CN113904578B - Weight coefficient-free model predictive control method for single-phase cascade H-bridge converter - Google Patents
Weight coefficient-free model predictive control method for single-phase cascade H-bridge converter Download PDFInfo
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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
- H02M7/53—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 using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
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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
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Abstract
The application provides a model predictive control method without weight coefficient for a single-phase cascade H-bridge converter, which comprises the following steps: constructing a two-dimensional control area based on the switching function; dividing a two-dimensional control area into 2N+2 subareas according to the input voltage of the converter, wherein N is the total number of cascading units of the single-phase cascading H-bridge converter; determining a subarea where an input voltage reference value is located as an optimal subarea, and taking all switch states corresponding to two voltage vectors in the optimal subarea as candidate switch states of the corresponding voltage vectors; constructing a cost function based on current errors, and calculating the optimal acting time corresponding to each voltage vector; constructing a cost function based on voltage errors, and respectively selecting the corresponding optimal switch states from the candidate switch states corresponding to the two voltage vectors; and taking the optimal action time and the optimal switching state as output to control the single-phase cascade H-bridge converter. The method can avoid the parameter setting and readjusting process of the controller, has simple control structure and reduces the algorithm execution time.
Description
Technical Field
The application relates to the technical field of power electronics and converter control, in particular to a model predictive control method without weight coefficient for a single-phase cascade H-bridge converter.
Background
The cascade H-bridge converter is one of main topological structures of the multi-level converter, has the advantages of low harmonic distortion rate of alternating-current output voltage, good electromagnetic environment, high modularization degree, easy expansion and the like, and is widely applied to middle-high voltage and high-power occasions such as traction transmission, photovoltaic power generation, wind power generation and the like.
The main control objective of the cascade H-bridge converter is to realize the rapid tracking of the current at the power grid side and the balanced control of the capacitor voltage at the direct current side. The traditional control strategy comprises PI control, hysteresis control and the like, wherein the parameter setting of the PI control strategy is complex, the dynamic response speed of the system is slow, and the delay of the PI controller causes steady-state errors in current tracking; the hysteresis control strategy has simple design and good transient performance, but has the following defects: the switching frequency is not fixed, the controllability is poor, the harmonic frequency spectrum is wider, and the design difficulty of a post-stage filter is high. In recent years, the predictive control strategy is widely applied due to the advantages of high model tolerance, good stability, quick dynamic response and the like, such as dead beat control and model predictive control; the dead beat control strategy control has high precision but does not comprise constraint conditions; the model predictive control can solve the problems of multiple variables and constraint, and has a good development prospect in the field of power converter control.
The model prediction control predicts the future output state of the system according to the system model, constructs a cost function according to the deviation of the predicted output and the expected output, obtains the current optimal control strategy by using the minimized cost function, effectively overcomes the uncertainty of the system parameters, and has better robustness when the load fluctuates or the system parameters change. When a plurality of control targets are involved, weight coefficients are introduced into model predictive control to balance the tracking performance of each variable, so that balance control of multi-target performance is realized. For cascaded H-bridge converters, a trade-off between grid-side current and capacitor voltage is required using a weight coefficient. Because the magnitude of the weight coefficient has a large dependence on the structural parameters of the current transformer, the design and adjustment process of the current transformer can occupy a large amount of time, and therefore the current-voltage nominal value ratio is adopted for representing, so that the algorithm operation speed is improved. In the motor drive system torque control research, a model predictive control strategy without a weight coefficient is provided. However, due to the fact that the number of switches of the cascade H bridge is large, a control strategy based on model prediction is complex, and response speed is low. Meanwhile, because the network side current and the direct current voltage need to be controlled, the weight coefficient is usually introduced into the cost function in the traditional predictive control, a great amount of time is required in the design process, and when the system parameters or the working state change, the weight coefficient needs to be designed secondarily, so that the difficulty of the controller is increased.
Therefore, there is a need for a method that can improve the accuracy and response speed of system control while avoiding the introduction of weight coefficients, and improve the accuracy and robustness of system control when system parameters are changed.
Disclosure of Invention
The application provides a model predictive control method without a weight coefficient for a single-phase cascade H-bridge converter, which aims to solve the defects in the prior art.
In order to achieve the above purpose, the present application adopts the following technical scheme.
The embodiment provides a weight coefficient-free model prediction control method of a single-phase cascade H-bridge converter, which comprises the following steps:
s1, constructing a two-dimensional control area based on a switching function;
s2, dividing a two-dimensional control area into 2N+2 subareas according to the input voltage of the converter, wherein N is the total number of cascading units of the single-phase cascading H-bridge converter;
s3, determining a subarea where the input voltage reference value is located as an optimal subarea, and taking all switch states corresponding to two voltage vectors in the optimal subarea as candidate switch states of the corresponding voltage vectors;
s4, constructing a cost function based on the current error, and calculating the optimal acting time corresponding to each voltage vector according to the cost function based on the current error;
s5, constructing a cost function based on voltage errors, and respectively selecting the corresponding optimal switch states from the candidate switch states corresponding to the two voltage vectors according to the cost function based on the voltage errors;
and S6, taking the obtained optimal action time and optimal switching state of the two voltage vectors as output to control the single-phase cascade H-bridge converter.
Preferably, the switching function is represented by the following formula (1):
wherein S is a Is the sum of the switching functions of the left bridge arm of the cascade H bridge, S b Is the sum of the switching functions of the right bridge arm of the cascade H bridge, S an Is the switch state of the upper switch of the left bridge arm of the nth unit,S bn the switch state of the upper switch of the right bridge arm of the nth unit is that the two switch states meet S an ,S bn ∈{0,1}。
Preferably, the step S1 includes: with the S a Is x-axis, with the S b Establishing a two-dimensional control area for the y-axis, wherein the two-dimensional control area comprises (N+1) 2 A switch pair corresponding to 4 N A switch state.
Preferably, the step S2 includes:
according to the input voltage of the converter, connecting switch pairs with the same input voltage vector of the converter in the two-dimensional control area;
and dividing the single-phase cascade H-bridge converter with the total number of the cascade units being N into 2N+2 subareas according to the equipotential lines.
Preferably, determining the sub-region in which the input voltage reference value is located as an optimal sub-region includes: numbering the sub-regions in sequence according to the sequence of the input voltage of the converter from small to large, and obtaining the optimal sub-region number where the input voltage reference value is located according to the following formula (2):
wherein z is the optimal sub-region number, floor (·) represents a rounding down,for the input voltage reference value of the converter, v d For a single cell capacitor voltage.
Preferably, the cost function based on the current error is shown in the following formula (3):
wherein J is i (k) As a cost function of the current error at time k,time k+1Network side current reference predicted value;
i s and (k+1) is a net side current predicted value at the time of k+1, and the calculation formula is as follows (4):
wherein i is s (k) For the network side current at time k, t ρ The action time of the rho-th voltage vector;
f is,ρ the network side current change rate corresponding to the rho voltage vector selected in the optimal subarea is shown as the following formula (5):
wherein L is s R is the equivalent inductance of the net side s V is the equivalent resistance of the net side s For the mains voltage, i s For net side current, v in,ρ The voltage is input to the rho-th converter.
Preferably, calculating the optimal action time corresponding to each voltage vector according to the cost function based on the current error includes:
one of the two voltage vectors v in,1 The optimal time of action of (2) is obtained by deriving the current error cost function from the derivative of 0 according to the following equation (6):
wherein J is i To be the cost function of current error, t 1 Is the voltage vector v in,1 Is used for the action time of the (a);
another voltage vector v in,2 The optimal time of action of (2) is obtained according to the following formula (7):
t 2 ＝T s -t 1 (7)
wherein t is 2 Is the voltage vector v in,2 Is used for the action of (a)Between T s For the sampling time, i.e. one control period.
Preferably, the cost function based on the voltage error is shown in the following formula (8):
wherein J is v (k) As a cost function of the voltage error at time k,reference predicted value of capacitance voltage of nth unit at k+1 time, v d,n (k+1) is a predicted value of the capacitance voltage of the nth cell at time k+1.
Preferably, selecting the corresponding optimal switch state from the candidate switch states corresponding to the two voltage vectors according to the cost function based on the voltage error, respectively, including: and respectively selecting the switch state with the minimum corresponding voltage error cost function from the candidate switch states of the two voltage vectors as the optimal switch state.
Preferably, the converter input voltage is calculated according to the following formula (9):
wherein v is d ＝v d,n (n=1, 2,) N is the single cell capacitor voltage.
According to the technical scheme provided by the model predictive control method without the weight coefficient for the single-phase cascade H-bridge converter, the method is characterized in that a two-dimensional control area is constructed based on a switching function, and the control area is divided into a plurality of subareas according to the input voltage reference value of the converter; firstly, considering network side current tracking control, determining a subarea where an input voltage reference value is located as an optimal subarea, taking a switching state corresponding to two voltage vectors in the subarea as a candidate switching state, constructing a cost function based on current errors, and calculating the optimal action time of the two vectors; further considering capacitor voltage balance control, constructing a cost function based on voltage errors, and selecting the optimal switching state corresponding to the two vectors from the candidate switching states as an output control converter. The method can realize the step-by-step independent control of the network side current and the capacitor voltage, eliminates the weight coefficient, avoids the parameter setting and readjusting process of the controller, has simple control structure and reduces the algorithm execution time. The method has the advantages of good steady state and dynamic performance, high control precision, high response speed and strong robustness to the change of the internal parameters of the system.
Additional aspects and advantages of the application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the application.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for the description of the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
Fig. 1 is a schematic flow chart of a control method for predicting a model without a weight coefficient of a single-phase cascaded H-bridge converter according to an embodiment;
fig. 2 is a main circuit topology structure diagram of a single-phase cascaded H-bridge converter provided in an embodiment;
fig. 3 is a schematic diagram of a two-dimensional control area of a single-phase three-unit cascaded H-bridge converter according to an embodiment;
fig. 4 is a schematic diagram of a two-dimensional control area division result of the single-phase three-unit cascaded H-bridge converter provided in the embodiment;
FIG. 5 is a diagram showing waveforms of the grid side current, the capacitor voltage and the input voltage of the converter according to the embodiment provided with different weight coefficients;
fig. 6 is a waveform diagram of the grid-side current, the capacitor voltage and the input voltage of the converter according to the different control methods provided in the embodiments.
Detailed Description
Embodiments of the present application are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals refer to the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the drawings are exemplary only for explaining the present application and are not to be construed as limiting the present application.
As used herein, the singular forms "a", "an", "the" and "the" are intended to include the plural forms as well, unless expressly stated otherwise, as understood by those skilled in the art. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, and/or operations, but do not preclude the presence or addition of one or more other features, integers, steps, and/or groups thereof. It will be understood that the term "and/or" as used herein includes any and all combinations of one or more of the associated listed items.
It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
For the purpose of facilitating an understanding of the embodiments of the present application, reference will now be made to the drawings, by way of example, and not to the limitation of the embodiments of the present application.
Examples
Fig. 1 is a flow chart of a control method for predicting a model without a weight coefficient of a single-phase cascaded H-bridge converter according to the embodiment, and referring to fig. 1, the method includes the following steps:
s1, constructing a two-dimensional control area based on a switching function.
Fig. 2 is a main circuit topology diagram of a single-phase cascaded H-bridge converter according to this embodiment, and referring to fig. 2, a cascaded H-bridge is formed by connecting N units in series. The upper part of the left bridge arm and the right bridge arm of the nth unitS for the switching states of two switches an 、S bn (n=1, 2,., N) which satisfies the following formula (1)
S an ,S bn ∈{0,1} (1)
Because the upper and lower switches of the same bridge arm cannot be conducted simultaneously, each unit has 4 switch states, the power supply is used (S an ,S bn ) Can be expressed in the form of (1, 0), (0, 1), (1, 1) and (0, 0).
The switching function is shown in the following formula (2):
wherein S is a Is the sum of the switching functions of the left bridge arm of the cascade H bridge, S b Is the sum of the switching functions of the right bridge arm of the cascade H bridge.
With the S a Is x-axis, with the S b For the y-axis, a two-dimensional control region is established, for example, a single-phase three-cell cascaded H-bridge converter, the control region of which is shown in FIG. 3, comprising 16 switch pairs (S a ,S b ) The number of the corresponding switch states is the number of the rectangular frames in the upper right corner of each group of switch pairs, and the total number of the switch states is 64 in the control area. Correspondingly, for the single-phase N-cell cascade H-bridge converter, the two-dimensional control area comprises (N+1) 2 A switch pair corresponding to 4 N A switch state.
S2, dividing the two-dimensional control area into 2N+2 subareas according to the input voltage of the converter, wherein N is the total number of cascading units of the single-phase cascading H-bridge converter.
According to the structure of fig. 2, using kirchhoff's voltage law and the euler forward method, the net side current expression is:
wherein i is s (k+1) is a net-side current predicted value at time k+1, L s R is the equivalent inductance of the net side s Is the equivalent resistance of the net side, T s In order to sample the time of the sample,i.e. a control period, v s (k) For the network side supply voltage at time k, v in (k) Input voltage of current transformer at k moment, i s (k) The net side current at time k.
With the switching state, the converter input voltage is calculated according to the following equation (4):
wherein v is d ＝v d,n (n=1, 2,) N is the single cell capacitor voltage.
According to the formulas (3) and (4), when the input voltages of the corresponding converters of different switch pairs are the same, the output results of the control signals are identical. The switch pairs with the same input voltage vector of the converter are connected, and the control area is divided by using equipotential lines. Fig. 4 is a schematic diagram of a two-dimensional control region division result of the single-phase three-unit cascaded H-bridge converter provided in this embodiment, which includes 8 sub-regions (corresponding numbers I to VIII). In the figure, the numbers in the circles correspond to 1-16 switch pairs respectively, and the dotted lines represent different input voltages (-3 v) of the converter d ,-2v d ,...,3v d ). Accordingly, for a single-phase N-cell cascaded H-bridge converter, the control region may be divided into 2n+2 sub-regions.
S3, determining a subarea where the input voltage reference value is located as an optimal subarea, and taking all switch states corresponding to two voltage vectors in the optimal subarea as candidate switch states of the corresponding voltage vectors.
Considering first the grid-side current tracking control, since the grid-side current is only determined by the converter input voltage, the input voltage reference value can be used to select the optimal sub-region. The converter input voltage reference value may be expressed as:
wherein, the liquid crystal display device comprises a liquid crystal display device,input voltage reference value for the converter, +.>Is the net side current reference.
Numbering the sub-regions in sequence according to the sequence of the input voltage of the converter from small to large, and obtaining the optimal sub-region number where the input voltage reference value is located according to the following formula (6):
wherein z is the optimal sub-region number, floor (·) represents a rounding down,for the input voltage reference value of the converter, v d For a single cell capacitor voltage.
S4, constructing a cost function based on the current error, and calculating the optimal acting time corresponding to each voltage vector according to the cost function based on the current error.
In order to improve the system performance, a double-vector model predictive control is adopted to select two voltage vectors v in an optimal subarea in,1 And v in,2 The expected values are synthesized.
According to equation (3), the net side current change rate for each voltage vector is:
wherein f is,ρ For the network side current change rate (ρ=1, 2) corresponding to the ρ -th voltage vector selected in the optimal sub-region, L s R is the equivalent inductance of the net side s V is the equivalent resistance of the net side s For the mains voltage, i s For net side current, v in,ρ The voltage is input to the rho-th converter.
Further obtaining a network side current predicted value at the time k+1, wherein the calculation formula is as follows (8):
wherein i is s (k) For the network side current at time k, t ρ The action time of the rho-th voltage vector;
constructing a cost function based on the current error as shown in the following formula (9):
wherein J is i (k) As a cost function of the current error at time k,the predicted value is referenced to the grid-side current at time k+1.
In order to realize tracking of the network side current reference value, the optimal control effect is achieved, namely, the current error cost function is minimum, and one voltage vector v of the two voltage vectors in,1 The optimal time of action of (2) is obtained by deriving the current error cost function from the derivative of 0 according to the following equation (10):
wherein J is i To be the cost function of current error, t 1 Is the voltage vector v in, 1, the action time of the catalyst;
substituting formulas (8) and (9) into (10) to obtain
According to formula (11), whenWhen the current error cost function has a minimum value. Further, taking into account the actual system constraints, it is possible toTo the point of
Wherein t is 2 Is the voltage vector v in,2 Is used for the action time of the (a).
Thus, two voltage vectors v in,1 And v in,2 Corresponding optimal action time t 1,opt And t 2,opt The method comprises the following steps of:
if the input voltage reference value of the converter is located in the region III in FIG. 4, the two voltage vectors are v in,1 ＝-2v d 、v in,2 ＝-v d The corresponding optimal action time can be obtained according to the formula (13); if the input voltage reference value of the converter is located in the region I or the region VIII, the two voltage vectors are v respectively in,1 ＝v in,2 ＝-3v d Or v in,1 ＝v in,2 ＝3v d Voltage vector v in,1 The corresponding optimal action time is t 1,opt ＝T s Voltage vector v in,2 The corresponding optimal action time is t 2,opt ＝0。
S5, constructing a cost function based on the voltage error, and respectively selecting the corresponding optimal switch states from the candidate switch states corresponding to the two voltage vectors according to the cost function based on the voltage error.
According to the analysis, the network side current can be effectively tracked in the optimal subarea. Further, it is necessary to implement balance control of capacitor voltage, and construct a cost function J based on voltage error v The following formula (14):
wherein J is v (k) For the voltage error cost function at time k，Reference predicted value of capacitance voltage of nth unit at k+1 time, v d,n (k+1) is a predicted value of the capacitance voltage of the nth cell at time k+1.
And respectively selecting the switch state with the minimum corresponding voltage error cost function from the candidate switch states of the two voltage vectors as the optimal switch state.
If the converter input voltage reference value is located in region III in FIG. 4, then J can be enabled in the switch state corresponding to switch pair {2,3} v The smallest switching state being the voltage vector v in,1 In the optimal switching state of the switch pair {4,5,6}, J can be made to be v The smallest switching state being the voltage vector v in,2 Is provided for the optimum switching state of (a).
And S6, taking the obtained optimal action time and optimal switching state of the two voltage vectors as output to control the single-phase cascade H-bridge converter.
In summary, for the single-phase three-unit cascaded H-bridge converter in this embodiment, in the unit control period, the conventional model predictive control algorithm needs to traverse all the switching states (64 in total), and the application of the method only needs to select the optimal switching state from 21 candidate switching states, so that the algorithm execution time is greatly shortened.
The following is a comparative example of the method of the present application and the method adopted in the prior art, and the simulation circuit is built according to the single-phase three-unit cascade H-bridge converter structure through MATLAB/Simulink simulation platform, so as to verify the tracking performance and response speed of the method of the present application.
According to the main circuit topology structure diagram of the single-phase cascade H-bridge converter of FIG. 2, a single-phase three-unit cascade H-bridge converter simulation model is built in a MATLAB/Simulink simulation platform. The key parameters for the simulation are shown in table 1 below.
TABLE 1
Firstly, the influence of the weight coefficient on the system stability is analyzed, and the network side current and capacitance voltage waveforms obtained under different weight coefficients lambda are obtained by adopting a traditional model prediction method, as shown in figure 5. From fig. 5 (a), as λ gradually increases from 0.0001 to 0.1, the net side current total harmonic distortion increases from 4.29% to 5.90%. From fig. 5 (b), when λ is relatively small (0.0001), the dc side capacitance voltage is extremely unbalanced; when λ is relatively large (0.1), the controller can effectively track the capacitance voltage reference value. Therefore, the weight coefficient directly determines the tracking effect of the network side current and the balance degree of the capacitor voltage, the value of the weight coefficient is selected according to different system parameters and control targets by comprehensively considering the total harmonic distortion rate of the network side current and the capacitor voltage, and the execution time and the complexity of an algorithm are obviously increased.
Based on the above analysis, the weight coefficient of the conventional model prediction method is set to be 0.03, and performance comparison analysis is performed with the method of this embodiment, and as a result, as shown in fig. 6, it can be seen by comparison that both methods can effectively track the network side current and the capacitor voltage. As can be seen from fig. 6, the method of the present embodiment can reduce the total harmonic distortion of the grid-side current from 5.84% to 3.39%, and reduce the total harmonic distortion of the input voltage of the converter from 50.70% to 38.63%. Therefore, compared with the traditional model prediction method, the method of the embodiment does not need to introduce weight coefficients, has low system complexity and high control precision, and effectively reduces the total harmonic distortion rate of network side current and converter input voltage.
It will be appreciated by those skilled in the art that the above application types are merely examples, and that other application types that may be present in the present application or that may be present in the future are intended to be within the scope of the present application as applicable thereto and are hereby incorporated by reference herein.
It should be understood by those skilled in the art that the above-mentioned decision to invoke a policy according to user information is merely a better illustration of the technical solution of the embodiments of the present application, and is not a limitation of the embodiments of the present application. Any method for determining a calling policy based on user attributes is included in the scope of the embodiments of the present application.
From the above description of embodiments, it will be apparent to those skilled in the art that the present application may be implemented in software plus a necessary general hardware platform. Based on such understanding, the technical solution of the present application may be embodied essentially or in a part contributing to the prior art in the form of a software product, which may be stored in a storage medium, such as a ROM/RAM, a magnetic disk, an optical disk, etc., including several instructions for causing a computer device (which may be a personal computer, a server, or a network device, etc.) to execute the method described in the embodiments or some parts of the embodiments of the present application.
The present application is not limited to the above-mentioned embodiments, and any changes or substitutions that can be easily understood by those skilled in the art within the technical scope of the present application are intended to be included in the scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.
Claims (2)
1. A weight coefficient-free model prediction control method of a single-phase cascade H-bridge converter is characterized by comprising the following steps of:
s1, constructing a two-dimensional control area based on a switching function; the switching function is represented by the following formula (1):
wherein, the cascade H bridge is formed by connecting N units in series, S a Is the sum of the switching functions of the left bridge arm of the cascade H bridge, S b Is the sum of the switching functions of the right bridge arm of the cascade H bridge, S an Is the switch state of the upper switch of the left bridge arm of the nth unit, S bn The switch state of the upper switch of the right bridge arm of the nth unit is that the two switch states meet S an ,S bn E {0,1}; the step S1 comprises the following steps: with the S a Is x-axis, with the S b Establishing a two-dimensional control area for the y-axis, two-dimensionalThe control region contains (N+1) 2 A switch pair corresponding to 4 N A plurality of switch states;
s2, dividing a two-dimensional control area into 2N+2 subareas according to the input voltage of the converter, wherein N is the total number of cascading units of the single-phase cascading H-bridge converter; the method specifically comprises the following steps: according to the input voltage of the converter, connecting switch pairs with the same input voltage vector of the converter in the two-dimensional control area;
dividing a single-phase cascade H-bridge converter with the total number of cascade units being N into 2N+2 subareas according to equipotential lines;
s3, determining a subarea where the input voltage reference value is located as an optimal subarea, and taking all switch states corresponding to two voltage vectors in the optimal subarea as candidate switch states of the corresponding voltage vectors; the determining the sub-area where the input voltage reference value is located as the optimal sub-area includes: numbering the sub-regions in sequence according to the sequence of the input voltage of the converter from small to large, and obtaining the optimal sub-region number where the input voltage reference value is located according to the following formula (2):
wherein z is the optimal sub-region number, floor (·) represents a rounding down,for the input voltage reference value of the converter, v d A single cell capacitor voltage;
s4, constructing a cost function based on the current error, and calculating the optimal acting time corresponding to each voltage vector according to the cost function based on the current error; the cost function based on the current error is shown in the following formula (3):
wherein J is i (k) For current error at time kThe difference cost function is used to determine the difference cost function,the current reference predicted value of the network side at the moment k+1;
i s and (k+1) is a net side current predicted value at the time of k+1, and the calculation formula is as follows (4):
wherein i is s (k) For the network side current at time k, t ρ The action time of the rho-th voltage vector;
f is,ρ the network side current change rate corresponding to the rho voltage vector selected in the optimal subarea is shown as the following formula (5):
wherein L is s R is the equivalent inductance of the net side s V is the equivalent resistance of the net side s For the mains voltage, i s For net side current, v in,ρ The voltage is input to the rho-th converter;
s5, constructing a cost function based on voltage errors, and respectively selecting the corresponding optimal switch states from the candidate switch states corresponding to the two voltage vectors according to the cost function based on the voltage errors; the method comprises the steps of calculating the optimal acting time corresponding to each voltage vector according to a cost function based on current errors, and specifically comprises the following steps:
one of the two voltage vectors v in,1 The optimal time of action of (2) is obtained by deriving the current error cost function from the derivative of 0 according to the following equation (6):
wherein J is i To be the cost function of current error, t 1 Is the voltage vector v in,1 Is used for the action time of the (a);
another voltage vector v in,2 The optimal time of action of (2) is obtained according to the following formula (7):
t 2 ＝T s -t 1 (7)
wherein t is 2 Is the voltage vector v in,2 Time of action, T s Sampling time, namely a control period;
the cost function based on the voltage error is shown in the following formula (8):
wherein J is v (k) As a cost function of the voltage error at time k,reference predicted value of capacitance voltage of nth unit at k+1 time, v d,n (k+1) is a capacitance voltage predicted value of the nth cell at time k+1;
s6, taking the obtained optimal action time and optimal switching state of the two voltage vectors as output to control the single-phase cascade H-bridge converter, wherein the method comprises the following steps of: and respectively selecting the switch state with the minimum corresponding voltage error cost function from the candidate switch states of the two voltage vectors as the optimal switch state.
2. The method of claim 1, wherein the converter input voltage is calculated according to the following equation (9):
wherein v is d ＝v d,n (n=1, 2,) N is the single cell capacitor voltage.
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