Open-circuit Fault Tolerant Control and Neutral Point Voltage Balancing Method for a T-Type Three-Level Converter

Abstract

An open-circuit fault tolerance control and neutral point voltage balancing method for a T-type three-level converter, comprising the following steps: The first step: injecting a zero-sequence voltage into the faulty phase; The second step: calculating the reference neutral point current; The third step: calculating the zero-level duty ratio of the non-faulty phase; The fourth step: calculating the modulation signals of double-modulation-wave PWM and realizing the neutral point voltage balance by the T-type three-level converter outputting a specific voltage. Compared with the existing open-circuit fault tolerance control method and neutral point voltage balance control method for T-type three-level converters based on the zero-sequence voltage freedom, which can only be used for horizontal tube faults, the fault tolerance method based on double-modulation-wave PWM proposed by the present invention develops a new control freedom, calculates the zero-level duty ratio according to the reference neutral point current, and then obtains the expression of double-modulation-wave PWM. Therefore, when a vertical tube open-circuit fault occurs in the T-type three-level converter, open-circuit fault tolerance control and neutral point voltage balance can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of T-type three-level converters, and particularly relates to an open-circuit fault tolerant control and neutral point voltage balancing method for a T-type three-level converter. Background Art

[0002] Neutral point clamped multi-level converters have been widely used in many medium-voltage and large-capacity fields due to their outstanding advantages such as few switching devices, high power density, and no passive devices. Among them, T-type three-level converters have been widely used in fields such as photovoltaics. In practice, various fault problems may occur in T-type three-level converters. The most common ones are short-circuit faults and open-circuit faults of switching devices, which will have a great impact on the converter system. Short-circuit faults usually bring a large short-circuit current, which will damage the system in a very short time. Therefore, if a short-circuit fault occurs, the system will stop immediately, and there is basically no problem of fault tolerant control. Open-circuit faults will not cause a large short-circuit current, and the system can still maintain its operating state under open-circuit faults, that is, fault tolerant control. Developing a good open-circuit fault tolerant control method is of great significance for improving the reliability of T-type three-level converters. At the same time, there is also a problem of neutral point voltage balance control in T-type three-level converters. How to achieve coordinated control of the neutral point voltage on the basis of open-circuit fault tolerant control is crucial and is also a major problem that needs to be solved urgently at present.

[0003] When an open-circuit fault occurs in the horizontal tube or vertical tube, the T-type three-level converter will not be able to operate normally. Therefore, it is necessary to study the fault tolerant control method of the T-type three-level converter when the switching device has an open-circuit fault (that is, the ability to still ensure the desired output value in the tube fault state). For T-type three-level converters, the carrier-based pulse width modulation technology (PWM) is the most widely used method. When a horizontal tube fails, due to the failure of the horizontal tube, it cannot output the intermediate level. Therefore, only two-level PWM can be used to modulate the faulty phase. However, the remaining two normal phases can still use three-level carrier-based PWM.

[0004] On the premise of ensuring normal three-phase input voltages during open-circuit faults, it is also necessary to study the neutral point voltage balance control problem of T-type three-level converters, because the access of the zero level will introduce a neutral point current, causing the voltage values of the upper and lower two capacitors to deviate from the reference voltage value, resulting in output waveform distortion. To achieve capacitor voltage balance control, a balance control method of injecting zero-sequence voltage can be adopted. Zero-sequence voltage refers to triple-frequency harmonic voltages such as 3, 9, 15, etc. In a three-phase system, injecting zero-sequence voltage can change the reference signal of the phase voltage but does not affect the value of the line voltage. The basic principle of the balance control method of injecting zero-sequence voltage is to generate the required neutral point current. After injecting zero-sequence voltage, the reference voltages of the three-phase voltages will also change, and the generated neutral point current will also change, which can be used to balance the neutral point voltage.

[0005] When an open - circuit fault occurs in the horizontal switch of a three - phase T - type three - level converter, the method of injecting zero - sequence voltage is still used to change the magnitude of the neutral - point current, so as to ensure the balance control of the capacitor voltages above and below the bus. However, different from the condition of injecting zero - sequence voltage to maintain balance control without faults, at this time, due to the open - circuit fault of the horizontal switch in phase A, zero - level voltage cannot be generated, so the required neutral - point current can only be generated by the zero - level action time of phase B and phase C. Therefore, it is necessary to find the optimal zero - sequence voltage and inject it, and then change the reference voltage to change the neutral - point current. If the generated neutral - point current is close to the neutral - point current required to achieve neutral - point voltage balance, the neutral - point voltage balance control can be achieved under the condition of the open - circuit of the horizontal switch.

[0006] However, the existing fault - tolerant control method for open - circuit faults and the mid - point voltage balance control method of the T - type three - level converter only aim at the open - circuit fault of the horizontal switch tube. When an open - circuit fault occurs in the vertical switch tube, the degree of freedom of zero - sequence voltage in carrier - based PWM cannot be used to achieve the mid - point voltage balance during the open - circuit fault of the vertical switch tube of the T - type three - level converter, and new degrees of freedom need to be developed. Summary of the Invention

[0007] The purpose of the present invention is to provide a fault - tolerant control for open - circuit faults and a mid - point voltage balance method of a T - type three - level converter, so as to solve the problem that the existing fault - tolerant control method for open - circuit faults and the mid - point voltage balance control method of the T - type three - level converter can only be aimed at the open - circuit fault of the horizontal switch tube.

[0008] To solve the above - mentioned technical problems, the present invention adopts the following technical solutions:

[0009] A fault - tolerant control for open - circuit faults and a mid - point voltage balance method of a T - type three - level converter, including the following steps:

[0010] The first step: Inject zero - sequence voltage into the faulty phase;

[0011] The second step: Calculate the reference neutral - point current;

[0012] The third step: Calculate the zero - level duty cycle of the non - faulty phase;

[0013] The fourth step: Calculate the modulation signals of double - modulation - wave PWM and output a specific voltage of the T - type three - level converter to achieve mid - point voltage balance.

[0014] A further technical solution is that the specific steps of the first step include: When open - circuit faults occur in both vertical switches of the T - type three - level converter, inject zero - sequence voltage into the faulty phase and clamp it to zero - level. Assume the faulty phase is phase A, and the other two normal phases output three - level phase voltages. Assume the other two normal phases are phase B and phase C respectively. At this time, the magnitude of the injected zero - sequence voltage is the opposite of the reference voltage of phase A. The three - phase reference voltages after injection are as follows:

[0015] ,

[0016] Among them, , , are the three-phase reference voltages after injecting zero-sequence voltage respectively;

[0017] The specific steps of the second step include: calculating the reference neutral-point current to be generated for phases B and C ;

[0018] The specific steps of the third step include: setting the duty cycle of phase B at zero level as , and the duty cycle of phase C at zero level as , then the required reference neutral-point current needs to meet the following conditions:

[0019] ,

[0020] Among them, is the output current of phase B, is the output current of phase C;

[0021] The specific steps of the fourth step include: setting as the original reference voltage of phase X after injecting zero-sequence voltage in the first step, where phase X is any one of the two non-faulty phases, and are the reference voltages of the two vertical tubes of phase X after decomposition respectively. To ensure the volt-second product balance and ensure that the duty cycle of the lower tube in the two vertical tubes of this phase is greater than that of the upper tube, the process of decomposing the modulation wave of phase X is as follows:

[0022] ,

[0023] Among them, is the duty cycle of phase X at zero level;

[0024] Finally, substituting the reference voltage of phase X obtained after injecting zero-sequence voltage in the first step and the duty cycle of zero level obtained in the third step into the modulation wave decomposition formula of phase X, the expression of the double modulation wave of phase X can be obtained:

[0025] .

[0026] A further technical solution is that in the second step, the calculation of the reference neutral-point current is related to the difference between the upper and lower capacitor voltages of the T-type three-level converter and the output current of phase A. Therefore, the calculation formula of the reference neutral-point current is as follows:

[0027] ,

[0028] Among them, is the reference neutral point current of phase B, is the reference neutral point current of phase C, is the upper DC bus capacitor voltage of the T-type three-level converter, is the lower DC bus capacitor voltage of the T-type three-level converter, is the control period, is the output current of phase A.

[0029] A further technical solution is that in the third step, when the directions of the currents of phase B and phase C are both the same as the direction of the reference neutral point current direction, if the neutral point current generated by the output current of phase B at the maximum duty cycle is greater than the required reference neutral point current , then the duty cycle of the zero level of phase C is 0, and the duty cycle of the zero level of phase B is:

[0030] .

[0031] A further technical solution is that in the third step, when the directions of the currents of phase B and phase C are both the same as the direction of the reference neutral point current direction, if the current of phase B generates a neutral point current less than the required reference neutral point current at the maximum duty cycle, then the duty cycle of the zero level of phase B at this time is:

[0032] ;

[0033] The duty cycle of the zero level of phase C is:

[0034] .

[0035] A further technical solution is that in the third step, when only one of the directions of the currents of phase B and phase C is the same as the direction of the reference neutral point current to be generated, assuming that the current direction of phase B is the same as the direction of the reference neutral point current at this time, judge the ratio of the neutral point current generated by the current of phase B at the maximum duty cycle to the required neutral point current , and judge the duty cycle of the zero level of phase B according to the result, that is:

[0036] ,

[0037] At this time, since the current direction of phase C is opposite to the direction of the required neutral point current , the duty cycle of the zero level of phase C is 0.

[0038] A further technical solution is that in the third step, when the directions of the B-phase and C-phase currents are both opposite to the reference current direction, in order to avoid further deteriorating the offset of the midpoint voltage, the duty ratios of the zero levels of the B-phase and C-phase should both be 0, that is:

[0039] .

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0041] Compared with the open-circuit fault tolerance control method and the midpoint voltage balance control method based on the zero-sequence voltage freedom, which can only be used for horizontal tube faults at present, the fault tolerance method based on double modulation wave PWM proposed by the present invention develops new control freedoms. The duty ratio of the zero level is calculated according to the reference midpoint current, and then the expression of the double modulation wave PWM is obtained, without being limited to the zero-sequence voltage freedom that cannot be used in some fault states. Therefore, open-circuit fault tolerance control and midpoint voltage balance can be achieved when a vertical tube open-circuit fault occurs in a T-type three-level converter, and it has a wider application range. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is a schematic diagram of a T-type three-level topology structure in the prior art;

[0043] Figure 2 is a schematic diagram of various open-circuit fault states of a T-type three-level topology in the prior art;

[0044] Figure 3 is the waveform of the three-phase reference voltage after injecting zero-sequence voltage in the present invention;

[0045] Figure 4 is a schematic diagram of the modulation process of the double modulation wave PWM waveform in the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0047] Figures 3 to 4 This is an embodiment of the present invention.

[0048] Embodiment:

[0049] An open-circuit fault tolerance control and midpoint voltage balance method for a T-type three-level converter, comprising the following steps:

[0050] The first step: Inject zero-sequence voltage into the faulty phase:

[0051] When open - circuit faults occur in both vertical tubes of the T - type three - level converter, the faulty phase is clamped to the zero - level by injecting a zero - sequence voltage. Assume the faulty phase is phase A, and the other two normal phases output three - level phase voltages. Assume these two normal phases are phase B and phase C respectively. Since the magnitudes of the three - phase reference voltages input are as follows:

[0052] ,

[0053] To make phase A always output a zero - level voltage, the magnitude of the injected zero - sequence voltage at this time is the opposite of the reference voltage of phase A. The three - phase reference voltages after injection are as follows:

[0054] ,

[0055] where, , , are the three - phase reference voltages after injecting the zero - sequence voltage respectively.

[0056] Step 2: Calculation of the reference neutral - point current:

[0057] Since the neutral - point voltage of the T - type three - level converter is closely related to the neutral - point current, the prerequisite for achieving neutral - point voltage balance is to calculate the neutral - point current that needs to be generated. Since both vertical tubes of phase A have failed and the output neutral - point current magnitude cannot be effectively controlled, for the two non - faulty phases, namely phase B and phase C, calculate the reference neutral - point currents that phases B and C need to generate ;

[0058] where, the reference neutral - point current is related to the difference between the upper and lower capacitor voltages of the T - type three - level converter at the current moment and the current of phase A. Since there is a vertical - tube fault in phase A and it always outputs a zero - level, the output current of phase A has always been the neutral - point current of phase A. In summary, in order to balance the capacitor voltages, the calculation formulas for the reference neutral - point currents that phases B and C need to generate are as follows:

[0059] ,

[0060] where, is the reference neutral - point current of phase B, is the reference neutral - point current of phase C, is the voltage of the upper DC - bus capacitor of the T - type three - level converter, is the voltage of the lower DC - bus capacitor of the T - type three - level converter, is the control period, is the output current of phase A.

[0061] Step 3: Calculation of the zero - level duty cycle of the non - faulty phases:

[0062] Assume that the duty cycle of phase B at zero level is , and the duty cycle of phase C at zero level is . Then the following conditions need to be satisfied to generate the required reference midpoint current :

[0063] ,

[0064] where is the output current of phase B, is the output current of phase C,

[0065] Since this formula has two variables but only one equation, it is difficult to obtain the exact solutions of the two unknowns simultaneously. Therefore, it is necessary to conduct a classified discussion on it:

[0066] Case 1: When the directions of the currents in both phase B and phase C are the same as the direction of the reference midpoint current .

[0067] At this time, the ability of the phase B current to generate the reference current needs to be considered first. If the midpoint current generated by the phase B current at the maximum duty cycle is greater than the reference midpoint current , it means that phase B has the ability to generate the midpoint current completely by itself. Then the duty cycle of phase C at zero level is 0, and the duty cycle of phase B at zero level is:

[0068] ,

[0069] On the contrary, if the midpoint current generated by phase B at the maximum duty cycle is less than the required reference midpoint current , then phase B should use the maximum duty cycle to generate as much midpoint current as possible. Then the duty cycle of phase B at this time is:

[0070] ,

[0071] At this time, the remaining midpoint current should be generated by phase C, that is, the duty cycle of phase C at zero level is:

[0072] ,

[0073] Case 2: When only one of the directions of the currents in phase B and phase C is the same as the direction of the reference midpoint current

[0074] Assume that at this time the direction of the phase B current is the same as the direction of the reference midpoint current. Judge the ratio of the midpoint current generated by the phase B current at the maximum duty cycle to the required reference midpoint current . According to the result, judge the duty cycle of phase B at zero level, that is:

[0075] , ​

[0076] At this time, since the direction of the C-phase current is opposite to the required reference neutral-point current direction, the duty cycle of the C-phase is 0.

[0077] Case 3: When the directions of the B-phase and C-phase currents are both opposite to the reference neutral-point current opposite

[0078] At this time, in order to avoid further deteriorating the offset of the neutral-point voltage, the duty cycles of the zero levels of the B-phase and C-phase should both be 0, that is:

[0079] .

[0080] Fourth step: Calculation of the modulation signals of the dual-modulation-wave PWM and realization of the neutral-point voltage balance by the T-type three-level converter outputting a specific voltage:

[0081] Let be the reference voltage signal of the X-phase after the zero-sequence voltage injection in the first step, where the X-phase is any one of the two non-faulty phases, and are the reference voltage signals of the two vertical tubes of the X-phase respectively after decomposition. In order to ensure the volt-second product balance and ensure that the duty cycle of the lower tube in the two vertical tubes of this phase is greater than that of the upper tube, the process of decomposing the modulation wave of the X-phase is as follows:

[0082] ,

[0083] Finally, substituting the reference voltage signal of the X-phase obtained after the zero-sequence voltage injection in the first step and the duty cycle of the zero level obtained in the third step into the modulation-wave decomposition formula of the X-phase, the expression of the dual-modulation wave of the X-phase can be obtained:

[0084] .

[0085] Then, by the T-type three-level converter outputting the calculated voltage, the neutral-point voltage balance can be achieved.

[0086] Although the present invention has been described herein with reference to a number of illustrative embodiments of the invention, it should be understood that those skilled in the art can design many other modifications and embodiments that will fall within the scope of the principles and spirit of the disclosure of this application. More specifically, within the scope of the drawings and claims of this application, various variations and improvements can be made to the components and / or the layout of the subject combination layout. In addition to the variations and improvements made to the components and / or the layout, other uses will also be apparent to those skilled in the art.

Claims

1. A method for open-circuit fault tolerance control and midpoint voltage balancing of a T-type three-level converter, characterized in that: The following steps are involved: Step 1: Inject zero-sequence voltage into the fault phase; Step 2: Reference midpoint current calculation; Step 3: Calculate the zero-level duty cycle of the non-fault phase; Step 4: Calculate the modulation signal of the dual modulation wave PWM and output a specific voltage of the T-type three-level converter to achieve midpoint voltage balance; The specific steps of the first step include: when both vertical tubes of the T-type three-level converter have an open circuit fault, the zero-sequence voltage injected into the faulty phase is clamped to zero level, the faulty phase is assumed to be phase A, and the other two normal phases output three-level phase voltages, and the other two normal phases are assumed to be phase B and phase C respectively. At this time, the magnitude of the injected zero-sequence voltage is the opposite of the reference voltage of phase A, and the three-phase reference voltage after injection is as follows: ， in, They are the three-phase reference voltages after zero-sequence voltage is injected; The specific steps of the second step include: calculating the reference midpoint current to be generated for phase B and phase C ; The specific steps of the third step include: assuming that the duty cycle of phase B is zero level , the duty cycle of phase C zero level is , then the reference midpoint current The following conditions must be met: ， in, is the output current of phase B, is the output current of phase C; The specific steps of the fourth step include: is the original reference voltage of phase X after the first step of zero-sequence voltage injection, where phase X is any one of the two non-fault phases, and It is the reference voltage of the two vertical tubes of the X phase after decomposition. In order to ensure the balance of the volt-second product and ensure that the duty cycle of the lower tube of the two vertical tubes of this phase is greater than that of the upper tube, the process of decomposing the modulation wave of the X phase is as follows: ， in, is the zero-level duty cycle of phase X; Finally, the reference voltage obtained after the zero-sequence voltage injection of the X phase in the first step and the zero-level duty cycle obtained in the third step are substituted into the modulation wave decomposition formula of the X phase, and the expression of the X phase dual modulation wave can be obtained: 。 2. The method for open-circuit fault tolerance control and midpoint voltage balancing of a T-type three-level converter according to claim 1, characterized in that: In the second step, the reference midpoint current The calculation of is related to the difference between the upper and lower capacitor voltages of the T-type three-level converter and the output current of phase A, so the reference midpoint current The calculation formula is as follows: ， in, is the reference midpoint current of phase B, is the reference midpoint current of phase C, is the upper DC bus capacitor voltage of the T-type three-level converter, is the lower DC bus capacitor voltage of the T-type three-level converter, T is the control period, is the output current of phase A.

3. The method for open-circuit fault tolerance control and midpoint voltage balancing of a T-type three-level converter according to claim 1, characterized in that: In the third step, when the directions of the currents of phases B and C are both consistent with the reference midpoint current When the directions are consistent, if the output current of phase B is The midpoint current generated at the maximum duty cycle is greater than the required reference midpoint current , then the duty cycle of the C phase zero level is 0, and the duty cycle of the B phase zero level is: 。 4. The method for open-circuit fault tolerance control and midpoint voltage balancing of a T-type three-level converter according to claim 1, characterized in that: In the third step, when the directions of the currents of phases B and C are both consistent with the reference midpoint current When the directions are consistent, if the output current of phase B is The midpoint current generated at the maximum duty cycle is less than the required reference midpoint current , then the duty cycle of phase B zero level is: ； The duty cycle of phase C zero level is: 。 5. The method for open-circuit fault tolerance control and midpoint voltage balancing of a T-type three-level converter according to claim 1, characterized in that: In the third step, when only one of the directions of the currents of phase B and phase C is consistent with the reference midpoint current When the directions are consistent, it is assumed that the direction of the current of phase B is consistent with the reference midpoint current. The direction is consistent, judging the midpoint current generated by the B phase output current at the maximum duty cycle and the required midpoint current The duty cycle of phase B zero level is determined based on the result, that is: ， At this time, since the direction of the C phase current is consistent with the required reference midpoint current If the direction is opposite, the duty cycle of the C phase zero level is 0.

6. The method for open-circuit fault tolerance control and midpoint voltage balancing of a T-type three-level converter according to claim 1, characterized in that: In the third step, when the directions of the currents of phases B and C are consistent with the required reference current When the direction is opposite, in order to avoid further deterioration of the offset of the midpoint voltage, the duty cycle of the zero level of phase B and phase C should be 0, that is: 。

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