Fault-tolerant control method for T-type three-level inverter under unbalanced neutral point voltage

Abstract

The present invention provides a fault-tolerant control method for a T-type three-level inverter under unbalanced midpoint voltage conditions. The method comprises defining an unbalance factor; drawing a spatial vector diagram based on the unbalance factor, dividing the space into different sectors, and determining the sector in which the reference voltage is located under unbalanced DC side midpoint voltage conditions; calculating the three-phase action time of each sector; obtaining a midpoint current model under fault conditions based on the current path under fault conditions and the three-phase action time; calculating the required zero-sequence component; and injecting the zero-sequence component into a three-phase modulated wave to change the triangular carrier of the faulty phase and generate a PWM drive signal. The present invention can maintain a low total harmonic distortion rate for the three-phase output current and a given unbalanced midpoint voltage, while simultaneously achieving unbalanced DC side midpoint voltage control and fault-tolerant control during switch tube faults.

Description

Technical Field

[0001] The present invention belongs to the technical field of fault-tolerant control of T-type three-level inverters, and in particular relates to a fault-tolerant control method of a T-type three-level inverter under a midpoint voltage imbalance working condition. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] In photovoltaic power generation systems, T-type three-level inverters are widely used in medium-voltage and high-power applications due to their advantages such as small number of switches, uniform power loss and high efficiency.

[0004] A balanced DC midpoint voltage is crucial for the safe and stable operation of T-type three-level inverters. However, in some special cases, T-type three-level inverters must operate under conditions of unbalanced DC midpoint voltage. For example, to improve the efficiency of centralized photovoltaic inverter systems, independent maximum power point tracking (MPPT) control strategies are employed for photovoltaic modules to achieve maximum power generation efficiency. Furthermore, some hybrid photovoltaic energy storage systems also require midpoint voltage imbalance control to maximize power utilization from multiple sources.

[0005] However, unbalanced midpoint voltage in T-type three-level inverters can have a number of negative consequences. Firstly, if appropriate control methods are not employed, the unbalanced midpoint voltage can distort the inverter's output current, causing the total harmonic distortion (THD) to exceed grid-connected requirements. Secondly, the unbalanced midpoint voltage inevitably subjects power devices to varying voltage stresses, which can easily lead to switch failures and endanger system and personnel safety.

[0006] According to the inventors' understanding, the existing midpoint voltage imbalance control method is only applicable to normal operating conditions of the switching tube, and will produce serious current distortion when a fault occurs; the existing fault-tolerant control method is only applicable to conditions where the midpoint voltage is balanced, and due to different modeling, the midpoint voltage cannot be maintained at the required imbalance value. Summary of the Invention

[0007] In order to solve the above problems, the present invention proposes a fault-tolerant control method for a T-type three-level inverter under unbalanced midpoint voltage conditions. The present invention can achieve a low total harmonic distortion rate of the three-phase output current and keep the midpoint voltage at a given unbalanced value, while achieving DC side midpoint voltage unbalance control and fault-tolerant control in the event of a switch tube failure.

[0008] According to some embodiments, the present invention adopts the following technical solutions:

[0009] A fault-tolerant control method for a T-type three-level inverter under a midpoint voltage imbalance condition comprises the following steps:

[0010] Calculate the unbalance factor based on the voltages of the upper and lower capacitors on the DC side;

[0011] Draw a space vector diagram based on the imbalance factor, divide the space vector diagram into different sectors according to the position of the midpoint vector, and determine the sector where the reference voltage is located under the DC side midpoint voltage unbalance condition;

[0012] Calculate the three-phase action time of each sector;

[0013] According to the current path of the T-type three-level inverter switch tube in the case of a fault, combined with the three-phase action time, a midpoint current model in the fault condition is obtained;

[0014] In the case of unbalanced capacitor voltage on the DC side of a T-type three-level inverter, the required zero-sequence component is calculated based on the midpoint current model under fault conditions;

[0015] The zero-sequence component is injected into the three-phase modulation wave to change the triangular carrier of the fault phase and generate a PWM drive signal.

[0016] As an optional implementation, 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, in the [P] state, the output voltage of the bridge arm is equal to the voltage value of the upper capacitor, in the [O] state, the output voltage of the bridge arm is equal to zero, and in the [N] state, the output voltage of the bridge arm is equal to the negative value of the voltage of the lower capacitor.

[0017] As an optional implementation, the specific process of calculating the unbalance factor based on the voltages of the upper and lower capacitors on the DC side includes:

[0018] The voltage of the capacitor on the DC side is equal to V P , the lower capacitor voltage is equal to V N , the sum of the two capacitor voltages is equal to V dc To characterize the imbalance degree of the two capacitor voltages on the DC side, the unbalance factor of the two capacitor voltages is introduced. k , expressed as

[0019] .

[0020] As an optional implementation, a space vector diagram is drawn according to the imbalance factor, which is divided into six sectors according to the position of the center vector. The sector where the reference voltage is located is determined based on the relationship between the angle between the center vector [PON] and the large vector [PNN] and the phase angle of the reference voltage.

[0021] As a further embodiment, the phase angle of the reference voltage is θ, the process of determining the sector where the reference voltage is located is:

[0022]

[0023] in, α is the angle between the middle vector [PON] and the large vector [PNN].

[0024] As an optional implementation, the specific process of calculating the three-phase action time of each sector includes calculating the three-phase action time of each sector according to the volt-second balance principle, and the volt-second balance principle is:

[0025] ;

[0026] in, m x is the initial modulation wave, V dc is the sum of the voltages of the upper and lower capacitors on the DC side, T s is the sampling time, k is the unbalance factor of the two capacitor voltages defined, n x is the benchmark function, d x is the duty cycle of the [O] state, m z is the zero-sequence component that should be injected.

[0027] As an optional implementation, the midpoint current model under fault conditions is:

[0028]

[0029] in, i x is the current of phase x, k is the unbalance factor of the two capacitor voltages defined, m x is the initial modulation wave, n x is the benchmark function.

[0030] As an optional implementation method, according to the midpoint current model under fault conditions, the specific process of calculating the required zero-sequence component includes: the zero-sequence component is expressed as

[0031]

[0032] in, i x is the current of phase x, k is the unbalance factor of the two capacitor voltages defined, mx is the initial modulation wave, n x is the benchmark function, is the reference current value.

[0033] As a further embodiment, the calculated required zero sequence component is limited so that ,in:

[0034]

[0035] k is the unbalance factor of the two capacitor voltages defined, m x is the initial modulation wave, n x is the benchmark function.

[0036] A fault-tolerant control system for a T-type three-level inverter under a midpoint voltage imbalance condition, comprising:

[0037] The unbalance factor calculation module is configured to calculate the unbalance factor according to the voltages of the upper and lower capacitors on the DC side;

[0038] a sector determination module configured to draw a space vector diagram based on the imbalance factor, divide the space vector diagram into different sectors based on the position of the midpoint vector, and determine the sector where the reference voltage is located under the DC side midpoint voltage unbalance condition;

[0039] an action time calculation module, configured to calculate the three-phase action time of each sector;

[0040] A midpoint current calculation module is configured to obtain a midpoint current model under fault conditions based on a current path of a T-type three-level inverter switch tube under fault conditions and in combination with the three-phase action time;

[0041] a zero-sequence component calculation module configured to calculate a required zero-sequence component according to a midpoint current model under a fault condition when a capacitor voltage on a DC side of a T-type three-level inverter is unbalanced;

[0042] The carrier reconstruction module is configured to inject a zero-sequence component into the three-phase modulation wave, change the triangular carrier of the fault phase, and generate a PWM drive signal.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] The present invention implements fault-tolerant control of a T-type three-level inverter under conditions of DC side imbalance, maintaining a low total harmonic distortion (THD) of the three-phase output current. The midpoint current model is derived from the volt-second balance principle and is also applicable to conditions where the voltages of the two capacitors on the DC side are balanced. The method described in the present invention enables the T-type three-level inverter system to output a good current at any DC side midpoint voltage, thereby improving system efficiency. The method described in the present invention enables the T-type three-level inverter system to maintain safe and stable operation even in the event of a switch failure, thereby ensuring system reliability. Compared to traditional midpoint voltage imbalance control methods, the present invention can achieve midpoint voltage imbalance control in the event of a switch failure while maintaining a low output current harmonic distortion. Compared to traditional fault-tolerant control methods, the present invention can simultaneously achieve fault-tolerant control in the event of a DC side midpoint imbalance, achieving decoupling between the two and maintaining efficient and reliable system operation. To make the above-mentioned objects, features, and advantages of the present invention more readily apparent, preferred embodiments are described below in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0046] Figure 1 A T-type three-level inverter system in dual MPPT mode provided by an embodiment of the present invention;

[0047] Figure 2 The T-type three-level inverter provided by the embodiment of the present invention is k >0 when the space vector diagram;

[0048] Figure 3 A carrier modulation method for a non-fault phase provided in an embodiment of the present invention;

[0049] Figure 4 The switch tube provided in the embodiment of the present invention S a2 The current path in the event of a fault;

[0050] Figure 5 A carrier modulation method for a fault phase provided by an embodiment of the present invention;

[0051] Figure 6 A block diagram of fault-tolerant control under midpoint voltage imbalance provided by an embodiment of the present invention;

[0052] Figure 7 Output waveform diagram when a fault occurs under unbalanced DC side working conditions provided by an embodiment of the present invention;

[0053] Figure 8 The embodiment of the present invention provides k Dynamic response output waveform changing from 0.25 to 0. DETAILED DESCRIPTION

[0054] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0055] It should be noted that the following detailed descriptions are illustrative and 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 skilled in the art to which the present invention belongs.

[0056] It should be noted that the terms used herein are only for describing specific embodiments 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 intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0057] The present invention is directed to a T-type three-level inverter system. Figure 1 The topology diagram of a T-type three-level inverter system includes the A, B, and C phases. Each phase includes four power switches: two half-bridge (HB) switches and two neutral-point (NP) switches. The DC side includes two series-connected capacitors, each connected to a photovoltaic module. The center of the two capacitors forms a neutral point.

[0058] Specifically, each phase bridge arm of the T-type three-level inverter system has three operating states: [P], [O], and [N]. Taking the neutral point of the two capacitors on the DC side as the reference point, in the [P] state, the bridge arm output voltage is equal to the voltage value of the upper capacitor; in the [O] state, the bridge arm output voltage is equal to zero; and in the [N] state, the bridge arm output voltage is equal to the negative value of the lower capacitor voltage.

[0059] Table 1 shows the switching state, output voltage and working conditions of each power switch tube in each phase bridge arm of the T-type three-level inverter system.

[0060] Table 1 Switching status, output voltage and switching tube working conditions of T-type three-level inverter

[0061]

[0062] Assume that the voltage of the capacitor on the DC side is equal to V P , the lower capacitor voltage is equal to V N , the sum of the two capacitor voltages is equal to Vdc To characterize the imbalance degree of the two capacitor voltages on the DC side, the unbalance factor of the two capacitor voltages is introduced. k , expressed as

[0063] (1)

[0064] The expressions of the upper and lower capacitor voltages on the DC side are obtained as follows:

[0065] (2)

[0066] The T-type three-level inverter consists of 27 space voltage vectors, including large vectors, medium vectors, small vectors and zero vectors. Figure 2 Shown k The space vector diagram when P > 0 is divided into six sectors according to the position of the center vector, and the angle between the center vector [PON] and the large vector [PNN] is defined as α , expressed as

[0067] (3)

[0068] The phase angle of the reference voltage is θ , the method to determine the sector where the reference voltage is located is

[0069] (4)

[0070] The present invention controls the on-off of a power switch tube in a T-type three-level inverter system based on a carrier modulation method. Figure 3 The figure shows the carrier modulation method, which generates a PWM signal by comparing the modulated wave with two carrier waves with an amplitude of 1.

[0071] Define the benchmark function as

[0072] (5)

[0073] The duty cycle of the [O] state is defined as

[0074] (6)

[0075] To calculate the zero-sequence component, according to the volt-second balance principle,

[0076] (7)

[0077] Table 2 shows the calculated three-phase action time of each sector.

[0078] Table 2 Three-phase action time

[0079]

[0080] When an HB diode fails, the modulation index must be reduced to achieve fault-tolerant control, which is unacceptable in grid-connected inverter systems. Furthermore, the unbalanced midpoint voltage causes uneven voltage stress on the NP diodes, but has no effect on the HB diodes, increasing the probability of NP diode failure. Therefore, this invention only considers the case of an NP diode open circuit failure. When an NP switching diode fails, the fault phase current path changes. Figure 4 The switch tube is shown S a2 When a fault occurs, the current path cannot be generated in the [O] state. In this case, if a non-existent vector is used to synthesize the reference vector, the output current and voltage will be distorted, even exceeding the grid connection requirements.

[0081] The present invention realizes fault-tolerant control by modifying the carrier of the fault phase. Figure 5 The figure shows the modified carrier wave. A carrier wave with an amplitude of 2 replaces two carrier waves with an amplitude of 1. This allows the faulty phase to output only the [P] and [N] states but not the [O] state. The normal phase still outputs the [P], [O], and [N] states, thus avoiding the use of non-existent space vectors and ensuring the output current quality.

[0082] Since phase A does not produce the [O] state, it has no effect on the midpoint current. The midpoint current at this time is expressed as

[0083] (8)

[0084] Substituting the action time in Table 2 and summarizing it, we can get

[0085] (9)

[0086] The present invention uses a PI controller to obtain the reference current , the zero sequence component is expressed as

[0087] (10)

[0088] In order to avoid overmodulation, the zero-sequence component should be limited, which is expressed as

[0089] (11)

[0090] in

[0091] (12)

[0092] Injecting the limited zero-sequence component into the initial modulation wave, the final modulation wave is:

[0093] (13)

[0094] Based on the above method, the fault-tolerant control of the T-type three-level inverter under the condition of unbalanced DC side midpoint voltage is realized. Figure 6 This is the control block diagram of the system.

[0095] Figure 7 The output waveform of the T-type three-level inverter system when this method is used includes the three-phase output current, the phase voltage of the fault phase and the voltage of the two capacitors on the DC side. S a2 When a fault occurs, the three-phase output current is distorted and the total harmonic distortion rate exceeds the grid-connected requirement; after adopting the fault-tolerant control method of the present invention, the three-phase output current harmonic distortion rate is maintained at a low level, and the DC side midpoint voltage imbalance control can be achieved at the same time.

[0096] Figure 8 This is the dynamic response output waveform of this method. It can be seen that this method can simultaneously realize midpoint voltage unbalance control and fault-tolerant control, and is suitable for the DC side capacitor voltage balance condition.

[0097] In this invention, by modifying the carrier, the use of non-existent space vectors is avoided, achieving fault-tolerant control. By establishing a midpoint current model under fault conditions, calculating the zero-sequence component, and injecting a modulated wave, unbalanced DC midpoint voltage control is achieved. Therefore, the proposed method decouples midpoint voltage control from fault-tolerant control. It should be noted that the proposed method is also applicable to DC midpoint voltage balanced conditions.

[0098] It will be understood by those skilled in the art that embodiments of the present invention may be provided as methods, systems, or computer program products. Thus, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention may take the form of one or more computer-usable storage media (including but not limited to disk storage, CD - ROM , optical storage, etc.).

[0099] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowcharts and / or block diagrams, as well as combinations of processes and / or blocks in the flowcharts and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowcharts and / or block diagrams. Figure 1 a process or multiple processes and / or boxes Figure 1A device that provides the functions specified in a block or multiple blocks.

[0100] These computer program instructions may also be stored in a computer readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 a process or multiple processes and / or boxes Figure 1 The function specified in one or more boxes.

[0101] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing the instructions executed on the computer or other programmable device for implementing the process. Figure 1 a process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0102] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made by those skilled in the art that fall within the spirit and principles of the present invention and do not require creative effort are intended to be within the scope of protection of the present invention.

Claims

1. A fault-tolerant control method for a T-type three-level inverter under a midpoint voltage imbalance condition, characterized in that: The following steps are involved: Calculate the unbalance factor based on the voltages of the upper and lower capacitors on the DC side; Draw a space vector diagram based on the imbalance factor, divide the space vector diagram into different sectors according to the position of the midpoint vector, and determine the sector where the reference voltage is located under the DC side midpoint voltage unbalance condition; Calculate the three-phase action time of each sector; According to the current path of the T-type three-level inverter switch tube in the case of a fault, combined with the three-phase action time, a midpoint current model in the fault condition is obtained; In the case of unbalanced capacitor voltage on the DC side of a T-type three-level inverter, the required zero-sequence component is calculated based on the midpoint current model under fault conditions; The zero-sequence component is injected into the three-phase modulation wave to change the triangular carrier of the fault phase and generate a PWM drive signal.

2. The fault-tolerant control method for a T-type three-level inverter under a midpoint voltage imbalance condition according to claim 1, wherein: Each phase bridge arm of the T-type three-level inverter system has three operating states: [P], [O], and [N]. Taking the neutral point of the two capacitors on the DC side as the reference point, in the [P] state, the bridge arm output voltage is equal to the voltage value of the upper capacitor; in the [O] state, the bridge arm output voltage is equal to zero; and in the [N] state, the bridge arm output voltage is equal to the negative value of the lower capacitor voltage.

3. The fault-tolerant control method for a T-type three-level inverter under a midpoint voltage imbalance condition according to claim 1, wherein: The specific process of calculating the unbalance factor based on the voltage of the upper and lower capacitors on the DC side includes: The voltage of the capacitor on the DC side is equal to V P , the lower capacitor voltage is equal to V N , the sum of the two capacitor voltages is equal to V dc To characterize the imbalance degree of the two capacitor voltages on the DC side, the imbalance factor k of the two capacitor voltages is introduced, which is expressed as 。 4. The fault-tolerant control method for a T-type three-level inverter under a midpoint voltage imbalance condition according to claim 1, wherein: Draw a space vector diagram based on the imbalance factor, divide a fundamental wave cycle into six sectors according to the position of each mid-vector, and determine the sector where the reference voltage is located based on the relationship between the angle between the mid-vector [PON] and the large vector [PNN] and the reference voltage phase angle. The specific process is as follows: The phase angle of the reference voltage is θ , the process of determining the sector where the reference voltage is located is: in, α is the angle between the middle vector [PON] and the large vector [PNN].

5. The fault-tolerant control method for a T-type three-level inverter under a midpoint voltage imbalance condition as claimed in claim 2, wherein: The specific process of calculating the three-phase action time of each sector includes calculating the three-phase action time of each sector according to the volt-second balance principle, and the volt-second balance principle is: ； in, m x is the initial modulation wave, V dc is the sum of the voltages of the upper and lower capacitors on the DC side, T s is the sampling time, k is the unbalance factor of the two capacitor voltages defined, n x is the benchmark function, d x is the duty cycle of the [O] state, m z is the zero-sequence component that should be injected.

6. The fault-tolerant control method for a T-type three-level inverter under a midpoint voltage imbalance condition according to claim 1, wherein: The midpoint current model under fault conditions is: in, i x for x Phase current, k is the unbalance factor of the two capacitor voltages defined, m x is the initial modulation wave, n x is the benchmark function.

7. The fault-tolerant control method for a T-type three-level inverter under a midpoint voltage imbalance condition according to claim 1, wherein: According to the midpoint current model under fault conditions, the specific process of calculating the required zero-sequence component includes: the zero-sequence component is expressed as in, i x for x Phase current, k is the unbalance factor of the two capacitor voltages defined, m x is the initial modulation wave, n x is the benchmark function, is the reference current value.

8. The fault-tolerant control method for a T-type three-level inverter under a midpoint voltage imbalance condition according to claim 1, wherein: It also includes limiting the calculated desired zero-sequence component so that, where: k is the unbalance factor of the two capacitor voltages defined, m x is the initial modulation wave, n x is the benchmark function.

9. The fault-tolerant control method for a T-type three-level inverter under a midpoint voltage imbalance condition as claimed in claim 2, wherein Carrier reconstruction is performed on the fault phase. The specific process is as follows: only a carrier with an amplitude of 2 is used in the fault phase, and only the [P] and [N] states are output to avoid the use of the [O] state that does not exist due to the switch failure.

10. A fault-tolerant control system for a T-type three-level inverter under unbalanced midpoint voltage conditions, characterized in that: include: The unbalance factor calculation module is configured to calculate the unbalance factor according to the voltages of the upper and lower capacitors on the DC side; a sector determination module configured to draw a space vector diagram based on the imbalance factor, divide the space vector diagram into different sectors based on the position of the midpoint vector, and determine the sector where the reference voltage is located under the DC side midpoint voltage unbalance condition; an action time calculation module, configured to calculate the three-phase action time of each sector; A midpoint current calculation module is configured to obtain a midpoint current model under fault conditions based on a current path of a T-type three-level inverter switch tube under fault conditions and in combination with the three-phase action time; a zero-sequence component calculation module configured to calculate a required zero-sequence component according to a midpoint current model under a fault condition when a capacitor voltage on a DC side of a T-type three-level inverter is unbalanced; The carrier reconstruction module is configured to inject a zero-sequence component into the three-phase modulation wave, change the triangular carrier of the fault phase, and generate a PWM drive signal.

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