CN116526556A - Model-free predictive control method for parallel T-shaped three-level rectifier - Google Patents
Model-free predictive control method for parallel T-shaped three-level rectifier Download PDFInfo
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- H—ELECTRICITY
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Abstract
The invention provides a model-free predictive control method for a parallel T-shaped three-level rectifier. The method adopts double closed-loop control, and the outer loop adopts self-adaptive droop control with voltage feedforward to realize the stabilization of direct-current voltage, high power factor and load power distribution; the inner loop adopts model-free predictive control, firstly, a discrete mathematical model of current, midpoint voltage and circulation of an inner loop control target power grid is established; on the basis, a model-free predictive control implementation method of three control targets is designed, wherein a synchronous update method is adopted for updating the current difference; finally, the multi-objective optimization of the inner ring model-free predictive control is realized. The model-free predictive control is based on data driving, does not depend on any hardware parameters and circuit models, and has strong parameter robustness; the multi-objective model-free predictive control of the current, the midpoint voltage and the circulation of the power grid in the parallel system is realized; a synchronous update method is used to update the current difference data, while the current difference is updated more frequently than a single rectifier due to the relationship of the circulating current and the common mode voltage. The whole scheme realizes the multi-objective optimization control of the parallel operation system, and greatly improves the parameter robustness of the system.
Description
Technical Field
The invention belongs to the technical field of power electronic converter control, and particularly relates to a model-free predictive control method for a parallel T-shaped three-level rectifier.
Background
At present, along with the wide application of new energy automobiles, a large-scale direct current charging technology becomes increasingly a development trend of new energy automobile charging, and in addition, high-power rectifiers are required in the fields of railway locomotive traction, mine lifting systems and wind power generation systems. The power density and current harmonic superiority of the T-shaped three-level rectifier promote the T-shaped three-level rectifier to be applied to high-power occasions. When a single T-type three-level rectifier fails to meet load power demands, a parallel configuration is often required to further increase power levels. However, parallel T-type three-level rectifiers face the problems of power distribution, dc voltage stabilization, power factor, midpoint voltage balancing, and loop current suppression.
The model predictive control predicts state variables through a system model, and selects an optimal solution through a design cost function, so that the model predictive control has obvious control advantages in a multi-target complex control system, but the control effect of the model predictive control depends on the accuracy of the system model, and is difficult to play a role when parameters are mismatched and the circuit relationship is unknown. The existing control method for improving the robustness of the model predictive control parameters is complex in calculation process, and the model-free control method with multi-focus on current tracking ignores the multi-target optimization function of model-free control. Therefore, a model-free predictive control method suitable for the parallel rectifier system needs to be researched, multi-objective control of the parallel rectifier system is realized, and the parameter robustness of model predictive control is improved.
Droop control is a common method of achieving rectifier power distribution, dc voltage stabilization, high power factor, the basic principle of which is to linearly decrease the voltage as the output current increases, but the V-I droop control curve determines that droop control inevitably results in droop of the dc voltage. Therefore, an adaptive droop control method is needed to reduce the droop of the dc voltage.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a model-free predictive control method for a parallel T-shaped three-level rectifier.
In order to achieve the aim of the invention, the whole technical scheme adopted by the invention is double closed loop control, the outer loop adopts self-adaptive droop control with voltage feedforward to realize power distribution, high power factor and stable direct current voltage, and the inner loop adopts model-free predictive control to realize balance of midpoint voltage, circulation suppression and tracking of grid current.
A model-free predictive control method for parallel T-shaped three-level rectifiers is provided, wherein the adaptive droop control of the outer loop voltage feedforward comprises the following specific contents:
droop control of the direct current voltage controls active power input to the direct current network by detecting a difference between the direct current voltage and a reference voltage, thereby achieving power balance and voltage stabilization. In the invention, the V-I characteristic droop control is adopted, and the V-I characteristic droop control can be expressed as:
u in dc ，Is a direct current voltageReference value, I dc ，/>Is direct current and reference value, K d For the V-I characteristic sag factor, sag factor K d Changing to an adaptive droop coefficient, a system operation reference point is typically set as: when the transmission power is 0, the voltage of the direct current bus is the rated voltage of the system; i.e. set->Is rated voltage. After adding the adaptive droop coefficient, the V-I characteristic droop control is changed into:
wherein P is N The droop coefficient changes along with the change of the direct current, representing the rated power of the rectifier, and the adaptive droop control is realized. When the two rectifiers are connected in parallel, the principle of droop control to realize power distribution is shown in fig. 1.
Due to the principle defect of droop control, the direct current voltage has a certain droop. To improve this droop characteristic, a feedforward amount is added based on the difference between the dc voltage and the reference value to compensate for the effect of droop control on the dc voltage.
A model-free predictive control method for parallel T-shaped three-level rectifiers comprises the following specific steps:
s1, establishing a discretization mathematical model of current, midpoint voltage and circulation of an inner loop control target grid of a parallel T-shaped three-level rectifier, and determining a model-free predictive control method of the inner loop control target;
s2, sampling the power grid current i at the moment k n1 (k) Switch state S n1 (k) DC side capacitor voltage and calculating to obtain midpoint voltage deviation Deltav 1 (k) Converting the sampled power grid current from a three-phase coordinate system to a two-phase rotation coordinate system, and utilizing the power grid current and time delay under the two-phase rotation coordinate systemCalculating current difference delta i at time k in link 1m |V(k)；
S3, updating a current difference data table according to the current difference sampled at the moment k and the switching state, and updating current difference data caused by the switching state of the coaxial lines in the 27 switching vector phase tables according to a model-free predictive control method of current tracking;
s4, predicting current differences delta i caused by different switch vectors k+1 according to the acquired data and the updated current difference data table 1m (k+1) substituting the current tracking cost function g 1j And calculating and sequencing current errors of the two-phase rotating coordinate system caused by different switching vectors, and traversing and selecting the optimal switching vector which is simultaneously compatible with current tracking, midpoint voltage balancing and circulation suppression according to the sequenced switching vectors.
Further, the establishing of the parallel T-shaped three-level rectifier power grid current discretization mathematical model for realizing model-free control of power grid current tracking specifically comprises the following steps:
the two rectifiers are identical in model structure except that the transmission power and hardware parameters are different, so the first rectifier is taken as an example for analysis. The mathematical model of the parallel T-shaped three-level rectifier under the two-phase rotation coordinate system is as follows:
i in 1d ，i 1q Dq-axis grid current for first rectifier, L 1 R is the inductance of the filter 1 E is parasitic resistance of the filter d ，e q For dq axis grid voltage, u 1d ，u 1q For the rectifier input voltage ω is the dq axis rotational angular velocity.
The discrete model of the parallel T-shaped three-level rectifier can be obtained by discretizing the established T-shaped three-level rectifier power grid current mathematical model by using an Euler formula, and is as follows:
wherein (k), (k+1) is the sampling time, T s Is the sampling period. Model-free predictive control of grid current tracking is established based on current difference data, and the calculation formula of the dq-axis grid current difference at the time k and the time k+1 is as follows
In Deltai 1m (m=d, q) is the dq axis grid current difference, i 1m Is the dq-axis component of the grid current. In the fifth formula, the current difference at the time k+1 cannot be obtained by sampling, so that the current difference at the time k+1 cannot be obtained by calculation, but under the condition that the sampling frequency is high enough, old data stored in a data table can be used for replacing the current difference caused by different switching vectors at the time k+1. The power grid current prediction equation for the different switching vectors at time k+1 is therefore expressed as:
i 1m (k+1)|V j ＝i 1m (k)+Δi 1m,old |V j ,V j ∈{V 1 ,...,V 27 six-piece (V-shaped)
I in 1m (k+1)|V j For the moment k+1, the switching vector V j Is a dq-axis grid current predictive value, Δi 1m，old |V j For switch vector V stored in data table j Induced current difference, i 1m (k) Is the sampled value of the grid current at time k. The cost function of the grid current tracking is:
in the middle of For the reference value of the dq axis grid current at the time k+1, i 1d (k+1)|V j ，i 1q (k+1)|V j When k+1 is usedSwitch vector V of the etch prediction j The resulting dq axis grid current. The grid current reference is obtained from adaptive droop control of the inner loop band voltage feedforward.
Further, the establishing a parallel T-shaped three-level rectifier midpoint voltage discretization mathematical model to realize model-free control of midpoint voltage balance specifically comprises the following steps:
the mathematical model of the midpoint voltage deviation on the direct current side is as follows:
i in np For the current flowing into the neutral point, i n1 (n=a, B, C) is three-phase grid current, S n1 In the three-phase switch state, C 11 Is the direct-current side capacitance, deltaV 1 Is the voltage deviation of the midpoint of the direct current side.
After the midpoint voltage deviation is discretized, the method comprises the following steps:
from the result of the formula nine, i np1 ×ΔV 1 (k) When the voltage is less than or equal to 0, the midpoint voltage is balanced, and model-free predictive control of the midpoint voltage can be realized according to the relation between the current flowing into the neutral point and the switch state.
Further, the establishing a mathematical model of the zero sequence circulation of the parallel T-shaped three-level rectifier, and realizing model-free control of the circulation inhibition specifically comprises the following steps:
according to kirchhoff's law of zero sequence circulation flow path, it can be obtained:
v in Nx (x=1, 2) is the dc side lower side capacitor voltage, U nxOx R is the voltage between the midpoint of the DC side and the input point of the rectifier x Parasitic resistance, L, of the filter inductance of the two rectifiers x Filter inductance i for two rectifiers nx (n=a, B, C) is the three-phase grid current of the two rectifiers. V (V) Nx And U nxOx Expressed as:
u in cmvx Is the common-mode voltage of two rectifiers, deltaV x Is the midpoint voltage deviation of the two rectifiers. According to the definition of zero sequence loop, the zero sequence loop can be expressed as:
i in z Is zero sequence circulation. The mathematical model for comprehensively obtaining the circulation is as follows:
thirteen can see that the main influencing factors of zero sequence circulation are midpoint voltage and common mode voltage difference of the two rectifiers. According to the relation between the common-mode voltage and the switch state and the model-free predictive control of the midpoint voltage, the model-free predictive control of the circulation can be realized.
Further, the specific steps of updating the power grid current difference are as follows:
from a mathematical model of the current difference it can be seen that the current difference consists of the natural response of the zero-switch vector and the forced response of the non-zero vector:
wherein the method comprises the steps of
In delta i 1d0 ，δi 1q0 Natural response, δi, for zero vector 1d |V j ，δi 1q |V j For forced response caused by non-zero vectors, Δi 1d |V j ，Δi 1q |V j For switching vector V j Induced current difference, u 1d |V j ，u 1q |V j For switching vector V j The resulting rectifier input voltage. In combination with the phase diagrams of the 27 switching vectors (fig. 2), the current difference relationship of a set of switching vectors under in-phase conditions can be derived. In the updating process of the current difference, the current difference and the switching state obtained by sampling at the moment k can determine the switching vector current difference data to be updated, and a group of switching vectors with the same phase are updated according to the phase relation of the switching vectors, so that the current difference data updated at each moment is not one but one group, and the updating frequency of the current difference is greatly improved. In addition, due to the relation between the common-mode voltage and the switching state, the optimal switching vector in steady-state operation is basically limited to 7 switching vectors enabling the common-mode voltage to be 0, and the 7 switching vectors are divided into 4 different phase relation groups, so that the updating frequency of the current difference is further improved.
Further, the specific steps of selecting the optimal solution by the model-free predictive control are as follows:
the model-free predictive control execution flow chart is shown in fig. 3. And calculating current tracking errors caused by 27 switch vectors according to the cost function of current tracking, sequencing, and performing traversal selection on the sequenced switch vectors until the optimal switch vector is selected. The specific steps of the optimal switching vector selection are as follows: firstly judging the position of a switch vector in the ordered switch vector, wherein the ordered position represents the magnitude of a current tracking error, the current tracking error is smaller when the position is closer, judging whether the midpoint voltage balance and the circulation suppression are met under the condition of meeting the current tracking, and selecting an optimal solution if the midpoint voltage balance and the circulation suppression are met; if the current is not satisfied, the selection condition is relaxed, and the switching vector satisfying the current inhibition is selected as the optimal solution; if the current midpoint voltage deviation meets the requirement, the current midpoint voltage deviation is judged to be satisfied, if the current midpoint voltage deviation meets the requirement, the circulation is continued, and if the current midpoint voltage deviation does not meet the requirement, a switching vector which meets the midpoint balance and sacrifices a certain circulation inhibition is selected as an optimal solution. The selection of the optimal switching vector comprehensively considers three control targets of inner loop current tracking, midpoint voltage balancing and loop current inhibition.
Compared with the prior art, the invention has the beneficial effects that:
(1) The invention adopts a double closed-loop control strategy, so that voltage control and current control can have different time scales and can be realized in different controllers. The direct-current voltage stabilization, the power distribution and the high power factor are realized by placing the direct-current voltage stabilization, the power distribution and the high power factor in an outer ring, and the control target of model-free predictive control of the inner ring is simplified. The double closed-loop control realizes the multi-objective optimal control of the parallel T-shaped three-level rectifier.
(2) The model-free predictive control method for three control targets of inner loop current tracking, midpoint voltage balance and loop current inhibition is established, and the optimal solution selection of the inner loop is realized under the condition of not using weight factors. The model-free predictive control method does not need to use any prior knowledge of hardware parameters and circuit models, and greatly improves the parameter robustness of the system.
(3) The invention adopts the synchronous updating method of the current difference, updates the current differences of different switch vectors with the same phase according to the phase relation of the switch vectors, and improves the updating frequency of the current difference. In addition, due to the relation between the common-mode voltage and the switching state, the switching vector update group of the parallel T-shaped three-level rectifier in the steady-state process is reduced from 7 groups to 4 groups, and the update frequency of the current difference is further improved.
Drawings
Fig. 1 illustrates the droop control power allocation principle in the present invention;
fig. 2 is a diagram of 27 switching vector phases in the present invention;
FIG. 3 is a flow chart of the model-free predictive control selection of optimal solution execution in the present invention;
FIG. 4 is a schematic diagram of two parallel T-type three-level rectifier topologies in the present invention;
FIG. 5 is a control block diagram of a two parallel T-type three level rectifier system in accordance with the present invention;
FIG. 6 is a simulation diagram of 7 sets of current difference updates in the present invention; wherein the graph (a) is 4 groups of 7 switching vectors that bring the common mode voltage to 0, and the graph (b) is another 3 groups that do not substantially participate in the optimal solution selection in steady state.
FIG. 7 shows waveforms of DC voltage and AC power change in case of voltage reference value change in the present invention; wherein the graph (a) is the difference between the DC voltage and the reference value, and the graph (b) is the AC side power;
FIG. 8 is a graph comparing model-free predictive control with prior art experiments under the parameter adaptation conditions of the present invention; wherein the graph (a) is the power grid current control effect, and the graph (b) is the loop current inhibition and midpoint voltage balance control effect.
Detailed Description
The technical solution will be clearly and completely described below in connection with the preferred examples and the accompanying drawings of the present invention. It should be understood that the preferred examples are illustrative of the present invention and are not intended to limit the scope of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without making any inventive effort are within the scope of the present invention.
The invention provides a model-free predictive control method for a parallel T-shaped three-level rectifier. The embodiment of the invention is used for a parallel system of two T-shaped three-level rectifiers with different powers. The whole technical scheme adopted by the invention is double closed-loop control, the outer loop adopts self-adaptive droop control with voltage feedforward to realize power distribution, high power factor and stable direct-current voltage, and the inner loop adopts model-free predictive control to realize balance of midpoint voltage, circulation suppression and tracking of grid current.
A model-free predictive control method for parallel T-shaped three-level rectifiers is provided, wherein the adaptive droop control of the outer loop voltage feedforward comprises the following specific contents:
droop control of the direct current voltage controls active power input to the direct current network by detecting a difference between the direct current voltage and a reference voltage, thereby achieving power balance and voltage stabilization. In the invention, the V-I characteristic droop control is adopted, and the V-I characteristic droop control can be expressed as:
u in dc ，Is a direct current voltage and a reference value thereof, I dc ，/>Is direct current and reference value, K d For the V-I characteristic sag factor, sag factor K d Changing to an adaptive droop coefficient, a system operation reference point is typically set as: when the transmission power is 0, the voltage of the direct current bus is the rated voltage of the system; i.e. set-> Is rated voltage. After adding the adaptive droop coefficient, the V-I characteristic droop control is changed into:
wherein P is N The droop coefficient changes along with the change of the direct current, representing the rated power of the rectifier, and the adaptive droop control is realized. When the two rectifiers are connected in parallel, the principle of droop control to realize power distribution is shown in fig. 1.
Due to the principle defect of droop control, the direct current voltage has a certain droop. To improve this droop characteristic, a feedforward amount is added based on the difference between the dc voltage and the reference value to compensate for the effect of droop control on the dc voltage.
A model-free predictive control method for parallel T-shaped three-level rectifiers comprises the following specific steps:
s1, establishing a discretization mathematical model of current, midpoint voltage and circulation of an inner loop control target grid of a parallel T-shaped three-level rectifier, and determining a model-free predictive control method of the inner loop control target;
s2, sampling the power grid current i at the moment k n1 (k) Switch state S n1 (k) DC side capacitor voltage and calculating to obtain midpoint voltage deviation DeltaV 1 (k) Converting the sampled power grid current from a three-phase coordinate system to a two-phase rotation coordinate system, and calculating the current difference delta i at the moment k by using the power grid current and a delay link under the two-phase rotation coordinate system 1m |V(k)；
S3, updating a current difference data table according to the current difference sampled at the moment k and the switching state, and updating current difference data caused by the switching state of the coaxial lines in the 27 switching vector phase tables according to a model-free predictive control method of current tracking;
s4, predicting current differences delta i caused by different switch vectors k+1 according to the acquired data and the updated current difference data table 1m (k+1) substituting the current tracking cost function g 1j And calculating and sequencing current errors of the two-phase rotating coordinate system caused by different switching vectors, and traversing and selecting the optimal switching vector which is simultaneously compatible with current tracking, midpoint voltage balancing and circulation suppression according to the sequenced switching vectors.
Further, the establishing of the parallel T-shaped three-level rectifier power grid current discretization mathematical model for realizing model-free control of power grid current tracking specifically comprises the following steps:
the two rectifiers are identical in model structure except that the transmission power and hardware parameters are different, so the first rectifier is taken as an example for analysis. The mathematical model of the parallel T-shaped three-level rectifier under the two-phase rotation coordinate system is as follows:
i in 1d ，i 1q Dq-axis grid current for first rectifier, L 1 R is the inductance of the filter 1 E is parasitic resistance of the filter d ，e q For dq axis grid voltage, u 1d ，u 1q For the rectifier input voltage ω is the dq axis rotational angular velocity.
The discrete model of the parallel T-shaped three-level rectifier can be obtained by discretizing the established T-shaped three-level rectifier power grid current mathematical model by using an Euler formula, and is as follows:
wherein (k), (k+1) is the sampling time, T s Is the sampling period. Model-free predictive control of grid current tracking is established based on current difference data, and the calculation formula of the dq-axis grid current difference at the time k and the time k+1 is as follows
In Deltai 1m (m=d, q) is the dq axis grid current difference, i 1m Is the dq-axis component of the grid current. In the fifth formula, the current difference at the time k+1 cannot be obtained by sampling, so that the current difference at the time k+1 cannot be obtained by calculation, but under the condition that the sampling frequency is high enough, old data stored in a data table can be used for replacing the current difference caused by different switching vectors at the time k+1. The power grid current prediction equation for the different switching vectors at time k+1 is therefore expressed as:
i 1m (k+1)|V j ＝i 1m (k)+Δi 1m,old |V j ,V j ∈{V 1 ,...,V 27 six-piece (V-shaped)
I in 1m (k+1)|V j For the moment k+1, the switching vector V j Is a dq-axis grid current predictive value, Δi 1m，old |V j For switch vector V stored in data table j Induced current difference, i 1m (k) Is the sampled value of the grid current at time k. The cost function of the grid current tracking is:
in the middle of For the reference value of the dq axis grid current at the time k+1, i 1d (k+1)|V j ，i 1q (k+1)|V j Switch vector V predicted for time k+1 j The resulting dq axis grid current. The grid current reference is obtained from adaptive droop control of the inner loop band voltage feedforward.
Further, the establishing a parallel T-shaped three-level rectifier midpoint voltage discretization mathematical model to realize model-free control of midpoint voltage balance specifically comprises the following steps:
the mathematical model of the midpoint voltage deviation on the direct current side is as follows:
i in np1 For the current flowing into the neutral point, i n1 (n=a, B, C) is three-phase grid current, S n1 In the three-phase switch state, C 11 Is the direct-current side capacitance, deltaV 1 Is the voltage deviation of the midpoint of the direct current side.
After the midpoint voltage deviation is discretized, the method comprises the following steps:
from the result of the formula nine, i np1 ×ΔV 1 (k) When the voltage is less than or equal to 0, the midpoint voltage is balanced, and model-free predictive control of the midpoint voltage can be realized according to the relation between the current flowing into the neutral point and the switch state.
Further, the establishing a mathematical model of the zero sequence circulation of the parallel T-shaped three-level rectifier, and realizing model-free control of the circulation inhibition specifically comprises the following steps:
according to kirchhoff's law of zero sequence circulation flow path, it can be obtained:
v in Nx (x=1, 2) is the dc side lower side capacitor voltage, U nxO R is the voltage between the midpoint of the DC side and the input point of the rectifier x Parasitic resistance, L, of the filter inductance of the two rectifiers x Filter inductance i for two rectifiers nx (n=a, B, C) is the three-phase grid current of the two rectifiers. V (V) Nx And U nxOx Expressed as:
u in cmvx Is the common-mode voltage of two rectifiers, deltaV x Is the midpoint voltage deviation of the two rectifiers. According to the definition of zero sequence loop, the zero sequence loop can be expressed as:
i in z Is zero sequence circulation. The mathematical model for comprehensively obtaining the circulation is as follows:
thirteen can see that the main influencing factors of zero sequence circulation are midpoint voltage and common mode voltage difference of the two rectifiers. According to the relation between the common-mode voltage and the switch state and the model-free predictive control of the midpoint voltage, the model-free predictive control of the circulation can be realized.
Table 1 shows the relationship between the switch state and the common mode voltage
Further, the specific steps of updating the power grid current difference are as follows:
from a mathematical model of the current difference it can be seen that the current difference consists of the natural response of the zero-switch vector and the forced response of the non-zero vector:
wherein the method comprises the steps of
In delta i 1d0 ，δi 1q0 Natural response, δi, for zero vector 1d |V j ，δi 1q |V j For forced response caused by non-zero vectors, Δi 1d |V j ，Δi 1q |V j For switching vector V j Induced current difference, u 1d |V j ，u 1q |V j For switching vector V j The resulting rectifier input voltage. In combination with the phase diagrams of the 27 switching vectors (fig. 2), the current difference relationship of a set of switching vectors under in-phase conditions can be derived. In the updating process of the current difference, the current difference and the switching state obtained by sampling at the moment k can determine the switching vector current difference data to be updated, and a group of switching vectors with the same phase are updated according to the phase relation of the switching vectors, so that the current difference data updated at each moment is not one but one group, and the updating frequency of the current difference is greatly improved. In addition, due to the relation between the common-mode voltage and the switching state, the optimal switching vector in steady-state operation is basically limited to 7 switching vectors enabling the common-mode voltage to be 0, and the 7 switching vectors are divided into 4 different phase relation groups, so that the updating frequency of the current difference is further improved.
Further, the specific steps of selecting the optimal solution by the model-free predictive control are as follows:
the model-free predictive control execution flow chart is shown in fig. 3. And calculating current tracking errors caused by 27 switch vectors according to the cost function of current tracking, sequencing, and performing traversal selection on the sequenced switch vectors until the optimal switch vector is selected. The specific steps of the optimal switching vector selection are as follows: firstly judging the position of a switch vector in the ordered switch vector, wherein the ordered position represents the magnitude of a current tracking error, the current tracking error is smaller when the position is closer, judging whether the midpoint voltage balance and the circulation suppression are met under the condition of meeting the current tracking, and selecting an optimal solution if the midpoint voltage balance and the circulation suppression are met; if the current is not satisfied, the selection condition is relaxed, and the switching vector satisfying the current inhibition is selected as the optimal solution; if the current midpoint voltage deviation meets the requirement, the current midpoint voltage deviation is judged to be satisfied, if the current midpoint voltage deviation meets the requirement, the circulation is continued, and if the current midpoint voltage deviation does not meet the requirement, a switching vector which meets the midpoint balance and sacrifices a certain circulation inhibition is selected as an optimal solution. The selection of the optimal switching vector comprehensively considers three control targets of inner loop current tracking, midpoint voltage balancing and loop current inhibition.
Fig. 4 is a topological block diagram of two rectifiers connected in parallel according to an advantageous embodiment of the present invention. Fig. 5 is a control block diagram of a parallel rectifier according to an embodiment of the present invention. The implementation process of the embodiment of the invention is as follows: the sampled direct current voltage and direct current are used for obtaining a power grid current d-axis reference value through a self-adaptive droop controller with voltage feedforward and a PI regulator, and in order to ensure the high power factor of the rectifier, the power grid current q-axis reference value is set to be 0. And sending the calculated power grid current reference value to a model-free predictive controller, and selecting an optimal switching state by the model-free predictive controller according to an inner loop control target and a model-free predictive control algorithm and outputting the optimal switching state to a T-type three-level rectifier switching sequence.
To verify the effectiveness of the control strategy provided by the present invention, two different power parallel T-type three-level rectifier systems employing the control scheme of fig. 5 are taken as examples. The grid current difference update simulation is shown in fig. 6. When the dc side voltage reference value and the load power change, the dc voltage and the ac side power of the shunt rectifier are as shown in fig. 7. When the filter parameters are out of order, model-free predictive control is compared with the prior art grid current tracking, loop suppression and midpoint voltage balancing effects such as those shown in fig. 8.
It can be seen from fig. 6 (a) that the current differences caused by the different sets of switching vectors correspond to the phase relationship diagram of the different switching vectors in fig. 2. It can be seen from fig. 6 (b) that after steady operation, the current difference caused by the other 3 sets of switching vectors is not updated, representing that the optimal solution is selected within 7 switch combinations that make the common mode voltage 0. As can be seen from fig. 7 (a), when the dc voltage reference value increases by 100V, the difference between the dc voltage and the reference value becomes-100V, and then after 0.5s, the dc voltage is restored to the vicinity of the dc voltage reference value, and the effectiveness of the adaptive droop control with voltage feedforward for dc voltage stabilization is exhibited. As can be seen from fig. 7 (b), the two rectifiers with rated powers of 5kW and 10kW respectively better realize power distribution, and the power factors of the two rectifiers are close to 1, so that the effectiveness of the self-adaptive droop control with voltage feedforward in power distribution and high power factor control is verified. As can be seen from fig. 8 (a), under the parameter adaptation condition, the conventional model prediction control current tracking effect is poor, and waveform distortion is serious. The model-free predictive control of the invention achieves the goal of current tracking. As can be seen from fig. 8 (b), under the parameter adaptation condition, the circulation effective value of the conventional model predictive control is 8.835a, whereas the circulation effective value of the model-free predictive control of the present invention is 137.1mA, which is a significant advantage in the circulation suppression effect. The influence of the parameter mismatch of the filter on the balance effect of the midpoint voltage is not great, the traditional model predictive control is equivalent to the model-free predictive control in control effect, and the midpoint voltage deviation is 1.4V.
While the principles and embodiments of the present invention have been described with reference to specific embodiments thereof, those skilled in the art will recognize that the foregoing embodiments are merely illustrative of the principles and features of the invention, and are provided to enable one of ordinary skill in the art to understand and practice the invention and to limit the scope of the invention, without departing from the spirit or scope of the invention, in which the general principles defined herein may be implemented in other embodiments. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein and is to be accorded the widest scope consistent with the principles and modifications described herein.
Claims (6)
1. The model-free predictive control method for the parallel T-shaped three-level rectifier is characterized by comprising the following steps of:
s1, the parallel rectifier has the multi-objective optimization problems of stable direct-current voltage, power distribution, high power factor, power grid current tracking, circulation suppression and neutral-point voltage balance, and therefore a double closed-loop control strategy is adopted to realize multi-objective optimization control;
s2, establishing a discretization mathematical model of the current, midpoint voltage and circulation of an inner loop control target grid of the parallel T-shaped three-level rectifier, and determining a model-free predictive control method of the inner loop control target;
s3, sampling the power grid current i at the moment k n1 (k) Switch state S n1 (k) DC side capacitor voltage and calculating to obtain midpoint voltage deviation DeltaV 1 (k) Converting the sampled power grid current from a three-phase coordinate system to a two-phase rotation coordinate system, and calculating the current difference delta i at the moment k by using the power grid current and a delay link under the two-phase rotation coordinate system 1m |V(k)；
S4, updating a current difference data table according to the current difference sampled at the moment k and the switching state, and updating current difference data caused by the switching state of the coaxial lines in the 27 switching vector phase tables according to a model-free predictive control method of current tracking;
s5, predicting the current difference delta i caused by different switch vectors k+1 according to the acquired data and the updated current difference data table 1m (k+1) substituting the current tracking cost function g 1j Calculating and sequencing current errors of two-phase rotation coordinate systems caused by different switching vectors, and switching according to sequencingAnd traversing and selecting the optimal switching vector which is compatible with current tracking, midpoint voltage balance and circulation suppression.
2. The model-free predictive control method for a parallel T-type three-level rectifier according to claim 1, wherein the S1 double closed loop control specifically comprises:
the integral control scheme of the parallel rectifier adopts double closed loop control, the outer loop adopts self-adaptive droop control with voltage feedforward to realize power distribution, high power factor and stable direct current voltage, the inner loop adopts model-free predictive control to realize balance of midpoint voltage, circulation suppression and tracking of grid current, and the specific steps are that the sampled direct current voltage and direct current flow obtain a grid current d-axis reference value through the self-adaptive droop controller with voltage feedforward and a PI regulator, and in order to ensure the high power factor of the rectifier, the grid current q-axis reference value is set to 0; and the calculated power grid current reference value is sent to a model-free predictive controller, the model-free predictive controller selects an optimal switching state according to an inner loop control target and a model-free predictive control algorithm and outputs the optimal switching state to a switching sequence of the T-shaped three-level rectifier, and the overall scheme realizes multi-target optimal control of the parallel T-shaped three-level rectifier.
3. The model-free predictive control method for a parallel T-type three-level rectifier according to claim 2, wherein the specific steps of the adaptive droop control with voltage feedforward are as follows:
the adaptive V-I droop control characteristics are:
u in dc ，Is a direct current voltage and a reference value thereof, I dc Is a direct current, P N Is a rectifierRated power, K d The droop coefficient of the self-adaptive V-I droop control characteristic is changed along with the change of the direct current, so that the self-adaptive droop control is realized;
in addition, due to the defect of the principle of sagging control, the direct-current voltage has certain sagging; to improve this droop characteristic, a feedforward amount is added based on the difference between the dc voltage and the reference value to compensate for the effect of droop control on the dc voltage.
4. The model-free predictive control method for parallel T-type three-level rectifiers according to claim 1, wherein the model-free predictive control method for S2 grid current, midpoint voltage and loop current is based on data driving, does not depend on any hardware parameters and circuit structures, realizes multi-objective optimization without using weight factors, and reduces the problem of update stagnation of current difference, and the inner loop model-free predictive control comprises the following specific steps:
the two rectifiers have the same model structure except that the transmission power and hardware parameters are different, so that the first rectifier is taken as an example for analysis;
the mathematical model of the parallel T-shaped three-level rectifier under the two-phase rotation coordinate system is as follows:
i in 1d ，i 1q Dq-axis grid current for first rectifier, L 1 R is the inductance of the filter 1 E is parasitic resistance of the filter d ，e q For dq axis grid voltage, u 1d ，u 1q V is the dq axis rotation angular velocity for the rectifier input voltage;
the discrete model for obtaining the parallel T-shaped three-level rectifier by utilizing Euler formula discretization is as follows:
where (k) is the sampling time, (k+1) is the predicted next time, T s Is the sampling period; model-free predictive control of current tracking is established based on current difference data, and the current difference caused by different switching vectors predicted at time k+1 is expressed as:
i 1m (k+1)|V j ＝i 1m (k)+Δi 1m,old |V j ,V j ∈{V 1 ,...,V 27 }
in Deltai 1m (m=d, q) is dq axis grid current, Δi 1m，old |V j For switch vector V stored in data table j A resulting current difference; i.e 1m (k) I is the sampling value of the current k moment of the power grid 1m (k+1)|V j Switch vector V predicted for time k+1 j A resulting grid current; under the condition that the sampling time is small enough, the old data stored in the data table can be used for predicting the current difference caused by the switching vector at the next moment; the cost function of current tracking is:
in the middle of A dq-axis power grid current reference value at the time k+1; i.e 1d (k+1)|V j ，i 1q (k+1)|V j Switch vector V predicted for time k+1 j The resulting dq-axis grid current;
the mathematical model of the midpoint voltage deviation on the direct current side is as follows:
i in np1 To flow into neutralityCurrent of point i n1 (n=a, B, C) is three-phase grid current, S n1 In the three-phase switch state, C 11 Is the direct-current side capacitance, deltaV 1 Is the voltage deviation of the midpoint of the direct current side;
after the midpoint voltage deviation is discretized, the method comprises the following steps:
from the discretized midpoint voltage deviation formula, at i np1 ×ΔV 1 (k) When the voltage is less than or equal to 0, the midpoint voltage is balanced, and model-free predictive control of the midpoint voltage can be realized according to the relation between the current flowing into the neutral point and the switching state;
according to the circulation channel of zero sequence circulation, the mathematical model for determining the circulation is as follows:
i in z Is zero sequence circulation, U cmvx (x=1, 20 is the common mode voltage of the two rectifiers, R x Parasitic resistance, L, of the filter inductance of the two rectifiers x Filter inductance, deltaV, for two rectifiers x The midpoint voltage deviation of the two rectifiers is obtained; the main influencing factors of zero sequence circulation can be seen as midpoint voltage and common-mode voltage difference of the two rectifiers; according to the relation between the common-mode voltage and the switch state and the model-free predictive control of the midpoint voltage, the model-free predictive control of the circulation can be realized.
5. The model-free predictive control method for a parallel T-type three-level rectifier according to claim 1, wherein the specific steps of S4 current difference update are as follows:
from a mathematical model of the current difference it can be seen that the current difference consists of the natural response of the zero-switch vector and the forced response of the non-zero vector:
wherein the method comprises the steps of
In delta i 1d0 ，δi 1q0 Natural response, δi, for zero vector 1d |V j ，δi 1q |V j For forced response caused by non-zero vectors, Δi 1d |V j ，Δi 1q |V j For switching vector V j Induced current difference, u 1d |V j ，u 1q |V j For switching vector V j A resulting rectifier input voltage; combining the phase diagrams of the 27 switch vectors, the current difference relation of a group of switch vectors under the same phase condition can be obtained; in the updating process of the current difference, the current difference and the switching state obtained by sampling at the moment k can determine the switching vector current difference data to be updated, and a group of switching vectors with the same phase are updated according to the phase relation of the switching vectors, so that the current difference data updated at each moment is not one but one group, and the updating frequency of the current difference is greatly improved; in addition, due to the relation between the common-mode voltage and the switching state, the optimal switching vector in steady-state operation is basically limited to 7 switching vectors enabling the common-mode voltage to be 0, and the 7 switching vectors are divided into 4 different phase relation groups, so that the updating frequency of the current difference is further improved.
6. The model-free predictive control method for a parallel T-type three-level rectifier according to claim 1, wherein the specific step of selecting the S5 optimal switching vector is as follows:
calculating current tracking errors caused by 27 switch vectors according to a cost function of current tracking, sequencing, and performing traversal selection on the sequenced switch vectors until an optimal switch vector is selected; the specific steps of the optimal switching vector selection are as follows: firstly judging the position of a switch vector in the ordered switch vector, wherein the ordered position represents the magnitude of a current tracking error, the current tracking error is smaller when the position is closer, judging whether the midpoint voltage balance and the circulation suppression are met under the condition of meeting the current tracking, and selecting an optimal solution if the midpoint voltage balance and the circulation suppression are met; if the current is not satisfied, the selection condition is relaxed, and the switching vector satisfying the current inhibition is selected as the optimal solution; if the current midpoint voltage deviation meets the requirement, the current midpoint voltage deviation is judged to be a current value, if the current midpoint voltage deviation meets the requirement, the current loop is continued, and if the current midpoint voltage deviation does not meet the requirement, a switching vector meeting the midpoint balance and sacrificing a certain loop inhibition is selected as an optimal solution; the selection of the optimal switching vector comprehensively considers three control targets of inner loop current tracking, midpoint voltage balancing and loop current inhibition.
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