rapidly affected by weather. When this fluctuation is unevenly distributed across different phases, it dynamically exacerbates or alters the degree of imbalance, making the problem more difficult to predict and control. Three-phase voltage imbalance can cause various serious hazards to the power system and electrical equipment. Voltage imbalance in the power grid can interfere with the normal operation of the converter's phase-locked loop (PLL), affecting the tracking accuracy and control stability of its grid-connected current. To cope with unbalanced voltage and meet grid connection requirements, the converter may output unexpected harmonic currents, leading to increased current waveform distortion and degraded power quality. Unbalanced voltage can cause instantaneous power fluctuations on the AC side of the converter, resulting in ripple at twice the power frequency on the DC bus voltage. This affects the lifespan and performance of DC-side equipment such as energy storage batteries and may also threaten the converter's own safety. Impedance sweep frequency analysis, especially methods based on positive-sequence components, typically assumes that the power grid is three-phase balanced. Under this assumption, the injected sweep frequency signal is usually a three-phase balanced positive-sequence signal, and the measured response signal mainly contains positive-sequence components. The calculated impedance is mainly positive-sequence impedance. Voltage imbalance means that there are significant negative-sequence components and possible zero-sequence components in the system. These components disrupt the system's symmetry. When a balanced positive-sequence sweep frequency current signal is injected into an unbalanced system, due to the system asymmetry, the injected positive-sequence current will not only produce a positive-sequence voltage response but will also couple to produce a negative-sequence voltage response and a zero-sequence voltage response. Similarly, the inherent unbalanced background voltage of the system also interacts with the injected current. Impedance sweep frequency analysis typically requires precise synchronization and phase measurement. PLLs are used to track the grid voltage phase. Voltage imbalance can cause phase jitter or tracking errors in the PLL output. Negative sequence components introduce a disturbance of twice the fundamental frequency into the PLL. This phase error directly affects the precise control of the injected sweep current phase and the accuracy of the measured voltage phase angle, thus leading to deviations in the calculated impedance phase. Impedance phase is crucial for stability analysis. Measurement errors may lead to underestimating or overestimating the system's impedance amplitude at a specific frequency. This can mask potential resonance points, where the impedance should be very low, resulting in the failure to identify resonance risks and causing subsynchronous / supersynchronous oscillations after the grid connection of new energy power plants.It is also possible to misjudge the existence of false resonant points, leading to unnecessary conservative control or increased grid connection costs, such as excessive addition of filters. Impedance sweep is a key method for evaluating the stability of interaction between renewable energy power plants and the grid. Inaccurate grid impedance models can lead to incorrect judgments on system stability margins, and the designed power plant controller parameters, such as PLL bandwidth, current loop parameters, and active damping strategies, may not be suitable for the actual operating conditions of unbalanced grids, increasing the risk of oscillation and instability. Based on this, this invention proposes an impedance sweep device based on dynamic grid-side voltage compensation under three-phase voltage imbalance conditions to solve the above problems. Summary of the Invention

[0003] Therefore, the purpose of this invention is to provide an impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions, so as to at least solve the above problems.

[0004] The technical solution adopted in this invention is as follows: An impedance scanning device based on grid-side voltage dynamic compensation under three-phase voltage imbalance conditions includes a three-phase rectifier, a DC bus capacitor, an inverter, a voltage compensation device, a frequency sweeping device, a circuit breaker, and a series transformer. The three-phase rectifier is connected in series with the inverter of the voltage compensation device and the frequency sweeping device. The secondary side of the series transformer of the voltage compensation device is connected in series to the grid side. The secondary side of the series transformer of the frequency sweeping device is connected in series to the grid connection point, and its primary side is connected to the inverter and coupled to the secondary side of the series transformer itself. Circuit breaker KM3 is connected to the voltage compensation device and the frequency sweeping device. Circuit breaker KM1 is connected to the series transformer of the voltage compensation device in parallel on the grid side. Circuit breaker KM2 is connected to the series transformer of the frequency sweeping device in parallel at the grid connection point. Among them, the three-phase rectifier and inverter are used to generate compensation voltage to balance the voltage on the grid side; Among them, the voltage compensation device series transformer is used to inject the compensation voltage into the grid side, and the frequency sweep device series transformer is used to inject the disturbance voltage into the grid connection point. Among them, the frequency sweeping device inverter is used to generate disturbance voltage and perform frequency sweeping; Among them, circuit breaker KM1 is used to control the activation and deactivation of the voltage compensation device, circuit breaker KM2 is used to control the activation and deactivation of the frequency sweeping device, and circuit breaker KM3 is used to control the opening and closing of the frequency sweeping device.

[0005] Furthermore, the device is used to perform the following steps: S1. In the initial state of the device, switches KM1 and KM2 are closed, and KM3 is open; S2. Testing conditions: Whether the ratio of the difference between the three-phase phase voltages to the reference value is less than or equal to 3%; S3. If the detection conditions are not met, disconnect switches KM1 and KM3, close switch KM2, and start the voltage compensation device to adjust the grid-side voltage. S4. The voltage compensation device injects compensation voltage into the grid side; S5. Repeat the test. If the test conditions are still not met, enter the loop. S6. When the detection conditions are met, close switches KM1 and KM3, open KM2, start the frequency sweeping device, and inject disturbance voltage into the grid connection point. S7. Perform FFT analysis on the data collected at the grid connection point and fit the impedance characteristic curve.

[0006] Furthermore, in step S1, in the initial state of the device, switches KM1 and KM2 are closed, KM3 is open, the voltage compensation device is disconnected, and the frequency sweep device is not connected to the circuit.

[0007] Furthermore, in step S2, the detection condition is whether the ratio of the difference between the three-phase phase voltages to the reference value is less than or equal to 3%, i.e., satisfying the following calculation formula: , If the detection conditions are met, proceed to step S6; otherwise, proceed to step S3.

[0008] Furthermore, if the detection conditions are not met in step S3, then switches KM1 and KM3 are disconnected, switch KM2 is closed, the voltage compensation device is started to adjust the grid-side voltage, and the frequency sweep device is disconnected.

[0009] Furthermore, in step S4, the voltage compensation device injects compensation voltage into the grid side, and the output voltage space vector expression is:

[0010] Using the output voltage space vector The inverter switching devices are controlled to achieve a three-phase symmetrical sinusoidal voltage output by using three-dimensional space vector modulation technology.

[0011] Furthermore, if the detection conditions are still not met after step S5, then proceed to step 3 again.

[0012] Furthermore, if the detection conditions are met in step S6, switches KM1 and KM3 are closed, KM2 is opened, and the frequency sweep device is started, injecting a disturbance voltage into the grid connection point. After the compensation device is put into operation, if the three-phase voltage meets the detection conditions, the frequency sweep device can also be started simultaneously for testing.

[0013] Furthermore, the three-phase rectifier is a three-phase full-bridge circuit composed of three single-phase half-bridge circuits connected in parallel, and the inverter topology of the voltage compensation device and frequency sweep device is a single-phase H-bridge inverter.

[0014] Furthermore, the control of each inverter submodule of the frequency sweep device adopts open-loop control. This is the amplitude command for the X-phase output phase voltage; The DC-side voltage of the X-phase submodule; according to and Calculate the modulation ratio of phase X. ; The frequency command value for the disturbance injection voltage is used to reduce the impact of DC-side voltage fluctuations on the output when the voltage disturbance injection device outputs a lower frequency voltage waveform. The initial phase command value of the X-phase disturbance voltage is used; the three-phase modulation waveform of the disturbance injection voltage is synthesized based on the modulation ratio, frequency command value and initial phase command value. The modulation wave is phase-shifted and modulated by a unipolar frequency doubling carrier to obtain the switching control signal of the single-phase H-bridge of the submodule.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. Voltage imbalance is a major source of interference in frequency sweep. Compensating the grid-side voltage to balance first significantly improves the accuracy of impedance measurements. Frequency sweeping under uncompensated imbalance conditions results in errors introduced by system asymmetry, which may lead to inaccurate subsequent analysis. This solution fundamentally eliminates this problem.

[0016] 2. When the power grid is balanced, the compensation stage is skipped, and the voltage compensation device is in standby or low-power state, significantly reducing the conduction losses, heat dissipation requirements, and power consumption of power devices, thus extending equipment life. Frequency sweeping is avoided in invalid or unbalanced states, saving energy consumption of the frequency sweeping device and potential signal injection costs.

[0017] 3. When the power grid is unbalanced, the voltage compensation device should prioritize voltage correction to eliminate potential risks such as overvoltage / undervoltage and neutral point offset, ensuring the system is in a safe and stable state before initiating frequency sweep. Frequency sweep in an unbalanced power grid may cause voltage over-limits due to coupling effects, triggering malfunctions of protection devices. Compensation first can avoid such cascading risks.

[0018] 4. In a balanced system, the decoupling of sequence components is more thorough, the post-processing algorithm for frequency sweep data is simpler and requires less computation, resulting in faster and more reliable output. If frequency sweeping is performed directly in an unbalanced power grid to obtain distorted results, multiple retries or manual intervention are often required for verification.

[0019] 5. Before the power plant is put into operation, the power grid may be unbalanced due to load imbalance during the construction period. Compensation followed by frequency sweeping can ensure accurate acquisition of the grid impedance characteristics, providing a reliable basis for controller parameter tuning (such as PLL bandwidth and current loop gain), and avoiding the risk of oscillation after grid connection. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only preferred embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 The flowchart illustrates the operation of an impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions, as provided in a specific embodiment of the present invention. Figure 2 The circuit topology of an impedance scanning device based on grid-side voltage dynamic compensation under three-phase voltage imbalance conditions is provided in a specific embodiment of the present invention. Figure 3 The block diagram of a three-phase rectifier control for a voltage compensation device based on a grid-side voltage dynamic compensation impedance scanning device under three-phase voltage imbalance conditions is provided in a specific embodiment of the present invention. Figure 4 The diagram below shows a single-stage H-bridge inverter control block diagram for a voltage compensation device based on a grid-side voltage dynamic compensation impedance scanning device under three-phase voltage imbalance conditions, as provided in a specific embodiment of the present invention. Figure 5 The diagram below shows a control block diagram of a single-stage H-bridge inverter with an impedance scanning device based on grid-side voltage dynamic compensation under three-phase voltage imbalance conditions, provided by a specific embodiment of the present invention. Figure 6 The waveforms of the grid-side voltage and switching signal of an impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions are shown in a specific embodiment of the present invention. Figure 7 The grid-connected voltage waveform diagram after the injection of disturbance voltage by an impedance scanning device based on grid-side voltage dynamic compensation under three-phase voltage imbalance conditions is provided in a specific embodiment of the present invention. Figure 8 The image shows the frequency sweep results of a three-phase voltage imbalance impedance scanning device based on grid-side voltage dynamic compensation under a specific embodiment of the present invention. Figure 9 The image shows the frequency sweep result after three-phase voltage balance of an impedance scanning device based on grid-side voltage dynamic compensation under three-phase voltage imbalance conditions, provided in a specific embodiment of the present invention. Detailed Implementation The principles and features of the present invention are described below with reference to the accompanying drawings. The listed embodiments are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0022] Reference Figures 1-2 This invention provides an impedance scanning device based on grid-side voltage dynamic compensation under three-phase voltage imbalance conditions. The device comprises a three-phase rectifier, a DC bus capacitor, an inverter, a voltage compensation device, a frequency sweeping device, a circuit breaker, and a series transformer. The three-phase rectifier is connected in series with the inverter of the voltage compensation device and the frequency sweeping device. The secondary side of the series transformer of the voltage compensation device is connected in series to the grid side. The secondary side of the series transformer of the frequency sweeping device is connected in series to the grid connection point, and its primary side is connected to the inverter and coupled to the secondary side of the series transformer itself. Circuit breaker KM3 is connected to the voltage compensation device and the frequency sweeping device. Circuit breaker KM1 is connected to the series transformer of the voltage compensation device in parallel with the grid side. Circuit breaker KM2 is connected to the series transformer of the frequency sweeping device in parallel with the grid connection point. Among them, the three-phase rectifier and inverter are used to generate compensation voltage to balance the voltage on the grid side; Among them, the voltage compensation device series transformer is used to inject the compensation voltage into the grid side, and the frequency sweep device series transformer is used to inject the disturbance voltage into the grid connection point. Among them, the frequency sweeping device inverter is used to generate disturbance voltage and perform frequency sweeping; Among them, circuit breaker KM1 is used to control the activation and deactivation of the voltage compensation device, circuit breaker KM2 is used to control the activation and deactivation of the frequency sweeping device, and circuit breaker KM3 is used to control the opening and closing of the frequency sweeping device.

[0023] For example, the frequency sweep device uses a three-phase series injection of disturbance voltage. According to the series voltage division principle, the disturbance voltage is mainly superimposed on the equipment under test, resulting in a high signal-to-noise ratio and high impedance measurement accuracy. The objective of the operation control method of the frequency sweep device is to achieve an output sinusoidal voltage frequency of 1-10000Hz, an effective value of the output sinusoidal voltage of 0%-10% of the system voltage, and a low distortion rate of the output sinusoidal voltage.

[0024] The device is used to perform the following steps: S1. In the initial state of the device, switches KM1 and KM2 are closed, and KM3 is open; S2. Testing conditions: Whether the ratio of the difference between the three-phase phase voltages to the reference value is less than or equal to 3%; S3. If the detection conditions are not met, disconnect switches KM1 and KM3, close switch KM2, and start the voltage compensation device to adjust the grid-side voltage. S4. The voltage compensation device injects compensation voltage into the grid side; S5. Repeat the test. If the test conditions are still not met, enter the loop. S6. When the detection conditions are met, close switches KM1 and KM3, open KM2, start the frequency sweeping device, and inject disturbance voltage into the grid connection point. S7. Perform FFT analysis on the data collected at the grid connection point and fit the impedance characteristic curve.

[0025] In step S1, in the initial state of the device, switches KM1 and KM2 are closed, KM3 is open, the voltage compensation device is disconnected, and the frequency sweep device is not connected to the circuit.

[0026] In step S2, the detection condition is whether the ratio of the difference between the three-phase phase voltages to the reference value is less than or equal to 3%, i.e., it satisfies the following calculation formula: , If the detection conditions are met, proceed to step S6; otherwise, proceed to step S3.

[0027] If the detection conditions are not met in step S3, disconnect switches KM1 and KM3, close switch KM2, start the voltage compensation device to adjust the grid-side voltage, and disconnect the frequency sweep device.

[0028] Step S4: The voltage compensation device injects compensation voltage into the grid side, and the output voltage space vector expression is:

[0029] Using the output voltage space vector The inverter switching devices are controlled to achieve a three-phase symmetrical sinusoidal voltage output by using three-dimensional space vector modulation technology.

[0030] If the detection conditions are still not met after step S5, proceed to step S3 again.

[0031] If the detection conditions are met in step S6, close switches KM1 and KM3, open KM2, and start the frequency sweep device. The frequency sweep device injects a disturbance voltage into the grid connection point. After the compensation device is put into operation, if the three-phase voltage meets the detection conditions, the frequency sweep device can also be started simultaneously for testing.

[0032] The three-phase rectifier is a three-phase full-bridge circuit composed of three single-phase half-bridge circuits connected in parallel. The inverter topology of the voltage compensation device and the frequency sweep device is a single-phase H-bridge inverter.

[0033] The frequency sweep device employs open-loop control for each inverter submodule. This is the amplitude command for the X-phase output phase voltage; The DC-side voltage of the X-phase submodule; according to and Calculate the modulation ratio of phase X. ; The frequency command value for the disturbance injection voltage is used to reduce the impact of DC-side voltage fluctuations on the output when the voltage disturbance injection device outputs a lower frequency voltage waveform. The initial phase command value of the X-phase disturbance voltage is used; the three-phase modulation waveform of the disturbance injection voltage is synthesized based on the modulation ratio, frequency command value and initial phase command value. The modulation wave is phase-shifted and modulated by a unipolar frequency doubling carrier to obtain the switching control signal of the single-phase H-bridge of the submodule.

[0034] For example, refer to Figure 3 Assuming the series transformer is a linear transformer and all switching devices in the circuit are ideal switching devices, the mathematical models of the rectifier and inverter in the three-phase stationary coordinate system can be obtained from KVL and KCL. Performing Clarke and Park transformations on these models yields the mathematical models in the synchronous rotating coordinate system. The mathematical model of the three-phase rectifier in the dq coordinate system can be expressed as:

[0035] In the formula: This is the equivalent output gain relative to the DC side; , These are the input currents on the d-axis and q-axis, respectively; For output filtering inductance of a three-phase rectifier; Its parasitic resistance; This is the DC side voltage; , These are the components of the three-phase rectifier output voltage along the d and q axes, respectively.

[0036] The mathematical model of a single-stage H-bridge inverter with voltage compensation device in the dq coordinate system is as follows:

[0037] In the formula: This is the output filter inductor on the inverter side of the voltage compensation device. Its parasitic resistance; and These are the inverter-side inductor currents in the dq coordinate system; , These are the components of the inverter-side output voltage on the dq axis.

[0038] The current in the AC filter capacitor on the inverter side can be expressed as:

[0039] In the formula: and These are the compensation currents of the inverter; This is the output filter capacitor.

[0040] From the mathematical model of a three-phase rectifier, we can see that... and Both are related. The DC-side voltage is stabilized using PI control, and the DC bus voltage is tracked using the d-axis current. The DC voltage controller can be expressed as:

[0041] In the formula: , These are the proportional and integral coefficients of the DC voltage PI controller, respectively. To compensate for system losses, a DC voltage controller is used to achieve stable control of the DC side voltage.

[0042] Since using a PI controller in a synchronous rotating coordinate system can reduce the steady-state error of the system, the current controller of the three-phase rectifier can be designed as follows:

[0043] In the formula: These are the proportional and integral coefficients of the current PI controller, respectively. These are the output voltages of the current PI controller.

[0044] Since the three-phase rectifier only synthesizes the positive sequence current component, the q-axis current command... It can be set to 0 to obtain a balanced sinusoidal current. d-axis current command. From the following formula, we can obtain:

[0045] In the formula: for d-axis components The DC output after low-pass filtering to eliminate the influence of harmonic current and reactive current.

[0046] Two-dimensional space vector modulation is used to control the on / off state of the bridge arms of a three-phase rectifier to obtain a three-phase symmetrical sinusoidal input current.

[0047] Reference Figure 4 To achieve faster dynamic response and higher steady-state compensation accuracy, the inverter of the voltage compensation device adopts dual-loop control of voltage and current. The inner loop tracks the inductor current feedback, while the outer loop controls the load voltage. The outer voltage loop is designed as follows:

[0048] In the formula: , These are the proportional and integral coefficients of the outer loop PI control, respectively; , These are the load compensation currents in the synchronous rotating coordinate system, respectively.

[0049] Since the d-axis coincides with the direction of the grid voltage vector, the q-axis voltage reference value... It can be set to 0, and thus the inner loop current control can be expressed as:

[0050] In the formula: The proportional gain of the inner loop P controller; , These are the inductor currents on the inverter side in the synchronous rotating coordinate system.

[0051] Using the output voltage space vector The inverter switching devices are controlled to achieve a three-phase symmetrical sinusoidal voltage output by using three-dimensional space vector modulation technology.

[0052] Reference Figure 5 H-bridge cascaded inverters often employ closed-loop control strategies such as PI control, hysteresis control, deadbeat control, and quasi-PR control to achieve high-quality output voltage waveforms. However, PI control struggles to track AC reference signals without steady-state error, especially high-frequency AC reference signals, thus failing to meet the wide-bandwidth voltage disturbance requirements of voltage disturbance injection devices. Hysteresis control is simple to implement, but its switching frequency is not fixed and requires precise selection of the loop width. Deadbeat control offers good dynamic characteristics, but it relies on an accurate mathematical model of the controlled object. Quasi-PR control is an improvement on traditional proportional-resonant control, maintaining high gain at the rated frequency while reducing the adverse effects of frequency offset. Considering the wide output voltage bandwidth and large amplitude variation range of the voltage disturbance injection device, using closed-loop control strategies such as PI control, hysteresis control, deadbeat control, and quasi-PR control would obviously lead to complex system debugging and reduced equipment reliability. Therefore, open-loop control is adopted for the control of each inverter submodule of the frequency sweep device.

[0053] This is the amplitude command for the X-phase output phase voltage; The DC-side voltage of the X-phase submodule; according to and The modulation ratio of phase X can be calculated. ; This is the frequency command value for the disturbance injection voltage, which can reduce the impact of DC-side voltage fluctuations on the output when the voltage disturbance injection device outputs a lower frequency voltage waveform; This represents the initial phase command value of the X-phase disturbance voltage. Based on the modulation ratio, frequency command value, and initial phase command value, a three-phase modulated waveform of the disturbance injection voltage can be synthesized. The modulated wave is then phase-shifted and modulated by a unipolar frequency-doubled carrier to obtain the switching control signal for the single-phase H-bridge in the submodule.

[0054] Reference Figures 6-9 , Figure 6 The waveforms are shown for the grid-side voltage and the switch signal. Voltage regulation is performed at this time, and the switch operates after successful regulation. Figure 7 The image shows the grid-connected voltage waveform after the disturbance voltage is injected. After the voltage is regulated to equilibrium, the frequency sweep device is activated to inject the disturbance voltage. The impedance characteristic curve obtained by sweeping under the three-phase voltage imbalance condition is shown below. Figure 8 As shown, the impedance characteristic curve obtained by scanning under the three-phase voltage balance state is as follows: Figure 9 As shown.

[0055] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions, characterized in that, The device includes a three-phase rectifier, a DC bus capacitor, an inverter, a voltage compensation device, a frequency sweeping device, a circuit breaker, and a series transformer. The three-phase rectifier is connected in series with the inverter of the voltage compensation device and the frequency sweeping device. The secondary side of the series transformer of the voltage compensation device is connected in series to the grid side. The secondary side of the series transformer of the frequency sweeping device is connected in series to the grid connection point, and its primary side is connected to the inverter and coupled to the secondary side of the series transformer itself. Circuit breaker KM3 is connected to the voltage compensation device and the frequency sweeping device. Circuit breaker KM1 is connected to the grid side and connected in parallel to the series transformer of the voltage compensation device. Circuit breaker KM2 is connected to the grid connection point and connected in parallel to the series transformer of the frequency sweeping device. Among them, the three-phase rectifier and inverter are used to generate compensation voltage to balance the voltage on the grid side; Among them, the voltage compensation device series transformer is used to inject the compensation voltage into the grid side, and the frequency sweep device series transformer is used to inject the disturbance voltage into the grid connection point. Among them, the frequency sweeping device inverter is used to generate disturbance voltage and perform frequency sweeping; Among them, circuit breaker KM1 is used to control the activation and deactivation of the voltage compensation device, circuit breaker KM2 is used to control the activation and deactivation of the frequency sweeping device, and circuit breaker KM3 is used to control the opening and closing of the frequency sweeping device.

2. An impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions according to claim 1, characterized in that, The device is used to perform the following steps: S1. In the initial state of the device, switches KM1 and KM2 are closed, and KM3 is open; S2. Testing conditions: Whether the ratio of the difference between the three-phase phase voltages to the reference value is less than or equal to 3%; S3. If the detection conditions are not met, disconnect switches KM1 and KM3, close switch KM2, and start the voltage compensation device to adjust the grid-side voltage. S4. The voltage compensation device injects compensation voltage into the grid side; S5. Repeat the test. If the test conditions are still not met, enter the loop. S6. When the detection conditions are met, close switches KM1 and KM3, open KM2, start the frequency sweeping device, and inject disturbance voltage into the grid connection point. S7. Perform FFT analysis on the data collected at the grid connection point and fit the impedance characteristic curve.

3. An impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions according to claim 2, characterized in that, In step S1, in the initial state of the device, switches KM1 and KM2 are closed, KM3 is open, the voltage compensation device is disconnected, and the frequency sweep device is not connected to the circuit.

4. An impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions according to claim 3, characterized in that, In step S2, the detection condition is whether the ratio of the difference between the three-phase phase voltages to the reference value is less than or equal to 3%, i.e., it satisfies the following calculation formula: , If the detection conditions are met, proceed to step S6; otherwise, proceed to step S3.

5. An impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions according to claim 4, characterized in that, If the detection conditions are not met in step S3, disconnect switches KM1 and KM3, close switch KM2, start the voltage compensation device to adjust the grid-side voltage, and disconnect the frequency sweep device.

6. An impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions according to claim 5, characterized in that, Step S4: The voltage compensation device injects compensation voltage into the grid side, and the output voltage space vector expression is: Using the output voltage space vector The inverter switching devices are controlled to achieve a three-phase symmetrical sinusoidal voltage output by using three-dimensional space vector modulation technology.

7. An impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions according to claim 6, characterized in that, If the detection conditions are still not met after step S5, proceed to step S3 again.

8. An impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions according to claim 7, characterized in that, If the detection conditions are met in step S6, close switches KM1 and KM3, open KM2, start the frequency sweep device, and inject disturbance voltage into the grid connection point.

9. An impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions according to claim 2, characterized in that, The three-phase rectifier is a three-phase full-bridge circuit composed of three single-phase half-bridge circuits connected in parallel. The inverter topology of the voltage compensation device and the frequency sweep device is a single-phase H-bridge inverter.

10. An impedance scanning device based on dynamic compensation of grid-side voltage under three-phase voltage imbalance conditions according to claim 9, characterized in that, The frequency sweep device employs open-loop control for each inverter submodule. This is the amplitude command for the X-phase output phase voltage; The DC-side voltage of the X-phase submodule; according to and Calculate the modulation ratio of phase X. ; The frequency command value for the disturbance injection voltage is used to reduce the impact of DC-side voltage fluctuations on the output when the voltage disturbance injection device outputs a lower frequency voltage waveform. The initial phase command value of the X-phase disturbance voltage is used; the three-phase modulation waveform of the disturbance injection voltage is synthesized based on the modulation ratio, frequency command value and initial phase command value. The modulation wave is phase-shifted and modulated by a unipolar frequency doubling carrier to obtain the switching control signal of the single-phase H-bridge of the submodule.

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