CN109951097B - MMC space vector modulation method based on submodule recombination and having fault tolerance - Google Patents

MMC space vector modulation method based on submodule recombination and having fault tolerance Download PDF

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CN109951097B
CN109951097B CN201910220489.6A CN201910220489A CN109951097B CN 109951097 B CN109951097 B CN 109951097B CN 201910220489 A CN201910220489 A CN 201910220489A CN 109951097 B CN109951097 B CN 109951097B
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白志红
陈浩奇
马皓
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Zhejiang University ZJU
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Abstract

The invention discloses an MMC space vector modulation method based on submodule recombination and with fault tolerance, which comprises the following steps: (1) combining any two or more sub-modules of each phase in the MMC to form a plurality of new three-phase control units called as sub-units; (2) when the MMC normally operates, a space vector modulation strategy is adopted to independently control each subunit, the sampling time of each subunit is staggered, and the balance of the capacitance and voltage of each submodule is kept through the alternation of switching signals of the submodules in the subunits and among the subunits; (3) when the MMC submodule has an open-circuit fault, the structure of the MMC is recombined according to the fault condition, and the redundant vector is used for acting on the subunit to ensure the fault-tolerant operation of the MMC. By using the invention, the fault-tolerant operation of the MMC can be realized through the flexibility of recombination and space vector modulation under the condition of not increasing additional redundant backup and peripheral circuits.

Description

MMC space vector modulation method based on submodule recombination and having fault tolerance
Technical Field
The invention belongs to the field of power electronic equipment modulation strategies, and particularly relates to an MMC space vector modulation method based on submodule recombination and having fault tolerance.
Background
The MMC is considered as one of the most typical power electronic conversion topologies applied to medium-high voltage occasions in the future by students, and has been developed rapidly in the field of flexible direct current transmission. The MMC not only has the advantages of high modular structure, low switching frequency, low output harmonic content and easy expansion of power level, but also has the more important advantage that the direct current side of the MMC can be supplied with power through a public direct current bus without a heavy and expensive phase-shifting transformer, so that the integral structure of the system is simpler.
The modulation strategy is the basis of normal operation of the MMC and has direct influence on the harmonic waves of the output voltage of the MMC. The modulation strategies currently used in MMC have the latest level approximation, multi-carrier based pulse width modulation and space vector modulation.
The recent level approximation method approximates the sine wave by using the step wave instantaneous value, is more suitable for occasions with larger number of submodules, and has serious output distortion caused by too few steps of output voltage of the recent level approximation method when the number of the submodules is less.
The pulse width modulation based on the multi-carrier has lower switching harmonic waves and is suitable for occasions with fewer sub-modules, but the switching mode of the converter is fixed, the flexibility is lower, and the direct current utilization rate is not high.
The space vector modulation mainly combines voltage vectors of different switch states of the converter, thereby realizing vector tracking of given voltage, and having the advantages of simple digital realization, small output harmonic wave, realization of redundancy fault tolerance, high direct current utilization rate and the like. But as the number of MMC submodules increases, the number of basis vectors increases by a factor of three, which greatly increases the complexity of modulation. Commonly used are vector decomposition methods and methods that introduce two-level space vector modulation. The vector decomposition method is that a series of complex decomposition is carried out on a reference voltage vector, and finally final synthesis is completed in two-level space vector distribution, but the decomposition process is complex and the calculation amount is large; the method of introducing two-level space vector modulation (such as the patents with application numbers of 201710092797.6 and 201810913697. X) can better introduce a space vector modulation method and improve the utilization rate of direct-current voltage, but is limited to two-level space vectors, has no redundant vectors, is relatively fixed in sub-module control, and cannot exert the flexibility and the redundancy of a space vector modulation strategy.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides an MMC space vector modulation method based on submodule recombination and having fault tolerance, which can realize fault-tolerant operation of the MMC through the flexibility of recombination and space vector modulation under the condition of not increasing additional redundant backup and peripheral circuits.
A Modular Multilevel Converter (MMC) space vector modulation method based on submodule recombination and with fault tolerance comprises the following steps:
(1) combining any two or more sub-modules of each phase in the MMC to form a plurality of new three-phase control units called as sub-units;
(2) when the MMC normally operates, a space vector modulation strategy is adopted to independently control each subunit, the sampling time of each subunit is staggered, and the balance of the capacitance and voltage of each submodule is kept through the alternation of switching signals of the submodules in the subunits and among the subunits;
(3) when the MMC submodule has an open-circuit fault, the structure of the MMC is recombined according to the fault condition, and the redundant vector is used for acting on the subunit to ensure the fault-tolerant operation of the MMC.
The invention combines two or more than two submodules in the same phase with the bridge arm to form a new three-phase control unit, which is called as a subunit, thereby not only facilitating the centralized control of the submodules, but also simplifying the realization of a space vector modulation algorithm; the invention achieves the capacitance-voltage balance of the sub-modules by rotating the switching signals of the sub-modules in each sub-unit and rotating the switching signals of the same bridge arm sub-unit; the invention recombines the MMC after the fault of the MMC, and the MMC system has certain fault-tolerant characteristic by using the redundant vector.
Preferably, in the step (1), two sub-modules of the same phase and the same bridge arm in the MMC are combined in pairs to form a plurality of sub-units. Taking two sub-module combinations per phase as an example, each sub-unit can output 2U per phasecap、UcapAnd three voltages of 0, corresponding to P, O, N switch states, respectively. The situation and modulation method of each phase of a plurality of sub-module combinations are analogized in turn.
The specific process of the step (2) is as follows:
(2-1) when the MMC operates normally, according to the topology of the MMC, the voltage U of the direct current bus is knowndcAnd an output phase voltage uioObtaining output reference voltages of each phase of an upper bridge arm and a lower bridge arm (i is u, v, w), wherein u, v and w are three phases respectively; for example, the output reference voltages of the u-phase upper and lower bridge arms are respectively:
Figure BDA0002003461190000031
and
Figure BDA0002003461190000032
(2-2) synthesizing a reference voltage vector u for the first subunit of the upper arm according to the output reference voltagerefAnd performing Clark transformation to obtain the value u of the reference voltage vector under α and β coordinatesα、uβSo as to obtain the included angle between the reference voltage vector and the α axis through triangular transformation
Figure BDA0002003461190000033
(2-3) dividing the space vector distribution of the subunits into 6 large sectors according to the basic vector, further dividing each large sector into 6 small areas according to the obtained included angle
Figure BDA0002003461190000034
And determining a large sector where the reference voltage vector is located, and uniformly calculating the reference voltage vector to the sector I according to the symmetry of the sector.
In the basic vectors, there are 3 zero vectors, which are (PPP), (OOO) and (NNN), respectively; there are 12 small vectors, respectively (POO), (ONN), (PPO), (OON), (OPO), (NON), (OPP), (NOO), (OOP), (NNO), (POP) and (ONO); there are 6 medium vectors, which are respectively (PON), (OPN), (NPO), (NOP), (ONP) and (PNO); there are 6 large vectors, which are (PNN), (PPN), (NPN), (NPP), (NNP), and (PNP), respectively. The way to uniformly reduce the reference voltage vector to the I-th sector is as follows:
(2-3-1) if
Figure BDA0002003461190000041
In this case, the reference voltage vector is located in sector I, and the pair u is not neededα、uβAnd
Figure BDA0002003461190000042
carry out a reduction, i.e.
Figure BDA0002003461190000043
u′α＝uα，u′β＝uβ；
(2-3-2) if
Figure BDA0002003461190000044
The reference voltage vector is located in the second sector and returns to the first sector
Figure BDA0002003461190000045
(2-3-3) if
Figure BDA0002003461190000046
The reference voltage vector is now in sector III, reduced to sector I, and now
Figure BDA0002003461190000047
(2-3-4) if
Figure BDA0002003461190000048
The reference voltage vector is now in sector IV, reduced to sector I, and now
Figure BDA0002003461190000049
u′α＝-uα，u′β＝-uβ；
(2-3-5) if
Figure BDA00020034611900000410
The reference voltage vector is located in the V-th sector and returns to the I-th sector
Figure BDA00020034611900000411
(2-3-6) if
Figure BDA00020034611900000412
When the reference voltage vector is located in the V-th sector, when
Figure BDA00020034611900000413
(2-4) based on the reduced included angle
Figure BDA00020034611900000414
And the post-calculation coordinate value u'αAnd u'βThe distance from the reference voltage vector to the sector edge is calculated to determine the small region number where the reference voltage vector is located, and the distance to the α Axis is Axis0 ═ u'β(ii) a To the left edge of sector I
Figure BDA00020034611900000415
Figure BDA00020034611900000416
To the farthest edge of the I sector
Figure BDA00020034611900000417
Reference distance
Figure BDA00020034611900000418
If Distance < Axis0 < ═ 2 Distance, then the reference voltage vector is located in small region 4;
if Distance < Axis1 < ═ 2 Distance, then the reference voltage vector is in small region 3;
if 0 < Axis2 < ═ Distance and
Figure BDA00020034611900000419
the reference voltage vector is located in a small area 1;
if 0 < Axis2 < ═ Distance and
Figure BDA00020034611900000420
the reference voltage vector is located in a small area 5;
if 0 < Axis0 < ═ Distance and 0 < Axis1 < ═ Distance and Distance < Axis2 < ^ 2 < ^ Distance and
Figure BDA0002003461190000051
the reference voltage vector is located in small region 2;
if 0 < Axis0 < ═ Distance and 0 < Axis1 < ═ Distance and Distance < Axis2 < ^ 2 < ^ Distance and
Figure BDA0002003461190000052
the reference voltage vector is now located in small area 6;
(2-5) after determining the small region where the reference voltage vector is located, selecting the nearest basic vector for synthesis by using a volt-second balance principle; the principle of selecting the order of action of the base vectors is as follows: the switching state of any phase avoids switching between the two states P, N directly as much as possible; when voltage space vectors are switched, the switching state of a certain phase is ensured to be changed as much as possible. Taking sector I as an example:
when the reference voltage vector is located in the small region 1, the vector action sequence is as follows: (ONN) → (OON) → (OOO) → (POO) → (OOO) → (OON) → (ONN);
when the reference voltage vector is located in the small region 2, the vector action sequence is as follows: (ONN) → (OON) → (PON) → (POO) → (PON) → (OON) → (ONN);
when the reference voltage vector is located in the small region 3, the vector action sequence is as follows: (ONN) → (PNN) → (PON) → (POO) → (PON) → (PNN) → (ONN);
when the reference voltage vector is located in the small region 4, the vector action sequence is as follows: (OON) → (PON) → (PPN) → (PPO) → (PPN) → (PON) → (OON);
when the reference voltage vector is located in the small region 5, the vector action sequence is: (OON) → (OOO) → (POO) → (PPO) → (POO) → (OOO) → (OON);
when the reference voltage vector is located in the small region 6, the vector acts in the order: (OON) → (PON) → (POO) → (PPO) → (POO) → (PON) → (OON);
(2-6) because the reference voltage vector is a rotating space vector, in order to achieve better output effect, different subunits of the same bridge arm sample the reference voltage vector in a staggered time manner, and the initial sampling time of the nth subunit is
Figure BDA0002003461190000053
Wherein M is the number of the sub-units of one bridge arm, and the sampling step length is delta T and T1Is the initial sampling time of the first subunit;
each subunit respectively performs space vector modulation operation according to the sampling result, and then respectively controls the sub-modules in the corresponding subunits;
and (2-7) achieving sub-module capacitor voltage balance by rotating switching signals inside the sub-units and between the sub-units. The rotation is divided into two aspects of the rotation inside the sub-units and the rotation between the sub-units.
For the sub-unit internal rotation, for example, each phase of one sub-unit is composed of two sub-modules, wherein the two sub-modules are respectively called as an upper sub-module and a lower sub-module. Switching signals of the upper submodule and the lower submodule are changed under each switching frequency, namely in a first switching period, the switching signal of the upper submodule obtained by the controller is sent to the upper submodule, and the switching signal of the lower submodule is sent to the lower submodule; but in the next switching period, the switching signal of the upper sub-module obtained by the controller is sent to the lower sub-module, and the switching signal of the lower sub-module obtained by the controller is sent to the upper sub-module and circulated. In the same subunit, three phases are rotated simultaneously.
For the switching signal alternation between the sub-modules, the switching signal of each sub-unit is alternated in each power frequency period. In a first power frequency period, a switching signal of a first subunit obtained by the controller is sent to the first subunit, a switching signal of a second subunit obtained by the controller is sent to the second subunit, and so on; in the next power frequency period, the switching signal of the first subunit obtained by the controller is actually sent to the second subunit, the switching signal of the second subunit obtained by the controller is actually sent to the third subunit, and so on, and the switching signal of the last subunit obtained by the controller is sent to the first subunit; and finally, each power frequency period is sequentially recurred and rotated.
In step (3) of the invention, for example, each phase of a subunit is composed of two submodules, and when the submodules adopt a half-bridge structure, each submodule has two power switching tubes, so that each subunit has four power switching tubes in a single phase, and the four power switching tubes are respectively named as T from top to bottom1、T2、T3And T4. When T is1Or T3When the power switch tube has an open-circuit fault, the phase has no P state and still has O, N state, which is called as type I fault; when T is2Or T4When the power switch tube has an open-circuit fault, the phase has no N state and still has P, O states, which is called type II fault.
According to the sub-module fault condition, the specific mode of carrying out recombination on the structure of the MMC is as follows:
(3-1) when only one submodule in the subunit fails, the submodule does not need to be recombined;
(3-2) when two sub-modules in the sub-units have faults, if the two faulty sub-modules are in the same phase, recombining the two fault sub-modules in the same phase into different sub-units respectively;
if the two faulty submodules are not in the same phase, if the fault types of the two faulty submodules are the same, recombination is not needed; if the failure types of the two failed sub-modules are different, the two failed sub-modules need to be recombined into different sub-units.
(3-3) when three sub-modules in the sub-units have faults, if two of the three sub-modules with faults are in the same phase, re-recombination is needed, and the sub-modules with two faults in the same phase are re-recombined into different sub-units;
if the three faulty submodules are not in the same phase, if the fault types of the three submodules are the same, recombination is not needed; if the sub-modules have different fault types, the sub-modules with different fault types need to be recombined into other sub-units, and the same number of sub-modules without faults or with the same fault types are introduced.
After the recombination is finished, each subunit continues to use space vector modulation, and still performs sampling in time error and independent modulation, but needs to adjust a specific action vector according to the fault type of each subunit:
if all the sub-units after being re-recombined are in I-type faults, no P state exists at the moment, derating operation is carried out for ensuring fault-tolerant operation of the MMC, namely the amplitude of the reference voltage vector is reduced, and the reference voltage vector only falls in a small hexagon in the middle of the sub-unit space vector distribution. The time of action of each base vector is obtained still according to the volt-second balance principle, and the action sequence of each sector vector is as follows:
when the reference voltage vector is located in the I-th sector, the vector acts in the order: (NNN) → (ONN) → (OON) → (OOO) → (OON) → (ONN) → (NNN);
when the reference voltage vector is located in the sector II, the vector action sequence is as follows: (NNN) → (NON) → (OON) → (OOO) → (OON) → (NON) → (NNN);
when the reference voltage vector is located in sector III, the vector acts in the order: (NNN) → (NON) → (NOO) → (OOO) → (NOO) → (NON) → (NNN);
when the reference voltage vector is located in the IV sector, the vector acts in the order: (NNN) → (NNO) → (NOO) → (OOO) → (NOO) → (NNO) → (NNN);
when the reference voltage vector is located in the V-th sector, the vector acts in the order: (NNN) → (NNO) → (ONO) → (OOO) → (ONO) → (NNO) → (NNN);
when the reference voltage vector is located in the VI-th sector, the vector acts in the order of: (NNN) → (ONN) → (ONO) → (OOO) → (ONO) → (ONN) → (NNN);
if all the sub-units after being re-recombined are in II type faults, no N state exists at the moment, derating operation is carried out in order to ensure fault-tolerant operation of the MMC, namely the amplitude of the reference voltage vector is reduced, and the reference voltage vector only falls in a small hexagon in the middle of the sub-unit space vector distribution. The time of action of each base vector is obtained still according to the volt-second balance principle, and the action sequence of each sector vector is as follows:
when the reference voltage vector is located in the I-th sector, the vector acts in the order: (PPP) → (PPO) → (POO) → (OOO) → (POO) → (PPO) → (PPP);
when the reference voltage vector is located in the sector II, the vector action sequence is as follows: (PPP) → (PPO) → (OPO) → (OOO) → (OPO) → (PPO) → (PPP);
when the reference voltage vector is located in sector III, the vector acts in the order: (PPP) → (OPP) → (OPO) → (OOO) → (OPO) → (OPP) → (PPP);
when the reference voltage vector is located in the IV sector, the vector acts in the order: (PPP) → (OPP) → (OOP) → (OOO) → (OOP) → (OPP) → (PPP);
when the reference voltage vector is located in the V-th sector, the vector acts in the order: (PPP) → (POP) → (OOP) → (OOO) → (OOP) → (POP) → (PPP);
when the reference voltage vector is located in the VI-th sector, the vector acts in the order of: (PPP) → (POP) → (POO) → (OOO) → (POO) → (POP) → (PPP);
compared with the prior art, the invention has the following beneficial effects:
1. the invention has high voltage utilization rate of space vector modulation, introduces a flexible modulation method into MMC, has simple principle and is easy to expand.
2. The invention can realize the fault-tolerant operation of the MMC without adding extra hardware circuits and redundant modules and only by adjusting the control object and the control method.
3. The method can be suitable for fault-tolerant operation of the MMC under the open-circuit fault of different numbers of sub-modules, and is strong in adaptability and easy to expand.
Drawings
FIG. 1 is a three-phase MMC circuit and reconfiguration example under normal operating conditions;
FIG. 2 is a diagram of a subunit space vector distribution;
FIG. 3 is a schematic diagram of a recombination process after three sub-modules of an upper bridge arm single-sub unit fail;
FIG. 4 is a graph of the subunit space vector distribution after a type I fault;
FIG. 5 is a graph of the subunit space vector distribution after a type II fault;
FIG. 6 is a waveform diagram of an experiment from a bridge arm midpoint to a direct current midpoint voltage under a normal operation condition of an MMC;
FIG. 7 is a waveform diagram of a three-phase line voltage experiment under normal operation of MMC;
FIG. 8 is a waveform diagram of an output phase current experiment under normal operation of MMC;
FIG. 9 is a capacitance-voltage experimental diagram of two sub-modules of an upper bridge arm and a lower bridge arm under the normal operation condition of the MMC.
Detailed Description
The invention will be described in further detail below with reference to the drawings and examples, which are intended to facilitate the understanding of the invention without limiting it in any way.
As shown in fig. 1, is an MMC topology with N sub-modules per leg, and most commonly a reorganization approach. Taking the combination of two sub-modules per phase as an example, the two sub-modules form one phase of the sub-unit, and the combination of three phases is completedA complete subunit. Each subunit can output 2Ucap、UcapAnd three voltages of 0, which may correspond to P, O, N states, respectively, and then space vector modulation may be applied to each subunit. Sampling moments of all the subunits are staggered with each other, and the subunits are controlled by independently applying a space vector modulation strategy according to self sampling reference. And finally, keeping the capacitance and voltage balance of the sub-modules through switching signals of the sub-modules in the sub-units and switching signals between the sub-units.
As shown in fig. 2, the vector diagram is a subunit space vector distribution diagram, and further contains a reference voltage vector sampled at a certain time by a certain submodule. The sub-unit space vector distribution may be divided into 6 large sectors by the base vector, and each large sector may be divided into 6 small regions. In the basic vectors, there are 3 zero vectors, which are (PPP), (OOO) and (NNN), respectively; there are 12 small vectors, respectively (POO), (ONN), (PPO), (OON), (OPO), (NON), (OPP), (NOO), (OOP), (NNO), (POP) and (ONO); there are 6 medium vectors, which are respectively (PON), (OPN), (NPO), (NOP), (ONP) and (PNO); there are 6 large vectors, which are (PNN), (PPN), (NPN), (NPP), (NNP), and (PNP), respectively. Due to the symmetry of each sector, when the reference voltage vector is located in other sectors, it can be reduced to sector I. The reference voltage vector in the figure is located in the I-th sector, small area 4, so the basic vector order of action is: (OON) → (PON) → (PPN) → (PPO) → (PPN) → (PON) → (OON).
As shown in FIG. 3, the re-organization process after the failure of three sub-modules in a sub-unit is illustrated, and only three sub-units are shown in the figure, wherein the middle sub-unit fails and SMu3Occurrence of type I failure, SMv3Occurrence of type II failure, SMv4A type II fault occurs. After recombination, SMu3Recombined into the first subunit, and SMu2Recombining into a second subunit; SMv4Recombined into a third subunit, and SMv5Recombining into a second subunit; SMv3And kept still. And the last three subunits are all operated in a derating mode.
As shown in fig. 4, in order to ensure the fault-tolerant operation of the MMC after the type I fault, the derating operation is performed, that is, the amplitude of the reference voltage vector is reduced, so that the reference voltage vector falls only in the middle small hexagon of the subunit space vector distribution. The time of action of each base vector is obtained still according to the volt-second balance principle, and the action sequence of each sector vector is as follows:
when the reference voltage vector is located in the I-th sector, the vector acts in the order: (NNN) → (ONN) → (OON) → (OOO) → (OON) → (ONN) → (NNN);
when the reference voltage vector is located in the sector II, the vector action sequence is as follows: (NNN) → (NON) → (OON) → (OOO) → (OON) → (NON) → (NNN);
when the reference voltage vector is located in sector III, the vector acts in the order: (NNN) → (NON) → (NOO) → (OOO) → (NOO) → (NON) → (NNN);
when the reference voltage vector is located in the IV sector, the vector acts in the order: (NNN) → (NNO) → (NOO) → (OOO) → (NOO) → (NNO) → (NNN);
when the reference voltage vector is located in the V-th sector, the vector acts in the order: (NNN) → (NNO) → (ONO) → (OOO) → (ONO) → (NNO) → (NNN);
when the reference voltage vector is located in the VI-th sector, the vector acts in the order of: (NNN) → (ONN) → (ONO) → (OOO) → (ONO) → (ONN) → (NNN).
As shown in fig. 5, in order to ensure the fault-tolerant operation of the MMC, derating operation is performed, i.e. the amplitude of the reference voltage vector is reduced, so that the reference voltage vector falls only in the middle small hexagon of the subunit space vector distribution. The time of action of each base vector is obtained still according to the volt-second balance principle, and the action sequence of each sector vector is as follows:
when the reference voltage vector is located in the I-th sector, the vector acts in the order: (PPP) → (PPO) → (POO) → (OOO) → (POO) → (PPO) → (PPP);
when the reference voltage vector is located in the sector II, the vector action sequence is as follows: (PPP) → (PPO) → (OPO) → (OOO) → (OPO) → (PPO) → (PPP);
when the reference voltage vector is located in sector III, the vector acts in the order: (PPP) → (OPP) → (OPO) → (OOO) → (OPO) → (OPP) → (PPP);
when the reference voltage vector is located in the IV sector, the vector acts in the order: (PPP) → (OPP) → (OOP) → (OOO) → (OOP) → (OPP) → (PPP);
when the reference voltage vector is located in the V-th sector, the vector acts in the order: (PPP) → (POP) → (OOP) → (OOO) → (OOP) → (POP) → (PPP);
when the reference voltage vector is located in the VI-th sector, the vector acts in the order of: (PPP) → (POP) → (POO) → (OOO) → (POO) → (POP) → (PPP).
The experimental platform parameters are shown in table 1.
TABLE 1
Figure BDA0002003461190000121
Fig. 6 shows waveforms of the midpoint voltage of the bridge arm to the midpoint voltage of the direct current under the space vector modulation strategy when the MMC works normally, and a total of 5 levels can be seen, wherein each level is 100V.
Fig. 7 is a waveform diagram of a three-phase line voltage experiment, and fig. 8 is a waveform diagram of an output phase current experiment, which shows a good sinusoidal state, the amplitude of the line voltage is about 380V, the phase current is close to 9A, and the direct current voltage utilization rate is 1.10.
Fig. 9 shows waveforms of capacitance voltages of two sub-modules of an upper bridge arm and a lower bridge arm under the recombination idea when the MMC works normally, and it can be seen that the capacitance voltages of the sub-modules can be kept balanced through a rotation strategy.
The embodiments described above are intended to illustrate the technical solutions and advantages of the present invention, and it should be understood that the above-mentioned embodiments are only specific embodiments of the present invention, and are not intended to limit the present invention, and any modifications, additions and equivalents made within the scope of the principles of the present invention should be included in the scope of the present invention.

Claims (6)

1. A MMC space vector modulation method based on submodule recombination and having fault tolerance is characterized by comprising the following steps:
(1) combining any more than two sub-modules of each phase in the MMC to form a plurality of new three-phase control units called as sub-units;
(2) when the MMC normally operates, a space vector modulation strategy is adopted to independently control each subunit, the sampling time of each subunit is staggered, and the balance of the capacitance and voltage of each submodule is kept through the alternation of switching signals of the submodules in the subunits and among the subunits;
(3) when an open-circuit fault occurs in the MMC sub-module, the structure of the MMC is recombined according to the fault condition, and the redundant vector is used for acting on the sub-module to ensure the fault-tolerant operation of the MMC; the specific way to recombine the structure of the MMC is as follows:
(3-1) when only one submodule in the subunit fails, the submodule does not need to be recombined;
(3-2) when two sub-modules in the sub-units have faults, if the two faulty sub-modules are in the same phase, recombining the two fault sub-modules in the same phase into different sub-units respectively;
if the two faulty submodules are not in the same phase, if the fault types of the two faulty submodules are the same, recombination is not needed; if the failure types of the two failure sub-modules are different, the two failure sub-modules need to be recombined into different sub-units;
(3-3) when three sub-modules in the sub-units have faults, if two of the three sub-modules with faults are in the same phase, re-recombination is needed, and the sub-modules with two faults in the same phase are re-recombined into different sub-units;
if the three faulty submodules are not in the same phase, if the fault types of the three submodules are the same, recombination is not needed; if the sub-modules have different fault types, the sub-modules with different fault types need to be recombined into other sub-units, and the same number of sub-modules without faults or with the same fault types are introduced.
2. The MMC space vector modulation method based on submodule recombination and having fault tolerance of claim 1, wherein in the step (1), two submodules of the same phase and the same bridge arm in the MMC are combined in pairs to form a plurality of subunits, and each subunit can output 2U of each phasecap、UcapAnd three voltages of 0, corresponding to P, O, N switch states, respectively.
3. The MMC space vector modulation method based on submodule reorganization and having fault tolerance of claim 2, wherein the specific process of the step (2) is as follows:
(2-1) when the MMC operates normally, according to the topology of the MMC, the voltage U of the direct current bus is knowndcAnd an output phase voltage uioObtaining output reference voltages of each phase of an upper bridge arm and a lower bridge arm (i is u, v, w), wherein u, v and w are three phases respectively;
(2-2) synthesizing a reference voltage vector u for the first subunit of the upper arm according to the output reference voltagerefAnd performing Clark transformation to obtain the value u of the reference voltage vector under α and β coordinatesα、uβSo as to obtain the included angle between the reference voltage vector and the α axis through triangular transformation
Figure FDA0002391645210000022
(2-3) dividing the space vector distribution of the subunits into 6 large sectors according to the basic vector, further dividing each large sector into 6 small areas according to the obtained included angle
Figure FDA0002391645210000023
Determining a large sector where a reference voltage vector is located, and uniformly calculating the reference voltage vector to the sector I according to the symmetry of the sector;
(2-4) based on the reduced included angle
Figure FDA0002391645210000024
And the post-calculation coordinate value u'αAnd u'βCalculating reference voltage vectorMeasuring the distance from the reference voltage vector to the edge of the sector, thereby judging the number of the small region where the reference voltage vector is located;
(2-5) selecting the nearest basic vector for synthesis by using a volt-second balance principle;
(2-6) different subunits of the same bridge arm sample the reference voltage vector in a staggered time mode, and the initial sampling time of the nth subunit is
Figure FDA0002391645210000021
Wherein M is the number of the sub-units of one bridge arm, and the sampling step length is delta T and T1Is the initial sampling time of the first subunit;
each subunit respectively performs space vector modulation operation according to the sampling result, and then respectively controls the sub-modules in the corresponding subunits;
and (2-7) achieving sub-module capacitor voltage balance by rotating switching signals inside the sub-units and between the sub-units.
4. The MMC space vector modulation method based on submodule reorganization and having fault tolerance of claim 3, wherein in the step (2-3), the unified reduction of the reference voltage vector to the I sector is as follows:
(2-3-1) if
Figure FDA00023916452100000313
The reference voltage vector is located in sector I, and the pair u is not neededα、uβAnd
Figure FDA0002391645210000032
carry out a reduction, i.e.
Figure FDA0002391645210000031
u′α＝uα，u′β＝uβ；
(2-3-2) if
Figure FDA00023916452100000312
The reference voltage vector is now in sector II, reduced to sector I, and
Figure FDA0002391645210000033
(2-3-3) if
Figure FDA00023916452100000311
The reference voltage vector is now in sector III, reduced to sector I, and then
Figure FDA0002391645210000034
(2-3-4) if
Figure FDA00023916452100000310
The reference voltage vector is now in sector IV, reduced to sector I, and now
Figure FDA0002391645210000035
u′α＝-uα，u′β＝-uβ；
(2-3-5) if
Figure FDA0002391645210000039
The reference voltage vector is located in sector V and returns to sector I
Figure FDA0002391645210000036
(2-3-6) if
Figure FDA0002391645210000038
The reference voltage vector is now located in sector V, at which time
Figure FDA0002391645210000037
5. The MMC space vector modulation method based on submodule reorganization and having fault tolerance of claim 3, wherein in step (2-5), when selecting the nearest base vector to synthesize, the principle of selecting the action order of the base vectors is as follows: the switching state of either phase avoids switching directly between the two switching states P, N; when the voltage space vector is switched, the switching state of a certain phase is ensured to be changed.
6. The MMC space vector modulation method based on submodule reorganization and having fault tolerance of claim 2, wherein in the step (3), the open-circuit faults are classified into type I faults and type II faults, and when the type I faults exist, the phase does not have a P state and still has an O, N state; in the case of type II fault, the phase has no N state and still has P, O state;
after the structure of the MMC is recombined, if the subunits are all I-type faults or all II-type faults, derating the subunits.
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