Neutral-point voltage full-range control system and method during T-type three-level inverter fault

Abstract

The invention relates to the technical field of inverter control, in particular to a neutral-point voltage full-range control system and method when a T-type three-level inverter breaks down, the T-type three-level inverter is important equipment in an energy router, and when a switch tube of the T-type three-level inverter breaks down, the problems of neutral-point voltage imbalance, current distortion and the like can be caused. According to the invention, the control capability of the neutral-point voltage is greatly improved by decomposing modulation waves when a switch tube fails, neutral-point voltage deviation elimination and oscillation suppression in a full modulation degree and full power factor range are realized, and fault-tolerant operation and neutral-point voltage balance control when the T-type three-level inverter fails are realized on the whole; safe and stable operation of the energy router system is realized, and safe operation of large and important electric power systems such as an electric power system, aerospace, metallurgical mining industry, rail transit and the like is guaranteed.

Description

Technical Field

[0001] The invention relates to the technical field of inverter control, in particular to a full-range control system and method for a midpoint voltage of a T-type three-level inverter when it fails. Background Art

[0002] The energy router system is an important solution to increase the consumption rate of new energy and improve the power quality of the power grid. The energy router consists of multiple DC and AC ports. Among them, the T-type three-level inverter is the core equipment in the energy router. It is widely used in the AC port of the energy router due to its advantages such as small number of switches, small conduction loss and uniform power loss.

[0003] Due to the high operating frequency and high number of switching times of the T-type three-level inverter switch tube, as well as the complex operating conditions and its own aging, the probability of failure of the system switch tube will also increase significantly. When the switch tube fails, the output current of the inverter will be seriously distorted, endangering the energy router system and personal safety. At the same time, the failure of a single device in the energy router may cause the entire system to be paralyzed. In places such as power systems, aerospace, metallurgy and mining, and rail transportation that have high requirements for power supply continuity, long-term power outages may cause significant economic losses or accidents. Therefore, the T-type three-level inverter is required to have fault tolerance so that the energy router system can still maintain safe and stable operation when the switch tube fails, thereby greatly improving the power supply continuity and reliability of the system.

[0004] In addition, the DC side midpoint voltage balance is also the key to ensure the safe and stable operation of the T-type three-level inverter. The midpoint voltage imbalance is divided into midpoint voltage oscillation and deviation, specifically: Midpoint voltage oscillation will generate a lot of harmonic pollution on the AC side, reducing the output current quality and system efficiency; midpoint voltage deviation will directly lead to overvoltage of DC capacitors and power devices, seriously threatening the stable operation of the system. Worse still, when the switch tube of the T-type three-level inverter fails, the problem of midpoint voltage imbalance will be further aggravated.

[0005] Due to the coupling between fault-tolerant control and midpoint voltage balance control, it is difficult to achieve oscillation suppression and deviation elimination of the midpoint voltage when a switch tube of a T-type three-level inverter fails within the full modulation index and full power factor range in the prior art due to the small degree of control freedom.

[0006] Based on the above reasons, the present invention designs a full-range control system and method for the midpoint voltage when a T-type three-level inverter fails. Within the full modulation index and full power factor range, the three-phase current distortion is eliminated, the midpoint voltage deviation is eliminated and the midpoint voltage oscillation is suppressed, thereby realizing fault-tolerant control and balanced control of the midpoint voltage when a T-type three-level inverter fails within the full range. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a full-range control system and method for the midpoint voltage when a T-type three-level inverter fails, which eliminates three-phase current distortion, eliminates the midpoint voltage deviation and suppresses the midpoint voltage oscillation within the full modulation index and full power factor range, thereby realizing fault-tolerant control and balanced control of the midpoint voltage when a T-type three-level inverter fails within the full range.

[0008] In order to achieve the above object, the present invention provides a full range control system for midpoint voltage when a T-type three-level inverter fails, including a half-cycle judgment module, a cycle optimization module and a fault-tolerant control module; The half-cycle judgment module is configured to: divide a fundamental wave cycle into an abnormal half-cycle and a normal half-cycle according to the direction of the fault phase current; The cycle optimization module is configured to: establish the midpoint current model of the abnormal half cycle and the normal half cycle respectively, perform cycle optimization within the value range of the three degrees of freedom of zero-sequence component, decomposed phase and decomposed quantity, and select a set of data with the strongest ability to control the midpoint voltage; The fault-tolerant control module is configured to: compare the modulated wave injected with the zero-sequence component and decomposed with the modified carrier wave to generate a PWM wave, avoid using a non-existent switching state, and implement fault-tolerant control.

[0009] The present invention also provides a method for controlling the midpoint voltage in a full range when a T-type three-level inverter fails, comprising the following steps: S1, determine the faulty switch tube through the fault diagnosis method, and then divide a fundamental wave cycle into an abnormal half cycle and a normal half cycle according to the direction of the fault phase current; S2, select one of the three-phase modulation waves for decomposition, where only two non-fault phase modulation waves can be selected for decomposition in the abnormal half-cycle, and all three phases can be decomposed in the normal half-cycle, and then establish the midpoint current model in the abnormal half-cycle and the normal half-cycle respectively, and perform cyclic optimization within the value range of the three control degrees of freedom of zero-sequence component, decomposition phase and decomposition amount to select a set of data with the strongest ability to control the midpoint voltage; S3, the modulation wave injected with the zero-sequence component and decomposed is compared with the carrier to generate a PWM wave. At the same time, in the abnormal half-cycle, the carrier of the faulty phase is modified to avoid using a non-existent switching state to achieve fault-tolerant control.

[0010] The specific steps of S1 are: When the switch tube fails, the current path before and after the switch tube failure is analyzed. When the fault phase current is in the forward direction, a non-existent switching state appears, which is defined as an abnormal half-cycle. When the fault phase current is in the reverse direction, it is consistent with the normal situation, which is defined as a normal half-cycle.

[0011] In S2, the modulation wave is decomposed into: If the modulation wave is greater than 0, the modulation wave Decompose into and , and compare with the upper and lower carriers respectively, by adjusting the decomposition amount , to adjust the duty cycle of the [O] state, and then control the midpoint current; if the modulation wave is less than 0, the modulation wave is decomposed into and , and compared with the upper and lower carriers respectively, the model of the midpoint current is modified as follows: ;in, is the midpoint current of the normal half cycle after decomposition, is the midpoint current of the abnormal half cycle after decomposition, is the decomposition amount, is the current of the decomposed phase.

[0012] The value range of zero-sequence component and decomposition quantity in S2 is expressed as: ;in, is the maximum value in the three-phase modulation wave, is the minimum value in the three-phase modulation wave, is the modulated wave of the decomposed phase, is the zero sequence component.

[0013] The specific steps of loop optimization in S2 are: S2-1, in the normal half cycle: by Quantify the midpoint current error, expressed as: ;in, is the DC side capacitance, is the voltage difference between the upper and lower capacitors on the DC side. is the sampling period; Traverse the zero-sequence component, the decomposed phase and the decomposed quantity within the value range of S2 to obtain the corresponding The smallest set of data can realize the optimal control of midpoint voltage, where phases A, B and C can all be used as decomposed phases; S2-2, in the abnormal half cycle: by Quantify the midpoint current error, expressed as: ; Traverse the zero-sequence component, decomposed phase and decomposition quantity within the value range of S2 to obtain the corresponding The smallest set of data, in which only two non-fault phases can be used as decomposed phases.

[0014] The specific steps of S3 are: In the normal half cycle: two carriers with an amplitude of 1 are used, and the three levels [P], [O], and [N] are still output; in the abnormal half cycle: a carrier with an amplitude of 2 is used, and only the two levels [P] and [N] are output, avoiding the use of the non-existent switching state [O], which is used to achieve fault-tolerant control.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention avoids the use of the non-existent switch state by modifying the fault phase carrier, realizes the fault-tolerant operation of the T-type three-level inverter when it fails, and improves the system reliability. By selecting the modulation wave for decomposition, the three degrees of freedom of controlling the midpoint voltage, namely the zero-sequence component, the decomposed phase and the decomposed amount, are constructed, and the control ability of the midpoint voltage is significantly increased. The present invention realizes the fault-tolerant operation and midpoint voltage balance control when the T-type three-level inverter fails as a whole, increases the control ability of the midpoint voltage when the switch tube fails, and realizes the elimination of the midpoint voltage deviation and the suppression of oscillation within the full modulation degree and the full power factor range.

[0016] In addition, the midpoint current model established by the present invention performs cyclic optimization within the value range of three degrees of freedom to obtain a set of data with the strongest ability to control the midpoint voltage, thereby achieving midpoint voltage deviation elimination and oscillation suppression within the full range. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of a T-type three-level inverter topology structure according to an embodiment of the present invention.

[0018] Figure 2 The current in the embodiment of the present invention is forward and reverse Current path diagram of phase A before and after the fault.

[0019] Figure 3 Schematic diagram of a carrier modification method when a switch tube fails according to an embodiment of the present invention.

[0020] Figure 4 Schematic diagram of a modulation wave decomposition method according to an embodiment of the present invention.

[0021] Figure 5 This is an overall control block diagram of the system according to an embodiment of the present invention.

[0022] Figure 6 When the modulation degree and power factor of the embodiment of the present invention are 1 Output waveform after open circuit fault.

[0023] Figure 7 In the embodiment of the present invention, the modulation degree is 0.5 and the power factor is 1 Output waveform after open circuit fault.

[0024] Figure 8 In the embodiment of the present invention, the modulation index is 1 and the power factor is 0. Output waveform after open circuit fault.

[0025] Fig. 9 In the embodiment of the present invention, the modulation index is 0.5 and the power factor is 0. Output waveform after open circuit fault. DETAILED DESCRIPTION

[0026] The present invention will now be further described with reference to the accompanying drawings.

[0027] T-type three-level inverter topology (such as Figure 1 As shown in Figure 1), each of the three phases A, B, and C includes four switching tubes, which are composed of two external switching tubes ( , ) and the internal switch tube ( , Each phase bridge arm has three switch states [P], [O], [N]. The switch state, output voltage and switch tube working conditions are shown in Table 1 below: Switch Status Output voltage The switch tube that is turned on (x=a, b, c) [P] <![CDATA[+V dc / 2]]> <![CDATA[S x1 , S x2 ]]> [O] 0 <![CDATA[S x2 , S x3 ]]> [N] <![CDATA[−V dc / 2]]> <![CDATA[S x3 , S x4 ]]> Table 1: Switching status, output voltage and switch tube working conditions of T-type three-level inverter In actual operation, the T-type three-level inverter is very prone to switch failure due to high operating frequency, complex operating conditions and device aging, which seriously threatens the safe operation of the system.

[0028] The types of switch tube failures are divided into short-circuit failures and open-circuit failures: When a short-circuit failure occurs, the instantaneous large current will threaten the entire inverter system, so the short-circuit failure is usually converted into an open-circuit failure for processing by connecting a series fuse or forcing a drive signal. When an open-circuit failure occurs, it will not instantly cause devastating damage to the power device or even the entire system, but the output current will be distorted, and it may even cause secondary damage to other components. Therefore, corresponding strategies need to be adopted to achieve fault-tolerant operation and reduce losses caused by downtime due to failures.

[0029] The switch tube open circuit failure is divided into external switch tube failure and internal switch tube failure. When the external switch tube fails, the modulation index needs to be reduced to Only in this way can the use of non-existent switching states be avoided, thereby achieving fault-tolerant control. However, this will cause the T-type three-level inverter to be disconnected from the grid or operate at a reduced rating, which is not allowed for most application scenarios.

[0030] Therefore, this embodiment only considers the case where the internal switch tube has an open circuit failure. Take the fault as an example for analysis, that is, phase A is the fault phase, Represents the second switch tube of phase A of the T-type three-level inverter (see Figure 1 The current path at this time is as follows Figure 2 shown.

[0031] like Figure 2 As shown in (a), when When [O] state of phase A is replaced by [N] state, if the non-existent switch state [O] is used to synthesize the reference vector, the output current will be distorted; like Figure 2 As shown in (b), when Therefore, according to the direction of the fault phase current, a fundamental wave cycle is divided into an abnormal half cycle ( ) and a normal half cycle ( ).

[0032] To achieve fault-tolerant control, the [O] state cannot be used for synthesis in the abnormal half cycle of phase A, so the fault phase carrier is modified as follows: Figure 3 In the abnormal half cycle, a carrier with an amplitude of 2 is used to output only two levels, [P] and [N], avoiding the use of the [O] state; in the normal half cycle, two carriers with an amplitude of 1 are still used to continue to output three levels, [P], [O], and [N].

[0033] After achieving fault-tolerant operation, the midpoint voltage of the T-type three-level inverter needs to be controlled. Therefore, the midpoint voltage (ΔV PN ) is the voltage difference between the upper and lower capacitors on the DC side. According to Kirchhoff’s current law, we can get: ;in, is the midpoint current, C is the DC side capacitor value, is the sampling period. It can be seen that the key to controlling the midpoint voltage is to obtain the desired midpoint current. The factors affecting the midpoint current are analyzed below: The three-phase initial modulation wave of the T-type three-level inverter is expressed as: ; where m is the modulation index, is the phase angle of phase A. Usually, the zero-sequence component is injected into the three-phase initial modulation wave ( ) to expand the modulation range and increase a degree of control freedom. The modulated wave after injection is expressed as: ; The midpoint current is determined by the [O] state duty cycle of each phase. In a normal half cycle, the midpoint current in a switching cycle can be expressed as: ; In the abnormal half cycle, phase A does not produce the [O] state due to the two-level modulation, and has no effect on the midpoint current. Therefore, the midpoint current is expressed as: ; It can be seen that the midpoint current at this time can only pass through the zero-sequence component ( ) for control, and the control freedom is very small. Moreover, after adopting fault-tolerant control, the average value of the midpoint current is asymmetric in the normal half cycle and the abnormal half cycle, which aggravates the imbalance of the midpoint voltage and increases the difficulty of midpoint voltage control. Therefore, in order to increase the midpoint voltage control capability, the modulation wave of one phase is selected for decomposition. The specific implementation method is as follows: Figure 4 If the modulation wave is greater than 0, the modulation wave Decompose into + and , and compare with the upper and lower carriers respectively, by adjusting the decomposition amount , the duty cycle of the [O] state can be adjusted to control the midpoint current. Similarly, if the modulation wave is less than 0, the modulation wave is decomposed into - and .

[0034] At this time, the selection of the decomposed phase and the value of the decomposition will affect the magnitude of the midpoint current, thereby obtaining two degrees of freedom to control the midpoint voltage, which can be used together with the zero-sequence component to control the midpoint voltage balance, thus achieving three-degree-of-freedom control. At this time, the midpoint current model is modified as follows: ; in is the midpoint current of the normal half cycle after decomposition, is the midpoint current of the abnormal half cycle after decomposition, is the current of the decomposed phase. If phase B is the decomposed phase, then .

[0035] In order to obtain a set of data for optimal control of the midpoint voltage, it is necessary to perform cyclic optimization within the value range of the three control degrees of freedom. The value range of the zero-sequence component and the decomposition amount is expressed as: ; is the maximum value in the three-phase modulation wave, is the minimum value in the three-phase modulation wave, It is the modulated wave of the decomposed phase.

[0036] In the normal half cycle, Quantify the midpoint current error, expressed as: ; Traverse the zero-sequence component, decomposed phase and decomposed quantity within the value range and obtain the corresponding The smallest set of data can realize the optimal control of midpoint voltage. Among them, phases A, B, and C can all be used as decomposed phases.

[0037] In the abnormal half cycle, Quantify the midpoint current error, expressed as: ; Similarly, traverse the three control degrees of freedom within the value range and obtain the corresponding The smallest set of data, in which phase A does not have the [O] state due to the use of fault-tolerant control and cannot be used as the decomposed phase to control the midpoint current. Therefore, only phases B and C can be decomposed.

[0038] The overall control block diagram of this embodiment is as follows Figure 5 shown.

[0039] Figure 6 for Output waveforms of the T-type three-level inverter after the proposed method is used during an open circuit fault, including the three-phase output current, the phase voltage of phase A, and the two DC side capacitor voltages.

[0040] in Figure 6 Indicates the working condition with modulation degree of 1 and power factor of 1; Figure 7 Indicates the working condition with modulation degree of 0.5 and power factor of 1; Figure 8 Indicates the working condition where the modulation index is 1 and the power factor is 0. Fig. 9 Indicates the working condition with a modulation index of 0.5 and a power factor of 0.

[0041] comprehensive Figure 6 to Figure 9 It can be seen that in After the fault, the three-phase current is severely distorted and the midpoint voltage is offset at low power factor, which seriously threatens the safe and stable operation of the system.

[0042] The present invention also provides a full-range control system for the midpoint voltage when a T-type three-level inverter fails, including a half-cycle judgment module, a cyclic optimization module and a fault-tolerant control module. Each module implements the full-range control method for the midpoint voltage when a T-type three-level inverter fails in an embodiment of the present invention.

[0043] The half-cycle judgment module is configured to: divide a fundamental wave cycle into an abnormal half-cycle and a normal half-cycle according to the direction of the fault phase current; The cycle optimization module is configured to: establish the midpoint current model of the abnormal half cycle and the normal half cycle respectively, perform cycle optimization within the value range of the three degrees of freedom of zero-sequence component, decomposed phase and decomposed quantity, and select a set of data with the strongest ability to control the midpoint voltage; The fault-tolerant control module is configured to: compare the modulated wave injected with the zero-sequence component and decomposed with the modified carrier wave to generate a PWM wave, avoid using a non-existent switching state, and implement fault-tolerant control.

[0044] The above are only preferred embodiments of the present invention, which are only used to help understand the method and core ideas of the present application. The protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should also be regarded as the protection scope of the present invention.

[0045] The present invention solves the problems in the prior art such as poor fault tolerance when the switch tube of the T-type three-level inverter fails, unbalanced DC side midpoint voltage, and current distortion, which cause unsafe and unstable risks to the energy router system, thereby affecting the normal operation of large and important equipment and potential unsafe factors. By decomposing the modulation wave when the switch tube fails, the control ability of the midpoint voltage is greatly increased, and the midpoint voltage deviation elimination and oscillation suppression within the full modulation index and full power factor range are achieved. The fault-tolerant operation and midpoint voltage balance control when the T-type three-level inverter fails are achieved as a whole, and the safe and stable operation of the energy router system is achieved, thereby providing a guarantee for the safe operation of large and important power systems such as power systems, aerospace, metallurgy and mining, and rail transportation.

Claims

1. A full range control system for midpoint voltage when a T-type three-level inverter fails, characterized in that: It includes a half-cycle judgment module, a cycle optimization module and a fault-tolerant control module; The half-cycle judgment module is configured to: divide a fundamental wave cycle into an abnormal half-cycle and a normal half-cycle according to the direction of the fault phase current; The cycle optimization module is configured to: establish midpoint current models of abnormal half-cycles and normal half-cycles respectively, perform cycle optimization within the value range of three degrees of freedom of zero-sequence component, decomposed phase and decomposed quantity, and select a set of data with the strongest ability to control midpoint voltage; The fault-tolerant control module is configured to: generate a PWM wave by comparing the modulated wave injected with the zero-sequence component and decomposed with the modified carrier wave, avoid using a non-existent switch state, and implement fault-tolerant control.

2. A method for controlling the midpoint voltage in a full range when a T-type three-level inverter fails, characterized in that: The following steps are involved: S1, determine the faulty switch tube through the fault diagnosis method, and then divide a fundamental wave cycle into an abnormal half cycle and a normal half cycle according to the direction of the fault phase current; S2, selecting one phase of the three-phase modulation wave for decomposition, wherein only two non-fault phase modulation waves can be selected for decomposition in the abnormal half cycle, and all three phases can be decomposed in the normal half cycle, and then respectively establishing the midpoint current model in the abnormal half cycle and the normal half cycle, and cyclically optimizing within the value range of the three control degrees of freedom of zero-sequence component, decomposition phase and decomposition amount, so as to select a set of data with the strongest ability to control the midpoint voltage; S3, comparing the modulated wave injected with the zero-sequence component and decomposed with the carrier wave to generate a PWM wave, and at the same time, in the abnormal half cycle, modifying the carrier wave of the faulty phase to avoid using a non-existent switching state, thereby realizing fault-tolerant control.

3. The method for controlling the midpoint voltage in a full range when a T-type three-level inverter fails according to claim 2, characterized in that: The specific steps of S1 are: When the switch tube fails, the current path before and after the switch tube failure is analyzed. When the fault phase current is in the forward direction, a non-existent switching state appears, which is defined as an abnormal half-cycle. When the fault phase current is in the reverse direction, it is consistent with the normal situation, which is defined as a normal half-cycle.

4. The method for controlling the midpoint voltage in a full range when a T-type three-level inverter fails according to claim 2, characterized in that: In S2, the modulation wave is decomposed into: If the modulation wave is greater than 0, the modulation wave Decompose into and , and compare with the upper and lower carriers respectively, by adjusting the decomposition amount , to adjust the duty cycle of the [O] state, and then control the midpoint current; if the modulation wave is less than 0, the modulation wave is decomposed into and , and compared with the upper and lower carriers respectively, the model of the midpoint current is modified as follows: ;in, is the midpoint current of the normal half cycle after decomposition, is the midpoint current of the abnormal half cycle after decomposition, is the decomposition amount, is the current of the decomposed phase.

5. The method for controlling the midpoint voltage in a full range when a T-type three-level inverter fails according to claim 2, characterized in that: The value range of the zero-sequence component and the decomposition amount in S2 is expressed as: ;in, is the maximum value in the three-phase modulation wave, is the minimum value in the three-phase modulation wave, is the modulated wave of the decomposed phase, is the zero sequence component.

6. The method for controlling the midpoint voltage in a full range when a T-type three-level inverter fails according to claim 2, characterized in that: The specific steps of loop optimization in S2 are: S2-1, in the normal half cycle: by Quantify the midpoint current error, expressed as: ;in, is the DC side capacitance, is the voltage difference between the upper and lower capacitors on the DC side. is the sampling period; Traverse the zero-sequence component, the decomposed phase and the decomposed quantity within the value range of S2 to obtain the corresponding The smallest set of data can realize the optimal control of midpoint voltage, where phases A, B and C can all be used as decomposed phases; S2-2, in the abnormal half cycle: by Quantify the midpoint current error, expressed as: ； Traverse the zero-sequence component, decomposed phase and decomposition quantity within the value range of S2 to obtain the corresponding The smallest set of data, in which only two non-fault phases can be used as decomposed phases.

7. The method for controlling the midpoint voltage in a full range when a T-type three-level inverter fails according to claim 2, characterized in that: The specific steps of S3 are: In the normal half cycle: two carriers with an amplitude of 1 are used, and the three levels [P], [O], and [N] are still output; in the abnormal half cycle: a carrier with an amplitude of 2 is used, and only the two levels [P] and [N] are output, avoiding the use of the non-existent switching state [O], which is used to achieve fault-tolerant control.

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