CN117498713A - Control method and control system of three-level converter - Google Patents
Control method and control system of three-level converter Download PDFInfo
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- CN117498713A CN117498713A CN202311439122.6A CN202311439122A CN117498713A CN 117498713 A CN117498713 A CN 117498713A CN 202311439122 A CN202311439122 A CN 202311439122A CN 117498713 A CN117498713 A CN 117498713A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H02M7/487—Neutral point clamped inverters
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Abstract
The embodiment of the invention provides a control method and a control system of a three-level converter, wherein the control method comprises the following steps: determining a first vector based on the calculated ideal discharge current and the actual discharge current, combined to the switching state of a switching tube in the three-level converter; determining a hexagonal sector diagram formed by each potential in the three-level converter; determining a second vector from the hexagonal sector diagram, wherein the second vector is obtained by comparing the sector position of the reference vector with the effective vector range; comparing the offset error with a preset offset error result, and determining a corresponding target mode; in the target mode, a hybrid vector is generated based on the first vector and the second vector to achieve control of the three-level converter. The embodiment of the invention can solve the problem of neutral point voltage offset of the direct current side capacitor in the circuit, saves time resource calculation of the DSP, realizes control of the three-level converter and ensures normal operation of the system.
Description
Technical Field
The embodiment of the invention relates to the technical field of converters, in particular to a control method and a control system of a three-level converter.
Background
Compared with the traditional two-level converter, the multi-level converter has the advantages of low bearing voltage of a switching device, small output voltage harmonic, low switching frequency and the like, so that the multi-level converter is widely applied to power electronic high-power application occasions. Three-level conversion is one of the most widely used because of its mature topology.
The circuit has the problem of capacitor neutral point voltage offset due to the unique structure and control strategy of the multilevel converter topology. The problem can lead to the increase of low harmonic wave output by the system, the efficiency is reduced, the service life of the capacitor at the direct current side can be shortened when the system is serious, and even the overvoltage and the damage of the power device can be caused due to the unbalanced voltage of the capacitor at the positive and negative bus bars at the direct current side, so that the normal operation of the system is damaged.
Therefore, it is desirable to provide a control method for a three-level converter to at least solve the problem of voltage offset in the capacitor in the circuit.
Disclosure of Invention
In view of the above, the embodiment of the invention provides a control method and a control system for a three-level converter, which can solve the problem of neutral point voltage offset of a direct-current side capacitor in a circuit and ensure the normal operation of the system.
In order to achieve the above object, the embodiment of the present invention provides the following technical solutions:
in a first aspect, an embodiment of the present invention provides a control method of a three-level converter, including:
determining a first vector based on the calculated ideal discharge current and the actual discharge current, combined to the switching state of a switching tube in the three-level converter;
determining a hexagonal sector diagram formed by each potential in the three-level converter;
determining a second vector from the hexagonal sector diagram, wherein the second vector is obtained by comparing the sector position of the reference vector with the effective vector range;
comparing the offset error with a preset offset error result, and determining a corresponding target mode;
in the target mode, a hybrid vector is generated based on the first vector and the second vector to achieve control of the three-level converter.
Optionally, the second vector is a combined virtual vector, and determining the second vector from the hexagonal sector map includes:
calculating a reference vector according to a current target, and determining the sector position of the reference vector in a hexagonal sector diagram according to a vector angle and a boundary condition, wherein the sector position comprises a first sector position and a second sector position, the current target is obtained by selecting potential points in a three-level converter by a user, the hexagonal sector diagram is divided into a first sector and a second sector, and the area of the first sector is larger than that of the second sector;
determining an optimal vector for current tracking according to the sector position of the reference vector and an effective vector range, wherein the effective vector range is a region with the shortest linear distance from the reference vector to any vector on a complex plane;
a combined virtual vector is generated based on the optimal vector, and the combined virtual vector is determined as a second vector.
Optionally, the determining the first vector based on the calculated ideal discharge current and the actual discharge current and the switching state of the switching tube combined in the three-level converter includes:
calculating ideal discharge current according to the charge offset error of a capacitor in the three-level converter, wherein the ideal discharge current is corresponding average discharge current of midpoint potential in a complete balance state;
determining an actual discharge current from the three-phase current of the three-level converter according to the current error;
the first vector of the balanced midpoint potential is determined in combination with the ideal discharge current, the actual discharge current, and the switching state of the switching tubes in the three-level converter.
Optionally, the comparing the offset error with a preset offset error result, determining a corresponding target mode includes:
comparing the offset error with a preset offset error result, and selecting a first mode when the comparison result shows that the current error and the midpoint potential offset error do not meet preset conditions;
when the comparison result is that the midpoint potential is balanced but the current error does not meet the preset condition, selecting a second mode;
and when the comparison result shows that the current error meets the preset condition and the midpoint potential offset error is larger, selecting a third mode.
Optionally, in the target mode, generating the hybrid vector based on the first vector and the second vector includes:
when the target mode is a first mode, splicing a first vector with a first duty ratio and a second vector with a second duty ratio to obtain a mixed vector, wherein the sum of the first duty ratio and the second duty ratio is 1;
when the target mode is the second mode, taking the second vector as a mixed vector;
when the target mode is the third mode, the first vector is taken as a mixed vector.
In a second aspect, an embodiment of the present invention provides a control system for a three-level converter, including:
the first processing module is used for determining a first vector based on the calculated ideal discharge current and the actual discharge current and combined with the switching state of a switching tube in the three-level converter;
the second processing module is used for determining a hexagonal sector diagram formed by each potential in the three-level converter; determining a second vector from the hexagonal sector diagram, wherein the second vector is obtained by comparing the sector position of the reference vector with the effective vector range;
the generation module is used for comparing the offset error with a preset offset error result and determining a corresponding target mode; in the target mode, a hybrid vector is generated based on the first vector and the second vector to achieve control of the three-level converter.
Optionally, the second vector is a combined virtual vector, and the second processing module is configured to determine the second vector from the hexagonal sector map, including:
calculating a reference vector according to a current target, and determining the sector position of the reference vector in a hexagonal sector diagram according to a vector angle and a boundary condition, wherein the sector position comprises a first sector position and a second sector position, the current target is obtained by selecting potential points in a three-level converter by a user, the hexagonal sector diagram is divided into a first sector and a second sector, and the area of the first sector is larger than that of the second sector;
determining an optimal vector for current tracking according to the sector position of the reference vector and an effective vector range, wherein the effective vector range is a region with the shortest linear distance from the reference vector to any vector on a complex plane;
a combined virtual vector is generated based on the optimal vector, and the combined virtual vector is determined as a second vector.
Optionally, the first processing module is configured to determine, based on the calculated ideal discharge current and the actual discharge current, a first vector based on a switching state of a switching tube coupled to the three-level converter, and includes:
calculating ideal discharge current according to the charge offset error of a capacitor in the three-level converter, wherein the ideal discharge current is corresponding average discharge current of midpoint potential in a complete balance state;
determining an actual discharge current from the three-phase current of the three-level converter according to the current error;
the first vector of the balanced midpoint potential is determined in combination with the ideal discharge current, the actual discharge current, and the switching state of the switching tubes in the three-level converter.
Optionally, the generating module is configured to compare the offset error with a preset offset error result, and determine a corresponding target mode, including:
comparing the offset error with a preset offset error result, and selecting a first mode when the comparison result shows that the current error and the midpoint potential offset error do not meet preset conditions;
when the comparison result is that the midpoint potential is balanced but the current error does not meet the preset condition, selecting a second mode;
and when the comparison result shows that the current error meets the preset condition and the midpoint potential offset error is larger, selecting a third mode.
Optionally, the generating module is configured to generate, in the target mode, a hybrid vector based on the first vector and the second vector, and includes:
when the target mode is a first mode, splicing a first vector with a first duty ratio and a second vector with a second duty ratio to obtain a mixed vector, wherein the sum of the first duty ratio and the second duty ratio is 1;
when the target mode is the second mode, taking the second vector as a mixed vector;
when the target mode is the third mode, the first vector is taken as a mixed vector.
The embodiment of the invention provides a control method of a three-level converter, which specifically comprises the following steps: determining a first vector based on the calculated ideal discharge current and the actual discharge current, combined to the switching state of a switching tube in the three-level converter; determining a hexagonal sector diagram formed by each potential in the three-level converter; determining a second vector from the hexagonal sector diagram, wherein the second vector is obtained by comparing the sector position of the reference vector with the effective vector range; comparing the offset error with a preset offset error result, and determining a corresponding target mode; in the target mode, a hybrid vector is generated based on the first vector and the second vector to achieve control of the three-level converter. Because the first vector is the minimum vector determined by the actual current in the three-level converter and the second vector is the optimal vector calculated by the three-level converter in the embodiment of the invention, when the mixed vector is generated by the first vector and the second vector set, the mixed vector can represent the actual current condition and the offset condition of the voltage offset of the midpoint of the capacitor, and when the mixed vector acts on the three-level converter, the control of the three-level converter can be realized.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required to be used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only embodiments of the present application, and that other drawings may be obtained according to the drawings provided without inventive effort to a person skilled in the art.
Fig. 1 is an alternative schematic diagram of a control system of a three-level converter according to an embodiment of the present invention.
Fig. 2 is an alternative schematic diagram of a three-level converter in an embodiment of the invention.
Fig. 3 is an alternative schematic diagram of a basic vector diagram corresponding to the three-level converter according to an embodiment of the present invention.
Fig. 4 is an alternative schematic diagram of a hexagonal sector diagram corresponding to a three-level converter according to an embodiment of the present invention.
Fig. 5 is a comparison chart of the relationship between the first sector and the second sector according to the embodiment of the present invention.
Fig. 6 is a schematic diagram of generating a hybrid vector according to an embodiment of the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and fully with reference to the accompanying drawings, in which it is evident that the embodiments described are only some, but not all, of the embodiments of the present application. All other embodiments, which can be made by one of ordinary skill in the art based on the embodiments herein without making any inventive effort, are intended to be within the scope of the present application.
The embodiment of the invention provides a control method of a three-level converter, which specifically comprises the following steps: determining a first vector based on the calculated ideal discharge current and the actual discharge current, combined to the switching state of a switching tube in the three-level converter; determining a hexagonal sector diagram formed by each potential in the three-level converter; determining a second vector from the hexagonal sector diagram, wherein the second vector is obtained by comparing the sector position of the reference vector with the effective vector range; comparing the offset error with a preset offset error result, and determining a corresponding target mode; in the target mode, a hybrid vector is generated based on the first vector and the second vector to achieve control of the three-level converter. Because the first vector is the minimum vector determined by the actual current in the three-level converter and the second vector is the optimal vector calculated by the three-level converter in the embodiment of the invention, when the mixed vector is generated by the first vector and the second vector set, the mixed vector can represent the actual current condition and the offset condition of the voltage offset of the midpoint of the capacitor, and when the mixed vector acts on the three-level converter, the accurate control of the three-level converter can be realized.
Further, referring to fig. 1, the control system of the three-level converter in the embodiment of the present invention may be shown in fig. 1, and the control system of the three-level converter in the embodiment of the present invention includes a three-level converter 10, a first processing module 11, a second processing module 12, a sampling module 13, and a generating module 14.
The first processing module 11 is configured to determine a first vector based on the calculated ideal discharge current and the actual discharge current, and a switching state of a switching tube coupled to the three-level converter;
the second processing module 12 is used for determining a hexagonal sector diagram composed of potentials in the three-level converter; determining a second vector from the hexagonal sector diagram, wherein the second vector is obtained by comparing the sector position of the reference vector with the effective vector range;
the generating module 14 is configured to compare the offset error with a preset offset error result, determine a corresponding target mode, and generate a hybrid vector based on the first vector and the second vector in the target mode, so as to implement control of the three-level converter.
In other optional implementations, the system further includes a sampling module 13, where the sampling module 13 is configured to collect the voltage and current integrated into the power grid, where the collected voltage is for phase locking, and abc/dq transformation is needed, and abc cannot be adjusted without static difference, i in fig. 1 d And i q Reference values for the active and reactive components of the current respectively;
because the first vector is the minimum vector determined by the actual current in the three-level converter and the second vector is the optimal vector calculated by the three-level converter in the embodiment of the invention, when the mixed vector is generated by the first vector and the second vector set, the mixed vector can well represent the actual current condition and the offset condition of the voltage offset in the middle point of the capacitor, and when the mixed vector acts on the three-level converter, the control of the three-level converter can be realized.
In the process of converting three-phase variables (such as current, voltage, etc.) from a three-phase coordinate system (A, B, C or α, β) to a two-phase coordinate system (D, Q or d, q), in the embodiment of the present invention, the three-level converter is shown in fig. 2, and the system equations under the α, β coordinate system are:
wherein u is α 、u β And e α 、e β Is the voltage component of the alpha beta-axis converter and the grid voltage component, i α And i β Is an alpha beta axis alternating current component, L s And R is s The filter inductance and the internal resistance are respectively.
C 1 And C 2 Upper and lower capacitors on DC side, and C 1 ＝C 2 ＝C，U cd I is capacitance voltage offset error o Is the current flowing into the midpoint.
Further, in order to implement the processing of the three-level converter, in the embodiment of the present invention, the FCS-MPC is used to determine a base vector of each potential in the three-level converter, and a second vector corresponding to the minimum cost function is searched from the base vectors, which specifically includes:
s1, discretizing and expressing a formula 1 and a formula 2 by adopting a forward Euler method to obtain a prediction equation of the system:
wherein i is α (k+1) and i β (k+1) is the component of the alpha, beta axis of the predicted current, u cd (k+1) is the DC side predicted capacitor voltage offset error, T s Is the sampling period.
Step S2, constructing a cost function by using the square value and the absolute error value of the predicted capacitor voltage offset error, wherein the minimum cost function is expressed as:
where λ is the weight factor of the capacitor voltage offset error, which value determines the priority of the controlled variable in global optimization.
Step S3, searching an optimal vector corresponding to the minimum cost function from the basic vector by adopting a vector enumeration algorithm, and obtaining an optimal voltage vector V opt (k) Expressed as:
V opt (k)＝V minf [i(V m )],m∈[1,2.....27]equation 6
Wherein V is min f[i(V m )]And a voltage vector subscript function corresponding to the minimum cost function.
In a static coordinate system, the switching decision voltage vector of the converter is expressed as:
s in equation 7 a 、S b And S is c The switching states of a phase, b phase and c phase in the three-level converter are respectively U dc Is a direct current side voltage.
In an alternative implementation of the embodiment of the present invention, a basic vector diagram corresponding to the three-level converter may be shown with reference to fig. 3. In FIG. 3, the position and switching state of 27 basic voltage vectors are included, where V 1 ～V 6 Is a large vector, V 7 ～V 12 Is a middle vector, V 13 ～V 24 Is a small vector, V 25 ～V 27 Is a zero vector.
In the embodiment of the invention, a hexagonal sector diagram formed by each potential in the three-level converter can also be determined; and determining a second vector from the hexagonal sector diagram, wherein the second vector is obtained by comparing the sector position of the reference vector with the effective vector range.
In an embodiment of the present invention, the second vector is a combined virtual vector, and determining the second vector from the hexagonal sector map includes:
calculating a reference vector according to a current target, and determining the sector position of the reference vector in a hexagonal sector diagram according to a vector angle and a boundary condition, wherein the sector position comprises a first sector position and a second sector position, the current target is obtained by selecting potential points in a three-level converter by a user, the hexagonal sector diagram is divided into a first sector and a second sector, and the area of the first sector is larger than that of the second sector;
determining an optimal vector for current tracking according to the sector position of the reference vector and an effective vector range, wherein the effective vector range is a region with the shortest linear distance from the reference vector to any vector on a complex plane;
a combined virtual vector is generated based on the optimal vector, and the combined virtual vector is determined as a second vector.
Therefore, in the embodiment of the invention, the optimal vector of current tracking can be directly determined through the hexagonal sector diagram without vector enumeration.
Specifically, the hexagonal sector diagram is shown with reference to fig. 4, where the entire vector complex plane is divided into 6 sectors (I-VI), and each sector is further divided into 6 sub-sectors (e.g., (1) - (6) in fig. 5) according to the effective vector radiation, and in the embodiment of the present invention, the sectors may be understood as a first sector, that is, a large sector, and the sub-sectors in one sector may be understood as a second sector, that is, a small sector.
Further, taking the first sector I as an example, the optimal vector of the second sector is shown in table 1:
TABLE 1
| Second sector | Optimal vector |
| ① | V 25 ,V 26 ,V 27 |
| ② | V 13 ,V 14 |
| ③ | V 15 ,V 16 |
| ④ | V 1 |
| ⑤ | V 7 |
| ⑥ | V 2 |
Further, the effective range of each basic vector is shown in fig. 5, wherein the dashed line in the hexagon is the perpendicular bisector of the connecting line of the two vectors. For example, the effective range of the zero vector is 1 area. When the reference vector is within 1 area, the linear distance of the reference vector to the zero vector is the shortest, which means that the error between the reference vector and the zero vector is the smallest.
To determine electricityThe optimal vector for flow tracking, i.e. the reference vector (V * (k) ) And determines its sector and sub-sector locations based on its vector angle and boundary conditions.
Wherein U in formula 8 * (k) And theta * (k) Respectively represent V * (k) Amplitude and phase of (a) in equation 9And->V respectively * (k) The alpha and beta axis components of (c).
Where N is the number of the large sector.
To reduce computational complexity, II-VI sectors are moved to the I-th sector.
θ r ＝θ * (k) - (n-1) pi/3 formula 11
Wherein θ r Is the vector angle rotated to the first sector.
After rotating to sector I, its alpha and beta axis componentsExpressed as:
according toAnd theta r Determination may determine V * (k) The location of the small sector is shown in fig. 5, and in conjunction with fig. 5, it can be understood that the boundary condition of (2) is determined as:
when V is * (k) In the sub-sector (2), θ r (k) Between 0 and pi/6. The boundary conditions for the small sector (2) are:
it can be seen that the optimal vector of current tracking in the embodiment of the present invention can be determined according to V * (k) The large and small sector positions of (a) are determined directly.
It should be noted that, the middle-small vectors will cause the middle-point potential to deviate, so it is necessary to further replace these basic vectors with combined virtual vectors, to achieve the same current tracking performance without affecting the middle-point potential, that is, to at least solve the capacitor middle-point voltage deviation, and to achieve efficient current tracking.
Table 2 shows that the second vector V is obtained based on the optimal vector combination using the first sector I as an example J (k) Is described in the following.
When V is * (k) When being positioned in the sub sector (2), V 13 And V 14 Half cycles are run separately to offset the midpoint potential offset error.
TABLE 2
Also, in the sub-sector (3), V 15 And V 16 The half cycles are run separately. When V is * (k) Is positioned at the sonWhen sector (5), V 7 Quilt V 1 And V 2 Is replaced by a hybrid vector of (c). V (V) 1 And V 2 Respectively run half period with V 7 The same current tracking effect is achieved, and the midpoint potential is not affected.
Thus, it can be according to V * (k) Is used for determining an optimal combination virtual vector V for tracking a target current without affecting a midpoint potential J (k)。
In the control method of the three-level converter provided by the embodiment of the invention, based on the calculated ideal discharge current and the actual discharge current, the first vector is determined by combining the switching state of the switching tube in the three-level converter and the second vector, so that the mixed vector is generated, and the neutral point potential balance without the need of weight factors and complex calculation is realized.
Specifically, in the embodiment of the present invention, the process of determining the first vector is:
step S31, calculating ideal discharge current according to the charge offset error of a capacitor in the three-level converter, wherein the ideal discharge current is the average discharge current corresponding to the midpoint potential in the complete balance state;
step S32, determining actual discharge current from three-phase current of the three-level converter according to the current error, wherein the actual discharge current is a current which does not exceed a preset difference value with the ideal discharge current;
step S33, determining a first vector of balanced midpoint potential according to the ideal discharge current, the actual discharge current and the switching state of the switching tube in the three-level converter.
Specifically, the ideal average discharge current (I (k)) for one cycle is expressed as:
I(k)＝(C 1 udc 1 (k)-C 2 udc 2 (k) Formula 15 of)/Ts
To determine the best actual average discharge current I from the three-phase current opt (k) Absolute current error i xe (k) The method comprises the following steps:
i xe (k)＝||I(k)|-|i x (k) |, x=a, b, c,0 equation 16
I opt (k) According to I (k), I xe (k) And three-phase current determination, expressed as:
I opt (k)＝tanh(I(k)*i minixe(k) (k))*i minixe(k) (k) Equation 17
Wherein, tanh is hyperbolic tangent function, i minixe (k) Is at least i xe (k) Corresponding current subscript functions.
Vector cost function (h) V ) From V * (k) And can generate a capacitor midpoint current I opt (k) Is formed by the absolute error between the basis vectors of (a), the vector cost function is expressed as:
thus, an optimal vector V that balances midpoint potential and has minimal negative impact on current tracking is determined by a small number of vector enumerations N (k) The subscript function of the minimum vector cost function is also denoted as:
Vz(k)＝V min h v (k) Equation 19
In order to fully consider current tracking and midpoint potential performance, the embodiment of the invention provides a mode of combining with a target mode and outputting a mixed vector, wherein the target mode comprises three selectable modes, namely a first mode, a second mode and a third mode, and the mode switching condition is designed according to current and capacitance voltage offset errors.
Capacitor voltage offset error u cd (k) Determined by equation 2. Error in current offset (i) αd(k) ,i βd(k) ) The error between the target current and the actual current is expressed as:
in addition, it is assumed that the current and capacitance voltage offset error tolerances are respectivelyAnd->
When the system has larger current error and midpoint potential offset error, a first mode is selected, V J (k) And V N (k) Half cycles are output separately to reduce the current error and the midpoint potential offset error. Thus, the operating conditions of the first mode are expressed as:
when the system has excellent neutral potential balance performance, but the current tracking error is relatively large, the second mode is selected. V (V) J (k) One cycle is output to track the target current without affecting the midpoint potential. Thus, the operating conditions of the second mode are expressed as:
when the potential offset error in the system is large but the current tracking performance is excellent, the third mode is selected. V (V) N (k) And outputting a period to reduce the offset error of the midpoint potential and simultaneously give consideration to current tracking. Thus, the operating conditions of the third mode are expressed as:
specifically, in the target mode, generating a hybrid vector based on the first vector and the second vector includes: when the target mode is a first mode, splicing a first vector with a first duty ratio and a second vector with a second duty ratio to obtain a mixed vector, wherein the sum of the first duty ratio and the second duty ratio is 1; when the target mode is the second mode, taking the second vector as a mixed vector; when the target mode is the third mode, the first vector is taken as a mixed vector.
With continued reference to fig. 6, fig. 6 is a schematic diagram of generating a hybrid vector according to an embodiment of the present invention. When the target mode is a first mode, the mixed vector comprises a first vector with a first duty ratio and a second vector with a second duty ratio; when the target mode is the second mode, the mixed vector is the second vector; when the target mode is the third mode, the mixed vector is the first vector.
In the control method of the three-level converter, the offset error is compared with a preset offset error result, and a corresponding target mode is determined; in the target mode, the mixed vector is generated based on the first vector and the second vector so as to realize the control of the three-level converter and realize the combined control in different modes.
Further, in the control method of the three-level converter provided by the embodiment of the invention, based on a hexagonal sector division method, an optimal vector for current tracking is determined under the condition of not carrying out vector enumeration, and a combined virtual vector is adopted to replace a basic vector, so that the influence on midpoint potential is avoided; further, a capacitance charge balance algorithm is adopted, and the optimal vector of neutral point potential balance is determined under the condition of not considering weight factors, so that negative influence on current tracking is minimized; finally, a mixed vector output mode is adopted, and a mixed vector is generated based on a first vector and a second vector in a target mode, so that the control of the three-level converter is realized. Therefore, in the control method of the three-level converter, the current tracking and midpoint potential balance performance are fully considered, and the control method at least can solve the problem of midpoint potential offset, and can also improve the prediction precision, so that the grid-connected current quality is improved.
In other alternative embodiments, a control system of a three-level converter is also provided, and the control system may be shown with reference to fig. 1, which is not described herein.
The first processing module in the control system of the three-level converter is used for determining a first vector based on the calculated ideal discharge current and the actual discharge current and combined with the switching state of the switching tube in the three-level converter;
the second processing module is used for determining a hexagonal sector diagram formed by each potential in the three-level converter; determining a second vector from the hexagonal sector diagram, wherein the second vector is obtained by comparing the sector position of the reference vector with the effective vector range;
the generation module is used for comparing the offset error with a preset offset error result and determining a corresponding target mode; in the target mode, a hybrid vector is generated based on the first vector and the second vector to achieve control of the three-level converter.
In the control system of the three-level converter provided by the embodiment of the present invention, the second vector is a combined virtual vector, and the second processing module is configured to determine the second vector from the hexagonal sector diagram, where the determining includes:
calculating a reference vector according to a current target, and determining the sector position of the reference vector in a hexagonal sector diagram according to a vector angle and a boundary condition, wherein the sector position comprises a first sector position and a second sector position, the current target is obtained by selecting potential points in a three-level converter by a user, the hexagonal sector diagram is divided into a first sector and a second sector, and the area of the first sector is larger than that of the second sector;
determining an optimal vector for current tracking according to the sector position of the reference vector and an effective vector range, wherein the effective vector range is a region with the shortest linear distance from the reference vector to any vector on a complex plane;
a combined virtual vector is generated based on the optimal vector, and the combined virtual vector is determined as a second vector.
In the control system of a three-level converter provided by the embodiment of the present invention, the first processing module is configured to determine, based on the calculated ideal discharge current and the actual discharge current, a first vector based on a switching state of a switching tube coupled to the three-level converter, where the first vector includes:
calculating ideal discharge current according to the charge offset error of a capacitor in the three-level converter, wherein the ideal discharge current is corresponding average discharge current of midpoint potential in a complete balance state;
determining an actual discharge current from the three-phase current of the three-level converter according to the current error;
the first vector of the balanced midpoint potential is determined in combination with the ideal discharge current, the actual discharge current, and the switching state of the switching tubes in the three-level converter.
In the control system of the three-level converter provided by the embodiment of the present invention, the generating module is configured to compare an offset error with a preset offset error result, and determine a corresponding target mode, where the generating module includes:
comparing the offset error with a preset offset error result, and selecting a first mode when the comparison result shows that the current error and the midpoint potential offset error do not meet preset conditions;
when the comparison result is that the midpoint potential is balanced but the current error does not meet the preset condition, selecting a second mode;
and when the comparison result shows that the current error meets the preset condition and the midpoint potential offset error is larger, selecting a third mode.
In the control system of the three-level converter provided by the embodiment of the invention, the generating module is configured to generate a hybrid vector based on a first vector and a second vector in a target mode, and includes:
when the target mode is a first mode, splicing a first vector with a first duty ratio and a second vector with a second duty ratio to obtain a mixed vector, wherein the sum of the first duty ratio and the second duty ratio is 1;
when the target mode is the second mode, taking the second vector as a mixed vector;
when the target mode is the third mode, the first vector is taken as a mixed vector.
The foregoing describes several embodiments of the present invention, and the various alternatives presented by the various embodiments may be combined, cross-referenced, with each other without conflict, extending beyond what is possible embodiments, all of which are considered to be embodiments of the present invention disclosed and disclosed.
Although the embodiments of the present invention are disclosed above, the present application is not limited thereto. Various changes and modifications may be made by one skilled in the art without departing from the spirit and scope of the invention, and the scope of the invention shall be defined by the appended claims.
Claims (10)
1. A control method of a three-level converter, comprising:
determining a first vector based on the calculated ideal discharge current and the actual discharge current, combined to the switching state of a switching tube in the three-level converter;
determining a hexagonal sector diagram formed by each potential in the three-level converter;
determining a second vector from the hexagonal sector diagram, wherein the second vector is obtained by comparing the sector position of the reference vector with the effective vector range;
comparing the offset error with a preset offset error result, and determining a corresponding target mode;
in the target mode, a hybrid vector is generated based on the first vector and the second vector to achieve control of the three-level converter.
2. The method of claim 1, wherein the second vector is a combined virtual vector, and wherein determining the second vector from the hexagonal sector map comprises:
calculating a reference vector according to a current target, and determining the sector position of the reference vector in a hexagonal sector diagram according to a vector angle and a boundary condition, wherein the sector position comprises a first sector position and a second sector position, the current target is obtained by selecting potential points in a three-level converter by a user, the hexagonal sector diagram is divided into a first sector and a second sector, and the area of the first sector is larger than that of the second sector;
determining an optimal vector for current tracking according to the sector position of the reference vector and an effective vector range, wherein the effective vector range is a region with the shortest linear distance from the reference vector to any vector on a complex plane;
a combined virtual vector is generated based on the optimal vector, and the combined virtual vector is determined as a second vector.
3. The method of claim 1, wherein determining the first vector based on the calculated ideal discharge current and the actual discharge current in combination with the switching states of the switching transistors in the three-level converter comprises:
calculating ideal discharge current according to the charge offset error of a capacitor in the three-level converter, wherein the ideal discharge current is corresponding average discharge current of midpoint potential in a complete balance state;
determining an actual discharge current from the three-phase current of the three-level converter according to the current error;
the first vector of the balanced midpoint potential is determined in combination with the ideal discharge current, the actual discharge current, and the switching state of the switching tubes in the three-level converter.
4. The method for controlling a three-level converter according to claim 1, wherein comparing the offset error with a preset offset error result, determining a corresponding target pattern, comprises:
comparing the offset error with a preset offset error result, and selecting a first mode when the comparison result shows that the current error and the midpoint potential offset error do not meet preset conditions;
when the comparison result is that the midpoint potential is balanced but the current error does not meet the preset condition, selecting a second mode;
and when the comparison result shows that the current error meets the preset condition and the midpoint potential offset error is larger, selecting a third mode.
5. The method according to claim 4, wherein generating the hybrid vector based on the first vector and the second vector in the target mode comprises:
when the target mode is a first mode, splicing a first vector with a first duty ratio and a second vector with a second duty ratio to obtain a mixed vector, wherein the sum of the first duty ratio and the second duty ratio is 1;
when the target mode is the second mode, taking the second vector as a mixed vector;
when the target mode is the third mode, the first vector is taken as a mixed vector.
6. A control system for a three-level converter, comprising:
the first processing module is used for determining a first vector based on the calculated ideal discharge current and the actual discharge current and combined with the switching state of a switching tube in the three-level converter;
the second processing module is used for determining a hexagonal sector diagram formed by each potential in the three-level converter; determining a second vector from the hexagonal sector diagram, wherein the second vector is obtained by comparing the sector position of the reference vector with the effective vector range;
the generation module is used for comparing the offset error with a preset offset error result and determining a corresponding target mode; in the target mode, a hybrid vector is generated based on the first vector and the second vector to achieve control of the three-level converter.
7. The control system of the three-level converter of claim 6, wherein the second vector is a combined virtual vector, and wherein the second processing module for determining the second vector from the hexagonal sector map comprises:
calculating a reference vector according to a current target, and determining the sector position of the reference vector in a hexagonal sector diagram according to a vector angle and a boundary condition, wherein the sector position comprises a first sector position and a second sector position, the current target is obtained by selecting potential points in a three-level converter by a user, the hexagonal sector diagram is divided into a first sector and a second sector, and the area of the first sector is larger than that of the second sector;
determining an optimal vector for current tracking according to the sector position of the reference vector and an effective vector range, wherein the effective vector range is a region with the shortest linear distance from the reference vector to any vector on a complex plane;
a combined virtual vector is generated based on the optimal vector, and the combined virtual vector is determined as a second vector.
8. The control system of claim 6, wherein the first processing module for determining the first vector based on the calculated ideal discharge current and the actual discharge current, the switching states of the switching tubes coupled to the three-level converter, comprises:
calculating ideal discharge current according to the charge offset error of a capacitor in the three-level converter, wherein the ideal discharge current is corresponding average discharge current of midpoint potential in a complete balance state;
determining an actual discharge current from the three-phase current of the three-level converter according to the current error;
the first vector of the balanced midpoint potential is determined in combination with the ideal discharge current, the actual discharge current, and the switching state of the switching tubes in the three-level converter.
9. The control system of the three-level converter according to claim 6, wherein the generating module, configured to compare the offset error with a preset offset error result, determines a corresponding target mode, includes:
comparing the offset error with a preset offset error result, and selecting a first mode when the comparison result shows that the current error and the midpoint potential offset error do not meet preset conditions;
when the comparison result is that the midpoint potential is balanced but the current error does not meet the preset condition, selecting a second mode;
and when the comparison result shows that the current error meets the preset condition and the midpoint potential offset error is larger, selecting a third mode.
10. The control system of the three-level converter according to claim 9, wherein the generating module for generating the hybrid vector based on the first vector and the second vector in the target mode includes:
when the target mode is a first mode, splicing a first vector with a first duty ratio and a second vector with a second duty ratio to obtain a mixed vector, wherein the sum of the first duty ratio and the second duty ratio is 1;
when the target mode is the second mode, taking the second vector as a mixed vector;
when the target mode is the third mode, the first vector is taken as a mixed vector.
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