Distributed power supply wireless synchronous grid-connected control method and system considering frequency difference

Abstract

The invention discloses a distributed power supply wireless synchronous grid-connected control method and system considering frequency difference, and the method comprises the steps: firstly, carrying out the real-time calculation of the phase of a far-end voltage through employing a virtual rotating shaft frequency phase conversion technology on the basis of considering the frequency difference between a near-end power grid and a far-end distributed power supply; secondly, an error coefficient is defined, and the compensation precision of the voltage phase in the synchronous grid connection process is quantified; and finally, improving the phase standard during grid connection of the distributed power supply by considering the error generated by the sampling frequency of the measuring device so as to ensure the seamless connection of the electric energy transmission between the distributed power supply and the power distribution network, reduce the energy loss, and monitor the voltage frequency conditions of the power distribution network side, the load side and the distributed power supply in real time. And the safety and reliability of power supply are ensured.

Description

Technical Field

[0001] The present invention relates to the field of distributed power grid-connected control, and in particular to a distributed power wireless synchronous grid-connected control method and system taking frequency differences into account. Background Art

[0002] In order to cope with climate change and reduce environmental pollution caused by fossil fuels, the world is actively promoting the development of distributed renewable energy such as photovoltaics and wind power. These clean energy sources not only help reduce greenhouse gas emissions, but also enhance the autonomy and security of national energy supply. With the continuous advancement of distributed power generation technology, more and more wind power and photovoltaic power generation systems are connected to the power grid. However, although this process has effectively promoted the popularization of renewable energy, it has also brought many technical challenges, especially in terms of the reliability of grid-connected operations.

[0003] When distributed power sources are connected to the grid, it is crucial to ensure that their voltage amplitude, phase and frequency are consistent with the grid. This is not only related to the stability and reliability of power supply, but also affects the safe operation of the grid. However, since many distributed power generation sites are located in remote areas and are far away from the grid, it is difficult to achieve real-time monitoring when detecting these important parameters. The problem of communication delay makes it difficult to obtain voltage information of distributed power sources and the grid at the same time, which in turn has a negative impact on phase adjustment and grid stability.

[0004] The patent with application number 202311186882.0 and name "Inverter and grid voltage phase synchronization, grid-connected method and control device" proposes to establish a preset phase of the grid voltage. When the phase of the grid voltage reaches the preset phase, an I / O signal jump is generated and the output voltage phase of the inverter is immediately obtained. The output voltage phase of the inverter is compared with the preset phase, and the output voltage phase of the inverter is adjusted to the preset phase by adjusting the output frequency of the inverter; the patent with application number 202310780289.2 and name "Phase synchronization method, device, system and electronic equipment of grid-connected converter" generates a first alternating current by obtaining the grid frequency and phase, and outputs the first alternating current to the connecting line on the grid side, and outputs a synchronization signal to the connecting line on the signal side, so that the second converter collects the synchronization signal from the signal side and outputs a second alternating current with the same frequency and phase as the first alternating current, and the distributed power supply is connected to the grid through the data of the second converter. However, the above-mentioned methods all take into account the errors caused by communication delays and lack an effective real-time phase monitoring and compensation mechanism, which hinders the smooth grid connection of distributed power sources. Summary of the invention

[0005] The purpose of the present invention is to propose a distributed power supply wireless synchronous grid-connected control method and system taking into account frequency differences, in order to address the time delay problem caused by wireless transmission when distributed power supplies are connected to the grid, so as to ensure seamless connection of power transmission between distributed power supplies and distribution networks, reduce energy losses, and be able to monitor the voltage and frequency conditions of the distribution network side, load side and distributed power supplies in real time to ensure the safety and reliability of power supply.

[0006] The technical solution to achieve the purpose of the present invention is:

[0007] A distributed power supply wireless synchronous grid-connected control method taking frequency difference into account, comprising:

[0008] A synchronous phasor measurement device is used to collect remote and grid voltage signals, and the remote grid voltage phase data with a time stamp is predicted and compensated based on the frequency, and converted into a virtual signal with the same time stamp as the near end;

[0009] When it is determined that there is no abnormality in the remote voltage sampling signals and the grid voltage sampling signals, the voltage difference is obtained by subtracting the amplitude of the collected remote voltage signal from the amplitude of the grid voltage;

[0010] If the absolute value of the voltage difference is less than the error threshold, the collected grid voltage frequency is subtracted from the distributed power supply frequency to obtain the grid-connected frequency difference;

[0011] If the absolute value of the grid-connected frequency difference meets the set condition, the grid voltage phase is subtracted from the converted remote phase virtual signal to obtain the grid-connected phase difference;

[0012] Calculate the standard value of the grid-connected phase. If the absolute value of the grid-connected phase difference is less than the standard value of the grid-connected phase, calculate the grid-connected closing moment and perform grid-connected control.

[0013] Furthermore, the remote grid voltage phase data with time stamp is converted into a virtual signal as follows:

[0014]

[0015] Where K is the error coefficient, f i-1 For the controller at T i Always receive the data with T i-1 The frequency signal of the time scale, f N is the frequency signal at the grid end, T i The virtual voltage phase at the moment, For the controller at T i Always receive the data with T i-1 Virtual voltage phase of the time scale, Δt i-(i-1) Indicates T i time, d is the increment symbol, is the compensated Ti The virtual voltage phase at the moment, is the compensated T i-1 Virtual voltage phase at moment.

[0016] Furthermore, the standard value of the grid-connected phase is 10°-Δθ, where Δθ is the phase error generated in one sampling period, which is:

[0017]

[0018] f is the frequency of the system to be tested, f s is the sampling frequency of the synchronized phasor measurement device.

[0019] Furthermore, the standard value of the grid-connected phase is 8.2°.

[0020] Furthermore, it is determined whether the remote voltage sampling signal and the grid voltage sampling signal are abnormal. The specific determination conditions are:

[0021]

[0022] If U g With f g If the above conditions are met at the same time, it is considered that there is no abnormality in the grid voltage and sampling link, among which, U n is the grid voltage, U g is the remote voltage, f g For the distributed power phase.

[0023] Furthermore, the absolute value of the voltage difference is less than the error threshold of 20% U n .

[0024] Furthermore, the setting condition satisfied by the absolute value of the grid-connected frequency difference is: 0.2≤Δf≤0.5 Hz, where Δf is the grid-connected frequency difference.

[0025] Furthermore, considering the action characteristics of the grid-connected device, the grid-connected closing time is t s +k·T s , where t s is the action time of the switch, T s is the grid-connected closing cycle, and k is the number of closing cycles.

[0026] Furthermore, in the fifth stable and repeated closing cycle, the time interval from the closing pulse to the start of contact between the moving and static contacts is T d , is the time before the switch is turned on, and the moment when the controller issues the closing command after the synchronization is completed is t s +5·T s -T d , the grid-connected switch is actuated and closed, and the contact moment of the moving and static contacts is t s +5·Ts , T s It is the grid-connected closing cycle.

[0027] Furthermore, it also includes: if there is an abnormality in the remote voltage sampling signal and the grid voltage sampling signal, solving the abnormality.

[0028] Furthermore, the method further includes: if the absolute value of the voltage difference is not less than the error threshold, adjusting the voltage difference by a controller until the absolute value of the voltage difference is less than the error threshold.

[0029] Furthermore, it also includes: if the absolute value of the grid-connected frequency difference does not meet the set condition, adjusting by the controller until the absolute value of the grid-connected frequency difference meets the set condition.

[0030] A distributed power wireless synchronous grid-connected control system taking frequency differences into account, comprising:

[0031] The acquisition unit uses a synchronous phasor measurement device to collect remote and grid voltage signals;

[0032] A virtual signal calculation unit predicts and compensates the remote-end time-stamped grid voltage phase data based on frequency, and converts it into a virtual signal at the same time scale as the near-end;

[0033] The voltage difference calculation unit, when determining that there is no abnormality in the remote voltage sampling signals and the grid voltage sampling signals, subtracts the amplitude of the collected remote voltage signal from the amplitude of the grid voltage to obtain the voltage difference;

[0034] The grid frequency difference calculation unit, if the absolute value of the voltage difference is less than the error threshold, subtracts the collected grid voltage frequency from the distributed power supply frequency to obtain the grid frequency difference;

[0035] A standard value calculation unit for a grid-connected phase, used for calculating a standard value for a grid-connected phase;

[0036] The grid-connected control unit calculates the grid-connected closing time and performs grid-connected control.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] The present invention firstly calculates the phase of the remote voltage in real time by using the virtual rotating shaft frequency phase conversion technology based on the frequency difference between the near-end power grid and the remote distributed power source; secondly, defines the error coefficient to quantify the compensation accuracy of the voltage phase during the synchronous grid-connected process; finally, takes into account the error caused by the sampling frequency of the measuring device itself, and improves the phase standard when the distributed power source is connected to the grid, so as to ensure seamless connection of the power transmission between the distributed power source and the distribution network, reduce energy loss, and be able to monitor the voltage and frequency conditions of the distribution network side, the load side and the distributed power source in real time to ensure the safety and reliability of power supply. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the schematic diagram of the distributed power supply synchronization and grid connection.

[0040] Figure 2 This is the principle diagram of frequency-phase conversion of virtual rotating axis.

[0041] Figure 3 This is the flow chart for the synchronization and grid connection of distributed power sources.

[0042] Figure 4 This is the switch action characteristic diagram. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0044] A distributed power supply wireless synchronous grid-connected control method taking frequency difference into account, specifically comprising:

[0045] The first step is to design a frequency-phase conversion method for a virtual rotating axis that takes frequency differences into account.

[0046] Figure 1 The schematic diagram of the distributed power supply synchronization and grid connection is shown in the figure. In this application, a synchronous phasor measurement unit (PMU) is used as a data acquisition and transmission module. The device uses the global positioning system (GPS) second pulse as a phasor measurement unit composed of a synchronous clock and has a built-in communication module to obtain real-time data and real-time time scales of voltage and current. The data transmission delay is usually between 1 millisecond and several milliseconds, which can effectively improve the success rate of distributed power supply synchronization and grid connection. PMUs are installed on the power grid side and the distributed power supply side respectively. The voltage measured on the power grid side is the proximal voltage, which is also used as the reference voltage when the distributed power supply is connected to the grid. The voltage measured on the distributed power supply side is the far-end voltage. The voltage data at both ends are transmitted to the controller. When the controller detects that the voltage amplitude, phase and frequency of the power grid and the distributed power supply meet the grid connection requirements, a control signal is sent to complete the grid connection of the distributed power supply.

[0047] However, since the distributed power source is too far away from the grid-connected bus, even with the use of advanced PMU, the problem of delay in data transmission still exists objectively, which will cause the distributed power source to fail to connect to the grid because the phase difference does not meet the grid connection standard. This application proposes a virtual rotating shaft frequency-phase conversion method that takes into account the frequency difference, predicts and compensates the grid voltage phase data with a time stamp at the far end based on the frequency, converts it into a virtual signal with the same time stamp as the near end, and transmits it to the controller via wired transmission to eliminate communication delays. The principle of this technology is as follows Figure 2 As shown in the figure and are the vectors of the near-end voltage and far-end voltage received by the controller, respectively. The length represents the voltage amplitude, and the angle represents the voltage phase. When the controller detects the time T0 When the phase meets the grid connection conditions, due to the existence of communication delay ΔT, The actual phase of is the phase at time T0+ΔT, thus missing the best time for grid connection. The phase of The actual phase is converted to the time the controller receives the data according to the data transmission time. The specific implementation principle of the virtual phase is as follows:

[0048] The sampling unit collects the remote voltage phase and frequency signals at time T0: and f0. Through wireless transmission, the controller receives the voltage phase and frequency signal with T0 time stamp at time T1. The virtual voltage phase at time T1 is calculated by the relationship between frequency, time delay and initial phase. for:

[0049]

[0050] Time T1 is the initial value of the controller remote signal sampling, there is no compensation phase, virtual voltage phase The error is large. At time T2, the controller receives the voltage phase and frequency signals with T1 time stamp, which are and f1, the virtual voltage phase at time T2 is calculated by the relationship between frequency, time delay, and initial phase for:

[0051]

[0052] The actual voltage phase at time T1 The virtual voltage phase at time T1 The difference is taken as the compensation voltage phase at time T2:

[0053]

[0054] Then the virtual voltage phase at time T2 after compensation is for:

[0055]

[0056] Similarly, the virtual voltage phase at time T3 after compensation is for:

[0057]

[0058] Therefore, at any time T i Collected with T i-1 The voltage phase and frequency signal of the time scale, i=0,1,2..., then the compensated T i The virtual voltage phase at this moment is:

[0059]

[0060] To express the delay compensation accuracy of wireless transmission signals, the error coefficient K is defined as the delay after compensation. i The virtual voltage vector at the moment is i The phase error of the sampled voltage of the time scale is shown as follows:

[0061]

[0062] Grid connection requires that the phases of the voltages on both sides of the switch must be the same, so the K value must not be greater than 0.01, otherwise the closing phase difference will be too large.

[0063] Limited by factors such as equipment performance, transmission distance, and crystal oscillator accuracy, if the error coefficient K>0.01 appears, the virtual space-time coordinate dynamic compensation technology needs to be corrected in time, and the compensation formula (6) becomes:

[0064]

[0065] The compensation accuracy is improved through negative feedback design. Formula (7) represents the reliability of continuous verification of compensation accuracy. During the synchronization check process, the error coefficient needs to always meet K≤0.01. If it is not met, the synchronization process is interrupted and the compensation phase is corrected through formula (8) until the error coefficient K≤0.01 is met, and the synchronization check process is restarted.

[0066] Grid connection requires that the voltage amplitude, frequency, and phase on both sides of the switch meet relevant regulations. Generally, the frequency difference is 0.5Hz, the voltage amplitude difference is within 20%, and the phase difference is within 10°. Due to the sampling characteristics of the measuring device itself, an error of one sampling cycle often occurs during the measurement process. This error originates from the fixed time interval used by the equipment when collecting data. This time interval limitation may result in the inability to accurately capture the true characteristics of the signal when the signal changes rapidly or the frequency is high, thereby affecting the accuracy of the measurement results. Based on this, it is necessary to improve the standard of phase difference when connecting to the grid to compensate for the error caused by the sampling frequency.

[0067] Assume the sampling frequency of the measuring device is f s , the frequency of the system to be measured is f, then the phase error Δθ generated in one sampling period is as shown in formula (9):

[0068]

[0069] At this time, the phase error standard for distributed power grid connection is modified to:

[0070]

[0071] The distributed power generation synchronization and grid connection process using the virtual rotating shaft frequency phase conversion method taking into account the frequency difference in the first step is as follows: Figure 3 As shown, not only can the synchronous grid connection be completed quickly, but also the grid connection failure caused by grid voltage fluctuation during the synchronization process can be avoided.

[0072] The second step is to determine the correctness of the remote grid voltage sampling signal;

[0073]

[0074] If U g With f g If the above conditions are met at the same time, it is considered that there is no abnormality in the grid voltage and the sampling link; if not, the cause needs to be found out until equation (11) is met.

[0075] The third step is to subtract the amplitude of the remote voltage signal collected by wire from the amplitude of the grid voltage:

[0076]

[0077] If the absolute value of the voltage difference does not satisfy ΔU ≤ 20% U n , then the output voltage of the distributed power supply is controlled and adjusted until equation (12) is satisfied.

[0078] The fourth step is to subtract the sampled grid-side voltage frequency from the distributed power frequency to obtain the grid-connected frequency difference:

[0079]

[0080] If the frequency difference does not satisfy 0.2≤Δf≤0.5Hz, the controller outputs a command to control the distributed power supply to adjust the output voltage frequency until equation (13) is satisfied.

[0081] The fifth step is to subtract the sampled grid-side voltage phase from the distributed power phase after the virtual rotating shaft frequency phase conversion to obtain the grid-connected phase difference:

[0082]

[0083] Step 6: Recalculate the grid-connected phase standard according to equations (9) and (10). Usually, the sampling frequency of PUM is f s =10000Hz, when the distributed power phase f g =50Hz, the phase error Δθ generated in one sampling cycle is 1.8°, and the phase difference when connected to the grid should not be greater than 10°-1.8°=8.2°, so the grid-connected phase standard is:

[0084]

[0085] If the phase difference does not satisfy The controller outputs instructions to control the distributed power supply to adjust the output voltage phase until equation (15) is satisfied.

[0086] Step 7: The voltage on both sides of the grid-connected switch S0 satisfies ΔU≤20%U n , 0.2≤Δf≤0.5Hz, After that, the grid connection conditions are met. Since a small frequency difference can easily lead to a long synchronization time, a large frequency difference can reduce the accuracy of the grid connection closing time and cause excessive impact current. Therefore, the maximum grid connection frequency difference is set to 0.2-0.5Hz, corresponding to the grid connection closing period T s The controller calculates the time for grid connection and closing as t s +k·T s , ts is the action time of the switch, that is, the time from receiving the signal to the switch action, in order to prevent the load mutation or power system failure from causing the grid frequency mutation, thus causing the grid connection failure. Set the grid connection within the stable and repeated fifth closing cycle, taking into account the switch action characteristics such as Figure 4 As shown in the figure, the time interval from the closing pulse to the start of contact between the moving and static contacts is T d , is called the time before the switch is turned on. After the synchronization is completed, the time when the controller issues the closing command is t s +5·T s -T d . Switch S0 is actuated and connected to the grid, and the contact moment between the moving and static contacts is t s +5·Ts .

[0087] On the basis of compensating for communication delays and inherent measurement errors caused by sampling frequency, this method further considers the impact of switching characteristics on system performance. The design not only corrects the errors caused by communication delays and sampling frequency mismatches, but also effectively evaluates and adjusts the non-ideal factors caused by switching characteristics, greatly enhancing the reliability of distributed power grid-connected operations, helping to improve the overall performance and responsiveness of the power system, and ensuring safe and stable operation under various dynamic loads and environmental conditions.

[0088] This embodiment also provides a distributed power supply wireless synchronous grid-connected control system taking frequency differences into account, including:

[0089] The acquisition unit uses a synchronous phasor measurement device to collect remote and grid voltage signals;

[0090] A virtual signal calculation unit predicts and compensates the remote-end time-stamped grid voltage phase data based on frequency, and converts it into a virtual signal at the same time scale as the near-end;

[0091] The voltage difference calculation unit, when determining that there is no abnormality in the remote voltage sampling signals and the grid voltage sampling signals, subtracts the amplitude of the collected remote voltage signal from the amplitude of the grid voltage to obtain the voltage difference;

[0092] The grid frequency difference calculation unit, if the absolute value of the voltage difference is less than the error threshold, subtracts the collected grid voltage frequency from the distributed power supply frequency to obtain the grid frequency difference;

[0093] A standard value calculation unit for a grid-connected phase, used for calculating a standard value for a grid-connected phase;

[0094] The grid-connected control unit calculates the grid-connected closing time and performs grid-connected control.

[0095] Finally, it should be noted that the above embodiments are only specific implementations of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The protection scope of the present invention is not limited thereto. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that any person skilled in the art can still modify the technical solutions recorded in the above embodiments within the technical scope disclosed by the present invention, or can easily think of changes, or make equivalent replacements for some of the technical features therein; and these modifications, changes or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A distributed power supply wireless synchronization grid-connected control method taking frequency differences into account, characterized in that: include: A synchronous phasor measurement device is used to collect remote and grid voltage signals, and the remote grid voltage phase data with a time stamp is predicted and compensated based on the frequency, and converted into a virtual signal with the same time stamp as the near end; When it is determined that there is no abnormality in the remote voltage sampling signals and the grid voltage sampling signals, the voltage difference is obtained by subtracting the amplitude of the collected remote voltage signal from the amplitude of the grid voltage; If the absolute value of the voltage difference is less than the error threshold, the collected grid voltage frequency is subtracted from the distributed power supply frequency to obtain the grid-connected frequency difference; If the absolute value of the grid-connected frequency difference meets the set condition, the grid voltage phase is subtracted from the converted remote phase virtual signal to obtain the grid-connected phase difference; Calculate the standard value of the grid-connected phase. If the absolute value of the grid-connected phase difference is less than the standard value of the grid-connected phase, calculate the grid-connected closing moment and perform grid-connected control.

2. A distributed power supply wireless synchronization grid-connected control method taking frequency difference into account according to claim 1, characterized in that: The virtual signal converted from the remote grid voltage phase data with time stamp is: Where K is the error coefficient, f i-1 For the controller at T i Always receive the data with T i-1 The frequency signal of the time scale, f N is the frequency signal at the grid end, T i The virtual voltage phase at the moment, For the controller at T i Always receive the data with T i-1 Virtual voltage phase of the time scale, Δt i-(i-1) Indicates T i time, d is the increment symbol, is the compensated T i The virtual voltage phase at the moment, is the compensated T i-1 Virtual voltage phase at moment.

3. A distributed power wireless synchronous grid-connected control method taking frequency difference into account according to claim 2, characterized in that: The standard value of the grid-connected phase is 10°-Δθ, where Δθ is the phase error generated in one sampling period: f is the frequency of the system to be tested, f s is the sampling frequency of the synchronized phasor measurement device.

4. A method for wireless synchronous grid-connected control of distributed power sources taking frequency differences into account according to claim 3, characterized in that: The standard value of the grid-connected phase is 8.2°.

5. The method for wireless synchronous grid-connected control of distributed power sources taking frequency differences into account according to claim 1 is characterized in that: Determine whether the remote and grid voltage sampling signals are abnormal. The specific judgment conditions are: If U g With f g If the above conditions are met at the same time, it is considered that there is no abnormality in the grid voltage and sampling link, among which, U n is the grid voltage, U g is the remote voltage, f g For the distributed power phase.

6. A method for wireless synchronous grid-connected control of distributed power sources taking frequency differences into account according to claim 5, characterized in that: The absolute value of the voltage difference is less than the error threshold of 20% U n .

7. A method for controlling wireless synchronization of distributed power sources in accordance with claim 6, characterized in that: The setting condition that the absolute value of the grid-connected frequency difference satisfies is: 0.2≤Δf≤0.5Hz, where Δf is the grid-connected frequency difference.

8. A distributed power supply wireless synchronization grid-connected control method taking frequency difference into account according to claim 7, characterized in that: Considering the action characteristics of the grid-connected device, the grid-connected closing time is t s +k·T s , where t s is the action time of the switch, T s is the grid-connected closing cycle, and k is the number of closing cycles.

9. A method for wireless synchronous grid-connected control of distributed power sources taking frequency differences into account according to claim 8, characterized in that: The grid is connected in the fifth stable and repeated closing cycle. The time interval from the closing pulse to the start of contact between the moving and static contacts is T d , is the time before the switch is turned on, and the moment when the controller issues the closing command after the synchronization is completed is t s +5·T s -T d , the grid-connected switch is actuated and closed, and the contact time of the moving and static contacts is t s +5·T s , T s It is the grid-connected closing cycle.

10. The method for wireless synchronous grid-connected control of distributed power sources taking frequency differences into account according to claim 1, characterized in that: Also includes: If there is an abnormality in the remote and grid voltage sampling signals, resolve the abnormality.

11. A distributed power supply wireless synchronization grid-connected control method taking frequency difference into account according to claim 1, characterized in that: Also includes: If the absolute value of the voltage difference is not less than the error threshold, the controller is used to adjust until the absolute value of the voltage difference is less than the error threshold.

12. A method for wireless synchronous grid-connected control of distributed power sources taking frequency differences into account according to claim 1, characterized in that: Also includes: If the absolute value of the grid-connected frequency difference does not meet the set conditions, the controller is adjusted until the absolute value of the grid-connected frequency difference meets the set conditions.

13. A wireless synchronous grid-connected control system for distributed power sources taking frequency differences into account to implement the method described in any one of claims 1 to 12, characterized in that: include: The acquisition unit uses a synchronous phasor measurement device to collect remote and grid voltage signals; A virtual signal calculation unit predicts and compensates the remote-end time-stamped grid voltage phase data based on frequency, and converts it into a virtual signal at the same time scale as the near-end; The voltage difference calculation unit, when determining that there is no abnormality in the remote voltage sampling signals and the grid voltage sampling signals, subtracts the amplitude of the collected remote voltage signal from the amplitude of the grid voltage to obtain the voltage difference; The grid frequency difference calculation unit, if the absolute value of the voltage difference is less than the error threshold, subtracts the collected grid voltage frequency from the distributed power supply frequency to obtain the grid frequency difference; A standard value calculation unit for a grid-connected phase, used for calculating a standard value for a grid-connected phase; The grid-connected control unit calculates the grid-connected closing time and performs grid-connected control.

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