A t-type three-level converter open-circuit fault diagnosis method with strong robustness

Abstract

The application discloses a kind of T type three-level converter open-circuit fault diagnosis method with strong robustness, belong to the field of power electronic equipment fault diagnosis, to distinguish different switch similar open-circuit fault characteristics and accurately locate open-circuit fault tube.Considering different modulation strategies and modulation modes, the phase voltage residual is generated through the phase voltage model, so as to extract the fault characteristics of the converter under various open-circuit faults.Analyze the influence of factors such as parameter error, dead time and delay time of the system on the phase voltage vector residual, and the adaptive residual vector amplitude threshold based on uncertainty propagation theory.A hierarchical fault diagnosis scheme is proposed, which can locate the group-level open-circuit fault according to the voltage residual vector amplitude and angle, switch the three-level modulation mode of all phases of the converter to two-level mode, to further identify the specific fault switch from the fault group.A reasonable balance between robustness and speed can be achieved, and the complexity and adaptability of the diagnosis algorithm are fully considered.

Description

Technical Field

[0001] This invention belongs to the field of power electronic equipment fault diagnosis technology, and more specifically, relates to a robust T-type three-level converter open-circuit fault diagnosis method. Background Technology

[0002] Multilevel converters have received widespread attention in medium-voltage and medium-switching-frequency applications, such as renewable energy generation, rail traction, and automotive drive systems. Among multilevel converters, three-level neutral-point clamp converters and T-type three-level converters are the most commonly used topologies due to their simplicity and flexibility. Compared to three-level neutral-point clamp converters, T-type three-level converters offer advantages in conversion efficiency and fault-tolerant control. Currently, ensuring the high reliability of three-level converters is a critical issue in practical applications, attracting considerable interest. Typically, short-circuit faults can be detected using hardware detection and protection circuits built into the driver board. With the help of short-circuit protection circuits, the drive signal is locked, the fault switch is turned off, and the short-circuit fault is converted into an open-circuit fault. Open-circuit faults typically manifest as current and voltage distortion. If open-circuit faults are not addressed immediately, they may cause further damage to other equipment. Therefore, a fast and robust open-circuit fault diagnosis method is worthy of research.

[0003] Open-circuit fault diagnosis methods for converters fall into two categories: current-based and voltage-based methods. Current-based methods primarily diagnose open-circuit faults based on characteristics such as the average current value, root mean square value, vector magnitude, zero-current characteristics, and reference current error. These methods are simple to implement and have low hardware requirements. However, when applied to three-level converters, they suffer from the inability to distinguish open-circuit faults, and their diagnostic speed is generally slow. Voltage-based methods, on the other hand, can achieve fault diagnosis by adding additional hardware circuitry to detect pole voltages, line voltages, and gate conduction voltages. These methods improve diagnostic speed but have higher costs. In recent years, voltage-based fault diagnosis methods have received widespread attention. These methods observe the phase voltage, line voltage, and DC-side capacitor voltage of the converter through system parameters and switching states, and diagnose faults based on the voltage residual characteristics between these values ​​and the actual values. Furthermore, increasing the sampling rate can further improve the fault diagnosis speed, but its anti-interference capability is limited. Voltage-based fault diagnosis methods are sensitive to changes in system parameters, and threshold selection is one of the key issues in ensuring effectiveness and improving robustness.

[0004] Based on the above analysis, existing fault diagnosis methods struggle to achieve a reasonable balance between robustness and speed. Furthermore, the complexity and adaptability of fault diagnosis methods require further investigation. Summary of the Invention

[0005] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention proposes a robust open-circuit fault diagnosis method for T-type three-level converters. The proposed fault diagnosis method aims to achieve a reasonable balance between robustness, safety and speed.

[0006] To achieve the above objectives, the present invention provides a robust method for diagnosing open-circuit faults in a T-type three-level converter, comprising:

[0007] S1: Calculate the desired output phase voltage u of the T-type three-level converter under different modulation strategies and modulation modes. Xn_ref ;

[0008] S2: Calculate the actual output phase voltage u of the T-type three-level converter. Xn_real ;

[0009] S3: Determined by the actual output phase voltage u Xn_real and the desired output phase voltage u Xn_ref Calculate the output phase voltage vector residual of the T-type three-level converter;

[0010] S4: The magnitude and phase of the output phase voltage vector residual of the T-type three-level converter are used as open-circuit fault characteristics;

[0011] S5: Considering the impact of system parameter errors, dead time, and delay time on the phase voltage vector residual, design an adaptive residual vector magnitude threshold based on uncertainty propagation theory;

[0012] S6: Perform group-level open-circuit fault diagnosis based on the magnitude, phase, and adaptive residual vector magnitude threshold of the output phase voltage vector residual. Simultaneously with the completion of group-level open-circuit fault diagnosis, switch the three-level modulation mode of the T-type three-level converter to the two-level modulation mode. Perform device-level open-circuit fault diagnosis based on the group-level open-circuit fault diagnosis results, the magnitude of the phase voltage vector residual after the modulation mode switch, and the adaptive residual vector magnitude threshold.

[0013] In some alternative implementations, in step S1:

[0014] The desired output phase voltage u of the T-type three-level converter under sinusoidal pulse width modulation (SPWM) strategy and three-level 3L modulation mode Xn_ref for: Where, m X For each phase, the normalized modulation ratio is X = phases A, B, and C, u dc1 C is the DC-side capacitor of the T-type three-level converter. high The voltage value, u dc2 C is the DC-side capacitor of the T-type three-level converter. low The voltage value;

[0015] The desired output phase voltage u of the T-type three-level converter under sinusoidal pulse width modulation (SPWM) strategy and two-level 2L modulation mode Xn_ref for: Where, m X For each phase, the normalized modulation ratio is X = phases A, B, and C, u dc This refers to the DC-side voltage value of a T-type three-level converter.

[0016] The desired output phase voltage u of the T-type three-level converter under space vector pulse width modulation (SVPWM) strategy and three-level 3L modulation mode Xn_ref for: Among them, V nor_α and V nor_β These are the desired output phase voltage vectors V nor Voltage components on the α and β axes, Where T1, T2, and T3 are the duty cycles of the corresponding basic voltage vectors in the 3L modulation mode, u dc1 C is the DC-side capacitor of the T-type three-level converter. high The voltage value;

[0017] The desired output phase voltage u of the T-type three-level converter under space vector pulse width modulation (SVPWM) strategy and three-level 2L modulation mode Xn_ref for: V nor_α and V nor_β These are the desired output phase voltage vectors V nor Voltage components on the α and β axes, Among them, T cm1 T cm2 T cm3 u represents the duty cycle of the corresponding basic voltage vector in 2L modulation mode. dc This represents the DC-side voltage value of a T-type three-level converter.

[0018] In some alternative implementations, in step S2, u Xn_real =R X i X +L X di X / dt+u xn Determine the actual output phase voltage u Xn_real , where R X L is the line resistance. X For line inductance, u xn Let i be the load phase voltage. X This represents the phase current.

[0019] In some alternative implementations, in step S3, Δu Xn =u Xn_ref -uXn_real Determine the output phase voltage vector residual of the T-type three-level converter.

[0020] In some alternative implementations, in step S4, by The vector expression of the output phase voltage vector residual is obtained, where Δu α and Δu β The output phase voltage vector residual Δu Xn Voltage components along the α and β axes, where θ is the phase voltage vector residual V. r The vector angle, the magnitude of the output phase voltage vector residual is

[0021] In some optional implementations, in step S5, the adaptive residual vector magnitude threshold is: Among them, the variable ΔV α and ΔV β for: Among them, the inherent phase voltage vector residual Δv Xn For: Δv Xn =ΔL X ·di X / dt+Δu Xn_dt ΔL X The inherent phase voltage vector residual component Δu is the error in line inductance parameters caused by dead time, rise and fall delay times. Xn_dt for: t dead Represents dead time, t on and t off T represents the rise time and the fall time of the switching transistor, respectively. s Indicates the switching cycle.

[0022] In some alternative implementations, in step S6, the fault flag is:

[0023] If the fault flag F_flag is 1, then group-level fault diagnosis begins. The group-level open-circuit fault diagnosis rules are as follows:

[0024]

[0025] Group-level fault flag F X1 / 4 A value of 1 indicates that the fault occurred at switch S. X1 and S X4 In the middle, F X2 / 3 A value of 1 indicates that the fault occurred at switch S. X2 and S X3 middle.

[0026] In some alternative implementations, in step S6:

[0027] Once the group-level open-circuit fault is resolved, a modulation mode switching command is issued to switch the modulation mode from 3L mode to 2L mode.

[0028] Based on the group-level open-circuit fault diagnosis results, the amplitude of the phase voltage vector residual after modulation mode switching, and the adaptive residual vector amplitude threshold, device-level open-circuit fault diagnosis is performed. The device-level open-circuit fault diagnosis rules are as follows:

[0029]

[0030] Among them, T ac This is the fundamental period of the output voltage / current.

[0031] In some alternative implementations, in step S1:

[0032] The formula for calculating the desired output phase voltage switches in real time depending on the modulation strategy and modulation mode. When the system is in SVPWM modulation strategy and 3L modulation mode, the output phase voltage is: When the system detects a group-level open-circuit fault and switches to the 2L modulation mode of the SVPWM modulation strategy, the output phase voltage is:

[0033] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0034] (1) A hierarchical open-circuit fault diagnosis method is proposed, which includes two steps. First, the group-level fault can be located based on the voltage residual vector magnitude and angle. Next, the three-level modulation mode of all phases is switched to two-level mode to further identify the faulty switch from the fault group.

[0035] (2) Compared with traditional voltage model-based diagnostic methods, using voltage residual vector magnitude as an adaptive threshold is faster than methods based on multiple threshold variables, thus improving the robustness of the diagnostic method.

[0036] (3) A fault location method based on modulation mode switching is proposed. Compared with the specific switching signal injection method, it effectively avoids the risk of system overcurrent and improves the reliability of diagnosis.

[0037] (4) The proposed method achieves a reasonable balance between robustness and speed. At the same time, the complexity and adaptability of the method are also taken into account. Attached Figure Description

[0038] Figure 1 This is a flowchart illustrating a robust open-circuit fault diagnosis method for a T-type three-level converter provided in an embodiment of the present invention.

[0039] Figure 2 This is a schematic diagram of the robustness test results under varying load conditions provided by an embodiment of the present invention, wherein (a) represents the per-unit value of the desired output phase voltage of the converter, (b) represents the per-unit value of the actual output phase voltage, and (c) represents the actual output phase current i. X The per-unit value, (d) represents the magnitude of the output phase voltage residual vector |V r | _pu The per-unit value V of the adaptive residual vector magnitude threshold th_pu ;

[0040] Figure 3 This is an embodiment of the present invention provided in S A1 A schematic diagram of the fault diagnosis experiment results under open-circuit fault conditions, where (a) represents the per-unit value of the expected output phase voltage of the converter, (b) represents the per-unit value of the actual output phase voltage, and (c) represents S. A1 The per-unit values ​​of the phase currents before and after an open-circuit fault, where (d) represents the magnitude of the output phase voltage residual vector |V r | _pu The per-unit value V of the adaptive residual vector magnitude threshold th_pu (e) represents the fault status flag F_flag, (f) represents the phase of the output phase voltage residual vector, (g) represents the fault phase status flag F_phase, and (h) represents the group-level fault flag F_flag. X1 / 4 and F X2 / 3 , (i) represents the device-level fault identifier F_switch. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0042] The adaptive threshold proposed in this invention improves robustness under various operating conditions. Furthermore, using the voltage residual vector amplitude as the adaptive threshold is theoretically faster than existing methods based on multiple threshold variables, because fault diagnosis methods based on multiple threshold variables must satisfy all thresholds simultaneously. Although the fault location method based on modulation mode switching has a lower diagnosis speed compared to specific switch signal injection methods, it can effectively avoid the risk of system overcurrent.

[0043] like Figure 1 As shown, the present invention provides a robust method for diagnosing open-circuit faults in a T-type three-level converter, comprising the following steps:

[0044] S1: Calculate the desired output phase voltage u of the T-type three-level converter under different modulation strategies and modulation modes. Xn_ref ;

[0045] In this embodiment of the invention, the desired output phase voltage u of the T-type three-level converter under sinusoidal pulse width modulation (SPWM) strategy and three-level 3L modulation mode is... Xn_ref The calculation formula is:

[0046]

[0047] Where, m X For each phase, the normalized modulation ratio is X = phases A, B, and C, u dc1 C is the DC-side capacitor of the T-type three-level converter. high The voltage value, u dc2 C is the DC-side capacitor of the T-type three-level converter. low The voltage value.

[0048] The desired output phase voltage u of the T-type three-level converter under sinusoidal pulse width modulation (SPWM) strategy and two-level 2L modulation mode Xn_ref The calculation formula is:

[0049]

[0050] Where, m X For each phase, the normalized modulation ratio is X = phases A, B, and C, u dc This represents the DC-side voltage value of a T-type three-level converter.

[0051] The desired output phase voltage u of the T-type three-level converter under space vector pulse width modulation (SVPWM) strategy and three-level 3L modulation mode Xn_ref The calculation formula is:

[0052]

[0053] Among them, V nor_α and V nor_β These are the desired output phase voltage vectors V nor Voltage components along the α and β axes, V nor The calculation formula is:

[0054]

[0055] Where T1, T2, and T3 are the duty cycles of the corresponding basic voltage vectors in the 3L modulation mode, u dc1 C is the DC-side capacitor of the T-type three-level converter. high The voltage value.

[0056] The desired output phase voltage u of the T-type three-level converter under space vector pulse width modulation (SVPWM) strategy and three-level 2L modulation mode Xn_ref The calculation formula is:

[0057]

[0058]

[0059] Among them, T cm1 T cm2 T cm3 u represents the duty cycle of the corresponding basic voltage vector in 2L modulation mode. dc This represents the DC-side voltage value of a T-type three-level converter.

[0060] S2: Calculate the actual output phase voltage u of the T-type three-level converter. Xn_real ;

[0061] Actual output phase voltage u Xn_real The calculation formula is:

[0062] u Xn_real =R X i X +L X di X / dt+u xn

[0063] Among them, R X L is the line resistance. X For line inductance, u xn Let i be the load phase voltage. X This represents the phase current.

[0064] S3: Calculate the output phase voltage vector residual of the T-type three-level converter;

[0065] The formula for calculating the output phase voltage vector residual of a T-type three-level converter is:

[0066] Δu Xn =u Xn_ref -u Xn_real

[0067] S4: Calculate the magnitude and phase of the output phase voltage vector residual of the T-type three-level converter;

[0068] The formulas for calculating the magnitude and phase of the output phase voltage vector residual of a T-type three-level converter are as follows:

[0069]

[0070] Where, Δu α and Δu β The output phase voltage vector residual ΔuXn Voltage components along the α and β axes, where θ is the phase voltage vector residual V. r The vector angle, the magnitude of the output phase voltage vector residual is

[0071] S5: Calculate the adaptive residual vector magnitude threshold;

[0072] The formula for calculating the adaptive residual vector magnitude threshold is:

[0073]

[0074] Among them, the variable ΔV α and ΔV β The calculation formula is:

[0075]

[0076] Among them, the inherent phase voltage vector residual Δv Xn The calculation formula is:

[0077] Δv Xn =ΔL X ·di X / dt+Δu Xn_dt

[0078] Where, ΔL X The inherent phase voltage vector residual component Δu is the error in line inductance parameters caused by dead time, rise and fall delay times. Xn_dt The calculation formula is:

[0079]

[0080] Among them, t dead Represents dead time, t on and t off T represents the rise time and the fall time of the switching transistor, respectively. s Indicates the switching cycle.

[0081] S6: Based on the phase voltage vector residual V r The amplitude, phase and adaptive residual vector amplitude threshold are used to perform group-level open circuit fault diagnosis. While completing the group-level open circuit fault diagnosis, the three-level modulation mode of the T-type three-level converter is switched to the two-level modulation mode.

[0082] The formula for calculating fault indicators is:

[0083]

[0084] If the fault flag F_flag is 1, then group-level fault diagnosis begins. The group-level open-circuit fault diagnosis rules are as follows:

[0085] Table 1 Group-level Open Circuit Fault Diagnosis Rules

[0086]

[0087] Group-level fault flag F X1 / 4 A value of 1 indicates that the fault occurred at switch S. X1 and S X4 In the middle, F X2 / 3 A value of 1 indicates that the fault occurred at switch S. X2 and S X3 middle.

[0088] S7: Perform device-level open-circuit fault diagnosis based on the group-level open-circuit fault diagnosis results, the amplitude of the phase voltage vector residual after modulation mode switching, and the adaptive residual vector amplitude threshold.

[0089] Table 2. Device-level Open Circuit Fault Diagnosis Rules

[0090]

[0091] Among them, T ac The fundamental period of the output voltage or current.

[0092] To describe this embodiment more clearly, Figure 2 and Figure 3 The experimental results for this embodiment are presented, and the main parameters used in the experimental results are shown in Table 3.

[0093] Table 3

[0094]

[0095] Figure 2 and Figure 3 Indices of parameters _pu This represents the per-unit value. The reference values ​​for voltage and current are 2V and 2A, respectively.

[0096] Figure 2 It is the current command i under closed-loop current control. d_ref_pu Robustness test results from 0.1 to 0.9. Figure 2 As can be seen from (a) and (b), under light load conditions, u Xn_ref_pu and u Xn_pu There is a small inherent voltage residual between them. However, |V r | _pu Always below the adaptive threshold V th_pu [See Figure 2 [middle (d)]. At the load change time t = 20 ms, i X_pu and |V r | _pu They all underwent sudden changes [see] Figure 2 [In (a) and (b)]. Due to V th_pu It is a function of the current slope, therefore V th_pu The corresponding increase will be made, such as Figure 2 As shown in (d), it can be seen that |V r | _pu Under transient and steady heavy load conditions (i d_ref_pu =0.9) is always below the threshold V th_pu . Figure 2 (c) represents the actual output phase current i X The per-unit value. From Figure 2 (c) shows the changes in the actual output phase current before and after the change in the current command.

[0097] Figure 3 The method was evaluated under light load conditions (i d_ref_pu =0.1) and S A1 The validity of the OC fault occurring at t=20ms. From... Figure 3 As can be seen from (a) and (b), the distorted phase voltage u An_pu Significant deviation from reference value u An_ref_pu At the same time, the corresponding phase current i A_pu Significantly affected. Threshold V th_pu Under healthy conditions, above |V r | _pu However, under fault conditions, it is lower than |V r | _pu [See Figure 3 [middle (d)], F_flag is immediately set to 1, such as Figure 3 As shown in (e), group-level fault diagnosis is enabled at this time. Figure 3 As shown in (f), the vector angle θ is approximately zero (between -π / 6 and π / 6), which means the fault phase is A (F_phase = 1) and F X1 / 4 =1(S A1 or S A4 (An over-the-air (OC) fault occurs at time t = 23ms, according to Table 1. Once the group-level fault diagnosis is completed, the 3L mode switches to the 2L mode, and the formula for calculating the expected voltage also changes from the formula used in the 3L mode to the formula used in the 2L mode.) Figure 3 As shown in (e) and (h), in the equipment-level fault diagnosis region [0.1T] ac ,T ac Within [time period], the fault state did not disappear (F_flag = 1). F_switch is set to 1 at t = 25ms, as shown below. Figure 3 As shown in (i). Figure 3 (c) shows S A1 Changes in phase current before and after an open-circuit fault. Figure 3(g) The fault status indicator F_phase is displayed in S A1 Changes before and after the open circuit fault.

[0098] The above results demonstrate that the robust open-circuit fault diagnosis method for T-type three-level converters proposed in this invention can accurately locate open-circuit faults, and the proposed method using adaptive thresholds exhibits strong robustness under various load transients.

[0099] The above embodiments are described for specific open-circuit faults; the analysis results for other open-circuit faults are the same as those in the embodiments. The above description of the embodiments is intended to enable those skilled in the art to understand and apply the present invention. Those skilled in the art will readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made to the present invention by those skilled in the art based on the disclosure thereof should be within the scope of protection of the present invention.

Claims

1. A robust method for diagnosing open-circuit faults in a T-type three-level converter, characterized in that, include: S1: Calculate the desired output phase voltage u of the T-type three-level converter under different modulation strategies and modulation modes. Xn_ref ; S2: Calculate the actual output phase voltage u of the T-type three-level converter. Xn_real ; S3: Determined by the actual output phase voltage u Xn_real and the desired output phase voltage u Xn_ref Calculate the output phase voltage vector residual of the T-type three-level converter; S4: The magnitude and phase of the output phase voltage vector residual of the T-type three-level converter are used as open-circuit fault characteristics; S5: Considering the impact of system parameter errors, dead time, and delay time on the phase voltage vector residual, design an adaptive residual vector magnitude threshold based on uncertainty propagation theory; S6: Perform group-level open-circuit fault diagnosis based on the magnitude, phase, and adaptive residual vector magnitude threshold of the output phase voltage vector residual. Simultaneously with the completion of group-level open-circuit fault diagnosis, switch the three-level modulation mode of the T-type three-level converter to the two-level modulation mode. Perform device-level open-circuit fault diagnosis based on the group-level open-circuit fault diagnosis results, the magnitude of the phase voltage vector residual after the modulation mode switch, and the adaptive residual vector magnitude threshold.

2. The robust T-type three-level converter open-circuit fault diagnosis method according to claim 1, characterized in that, In step S1: The desired output phase voltage u of the T-type three-level converter under sinusoidal pulse width modulation (SPWM) strategy and three-level 3L modulation mode Xn_ref for: Where, m X The normalized modulation ratio for each phase, X = phases A, B, and C, u dc1 C is the DC-side capacitor of the T-type three-level converter. high The voltage value, u dc2 C is the DC-side capacitor of the T-type three-level converter. low The voltage value; The desired output phase voltage u of the T-type three-level converter under sinusoidal pulse width modulation (SPWM) strategy and two-level 2L modulation mode Xn_ref for: Where, m X The normalized modulation ratio for each phase, X = phases A, B, and C, u dc This refers to the DC-side voltage value of a T-type three-level converter. The desired output phase voltage u of the T-type three-level converter under space vector pulse width modulation (SVPWM) strategy and three-level 3L modulation mode Xn_ref for: Among them, V nor_α and V nor_β These are the desired output phase voltage vectors V nor Voltage components on the α and β axes, Where T1, T2, and T3 are the duty cycles of the corresponding basic voltage vectors in the 3L modulation mode, u dc1 C is the DC-side capacitor of the T-type three-level converter. high The voltage value; The desired output phase voltage u of the T-type three-level converter under space vector pulse width modulation (SVPWM) strategy and three-level 2L modulation mode Xn_ref for: V nor_α and V nor_β These are the desired output phase voltage vectors V nor Voltage components on the α and β axes, Among them, T cm1 T cm2 T cm3 u represents the duty cycle of the corresponding basic voltage vector in 2L modulation mode. dc This represents the DC-side voltage value of a T-type three-level converter.

3. The robust T-type three-level converter open-circuit fault diagnosis method according to claim 2, characterized in that, In step S2, by u Xn_real =R X i X +L X di X / dt+u xn Determine the actual output phase voltage u Xn_real , where R X L is the line resistance. X For line inductance, u xn i is the load phase voltage. X This represents the phase current.

4. The robust open-circuit fault diagnosis method for a T-type three-level converter according to claim 3, characterized in that, In step S3, by Δu Xn =u Xn_ref -u Xn_real Determine the output phase voltage vector residual of the T-type three-level converter.

5. The robust T-type three-level converter open-circuit fault diagnosis method according to claim 4, characterized in that, In step S4, by The vector expression of the output phase voltage vector residual is obtained, where Δu α and Δu β The output phase voltage vector residual Δu Xn Voltage components along the α and β axes, where θ is the phase voltage vector residual V. r The vector angle, the magnitude of the output phase voltage vector residual is 6. The robust T-type three-level converter open-circuit fault diagnosis method according to claim 5, characterized in that, In step S5, the adaptive residual vector magnitude threshold is: Among them, the variable ΔV α and ΔV β for: Among them, the inherent phase voltage vector residual Δv Xn For: Δv Xn =ΔL X ·di X / dt+Δu Xn_dt ΔL X The inherent phase voltage vector residual component Δu is the error in line inductance parameters caused by dead time, rise and fall delay times. Xn_dt for: t dead Represents dead time, t on and t off T represents the rise time and the fall time of the switching transistor, respectively. s Indicates the switching cycle.

7. The robust T-type three-level converter open-circuit fault diagnosis method according to claim 6, characterized in that, In step S6, the fault flag is: If the fault flag F_flag is 1, then group-level fault diagnosis begins. The group-level open-circuit fault diagnosis rules are as follows: Group-level fault flag F X1 / 4 A value of 1 indicates that the fault occurred at switch S. X1 and S X4 In the middle, F X2 / 3 A value of 1 indicates that the fault occurred at switch S. X2 and S X3 middle.

8. The robust T-type three-level converter open-circuit fault diagnosis method according to claim 7, characterized in that, In step S6: After the group-level open circuit fault is completed, a modulation mode switching command is issued to switch the modulation mode from 3L mode to 2L mode; Based on the group-level open-circuit fault diagnosis results, the amplitude of the phase voltage vector residual after modulation mode switching, and the adaptive residual vector amplitude threshold, device-level open-circuit fault diagnosis is performed. The device-level open-circuit fault diagnosis rules are as follows: Among them, T ac This is the fundamental period of the output voltage / current.

9. The robust T-type three-level converter open-circuit fault diagnosis method according to claim 2, characterized in that, In step S1: The formula for calculating the desired output phase voltage switches in real time depending on the modulation strategy and modulation mode. When the system is in SVPWM modulation strategy and 3L modulation mode, the output phase voltage is: When the system detects a group-level open-circuit fault and switches to the 2L modulation mode of the SVPWM modulation strategy, the output phase voltage is:

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