A leakage current centralized suppression method for a transformer area level photovoltaic inverter

Abstract

The application is suitable for the field of photovoltaic inverter leakage current suppression, and provides a method for centralized suppression of leakage current of a photovoltaic inverter at a transformer area level, which comprises the following steps: counting the number of photovoltaic inverters in the transformer area, and setting initial values of carrier phase of each photovoltaic inverter; acquiring leakage current signals of each photovoltaic inverter in the transformer area, and monitoring the overall leakage current of the transformer area in real time; according to the overall leakage current of the transformer area, optimizing the carrier phase shift angle through an analog annealing algorithm to obtain compensation values of the carrier phase of each photovoltaic inverter; and sending the generated carrier phase compensation instructions to local control units of each photovoltaic inverter in the transformer area, and adjusting the carrier phase of each photovoltaic inverter according to the received instructions to realize centralized suppression of leakage current of the photovoltaic inverters at the transformer area level. The application has the beneficial effects that centralized suppression of leakage current of the photovoltaic inverters at the transformer area level can be realized, and the suppression precision of leakage current of the photovoltaic inverters at the transformer area level can be improved.

Description

Technical Field

[0001] This invention belongs to the field of leakage current suppression for photovoltaic inverters, and particularly relates to a method for centralized suppression of leakage current in photovoltaic inverters at the distribution area level. Background Technology

[0002] With rapid economic development, the demand for energy in today's society is increasing day by day. Traditional fossil fuels have limited reserves and bring many environmental problems. Solar energy, as one of the clean energy sources, has the advantages of low pollution, abundant energy storage, and convenient collection. It has now been used on a large scale, and the total installed capacity of photovoltaics in China has been steadily increasing in recent years.

[0003] Distributed photovoltaic (PV) inverters, as core equipment for energy conversion and power quality control in grid-connected PV systems, play a crucial role throughout the entire system, with their various indicators directly or indirectly affecting power quality. PV inverters are categorized into isolated and non-isolated types based on whether they employ transformers for DC-AC electrical isolation. Non-isolated PV inverters offer advantages such as simple structure, small size, and light weight, making them widely applicable. However, unlike isolated PV inverters, non-isolated PV inverters have a direct electrical connection between the PV units and the AC grid. Due to the significant parasitic capacitance between the PV panels and the ground, a loop is formed between the PV panels, parasitic capacitance, ground, and grid. This parasitic capacitance is excited by high-frequency changing common-mode voltage, causing substantial leakage current in the common-mode loop. This leakage current severely impacts the output power quality of the grid-connected PV system and can even endanger personal safety. Summary of the Invention

[0004] The purpose of this invention is to provide a method for centralized suppression of leakage current in regional photovoltaic inverters, aiming to solve the problems mentioned in the background art.

[0005] The present invention is implemented as follows: a method for centralized suppression of leakage current in a photovoltaic inverter at the distribution area level, the method comprising the following steps:

[0006] S01: Count the number of photovoltaic inverters in the distribution area and set the initial value of the carrier phase for each photovoltaic inverter;

[0007] S02: Acquire leakage current signals of each photovoltaic inverter in the distribution area and monitor the overall leakage current of the distribution area in real time;

[0008] S03: Based on the overall leakage current of the transformer area, the carrier phase shift angle is optimized through simulated annealing algorithm to obtain the carrier phase compensation value for each photovoltaic inverter;

[0009] S04: The generated carrier phase compensation command is sent to the local control unit of each photovoltaic inverter. The local control unit of each photovoltaic inverter adjusts its own carrier phase according to the received command to achieve centralized suppression of leakage current of the inverter at the distribution area level.

[0010] This invention provides a method for centralized suppression of leakage current in photovoltaic inverters at the distribution area level. Based on a carrier phase-shifting inverter control strategy, it achieves centralized suppression of leakage current in the distribution area inverters by rationally configuring the carrier phase of each inverter. Furthermore, by real-time monitoring of the overall leakage current status within the distribution area, the carrier phase is adjusted and compensated in a timely manner to improve the leakage current suppression accuracy of the photovoltaic inverters at the distribution area level. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the structure of a photovoltaic inverter system in a method for centralized suppression of leakage current in a photovoltaic inverter at the distribution level.

[0012] Figure 2 This is a common-mode equivalent model diagram of leakage current in a method for centralized suppression of leakage current in photovoltaic inverters at the distribution area level.

[0013] Figure 3 This is a schematic diagram of carrier phase shifting in a method for centralized suppression of leakage current in a photovoltaic inverter at the distribution area level.

[0014] Figure 4 This is a flowchart of carrier phase shift control in a method for centralized suppression of leakage current in a photovoltaic inverter at the distribution area level;

[0015] Figure 5 This is a flowchart of a simulated annealing algorithm used in a centralized method for suppressing leakage current in a photovoltaic inverter at the distribution level. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0017] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0018] The present invention provides a method for centralized suppression of leakage current in a photovoltaic inverter at the distribution level, which solves the technical problems in the background art.

[0019] like Figure 1The diagram shown is a schematic of the photovoltaic inverter system structure of the present invention. It includes: photovoltaic cells (PV), DC-side filter capacitors, photovoltaic inverter, LCL filter, photovoltaic cell parasitic capacitance, inverter controller, online monitoring system, line impedance, grid connection point, and power grid.

[0020] The following explanations are provided for the relevant labels in the diagram: PV represents the photovoltaic unit; Cdck (k = 1, 2, ..., a) represents the DC-side capacitor of the photovoltaic inverter; Cpvk (k = 1, 2, ..., a) represents the parasitic capacitance of the photovoltaic unit; ileak (k = 1, 2, ..., a) represents the common-mode leakage current; PCC represents the common coupling point; Zg represents the grid line impedance; k (k = 1, 2, ..., a) represents the carrier phase compensation value; GND represents the grounding device; (vAO, vBO, vCO) represent the common-mode equivalent excitation source of leakage current; L1 represents the photovoltaic inverter-side filter inductance; Lg represents the photovoltaic inverter grid-side filter inductance; Cf represents the photovoltaic inverter filter capacitor; O represents the virtual neutral point on the DC side of the photovoltaic output; eg represents the grid voltage; T represents the initial temperature of the simulated annealing algorithm; Tstop represents the termination temperature of the simulated annealing algorithm; represents the annealing rate; represents the standard maximum value of the total leakage current in the transformer area; k represents the number of iterations at each temperature; Si represents the random carrier phase compensation value; S0 represents the carrier phase compensation value.

[0021] Figure 2 This is the common-mode equivalent model diagram of the leakage current of a photovoltaic inverter. It includes: the virtual neutral point O on the DC side of the photovoltaic unit output, the voltage vAO from point A to point O at the inverter output, the voltage vBO from point B to point O at the inverter output, the voltage vCO from point C to point O at the inverter output, the filter inductor L1 on the photovoltaic inverter side, the filter inductor Lg on the photovoltaic inverter grid side, the filter capacitor Cf on the photovoltaic inverter side, and the grid voltage eg.

[0022] In non-isolated grid-connected photovoltaic (PV) systems, there is no electrical isolation between the DC and AC sides. Due to the influence of PV panel materials, temperature, humidity, and other factors, PV units have significant parasitic capacitance to ground, resulting in a common-mode voltage between the PV array and the grid. This common-mode voltage acts on the parasitic capacitance through the common-mode loop, forming a common-mode leakage current. Since the grid voltage is a low-frequency signal (50Hz), its impact on the leakage current in the common-mode loop is negligible; therefore, the grid side can be effectively short-circuited.

[0023] The inverter control strategy employs a space vector pulse width modulation (SVPWM) strategy. Using a two-sided Fourier transform, combined with the nodal voltage method and the superposition theorem, the expression for the leakage current (ileakage) of a single photovoltaic inverter can be derived as follows:

[0024]

[0025] In equation (1), Mr is the modulation ratio, which is the ratio of the modulation wave amplitude to the triangular carrier amplitude; ωsw is the angular frequency of the triangular carrier; ω0 is the angular frequency of the modulation wave; Vdc is the DC side voltage; Amn is the harmonic amplitude; and K is the passive parameter expression of the loop, the specific form of which is:

[0026]

[0027] In equation (2), L1 is the filter inductance on the photovoltaic inverter side, Lg is the filter inductance on the grid side of the photovoltaic inverter, Cf is the filter capacitor of the photovoltaic inverter, and Cpv is the parasitic capacitance of the photovoltaic unit.

[0028] Assuming that the passive parameters of each inverter are the same, the expression for the sum of the leakage currents of inverters a, ilea_sum1, is:

[0029]

[0030] Figure 3 This is a schematic diagram of the initial phase of the triangular carrier of each inverter after using the carrier phase-shift control strategy described in this invention.

[0031] Figure 4 The flowchart of the carrier phase shift control described in this invention is shown below. The specific method for centralized suppression of leakage current in a photovoltaic inverter at the distribution area level described in this invention is as follows:

[0032] Step S01: Count the number of photovoltaic inverters in the distribution area and set the initial value of the carrier phase for each photovoltaic inverter.

[0033] Count the total number of photovoltaic inverters ('a') in the distribution area, and set the initial carrier phase value for each photovoltaic inverter. The carrier phase of the first photovoltaic inverter remains unchanged; the carrier phase of the second photovoltaic inverter shifts relative to the first. The carrier phase of the third photovoltaic inverter is offset compared to the second one. And so on, the carrier phase of the a-th photovoltaic inverter is offset from that of the first one.

[0034] Combining equation (1), substituting the initial carrier phase into the leakage current expression for each inverter, the expression for the leakage current ilea_a of the a-th (a≥2) inverter is:

[0035]

[0036] Assuming all inverters have the same passive parameters, let the sum of the leakage currents of a (a≥2) inverters be denoted as ilea_sum2. Considering the parity of the constant a and the values ​​of m and n, the harmonic components in the expression for the sum of leakage currents, except for m=ka (k=1,2,3,…), can all be eliminated through mathematical simplification, i.e.:

[0037]

[0038] Comparing ilea_sum1 and ilea_sum2 in equations (3) and (5), it is clear that after using the carrier phase-shift control strategy, the harmonic terms in the total leakage current expression of the photovoltaic inverter cancel each other out, the number of terms decreases, and the amplitude of the total leakage current decreases. Theoretically, this proves that the carrier phase-shift control strategy can achieve centralized suppression of leakage current in the transformer area inverter, and the effect is significant.

[0039] Step S02: Obtain the leakage current signal of each photovoltaic inverter in the distribution area and monitor the overall leakage current of the distribution area in real time.

[0040] Leakage current of each photovoltaic inverter, such as Figure 1 As shown, the leakage currents are represented sequentially as ilea1, ilea2, ..., ileaa. The leakage current data of each photovoltaic inverter is sent to the monitoring system in real time for further processing.

[0041] Step S03: Based on the real-time monitoring of leakage current, the system optimizes the carrier phase shift angle using a simulated annealing algorithm to obtain the carrier phase compensation value for each photovoltaic inverter.

[0042] Because photovoltaic inverters within a distribution area have different DC voltages and power ratings, the leakage current amplitudes of each inverter will vary. Therefore, precise suppression of leakage current in the distribution area is difficult to achieve by simply shifting the carrier phase at a specific angle. This invention considers the differences in leakage current amplitudes caused by different DC voltages and power ratings and optimizes the carrier phase of the leakage current for each photovoltaic inverter. According to the standard requirements for total leakage current in the distribution area, the system presets a maximum total leakage current of η. When the sum of the leakage currents of the photovoltaic inverters in the distribution area exceeds this value, the system will perform real-time optimization and compensation of the leakage current phase of each inverter based on its initial phase using a simulated annealing algorithm.

[0043] The flowchart of the simulated annealing algorithm is as follows: Figure 5 As shown, the specific execution steps are as follows:

[0044] First, initialize all parameters, including initial temperature T, termination temperature Tstop, annealing rate α, and number of iterations k at each temperature;

[0045] Secondly, inverter carrier phase data groups are randomly generated between [0, 2π]. Each phase data is substituted into the leakage current expression, and the sum of the leakage currents of the inverters in the distribution area is calculated. The Metropolis criterion is used to determine whether to accept the inverter carrier phase data group. The Metropolis criterion is as follows:

[0046]

[0047] Where p represents the probability of receiving this set of inverter carrier phase data;

[0048] Furthermore, p is compared with the random number generated in [0,1]. If p is less than the random number, the algorithm will accept the current carrier phase data and compare the sum of the leakage current of the inverters in the current distribution area with the standard maximum value of the total leakage current in the distribution area. If it is less than the standard maximum value, the loop will be exited and the carrier phase data of this set of inverters will be designated as the carrier phase compensation value. If p is not less than the random number, the current phase data will be discarded and the loop will continue until the ideal carrier phase compensation value is obtained.

[0049] In step S04, the generated carrier phase compensation command is sent to the local control unit of each photovoltaic inverter. Each photovoltaic inverter adjusts its own carrier phase according to the received command, thereby realizing the centralized suppression of leakage current of the transformer substation-level inverter.

[0050] The carrier phase compensation values ​​Dj10, Dj20, ..., Dja0 generated in step S03 are sent to the local control unit of each photovoltaic inverter. By changing the phase of the triangular carrier in the space vector pulse width modulation (SVPWM), the sum of the leakage current of the photovoltaic inverters in the distribution area is lower than the maximum standard value of the leakage current in the distribution area, thereby realizing the centralized suppression of leakage current of the grid-connected photovoltaic inverters in the distribution area.

[0051] The above embodiments of the present invention provide a method for centralized suppression of leakage current in photovoltaic inverters at the distribution area level. Based on a carrier phase-shifting inverter control strategy, the method achieves centralized suppression of leakage current in the distribution area inverters by rationally configuring the carrier phase of each inverter. Furthermore, by real-time monitoring of the overall leakage current status within the distribution area, the carrier phase is adjusted and compensated in a timely manner to improve the leakage current suppression accuracy of the photovoltaic inverters at the distribution area level.

[0052] In order for the above methods and systems to operate smoothly, the system may include more or fewer components than those described above, or combine certain components, or different components, in addition to the various modules mentioned above. For example, it may include input / output devices, network access devices, buses, processors, and memory.

[0053] The processor referred to can be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor can be a microprocessor or any conventional processor. This processor is the control center of the system, connecting various parts through various interfaces and lines.

[0054] The aforementioned memory can be used to store computer and system programs and / or modules. The aforementioned processor implements the various functions mentioned above by running or executing the computer programs and / or modules stored in the memory, and by calling the data stored in the memory. The memory may mainly include a program storage area and a data storage area. The program storage area may store the operating system, at least one application program required for a function (such as information collection template display function, product information publishing function, etc.). The data storage area may store data created based on the use of the berth status display system (such as product information collection templates corresponding to different product types, product information that different product providers need to publish, etc.). In addition, the memory may include high-speed random access memory, and may also include non-volatile memory, such as hard disk, memory, plug-in hard disk, smart media card (SMC), secure digital (SD) card, flash card, at least one disk storage device, flash memory device, or other volatile solid-state storage device.

[0055] It should be understood that although the steps in the flowcharts of the various embodiments of the present invention are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the various embodiments may include multiple sub-steps or multiple stages. These sub-steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these sub-steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least a portion of the sub-steps or stages of other steps.

[0056] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0057] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

[0058] 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, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for centralized suppression of leakage current in a distribution-level photovoltaic inverter, characterized in that, The method includes the following steps: S01: Count the number of photovoltaic inverters in the distribution area and set the initial value of the carrier phase for each photovoltaic inverter; S02: Acquire leakage current signals of each photovoltaic inverter in the distribution area and monitor the overall leakage current of the distribution area in real time; S03: Based on the overall leakage current of the transformer area, the carrier phase shift angle is optimized through simulated annealing algorithm to obtain the carrier phase compensation value for each photovoltaic inverter; S04: The generated carrier phase compensation command is sent to the local control unit of each photovoltaic inverter. The local control unit of each photovoltaic inverter adjusts its own carrier phase according to the received command to achieve centralized suppression of leakage current of the inverter at the distribution area level.

2. The method for centralized suppression of leakage current in a photovoltaic inverter at the distribution area level according to claim 1, characterized in that, In step S01, the initial values ​​for the carrier phase of each photovoltaic inverter are: the carrier phase of the first photovoltaic inverter remains unchanged; the carrier phase of the second photovoltaic inverter is offset compared to the first inverter. The carrier phase of the third photovoltaic inverter is offset compared to the first one. And so on, the first The carrier phase of the photovoltaic inverter is offset compared to the first one. in This represents the total number of photovoltaic inverters in the distribution area.

3. The method for centralized suppression of leakage current in a photovoltaic inverter at the distribution area level according to claim 2, characterized in that, Substituting the initial carrier phase value into the leakage current expression for each inverter, then the first... The expression for the leakage current of the inverter is: ， in, ≥2, Mr is the modulation ratio; ωsw is the angular frequency of the triangular carrier wave; ω0 is the angular frequency of the modulating wave; Vdc is the DC side voltage; Amn is the harmonic amplitude; K is the expression for the passive parameters of the loop, and its specific form is: ， In the formula, L1 is the filter inductance on the photovoltaic inverter side, Lg is the filter inductance on the grid side of the photovoltaic inverter, Cf is the filter capacitor of the photovoltaic inverter, and Cpv is the parasitic capacitance of the photovoltaic unit.

4. The method for centralized suppression of leakage current in a distribution-level photovoltaic inverter according to claim 1, characterized in that, The method also includes: sending the leakage current information of each inverter to the monitoring center in real time to monitor the overall leakage current of the transformer area.

5. The method for centralized suppression of leakage current in a photovoltaic inverter at the distribution area level according to any one of claims 1-4, characterized in that, In step S03, the leakage current information of each photovoltaic inverter in the distribution area and the overall leakage current situation of the distribution area are clarified, and the leakage current carrier phase is optimized by simulated annealing algorithm.

6. The method for centralized suppression of leakage current in a distribution-level photovoltaic inverter according to claim 5, characterized in that, Optimizing the leakage current carrier phase using the simulated annealing algorithm specifically includes: S11: Initialize each parameter, including the initial temperature T, the termination temperature Tstop, the annealing rate α, and the number of iterations k at each temperature; S12: For a set of inverter carrier phase data randomly generated between [0, 2π], substitute each phase data into the leakage current expression and calculate the sum of the leakage currents of the inverters in the distribution area. Determine whether to accept the set of inverter carrier phase data according to the Metropolis criterion. The Metropolis criterion is as follows: ， Where p represents the probability of receiving this set of inverter carrier phase data; S13: Compare p with the random number generated in [0,1]. If p is less than the random number, the algorithm will accept the current carrier phase data and compare the sum of the leakage current of the inverters in the current distribution area with the standard maximum value of the total leakage current of the distribution area. If it is less than the standard maximum value, the loop will exit and the carrier phase data of this set of inverters will be designated as the carrier phase compensation value. If p is not less than the random number, the current phase data will be discarded and the loop will continue until the ideal carrier phase compensation value is obtained.

7. The method for centralized suppression of leakage current in a distribution-level photovoltaic inverter according to claim 1, characterized in that, In S04, the local control unit of each photovoltaic inverter adjusts its own carrier phase according to the received carrier phase compensation value instruction, so that the leakage current is lower than the maximum value of the set leakage current standard for the area, thereby realizing the centralized suppression of leakage current of the photovoltaic inverter at the area level.

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