Fault Tolerant Control Method for T-Type Inverter Considering Reduction of Current Ripple and Switching Loss

Abstract

The present invention belongs to the technical field of inverter fault-tolerant control, and provides a T-type inverter fault-tolerant control method that takes into account both current ripple and reduction of switching losses. The technical solution is as follows: The DPWM strategy is adopted to inject a zero-sequence component into the initial modulation wave, and one of the three phases is clamped at any moment; analyze the current paths in different switching tube fault cases of the inverter to obtain non-existent switching states, and divide a fundamental wave period into a healthy half-cycle and a fault half-cycle according to the direction of the fault-phase current; in the fault half-cycle, select the clamping state or reconstruct the carrier to avoid the use of non-existent switching states to achieve fault-tolerant control; obtain the relationship between the current ripple and the acting vector, calculate the current ripple according to the current ripple slope and the vector acting time, and select the clamping state with the minimum current ripple; establish a neutral point current model, and when the neutral point voltage exceeds the defined threshold, select the clamping state according to the magnitude of the neutral point current to achieve neutral point voltage balance control. In summary, the present invention takes into account both current ripple and reduction of switching losses while realizing fault-tolerant control, and has high engineering application value.

Description

Technical Field

[0001] The present invention belongs to the technical field of inverter fault-tolerant control, and particularly relates to a T-type inverter fault-tolerant control method that takes into account both current ripple and reduction of switching losses. Background Art

[0002] The statements in this part only provide background technical information related to the present invention, and do not necessarily constitute prior art.

[0003] As an interface between a generator and a power grid or a load, an inverter plays a crucial role in meeting the requirements of high efficiency, high reliability, and high power density of renewable energy power generation systems. Among them, the T-type three-level inverter has been widely used due to its advantages such as simple structure, small loss, and few harmonic components in the output waveform.

[0004] On the one hand, efficiency is an important indicator to measure the performance of a system, and the efficient operation of an inverter is an effective means to save energy. Among them, the modulation strategy is an important factor affecting the working efficiency of an inverter. At present, the Discontinuous Pulse Width Modulation (DPWM) strategy is an important means to reduce the switching losses of an inverter system. Compared with the traditional Sinusoidal PWM (SPWM) and Space Vector Modulation (SVPWM), DPWM can clamp the voltage of a certain phase for a period of time within a cycle, so that the switching tubes of that phase do not act when the voltage is clamped, thereby reducing the switching losses and improving the efficiency of the inverter system.

[0005] On the other hand, due to the increase in the number of switching tubes in the T-type three-level inverter and its complex working environment, the probability of switching tube failures increases significantly. When a switching tube fails, the output current of the system is severely distorted, which may even endanger the system and personal safety, and seriously reduce the reliability of the inverter system. The fault types of switching tubes are divided into two cases: short circuit and open circuit. When a short-circuit fault occurs, the instantaneous large current generated in the power loop usually threatens the entire converter system. Therefore, an auxiliary circuit is generally used for isolation to convert the short-circuit fault into an open-circuit fault. When an open-circuit fault occurs, it will not instantaneously cause devastating damage to the power devices and even the entire system. The corresponding fault can be eliminated through a control method to reduce the losses caused by fault shutdown. Therefore, fault-tolerant control under inverter switching tube failures is particularly important.

[0006] However, both the DPWM strategy and fault-tolerant control result in a reduction in the number of available space voltage vectors, causing large ripples in the output current. Therefore, it is necessary to simultaneously consider the suppression of current ripple to reduce the volume of the filter and thereby improve the power density of the system. Summary of the Invention

[0007] To solve at least one of the technical problems existing in the above background art, the present invention provides a fault-tolerant control method for a T-type inverter that takes into account both current ripple and reduction of switching losses. It adopts the DPWM strategy to reduce switching losses, realizes fault-tolerant control when a switching tube fails by selecting a clamping state or modifying the carrier, and selects the optimal clamping state according to the calculated current ripple to reduce the output current ripple, while achieving the balance of the midpoint voltage on the DC side.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] The first aspect of the present invention provides a fault-tolerant control method for a T-type inverter that takes into account both current ripple and reduction of switching losses, including the following steps:

[0010] Inject a zero-sequence component into the three-phase initial modulation wave, and clamp one of the three phases at any moment to obtain all candidate clamping states;

[0011] Analyze the current path when a switching tube of the T-type three-level inverter fails to obtain the switching states in which the corresponding faults do not exist, and divide the fundamental wave period into a healthy half-cycle and a fault half-cycle according to the direction of the fault-phase current;

[0012] In the fault half-cycle, according to the fault type of the switching tube, select the clamping state or reconstruct the carrier to avoid using the non-existent switching states;

[0013] After fault-tolerant control, reorder all vectors in the space vector diagram according to the magnitudes of the three-phase phase voltages, calculate the current ripple slope and the vector action time based on the sorted vectors, calculate the current ripple value of each candidate clamping state according to the current ripple slope and the vector action time, and obtain the clamping state with the minimum current ripple value;

[0014] Establish a midpoint current model for the healthy half-cycle and the fault half-cycle respectively. When the midpoint voltage is within the set voltage threshold range, select the switching state with the minimum current ripple value. When the midpoint voltage exceeds the set voltage threshold range, select the switching state according to the midpoint current model.

[0015] As an implementation, selecting the clamping state or reconstructing the carrier according to the fault type of the switching tube to avoid using the non-existent switching states includes: if an external switching tube fails, reduce the modulation index to the set modulation index threshold and select the clamping state; if an internal switching tube fails, reconstruct the carrier to avoid using the non-existent switching states.

[0016] As an implementation, when an external switching tube fails, reduce the modulation index to below 0.58, and at the same time discard some candidate clamping states to avoid using the non-existent [P] state.

[0017] As an implementation, when the internal switching tube fails, the carrier is reconstructed, including still using two carriers with an amplitude of 1 in the healthy half-cycle, while changing to using one carrier with an amplitude of 2 in the faulty half-cycle, avoiding the use of the non-existent [O] state.

[0018] As an implementation, the expression for injecting the zero-sequence component is:

[0019]

[0020]

[0021]

[0022] where is the initial modulation wave of the x-phase, m z is the injected zero-sequence component, m x is the final modulation wave after injecting the zero-sequence component. In one switching cycle, the x-phase is clamped to [P], [O], [N], which are represented as X_P, X_O, X_N respectively.

[0023] As an implementation, the calculation formula for the current ripple slope is:

[0024]

[0025]

[0026]

[0027]

[0028] where v x represents the normalized phase voltage, l, m, n respectively represent the initial constant terms of the three-phase current ripple slopes, b x represents the final constant term of the current ripple slope, k x represents the corresponding current ripple slope of each vector in the vector sequence, L is the inductance value, V dc is the DC-side voltage.

[0029] As an implementation, when the vector sequence is [P, N, N]-[P, O, N]-[P, O, O]-[P, O, N]-[P, N, N], the action time is calculated as:

[0030]

[0031] where T s represents the switching cycle, and t1, t2, t3 respectively represent half of the action time of each vector in one switching cycle.

[0032] As an implementation manner, the midpoint current model of the healthy half cycle is:

[0033] i NPh =-|m a |·i a -|m b |·i b -|m c |·i c ,

[0034] The midpoint current model of the faulty half cycle is:

[0035] i NPf =-i a -|m b |·i b -|m c |·i c ,

[0036] wherein, i x is the phase current of phase x.

[0037] As an implementation manner, a midpoint voltage threshold V TH is set. When the midpoint voltage ΔV PN is within an acceptable range, that is, -V TH <ΔV PN <V TH , the clamping state with the optimal current ripple is still selected; when ΔV PN >V TH , the clamping state corresponding to the maximum midpoint current is selected; when ΔV PN <-V TH , the clamping state corresponding to the minimum midpoint current is selected.

[0038] The second aspect of the present invention provides a T-type inverter fault-tolerant control system that takes into account current ripple and reduction of switching losses, including:

[0039] A zero-sequence injection module, configured to inject a zero-sequence component into the three-phase initial modulation wave, and clamp one of the three phases at any moment to obtain all candidate clamping states;

[0040] A current path analysis module, configured to analyze the current path when a switching tube of a T-type three-level inverter fails, obtain the switching states in which the corresponding faults do not exist, and divide the fundamental wave period into a healthy half cycle and a faulty half cycle according to the direction of the fault phase current;

[0041] A fault-tolerant control module, configured to, in the faulty half cycle, select the clamping state or reconstruct the carrier according to the fault type of the switching tube to avoid using the non-existent switching states;

[0042] The current ripple suppression module is configured to reorder all vectors in the space vector diagram according to the magnitudes of the three-phase phase voltages after fault-tolerant control, calculate the current ripple slope and the vector action time based on the sorted vectors, calculate the current ripple value of each candidate clamping state according to the current ripple slope and the vector action time, and obtain the clamping state with the minimum current ripple value.

[0043] The midpoint voltage control module is configured to establish a midpoint current model for the healthy half-cycle and the faulty half-cycle respectively. When the midpoint voltage is within the set voltage threshold range, select the switching state with the minimum current ripple value. When the midpoint voltage exceeds the set voltage threshold range, select the switching state according to the midpoint current model.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] 1. While taking into account the reduction of current ripple and switching losses, the present invention realizes the fault-tolerant control of the T-type three-level inverter. When a switch tube fails, the distortion of the three-phase output current is eliminated, the total harmonic distortion rate of the current is reduced, the switching times of the switch tubes are reduced, and the balance of the DC-side midpoint voltage is maintained.

[0046] 2. Through zero-sequence component injection, the present invention clamps one of the three phases at any moment, reduces the switching times of the switch tubes and the switching losses, and improves the inverter efficiency.

[0047] 3. By selecting the clamping state or performing carrier reconstruction under different switch tube fault conditions, the present invention avoids the use of non-existent switching states, realizes the fault-tolerant control of the inverter, and thus improves the reliability of the system.

[0048] 4. By obtaining the relationship between the current ripple and the acting vector, calculating the current ripple value according to the current ripple slope and the vector action time, and selecting the clamping state with the minimum current ripple to suppress the current ripple, the present invention reduces the volume of the filter, thereby improving the system power density.

[0049] 5. By establishing a midpoint current model and selecting the clamping state according to the midpoint current when the midpoint voltage exceeds the acceptable range, the present invention realizes the balance control of the DC-side midpoint voltage and ensures the safe and stable operation of the system.

[0050] Some of the advantages of the additional aspects of the present invention will be given in the following description, some will become obvious from the following description, or will be understood through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The specification drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0052] Figure 1 is the T-type three-level inverter system provided by the embodiment of the present invention;

[0053] Figure 2 is the three-level space vector diagram provided by the embodiment of the present invention;

[0054] Figure 3 is the current path diagram of phase A before and after the switch tube fails provided by the embodiment of the present invention;

[0055] Figure 4 is the S provided by the embodiment of the present invention a2 carrier reconstruction diagram during fault;

[0056] Figure 5 is the system control block diagram provided by the embodiment of the present invention;

[0057] Figure 6 is the S provided by the embodiment of the present invention a1 output waveform diagram of different methods during fault;

[0058] Figure 7 is the S provided by the embodiment of the present invention a2 output waveform diagram of different methods during fault. Detailed implementation mode

[0059] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0060] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0061] It should be noted that the terms used herein are only for describing specific implementation manners and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0062] Embodiment 1

[0063] This embodiment provides a fault-tolerant control method for a T-type inverter that takes into account both current ripple and reduced switching losses, including the following steps:

[0064] Step 1: Inject a zero-sequence component into the three-phase initial modulation wave, clamp one of the three phases at any moment, and obtain the candidate clamping states for each region;

[0065] Step 2: Analyze the current path in the case of switch tube faults in the T-type three-level inverter, obtain the switch states that do not exist during switch tube faults, and divide a fundamental wave period into a healthy half-cycle and a fault half-cycle according to the direction of the fault phase current;

[0066] Step 3: In the fault half-cycle, select the clamping state according to different switch tube fault conditions or reconstruct the carrier to avoid using non-existent switch states and achieve fault-tolerant control;

[0067] Step 4: Reorder all vectors according to the magnitudes of the three-phase phase voltages, calculate the current ripple slope and the vector action time, and then calculate the current ripple. Select the clamping state with the smallest current ripple to suppress the current ripple;

[0068] Step 5: Establish the midpoint current models for the healthy half-cycle and the fault half-cycle respectively. When the midpoint voltage exceeds the acceptable range, select the clamping state according to the magnitude of the midpoint current to achieve midpoint voltage balance control.

[0069] While taking into account the reduction of current ripple and switching losses, the present invention realizes the fault-tolerant control of the T-type three-level inverter. When a switch tube fails, the distortion of the three-phase output current is eliminated, the total harmonic distortion rate of the current is reduced, the switching times of the switch tubes are reduced, and the balance of the DC-side midpoint voltage is maintained.

[0070] As Figure 1 shown is the topological structure diagram of the T-type three-level inverter system, including the A-phase, B-phase, and C-phase bridge arms. Each phase bridge arm includes four power switch tubes, which are composed of two outer switch tubes (S x1 , S x4 ) and two inner switch tubes (S x2 , S x3 ). The DC side includes two capacitors C1 and C2 connected in series, and a neutral point is formed in the middle of the two capacitors.

[0071] Specifically, each phase bridge arm of the T-type three-level inverter system has three working states [P], [O], and [N]. Taking the neutral point of the two capacitors on the DC side as the reference point, when in the [P] state, the output voltage of the bridge arm is equal to the voltage value of the upper capacitor; when in the [O] state, the output voltage of the bridge arm is equal to zero; when in the [N] state, the output voltage of the bridge arm is equal to the negative value of the voltage of the lower capacitor.

[0072] Table 1 shows the switch states, output voltages, and the working conditions of each power switch tube in each phase bridge arm of the T-type three-level inverter system.

[0073] Table 1 Switch States, Output Voltages, and Switching Tube Working Conditions of the T-Type Three-Level Inverter

[0074]

[0075] Different switching states constitute 27 vectors, including 3 zero vectors, 12 small vectors, 6 medium vectors and 6 large vectors. The space vector diagram is as shown in Figure 2 Figure []. The 6 large vectors divide the entire vector diagram into 6 large sectors, and each large sector is further divided into 4 sub-sectors: A, B, C, D.

[0076] Three-phase initial modulation wave is expressed as:

[0077]

[0078] where ωt is the phase angle of phase A and M is the modulation degree.

[0079] Sorting the initial modulation waves by magnitude gives:

[0080]

[0081] i max 、i mid 、i min are the corresponding phase currents respectively. Within one switching period, the x-phase can be clamped to [P], [O], [N], which are represented as X_P, X_O, X_N respectively. The injected zero-sequence component is expressed as:

[0082]

[0083] The final modulation wave m x is expressed as:

[0084]

[0085] By injecting the zero-sequence component, the present invention clamps one of the three phases at any moment, reduces the switching times of the switching tubes and lowers the switching loss, thereby improving the efficiency of the inverter.

[0086] All the clamping states are summarized in Table 2.

[0087] Table 2 All clamping states and distribution sub-sectors

[0088]

[0089] The switching tube faults of the T-type three-level inverter are divided into outer switching tube faults and inner switching tube faults. Taking the S a1 fault and the S a2 fault as examples for analysis, the current path is as shown in Figure 3 Figure [].

[0090] When i a >0, the [P] state of phase A cannot be generated during the S a1 fault, and the [O] state of phase A during the Sa2 It cannot be generated during a fault. At this time, if the reference vector is synthesized with a non-existent switch state, the output current will be distorted; when i a < 0, the current path is the same as in the normal situation. Therefore, according to the direction of the faulty-phase current, a fundamental wave period is divided into a faulty half-period (i a > 0) and a healthy half-period (i a < 0).

[0091] S a1 During a fault, first reduce the modulation index to below 0.58. At this time, the reference vector is always located in sub-sector A, and only some of the clamping states are selected for use. For example, when m a = m max , MAX_P1, MID_O, and MIN_O cannot be used, and only MAX_O and MIN_N3 can be selected to avoid the use of the [P] state of phase A to achieve fault-tolerant control. S a2 During a fault, there is no need to reduce the modulation index. Two carriers with an amplitude of 1 are still used in the healthy half-period, while in the faulty half-period, a carrier with an amplitude of 2 is used instead, as Figure 4 shown, avoiding the use of non-existent [O] states and achieving fault-tolerant control.

[0092] The present invention realizes the fault-tolerant control of the inverter by selecting the clamping state or performing carrier reconstruction under different switch tube fault conditions, avoiding the use of non-existent switch states, and thus improving the reliability of the system.

[0093] Since both the fault-tolerant control and the DPWM result in a reduction in the number of available space vectors, leading to an increase in the output current ripple, it is necessary to select the optimal switch state to reduce the current ripple.

[0094] According to Kirchhoff's voltage law, we have:

[0095]

[0096] where L is the inductance value, V xO is the phase voltage, V dc is the DC-side voltage, V nO is the voltage between point n and point O, expressed as:

[0097]

[0098] According to the symmetry of the space vector diagram, all vectors can be reconstructed as [V max , V mid , V min according to the magnitudes of the three-phase phase voltages, representing the maximum, intermediate, and minimum voltage values of each vector switch state respectively. For example, in the second sector, it always satisfies m b>m a >m c , then at this time [V max , V mid , V min corresponds to [V bO , V aO , V cO ; Define the normalized three-phase phase voltage as:

[0099]

[0100] The initial constant term of the three-phase current ripple slope is calculated as:

[0101]

[0102] Due to carrier reconstruction for fault-tolerant control during S a2 fault, some error sequence vectors will appear. For example, when the clamping state is MAX_P3 and m a = m min , the switch sequence after fault-tolerant control becomes [P, N, N]-[P, O, N]-[P, O, P]-[P, O, N]-[P, N, N], and [P, O, P] is the error sequence vector. Therefore, the ripple slope constant term needs to be modified and finally expressed as:

[0103]

[0104] The final current ripple slope is expressed as:

[0105]

[0106] The ripple current of the x-phase is expressed as:

[0107] i x_rip = i x - i x_ref (11)

[0108] where, i x_ref represents the ideal current. Due to the symmetry of the current ripple within a switching period, only half of the root mean square of the current ripple needs to be calculated, expressed as:

[0109]

[0110] where, T s represents the switching period, t1, t2, t3 respectively represent half of the action time of each vector in a switching period, and k1, k2, k3 respectively represent the corresponding current ripple slopes of each vector in the vector sequence.

[0111] The average value of the three-phase current ripple is expressed as:

[0112]

[0113] When the vector sequence is [P, N, N]-[P, O, N]-[P, O, O]-[P, O, N]-[P, N, N], the action time is calculated as follows:

[0114]

[0115] The present invention obtains the relationship between the current ripple and the action vector, calculates the current ripple value of each clamping state from the slope of the current ripple and the vector action time, and selects the clamping state with the minimum current ripple to achieve the suppression of the current ripple, reducing the volume of the filter, thereby improving the system power density.

[0116] At the same time, the balance of the DC-side midpoint voltage is the key to ensuring the safe and stable operation of the T-type three-level inverter. The midpoint voltage is affected by the midpoint current, and the midpoint current in a healthy half-cycle can be expressed as:

[0117] i NPh =(1 - |m a |)·i a +(1 - |m b |)·i b +(1 - |m c |)·i c (15)

[0118] Since the balanced three-phase currents have the following relationship:

[0119] i a +i b +i c = 0 (16)

[0120] Then equation (15) can be rewritten as:

[0121] i NPh =-|m a |·i a -|m b |·i b -|m c |·i c (17)

[0122] In the faulty half-cycle during an S a2 fault, since there is no [O] state in phase A after fault-tolerant control and it has no influence on the midpoint current, the midpoint current model at this time is expressed as:

[0123] i NPf =-i a -|m b |·i b -|m c|·i c (18)

[0124] Set the midpoint voltage threshold V TH , when the midpoint voltage ΔV PN is within the acceptable range, that is, -V TH < ΔV PN < V TH , still select the clamping state with the optimal current ripple; when ΔV PN > V TH , select the clamping state corresponding to the maximum midpoint current; when ΔV PN < -V TH , select the clamping state corresponding to the minimum midpoint current. Through this method, the balanced control of the DC midpoint voltage is achieved.

[0125] The present invention realizes the balanced control of the DC-side midpoint voltage by establishing a midpoint current model and selecting the clamping state according to the midpoint current when the midpoint voltage exceeds the acceptable range, ensuring the safe and stable operation of the system.

[0126] The above method realizes the fault-tolerant control of a T-type three-level inverter that takes into account both current ripple and reduction of switching losses. Figure 5 The control block diagram of the system is shown as follows.

[0127] Figure 6 and Figure 7 are respectively the output waveform diagrams of the T-type three-level inverter system when S a1 and S a2 fail, including three-phase output current, phase voltage of phase A, line voltage between phase A and phase B, and voltages across two DC-side capacitors. It can be seen that the fault-tolerant control method proposed by the present invention can eliminate the three-phase current distortion caused by the fault, the phase voltage is clamped for a period of time, the midpoint voltage remains balanced, and the THDi is significantly reduced compared with the traditional fault-tolerant control method. It is proved that the method proposed by the present invention realizes fault-tolerant control, reduction of current ripple, reduction of switching losses, and balance of midpoint voltage at the same time.

[0128] Embodiment 2

[0129] This embodiment provides a fault-tolerant control system for a T-type inverter that takes into account both current ripple and reduction of switching losses, including:

[0130] A zero-sequence injection module, configured to inject a zero-sequence component into the three-phase initial modulation wave, clamp one of the three phases at any moment, and obtain all candidate clamping states;

[0131] The current path analysis module is configured to analyze the current path when a switch tube of a T-type three-level inverter fails, obtain the switch states in which the corresponding faults do not exist, and divide the fundamental wave period into a healthy half-cycle and a faulty half-cycle according to the direction of the faulty phase current;

[0132] The fault-tolerant control module is configured to, in the faulty half-cycle, select the clamping state or reconstruct the carrier according to the fault type of the switch tube to avoid using the non-existent switch states;

[0133] The current ripple suppression module is configured to, after the fault-tolerant control, reorder all vectors in the space vector diagram according to the magnitudes of the three-phase phase voltages, calculate the current ripple slope and the vector action time based on the sorted vectors, calculate the current ripple value of each candidate clamping state according to the current ripple slope and the vector action time, and obtain the clamping state with the minimum current ripple value;

[0134] The neutral point voltage control module is configured to establish a neutral point current model for the healthy half-cycle and the faulty half-cycle respectively. When the neutral point voltage is within the set voltage threshold range, select the switch state with the minimum current ripple value. When the neutral point voltage exceeds the set voltage threshold range, select the switch state according to the neutral point current model.

[0135] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A fault-tolerant control method for a T-type inverter that takes into account both current ripple and switching loss reduction, characterized in that: Including: Injecting a zero-sequence component into the three-phase initial modulation wave, clamping one of the three phases at any moment to obtain all candidate clamping states; Analyzing the current path during the failure of the switching tubes of the T-type three-level inverter, obtaining the switching states in which the corresponding failures do not exist, and dividing the fundamental wave period into a healthy half-cycle and a faulty half-cycle according to the direction of the faulty-phase current; In the faulty half-cycle, according to the fault type of the switching tube, selecting the clamping state or reconstructing the carrier to avoid using the non-existent switching states; After fault-tolerant control, reordering all vectors in the space vector diagram according to the magnitudes of the three-phase phase voltages, calculating the current ripple slope and the vector action time based on the sorted vectors, and calculating the current ripple value of each candidate clamping state according to the current ripple slope and the vector action time to obtain the clamping state with the minimum current ripple value; Establishing the midpoint current models for the healthy half-cycle and the faulty half-cycle respectively. When the midpoint voltage is within the set voltage threshold range, selecting the switching state with the minimum current ripple value. When the midpoint voltage exceeds the set voltage threshold range, selecting the switching state according to the midpoint current model; Wherein, the expression for injecting the zero-sequence component is: ， ， ， Among them, , , are obtained by sorting the initial modulation waves according to their magnitudes. is the x -phase initial modulation wave. x = a, b, c , m z is the injected zero-sequence component. m x is the final modulation wave after injecting the zero-sequence component. Within one switching period, the x phase is clamped to [P], [O], [N], which are represented as X_P, X_O, X_N respectively.

2. The fault-tolerant control method for a T-type inverter that takes into account both current ripple and reduction of switching losses as described in claim 1, characterized in that, Selecting the clamping state or reconstructing the carrier according to the fault type of the switching tube to avoid using the non-existent switching states, including: if it is an external switching tube fault, reducing the modulation index to the set modulation index threshold and selecting the clamping state; if it is an internal switching tube fault, reconstructing the carrier to avoid using the non-existent switching states.

3. The T-type inverter fault-tolerant control method that takes into account both current ripple and reduction of switching losses as described in claim 2, wherein When an external switching tube fails, reducing the modulation index to below 0.58, and at the same time discarding some candidate clamping states to avoid using the non-existent [P] state.

4. The fault-tolerant control method for a T-type inverter that takes into account both current ripple and reduction of switching losses as claimed in claim 2, wherein, When an internal switching tube fails, reconstructing the carrier, including still using two carriers with an amplitude of 1 in the healthy half-cycle, and changing to using one carrier with an amplitude of 2 in the faulty half-cycle, avoiding using the non-existent [O] state.

5. The fault-tolerant control method for a T-type inverter that takes into account both current ripple and reduction of switching losses as described in claim 1, characterized in that, The calculation formula for the current ripple slope is: ， ， ， ， Among them, v x represents the normalized phase voltage, l , m , n respectively represent the initial constant terms of the three-phase current ripple slopes, b x represents the final constant term of the current ripple slope, k x represents the corresponding current ripple slope of each vector in the vector sequence, L is the inductance value, V dc is the DC-side voltage.

6. The fault-tolerant control method for a T-type inverter that takes into account both current ripple and reduction of switching losses as described in claim 1, characterized in that, When the vector sequence is [P, N, N] - [P, O, N] - [P, O, O] - [P, O, N] - [P, N, N], the action time is calculated as: ， Among them, T s represents the switching period, t 1, t 2, t 3 respectively represent half of the action time of each vector in a switching period.

7. The fault-tolerant control method for a T-type inverter that takes into account both current ripple and reduction of switching losses as described in claim 1, characterized in that, The midpoint current model for the healthy half-cycle is: ， The midpoint current model for the faulty half-cycle is: ， Among them, i x is x the phase current of the phase.

8. The fault-tolerant control method for a T-type inverter that takes into account both current ripple and reduction of switching losses as claimed in claim 1, characterized in that, Set the midpoint voltage threshold V TH , when the midpoint voltage ΔV PN is within the acceptable range, that is, − V TH < ΔV PN < V TH , still select the clamping state with the optimal current ripple; when ΔV PN > V TH , select the clamping state corresponding to the maximum midpoint current; when ΔV PN < − V TH , select the clamping state corresponding to the minimum midpoint current.

9. A fault-tolerant control system for a T-type three-level inverter that takes into account both current ripple and switching losses, using the control method described in any one of claims 1-8, characterized in that Including: A zero-sequence injection module configured to inject a zero-sequence component into the three-phase initial modulation wave, clamping one of the three phases at any moment to obtain all candidate clamping states; A current path analysis module configured to analyze the current path during the failure of the switching tubes of the T-type three-level inverter, obtaining the switching states in which the corresponding failures do not exist, and dividing the fundamental wave period into a healthy half-cycle and a faulty half-cycle according to the direction of the faulty-phase current; A fault-tolerant control module configured to, in the faulty half-cycle, select the clamping state or reconstruct the carrier according to the fault type of the switching tube to avoid using the non-existent switching states; The current ripple suppression module is configured to reorder all vectors in the space vector diagram according to the magnitudes of the three-phase phase voltages after fault-tolerant control, calculate the current ripple slope and the vector action time based on the sorted vectors, calculate the current ripple value of each candidate clamping state according to the current ripple slope and the vector action time, and obtain the clamping state with the minimum current ripple value; The neutral point voltage control module is configured to establish a neutral point current model for the healthy half-cycle and the fault half-cycle respectively. When the neutral point voltage is within the set voltage threshold range, select the switching state with the minimum current ripple value. When the neutral point voltage exceeds the set voltage threshold range, select the switching state according to the neutral point current model.

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