Detection device for neutral line connection of split-phase power grid inverter

Abstract

The invention discloses a neutral line connection detection device for a split-phase power grid inverter, which relates to the technical field of split-phase power grid inverters, and comprises a relay group, a filtering unit, a voltage sampling unit and a control unit, the filtering unit adopts an inherent output filtering capacitor of the DC-AC conversion module; the control unit can control on-off of the corresponding relay, the inherent filter capacitor is connected to a power grid to form an asymmetric load, and the neutral line connection state is judged by comparing the two-phase voltage difference value. According to the invention, additional detection hardware is not needed, the system cost and the power grid disturbance are reduced, the detection reliability is improved, and the overvoltage damage of the electric equipment can be effectively avoided.

Description

Technical Field

[0001] This invention relates to the field of split-phase power grid inverter technology, specifically to a device and method for detecting the neutral line connection of a split-phase power grid inverter. Background Technology

[0002] Split-phase power grids, a common residential power supply system widely used in North America, utilize two phase lines with a 180° phase difference from the distribution transformer. These, along with a neutral line, provide two voltage levels: a lower voltage for general household appliances such as lighting and common sockets, and a higher voltage for high-power devices like air conditioners and water heaters. In this system, the inverter, as a key device for connecting to distributed energy sources or backup power, must have its output neutral line reliably connected to the split-phase power grid's neutral line. Failure to reliably connect the inverter's neutral line to the split-phase power grid's neutral line will result in an abnormal system voltage reference point, causing voltage drift in each phase. This could lead to overvoltage damage to equipment connected to the low-voltage line, posing a significant safety hazard.

[0003] In existing technologies, to solve the voltage anomaly problem caused by a disconnected neutral wire, a detection circuit based on external impedance is typically used, such as... Figure 1 As shown, this scheme uses an additional detection circuit inside the inverter to connect a specific impedance in parallel to the voltage port of one phase by using the closing action of a relay, artificially creating an asymmetrical impedance load between the two phases. Then, the effective values ​​of the two phase voltages are detected. If a significant voltage difference is detected between the two, it is determined that the inverter neutral line is not connected to the grid neutral line; if the effective values ​​of the two phase voltages are basically equal, it is determined that the neutral line is in a normal connection state.

[0004] However, this existing technology has several obvious drawbacks: First, it relies on additional hardware circuits, which not only increases the complexity and number of components of the system, but also increases manufacturing costs and potential failure points; second, this method is an active detection method, which requires applying an unbalanced load by switching the relay state at a specific time. This process may cause instantaneous disturbances in the grid voltage, affecting connected sensitive equipment; in addition, the judgment logic relies on artificially created unbalanced operating conditions, and its ability to distinguish and its reliability under certain specific load conditions are still insufficient.

[0005] Therefore, it is necessary to develop a new detection device that is simpler, more reliable, and does not require the introduction of additional disturbances to overcome the shortcomings of existing technologies. Summary of the Invention

[0006] In view of the problems existing in the prior art, the purpose of the present invention is to provide a neutral line connection detection device and detection method for split-phase power grid inverters. It eliminates the need for additional dedicated detection hardware and can reliably detect the neutral line connection status using existing components within the inverter system, while reducing power grid disturbances and improving system operation stability.

[0007] A neutral line connection detection device for a split-phase power grid inverter, wherein the split-phase power grid includes a first phase line, a second phase line and a power grid neutral line, the inverter includes a grid-connected interface connected to the split-phase power grid and a DC-AC conversion module, and the detection device includes a relay group, a filter unit, a voltage sampling unit and a control unit;

[0008] The relay group includes a first relay, a second relay, and a third relay; the first relay is connected in series between the first phase line and the first phase input terminal of the DC-AC converter module, the second relay is connected in series between the neutral line of the power grid and the neutral line input terminal of the DC-AC converter module, and the third relay is connected in series between the second phase line and the second phase input terminal of the DC-AC converter module.

[0009] The filtering unit includes a first filtering capacitor and a second filtering capacitor. The first filtering capacitor is connected in parallel between the first phase input terminal and the neutral input terminal of the DC-AC converter module, and the second filtering capacitor is connected in parallel between the second phase input terminal and the neutral input terminal of the DC-AC converter module. The first filtering capacitor and the second filtering capacitor are inherent output filtering elements of the DC-AC converter module.

[0010] The voltage sampling unit is electrically connected to the first phase line, the second phase line, and the neutral line of the power grid, respectively, and is used to collect the effective values ​​of the first phase voltage between the first phase line and the neutral line of the power grid and the second phase voltage between the second phase line and the neutral line of the power grid in real time.

[0011] The control unit is electrically connected to the relay group and the voltage sampling unit respectively. The control unit is used to control the first and second relays to close, or control the second and third relays to close, before the inverter is connected to the grid or during system self-test, so as to connect the corresponding filter capacitor between the corresponding phase line and the grid neutral line. It is used to receive the voltage collected by the voltage sampling unit and determine the connection status between the inverter neutral line and the grid neutral line according to the voltage change or voltage difference.

[0012] As a preferred embodiment of the present invention, the first relay, the second relay, and the third relay are all normally open in their initial state.

[0013] In a preferred embodiment of the present invention, the voltage sampling unit includes a first voltage sampling branch and a second voltage sampling branch; the first voltage sampling branch is connected in parallel between the first phase line and the neutral line of the power grid, and the second voltage sampling branch is connected in parallel between the second phase line and the neutral line of the power grid.

[0014] As a preferred embodiment of the present invention, when the control unit determines that the inverter neutral line is properly connected to the grid neutral line, it controls the relay group to close, so that the inverter enters the grid-connected operation state; when it determines that the inverter neutral line is not reliably connected to the grid neutral line, it triggers the protection action, locks the inverter output, issues an alarm signal, and interrupts the grid connection process.

[0015] As a preferred embodiment of the present invention, the first relay, the second relay, and the third relay are all inherent switching elements of the inverter grid-connected circuit.

[0016] A method for detecting the neutral connection of a split-phase power grid inverter includes the following steps:

[0017] Step S1: The control unit controls the first relay, the second relay, and the third relay to remain open. The voltage sampling unit collects the effective value of the first phase voltage between the first phase line and the neutral line of the power grid, and / or the effective value of the second phase voltage between the second phase line and the neutral line of the power grid, and / or the effective value of the line voltage between the first phase line and the second phase line, as the initial voltage value.

[0018] Step S2: The control unit controls the first relay and the second relay to close, connecting the first filter capacitor between the first phase line and the neutral line of the power grid; or controls the second relay and the third relay to close, connecting the second filter capacitor between the second phase line and the neutral line of the power grid.

[0019] Step S3: After the system stabilizes, the corresponding effective values ​​of the first phase voltage and / or the second phase voltage and / or the line voltage are collected again through the voltage sampling unit as the detection voltage values;

[0020] Step S4: The control unit determines the connection status between the inverter neutral line and the grid neutral line based on the voltage change between the initial voltage value and the detected voltage value, or the change in the voltage combination difference calculated from the initial voltage value and the detected voltage value.

[0021] The method for determining the connection status in step S4 includes any of the following:

[0022] Method 1: Calculate the absolute value of the difference between the first phase voltage before and after step S1 and step S2. If the difference is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line; otherwise, it is determined that the connection is normal.

[0023] Method 2: Calculate the absolute value of the difference between the second phase voltage before and after step S1 and step S2. If the difference is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line; otherwise, it is determined that the connection is normal.

[0024] Method 3: Calculate the change in the absolute value of the difference between the first phase voltage and the line voltage before and after steps S1 and S2. If the change is greater than a preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line; otherwise, it is determined that the connection is normal.

[0025] Method 4: Calculate the change in the absolute value of the difference between the second phase voltage and the line voltage before and after steps S1 and S2. If the change is greater than a preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line; otherwise, it is determined that the connection is normal.

[0026] By adopting the above technical solution, the present invention has the following beneficial effects:

[0027] This invention completely eliminates the need for additional detection impedance, dedicated relays and their drive circuits required in traditional solutions, directly reducing the material cost of the inverter; fewer components mean a simpler layout, lower assembly costs and fewer potential failure points, thereby reducing manufacturing and maintenance costs; by eliminating additional detection circuits, the overall system architecture is simplified; compared to the traditional solution that artificially creates significant instantaneous load imbalance by connecting impedance, this solution uses capacitive components for detection, resulting in less instantaneous impact and voltage disturbance on the power grid, and has better grid friendliness. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of neutral line detection in a split-phase power grid inverter in the existing technology;

[0029] Figure 2 This is a schematic diagram of the circuit structure of the present invention;

[0030] Figure 3 This is a complete circuit topology diagram of Embodiment 1 of the present invention;

[0031] Figure 4 This is a schematic diagram of the process of Embodiment 1 of the present invention;

[0032] Figure 5 This is a schematic diagram of the process of Embodiment 2 of the present invention;

[0033] Figure 6 This is a schematic diagram of the process of Embodiment 3 of the present invention;

[0034] Figure 7 This is a schematic diagram of the process of Embodiment 4 of the present invention;

[0035] Figure 8This is a schematic diagram of the process of Embodiment 5 of the present invention;

[0036] Figure 9 This is a schematic diagram of the process of Embodiment 6 of the present invention.

[0037] In the diagram: 1. First relay; 2. Second relay; 3. Third relay; 4. First phase line; 5. Neutral line of the power grid; 6. Second phase line; 7. First filter capacitor; 8. Second filter capacitor; 9. DC-AC converter module; 10. First voltage sampling branch; 11. Second voltage sampling branch. Detailed Implementation

[0038] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0039] Example 1

[0040] like Figure 2 , 3 As shown, this embodiment discloses a neutral line connection detection device for a split-phase power grid inverter. The split-phase power grid includes a first phase line 4, a second phase line 6, and a power grid neutral line 5. The inverter includes a grid-connected interface connected to the split-phase power grid and a DC-AC conversion module 9. The detection device is integrated on the grid-connected interface side of the inverter and includes a relay group, a filter unit, a voltage sampling unit, and a control unit.

[0041] The relay group includes a first relay 1, a second relay 2, and a third relay 3. The first relay 1 is connected in series between the first phase line 4 and the first phase input terminal of the DC-AC converter module 9. The second relay 2 is connected in series between the grid neutral line 5 and the neutral line input terminal of the DC-AC converter module 9. The third relay 3 is connected in series between the second phase line 6 and the second phase input terminal of the DC-AC converter module 9. The first relay 1, the second relay 2, and the third relay 3 are all normally open in their initial state and are all inherent switching elements of the inverter grid-connected circuit.

[0042] The filtering unit includes a first filtering capacitor 7 and a second filtering capacitor 8. The first filtering capacitor 7 is connected in parallel between the first phase input terminal and the neutral input terminal of the DC-AC converter module 9, and the second filtering capacitor 8 is connected in parallel between the second phase input terminal and the neutral input terminal of the DC-AC converter module 9. The first filtering capacitor 7 and the second filtering capacitor 8 are inherent output filtering elements of the DC-AC converter module 9.

[0043] The voltage sampling unit is electrically connected to the first phase line 4, the second phase line 6, and the neutral line 5 of the power grid, respectively, and is used to collect the effective values ​​of the first phase voltage between the first phase line 4 and the neutral line 5 of the power grid, and the second phase voltage between the second phase line 6 and the neutral line 5 of the power grid in real time. The voltage sampling unit includes a first voltage sampling branch 10 and a second voltage sampling branch 11. The first voltage sampling branch 10 is connected in parallel between the first phase line 4 and the neutral line 5 of the power grid, and the second voltage sampling branch 11 is connected in parallel between the second phase line 6 and the neutral line 5 of the power grid. Each voltage sampling branch includes a signal conditioning circuit, which consists of an operational amplifier, resistors, and capacitors, and is used to filter, amplify, and level-shift the collected voltage signal to match the input range of the analog-to-digital converter.

[0044] The control unit is electrically connected to the relay group and the voltage sampling unit respectively. The control unit includes a microcontroller (MCU), which has a built-in or external analog-to-digital converter (ADC) interface, a general-purpose input / output (GPIO) interface and a communication interface. The output terminal of the voltage sampling unit is connected to the ADC interface, and the control terminal of the relay group is connected to the GPIO interface through a relay drive circuit. The microcontroller is connected to an external monitoring system through the communication interface to output alarm signals or receive external commands.

[0045] Principle Explanation: After performing the above detection operations, the system will exhibit different electrical characteristics based on its circuit structure:

[0046] When the inverter neutral line is not reliably connected to the grid neutral line 5, closing the first relay 1 and the second relay 2 will connect the first filter capacitor 7 between the first phase line 4 and the grid neutral line 5. The connection of this capacitive element destroys the original symmetry between the first phase and the second phase, causing the equivalent impedance between the first phase line and the grid neutral line to change, resulting in a significant difference in the first phase voltage before and after the capacitor is connected.

[0047] When the inverter neutral line is reliably connected to the grid neutral line 5, the grid neutral line provides a stable reference potential, which clamps the first phase voltage. At this time, even if the first filter capacitor 7 is connected, the first phase voltage will remain basically consistent before and after the connection. Therefore, by comparing the change in the first phase voltage before and after the capacitor is connected with the preset threshold, the connection status of the neutral line can be accurately determined.

[0048] The specific steps are as follows:

[0049] like Figure 4 As shown, firstly, the control unit controls the first relay 1, the second relay 2 and the third relay 3 to remain in the open state, and measures and records the initial effective value of the voltage between the first phase line 4 and the neutral line 5 of the power grid through the voltage sampling unit, which is denoted as V1;

[0050] Subsequently, the control unit controls the first relay 1 and the second relay 2 to close, while keeping the third relay 3 open, and connects the first filter capacitor 7 between the first phase line 4 and the power grid neutral line 5; after the system stabilizes, the effective value of the first phase voltage is measured again and recorded as V2;

[0051] The control unit calculates the absolute value of the voltage difference between the two measurements, ΔV = |V1 - V2|, and compares ΔV with a preset threshold: if ΔV is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line 5; if ΔV is less than or equal to the preset threshold, it is determined that the neutral line connection is normal.

[0052] Example 2

[0053] The difference between this embodiment and Embodiment 1 lies in the object being measured. For example... Figure 5 As shown, this embodiment also controls the first relay 1 and the second relay 2 to close and connect to the first filter capacitor 7, but it measures the change in the second phase voltage. The specific steps are as follows:

[0054] First, disconnect all relays, measure and record the initial effective voltage value between the second phase line 6 and the neutral line 5 of the power grid, and denot it as V1;

[0055] Subsequently, the first relay 1 and the second relay 2 are closed, and the first filter capacitor 7 is connected between the first phase line 4 and the neutral line 5 of the power grid; after the system stabilizes, the effective value of the second phase voltage is measured again and recorded as V2.

[0056] Calculate the absolute value of the voltage difference between the two measurements, ΔV = |V1 - V2|, and compare ΔV with a preset threshold: if ΔV is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line 5; otherwise, it is determined that the connection is normal.

[0057] Example 3

[0058] The difference between this embodiment and embodiments 1 and 2 lies in the type of filter capacitor used. For example... Figure 6 As shown, in this embodiment, the second relay 2 and the third relay 3 are closed, the second filter capacitor 8 is connected between the second phase line 6 and the neutral line 5 of the power grid, and the change in the first phase voltage is measured. The specific steps are as follows:

[0059] First, control all relays to disconnect, measure and record the initial effective value of the voltage between the first phase line 4 and the neutral line 5 of the power grid, and denot it as V1;

[0060] Subsequently, control the second relay 2 and the third relay 3 to close, and connect the second filter capacitor 8 between the second phase line 6 and the power grid neutral line 5; after the system stabilizes, measure the effective value of the first phase voltage again and record it as V2;

[0061] Calculate the absolute value of the voltage difference between the two measurements, ΔV = |V1 - V2|, and compare ΔV with a preset threshold: if ΔV is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line 5; otherwise, it is determined that the connection is normal.

[0062] Example 4

[0063] The difference between this embodiment and Embodiment 3 lies in the object being measured. For example... Figure 7 As shown, this embodiment also controls the second relay 2 and the third relay 3 to close and connect to the second filter capacitor 8, but it measures the change in the second phase voltage. The specific steps are as follows:

[0064] First, disconnect all relays, measure and record the initial effective voltage value between the second phase line 6 and the neutral line 5 of the power grid, and denot it as V1;

[0065] Subsequently, control the second relay 2 and the third relay 3 to close, and connect the second filter capacitor 8 between the second phase line 6 and the power grid neutral line 5; after the system stabilizes, measure the effective value of the second phase voltage again and record it as V2;

[0066] Calculate the absolute value of the voltage difference between the two measurements, ΔV = |V1 - V2|, and compare ΔV with a preset threshold: if ΔV is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line 5; otherwise, it is determined that the connection is normal.

[0067] Example 5

[0068] This embodiment uses a combination of voltage difference comparisons for determination. For example... Figure 8 As shown, the specific steps are as follows:

[0069] First, control all relays to disconnect, measure the effective voltage value VL1-N between the first phase line 4 and the neutral line 5 of the power grid, and the effective line voltage value VL1-L2 between the first phase line 4 and the second phase line 6, and calculate the absolute value of the difference between the two, D1=|VL1-N-VL1-L2|.

[0070] Subsequently, control the first relay 1 and the second relay 2 to close, and connect the first filter capacitor 7 between the first phase line 4 and the power grid neutral line 5; after the system stabilizes, measure VL1-N and VL1-L2 again, and calculate the absolute value of the difference D2=|VL1-N-VL1-L2|.

[0071] Calculate the difference between the two values, ΔD = |D1 - D2|, and compare ΔD with a preset threshold: if ΔD is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line 5; otherwise, it is determined that the connection is normal.

[0072] Example 6

[0073] This embodiment is similar to Embodiment 5, but uses a different voltage combination. For example... Figure 9 As shown, the specific steps are as follows:

[0074] First, control all relays to disconnect, measure the effective voltage value VL2-N between the second phase line 6 and the neutral line 5 of the power grid, and the effective line voltage value VL1-L2 between the first phase line 4 and the second phase line 6, and calculate the absolute value of the difference between the two, D1=|VL2-N-VL1-L2|.

[0075] Subsequently, control the first relay 1 and the second relay 2 to close, and connect the first filter capacitor 7 between the first phase line 4 and the power grid neutral line 5; after the system stabilizes, measure VL2-N and VL1-L2 again, and calculate the absolute value of the difference D2=|VL2-N-VL1-L2|.

[0076] Calculate the difference between the two values, ΔD = |D1 - D2|, and compare ΔD with a preset threshold: if ΔD is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line 5; otherwise, it is determined that the connection is normal.

[0077] It should be noted that the subsequent operations of the determination results in the above embodiments 2-6 are the same as those in embodiment 1: if the connection is determined to be normal, the control unit continues to execute the grid connection process, closes the third relay 3, and enables the inverter to enter the normal power generation operation state; if the connection is determined to be not connected, the control unit triggers the protection action, locks the inverter output, issues an alarm signal, and prohibits the inverter from grid connection.

[0078] All components mentioned in this article are general standard parts or components known to those skilled in the art. Their structures and principles can be learned by those skilled in the art through technical manuals or conventional experimental methods, so they will not be described in detail here.

[0079] While the specific embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention, and modifications or variations without creative effort are still within the protection scope of the present invention.

Claims

1. A device for detecting the neutral connection of a split-phase power grid inverter, characterized in that, include: The relay group includes a first relay (1), a second relay (2), and a third relay (3); the split-phase power grid includes a first phase line (4), a second phase line (6), and a power grid neutral line (5); the inverter includes a grid connection interface connected to the split-phase power grid and a DC-AC conversion module (9); the first relay (1) is connected in series between the first phase line (4) and the first phase input terminal of the DC-AC conversion module (9); the second relay (2) is connected in series between the power grid neutral line (5) and the neutral line input terminal of the DC-AC conversion module (9); and the third relay (3) is connected in series between the second phase line (6) and the second phase input terminal of the DC-AC conversion module (9). The filtering unit includes a first filtering capacitor (7) and a second filtering capacitor (8). The first filtering capacitor (7) is connected in parallel between the first phase input terminal and the neutral input terminal of the DC-AC converter module (9), and the second filtering capacitor (8) is connected in parallel between the second phase input terminal and the neutral input terminal of the DC-AC converter module (9). The first filtering capacitor (7) and the second filtering capacitor (8) are inherent output filtering elements of the DC-AC converter module (9). The voltage sampling unit is electrically connected to the first phase line (4), the second phase line (6), and the neutral line (5) of the power grid, respectively, and is used to collect the first phase voltage between the first phase line (4) and the neutral line (5) of the power grid and the second phase voltage between the second phase line (6) and the neutral line (5) of the power grid in real time; The control unit is electrically connected to the relay group and the voltage sampling unit respectively. The control unit is used to control the first relay (1) and the second relay (2) to close, or to control the second relay (2) and the third relay (3) to close, before the inverter is connected to the grid or during the system self-test, so as to connect the corresponding filter capacitor between the corresponding phase line and the neutral line of the power grid (5). Used to receive the voltage collected by the voltage sampling unit, and to determine the connection status of the inverter neutral line and the grid neutral line (5) based on the voltage change or voltage difference.

2. The neutral connection detection device for a split-phase power grid inverter according to claim 1, characterized in that: The first relay (1), the second relay (2), and the third relay (3) are all normally open in their initial state.

3. The neutral line connection detection device for a split-phase power grid inverter according to claim 1, characterized in that: The voltage sampling unit includes a first voltage sampling branch (10) and a second voltage sampling branch (11); the first voltage sampling branch (10) is connected in parallel between the first phase line (4) and the power grid neutral line (5), and the second voltage sampling branch (11) is connected in parallel between the second phase line (6) and the power grid neutral line (5).

4. The neutral connection detection device for a split-phase power grid inverter according to claim 1, characterized in that: When the control unit determines that the inverter neutral line and the grid neutral line (5) are connected normally, it controls the relay group to close, so that the inverter enters the grid-connected operation state; when it determines that the inverter neutral line and the grid neutral line (5) are not reliably connected, it triggers the protection action, issues an alarm signal and prohibits the inverter from being connected to the grid.

5. The neutral line connection detection device for a split-phase power grid inverter according to claim 1, characterized in that: The first relay (1), the second relay (2), and the third relay (3) are all inherent switching elements of the inverter grid-connected circuit.

6. A method for detecting the neutral connection of a split-phase power grid inverter based on the device described in any one of claims 1-5, characterized in that, Includes the following steps: Step S1: The control unit controls the first relay (1), the second relay (2), and the third relay (3) to remain open, and collects the effective value of the first phase voltage between the first phase line (4) and the neutral line (5) of the power grid, and / or the effective value of the second phase voltage between the second phase line (6) and the neutral line (5) of the power grid, and / or the effective value of the line voltage between the first phase line (4) and the second phase line (6) through the voltage sampling unit, as the initial voltage value; Step S2: The control unit controls the first relay (1) and the second relay (2) to close, connecting the first filter capacitor (7) between the first phase line (4) and the power grid neutral line (5); or controls the second relay (2) and the third relay (3) to close, connecting the second filter capacitor (8) between the second phase line (6) and the power grid neutral line (5); Step S3: After the system stabilizes, the corresponding effective values ​​of the first phase voltage and / or the second phase voltage and / or the line voltage are collected again through the voltage sampling unit as the detection voltage values; Step S4: The control unit determines the connection status of the inverter neutral line and the grid neutral line (5) based on the voltage change between the initial voltage value and the detected voltage value, or the change in the voltage combination difference calculated from the initial voltage value and the detected voltage value.

7. The method for detecting the neutral connection of a split-phase power grid inverter according to claim 6, characterized in that, The method for determining the connection status in step S4 includes any of the following: Method 1: Calculate the absolute value of the difference between the first phase voltage before and after step S1 and step S2. If the difference is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line (5). Otherwise, it is determined that the connection is normal. Method 2: Calculate the absolute value of the difference between the second phase voltage before and after step S1 and step S2. If the difference is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line (5). Otherwise, it is determined that the connection is normal. Method 3: Calculate the absolute value of the difference between the first phase voltage and the line voltage before and after step S1 and step S2. If the change is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line (5). Otherwise, it is determined that the connection is normal. Method 4: Calculate the absolute value of the difference between the second phase voltage and the line voltage before and after step S1 and step S2. If the change is greater than the preset threshold, it is determined that the inverter neutral line is not reliably connected to the grid neutral line (5). Otherwise, it is determined that the connection is normal.

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