A vector modulation method for uninterrupted operation of cascaded H-bridge converters under fault conditions

Abstract

The present invention discloses a vector modulation method for uninterrupted operation of a cascaded H-bridge converter under fault conditions, belonging to the technical field of motor drive. Each phase of the cascaded H-bridge converter is composed of n identical H-bridge modules connected in cascade, and the DC voltage input to each H-bridge module is equal to V dc Each H-bridge module includes four power switch devices. The modulation method includes: defining the fault type according to the output voltage characteristics of the submodule, and obtaining the spatial vector diagram of the output voltage under different fault conditions; synthesizing the voltage reference vector V according to the three-phase reference voltage ref , further selecting appropriate vectors for synthesis in the spatial vector diagram of the output voltage under fault. The present invention can determine the selection range of the switch state, the method is simple, and it has strong versatility for different faults. When synthesizing vectors, voltage vectors with small common-mode voltage are used as much as possible and a five-segment sequence generation method is used, which reduces the damage of the common-mode voltage to the system and improves the power conversion efficiency of the entire system.

Description

Technical Field

[0001] The invention belongs to the technical field of motor drive, and in particular relates to a vector modulation method for uninterrupted operation of a cascaded H-bridge converter under fault. Background Art

[0002] As the application of power electronics technology becomes more and more widespread, the voltage that power electronic devices need to handle is also increasing. Limited by the voltage level of current power tube switching devices, traditional two-level inverters are no longer suitable for medium and high voltage levels. As a topological structure for high-voltage and high-power applications, the cascaded H-bridge multi-level converter has the advantages of low switching device stress, low output voltage harmonics, and high DC bus voltage utilization. It has been widely studied in the fields of renewable energy power generation, static VAR generators, and motor drives.

[0003] However, as the number of power switching devices increases, the structure of the converter becomes more complex and the possibility of system failure increases. The failure of the power switch tube will cause a certain degree of distortion in the output of the system, which may impact the entire large power grid in the grid-connected system and affect the quality of the power grid. In industrial systems, converter failure may cause the entire circuit system to stop working, which may cause the factory to stop production or cause serious safety accidents, causing huge losses to the user department and society. Therefore, it is very meaningful to study the fault-tolerant control technology for uninterrupted operation of the converter after the fault, to ensure the symmetrical output of the three-phase electrical quantity, adjustable voltage, and minimal impact on the power quality, so as to have the necessary emergency handling capabilities. After detecting the faulty power switching device, it is necessary to design a fault-tolerant control operation method to achieve uninterrupted operation of the converter when the fault occurs, and make full use of the remaining normal power switching devices to ensure the power quality of the converter operation.

[0004] From the perspective of implementation, the current fault-tolerant control operation methods after a module failure in the system can be divided into two categories, namely, those based on adding hardware and those based on software control algorithms. In the strategy based on adding hardware, it is necessary to add additional H-bridge modules to the cascaded H-bridge multilevel converter circuit structure, which increases the system cost. The software control strategy does not require the addition of additional modules, but it increases the complexity of control. Focusing on the software control strategy, the fault-tolerant modulation algorithms under system failure are mainly divided into space vector modulation using redundant switching states and carrier phase shift modulation with zero-sequence voltage injection.

[0005] Existing literature 1F.Carnielutti,H.Pinheiro and C.Rech,"Generalized Carrier-Based Modulation Strategy for Cascaded Multilevel Converters Operating UnderFault Conditions,"in IEEE Transactions on Industrial Electronics,vol.59,no.2,pp.679-689,Feb.2012 uses a modulation algorithm that adds a modulated wave to a load wave, and injects a common-mode component into the modulated wave to ensure reliable operation of the system after a fault. First, the common-mode voltage range that needs to be injected to ensure reliable operation of the system under different fault conditions is calculated based on the different DC input voltages of the three phases after the fault, and then the average value within the range is selected as the final injected common-mode voltage. This method is simple to implement and is applicable to a variety of fault conditions, but it will cause the common-mode voltage of the system to increase, the overall efficiency to decrease, and the phase-to-phase level jump to increase.

[0006] Existing literature 2L. GGPde Castro and MBR "Afast space-vector algorithm for cascaded H-bridge converters under faulty conditions," 2012 10th IEEE / IAS International Conference on Industry Applications, 2012, pp. 1-7 uses a space vector modulation strategy and simplifies the vector selection process under gh coordinates, which can be used under different fault conditions. However, this method does not provide a method for generating a vector sequence, is complex to implement, and does not optimize common-mode voltage, system efficiency and other indicators.

[0007] Existing literature 3H.Lin,M.Mehrabankhomartash,F.Yang,M.Saeedifard,J.Yang andZ.Shu,"AFlexible Space Vector Modulation Scheme for Cascaded H-BridgeMultilevel Inverters Under Failure Conditions,"in IEEE Transactions onIndustrial Electronics,vol.69,no.12,pp.11856-11867,Dec.2022 uses space vector modulation strategy to ensure the normal operation of the system under faults. Different switching sequences can be given according to different optimization objectives such as: small switching times, good harmonic characteristics, and power sharing. However, it is not possible to unify vector generation and sequence generation for different fault conditions, and the implementation is complicated. Summary of the invention

[0008] In view of the defects and shortcomings of the above-mentioned modulation method in fault-tolerant control under the fault of the cascaded H-bridge converter, the present invention proposes a vector modulation method for uninterrupted operation under the fault of the cascaded H-bridge converter. The method can ensure the uninterrupted operation of the converter under different fault conditions, and can output a line voltage with three-phase symmetry and the highest DC voltage utilization rate. At the same time, a general switching sequence generation method is given to select appropriate switching states and switching sequences to reduce the common-mode voltage and improve system efficiency.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is:

[0010] A vector modulation method for uninterrupted operation of a cascaded H-bridge converter under fault conditions, wherein each phase of the cascaded H-bridge converter is composed of n identical H-bridge modules in cascade, and the DC voltage input to each H-bridge module is equal to V dc Each H-bridge module includes four power switching devices, which are defined as W xn1 ,W xn2 ,W xn3 ,W xn4 , x = a, b, c, n is the number of cascaded full bridges in each phase;

[0011] The modulation method comprises:

[0012] According to the output voltage characteristics of the submodules, the fault type is defined, and the spatial vector diagram of the output voltage under different fault conditions is obtained;

[0013] The voltage reference vector V is obtained by synthesizing the three-phase reference voltage ref, and further select appropriate vectors for synthesis in the spatial vector diagram of the output voltage under fault.

[0014] As a further improvement of the present invention, different three-phase switch tube fault conditions are quantified into different three-phase voltage output ranges, and then H i D i The simplified characteristics of the coordinate system select a five-segment switching sequence with a small common-mode voltage from a variety of synthesis sequences within the three-phase voltage output range as the final output switching sequence for synthesizing the reference vector.

[0015] As a further improvement of the present invention, H i D i The rotationally symmetric coordinate system transforms the three-phase ABC coordinate system and converts the reference vector V ref Always locate in a rhombus, select the rhombus vertex closest to the origin as the relative zero vector V 0 The coordinate position of V 0 H i D i Coordinate value; then determine whether the reference vector is located in the regular triangle or inverted triangle of the rhombus, determine the three vectors closest to the reference vector in the modulation triangle as the synthetic basic vectors, and calculate the action time of the three vectors.

[0016] As a further improvement of the present invention, the relative zero vector V 0 The switching state of the phase is taken as the starting state of the sequence, and it is ensured that at most one phase of the switching state is changed at any time in the switching sequence, and only one level is jumped at a time; a five-segment sequence is generated by discontinuous PWM, starting from the relative zero vector, a five-segment subtraction sequence and a five-segment addition sequence are obtained.

[0017] As a further improvement of the present invention, when V ref After synthesizing the switching state range, select the switching state with the smallest common mode voltage; select the switching sequence with smaller common mode voltage under the two generation modes of five-segment addition sequence and five-segment subtraction sequence as the switching sequence of the synthesized reference vector.

[0018] As a further improvement of the present invention, different fault conditions are attributed to different ABC three-phase voltage output ranges; the ABC three-phase voltage output range is used to restrict the selection of the three-phase switch state in the switch sequence, so as to obtain a reference vector V that satisfies the fault condition. ref The synthetic three-phase switch state selection range.

[0019] As a further improvement of the present invention, for different sequence generation modes, different regions and different modulation triangles, a certain phase switch state k relative to the zero vector i The selection range has the following constraints:

[0020]

[0021]

[0022] Among them, for H 1 D 1 It is phase A, k 1 ; for H 2 D 2 is phase B, k 2 ; for H 3 D 3 It is phase C, k 3 , -n aL and n aH It is the lower and upper limits of the output level after a fault occurs in phase A. bL and n bH It is the lower and upper limits of the output level after a fault occurs in phase B. cL and n cH are the lower and upper limits of the output level after a fault occurs in phase C; f H1 and f D1 is the relative zero vector at H 1 D 1 The HD coordinate value in the region represents the relationship between the three-phase level states of ABC. Similarly, f H2 and f D2 is the relative zero vector at H 2 D 2 HD coordinate value within the area, f H3 and f D3 is the relative zero vector at H 3 D 3 The HD coordinates within the area.

[0023] As a further improvement of the present invention, in a certain phase switch state k relative to the zero vector i In the selection range of , the switching sequence common mode voltage is small k i The selection method is as follows:

[0024]

[0025] where k imin and k imax k is the value of k under different fault conditions i Optional minimum and maximum values, k icmv_min and k icmv_max The minimum and maximum values ​​of the switching states in which the common-mode voltage is closest to 0 are calculated to obtain the space vectors that maximize and minimize the common-mode voltage in the switching sequence.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The present invention adopts H i D i The rotational symmetric coordinate system is used for coordinate transformation, and the symmetry is also very strong in the subsequent selection of switch states and generation of switch sequences, which simplifies the implementation process of the entire algorithm; the five-segment sequence generation method is used, and there is a one-phase output voltage clamp in each switching cycle, which can reduce the number of switches and improve the power conversion efficiency of the system; the proposed switch sequence generation method changes the switch state of at most one phase at any time, and only jumps one level at a time, which reduces the number of switch tube actions and improves the reliability of system operation; the three-phase switch state is combined with the three-phase voltage output range under the fault, and the selection range of the switch state can be determined through basic inequality constraints. The method is simple and has strong versatility for different faults; the optimization goal of small common mode voltage is considered when selecting the switch state, and the switch state with small common mode voltage is used to generate the switch sequence, which greatly reduces the damage of the common mode voltage to the entire system. The present invention can determine the selection range of the switch state, the method is simple, and has strong versatility for different faults. In vector synthesis, voltage vectors with small common-mode voltage are used as much as possible and a five-segment sequence generation method is used, which reduces the damage of common-mode voltage to the system and improves the power conversion efficiency of the entire system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A topological structure diagram of a (2n+1)-level cascaded H-bridge multi-level converter adopted by the present invention;

[0029] Figure 2 The topological structure diagram of a single H-bridge module of the circuit analyzed by the present invention, (a) is the topological structure diagram of a single H-bridge module of the circuit analyzed by the present invention, (b) is the failure of the entire submodule, (c) is a failure condition of the P-HB module, and (d) is a failure condition of the N-HB module;

[0030] Figure 3 is the spatial vector diagram of the output voltage of the five-level converter used in the analysis of the present invention under different fault conditions, where Figure 3 (a) A module in phase A has a P-HB module fault due to bridge arm damage. Figure 3 (b) A phase fault is a whole module, Figure 3 (c) One whole module is faulty in phase A and two whole modules are faulty in phase B;

[0031] Figure 4 The present invention is based on H i D i Space vector diagram in (i=1,2,3) coordinate system;

[0032] Figure 5There are two switch sequence generation methods used in the present invention, where (a) is to select the vector clockwise, which is defined as a five-segment subtraction sequence; (b) is to select the vector counterclockwise, which is defined as a five-segment addition sequence;

[0033] Figure 6 It is a flowchart of the overall process of realizing the vector modulation method for uninterrupted operation of the cascaded H-bridge converter under fault conditions proposed in the present invention;

[0034] Figure 7 The simulation result diagram of the phase voltage, line voltage and load current output by the converter according to the maximum DC voltage bus utilization rate under different fault conditions using the method of the present invention;

[0035] Figure 8 The simulation result diagram of the phase voltage, line voltage and load current output by the converter with the same modulation index under different fault conditions using the method of the present invention;

[0036] Fig. 9 The figure is a simulation result diagram of the common mode voltage output by the converter with different fault-tolerant modulation methods;

[0037] Fig.10 This is the phase voltage simulation result diagram of different fault-tolerant modulation methods when the output power quality is basically the same. DETAILED DESCRIPTION

[0038] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0039] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0040] The present invention provides a vector modulation method for uninterrupted operation of a cascaded H-bridge converter under fault conditions. Each phase of the cascaded H-bridge converter is composed of n identical H-bridge modules in cascade, and the DC voltage input to each H-bridge module is equal to V dc Each H-bridge module includes four power switching devices, which are defined as W xn1 ,W xn2 ,W xn3 ,W xn4 , x = a, b, c, n is the number of cascaded full bridges in each phase;

[0041] The modulation method comprises:

[0042] According to the output voltage characteristics of the submodules, the fault type is defined, and the spatial vector diagram of the output voltage under different fault conditions is obtained;

[0043] The voltage reference vector V is obtained by synthesizing the three-phase reference voltage ref , and further select appropriate vectors for synthesis in the spatial vector diagram of the output voltage under fault.

[0044] Furthermore, the three-phase different switch tube fault conditions are quantified into different three-phase voltage output ranges, and then H i D i The simplified characteristics of the coordinate system select a five-segment switching sequence with a small common-mode voltage from a variety of synthesis sequences within the three-phase voltage output range as the final output switching sequence for synthesizing the reference vector.

[0045] Furthermore, using H i D i The rotationally symmetric coordinate system transforms the three-phase ABC coordinate system and converts the reference vector V ref Always locate in a rhombus, select the rhombus vertex closest to the origin as the relative zero vector V 0 The coordinate position of V 0 H i D i Coordinate value; then determine whether the reference vector is located in the regular triangle or inverted triangle of the rhombus, determine the three vectors closest to the reference vector in the modulation triangle as the synthetic basic vectors, and calculate the action time of the three vectors.

[0046] Furthermore, the relative zero vector V 0 The switching state of the phase is taken as the starting state of the sequence, and it is ensured that at most one phase of the switching state is changed at any time in the switching sequence, and only one level is jumped at a time; a five-segment sequence is generated by discontinuous PWM, starting from the relative zero vector, a five-segment subtraction sequence and a five-segment addition sequence are obtained.

[0047] Further, under the condition that V refAfter synthesizing the switching state range, select the switching state with the smallest common mode voltage; select the switching sequence with smaller common mode voltage under the two generation modes of five-segment addition sequence and five-segment subtraction sequence as the switching sequence of the synthesized reference vector.

[0048] Furthermore, different fault conditions are attributed to different ABC three-phase voltage output ranges; the ABC three-phase voltage output range is used to restrict the selection of the three-phase switch state in the switch sequence, so as to obtain the reference vector V that meets the fault condition. ref The synthetic three-phase switch state selection range.

[0049] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments:

[0050] Figure 1 The topological structure diagram of the cascaded H-bridge multi-level converter with (2n+1) levels, each phase is composed of n identical H-bridge modules in cascade, and the DC voltage input to each module is assumed to be equal to V dc Each H-bridge module includes four power switching devices, which are defined as W xn1 ,W xn2 ,W xn3 ,W xn4 , x = a, b, c, n is the number of cascaded full bridges in each phase, such as Figure 2 The output voltage of each module in different switching states is given in Table 1.

[0051] Table 1 Module output voltage in different switching states

[0052]

[0053] The "1" and "0" in the table represent the on and off states of the device, respectively. According to the output voltage characteristics of the submodule, typical faults can be divided into three types. The first type is that the output voltage is 0, and the submodule must be bypassed because multiple switching devices are damaged, such as Figure 2 (b) The second output is 0 or V dc , for example, when W xn1 Because of the short circuit caused by damage, Figure 2 (c) In order to prevent the bridge arm from being directly turned on, W xn3 should remain off, but W xn2 and W xn4 Can be turned on or off, at this time the module cannot output -V dc , can only output 0 or V dc In the present invention, such a submodule is defined as P-HB; the third output is 0 or -V dc , for example, when W xn1 Because of damage and circuit breaker, Figure 2 As shown in (d), in order to prevent the bridge arm from being directly turned on, W xn3 Should remain open, but W xn2 and W xn4 Can be turned on or off, at this time the module cannot output V dc , can only output 0 or -V dc In the present invention, such a submodule is defined as N-HB. According to the output voltage characteristics of the submodule under different fault types, Figure 3 The spatial vector diagram of the output voltage of the five-level cascaded H-bridge converter under different fault conditions is given. Figure 3 (a) A module in phase A has a P-HB module fault due to bridge arm damage. Figure 3 (b) A phase fault is a whole module, Figure 3 (c) shows one whole module for phase A fault and two whole modules for phase B fault, where the red area is the voltage space vector area that can be output after the fault, and the black dotted circle is the reference vector area that can be synthesized.

[0054] The three-phase reference voltages are defined as V a ,V b and V c , the voltage reference vector can be calculated as:

[0055]

[0056] Where V ref and θ are the amplitude and phase angle of the voltage reference vector, respectively. The following are steps for synthesizing the voltage reference vector using the method of the present invention to ensure uninterrupted operation of the converter under converter fault conditions:

[0057] 1) Coordinate transformation

[0058] Figure 4 Given the H i D i The space vector diagram in the (i=1,2,3) coordinate system divides the original hexagonal vector area into three parts, namely H 1 D 1 ,H 2 D 2 and H 3 D 3 Among them, H 1 D 1 The transformation formula of the coordinate system is:

[0059]

[0060] H 2 D 2 The transformation formula of the coordinate system is:

[0061]

[0062] H 3 D 3 The transformation formula of the coordinate system is:

[0063]

[0064] 2) Modulation triangle positioning

[0065] For H i D i (i=1,2,3) V in any coordinate system ref The same process is done for the reference vector:

[0066]

[0067] It can be found that no matter V ref Where is it located, equation (5) can always obtain the vector V closest to the origin among the four voltage vectors in the diamond area 0 For any region, select V 0 is the relative zero vector, then V r ' ef The coordinates of are calculated as:

[0068]

[0069] V can be located by judging the dividing line of the two modulation triangles ref The modulation triangle in which we are located:

[0070]

[0071] 3) Calculation of vector action time

[0072] V 0 Assuming it as a zero vector, analyze V r ' ef The synthetic process of V ref The vector action time, when V r ' ef When it is in an inverted triangle, it is easy to know:

[0073]

[0074] Then use V 1 and V 2 You can synthesize V r ' ef :

[0075] V r ' ef =d 1 ·(Vright -V 0 )+d 2 ·(V down -V 0 ) (9)

[0076] In the formula, d 0 ,d 1 and d 2 V 0 ,V 1 and V 2 The action time of can be calculated by equation (6), equation (8) and equation (9):

[0077]

[0078] Substituting formula (8) into formula (9), we can get:

[0079] V ref =(1-d 1 -d 2 )·V 0 +d 1 ·V right +d 2 ·V down (11)

[0080] It can be found that d 0 ,d 1 and d 2 Also corresponding to V 0 ,V right and V down Similarly, when it is located in an equilateral triangle, there is;

[0081]

[0082] Also use V 1 and V 2 Synthetic V r ' ef :

[0083] V r ' ef =d 1 ·(V right -V 0 )+d 2 ·(V up -V 0 ) (13)

[0084] Easy to get:

[0085]

[0086] Among them, d 0,d 1 and d 2 Also corresponding to V 0 ,V right and V up action time.

[0087] 4) DPWM switching sequence generation

[0088] Define the relative zero vector V 0 The switching state of the ABC three-phase is (S a ,S b ,S c ), according to H i D i The coordinate transformation principle of the (i=1,2,3) coordinate system can obtain different regions S a ,S b ,S c The relationship between. Define V 0 In region 1, the switching state of phase A is k 1 , in region 2, the switching state of phase B is k 2 , in region 3, the switching state of phase C is k 3 , we can calculate V 0 Expressions for switch states in different regions.

[0089]

[0090] Get the relative zero vector V 0 After the switch state is determined, the relative zero vector V 0 The switching state generates a switching sequence to synthesize the vector. In order to reduce the number of switches and prevent sudden level changes, the present invention selects three nearest vectors to generate a switching sequence based on the modulation triangle where the reference vector is located, and ensures that at most one phase of the switching state is changed at any time in the sequence, and only one level is jumped at a time. The five-segment sequence generation method is based on DPWM, and there is a phase clamp in each switching cycle, which can further reduce the number of switches, thereby improving the power conversion efficiency of the system.

[0091] Since V 0 The position of has been determined. There are only two ways to generate a five-segment switch sequence. The first is to select the vector clockwise, which is defined as a five-segment reduction sequence, such as Figure 5 (a); the other is to select vectors counterclockwise, defined as a five-segment addition sequence, such as Figure 5 As shown in (b), when the region where the vector is located, the type of modulation triangle, and the sequence generation method are determined, a unique switching sequence can be obtained based on the reference vector.

[0092] 5) Selection of switch state for common mode voltage reduction

[0093] Reference vector V ref By V 0 ,V right and V down Synthesis, the five-segment addition sequence or the five-segment subtraction sequence can give the vector V 0 ,V right and V down The relative relationship between the respective switch states, if the reference zero vector V is known 0 The switch state can be known by the synthetic reference vector V ref The switching sequence of k i (i=1,2,3), the other switch state values ​​in the entire sequence can be calculated according to the sequence generation method.

[0094] V dc Normalized to 1, the output voltage range of phase A is defined as [-n aL ,n aH ], the B phase voltage output range is [-n bL ,n bH ], the C phase voltage output range is [-n cL ,n cH ]. According to the output voltage characteristics of the fault condition, the voltage range that the ABC three-phase voltage can output under different fault conditions can be obtained. For example, there are 5 H-bridge modules in phase A, one of which, a1, is completely bypassed due to a fault, and another module, a2, can only output 0 and V due to damage to the bridge arm. dc , that is, P-HB, then the voltage range that phase A can output is [-3,4].

[0095] After obtaining the output range of the three-phase voltage, the switch sequence for generating the reference vector needs to be within the voltage range. Taking the decreasing sequence of the equilateral triangle in region 1 as an example, in order to synthesize the vector, the ABC three-phase switch states of the entire sequence need to be within the ABC three-phase voltage output range, that is, to satisfy inequalities (16) and (17):

[0096]

[0097]

[0098] Simplifying, we can get k 1 The value range of is:

[0099]

[0100] Similarly, k i The value range of (i=1,2,3) can be calculated, as shown in Table 2.

[0101] Table 2 k in different modulation triangles i The value range of

[0102]

[0103]

[0104] according to Figure 1 The cascaded H-bridge topology diagram can be used to write the Kirchhoff voltage equation:

[0105]

[0106] Calculate the common mode voltage u on for:

[0107] u on =(u an +u bn +u cn ) / 3∝(S a +S b +S c ) / 3 (20)

[0108] In order to minimize the common-mode voltage, the common-mode voltage calculated from the switching states with the largest and smallest common-mode values ​​in the switching sequence should be as close to 0 as possible. For the five-stage reduction sequence:

[0109]

[0110] Simplified:

[0111]

[0112] Similarly, for the five-stage addition sequence:

[0113]

[0114] It can be seen from equations (22) and (23) that in k icmv_min and k icmv_max Select k between i The common-mode voltage can be reduced, but k icmv_min and k icmv_max Not necessarily an integer and k i The selection must be based on k imin and k imax Between as a premise, combined with k icmv_min ,k icmv_max ,k imin and k imax The size relationship between the common mode voltage and the minimum k i The selection is as shown in formula (24):

[0115]

[0116] Since there are two ways to generate the switch sequence, namely the subtraction sequence and the addition sequence, it is necessary to further determine the sequence generation method with a smaller common-mode voltage under the two sequence generation methods. Using the subtraction sequence generation method, we can obtain k using equation (24): i The value is k i1 ; Using the decreasing sequence generation method, we can get k using formula (24) i The value is k i2 , the expression of the common-mode voltage in the middle switching state of the two sequences can be obtained as:

[0117]

[0118] So the final common-mode voltage is the smallest k i The selection method is:

[0119]

[0120] The vector modulation method proposed by the present invention for continuous operation of the cascaded H-bridge converter under fault conditions is used to realize the synthesis of the reference vector V ref The overall process flow chart is as follows Figure 6 shown.

[0121] In order to verify the correctness of the theoretical analysis of the above fault-tolerant modulation method, the present invention provides a design example. The module input DC voltage V dc =1kV, number of modules per phase N = 5, DC side capacitance C f =400uF, fundamental frequency f o =50Hz, switching frequency f s =10kHz, load resistance R g =10Ω, load inductance L g =20mH.

[0122] Figure 7 The simulation results of the phase voltage, line voltage and load current output by the converter according to the maximum DC voltage bus utilization rate under different fault conditions are given. Condition 1 is a normal situation, and there are no faults in the three phases; under condition 2, the fault of phase A is one whole module and one P-HB module due to bridge arm damage, the fault of phase B is one P-HB module due to bridge arm damage, and phase C is faultless; under condition 3, the fault of phase A is two whole modules, the fault of phase B is one whole module, and phase C is faultless. It can be seen that under different fault types, the converter can output three-phase symmetrical line voltage with the highest DC voltage utilization rate.

[0123] Figure 8The simulation results of the phase voltage, line voltage and load current output by the converter with the same modulation index under the above different fault conditions are given. It can be seen that under different fault types, the converter can output three-phase symmetrical and constant line voltage and load current.

[0124] Fig. 9 The simulation results of the common mode voltage output by the converter of different fault tolerant modulation methods are given under the conditions of one whole module of phase A fault, one whole module of phase B fault, and no fault in phase C. It can be seen that the effective value of the common mode voltage can be significantly reduced by using the switch state selection and switch sequence generation method of the present invention.

[0125] Fig.10 The simulation results of phase voltage of different fault-tolerant modulation methods are given when the output power quality is basically the same. It can be seen that the phase voltage has level clamping when the switching sequence generation method of the present invention is used, which can reduce the number of switching times and improve the power conversion efficiency of the system.

[0126] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A vector modulation method for uninterrupted operation of a cascaded H-bridge converter under fault conditions, characterized in that: Each phase of the cascaded H-bridge converter is composed of n identical H-bridge modules in cascade, and the DC voltage input to each H-bridge module is equal to ; Each H-bridge module includes four power switching devices, which are defined as , n is the number of cascaded full bridges in each phase; The modulation method comprises: According to the output voltage characteristics of the submodules, the fault type is defined, and the spatial vector diagram of the output voltage under different fault conditions is obtained; The voltage reference vector is obtained by synthesizing the three-phase reference voltage , the corresponding vector is further selected in the spatial vector diagram of the output voltage under fault to be synthesized; The three-phase different switch tube fault conditions are quantified into different three-phase voltage output ranges, and then H i D i The simplified characteristics of the coordinate system select a five-segment switching sequence with a small common-mode voltage from a variety of synthesis sequences within the three-phase voltage output range as the final output switching sequence for synthesizing the reference vector; Using H i D i The rotationally symmetric coordinate system transforms the three-phase ABC coordinate system and converts the reference vector Always locate in a rhombus, select the rhombus vertex closest to the origin as the relative zero vector The coordinate position of H i D i coordinate values; then determine whether the reference vector is located in an equilateral triangle or an inverted triangle of a rhombus, determine the three vectors closest to the reference vector in the modulation triangle as the synthetic basic vectors, and calculate the action time of the three vectors; The relative zero vector The switching state of the phase is taken as the starting state of the sequence, and it is ensured that at any time in the switching sequence, only one phase of the switching state is changed, and only one level is jumped at a time; a five-segment sequence is generated by discontinuous PWM, starting from the relative zero vector, a five-segment subtraction sequence and a five-segment addition sequence are obtained; Under the satisfied failure After synthesizing the switch state range, select the switch state with the smallest common mode voltage; select the switch sequence with the smaller common mode voltage under the two generation modes of the five-segment addition sequence and the five-segment subtraction sequence as the switch sequence of the synthesized reference vector; Different fault conditions are attributed to different ABC three-phase voltage output ranges; the ABC three-phase voltage output range is used to restrict the selection of three-phase switch states in the switch sequence, so as to obtain the reference vector that meets the fault condition. The selection range of the synthesized three-phase switch state; The switching state of a phase relative to the zero vector in different sequence generation methods, different regions and different modulation triangles k i The selection range has the following constraints: Among them, H1D1 is phase A, which is k 1; for H2D2, it is phase B, k 2; For H3D3, it is C phase, k 3. It is the lower and upper limits of the output level after a fault occurs in phase A. It is the lower and upper limits of the output level after a fault occurs in phase B. It is the lower and upper limits of the output level after a fault occurs in phase C; is the relative zero vector at In the region HD The coordinate value represents the relationship between the three-phase level states of ABC. Similarly, is the relative zero vector at In the region HD Coordinate values, is the relative zero vector at In the region HD Coordinate value.

2. The vector modulation method for uninterrupted operation of the cascaded H-bridge converter under fault according to claim 1, characterized in that: In a phase switch state relative to zero vector k i In the selection range, the switching sequence common mode voltage is small k i The selection method is as follows: in and For different fault conditions k i The minimum and maximum values ​​of and The minimum and maximum values ​​of the switching states in which the common-mode voltage is closest to 0 are calculated to obtain the space vectors that maximize and minimize the common-mode voltage in the switching sequence.

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