A grid impedance detection method and grid-connected system

Abstract

The present application provides a grid impedance detection method and a grid-connected system, which are applied to a grid-connected system including multiple inverters. The method adjusts the amplitude of a target subharmonic signal according to the current of a grid-connected point; adjusts the phase of the target subharmonic signal according to the fundamental voltage signal at the connection point between the target inverter and the grid, and generates a target subharmonic signal using the amplitude of the target subharmonic signal and the phase of the target subharmonic signal; and adjusts the output current of the target inverter based on the target subharmonic signal and the fundamental current signal of the target inverter to obtain the impedance of the grid-connected system. Using the grid impedance detection method provided by the present application, the grid-connected point injects the target subharmonic signal in a closed loop as a control instruction, and performs phase synchronization according to the fundamental voltage signal at the connection point between the target inverter and the grid, so that the target subharmonic signals injected into each target inverter do not affect each other, thereby reducing the grid impedance detection calculation error and obtaining an accurate grid impedance.

Description

Technical Field

[0001] The present application provides a method for detecting power impedance and a grid-connected system. Background Art

[0002] Photovoltaic power generation, as a renewable and clean energy source, has been rapidly developing. As the core component of a photovoltaic power generation system, the photovoltaic inverter is responsible for converting the direct current (DC) generated by the photovoltaic modules into sinusoidal alternating current (AC) and feeding it into the power grid.

[0003] With the rapid expansion of photovoltaic inverter installations, the power grid is increasingly exhibiting weak grid characteristics. Increased grid impedance affects the inverter control loop gain, bandwidth, and control performance, adversely affecting the inverter's grid-connected power quality and stable operation, making it difficult to further increase the penetration rate of renewable energy grid integration. Therefore, accurately detecting grid impedance information is crucial for inverter operation and the stability of the grid-connected system.

[0004] Currently, grid impedance detection methods can be divided into active and passive measurement schemes. Active measurement schemes include specific harmonic injection, pulse signal injection, and power perturbation methods, while passive measurement schemes include least squares recursive estimation and Kalman filtering. Specifically, the specific harmonic injection method injects a certain voltage or current disturbance into the grid and performs frequency analysis on the disturbance and response signals to obtain their respective frequency components. Finally, the frequency domain characteristics of the grid impedance are obtained by deriving the frequency domain characteristics.

[0005] However, in current scenarios where multiple inverters are connected to the grid, if the impedance of each inverter is detected by inputting specific subharmonics, the specific subharmonics injected into each inverter will affect each other, resulting in errors in the measured impedance. Therefore, a grid impedance detection method is needed to accurately detect the voltage and current harmonics at the grid connection point, thereby obtaining an accurate grid impedance. Summary of the Invention

[0006] The present application provides a grid impedance detection method and a grid-connected system for accurately detecting voltage and current harmonics at a grid-connected point, thereby obtaining accurate grid impedance.

[0007] In a first aspect, the present application provides a grid impedance detection method, which is applied to a grid-connected system, the grid-connected system including multiple inverters, the method comprising: adjusting the amplitude of a target subharmonic signal based on the current at a grid connection point, the grid connection point being the connection point between the grid-connected system and the grid; adjusting the phase of the target subharmonic signal based on the fundamental voltage signal at the connection point between the target inverter and the grid, the target inverter being any one of the multiple inverters; generating a target subharmonic signal using the amplitude and phase of the target subharmonic signal; and adjusting the output current of the target inverter based on the target subharmonic signal and the fundamental current signal of the target inverter to obtain the impedance of the grid-connected system. In selecting the frequency of the target subharmonic signal, the selection principle is mainly to reduce the influence of grid voltage harmonics. In selecting the frequency, in order to reduce detection error, the frequency of the target subharmonic signal needs to be as close to the fundamental frequency as possible, and the injected frequency should be a frequency that does not exist in the grid before injection, so as to ensure that the injected target subharmonic current is completely generated by the injected target subharmonic voltage.

[0008] By using the grid impedance detection method provided in the embodiment of the present application, under the unified coordination of the controller, the current of the target subharmonic injected into the closed loop of the grid connection point is used as the control instruction. At the same time, the target inverters of each power station are phase-synchronized according to the phase of the fundamental voltage signal at the connection point between their respective ports and the grid, so that the target subharmonics injected into each target inverter do not affect each other, thereby effectively reducing the grid impedance detection and detection error, so as to obtain accurate grid impedance.

[0009] As a possible implementation, adjusting the amplitude of a target subharmonic signal based on the current at the grid connection point includes: obtaining the current value of the target subharmonic signal in the current at the grid connection point; and adjusting the amplitude of the target subharmonic signal using the current of the target subharmonic signal and the current of a preset target subharmonic signal. The grid connection point current can be obtained by a current sampling unit, which can include a current transformer. The extracted grid connection point current includes multiple harmonic components of different characteristic orders.

[0010] As a possible implementation method, the current of the target subharmonic signal and the current of the preset target subharmonic signal are used to adjust the amplitude of the target subharmonic signal, including: adjusting the amplitude of the target subharmonic signal through a proportional-integral controller based on the difference between the current of the target subharmonic signal and the current of the preset target subharmonic signal. The extracted target subharmonic signal component is compared with the preset target subharmonic signal value to obtain an error signal, and the error signal is subjected to proportional-integral control to obtain the amplitude of the target subharmonic signal. The amplitude output after proportional-integral control is related to the target subharmonic component of the current power grid and the preset target subharmonic component. In order to keep the target subharmonic component of the current power grid always within a fixed range, the more target subharmonic components the current power grid has, the fewer target subharmonic components injected into the current power grid during the cycle.

[0011] Since the impedance characteristics of each distributed grid connection point are different and the distances between the distributed grid connection points and the grid are different, if the target subharmonic signals with the same amplitude and phase are used as control signals, the sampling accuracy will be affected and they will affect each other. As a possible implementation method, the phase of the target subharmonic signal is adjusted according to the fundamental voltage signal at the connection point between the target inverter and the grid, including: adjusting the signal phase of the fundamental voltage signal at the connection point between the target inverter and the grid, and adjusting the signal phase of the fundamental voltage signal at the connection point between the target inverter and the grid to the phase of the target subharmonic signal. In order to prevent the specific subharmonics injected into each target inverter from affecting each other, the target subharmonic signal of the target inverter needs to be controlled to ensure that it has the same phase as the fundamental voltage signal at the connection point between the target inverter and the grid. A voltage sensor can be used to sample the voltage signal at the connection point between the target inverter and the grid to obtain a fundamental voltage signal, and the phase of the target subharmonic signal is adjusted according to the fundamental voltage signal. As a possible implementation, after obtaining the fundamental voltage signal at the connection point between the target inverter and the grid, the signal phase of the fundamental voltage signal at the connection point between the target inverter and the grid can be adjusted to the phase of the target subharmonic signal.

[0012] As a possible implementation method, the signal phase of the fundamental voltage signal at the connection point between the target inverter and the power grid is adjusted, and the signal phase of the fundamental voltage signal at the connection point between the target inverter and the power grid is adjusted to the phase of the target subharmonic signal, including: inputting the fundamental voltage signal at the connection point between the target inverter and the power grid into a phase-locked unit, and the phase of the synchronization signal output by the phase-locked unit is the phase of the target subharmonic signal. After sampling the fundamental voltage signal at the connection point between the target inverter and the power grid, a phase-locked loop can be used to obtain a synchronization signal with the same phase as the fundamental voltage signal. The phase of the synchronization signal is the same as the phase of the fundamental voltage signal at the connection point between the target inverter and the power grid.

[0013] As a possible implementation, adjusting the output current of the target inverter based on the target subharmonic signal and the fundamental current signal of the target inverter includes: adjusting the sum of the target subharmonic signal and the fundamental current signal of the target inverter to obtain a first signal; obtaining the difference between the first signal and the signal output by the target inverter to obtain a second signal; and using the second signal to change the output current of the target inverter. The first signal is a superposition signal of the target subharmonic signal and the fundamental current signal of the target inverter, and the second signal is the first signal and the signal currently output by the target inverter.

[0014] As a possible implementation, using a second signal to change the output current of a target inverter includes: inputting the second signal into a current loop to obtain a third signal; superimposing the third signal on a control signal for the target inverter, and changing the output current of the target inverter using the superimposed control signal. Inputting the second signal into the current loop to obtain the third signal is a method or control unit that connects the output current to a processing link using positive or negative feedback, primarily to improve system performance by enhancing current stability.

[0015] After adjusting the output current of the target inverter based on the target subharmonic signal and the fundamental current signal of the target inverter, the voltage and current at the grid connection point are measured to obtain the resistive component and the inductive component of the grid impedance in the grid.

[0016] As a possible implementation method, obtaining the impedance of the grid-connected system includes: obtaining the dq-axis voltage component and the dq-axis current component of the grid-connected system; and obtaining the inductive component and the resistive component of the resistance of the grid-connected system. The method of obtaining the resistive component of the resistance of the grid-connected system complies with the following formula:

[0017] r g_e is the resistive component of the grid-connected system resistance, U d_hx is the effective value of the d-axis voltage component of the grid-connected system, U q_hx is the effective value of the q-axis voltage component of the grid-connected system, I d_hx is the effective value of the d-axis current component of the grid-connected system, I q_hx is the effective value of the q-axis current component of the grid-connected system;

[0018] The method of obtaining the inductive component of the resistance of the grid-connected system conforms to the following formula:

[0019] L g_e It is the inductive component of the resistance of the grid-connected system.

[0020] As a possible implementation, the AC grid is a single-phase or three-phase AC grid. The dq axis, also known as the dq rotating coordinate system, obtains the voltage and current at the grid connection point and transforms them to obtain the corresponding harmonic voltage and harmonic current components in the stationary coordinate system.

[0021] In a second aspect, the present application provides a grid-connected system, which includes multiple inverters and an impedance detection device, each inverter including a controller; the impedance detection device is used to: adjust the amplitude of the target subharmonic signal according to the current of the grid-connected point, the grid-connected point being the connection point between the grid-connected system and the grid; the controller is used to: adjust the phase of the target subharmonic signal according to the fundamental voltage signal of the target inverter and the grid connection point, the target inverter being any one of the multiple inverters; generate the target subharmonic signal using the amplitude and phase of the target subharmonic signal; adjust the output current of the target inverter based on the target subharmonic signal and the fundamental current signal of the target inverter; the impedance detection device is further used to: obtain the impedance of the grid-connected system after adjusting the output current of the target inverter.

[0022] As a possible implementation, the impedance detection device includes: a harmonic current extraction unit, a first signal processing unit, and a proportional-integral control unit; the harmonic current extraction unit is used to obtain the current of the target subharmonic signal in the current of the grid-connected point; the first signal processing unit is used to obtain the difference between the current of the target subharmonic signal and the current of a preset target subharmonic signal; the proportional-integral control unit is used to adjust the amplitude of the target subharmonic signal based on the difference between the current of the target subharmonic signal and the current of the preset target subharmonic signal.

[0023] As a possible implementation manner, the controller included in each inverter specifically includes: a fundamental wave extraction unit, a phase-locking unit, and a target subharmonic signal generation unit; the fundamental wave extraction unit is used to: obtain the fundamental wave voltage signal at the connection point between the target inverter and the power grid, and input it into the phase-locking unit; the phase-locking unit is used to: output a synchronization signal based on the fundamental wave voltage signal at the connection point between the target inverter and the power grid; the target subharmonic signal generation unit is used to: generate the target subharmonic signal based on the phase of the synchronization signal and the amplitude of the target subharmonic signal input by the impedance detection device.

[0024] As a possible implementation, the controller included in each inverter also includes: a second signal processing unit, a current loop and a drive circuit; the second signal processing unit is used to adjust the sum of the target subharmonic signal and the fundamental current signal of the target inverter to obtain a first signal; to obtain the difference between the first signal and the signal output by the target inverter to obtain a second signal; the current loop is used to: generate a third signal based on the output second signal; the drive circuit is used to: superimpose the third signal on the control signal of the target inverter, and adjust the output current of the target inverter through the superimposed control signal.

[0025] As a possible implementation, the impedance detection device further includes: an impedance acquisition unit; the impedance acquisition unit is configured to: acquire dq axis voltage components and dq axis current components of the grid-connected system; and acquire inductive and resistive components in the resistance of the grid-connected system; wherein the method for acquiring the resistive component in the resistance of the grid-connected system complies with the following formula:

[0026] r g_e is the resistive component of the grid-connected system resistance, U d_hx is the effective value of the d-axis voltage component of the grid-connected system, U q_hx is the effective value of the q-axis voltage component of the grid-connected system, I d_hx is the effective value of the d-axis current component of the grid-connected system, I q_hx is the effective value of the q-axis current component of the grid-connected system; the method for obtaining the inductive component in the resistance of the grid-connected system conforms to the following formula:

[0027] L g_e It is the inductive component of the resistance of the grid-connected system.

[0028] For the description of the technical effects that can be achieved in the second aspect above, please refer to the description of the technical effects that can be achieved by any possible design in the first aspect above, and the repetitions will not be discussed. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural diagram of an existing grid-connected system;

[0030] Figure 2 A flowchart of the steps of a grid impedance detection method;

[0031] Figure 3 This is a structural diagram of a grid-connected system. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings. However, the example embodiments can be implemented in various forms and should not be understood as being limited to the embodiments set forth herein; on the contrary, these embodiments are provided to make the present application more comprehensive and complete, and to fully convey the concepts of the example embodiments to those skilled in the art. The same figure marks in the figures represent the same or similar structures, and thus their repeated descriptions will be omitted. The words expressing position and direction described in this application are all explained using the accompanying drawings as examples, but changes can be made as needed, and all changes are included in the scope of protection of this application. The drawings in this application are only used to illustrate relative position relationships and do not represent true proportions.

[0033] Photovoltaic power generation, as a renewable and clean energy source, has experienced rapid growth. With the continuous expansion of the grid-connected capacity of photovoltaic power generation systems, the connection of a large number of new power generation equipment and the increase in installed capacity, newly built photovoltaic power generation systems are generally located far from the power load center. Therefore, when transmitting electricity over long distances, there is often impedance between the photovoltaic power station and the grid. Grid impedance is one of the main factors leading to weak grids. Under the combined effects of nonlinear loads and line impedance, weak grids in practical applications can no longer be ignored, and they appear slightly inductive. Moreover, when the location of photovoltaic equipment connected to the grid changes, the grid inductive reactance relative to the common coupling point will also fluctuate accordingly. The system short circuit ratio (SCR) is generally used to characterize the degree of grid weakness. The lower the SCR, the weaker the grid, posing a significant challenge to grid stability. Therefore, if we want to increase the proportion of renewable energy integrated into the grid, it is crucial to effectively monitor the grid impedance information of grid-connected power stations.

[0034] Initial grid impedance measurement methods were mostly offline or required additional hardware, making them unsuitable for large-scale distributed generation systems. Current grid impedance measurement methods can be categorized as active and passive. Passive measurement methods include least squares recursive estimation and Kalman filtering, while active measurement methods include specific harmonic injection, pulse signal injection, and power perturbation.

[0035] Passive measurement methods detect and analyze existing disturbance signals in the power grid to obtain line impedance. For example, the Kalman filter method can be used to analyze the real-time measured voltage / current signals to obtain the inverter's feeder impedance. Alternatively, the equivalent impedance of the power grid can be obtained by repeatedly sampling the voltage / current values ​​of the inverter at different operating points. The advantage of passive detection is that it does not require the grid to inject disturbance signals, and does not add harmonic disturbances to the grid, thus having no impact on the grid. The disadvantage is that due to the low signal-to-noise ratio, the impedance identification accuracy is low. In addition, in stable power grid scenarios, the measurement accuracy of impedance detection is difficult to guarantee due to the small amount of disturbance itself.

[0036] The active detection method can measure line impedance by creating a momentary short circuit by connecting an external resistor or capacitor load at the common coupling point (grid connection point). This method can use capacitor switching to change the operating point of the power grid system, and finally analyze and process the voltage and current to obtain the output grid impedance. In addition, an external device can be used to input the fundamental current or harmonic disturbance current into the line to be measured, and then detect the voltage and current values ​​of the line to obtain the grid impedance. The specific principle is: using each inverter in the system to inject specific subfrequency harmonics into the grid, while measuring the voltage and current responses, and performing fast Fourier transform (FFT) analysis to obtain the impedance at the harmonic frequency. The advantages of the harmonic current injection method are accurate control and controllable harmonic current size. By injecting voltage harmonics of characteristic frequencies into the grid and extracting the harmonic currents of the grid, the grid impedance can be detected.

[0037] However, in photovoltaic grid-connected scenarios, multiple inverters are usually connected to the grid. If multiple inverters need to be controlled in parallel and coordinated, harmonics need to be injected into multiple inverters at the same time. When injecting harmonics into multiple inverters, the harmonics injected by each inverter have different phases, and the harmonics injected by each inverter will affect each other, resulting in large impedance detection errors.

[0038] To address the above issues, embodiments of the present application provide a grid impedance detection method and grid-connected system to accurately detect grid impedance in scenarios where multiple inverters operate in parallel. The grid impedance detection method and grid-connected system are based on the same concept, and because the method and system solve similar problems, their implementations can be referenced in conjunction with each other, and any repetitions will not be repeated.

[0039] Below, we first introduce the application scenarios of the embodiments of this application:

[0040] The embodiments of the present application can be applied to a grid-connected system in which multiple inverters are operated in parallel. Figure 1 As shown, Figure 1The grid-connected system in the system includes multiple inverters running in parallel. The AC output terminal of each inverter is connected to the grid connection point (point of common coupling, PCC). The grid-connected system is connected to the AC grid. Multiple inverters connected to the AC grid are not ideal grids. It is generally believed that the inverters are connected to the AC grid through an equivalent impedance Z. Each inverter includes a controller and a main circuit ( Figure 1 (not shown in the figure), the main circuit is used to implement the inverter's inversion function, thereby transmitting at a specific power. The controller is used to drive and control the main circuit to ensure stable operation of the inverter. Specifically, the controller may include a current control unit and a modulation unit. The current control unit is used to generate control instructions based on a given control signal; the modulation unit is used to convert the control instructions generated by the current control unit into drive signals for driving switching devices in the main circuit, thereby controlling the output current of the main circuit in the inverter.

[0041] If the power grid is a three-phase AC power grid, the inverter can be a three-phase grid-connected inverter. The three-phase grid-connected inverter can use a three-phase full-bridge topology as the inverter circuit. The circuit consists of six switching devices, and each phase consists of two switching devices connected in series and connected to the output end of the photovoltaic module.

[0042] The switching device may be one or more of various types of switching devices such as a relay, a metal oxide semiconductor field effect transistor (MOSFET), a bipolar junction transistor (BJT), an insulated gate bipolar transistor (IGBT), a silicon carbide (SiC) power tube, etc., which are not listed one by one in the embodiments of the present application. Each switching device may include a first electrode, a second electrode, and a control electrode, wherein the control electrode is used to control the closing or opening of the switch.

[0043] Figure 1 The grid-connected system shown can be connected to photovoltaic power generation, wind power generation, wind-solar hybrid power generation systems, and other systems. For example, in a photovoltaic power generation system, multiple photovoltaic power generation units (photovoltaic modules) are connected to multiple inverters in a one-to-one correspondence. The multiple photovoltaic power generation units are connected to the AC power grid through multiple inverters, which convert the DC power generated by the multiple photovoltaic power generation units into AC power, which is then transmitted to the AC power grid.

[0044] It should be noted that in Figure 1In the grid-connected system shown, the AC grid refers to an AC network composed of multiple devices. The number of devices in the AC grid is not limited in the embodiment of the present application. In addition, the AC grid can be a single-phase AC grid or a three-phase AC grid, and the embodiment of the present application does not make specific limitations on this. In addition, Figure 1 The grid-connected system can also be connected to an energy storage system, that is, multiple energy storage batteries in the energy storage system are connected to the AC power grid through multiple inverters.

[0045] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application will be described in further detail below with reference to the accompanying drawings. It should be noted that in the description of the present application, "at least one" refers to one or more, wherein "a plurality" refers to two or more. In view of this, "a plurality" may also be understood as "at least two" in the embodiment of the present invention. "And / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / ", unless otherwise specified, generally indicates that the previous and next associated objects are in an "or" relationship. In addition, it should be understood that in the description of the present application, words such as "first" and "second" are only used to distinguish the purpose of description, and cannot be understood as indicating or implying relative importance, nor can they be understood as indicating or implying order.

[0046] See Figure 2 As shown, Figure 2 A schematic diagram of a flow chart of a grid impedance detection method provided in an embodiment of the present application, Figure 2 The method shown can be applied to Figure 1 The grid-connected system shown includes multiple inverters. It should be noted that when grid impedance detection is required, at least one inverter in the parallel grid is in a grid-connected state.

[0047] The method comprises the following steps:

[0048] S201: Adjust the amplitude of the target subharmonic signal according to the current of the grid connection point, where the grid connection point is the connection point between the grid connection system and the power grid. Figure 1The connection point between the grid-connected system and the AC power grid, the grid-connected point current can be obtained through the current sampling unit. Exemplarily, the current sampling unit may include a current transformer (CT), which is composed of a closed iron core and a winding. Its primary side winding has very few turns (in this application, the primary side winding is the connecting cable between the grid-connected point and the AC power grid). The current transformer is placed on the cable to be measured in series in the circuit of the current to be measured, and the principle of electromagnetic mutual induction is used to detect the current on the cable to be measured. Among them, the extracted grid-connected point current includes a variety of harmonic components of different characteristic orders.

[0049] As a possible implementation method, the current value of the target subharmonic signal in the current of the grid connection point is obtained; the amplitude of the target subharmonic signal is adjusted using the current of the target subharmonic signal and the current of the preset target subharmonic signal. The target subharmonic signal component is extracted from the harmonic components of different characteristic orders by FFT. The extracted target subharmonic signal component is compared with the preset target subharmonic signal value to obtain an error signal, and the error signal is subjected to proportional integration differentiation (PI) control to obtain the amplitude of the target subharmonic signal. The amplitude output after proportional integration control is related to the target subharmonic component of the current power grid and the preset target subharmonic component. In order to keep the target subharmonic component of the current power grid always within a fixed range, the more existing target subharmonic components of the current power grid, the fewer target subharmonic components injected into the current power grid during the cycle.

[0050] The frequency of the target subharmonic signal is selected primarily to minimize the impact of grid voltage harmonics. To reduce detection error, the frequency of the target subharmonic signal needs to be as close to the fundamental frequency as possible. Furthermore, the injected frequency should be a frequency not present in the grid before injection. This ensures that the injected target subharmonic current is entirely generated by the injected target subharmonic voltage. However, due to the widespread presence of integer harmonic components in the power frequency grid (50 Hz), the frequency of the target subharmonic signal can be selected as 75 Hz or 125 Hz. This ensures that the frequency of the target subharmonic signal is close to the fundamental frequency, while the injected frequency is not present in the grid. On the one hand, the frequency of the 75 Hz or 125 Hz harmonic signal is very close to the fundamental frequency. On the other hand, there are no 75 Hz harmonic components in the grid. Of course, the frequency of the target subharmonic signal can also be selected as a high-frequency harmonic signal of 500 Hz, thereby reducing the time it takes to inject harmonics.

[0051] S202: Adjusting the phase of the target subharmonic signal according to a fundamental voltage signal at a connection point between a target inverter and a power grid, where the target inverter is any one of the multiple inverters.

[0052] Since the impedance characteristics of each distributed grid-connected point are different and the distances between the distributed grid-connected points and the grid are also different, if the target subharmonic signals with the same amplitude and phase are used as control signals, the sampling accuracy will be affected and they will affect each other.

[0053] In order to prevent the specific subharmonics injected into each target inverter from affecting each other, the target subharmonic signal of the target inverter needs to be controlled to ensure that it is in the same phase as the fundamental voltage signal between the target inverter and the power grid. The detection circuit can be used to sample the connection point between the target inverter and the power grid to obtain the fundamental voltage signal at the connection point between the target inverter and the power grid. The phase of the target subharmonic signal can be adjusted based on the fundamental voltage signal at the connection point between the target inverter and the power grid. As a possible implementation method, after obtaining the fundamental voltage signal at the connection point between the target inverter and the power grid, the signal phase of the fundamental voltage signal at the connection point between the target inverter and the power grid can be adjusted to the phase of the target subharmonic signal.

[0054] After sampling the fundamental voltage signal at the connection point between the target inverter and the grid, a phase-locked loop (PLL) can be used to obtain a synchronization signal with the same phase as the fundamental voltage signal. The phase of the synchronization signal is the same as the phase of the fundamental voltage signal.

[0055] S203: Generate a target subharmonic signal using the amplitude of the target subharmonic signal and the phase of the target subharmonic signal.

[0056] After obtaining the amplitude of the target subharmonic signal and the phase of the target subharmonic signal, the target subharmonic signal is generated by the phase of the target subharmonic signal and the amplitude of the target subharmonic signal. The phase of the target subharmonic signal is the same as the fundamental voltage signal, and the amplitude of the target subharmonic signal is the amplitude adjusted in step S201.

[0057] In this way, the amplitude of the target subharmonic signal used to control the output current of each target inverter is adjusted based on the target subharmonic component of the current power grid. The larger the target subharmonic component of the current power grid, the lower the amplitude of the target subharmonic signal; the smaller the target subharmonic component of the current power grid, the higher the amplitude of the target subharmonic signal. The phase of the target subharmonic signal controlling each target inverter is the same as the phase of the fundamental voltage signal at the connection point between the target inverter and the power grid, so they do not affect each other.

[0058] S204: Adjust the output current of the target inverter based on the target subharmonic signal and the fundamental current signal of the target inverter to obtain the impedance of the grid-connected system.

[0059] The target subharmonic signal, after being superimposed on the fundamental current signal of the target inverter, can be used as a control instruction for the target inverter, thereby causing the target inverter to generate a corresponding harmonic current (target subharmonic current).

[0060] As a possible implementation manner, adjusting the output current of the target inverter based on the target subharmonic signal and the fundamental current signal of the target inverter includes:

[0061] The method comprises adjusting the sum of the target subharmonic signal and the fundamental current signal of the target inverter to obtain a first signal, obtaining a difference between the first signal and the signal output by the target inverter to obtain a second signal, and using the second signal to change the output current of the target inverter.

[0062] The first signal is a superposition signal of the target subharmonic signal and the fundamental current signal of the target inverter, and the second signal is a difference between the first signal and a signal currently output by the target inverter.

[0063] Optionally, using the second signal to change the output current of the target inverter includes: inputting the second signal into the current loop to obtain a third signal; superimposing the third signal on the control signal of the target inverter, and changing the output current of the target inverter through the superimposed control signal.

[0064] Among them, the second signal (the difference between the first signal and the signal currently output by the target inverter) is input into the current loop to obtain the third signal. The current loop is also called the current loop proportional integral controller or current feedback system, which refers to a method or control unit that connects the output current to the processing link by positive feedback or negative feedback, mainly to improve the performance of the system by improving the stability of the current.

[0065] After adjusting the output current of the target inverter based on the target subharmonic signal and the fundamental current signal of the target inverter, the voltage and current at the grid connection point are measured to obtain the resistive component and the inductive component of the grid impedance in the grid.

[0066] As a possible implementation method, obtaining the impedance of the grid-connected system includes: obtaining the dq axis voltage component and the dq axis current component of the grid-connected system; obtaining the inductive component and the resistive component of the resistance of the grid-connected system;

[0067] The method for obtaining the resistive component in the resistance of the grid-connected system complies with the following formula:

[0068] r g_e is the resistive component in the resistance of the grid-connected system, and the U d_hx is the effective value of the d-axis voltage component of the grid-connected system, and U q_hx is the effective value of the q-axis voltage component of the grid-connected system, and I d_hx is the effective value of the d-axis current component of the grid-connected system, and I q_hx is the effective value of the q-axis current component of the grid-connected system;

[0069] The method for obtaining the inductive component in the resistance of the grid-connected system conforms to the following formula:

[0070] L g_e is the inductive component in the resistance of the grid-connected system.

[0071] Among them, the dq axis is also called the dq rotating coordinate system. The voltage and current of the grid-connected point are obtained, and they are transformed to obtain the corresponding harmonic voltage components and harmonic current components of the stationary coordinate system.

[0072] The instantaneous power of the nonlinear load is obtained by obtaining the formula of active power and reactive power under isovector transformation. The dq rotating coordinate system components of the nonlinear load current containing harmonic components are obtained according to power conservation and isovector transformation. The fundamental current component is obtained after current filtering. The dq rotating coordinate system components of the nonlinear load current are subtracted from the fundamental current components to obtain the harmonic current components in the dq rotating coordinate system. Similarly, the harmonic voltage components in the dq rotating coordinate system can also be obtained.

[0073] For example, if it is a three-phase AC grid (phases A, B, and C), the voltage sensor can be used to sample the grid connection point to obtain the line voltage U of the grid connection point. gAB , line voltage U gBC and line voltage U gCA , to obtain the phase voltage U of the three-phase power grid gA , phase voltage U gB and phase voltage U gC , the phase voltage U gA , phase voltage U gB and phase voltage U gC Transformed into the two-phase stationary dq coordinate system, the voltage U is obtained gD and voltage U gQ , to obtain the voltage U gD and voltage U gQ The effective value of the d-axis voltage component U is obtained by d_hx and the effective value of the q-axis voltage component Uq_hx .

[0074] Then use the current sensor to sample the grid point to obtain the three-phase current I A , three-phase current I B and three-phase current I C and the three-phase current I A , three-phase current I B and three-phase current I C Transformed into the two-phase stationary dq coordinate system, the two current components are current I D and current I Q , the current I D and current I Q The two current components in the synchronous rotating dq coordinate system are obtained by coordinate transformation and are current I d and current I q , to obtain the current I d and current I q The effective value of the d-axis current component I d_hx and the effective value of the q-axis current component I q_hx .

[0075] By using the grid impedance detection method provided in the embodiment of the present application, under the unified coordination of the controller, the current of the target subharmonic injected into the closed loop of the grid connection point is used as the control instruction. At the same time, the target inverters of each power station are phase-synchronized according to the phase of the fundamental voltage signal at the connection point between their respective ports and the grid, so that the target subharmonics injected into each target inverter do not affect each other, thereby effectively reducing the error in grid impedance detection and acquisition, so as to obtain accurate grid impedance.

[0076] Based on the same concept, this application also provides a grid-connected system, see Figure 3 As shown, Figure 3The schematic diagram of the structure of a grid-connected system is shown in FIG. 300. The grid-connected system 300 includes a plurality of inverters 301 and an impedance detection device 302. Each inverter 301 includes a controller 303. The impedance detection device 302 is used to adjust the amplitude of the target subharmonic signal according to the current of the grid-connected point. The grid-connected point is the connection point between the grid-connected system 300 and the grid 304. The controller 303 is used to adjust the amplitude of the target subharmonic signal according to the fundamental voltage signal of the connection point between the target inverter 3011 and the grid 304. phase, the target inverter 3011 is any one of the multiple inverters 301; using the amplitude of the target subharmonic signal and the phase of the target subharmonic signal, generating a target subharmonic signal; based on the target subharmonic signal and the fundamental current signal of the target inverter 3011, adjusting the output current of the target inverter 3011; the impedance detection device 302 is further used to: after adjusting the output current of the target inverter 3011, to obtain the impedance of the grid-connected system 300.

[0077] In addition, the grid-connected system 300 may further include an alternating current-direct current (AC-DC) conversion circuit 305 and a direct current-alternating current (DC-AC) conversion circuit 306. The AC-DC conversion circuit 305 is configured to convert the first AC voltage output by the inverter 301 into a first DC voltage, and the DC-AC conversion circuit 306 is configured to convert the first DC voltage input by the AC-DC conversion circuit 305 into a second AC voltage that is input to the grid 304.

[0078] As a possible implementation, continue to refer to Figure 3 As shown, the impedance detection device 302 includes: a harmonic current extraction unit 3021, a first signal processing unit 3022 and a proportional-integral control unit 3023; the harmonic current extraction unit 3021 is used to obtain the current of the target subharmonic signal in the current of the grid-connected point; the first signal processing unit 3022 is used to obtain the difference between the current of the target subharmonic signal and the current of a preset target subharmonic signal; the proportional-integral control unit 3023 is used to adjust the amplitude of the target subharmonic signal based on the difference between the current of the target subharmonic signal and the current of the preset target subharmonic signal.

[0079] As a possible implementation, continue to refer to Figure 3As shown, the controller 303 included in each inverter 301 specifically includes: a fundamental wave extraction unit 3031, a phase-locked unit 3032 and a target subharmonic signal generation unit 3033; the fundamental wave extraction unit 3031 is used to: obtain the fundamental wave voltage signal of the connection point between the target inverter 3011 and the power grid 304, and input the fundamental wave voltage signal into the phase-locked unit 3032; the phase-locked unit 3032 is used to: output a synchronization signal according to the fundamental wave voltage signal of the connection point between the target inverter 3011 and the power grid 304; the target subharmonic signal generation unit 3033 is used to: generate the target subharmonic signal based on the phase of the synchronization signal and the amplitude of the target subharmonic signal input by the impedance detection device 302.

[0080] As a possible implementation, continue to refer to Figure 3 As shown, the controller 303 included in each inverter 301 also includes: a second signal processing unit 3034, a current loop 3035 and a drive circuit 3036; the second signal processing unit 3034 is used to adjust the sum of the target subharmonic signal and the fundamental current signal of the target inverter to obtain a first signal; to obtain the difference between the first signal and the signal output by the target inverter 3011 to obtain a second signal; the current loop 3035 is used to: generate a third signal based on the output second signal; the drive circuit 3036 is used to: superimpose the third signal on the control signal of the target inverter 3011, and adjust the output current of the target inverter 3011 through the superimposed control signal.

[0081] As a possible implementation, continue to refer to Figure 3 As shown, the impedance detection device 302 further includes an impedance calculation unit 3024; the impedance calculation unit 3024 is configured to obtain the dq axis voltage component and the dq axis current component of the grid-connected system 300; and to obtain the inductive component and the resistive component of the resistance of the grid-connected system 300; wherein the method for obtaining the resistive component of the resistance of the grid-connected system complies with the following formula:

[0082] r g_e is the resistive component in the resistance of the grid-connected system 300, and the U d_hx is the effective value of the d-axis voltage component of the grid-connected system 300, and U q_hx is the effective value of the q-axis voltage component of the grid-connected system 300, and the I d_hx is the effective value of the d-axis current component of the grid-connected system 300, and the I q_hx is the effective value of the q-axis current component of the grid-connected system 300;

[0083] The method for obtaining the inductive component in the resistance of the grid-connected system 300 complies with the following formula:

[0084] L g_e is the inductive component in the resistance of the grid-connected system 300 .

[0085] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or machine program products. Therefore, the present application may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Furthermore, the present application may take the form of a machine program product implemented on one or more machine-usable storage media (including but not limited to magnetic disk storage, CD-ROM, optical storage, etc.) containing machine-usable program code.

[0086] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and acquisition machine program products according to the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by acquisition machine program instructions. These acquisition machine program instructions can be provided to a processor of a general-purpose acquisition machine, a special-purpose acquisition machine, an embedded processing machine, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the acquisition machine or other programmable data processing device generate instructions for implementing the processes in the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0087] These acquisition machine program instructions may also be stored in an acquisition machine readable memory that can direct an acquisition machine or other programmable data processing device to operate in a specific manner, so that the instructions stored in the acquisition machine readable memory produce an article of manufacture including an instruction device that implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0088] These acquisition machine program instructions can also be loaded onto an acquisition machine or other programmable data processing device so that a series of operation steps are executed on the acquisition machine or other programmable device to produce the processing implemented by the acquisition machine, thereby providing instructions for executing on the acquisition machine or other programmable device to implement the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0089] Obviously, those skilled in the art may make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalents, this application is intended to include these modifications and variations.

Claims

1. A method for detecting grid impedance, characterized in that: The method is applied to a grid-connected system, wherein the grid-connected system includes a plurality of inverters, and the method includes: adjusting the amplitude of the target subharmonic signal according to the current at the grid connection point, wherein the grid connection point is the connection point between the grid connection system and the power grid; adjusting the phase of the target subharmonic signal according to a fundamental voltage signal at a connection point between a target inverter and a power grid, wherein the target inverter is any one of the plurality of inverters; generating a target subharmonic signal by using the amplitude of the target subharmonic signal and the phase of the target subharmonic signal; adjusting an output current of the target inverter based on the target subharmonic signal and a fundamental current signal of the target inverter; The impedance of the grid-connected system is obtained according to the dq axis voltage components and the dq axis current components of the grid-connected system.

2. The method according to claim 1, characterized in that Adjusting the amplitude of the target subharmonic signal according to the current of the grid connection point includes: Obtaining a current value of the target subharmonic signal in the current of the grid connection point; The amplitude of the target subharmonic signal is adjusted by using the current of the target subharmonic signal and the current of a preset target subharmonic signal.

3. The method according to claim 2, characterized in that Adjusting the amplitude of the target subharmonic signal by using the current of the target subharmonic signal and the current of a preset target subharmonic signal includes: Based on the difference between the current of the target subharmonic signal and the current of a preset target subharmonic signal, the amplitude of the target subharmonic signal is adjusted by a proportional-integral controller.

4. The method according to any one of claims 1 to 3, characterized in that: Adjusting the phase of the target subharmonic signal according to the fundamental voltage signal at the connection point between the target inverter and the power grid includes: The signal phase of the fundamental voltage signal at the connection point between the target inverter and the grid is adjusted to the phase of the target subharmonic signal.

5. The method according to claim 4, characterized in that Adjusting the signal phase of the fundamental voltage signal at the connection point between the target inverter and the grid, and adjusting the signal phase of the fundamental voltage signal at the connection point between the target inverter and the grid to the phase of the target subharmonic signal, includes: The fundamental voltage signal at the connection point between the target inverter and the power grid is input into a phase-locked unit, and the phase of the synchronization signal output by the phase-locked unit is the phase of the target subharmonic signal.

6. The method according to any one of claims 1 to 3 or 5, characterized in that: Adjusting the output current of the target inverter based on the target subharmonic signal and the fundamental current signal of the target inverter includes: Adjusting the sum of the target subharmonic signal and the fundamental current signal of the target inverter to obtain a first signal; Obtaining a difference between the first signal and a signal output by the target inverter to obtain a second signal; The output current of the target inverter is changed using the second signal.

7. The method according to claim 6, characterized in that The changing the output current of the target inverter by using the second signal includes: inputting the second signal into a current loop to obtain a third signal; The third signal is superimposed on the control signal of the target inverter, and the output current of the target inverter is changed by the superimposed control signal.

8. The method according to any one of claims 1 to 3, 5 or 7, characterized in that: To obtain the impedance of the grid-connected system, including: Obtaining the inductive component and the resistive component of the resistance of the grid-connected system; The method for obtaining the resistive component in the resistance of the grid-connected system complies with the following formula: , is the resistive component in the resistance of the grid-connected system, is the effective value of the d-axis voltage component of the grid-connected system, is the effective value of the q-axis voltage component of the grid-connected system, is the effective value of the d-axis current component of the grid-connected system, is the effective value of the q-axis current component of the grid-connected system; The method for obtaining the inductive component in the resistance of the grid-connected system conforms to the following formula: , is the inductive component in the resistance of the grid-connected system.

9. The method according to claim 1, characterized in that The power grid is a single-phase AC power grid or a three-phase AC power grid.

10. A grid-connected system, characterized in that: It includes multiple inverters and impedance detection devices, and each inverter includes a controller; The impedance detection device is used to adjust the amplitude of the target subharmonic signal according to the current of the grid connection point, wherein the grid connection point is the connection point between the grid connection system and the power grid; The controller is configured to: adjust the phase of the target subharmonic signal according to a fundamental voltage signal at a connection point between a target inverter and a power grid, wherein the target inverter is any one of the plurality of inverters; and generate a target subharmonic signal using the amplitude and phase of the target subharmonic signal; adjusting an output current of the target inverter based on the target subharmonic signal and a fundamental current signal of the target inverter; The impedance detection device is further used to obtain the impedance of the grid-connected system based on the dq axis voltage components and dq axis current components of the grid-connected system after adjusting the output current of the target inverter.

11. The grid-connected system according to claim 10, characterized in that: The impedance detection device includes: a harmonic current extraction unit, a first signal processing unit and a proportional integral control unit; The harmonic current extraction unit is used to: obtain the current of the target subharmonic signal in the current of the grid connection point; The first signal processing unit is configured to obtain a difference between the current of the target subharmonic signal and the current of a preset target subharmonic signal; The proportional-integral control unit is configured to adjust the amplitude of the target subharmonic signal based on a difference between the current of the target subharmonic signal and the current of a preset target subharmonic signal.

12. The grid-connected system according to claim 10 or 11, characterized in that: The controller included in each inverter specifically includes: a fundamental wave extraction unit, a phase locking unit and a target subharmonic signal generation unit; The fundamental wave extraction unit is used to: obtain the fundamental wave voltage signal of the connection point between the target inverter and the power grid, and input the fundamental wave voltage signal into the phase lock unit; The phase-locked unit is configured to: output a synchronization signal according to a fundamental voltage signal at a connection point between the target inverter and the power grid; The target subharmonic signal generating unit is configured to generate the target subharmonic signal based on the phase of the synchronization signal and the amplitude of the target subharmonic signal input by the impedance detection device.

13. The grid-connected system according to claim 10 or 11, characterized in that: The controller included in each inverter further includes: a second signal processing unit, a current loop and a drive circuit; The second signal processing unit is configured to adjust the sum of the target subharmonic signal and the fundamental current signal of the target inverter to obtain a first signal; and to obtain a difference between the first signal and a signal output by the target inverter to obtain a second signal; The current loop is configured to: generate a third signal based on the output second signal; The driving circuit is configured to superimpose the third signal on the control signal of the target inverter, and adjust the output current of the target inverter through the superimposed control signal.

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