A harmonic synchronous droop control method for impedance measurement

Abstract

The present invention discloses a harmonic synchronous droop control method for impedance measurement, which relates to the technical field of grid-connected inverter control. The system studied by the method includes a DC side voltage source, a three-phase full-bridge inverter circuit, a three-phase LC filter, and a three-phase line impedance. The method does not require an additional communication system and only detects the harmonic voltage and current at the PCC point. When injecting harmonic disturbances to measure impedance, it can achieve synchronization of harmonic disturbance injection into multiple inverters without adding additional costs.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected inverter control, and in particular relates to a harmonic synchronous droop control method for impedance measurement. Background Art

[0002] As the penetration of renewable energy generation continues to increase, the power grid is gradually shifting from being primarily driven by synchronous generators to being primarily driven by inverters. Inverter control methods primarily include grid-following and grid-forming. The inverter's operating mode must be adjusted based on the grid's impedance to maintain system stability. Grid impedance detection technology is crucial in this regard. To improve the accuracy of impedance detection, it is necessary to actively inject harmonic disturbances into the grid.

[0003] At present, there are many academic papers on the active injection method of grid impedance detection, such as:

[0004] 1. "Grid Impedance Estimation Method Based on Complex Filters and Non-Characteristic Subharmonic Injection," Power System Technology, Issue 10, 2013, pp. 2796-2801. This article uses non-characteristic subharmonic injection to measure grid impedance, but considers harmonic injection from the perspective of only one inverter, not the case where multiple inverters inject harmonic disturbances simultaneously.

[0005] 2. The Chinese invention patent publication number CN109902347A discloses a method and device for measuring the short-circuit ratio of a grid-connected system based on fundamental impedance identification. This method proposes a method for measuring the short-circuit ratio of a grid-connected system based on harmonic disturbance injection. This method injects disturbances into the power grid through multiple inverters and calculates the short-circuit ratio of the system based on the voltage and current responses. However, the present invention does not consider the synchronization problem of the harmonic disturbance signals of multiple inverters.

[0006] 3. An article titled “An Active Islanding Detection Strategy With Zero Nondetection Zone for Operation in Single and Multiple Inverter Mode Using GPS Synchronized Pattern,” Deepthi Sivadas, Krishna Vasudevan, IEEE Transactions on Industrial Electronics, 2020: 5554-5564 (published online in 2020) proposes using the Global Positioning System (GPS) to synchronize the harmonic disturbances of multiple inverters. However, this would place higher demands on the inverter’s communication system, significantly increasing costs.

[0007] Based on the above literature, the existing technology has the following deficiencies:

[0008] 1. The existing impedance estimation method based on disturbance injection only injects harmonic disturbance from the perspective of a single machine. It is necessary to study the synchronization method of harmonic disturbance when multiple machines are injected simultaneously.

[0009] 2. The existing method of multi-machine harmonic phase synchronization requires high-precision communication to be implemented, which greatly increases the cost. It is necessary to study a multi-machine harmonic synchronization method that does not require communication. Summary of the Invention

[0010] The technical problem to be solved by the present invention is the harmonic voltage synchronization problem when multiple inverters simultaneously inject harmonic disturbance voltages under the existing impedance measurement technology. Specifically, the present invention adopts a harmonic synchronization droop control method. This method does not require an additional communication system. By simply sampling the harmonic voltage and current, the phase synchronization of the harmonic signals injected simultaneously by multiple inverters can be achieved by controlling the droop of the harmonic voltage in the disturbance injection control of the inverter, without increasing additional communication costs.

[0011] The present invention provides a droop control method for harmonic synchronization, the steps of which are as follows:

[0012] For each inverter running in parallel, the harmonic voltage at the output end of the three-phase LC filter capacitor is sampled and separated, and the harmonic current flowing through the three-phase line inductance is sampled;

[0013] The collected harmonic voltage is transformed from a three-phase stationary coordinate system to a two-phase stationary coordinate system to obtain the αβ axis components of the harmonic voltage; the collected harmonic current is transformed from a three-phase stationary coordinate system to a two-phase stationary coordinate system to obtain the αβ axis components of the harmonic current;

[0014] According to the αβ-axis components of the harmonic voltage and the αβ-axis components of the harmonic current, the harmonic active power output by each inverter is obtained;

[0015] According to the harmonic active power and harmonic reactive power output by each inverter, a harmonic frequency command signal and a harmonic D-axis voltage command signal of each inverter are obtained;

[0016] SPWM modulation is performed according to the obtained command signal to generate the switching signal of each inverter power device and control the output of each inverter.

[0017] Preferably, the coordinate transformation equations of the harmonic voltage and harmonic current are:

[0018]

[0019] Among them, u har_A ,u har_B ,u har_C is the harmonic voltage at the output of the three-phase LC filter capacitor, i har_A ,i har_B ,i har_C is the harmonic current of the three-phase line inductance, u har_α ,u har_β is the αβ axis component of the harmonic voltage, i har_α ,i har_β is the αβ axis component of the harmonic current.

[0020] Preferably, the harmonic active power and harmonic reactive power of the inverter are obtained by calculating the average power output by the grid-connected inverter:

[0021]

[0022] Where, P har is the harmonic active power, Q har is the harmonic reactive power, T LPF is the low-pass filter time constant, s is the Laplace operator, u har_α ,u har_β is the αβ axis component of the harmonic voltage, i har_α ,i har_β is the αβ axis component of the harmonic current.

[0023] Preferably, the harmonic frequency command signal and the harmonic d-axis voltage command signal of the inverter are obtained through the harmonic power loop droop control equation:

[0024]

[0025] Where, ω har_ref is the harmonic frequency command signal, u har_dref is the harmonic d-axis voltage command signal, P har_ref is the rated active power of harmonics, Q har_ref is the harmonic rated reactive power, ω har_n is the rated angular frequency of harmonics, u har_n is the rated amplitude of harmonics, m har is the active droop coefficient of the harmonic power loop, n har is the reactive droop coefficient of the harmonic power loop, P har is the harmonic active power, Q har is the harmonic reactive power.

[0026] The present invention also provides a grid-connected inverter power control system, characterized in that it includes a controller and a grid-connected inverter; the grid-connected inverter operates in a harmonic generation mode; the controller adopts the above-mentioned harmonic synchronous droop control method for impedance measurement to control the power devices of each inverter and control the output of each inverter so that the grid-connected inverter outputs harmonics with specified frequency, amplitude and phase synchronization.

[0027] Preferably, the main circuit topology of the inverter includes a DC side voltage source, a three-phase full-bridge inverter circuit, a three-phase LC filter, a three-phase line impedance and a three-phase power grid. The DC side voltage source is connected to the three-phase full-bridge inverter circuit, and the three-phase full-bridge inverter circuit is connected to the three-phase line impedance through the three-phase LC filter and then connected to the three-phase power grid.

[0028] The present invention also provides a readable storage medium having a computer program stored thereon, and when the computer program is executed, the harmonic synchronous droop control method for impedance measurement is implemented.

[0029] The present invention also provides a device comprising a processor, a memory, and a computer program stored in the memory and executable on the processor, wherein the processor implements the above-mentioned harmonic synchronous droop control method for impedance measurement when executing the computer program.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The present invention is simple to implement. It only needs to perform corresponding calculations when each inverter generates a disturbance signal instruction to achieve the synchronization of harmonic disturbance signals among multiple inverters.

[0032] 2. Compared with the method using GPS synchronization, the present invention does not require additional hardware, reduces costs, and simplifies the structure of the inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The main circuit topology diagram of the grid-connected inverter using this method.

[0034] Figure 2 4 is an implementation flow chart of the control method of the present invention.

[0035] Figure 3 This is a phase diagram of the harmonic voltage output by two inverters according to an embodiment of the present invention.

[0036] Figure 4 This is a frequency diagram of the harmonic voltage output by two inverters according to an embodiment of the present invention. DETAILED DESCRIPTION

[0037] This embodiment will be described in detail below with reference to the accompanying drawings.

[0038] Example 1

[0039] Figure 1 1 is a topological structure diagram of the main circuit of the grid-connected inverter applying this embodiment. As can be seen from the figure, the topological structure includes a DC side voltage source 10, a three-phase full-bridge inverter circuit 20, a three-phase LC filter 30, and a three-phase line impedance 40.

[0040] The DC side voltage source 10 is connected to the three-phase full-bridge inverter circuit 20, and the three-phase full-bridge inverter circuit 20 is connected to the three-phase line impedance 40 via the three-phase LC filter 30. Figure 1 In, V dc is the DC side voltage source 10, L f is the 30 Ω bridge arm side inductance of the three-phase LC filter, C f is the filter capacitor of the three-phase LC filter 30, r is the passive damping resistor of the three-phase LC filter 30, L g is the inductance in the three-phase line impedance 40.

[0041] The main circuit parameters in this embodiment are: DC side voltage V dc The rated output line voltage of the inverter is 770V / 50Hz, the rated power of a single inverter is 20kW, and the filter inductor L f is 0.9mH, the filter capacitor C f The passive damping resistor r is 11.6uF / 2.18Ω, and the simulated line inductance L g It is 11.75mH.

[0042] A droop control method for harmonic synchronization of the present invention comprises the following steps:

[0043] Step 1: For each inverter running in parallel, sample and separate the harmonic voltage at the output end of the capacitor of the three-phase LC filter (30) and record it as u har_A ,u har_B ,u har_C , the harmonic current flowing through the three-phase line inductance (40) is sampled and recorded as i har_A ,i har_B ,i har_C .

[0044] Step 2: Each inverter running in parallel generates the harmonic voltage u collected in step 1. har_A ,u har_B ,u har_C Transform the three-phase stationary coordinate system to the two-phase stationary coordinate system to obtain the αβ axis component u of the harmonic voltage har_α ,u har_β ; Collect the harmonic current i in step 1 har_A ,i har_B ,i har_C Transform the three-phase stationary coordinate system to the two-phase stationary coordinate system to obtain the αβ axis components of the harmonic current i har_α ,i har_β .

[0045] In this embodiment, the coordinate transformation equation is:

[0046]

[0047] Step 3: Each inverter is connected to the harmonic voltage according to the αβ axis component u har_α ,u har_β αβ axis components of the harmonic current i har_α ,i har_β , the harmonic active power P output by each inverter is obtained by the average power calculation equation of the grid-connected inverter output har and harmonic reactive power Q har .

[0048] In this embodiment, the calculation equation for the average power output by each grid-connected inverter is:

[0049]

[0050] Where, T LPF is the low-pass filter time constant, and s is the Laplace operator.

[0051] In this example, T LPF =0.013s.

[0052] Step 4: According to the harmonic active power P output by each inverter har And the output harmonic reactive power Qhar , the respective harmonic frequency command signals ω are obtained through the harmonic power loop droop control equation har_ref and harmonic d-axis voltage command signal u har_dref .

[0053] In this embodiment, the power loop droop control equation is:

[0054]

[0055] Where, P har_ref is the rated active power of harmonics, Q har_ref is the harmonic rated reactive power, ω har_n is the rated angular frequency of harmonics, u har_n is the rated amplitude of harmonics, m har is the active droop coefficient of the harmonic power loop, n har is the reactive droop coefficient of the harmonic power loop.

[0056] In this example, P har_ref =0,Q har_ref =20Var,ω har_n =150π,u har_n =311V,m har =10,n har =642.

[0057] Step 5: Perform SPWM modulation according to the command signal obtained in step 4 to generate the switching signal of each inverter power device, control the output of each inverter, and wait for the controller to issue the next voltage and current sampling instruction, then return to step 1 and enter the next control loop.

[0058] In order to verify the effect of the present invention, MATLAB / simulink simulation was performed on the present invention.

[0059] Figure 3 This is the synchronization of the 75H harmonics in the present invention when two inverters are connected in parallel. As can be seen from the figure, the harmonics of the two inverters have completed the synchronization of the harmonic phases, and the phases of the two are exactly the same.

[0060] Figure 4 This is the synchronization of the 75Hz harmonics in this example when two inverters are connected in parallel. As can be seen from the figure, after the transient process, the frequency of the harmonic voltage output by both inverters is the same, 75Hz.

[0061] Figure 3 、 Figure 4 This can well demonstrate the effectiveness of the method proposed in this invention.

[0062] Example 2

[0063] A grid-connected inverter power control system comprises a controller and a grid-connected inverter; the grid-connected inverter operates in a harmonic generation mode; the controller uses a harmonic synchronous droop control method for impedance measurement according to any one of claims 1 to 4 to control the power devices of each inverter and the output of each inverter, so that the grid-connected inverter outputs harmonics with a specified frequency, amplitude and phase synchronization.

[0064] Figure 1 1 is a topological structure diagram of the main circuit of the grid-connected inverter using this embodiment. The main circuit topology of the inverter includes a DC side voltage source, a three-phase full-bridge inverter circuit, a three-phase LC filter, a three-phase line impedance, and a three-phase grid. The DC side voltage source is connected to the three-phase full-bridge inverter circuit, and the three-phase full-bridge inverter circuit is connected to the three-phase line impedance via the three-phase LC filter and then connected to the three-phase grid.

[0065] Example 3

[0066] An embodiment of the present application may also be a readable storage medium having computer program instructions stored thereon, which, when executed by a processor, causes the processor to execute the steps of the decision-making behavior decision-making method according to various embodiments of the present application described in the above method of this specification.

[0067] The computer-readable storage medium can adopt any combination of one or more readable media. The readable medium can be a readable signal medium or a readable storage medium. The readable storage medium can, for example, include but is not limited to a system, device or component of electricity, magnetism, light, electromagnetic, infrared, or semiconductor, or any combination thereof. More specific examples (non-exhaustive list) of readable storage media include: an electrical connection with one or more wires, a portable disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination thereof.

[0068] Example 4

[0069] An embodiment of the present application may also be a device, which includes a processor, a memory, and a computer program stored in the memory and run on the processor. When the processor executes the computer program, it implements the steps of the decision-making behavior decision-making method according to various embodiments of the present application described in the above method of this specification.

[0070] The circuit topology and elimination method of the present invention described above can be viewed as hardware embodiments of the circuit topology alone, as software embodiments of the elimination method alone, or as a combination of hardware and software embodiments based on the circuit topology and module elimination method. Furthermore, the elimination method portion of the present invention can be implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROMs, optical storage, etc.) containing computer-usable program code, in the form of a computer program product. The elimination method can be implemented in various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0071] Furthermore, the embodiments of the present invention are described in conjunction with flowcharts and / or block diagrams. It should be understood that each process and / or block in the flowcharts and / or block diagrams of the present invention, as well as the combination of the processes and / or blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the present invention. Figure 1 a process or multiple processes and / or boxes Figure 1 These computer program instructions can also be stored in a computer-readable memory that can guide a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory produce a product including an instruction device, which implements the function specified in the process of the present invention. Figure 1 a process or multiple processes and / or boxes Figure 1 These computer program instructions can also be loaded onto a computer or other programmable data processing device, so that a series of operation steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process of the present invention. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0072] Therefore, the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Changes and modifications made by those skilled in the art based on the specific implementation methods of the present invention and the above circumstances should be regarded as equivalent solutions of this application and should fall within the scope of protection of the present invention.

Claims

1. A harmonic synchronous droop control method for impedance measurement, characterized in that: Here are the steps: For each inverter running in parallel, the harmonic voltage at the output end of the three-phase LC filter capacitor is sampled and separated, and the harmonic current flowing through the three-phase line inductance is sampled; The collected harmonic voltage is transformed from a three-phase stationary coordinate system to a two-phase stationary coordinate system to obtain the αβ axis components of the harmonic voltage; the collected harmonic current is transformed from a three-phase stationary coordinate system to a two-phase stationary coordinate system to obtain the αβ axis components of the harmonic current; According to the αβ-axis components of the harmonic voltage and the αβ-axis components of the harmonic current, the harmonic active power output by each inverter is obtained; According to the harmonic active power and harmonic reactive power output by each inverter, the harmonic frequency command signal and harmonic d-axis voltage command signal of each inverter are obtained through the harmonic power loop droop control equation; SPWM modulation is performed according to the obtained command signal to generate the switching signal of each inverter power device and control the output of each inverter.

2. A harmonic synchronous droop control method for impedance measurement according to claim 1, characterized in that: The coordinate transformation equations of the harmonic voltage and harmonic current are: Among them, u har_A ,u har_B ,u har_C is the harmonic voltage at the output of the three-phase LC filter capacitor, i har_A ,i har_B ,i har_C is the harmonic current of the three-phase line inductance, u har_α ,u har_β is the αβ axis component of the harmonic voltage, i har_α ,i har_β is the αβ axis component of the harmonic current.

3. The harmonic synchronous droop control method for impedance measurement according to claim 1, characterized in that: The harmonic active power and harmonic reactive power of the inverter are obtained by calculating the average power output of the grid-connected inverter: Where, P har is the harmonic active power, Q har is the harmonic reactive power, T LPF is the low-pass filter time constant, s is the Laplace operator, u har_α ,u har_β is the αβ axis component of the harmonic voltage, i har_α ,i har_β is the αβ axis component of the harmonic current.

4. The harmonic synchronous droop control method for impedance measurement according to claim 1, characterized in that: The harmonic frequency command signal and the harmonic d-axis voltage command signal of the inverter are obtained through the harmonic power loop droop control equation: Where, ω har_ref is the harmonic frequency command signal, u har_dref is the harmonic d-axis voltage command signal, P har_ref is the rated active power of harmonics, Q har_ref is the harmonic rated reactive power, ω har_n is the rated angular frequency of harmonics, u har_n is the rated amplitude of harmonics, m har is the active droop coefficient of the harmonic power loop, n har is the reactive droop coefficient of the harmonic power loop, P har is the harmonic active power, Q har is the harmonic reactive power.

5. A grid-connected inverter power control system, characterized in that: The invention comprises a controller and a grid-connected inverter; the grid-connected inverter operates in a harmonic generation mode; the controller adopts a harmonic synchronous droop control method for impedance measurement according to any one of claims 1 to 4 to control the power devices of each inverter and control the output of each inverter so that the grid-connected inverter outputs harmonics with a specified frequency, amplitude and phase synchronization.

6. A grid-connected inverter power control system according to claim 5, characterized in that: The main circuit topology of the inverter includes a DC side voltage source, a three-phase full-bridge inverter circuit, a three-phase LC filter, a three-phase line impedance and a three-phase power grid. The DC side voltage source is connected to the three-phase full-bridge inverter circuit, and the three-phase full-bridge inverter circuit is connected to the three-phase line impedance through the three-phase LC filter and then connected to the three-phase power grid.

7. A readable storage medium, characterized in that: A computer program is stored thereon, and when the computer program is executed, a harmonic synchronous droop control method for impedance measurement according to any one of claims 1 to 4 is implemented.

8. A device, characterized in that It includes a processor, a memory, and a computer program stored in the memory and capable of running on the processor. When the processor executes the computer program, it implements a harmonic synchronous droop control method for impedance measurement as described in any one of claims 1 to 4.

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