Grid-connected inverter switching tube open-circuit fault diagnosis method

Abstract

The invention relates to a grid-connected inverter switching tube open-circuit fault diagnosis method. The method comprises the following steps: step 1, analyzing abnormal influence factors of electric energy quality of a photovoltaic power station; 2, analyzing a resonance mechanism of a parallel inverter LCL filter circuit; 3, analyzing a common mode interference mechanism of the grid-connected inverter; 4, grid-connected inverter switching tube open-circuit fault diagnosis is carried out; the method has the advantages of current characteristic analysis, resonance phenomenon analysis and common mode interference consideration.

Description

Technical Field

[0001] The invention belongs to the technical field of grid-connected inverter fault diagnosis, and in particular relates to a method for diagnosing open-circuit faults of switch tubes of a grid-connected inverter. Background Art

[0002] The core device responsible for generating grid-connected current in a photovoltaic power station is the inverter. Whether the inverter can work normally is the core factor affecting the power quality of the grid-connected current. Since the power generation mechanism of the photovoltaic power station is through the high-frequency switching action of switching devices such as IGBT in the inverter of the power station, the direct current generated by the photovoltaic array through the photovoltaic effect is inverted into alternating current. In the actual operation of the inverter, since the switching tubes such as IGBT are in a high-frequency switching state for a long time, switch tube failure is prone to occur. When the switch tube short-circuit failure occurs, the protection mechanism inside the inverter will remove the short-circuit device, thereby converting the short-circuit failure into an open-circuit failure. Unlike the switch tube short-circuit failure, the inverter will not shut down immediately after the open-circuit failure occurs, but will continue to operate in an abnormal state. In this special case, the power quality of the grid-connected current output by the inverter will be greatly deteriorated, thereby endangering the stable operation of the power grid; therefore, it is very necessary to provide a grid-connected inverter switch tube open-circuit fault diagnosis method based on current characteristic analysis, resonance phenomenon analysis, and consideration of common mode interference. Summary of the invention

[0003] The purpose of the present invention is to overcome the deficiencies of the prior art and to provide a method for diagnosing open-circuit faults of a grid-connected inverter switch tube by analyzing current characteristics, analyzing resonance phenomena, and taking common-mode interference into consideration.

[0004] The object of the present invention is achieved by: a method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter, the method comprising the following steps:

[0005] Step 1: Analysis of abnormal factors affecting the power quality of photovoltaic power stations;

[0006] Step 2: Analysis of resonance mechanism of LCL filter circuit of parallel inverter;

[0007] Step 3: Analysis of common mode interference mechanism of grid-connected inverter;

[0008] Step 4: Diagnosis of open circuit fault of switch tube of grid-connected inverter.

[0009] The analysis of abnormal factors affecting the power quality of the photovoltaic power station in step 1 includes: open circuit failure of the switch tube in the three-level inverter, LCL filter resonance phenomenon and high-frequency common mode current of the non-isolated grid-connected inverter.

[0010] The resonance mechanism analysis of the LCL filter circuit of the parallel inverter in step 2 is as follows: the LCL filter is a typical third-order system, and it is prone to high-frequency resonance problems because it has an inherent resonance point, which causes the specific harmonic content in the grid-connected current to be abnormally amplified. The fundamental reason for the resonance of the LCL filter is that it is insufficiently damped at the inherent resonance frequency, and the inverter output voltage U i , for the grid-connected current i grid The transfer function is: Among them, ω r is the resonant angular frequency,

[0011] The resonance mechanism analysis of the LCL filter circuit of the parallel inverter in step 2 also needs to consider the situation where the transmission line impedance is large when the power grid is weak: Assume that the power grid line impedance is usually inductive L g Mainly represented by, at this time the output voltage U i To the grid current i grid The transfer function is: The resonance point is:

[0012]

[0013] The analysis of the common-mode interference mechanism of the grid-connected inverter in step 3 is as follows: the bus parasitic capacitance voltage in the common-mode current flow path of the non-isolated three-level photovoltaic inverter is U pv1 ; The distributed capacitance of the photovoltaic array to the ground in the LCL filter is C pp ; Common mode current is i com ; Through KVL loop, we get: In the formula, u ko is the bridge arm output voltage; i k is the inverter output current; U g is the grid voltage; i gk is the inverter grid-connected current; where k = a, b, c, considering the three-phase symmetry of the grid, we have: U ga +U gb +U gc =0(6), from equations (5) and (6), we can get: Since point O is not electrically connected to the midpoint of the capacitor, no zero-sequence current will flow through the filter capacitor C, and the common-mode current i com The expression is: It can also be written as

[0014]

[0015] The common mode interference mechanism analysis of the grid-connected inverter in step 3 PV According to formula (9), we can get: Midpoint voltage difference uNO The expression is: From formula (11), we can get: Substituting equations (8), (10), and (12) into equation (7), we can obtain:

[0016]

[0017] The common mode voltage of the grid-connected inverter common mode interference mechanism analysis in step 3 is the average value of the three-phase bridge arm output voltage, that is: Define the DC voltage differential mode component: U dm =U dc1 -U dc2 (15), Substituting equations (13) and (14) into equation (15), we can obtain:

[0018]

[0019] There are two voltage sources in the common mode model of the non-isolated three-level photovoltaic inverter in the common mode interference mechanism analysis of the grid-connected inverter in step 3: one is the differential mode component 0.5U of the three-level inverter dm , the second is the common mode voltage of the three-level inverter 0.5U com , the transfer function of common mode current to common mode voltage H(s): At this time, the network resonant frequency is:

[0020]

[0021] The grid-connected inverter switch tube open circuit fault diagnosis in step 4 is specifically as follows: Since the NPC three-level inverter is a three-phase symmetrical structure, when the inverter is in normal operation, it has three working states P, N, and O. Here, it is stipulated that when the current flows from the inverter to the grid, i>0. When the NPC three-level inverter has an open circuit fault, the current conduction path of each state will inevitably change. It is assumed that the current flows from the inverter to the grid as positive. Assume that S a1 , S a2 , S a3 , S a4 For the 4 switch tubes of phase A, VD 1 、VD 2 There are two clamping diodes; D a1 , D a2 , D a3 , D a4 There are 4 freewheeling diodes; when the photovoltaic power station is in a normal power generation state with a power factor of almost 1, due to the influence of the grid voltage, the clamping diode VD 1 、VD 2 Turn off, so that the switch tube S a1 With S a2The fault current characteristics are the same as those when the circuit is open. The switch tube S a3 With S a4 The fault current characteristics when the circuit is open are the same, so the open circuit fault of the switch tube of the grid-connected inverter can be diagnosed.

[0022] In step 4, when the inverter is in the P state and the output current i A >0, the output current flows from the switch tube S a1 and S a2 A path is formed in the A When <0, the current flows from the switch tube S a1 and S a2 flows through the anti-parallel diode; when the inverter is in the O state and the output current i A >0, the output current flows from the clamping diode VD 1 And switch tube S a2 When the output current i A When <0, the current flows from the switch tube S a3 and clamping diode VD 2 When the inverter is in the N state and the output current i A >0, the output current flows from the switch tube S a3 and S a4 flows through the anti-parallel diode; when the output current i A When <0, the current flows from the switch tube S a3 and S a4 In circulation.

[0023] The beneficial effects of the present invention are as follows: the present invention is a method for diagnosing an open-circuit fault of a grid-connected inverter switch tube. During use, the present invention starts from the analysis of abnormal influencing factors of the power quality of a photovoltaic power station, analyzes the resonance mechanism of the LCL filter circuit of a parallel inverter and the common-mode interference mechanism of a grid-connected inverter, diagnoses the open-circuit fault of the grid-connected inverter switch tube, analyzes the fault generation mechanism, extracts and identifies the inverter signal of the switch tube fault, collects and analyzes the fault characteristics, extracts the fault feature vector through Fourier transform, and performs fault discrimination on the extracted fault characteristics through a BP neural network, diagnoses and identifies the fault type, and realizes the open-circuit fault diagnosis of the inverter switch tube. The present invention has the advantages of current feature analysis, resonance phenomenon analysis, and consideration of common-mode interference. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is the Bode diagram of the LCL filter of the present invention.

[0025] Figure 2 This is the equivalent circuit diagram of LCL filtering under impedance consideration of the present invention.

[0026] Figure 3This is the Bode diagram of the LCL filter considering the line resistance of the present invention.

[0027] Figure 4 It is a schematic diagram of the common-mode current loop of the non-isolated photovoltaic grid-connected system of the present invention.

[0028] Figure 5 It is a schematic diagram of a non-isolated three-phase photovoltaic inverter of the present invention.

[0029] Figure 6 It is a common-mode equivalent model diagram of the three-level photovoltaic inverter of the present invention.

[0030] Figure 7 This is a common-mode equivalent simplified model diagram of the three-level photovoltaic inverter of the present invention.

[0031] Figure 8 Schematic diagram of current flow in each state when the switch tube of the present invention is normal Figure 1 .

[0032] Fig. 9 Schematic diagram of current flow in each state when the switch tube of the present invention is normal Figure 2 .

[0033] Fig.10 Schematic diagram of current flow in each state when the switch tube of the present invention is normal Figure 3

[0034] Fig.11 The switch tube S of the present invention a1 Schematic diagram of the current flow in an open circuit fault.

[0035] Fig.12 The switch tube S of the present invention a2 Schematic diagram of the current flow in an open circuit fault.

[0036] Fig.13 The switch tube S of the present invention a3 Schematic diagram of the current flow in an open circuit fault.

[0037] Fig.14 The switch tube S of the present invention a4 Schematic diagram of the current flow in an open circuit fault. DETAILED DESCRIPTION

[0038] The present invention will be further described below in conjunction with the accompanying drawings.

[0039] Example 1

[0040] like Figure 1-14 As shown, a method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter is provided, the method comprising the following steps:

[0041] Step 1: Analysis of abnormal factors affecting the power quality of photovoltaic power stations;

[0042] Step 2: Analysis of resonance mechanism of LCL filter circuit of parallel inverter;

[0043] Step 3: Analysis of common mode interference mechanism of grid-connected inverter;

[0044] Step 4: Diagnosis of open circuit fault of switch tube of grid-connected inverter.

[0045] The analysis of abnormal factors affecting the power quality of the photovoltaic power station in step 1 includes: open circuit failure of the switch tube in the three-level inverter, LCL filter resonance phenomenon and high-frequency common mode current of the non-isolated grid-connected inverter.

[0046] In this embodiment, ① open circuit fault of the switch tube in the three-level inverter: compared with the traditional two-level inverter, the three-level inverter has the advantages of evenly dividing the large voltage by connecting the power switch tubes in series, small switching loss, low harmonic content of the output waveform, small electromagnetic interference and high working efficiency. However, since the structure of the three-level inverter is more complicated than that of the two-level inverter, the power switch devices of the NPC three-level inverter are doubled from 6 in the two-level inverter to 12. Therefore, even if the probability of failure of a single power device is the same, the possibility of open circuit or short circuit fault of the power device in the inverter as a whole is greatly increased; and when the short circuit fault of the power switch tube occurs, the short circuit protection of the inverter will immediately remove the short circuit fault to avoid fire and other situations, thereby converting the short circuit fault of the switch tube into an open circuit fault of the switch tube. Therefore, the most common hardware fault in the use of the inverter is the open circuit fault of the switch tube in the inverter;

[0047] ②LCL filter resonance phenomenon: The output current of the photovoltaic power station inverter needs to be filtered by the filter to remove the high-order harmonics in it, so as to meet the power quality requirements of the grid. The high-frequency attenuation characteristics of the LCL filter make it possible to greatly save the inductor cost and reduce the switching frequency of the inverter to reduce the loss of the switch tube under the same filtering effect. However, as a third-order system, the LCL filter has an inherent resonant frequency. Under the complex harmonic background of the grid, it is easy to have serious resonance phenomenon, which seriously deteriorates the power quality of the grid-connected current.

[0048] ③ High-frequency common-mode current of non-isolated grid-connected inverters: In non-isolated grid-connected inverters, the removal of the transformer makes the common-mode current flow in a loop. The installation method of the photovoltaic array determines that it has an inherent parasitic capacitance. The parasitic capacitance of the silicon solar panel to the ground is generally 50-150nF / kW; because in the common-mode current loop, the high-frequency switching switch tube will generate an excitation source generated by the high-frequency change of the common-mode voltage. This excitation source will continuously charge and discharge the photovoltaic cell's capacitance to the ground at a high frequency, thus generating a high-frequency common-mode current; the common-mode current will increase the harmonic content of the grid-connected current, enhance electromagnetic interference, and even threaten the safety of photovoltaic power station operation and maintenance personnel.

[0049] The resonance mechanism analysis of the LCL filter circuit of the parallel inverter in step 2 is as follows: the LCL filter is a typical third-order system, and it is prone to high-frequency resonance problems because it has an inherent resonance point, which causes the specific harmonic content in the grid-connected current to be abnormally amplified. The fundamental reason for the resonance of the LCL filter is that it is insufficiently damped at the inherent resonance frequency, and the inverter output voltage U i , for the grid-connected current i grid The transfer function is: Among them, ω r is the resonant angular frequency,

[0050] In this embodiment, Figure 1 is the Bode diagram of the transfer function G(s). By analyzing the Bode diagram, we can get: r There is a resonance peak at , and the phase of G(s) crosses the -180° line, which means that from the control perspective, a pair of closed-loop nodes in the right half plane will be generated at this time, resulting in the instability of the grid-connected inverter to operate; if the power grid has a complex harmonic background, once it contains a harmonic voltage of the same frequency as the resonant frequency point, the LCL filter will resonate, causing the output current to amplify the harmonic component, thereby making the grid-connected current power quality of the grid-connected inverter substandard.

[0051] The resonance mechanism analysis of the LCL filter circuit of the parallel inverter in step 2 also needs to consider the situation where the transmission line impedance is large when the power grid is weak, such as Figure 2 As shown: Assume that the grid line impedance is usually inductive L g Mainly represented by, at this time the output voltage U i To the grid current i grid The transfer function is: The resonance point is:

[0052]

[0053] In this embodiment, it is known from the above formula that the grid line impedance L gThe existence of makes the system's resonant angular frequency ω r The resonance point of the system is reduced due to the line impedance L g The Bode diagram comparison of the two states is shown in the figure below. Figure 3 shown.

[0054] The present invention starts from the analysis of abnormal influencing factors of the power quality of the photovoltaic power station, analyzes the resonance mechanism of the LCL filter circuit of the parallel inverter and the common-mode interference mechanism of the grid-connected inverter, diagnoses the open-circuit fault of the switch tube of the grid-connected inverter, analyzes the fault generation mechanism, extracts and identifies the inverter signal from the switch tube fault, collects and analyzes the fault characteristics, extracts the fault feature vector through Fourier transform, and performs fault discrimination on the extracted fault characteristics through the BP neural network, diagnoses and identifies the fault type, and realizes the open-circuit fault diagnosis of the inverter switch tube; the present invention has the advantages of current characteristic analysis, resonance phenomenon analysis, and consideration of common-mode interference.

[0055] Example 2

[0056] like Figure 1-14 As shown, a method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter is provided, the method comprising the following steps:

[0057] Step 1: Analysis of abnormal factors affecting the power quality of photovoltaic power stations;

[0058] Step 2: Analysis of resonance mechanism of LCL filter circuit of parallel inverter;

[0059] Step 3: Analysis of common mode interference mechanism of grid-connected inverter;

[0060] Step 4: Diagnosis of open circuit fault of switch tube of grid-connected inverter.

[0061] The analysis of the common mode interference mechanism of the grid-connected inverter in step 3 is as follows: the common mode current flow path is as follows: Figure 4 As shown in Figure 5 It is a typical structure of a non-isolated three-level photovoltaic inverter. pv1 Indicates the bus parasitic capacitance voltage; in LCL filter, C pp Represents the distributed capacitance of the photovoltaic array to the ground; i com is the common mode current; through KVL, we can get the following result for the loop in the figure: In the formula, u ko is the bridge arm output voltage; i k is the inverter output current; U g is the grid voltage; i gk is the inverter grid-connected current; where k = a, b, c, considering the three-phase symmetry of the grid, we have: U ga +U gb +U gc=0(6), from equations (5) and (6), we can get: Since point O is not electrically connected to the midpoint of the capacitor, no zero-sequence current will flow through the filter capacitor C, and the common-mode current i com The expression is: It can also be written as

[0062] The common mode interference mechanism analysis of the grid-connected inverter in step 3 PV According to formula (9), we can get: Midpoint voltage difference u NO The expression is: From formula (11), we can get: Substituting equations (8), (10), and (12) into equation (7), we can obtain:

[0063]

[0064] The common mode voltage of the grid-connected inverter common mode interference mechanism analysis in step 3 is the average value of the three-phase bridge arm output voltage, that is: Define the DC voltage differential mode component: U dm =U dc1 -U dc2 (15), Substituting equations (13) and (14) into equation (15), we can obtain:

[0065]

[0066] There are two voltage sources in the common mode model of the non-isolated three-level photovoltaic inverter in the common mode interference mechanism analysis of the grid-connected inverter in step 3: one is the differential mode component 0.5U of the three-level inverter dm , the second is the common mode voltage of the three-level inverter 0.5U com , the transfer function of common mode current to common mode voltage H(s): At this time, the network resonant frequency is:

[0067]

[0068] In this embodiment, the common-mode equivalent model diagram of the non-isolated three-level photovoltaic inverter is derived from the above formula, as shown in Figure 6 As shown by Figure 6 It can be seen that there are two voltage sources in the common-mode model of the non-isolated three-level photovoltaic inverter: one is the differential mode component of the three-level inverter 0.5U dm , the second is the common mode voltage of the three-level inverter 0.5U com ;

[0069] Due to the unbalanced midpoint of the DC side capacitor of the three-level inverter, a differential mode excitation source of the DC voltage of the three-level inverter is generated, 0.5U dm Therefore, this excitation source can be suppressed by adopting the midpoint balance strategy in the inverter wave generation, so this influence can be ignored; therefore, the final common-mode equivalent simplified model is as follows Figure 7 As shown by Figure 7 It can be known that the common mode current is generated by a network composed of the bridge arm side inductance, the grid side inductance and the parasitic capacitance, with the common mode voltage as the excitation source; this network has an amplifying effect on the harmonic current of a specific harmonic frequency, which will have a negative impact on the power quality of the grid; according to Figure 7 , we can get the transfer function H(s) of common-mode current to common-mode voltage: At this time, the network resonant frequency is:

[0070]

[0071] The grid-connected inverter switch tube open circuit fault diagnosis in step 4 is specifically as follows: Since the NPC three-level inverter is a three-phase symmetrical structure, the normal working state of the inverter is analyzed with phase A as a reference. When the inverter is in normal operation, it has three working states: P, N, and O. The output current diagram of each state is as follows: Figure 8-10 As shown, it is stipulated here that when the current flows from the inverter to the grid, i>0. First, the normal working state of the inverter is analyzed:

[0072] ①When the inverter is in P state and the output current i A >0, the output current flows from the switch tube S a1 and S a2 A path is formed in the A When <0, the current flows from the switch tube S a1 and S a2 Flows through the anti-parallel diode;

[0073] ②When the inverter is in the O state and the output current i A >0, the output current flows from the clamping diode VD 1 And switch tube S a2 When the output current i A When <0, the current flows from the switch tube S a3 and clamping diode VD 2 medium circulation;

[0074] ③When the inverter is in N state and the output current i A >0, the output current flows from the switch tube S a3 and S a4 flows through the anti-parallel diode; when the output current i A When <0, the current flows from the switch tube Sa3 and S a4 medium circulation;

[0075] When an open-circuit fault occurs in the NPC three-level inverter, the current conduction path of each state will inevitably change. Assuming that the current flows from the inverter to the grid as positive, the following takes phase A as an example to analyze in detail the working state of the inverter when an open-circuit fault occurs in the switch tube: Assuming S a1 , S a2 , S a3 , S a4 For the 4 switch tubes of phase A, VD 1 、VD 2 There are two clamping diodes; D a1 , D a2 , D a3 , D a4 There are 4 freewheeling diodes; since the double-tube fault can be regarded as the superposition of two single-tube fault types, the circuit working mode when the single tube is open is first analyzed in detail, and the working mode of the double-tube open circuit can be superimposed on the working mode of the single tube open circuit:

[0076] ①S a1 Open circuit fault: If S a1 When an open circuit fault occurs, the original switch state [P] has lost its forward conduction path, and the forward conduction path functionally only has the switch state [O]. However, the photovoltaic power station is generally in the grid-connected power generation state, and its power factor is approximately 1. The clamping diode VD 1 Due to the positive voltage of the grid, it is reverse biased, so the forward current is almost 0; while the reverse conduction path in the [P] state is normal, and the current passes through the freewheeling diode D a1 With D a2 Flowing to the DC side, the fault current flow diagram is as follows Fig.11 As shown;

[0077] ②S a2 Open circuit fault: If S a2 When an open circuit fault occurs, the original switch states [P] and [O] have lost their forward conduction paths. Functionally, the forward conduction path only has the switch state [N], and the current path only passes through D a3 and D a4 , but at this time D a3 and D a4 Since the positive voltage of the power grid is reverse biased, the positive current of the fault phase is also 0; while in the [P] state, the reverse conduction path is normal, and the current flows through the freewheeling diode D a1 , D a2 Flows to the DC side. In the [O] state, the reverse conduction path is normal and the current flows through the freewheeling diode VD 2 And the switch tube S a3Flowing to the DC side, the fault current flow diagram is as follows Fig.12 As shown;

[0078] ③S a3 Open circuit fault: If S a3 When an open circuit fault occurs, in addition to the current direction, the overall analysis is the same as S a2 The open circuit fault is basically the same; the original switch states [N] and [O] have lost their reverse conduction paths, and the functional reverse conduction path only has the switch state [P], and the current path only passes through D a1 and D a2 , but at this time D a1 and D a2 Due to the reverse bias, the fault reverse current is 0; in the [N] state, the forward conduction path of the current is normal, and the current passes through the freewheeling diode D a3 , D a4 Flows to the DC side, while in the [O] state the current reverse conduction path is normal, and the current flows through the freewheeling diode VD 2 , switch tube S a2 Flowing to the DC side, the fault current flow diagram is as follows Fig.13 As shown;

[0079] ④S a4 Open circuit fault: If S a4 When an open circuit fault occurs, the original switch state [N] has lost its reverse conduction path, and the reverse conduction path functionally only has the switch state [O]. However, the photovoltaic power station is generally in the grid-connected power generation state, and its power factor is approximately 1. The clamping diode VD 2 The reverse current is almost zero due to the reverse voltage of the power grid. The forward conduction path of the [N] state is normal, and the current flows through the freewheeling diode D a3 , D a4 Flowing to the DC side, the fault current flow diagram is as follows Fig.14 As shown;

[0080] Comprehensive analysis of the fault mechanisms of the above four types of switch tube open circuits shows that when the photovoltaic power station is in a normal power generation state with a power factor of almost 1, due to the influence of the grid voltage, the clamping diode VD 1 、 VD 2 shut down, so that Switching tube S a 1 and S a 2 The fault current characteristics are the same as those when the circuit is open. S a 3 With S a4 The fault current characteristics when the circuit is open are the same, so the open circuit fault of the switch tube of the grid-connected inverter can be diagnosed.

[0081] The present invention starts from the analysis of abnormal influencing factors of the power quality of the photovoltaic power station, analyzes the resonance mechanism of the LCL filter circuit of the parallel inverter and the common-mode interference mechanism of the grid-connected inverter, diagnoses the open-circuit fault of the switch tube of the grid-connected inverter, analyzes the fault generation mechanism, extracts and identifies the inverter signal from the switch tube fault, collects and analyzes the fault characteristics, extracts the fault feature vector through Fourier transform, and performs fault discrimination on the extracted fault characteristics through the BP neural network, diagnoses and identifies the fault type, and realizes the open-circuit fault diagnosis of the inverter switch tube; the present invention has the advantages of current characteristic analysis, resonance phenomenon analysis, and consideration of common-mode interference.

Claims

1. A method for diagnosing an open-circuit fault of a switch tube of a grid-connected inverter, characterized in that: The method comprises the following steps: Step 1: Analysis of abnormal factors affecting the power quality of photovoltaic power stations; Step 2: Analysis of resonance mechanism of LCL filter circuit of parallel inverter; Step 3: Analysis of common mode interference mechanism of grid-connected inverter; Step 4: Diagnosis of open circuit fault of switch tube of grid-connected inverter.

2. A method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter according to claim 1, characterized in that: The analysis of abnormal factors affecting the power quality of the photovoltaic power station in step 1 includes: open circuit failure of the switch tube in the three-level inverter, LCL filter resonance phenomenon and high-frequency common mode current of the non-isolated grid-connected inverter.

3. A method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter according to claim 1, characterized in that: The resonance mechanism analysis of the LCL filter circuit of the parallel inverter in step 2 is as follows: the LCL filter is a typical third-order system, and it is prone to high-frequency resonance problems because it has an inherent resonance point, which causes the specific harmonic content in the grid-connected current to be abnormally amplified. The fundamental reason for the resonance of the LCL filter is that it is insufficiently damped at the inherent resonance frequency, and the inverter output voltage U i , for the grid-connected current i grid The transfer function is: Among them, ω r is the resonant angular frequency, 4. A method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter according to claim 3, characterized in that: The resonance mechanism analysis of the LCL filter circuit of the parallel inverter in step 2 also needs to consider the situation where the transmission line impedance is large when the power grid is weak: Assume that the power grid line impedance is usually inductive L g Mainly represented by, at this time the output voltage U i To the grid current i grid The transfer function is: The resonance point is:

5. A method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter according to claim 1, characterized in that: The analysis of the common-mode interference mechanism of the grid-connected inverter in step 3 is as follows: the bus parasitic capacitance voltage in the common-mode current flow path of the non-isolated three-level photovoltaic inverter is U pv1 ; The distributed capacitance of the photovoltaic array to the ground in the LCL filter is C pp ; Common mode current is i com ; Through KVL loop, we get: In the formula, u ko is the bridge arm output voltage; i k is the inverter output current; U g is the grid voltage; i gk is the inverter grid-connected current; where k = a, b, c, considering the three-phase symmetry of the grid, we have: U ga +U gb +U gc =0(6), from equations (5) and (6), we can get: Since point O is not electrically connected to the midpoint of the capacitor, no zero-sequence current will flow through the filter capacitor C, and the common-mode current i com The expression is: It can also be written as 6. A method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter according to claim 5, characterized in that: The common mode interference mechanism analysis of the grid-connected inverter in step 3 PV According to formula (9), we can get: Midpoint voltage difference u NO The expression is: From formula (11), we can get: Substituting equations (8), (10), and (12) into equation (7), we can obtain:

7. A method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter according to claim 6, characterized in that: The common mode voltage of the grid-connected inverter common mode interference mechanism analysis in step 3 is the average value of the three-phase bridge arm output voltage, that is: Define the DC voltage differential mode component: U dm =U dc1 -U dc2 (15), Substituting equations (13) and (14) into equation (15), we can obtain:

8. A method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter according to claim 7, characterized in that: There are two voltage sources in the common mode model of the non-isolated three-level photovoltaic inverter in the common mode interference mechanism analysis of the grid-connected inverter in step 3: one is the differential mode component 0.5U of the three-level inverter dm , the second is the common mode voltage of the three-level inverter 0.5U com , the transfer function of common mode current to common mode voltage H(s): At this time, the network resonant frequency is:

9. A method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter according to claim 1, characterized in that: The grid-connected inverter switch tube open circuit fault diagnosis in step 4 is specifically as follows: Since the NPC three-level inverter is a three-phase symmetrical structure, when the inverter is in normal operation, it has three working states P, N, and O. Here, it is stipulated that when the current flows from the inverter to the grid, i>0. When the NPC three-level inverter has an open circuit fault, the current conduction path of each state will inevitably change. It is assumed that the current flows from the inverter to the grid as positive. Assume that S a1 , S a2 , S a3 , S a4 There are four switch tubes for phase A, VD1 and VD2 are two clamping diodes; D a1 , D a2 , D a3 , D a4 There are 4 freewheeling diodes; when the photovoltaic power station is in a normal power generation state with a power factor of almost 1, due to the influence of the grid voltage, the clamping diodes VD1 and VD2 are turned off, so that the switch tube S a1 With S a2 The fault current characteristics are the same as those when the circuit is open. The switch tube S a3 With S a4 The fault current characteristics when the circuit is open are the same, so the open circuit fault of the switch tube of the grid-connected inverter can be diagnosed.

10. A method for diagnosing an open circuit fault of a switch tube of a grid-connected inverter according to claim 9, characterized in that: In step 4, when the inverter is in the P state and the output current i A >0, the output current flows from the switch tube S a1 and S a2 Forming a pathway; When the output current i A When <0, the current flows from the switch tube S a1 and S a2 flows through the anti-parallel diode; when the inverter is in the O state and the output current i A >0, the output current flows from the clamping diode VD1 and the switch tube S a2 When the output current i A When <0, the current flows from the switch tube S a3 and clamping diode VD2; when the inverter is in the N state and the output current i A >0, the output current flows from the switch tube S a3 and S a4 Flows through the anti-parallel diode; When the output current i A When <0, the current flows from the switch tube S a3 and S a4 In circulation.

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