A spatial voltage vector model predictive control method and system for APF

Abstract

The present invention provides a spatial voltage vector model predictive control method and system for an APF, the method comprising: establishing a neutral point voltage balance index, a zero-sequence current compensation index and a load harmonic compensation index of an NPC three-level APF, and constructing a hierarchical analysis evaluation framework for an APF spatial voltage vector. The index performance of 27 spatial voltage vectors of the NPC three-level is classified and evaluated through the hierarchical analysis evaluation framework, and the spatial voltage vectors include: a long vector, a middle vector, a short vector and a zero vector. The spatial voltage vector combination is reconstructed according to the comprehensive evaluation of the classified index performance of the spatial vector, and a spatial voltage vector model suitable for APF harmonic control is obtained, and the optimal switch combination state is predicted, and then the output current prediction value corresponding to each switch state is calculated. The value function of the neutral point voltage balance is calculated according to the predicted value, and the optimal voltage vector is obtained after multiple cycles. The present invention can improve the control efficiency of the APF.

Description

Technical Field

[0001] The present invention relates to the technical field of active power filters, and in particular to a spatial voltage vector model predictive control method and system for an APF. Background Art

[0002] With the development of industrial technology, power electronic devices represented by frequency converters are widely used in cigarette production. However, the use of power electronic devices will increase the nonlinear loads in the power grid and generate a large number of harmonics. A large number of harmonics injected into the power grid will cause the voltage and current waveforms of the power grid to be distorted, and the power quality will deteriorate. At the same time, it will also shorten the service life of cigarette factory equipment and even cause abnormal power outages and equipment interruption accidents. In order to eliminate the negative impact of harmonics, active power filters (APF, Active Power System) have received great attention. APF is a widely used device that can effectively suppress harmonics. Unlike passive filters, it is a device that actively compensates for harmonics. However, in the process of APF compensating harmonics, the switching devices are easily affected by temperature, humidity, excessive voltage, current and switching frequency, resulting in large electromagnetic interference, large dv / dt, high switching frequency and low inverter efficiency. Therefore, it is of great significance to optimize the switch combination of APF for harmonic control. Summary of the invention

[0003] The present invention provides a spatial voltage vector model predictive control method and system for an APF, which solves the problem that the existing APF has low control effect due to the influence of the performance of switching devices, can optimize the selection of switching vectors, use a smaller amount of calculation, achieve better control effect, and improve the control efficiency of the APF.

[0004] To achieve the above objectives, the present invention provides the following technical solutions:

[0005] A spatial voltage vector model predictive control method for an APF, comprising:

[0006] The midpoint voltage balance index, zero-sequence current compensation index and load harmonic compensation index of NPC three-level APF are established, and the APF space voltage vector hierarchical analysis evaluation framework is constructed;

[0007] The performance of 27 space voltage vectors of NPC three-level is classified and evaluated through the hierarchical analysis evaluation framework, and the space voltage vectors include: long vector, medium vector, short vector and zero vector;

[0008] According to the comprehensive evaluation of the performance of the classification index of the space vector, the space voltage vector combination is reconstructed to obtain the space voltage vector model suitable for APF harmonic control, and the optimal switch combination state is predicted, and then the output current prediction value corresponding to each switch state is calculated;

[0009] The value function of neutral point voltage balance is calculated according to the predicted value, and the optimal voltage vector is obtained after multiple cycles.

[0010] Preferably, it also includes:

[0011] Obtain the three-phase voltage and load current of the grid connection point, and calculate the three-phase harmonic compensation reference current based on the instantaneous power theory;

[0012] After the three-phase harmonic compensation reference current is Clark transformed, it is sent to the reconstructed space voltage vector model for prediction calculation.

[0013] Preferably, it also includes:

[0014] Obtaining a compensation reference current currently output by the APF, and performing Clark transformation on the compensation reference current and sending it to the space voltage vector model;

[0015] The space voltage vector model traverses all voltage vectors according to the compensation reference current, predicts the compensated output current at the next moment, determines the optimal voltage vector through the value function, obtains the control signal of the bridge arm switch, and outputs the harmonic compensation current.

[0016] Preferably, an APF space voltage vector hierarchical analysis evaluation framework is constructed, including:

[0017] Constructing a voltage vector hierarchical structure, taking the space voltage vector combination as the first layer, the voltage vector as the second layer, the midpoint voltage balance index, the zero-sequence current compensation index and the load harmonic compensation index as the third layer, and the optimal switch combination state as the fourth layer;

[0018] The voltage vector is hierarchically analyzed according to the voltage vector hierarchical structure, and the space voltage vector is reconstructed to obtain the optimal space vector combination.

[0019] Preferably, the performing hierarchical analysis on the voltage vector according to the voltage vector hierarchical architecture includes:

[0020] According to the influence factors of the set discrimination scale on the midpoint voltage balance index, the zero-sequence current compensation index and the load harmonic compensation index, the discrimination scale is set to: generally important, relatively important, particularly important, very important and extremely important;

[0021] The spatial voltage vector discriminant matrix corresponding to each index is constructed respectively, the eigenvector and eigenvalue of the discriminant matrix characteristics are calculated, and after normalization processing, the index weight coefficient of the spatial voltage vector is obtained;

[0022] Scoring the space voltage vector combination according to the weight coefficient to obtain an optimal space voltage vector combination;

[0023] An optimal switch combination state is obtained according to the optimal space voltage vector combination.

[0024] Preferably, the comprehensive weight coefficient of the space voltage vector is calculated according to the following formula:

[0025]

[0026] In the formula, F factor_i is the weight coefficient of index i (i = 1, 2, 3), F n_fctor_i is the weight coefficient of each type of vector in index i.

[0027] Preferably, reconstructing the space voltage vector combination according to the comprehensive evaluation of the classification index performance of the space vector includes:

[0028] (1) Space voltage vector combination 1 (zero vector, short 1 vector, long vector);

[0029] (2) Space voltage vector combination 2 (zero vector, short_0 vector, medium vector);

[0030] (3) Space voltage vector combination 3 (zero vector, short_1 vector, long vector);

[0031] (4) Space voltage vector combination 4 (zero vector, long vector, middle vector);

[0032] (5) Space voltage vector combination 5 (zero vector, short_0 vector, medium vector);

[0033] (6) Space voltage vector combination 6 (zero vector, short_0 vector, short_1 vector).

[0034] The present invention also provides a space voltage vector model prediction control system of an APF, comprising: an APF main circuit and a space voltage vector model;

[0035] The DC side neutral point in the APF main circuit is connected to the neutral line of the power supply grid, the output end of the APF main circuit is connected to the input end of the nonlinear load, the nonlinear load input end is also connected to the output end of the power supply grid, the output end of the APF main circuit is also connected to the input end of the space voltage vector model, and the output end of the space voltage vector module is connected to the control end of the APF main circuit;

[0036] The spatial voltage vector model is constructed with the midpoint voltage balance index, zero-sequence current compensation index and load harmonic compensation index of the APF main circuit. The midpoint voltage balance index represents the ability of the zero-sequence current to affect the midpoint voltage. The zero-sequence current compensation index represents the ability of the APF to output zero-sequence compensation current. The load harmonic compensation index represents the ability of the voltage vector to output harmonic compensation current.

[0037] The space voltage vector model divides 27 voltage vectors into long vectors, medium vectors, short vectors and zero vectors according to the output voltage vector amplitude of the APF main circuit, and predicts the optimal switch combination state through the space voltage vector combination of the APF switch, and then calculates the output current prediction value corresponding to each switch state;

[0038] The space voltage vector model calculates the value function of neutral point voltage balance according to the predicted value, and obtains the optimal voltage vector after multiple cycles to control the compensation current output by the APF main circuit.

[0039] Preferably, it further comprises: an instantaneous power extraction module and a first Clark transformation module;

[0040] The instantaneous power extraction module is used to obtain the three-phase voltage and load current of the grid connection point, and calculate the three-phase harmonic compensation reference current according to the instantaneous power theory;

[0041] The first Clark transformation module performs Clark transformation on the three-phase harmonic compensation reference current and then sends the result to the space voltage vector model for prediction calculation.

[0042] Preferably, it further comprises: a second Clark transformation module;

[0043] The second Clark transformation module performs Clark transformation on the compensation reference current output by the current APF main circuit and sends the result to the space voltage vector model;

[0044] The space voltage vector model traverses all voltage vectors according to the compensation reference current, predicts the compensated output current at the next moment, determines the optimal voltage vector through the value function, obtains the control signal of the bridge arm switch, and outputs the harmonic compensation current.

[0045] The present invention provides a spatial voltage vector model predictive control method and system for an APF, establishes a midpoint voltage balance index, a zero-sequence current compensation index, and a load harmonic compensation index of an NPC three-level APF, designs a hierarchical analysis evaluation framework, reconstructs a spatial voltage vector model of a switch state, analyzes the index performance of the APF spatial voltage vector, evaluates and scores the reconstructed spatial voltage vector combination, and obtains an optimal spatial voltage vector combination. The problem that the existing APF is affected by the performance of the switching device and has a low control effect is solved, the selection of the switching vector can be optimized, a smaller amount of calculation is used, a better control effect is achieved, and the control efficiency of the APF can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the specific embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below.

[0047] Figure 1 It is a schematic diagram of a spatial voltage vector model predictive control method of an APF provided by the present invention.

[0048] Figure 2 This is a schematic diagram of APF harmonic control provided by the present invention.

[0049] Figure 3 The figure is a schematic diagram of the DC side wiring of an NPC three-level APF provided in an embodiment of the present invention.

[0050] Figure 4 It is a schematic diagram of the output space voltage vector of the NPC three-level APF provided by the present invention.

[0051] Figure 5 (a) to (f) are schematic diagrams of the spatial voltage vector combination reconstructed by the NPC three-level APF provided by the present invention.

[0052] Figure 6 It is a schematic diagram of the spatial voltage vector hierarchical analysis architecture provided by the present invention.

[0053] Figure 7 It is a predictive control flow chart of the reconstructed space voltage vector model provided by the present invention.

[0054] Figure 8 It is a schematic diagram of a spatial voltage vector model predictive control system of an APF provided by the present invention. DETAILED DESCRIPTION

[0055] In order to enable persons skilled in the art to better understand the solutions of the embodiments of the present invention, the embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings and implementation modes.

[0056] In view of the problem that the current active filter is affected by the performance of the switching device and has a low control effect, the present invention provides a spatial voltage vector model predictive control method and system for APF, which constructs a spatial voltage vector model of the switch state through the midpoint voltage balance index, the zero-sequence current compensation index and the load harmonic compensation index to form a spatial voltage vector combination of the switch to predict the optimal switch combination state, and then obtain the optimal voltage vector. The problem that the existing APF is affected by the performance of the switching device and has a low control effect can be solved, the selection of the switching vector can be optimized, a smaller amount of calculation is used, a better control effect is achieved, and the control efficiency of the APF can be improved.

[0057] like Figure 1 As shown, a spatial voltage vector model predictive control method of an APF includes:

[0058] S1: Establish the midpoint voltage balance index, zero-sequence current compensation index and load harmonic compensation index of NPC three-level APF, and construct the APF space voltage vector hierarchical analysis evaluation framework;

[0059] S2: Classify and evaluate the performance of 27 spatial voltage vectors of NPC three-level through the hierarchical analysis evaluation framework, the spatial voltage vectors include: long vector, medium vector, short vector and zero vector;

[0060] S3: Reconstruct the space voltage vector combination according to the comprehensive evaluation of the performance of the space vector classification index, obtain the space voltage vector model suitable for APF harmonic control, predict the optimal switch combination state, and then calculate the output current prediction value corresponding to each switch state;

[0061] S4: Calculate the value function of neutral point voltage balance according to the predicted value, and obtain the optimal voltage vector after multiple cycles.

[0062] Specifically, when performing harmonic control in hybrid vector control, firstly, a value function is designed to comprehensively evaluate the three performance indicators of midpoint voltage deviation, zero-sequence current suppression and current tracking deviation, and the three performance indicators of voltage vector are evaluated; and according to the characteristics of various switch states, an optimized switch combination is used in the model predictive control link for harmonic control. Figure 2 As shown in the figure, the three-phase four-wire NPC three-level APF harmonic control control diagram, U dc is the DC bus voltage, L and R are the filter inductor and resistor respectively, I com ,I sys ,I load Represents APF compensation current, grid current and load harmonic current. The model predictive control strategy traverses all voltage vectors according to the harmonic compensation reference current, predicts the compensated output current at time k+1, determines the optimal voltage vector through the value function, obtains the control signal of the bridge arm switch, and outputs the harmonic compensation current.

[0063] According to Kirchhoff's current law, the grid current after harmonic control is as follows:

[0064] I sys =I load -I com .

[0065] like Figure 3 In a three-phase four-wire power grid, the DC side neutral point O of the NPC three-level APF is connected to the neutral line (N line) of the power grid. n is the zero-sequence compensation current output by APF, I o is the DC side capacitor current difference, I 0w is the output current of the zero-level bridge arm of the APF. o =I n -I 0w , the zero-sequence current causes the DC side capacitor to charge and discharge frequently, resulting in an imbalance in the midpoint voltage and making the APF compensation effect worse.

[0066] Define Z k (k=a,b,c) is the zero-level switch state, which represents the connection mode between phase k and the midpoint of the DC side:

[0067]

[0068] The zero-level bridge arm output current is:

[0069] I 0w =Z a *I a +Z b *I b +Z c *I c (3)

[0070] Substituting formula (3) into I o =I n -I 0w We can get:

[0071] I o =I n +Z a *I a +Z b *I b +Z c *I c (4)

[0072] According to the output voltage vector amplitude of the NPC three-level APF, the 27 voltage vectors are divided into four categories, such as Figure 4 As shown, they are:

[0073] (1) Long voltage vector (U21 ~U 26 ), the voltage amplitude is 2U dc , none of the three phases are connected to the midpoint of the DC side.

[0074] (2) Medium voltage vector (U 15 ~U 20 ), the voltage amplitude is One phase is connected to the midpoint of the DC side.

[0075] (3) Short_0 voltage vector (U4, U6, U8, U 10 , U 12 , U 14 ), the voltage amplitude is U dc , one phase is connected to the midpoint of the DC side.

[0076] (4) Short_1 voltage vector (U5, U7, U9, U 11 , U 13 , U 15 ), the voltage amplitude is U dc , the two phases are connected to the midpoint of the DC side.

[0077] (5) Zero voltage vector (U0, U1, U2), the voltage amplitude is 0, and all three phases are connected to the midpoint of the DC side.

[0078] The capacitance current difference corresponding to each type of voltage vector (taking sector 1 as an example):

[0079]

[0080] In the formula, I a0 , I b0 , I c0 is the zero-sequence component of the APF output three-phase current, I n0 It is 1 / 3 of the zero-sequence current on the N line, I a pn and I b pn These are the positive and negative sequence components of the A-phase and B-phase output currents of the APF.

[0081] According to formula (5), in addition to the zero voltage vector, the zero-sequence compensation current output by the APF also affects the charging and discharging of the capacitor. When the APF is in the long voltage vector state, the zero-sequence current has a greater impact on the midpoint voltage balance. When the APF is in the full zero vector, the zero-sequence current does not affect the midpoint voltage balance.

[0082] The method further includes:

[0083] S5: Obtain the three-phase voltage and load current of the grid connection point, and calculate the three-phase harmonic compensation reference current according to the instantaneous power theory;

[0084] S6: After Clark transformation of the three-phase harmonic compensation reference current, the three-phase harmonic compensation reference current is sent to the reconstructed space voltage vector model for prediction calculation.

[0085] The method further includes:

[0086] S7: obtaining the compensation reference current output by the current APF, and performing Clark transformation on the compensation reference current and sending it to the space voltage vector model;

[0087] S8: The space voltage vector model traverses all voltage vectors according to the compensation reference current, predicts the compensated output current at the next moment, determines the optimal voltage vector through the value function, obtains the control signal of the bridge arm switch, and outputs the harmonic compensation current.

[0088] Specifically, Figure 7 As shown in the figure, when APF harmonic control is performed, the grid-side voltage e is first collected. a 、e b 、e c and load current i a 、i b 、i c . According to the instantaneous power theory, the three-phase reference current i is calculated. a *、i b *、i c * And perform Clark transformation. Convert the reference values ​​of voltage and current in the abc coordinate system to the reference values ​​e in the αβ0 coordinate system α 、e β 、i αref 、i βref Finally, it is sent to the model predictive control module for calculation. According to the different switching states of the harmonic control device, there are 27 types and 5 categories of voltage vectors. Use model predictive control to traverse the spatial voltage vector combination and calculate the output current prediction value I corresponding to each switching state coma , I comb , I comc , and calculate the value function. g=|I refa -I coma |+|I refb -I comb |+|I refc -I comc |+λ|ΔU| is the value function for adding neutral point voltage balance. After multiple cycles, the optimal voltage vector is selected, where λ is the midpoint voltage balance weight coefficient.

[0089] Further, the space voltage vector combination constructed according to the midpoint voltage balance index, the zero-sequence current compensation index and the load harmonic compensation index includes:

[0090] (1) Space voltage vector combination 1 (zero vector, short 1 vector, long vector);

[0091] (2) Space voltage vector combination 2 (zero vector, short_0 vector, medium vector);

[0092] (3) Space voltage vector combination 3 (zero vector, short_1 vector, long vector);

[0093] (4) Space voltage vector combination 4 (zero vector, long vector, middle vector);

[0094] (5) Space voltage vector combination 5 (zero vector, short_0 vector, medium vector);

[0095] (6) Space voltage vector combination 6 (zero vector, short_0 vector, short_1 vector).

[0096] In practical applications, when the three-phase harmonic compensation current is balanced, the zero-sequence current does not affect the midpoint voltage balance. In practical applications, the single-phase harmonic source makes the three-phase harmonic compensation current unbalanced. To address this problem, the spatial voltage vector is reconstructed from the perspective of zero-sequence compensation current, and model prediction voltage balancing control is performed based on the reconstructed voltage vector to suppress the influence of zero-sequence current on midpoint voltage balance.

[0097] According to the above principles, if Figure 5 6 reconstructed space voltage vector combinations. (1) Space voltage vector combination 1 (zero vector, short 1 vector, long vector), such as Figure 5 (a); (2) space voltage vector combination 2 (zero vector, short _0 vector, medium vector), such as Figure 5 (b); (3) space voltage vector combination 3 (zero vector, short 1 vector, long vector), such as Figure 5 (c); (4) space voltage vector combination 4 (zero vector, long vector, middle vector), such as Figure 5 (d); (5) space voltage vector combination 5 (zero vector, short _0 vector, medium vector), such as Figure 5 (e); (6) space voltage vector combination 6 (zero vector, short_0 vector, short_1 vector), such as Figure 5 (f) A hierarchical analysis is performed on the six combinations of reconstructed spatial voltage vectors, and the optimal vector is selected for model predictive control.

[0098] Furthermore, a hierarchical analysis and evaluation framework of APF space voltage vector is constructed, including:

[0099] A voltage vector hierarchical structure is constructed, with the spatial voltage vector combination as the first layer, the voltage vector as the second layer, the midpoint voltage balance index, the zero-sequence current compensation index and the load harmonic compensation index as the third layer, and the optimal switch combination state as the fourth layer.

[0100] The voltage vector is hierarchically analyzed according to the voltage vector hierarchical structure, and the space voltage vector is reconstructed to obtain the optimal space vector combination.

[0101] Specifically, Figure 6 As shown in the figure, the spatial voltage vector is evaluated through hierarchical analysis and the spatial voltage vector is reconstructed. Among them, the midpoint voltage balance index represents the ability of zero-sequence current to affect the midpoint voltage. The zero-sequence current compensation index represents the ability of APF to output zero-sequence compensation current. The spatial voltage vector utilization index represents the ability of the voltage vector to output harmonic compensation current. The voltage vector hierarchical analysis framework is constructed based on the indicators, and the reconstructed voltage vector combination is scored according to the discrimination scale of the indicators to select the optimal spatial voltage vector combination.

[0102] Further, performing hierarchical analysis on the voltage vector according to the voltage vector hierarchical structure includes:

[0103] According to the influence factors of the set judgment scale on the midpoint voltage balance index, the zero-sequence current compensation index and the load harmonic compensation index, the judgment scale is set to: generally important, relatively important, particularly important, very important and extremely important.

[0104] The spatial voltage vector discriminant matrix corresponding to each index is constructed respectively, and the eigenvector and eigenvalue of the discriminant matrix characteristics are calculated. After normalization, the index weight coefficient of the spatial voltage vector is obtained.

[0105] The space voltage vector combination is scored according to the weight coefficient to obtain an optimal space voltage vector combination.

[0106] An optimal switch combination state is obtained according to the optimal space voltage vector combination.

[0107] Specifically, the APF harmonic governance hierarchical analysis index discrimination scale, midpoint voltage balancing capability (index 1), zero-sequence current compensation capability (index 2) and space voltage vector utilization (index 3) are obtained for different types of voltage vector discrimination scales. According to the index discrimination scale, an index discrimination matrix is ​​established to conduct hierarchical analysis and evaluation of the space voltage vector, and the space voltage vector combination suitable for APF harmonic governance is selected.

[0108] According to the hierarchical analysis method, the matrix discrimination scales 1, 3, 5, 7, and 9 represent the different importance of the influencing factors of the indicators, increasing from weak to strong, as shown in Table 1. The selection principles of the matrix discrimination scales of the three major indicators are as follows:

[0109] (1) To solve the contradiction between midpoint voltage balance and zero-sequence compensation, when selecting the scale between indicators, the vector indicator 1 (midpoint voltage balancing ability) and indicator 2 (zero-sequence current compensation energy) are compared to select scale 1. Indicator 3 (load harmonic compensation index) is the key indicator of harmonic compensation and is more important than indicators 1 and 2, so scale 3 is selected.

[0110] Table 1 AHP ​​discriminant scale

[0111]

[0112] (2) The smaller the zero-sequence current flowing through the DC side capacitor, the more stable the midpoint voltage. Compared with the long and medium vectors, the short vector only affects one split capacitor at the same sampling time, while the voltage of the other capacitor remains stable. Therefore, the voltage balancing ability of the short vector is better than that of the long and medium vectors.

[0113] For indicator 1, the discrimination scales of long vectors and middle vectors are 1 and 3, and the discrimination scales of short vectors are 5 and 7. The all-zero vector corresponds to a scale of 9, no zero-sequence current flows through the midpoint of the split capacitor, and the voltage remains stable. The voltage difference of the long vector at time k and k+1 is proportional to the three-phase zero-sequence current. In contrast, the middle vector is proportional to the two-phase zero-sequence current. Therefore, a long vector corresponds to a scale of 1, and a middle vector corresponds to a scale of 3. Applying the same analysis method, the short_0 vector corresponds to a scale of 5, and the short_1 vector corresponds to a scale of 7, see Table 2.

[0114] (3) For indicator 2, short_0 vector output zero-sequence voltage Corresponding to scale 9, zero-sequence voltage output by long vector and short_1 vector Corresponding to scale 5, the zero-sequence output voltage of the neutral vector and the zero vector is 0, corresponding to scale 1, see Table 2.

[0115] (4) For index 3, the voltage vector of the long vector output (2U dc ) corresponds to scale 9, the voltage vector modulus of the short vector output (U dc ) corresponds to scale 5, 2U dc with U dc The vector output voltage modulus between dc ) corresponds to scale 7, the voltage vector modulus of the zero vector output is 0, corresponding to scale 1, see Table 2.

[0116] Table 2 Space voltage vector discrimination matrix

[0117]

[0118] Discriminant matrix D:

[0119]

[0120] According to formula (6), the three index discriminant matrices shown in Tables 3 to 5 are obtained.

[0121] Table 3 Discriminant matrix of indicator 1

[0122]

[0123] Note: 1.λmax=5 2.CI=0 3.CR=0.

[0124] Table 4 Discriminant matrix of indicator 2

[0125]

[0126]

[0127] Note: 1.λmax=5 2.CI=0 3.CR=0.

[0128] Table 5 Discriminant matrix of indicator 3

[0129]

[0130] Note: 1.λmax=5 2.CI=0 3.CR=0.

[0131] Through hierarchical analysis programming and eigenvector and eigenvalue calculation, after normalization, the weight coefficients within the indicators shown in Table 6 are obtained.

[0132] Table 6 Indicator weight coefficients of space voltage vector

[0133]

[0134] According to the weight coefficients of the indicators in Table 6, the comprehensive weight coefficient of the space voltage vector is calculated, and the formula is as follows:

[0135]

[0136] In the formula, F factor_i is the weight coefficient of index i (i = 1, 2, 3) F n_fctor_i is the weight coefficient of each type of vector in index i.

[0137] In order to control the zero-sequence current compensation and midpoint voltage balance, the index 1 and the index 2 are analyzed according to formula (7), and the analysis results are listed in Table 7.

[0138] Table 7 Weight coefficients of index 1 and 2 of space voltage vector

[0139]

[0140] The changing trends of the weight coefficients of the space voltage vector indicators 1 and 2 show that the priorities of the vectors changed at 0.42 and 1.67 times, respectively, indicating that the short_0 and short_1 vectors are the best choices.

[0141] Table 8 Weight coefficients of indicators 1, 2, and 3 of space voltage vector

[0142]

[0143] When performing harmonic control, the spatial voltage vector utilization of index 3 is considered first, and the influence of indexes 1 and 2 is considered at the same time. According to formula (7), indexes 1, 2, and 3 are analyzed, and the analysis results are listed in Table 8. x1 represents the total weight coefficient of indexes 1 and 2, and y1 represents the weight coefficient of index 3. The analysis is as follows:

[0144] (1) The larger the ratio of y1 to x1, the higher the weight coefficient. The maximum modulus of the APF space voltage vector (2U dc ) and non-zero minimum zero-sequence voltage The ratio is 6.92.

[0145] (2) When the index ratio is in the range of [0, 1.6], the short_0 vector and the short_1 vector are optimal. When the index ratio is in the range of [1.6, 6.2], the long vector and the short_0 vector are optimal.

[0146] Table 9 Hierarchical analysis evaluation scores of spatial voltage vector combinations

[0147]

[0148]

[0149] The six spatial voltage vector combinations are evaluated by hierarchical analysis, and the spatial voltage vector combination 1 has the highest evaluation score. The evaluation results are shown in Table 9 above. Thus, by obtaining the weight coefficients in the current spatial voltage vector index and performing a comprehensive hierarchical analysis evaluation on the six types of reconstructed spatial voltage vector combinations, an optimized spatial voltage vector combination is obtained. The spatial voltage vector model traverses all voltage vectors according to the compensation reference current, predicts the compensated output current at the next moment, determines the optimal voltage vector through the value function, obtains the control signal of the bridge arm switch, and outputs the harmonic compensation current.

[0150] In practical applications, an NPC three-level harmonic control simulation model is built in PSCAD / EMTDC to verify the effectiveness of the predictive control strategy of the reconstructed space voltage vector model. The advantages of this control method are as follows: (1) Using hybrid switch vector control, the zero-sequence current suppression and midpoint voltage balance effect are better than the traditional method. (2) In actual engineering applications, when 27 voltage vectors are used for modulation, the control effect is often unsatisfactory due to the performance limitations of DSP, single-chip microcomputer and other equipment. This method achieves better control effect by optimizing the selection of switch vectors and using a smaller amount of calculation. (3) The selection of hybrid vectors can be dynamically optimized according to the DC side capacity of the converter to ensure the compensation effect.

[0151] It can be seen that the present invention provides a spatial voltage vector model predictive control method for APF, which constructs a spatial voltage vector model of the switch state through the midpoint voltage balance index, the zero-sequence current compensation index and the load harmonic compensation index to form a spatial voltage vector combination of switches to predict the optimal switch combination state, and then obtain the optimal voltage vector. The problem that the existing APF is affected by the performance of the switching device and causes low control effect can be solved, the selection of the switching vector can be optimized, a smaller amount of calculation is used, a better control effect is achieved, and the control efficiency of the APF can be improved.

[0152] Accordingly, if Figure 8 As shown, the present invention also provides a space voltage vector model predictive control system of APF, including: APF main circuit and space voltage vector model. The DC side neutral point in the APF main circuit is connected to the zero line of the power supply network, the output end of the APF main circuit is connected to the input end of the nonlinear load, the nonlinear load input end is also connected to the output end of the power supply network, the output end of the APF main circuit is also connected to the input end of the space voltage vector model, and the output end of the space voltage vector module is connected to the control end of the APF main circuit. The space voltage vector model is constructed with the midpoint voltage balance index, zero-sequence current compensation index and load harmonic compensation index of the APF main circuit. The midpoint voltage balance index represents the ability of zero-sequence current to affect the midpoint voltage, the zero-sequence current compensation index represents the ability of APF to output zero-sequence compensation current, and the load harmonic compensation index represents the ability of voltage vector to output harmonic compensation current. The space voltage vector model divides 27 voltage vectors into long vectors, medium vectors, short vectors and zero vectors according to the output voltage vector amplitude of the APF main circuit, and predicts the optimal switch combination state through the space voltage vector combination of the APF switch, and then calculates the output current prediction value corresponding to each switch state. The space voltage vector model calculates the value function of neutral point voltage balance according to the prediction value, and obtains the optimal voltage vector after multiple cycles to control the compensation current output by the APF main circuit.

[0153] The system also includes: an instantaneous power extraction module and a first Clark transformation module. The instantaneous power extraction module is used to obtain the three-phase voltage and load current of the grid connection point, and calculate the three-phase harmonic compensation reference current according to the instantaneous power theory. The first Clark transformation module performs Clark transformation on the three-phase harmonic compensation reference current and sends it to the space voltage vector model for prediction calculation.

[0154] The system also includes: a second Clark transformation module. The second Clark transformation module performs Clark transformation on the compensation reference current output by the current APF main circuit and then sends it to the space voltage vector model. The space voltage vector model traverses all voltage vectors according to the compensation reference current, predicts the compensation output current at the next moment, determines the optimal voltage vector through the value function, obtains the control signal of the bridge arm switch, and outputs the harmonic compensation current.

[0155] It can be seen that the present invention provides a spatial voltage vector model prediction control system for an APF, which constructs a spatial voltage vector model of a switch state through a midpoint voltage balance index, a zero-sequence current compensation index, and a load harmonic compensation index, so as to form a spatial voltage vector combination of switches to predict the optimal switch combination state, and then obtain the optimal voltage vector. This solves the problem that the existing APF is affected by the performance of the switching device and causes low control effect, can optimize the selection of the switching vector, use a smaller amount of calculation, achieve a better control effect, and can improve the control efficiency of the APF.

[0156] The above describes in detail the structure, features and effects of the present invention based on the embodiments shown in the drawings. The above is only a preferred embodiment of the present invention, but the present invention is not limited to the scope of implementation shown in the drawings. Any changes made in accordance with the concept of the present invention, or modifications to equivalent embodiments with equivalent changes, which still do not exceed the spirit covered by the description and drawings, should be within the protection scope of the present invention.

Claims

1. A spatial voltage vector model predictive control method for APF, characterized in that: include: The midpoint voltage balance index, zero-sequence current compensation index and load harmonic compensation index of NPC three-level APF are established, and the APF space voltage vector hierarchical analysis evaluation framework is constructed; The performance of 27 space voltage vectors of NPC three-level is classified and evaluated through the hierarchical analysis evaluation framework, and the space voltage vectors include: long vector, medium vector, short vector and zero vector; According to the comprehensive evaluation of the performance of the classification index of the space vector, the space voltage vector combination is reconstructed to obtain the space voltage vector model suitable for APF harmonic control, and the optimal switch combination state is predicted, and then the output current prediction value corresponding to each switch state is calculated; Calculate the value function of neutral point voltage balance according to the predicted value, and obtain the optimal voltage vector after multiple cycles; Construct an APF space voltage vector hierarchical analysis evaluation framework, including: Constructing a voltage vector hierarchical structure, taking the space voltage vector combination as the first layer, the voltage vector as the second layer, the midpoint voltage balance index, the zero-sequence current compensation index and the load harmonic compensation index as the third layer, and the optimal switch combination state as the fourth layer; Performing hierarchical analysis on the voltage vector according to the voltage vector hierarchical structure, reconstructing the space voltage vector to obtain an optimal space vector combination; The step of performing a hierarchical analysis on the voltage vector according to the voltage vector hierarchical structure includes: According to the influence factors of the set discrimination scale on the midpoint voltage balance index, the zero-sequence current compensation index and the load harmonic compensation index, the discrimination scale is set to: generally important, relatively important, particularly important, very important and extremely important; The spatial voltage vector discriminant matrix corresponding to each index is constructed respectively, the eigenvector and eigenvalue of the discriminant matrix characteristics are calculated, and after normalization processing, the index weight coefficient of the spatial voltage vector is obtained; Scoring the space voltage vector combination according to the weight coefficient to obtain an optimal space voltage vector combination; An optimal switch combination state is obtained according to the optimal space voltage vector combination.

2. The spatial voltage vector model predictive control method of APF according to claim 1, characterized in that: Also includes: Obtain the three-phase voltage and load current of the grid connection point, and calculate the three-phase harmonic compensation reference current based on the instantaneous power theory; After the three-phase harmonic compensation reference current is Clark transformed, it is sent to the reconstructed space voltage vector model for prediction calculation.

3. The spatial voltage vector model predictive control method of APF according to claim 2, characterized in that: Also includes: Obtaining a compensation reference current currently output by the APF, and performing Clark transformation on the compensation reference current and sending it to the space voltage vector model; The space voltage vector model traverses all voltage vectors according to the compensation reference current, predicts the compensated output current at the next moment, determines the optimal voltage vector through the value function, obtains the control signal of the bridge arm switch, and outputs the harmonic compensation current.

4. The spatial voltage vector model predictive control method of APF according to claim 3 is characterized in that: Calculate the comprehensive weight coefficient of the space voltage vector, the formula is as follows: In the formula, F factor_i is the weight coefficient of index i (i = 1, 2, 3), F n_fctor_i is the weight coefficient of each type of vector in index i.

5. The space voltage vector model predictive control method of APF according to claim 1, characterized in that: The space voltage vector combination is reconstructed based on the comprehensive evaluation of the performance of the space vector classification indicators, including: (1) Space voltage vector combination 1 (zero vector, short 1 vector, long vector); (2) Space voltage vector combination 2 (zero vector, short 0 vector, medium vector); (3) Space voltage vector combination 3 (zero vector, short 1 vector, long vector); (4) Space voltage vector combination 4 (zero vector, long vector, middle vector); (5) Space voltage vector combination 5 (zero vector, short 0 vector, medium vector); (6) Space voltage vector combination 6 (zero vector, short 0 vector, short 1 vector).

6. A space voltage vector model predictive control system of an APF, using the space voltage vector model predictive control method of an APF according to any one of claims 1 to 5, characterized in that: include: APF main circuit and space voltage vector model; The DC side neutral point in the APF main circuit is connected to the neutral line of the power supply grid, the output end of the APF main circuit is connected to the input end of the nonlinear load, the nonlinear load input end is also connected to the output end of the power supply grid, the output end of the APF main circuit is also connected to the input end of the space voltage vector model, and the output end of the space voltage vector module is connected to the control end of the APF main circuit; The spatial voltage vector model is constructed with the midpoint voltage balance index, zero-sequence current compensation index and load harmonic compensation index of the APF main circuit. The midpoint voltage balance index represents the ability of the zero-sequence current to affect the midpoint voltage. The zero-sequence current compensation index represents the ability of the APF to output zero-sequence compensation current. The load harmonic compensation index represents the ability of the voltage vector to output harmonic compensation current. The space voltage vector model divides 27 voltage vectors into long vectors, medium vectors, short vectors and zero vectors according to the output voltage vector amplitude of the APF main circuit, and predicts the optimal switch combination state through the space voltage vector combination of the APF switch, and then calculates the output current prediction value corresponding to each switch state; The space voltage vector model calculates the value function of neutral point voltage balance according to the predicted value, and obtains the optimal voltage vector after multiple cycles to control the compensation current output by the APF main circuit.

7. The spatial voltage vector model predictive control system of APF according to claim 6, characterized in that: Also included: an instantaneous power extraction module and a first Clark transformation module; The instantaneous power extraction module is used to obtain the three-phase voltage and load current of the grid connection point, and calculate the three-phase harmonic compensation reference current according to the instantaneous power theory; The first Clark transformation module performs Clark transformation on the three-phase harmonic compensation reference current and then sends the result to the space voltage vector model for prediction calculation.

8. The spatial voltage vector model predictive control system of APF according to claim 7, characterized in that: Also includes: The second Clark transformation module; The second Clark transformation module performs Clark transformation on the compensation reference current output by the current APF main circuit and sends the result to the space voltage vector model; The space voltage vector model traverses all voltage vectors according to the compensation reference current, predicts the compensated output current at the next moment, determines the optimal voltage vector through the value function, obtains the control signal of the bridge arm switch, and outputs the harmonic compensation current.

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