Weight coefficient-free midpoint potential control method and device for NPC type inverter

Abstract

The invention discloses a weight coefficient-free midpoint potential control method and device for an NPC type inverter, and the method comprises the steps: analyzing the influence of a three-level inverter output voltage vector on the midpoint potential based on an NPC type inverter topological structure, deducing the relation between the midpoint potential and the inverter output current, and carrying out the calculation of the NPC type inverter output current. The switching state capable of effectively reducing the neutral-point potential can be screened out by detecting the values of the current and the neutral-point potential; in the model predictive control, the neutral-point potential balance can be controlled by using a specific switch state, and a weight does not need to be added into a value function to calculate the neutral-point potential, so that the influence of the neutral-point potential on the output current in the model predictive control is greatly reduced, the waveform quality of the output current is improved while the neutral-point potential is controlled, and the control precision is improved. And meanwhile, the influence of the middle vector on the midpoint potential is suppressed. According to the method, the virtual vector is applied to model prediction control, so that the influence of the middle vector on the neutral point potential balance is reduced.

Description

Technical Field

[0001] The invention relates to the field of electric energy conversion device control, and in particular to a method and device for controlling a midpoint potential of an NPC type inverter without a weight coefficient. Background Art

[0002] In recent years, three-level inverters have been widely used in high-power applications such as motor drives and grid-connected converters. In the field of three-level inverter research, the main research directions are divided into the following two areas: First, in terms of hardware, the topology of the three-level inverter is optimized and innovated, which can be specifically divided into midpoint clamped type, flying capacitor type and cascaded H-bridge type. Second, the improvement of its software algorithm, one of which is the research on model predictive control.

[0003] Among them, the NPC type three-level inverter has been widely used because of its simple topology and not particularly complex control strategy. However, since the DC side voltage of the NPC type three-level inverter is composed of two sets of equivalent capacitors in series, the midpoint potential will be unbalanced due to the change of the upper and lower bus capacitor voltages, which will affect the output waveform quality to a certain extent. If the imbalance of the midpoint potential cannot be suppressed in time, it is even possible that the entire DC bus voltage is added to one capacitor, causing damage to the components.

[0004] In order to solve the problem of unbalanced midpoint potential, many scholars have conducted research on this and proposed their own solutions from different angles. In the field of model predictive control, the commonly used control method is to give the midpoint potential a certain weight coefficient and bring it into the value function, and calculate the modulation waveform that can stabilize the output reference current and reduce the midpoint potential through rolling optimization. However, this method will inevitably affect the control of other output quantities, such as output voltage and current, due to the introduction of midpoint potential for calculation, which will increase the harmonic rate of the output voltage and current waveforms. Summary of the invention

[0005] In view of the problems existing in the above-mentioned prior art, the present invention provides a method and device for controlling the midpoint potential of an NPC inverter without a weight coefficient. The present invention only needs to sample the output current of the inverter and perform calculation and analysis on the collected data. There is no need to introduce the midpoint potential value with a weight coefficient into the calculation formula of the value function, which not only reduces the impact on the output current of the inverter, but also has high algorithm execution efficiency and accuracy.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] In a first aspect, the present invention provides a method for controlling a midpoint potential of an NPC inverter without a weight coefficient, the method comprising the following steps:

[0008] Establish NPC inverter topology model;

[0009] Make the upper and lower bus capacitors C1 and C2 equal, take the DC side capacitors C1 and C2 as state variables, and derive the DC side voltage state equation of the NPC inverter;

[0010] By analyzing the relationship between the collected three-phase current and the DC side bus current, the relationship between the midpoint potential and the three-phase current is obtained;

[0011] The relationship between the midpoint potential and the three-phase current is discretized according to the forward Euler formula, and the relationship between the midpoint potential and the switch state is summarized.

[0012] According to the relationship between the midpoint potential and the switch state, the current sum corresponding to all redundant small vectors is calculated;

[0013] According to the current corresponding to each redundant small vector and the equivalent three-phase current and the current state of the midpoint potential, the correct redundant small vector is selected to control the midpoint potential of the DC side of the inverter;

[0014] According to the NPC inverter topology model, the value of the inverter output voltage at the current moment in the two-phase stationary coordinate system is calculated;

[0015] The sector in which the inverter is located is determined according to the voltage value currently output by the inverter, the switching state of the adjacent voltage vector is selected for rolling optimization, and the modulated wave is output.

[0016] In some embodiments, the DC side voltage state equation of the NPC inverter is:

[0017]

[0018] Among them, C1 and C2 are upper and lower bus capacitors, u dc1 、u dc2 is the voltage value of the upper and lower bus capacitors, i C1 、i C2 is the current flowing through the upper and lower busbar capacitor branches, i dc is the output current of the DC voltage source, i p 、i n is the upper and lower bus current;

[0019] Midpoint potential u o The upper and lower bus capacitor voltage u dc1 、u dc2 The relationship is as follows:

[0020] u o =u dc1 -u dc2

[0021] Substituting the current of the upper and lower bus capacitor branches into the relationship between the midpoint potential and the upper and lower bus capacitor voltages, the relationship between the midpoint potential and the bus current is obtained as follows:

[0022]

[0023] In some embodiments, the relationship between the collected three-phase current and the DC side bus current is analyzed to obtain a relationship between the midpoint potential and the three-phase current, including:

[0024] In the three-level inverter, the switching function of each phase bridge arm is defined as s a 、s b 、s c :

[0025]

[0026] Among them, V a1 、V a2 、V a3 、V a4 Represent the four switching devices of the inverter A phase bridge arm, V b1 、V b2 、V b3 、V b4 They represent the four switching devices of the inverter B phase bridge arm.

[0027] V c1 、V c2 、V c3 、V c4 They represent the four switching devices of the C-phase bridge arm of the inverter respectively;

[0028] According to the NPC inverter topology, the relationship between the three-phase current and the DC bus current is:

[0029]

[0030] Substitute the obtained relationship between the three-phase current and the DC side bus current into the relationship between the midpoint potential and the bus current. At this time, the relationship between the midpoint potential and the three-phase current is:

[0031]

[0032] In some embodiments, the relationship between the midpoint potential and the three-phase current is discretized according to the forward Euler formula, and the relationship between the midpoint potential and the switch state is summarized, including:

[0033] Assume that the sampling period of model predictive control is T s , according to the forward Euler formula The relationship between the midpoint potential and the three-phase current is discretized, and the relationship between the midpoint potential and the switch state is as follows:

[0034]

[0035] Among them, u o (k+1) is the midpoint potential value at the next sampling moment, u o (k) is the midpoint potential value i at the current sampling moment a (k), i b (k), i c (k) represent the three-phase current values ​​at the current sampling moment, i s (k) represents the sum of currents in one switching state;

[0036] According to the relationship between the midpoint potential and the switch state, it can be concluded that when the selected switch state makes the current sum positive, the switch state can reduce the midpoint potential at the next moment; if the currently selected switch state makes the current sum negative, the switch state can increase the midpoint potential at the next moment.

[0037] In some embodiments, the current and i corresponding to each redundant small vector s The current state of the equivalent three-phase current and the midpoint potential is used to select the correct redundant small vector to control the midpoint potential of the inverter DC side, including:

[0038] Divide all small vectors into six basic redundant small vectors V min 1. V min2 、V min3 、V min4 、V min5 、V min6 ; Where V3 is V min1 The positive small vector of V4 is V min 1, V5 is V min2 The positive small vector of V6 is V min2 The negative small vector, V7 is V min3 The positive small vector of V8 is V min3 The negative small vector, V9 is V min4 The small positive vector, V 10 V min4 The negative small vector, V 11 V min5 The small positive vector, V 12 V min5 The negative small vector, V 13 V min6 The small positive vector, V 14 V min6 The negative small vector of

[0039] The conditions for selecting redundant small vectors based on three-phase current and midpoint potential are:

[0040]

[0041] In some embodiments, the value of the inverter output voltage at the current moment in the two-phase stationary coordinate system is:

[0042]

[0043] Among them, u oαref (k),u oβref (k) is the value of the inverter output voltage in the two-phase stationary coordinate system at the current moment, i αref (k), i βref (k) is the value of the inverter output current sampled at the current moment in the two-phase stationary coordinate system, i αref (k-1), i βref (k-1) is the value of the inverter output current sampled at the last moment in the two-phase stationary coordinate system, e α (k), e β (k) is the value of the grid-side voltage sampled at the current moment in the two-phase stationary coordinate system.

[0044] In some embodiments, the value function of rolling optimization is:

[0045] J=|i αref (k+1)-i α (k+1)|+|i βref (k+1)-i β (k+1)|

[0046] Among them, i αref (k+1), i βref (k+1) is the reference value of the inverter output current in the two-phase stationary coordinate system at the next moment, i α (k+1), i β (k+1) is the predicted value of the inverter output current in the two-phase stationary coordinate system at the next moment.

[0047] In some embodiments, after selecting the correct redundant small vector to control the midpoint potential of the DC side of the inverter, the method further includes: synthesizing a virtual midpoint vector by using the basic small vector and the midpoint vector to construct a new space voltage vector and eliminate the midpoint potential difference generated by the basic vector;

[0048] The virtual vector synthesis formula is as follows:

[0049]

[0050] Among them, V 16new is the constructed virtual middle vector, V4 and V5 are basic small vectors, V 16 is the basic mean vector.

[0051] In a second aspect, the present invention provides a non-weighted coefficient midpoint potential control device for an NPC type inverter, the device comprising:

[0052] Topology model module, used to establish NPC inverter topology model;

[0053] The DC side voltage state equation module is used to make the upper and lower bus capacitors C1 and C2 equal, and take the DC side capacitors C1 and C2 as state variables to obtain the DC side voltage state equation of the NPC inverter;

[0054] The relational module is used to obtain the relational expression between the midpoint potential and the three-phase current by analyzing the relationship between the collected three-phase current and the DC side bus current;

[0055] The module for the relationship between the midpoint potential and the switch state is used to discretize the relationship between the midpoint potential and the three-phase current according to the forward Euler formula, and summarize the relationship between the midpoint potential and the switch state;

[0056] The calculation module 1 is used to calculate the current sum corresponding to all redundant small vectors according to the relationship between the midpoint potential and the switch state;

[0057] A redundant small vector selection control module is used to select a correct redundant small vector to control the midpoint potential of the inverter DC side according to the current corresponding to each redundant small vector and the current state of the equivalent three-phase current and the midpoint potential;

[0058] The calculation module 2 is used to calculate the value of the inverter output voltage at the current moment in the two-phase stationary coordinate system according to the NPC inverter topology structure model;

[0059] The optimization module is used to determine the sector where the inverter is located according to the voltage value output by the inverter at the current moment, select the switching state of the adjacent voltage vector for rolling optimization, and output the modulation wave.

[0060] In a third aspect, the present invention provides a computer-readable storage medium having computer program instructions stored thereon, which, when executed by a processor, implement the above-mentioned NPC type inverter non-weighted coefficient midpoint potential control method.

[0061] Compared with the prior art, the beneficial effects are:

[0062] The present invention only needs to analyze the relationship between the three-phase current and the midpoint potential at the sampling moment to select the switch state that can effectively suppress the midpoint potential at the next moment, which greatly reduces the influence of the calculated midpoint potential on the value function's ability to judge the current output, thereby achieving the purpose of reducing the model's predicted output current harmonic rate, and reduces the influence of the traditional midpoint vector switch state on the midpoint potential by configuring a virtual midpoint vector; the present invention has a simple structure, a small amount of calculation, and is easy to implement in hardware, and can achieve midpoint potential suppression while taking into account the quality of the output current. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The accompanying drawings are part of the present invention and are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention, but do not constitute an improper limitation of the present invention. Obviously, the accompanying drawings described below are only some embodiments. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without creative work.

[0064] Figure 1 A flow chart of a method for controlling midpoint potential of an NPC inverter without weight coefficient provided in one embodiment of the present invention;

[0065] Figure 2 A topological structure of an NPC inverter and a hardware diagram of a control circuit provided in one embodiment of the present invention;

[0066] Figure 3 A three-level inverter switch state and space voltage vector diagram provided in one embodiment of the present invention;

[0067] Figure 4 An improved three-level inverter space voltage vector diagram based on a virtual vector provided in one embodiment of the present invention;

[0068] Figure 5 A midpoint potential simulation waveform diagram of a weight-free model predictive control provided in one embodiment of the present invention;

[0069] Figure 6 A simulated waveform diagram of midpoint potential predicted by a weight-free coefficient model based on a virtual vector provided in one embodiment of the present invention;

[0070] Figure 7 It is a simulation waveform diagram of output current of the weight-free coefficient model predictive control based on virtual vector provided in one embodiment of the present invention;

[0071] Figure 8 It is a spectrum analysis diagram of the output current waveform predicted by a weight coefficient model when the upper and lower bus capacitances are equal provided in one embodiment of the present invention;

[0072] Fig. 9It is a spectrum analysis diagram of the output current waveform predicted by the model without weight coefficient provided in one embodiment of the present invention;

[0073] Fig.10 This is a spectrum analysis diagram of the output current waveform of the weight-free coefficient model predictive control based on a virtual vector provided in one embodiment of the present invention.

[0074] It should be noted that these drawings and textual descriptions are not intended to limit the conceptual scope of the present invention in any way, but are intended to illustrate the concept of the present invention for those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION

[0075] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0076] like Figure 1 As shown, in one embodiment of the present invention, a method for controlling the midpoint potential of an NPC type inverter without a weight coefficient is proposed, and the method includes the following steps:

[0077] S100: Establish an NPC inverter topology model.

[0078] Specifically, the constructed NPC inverter topology model refers to Figure 2 Shown is the control circuit hardware diagram.

[0079] Step S200: Make the upper and lower bus capacitances C1 and C2 equal, take the DC side capacitances C1 and C2 as state variables, and obtain the DC side voltage state equation of the NPC inverter.

[0080] Furthermore, the DC side voltage state equation of the NPC inverter is:

[0081]

[0082] Where: C1, C2 are upper and lower bus capacitors, u dc1 、u dc2 is the voltage value of the upper and lower bus capacitors, i C1 、i C2 is the current flowing through the upper and lower busbar capacitor branches, i dc is the output current of the DC voltage source, i p 、i n is the upper and lower bus current.

[0083] At this time, the relationship between the midpoint potential and the upper and lower bus capacitor voltages is as follows:

[0084] u o =u dc1 -u dc2 (3)

[0085] Substituting equations (1) and (2) into equation (3), we can obtain the relationship between the midpoint potential and the bus current:

[0086]

[0087] S300: By analyzing the relationship between the collected three-phase current and the DC side bus current, a relationship between the midpoint potential and the three-phase current is obtained.

[0088] In the embodiment of the present application, by analyzing the relationship between the collected three-phase current and the DC side bus current, the relationship between the midpoint potential and the three-phase current is obtained, including:

[0089] In the three-level inverter, the switching function of each phase bridge arm is defined as s a 、s b 、s c :

[0090]

[0091] Among them, V a1 、V a2 、V a3 、V a4 Represent the four switching devices of the inverter A phase bridge arm, V b1 、V b2 、V b3 、V b4 They represent the four switching devices of the inverter B phase bridge arm.

[0092] V c1 、V c2 、V c3 、V c4 They represent the four switching devices of the C-phase bridge arm of the inverter.

[0093] According to the NPC inverter topology, the relationship between the three-phase current and the DC bus current is:

[0094]

[0095] Substituting the above equation into equation (4), the relationship between the midpoint potential and the three-phase current is:

[0096]

[0097] S400: Discretize the relationship between the midpoint potential and the three-phase current according to the forward Euler formula, and summarize the relationship between the midpoint potential and the switch state.

[0098] For finite set model predictive control, formula (7) is discretized and a discrete model of the system is established to predict the system variables. Assume that the sampling period of the model predictive control is T s , according to the forward Euler formula The relationship between the midpoint potential and the three-phase current is discretized as follows:

[0099]

[0100] Among them, u o (k+1) is the midpoint potential value at the next sampling moment, u o (k) is the midpoint potential value i at the current sampling moment a (k), i b (k), i c (k) represent the three-phase current values ​​at the current sampling moment, i s (k) represents the sum of currents in one switching state.

[0101] Depend on Figure 3 As shown in the figure, only the small voltage vector contains redundant vectors, which proves that the midpoint potential imbalance can be effectively suppressed by selecting the switching state of the small vector. The current and i corresponding to the switching state of the three-phase bridge arm of each small vector s The equivalent relationship with the three-phase current is shown in Table 1:

[0102] Table 1

[0103]

[0104] It can be seen from Table 1 that when the selected switch state makes the current sum positive, the switch state can reduce the midpoint potential at the next moment, and if the currently selected switch state makes the current sum negative, the switch state can increase the midpoint potential at the next moment. Therefore, in order to select a small vector that can effectively suppress the midpoint potential, it is necessary to determine the positive and negative of the three-phase circuit and the midpoint potential under each switch state.

[0105] S500: Calculate the current sum corresponding to all redundant small vectors according to the relationship between the midpoint potential and the switch state.

[0106] Specifically, according to the relationship between the midpoint potential and the switch state shown in Table 1, the current sum corresponding to all redundant small vectors is analyzed.

[0107] S600: Select a correct redundant small vector to control the midpoint potential of the inverter DC side according to the current corresponding to each redundant small vector and the equivalent three-phase current and the current state of the midpoint potential.

[0108] Further, all small vectors are divided into six basic redundant small vectors V min 1. V min2 、V min3 、V min4 、V min5 、V min6 , where V3 is V min 1, V4 is V min 1, V5 is V min2 The positive small vector of V6 is V min2 The negative small vector, V7 is V min3 The positive small vector of V8 is V min3 The negative small vector, V9 is V min4 The small positive vector, V 10 V min4 The negative small vector, V 11 V min5 The small positive vector, V 12 V min5 The negative small vector, V 13 V min6 The small positive vector, V 14 V min6 The negative small vector of min1 For example, formula (8) can be deduced that when V min 1 When a small positive vector is selected, i s (k) = i a (k), if i a (k)>0 midpoint potential decreases if i a (k)<0 midpoint potential increases; when V min 1 When a small negative vector is selected, i s (k) = -i a (k), if i a (k)>0, the midpoint potential increases, if i a (k)<0 the midpoint potential decreases.

[0109] Therefore, the conditions for selecting redundant small vectors based on three-phase current and midpoint potential are:

[0110]

[0111] S700: Calculate the value of the inverter output voltage at the current moment in a two-phase stationary coordinate system according to the NPC inverter topology model.

[0112] Furthermore, the value of the inverter output voltage in the two-phase stationary coordinate system at the current moment is calculated as follows:

[0113]

[0114] Among them, u oαref (k),u oβref (k) is the value of the inverter output voltage in the two-phase stationary coordinate system at the current moment, i αref (k), i βref (k) is the value of the inverter output current sampled at the current moment in the two-phase stationary coordinate system, i αref (k-1), i βref (k-1) is the value of the inverter output current sampled at the last moment in the two-phase stationary coordinate system, e α (k), e β (k) is the value of the grid-side voltage sampled at the current moment in the two-phase stationary coordinate system.

[0115] S800: Determine the sector where the inverter is located according to the voltage value currently output by the inverter, select the switching state of the adjacent voltage vector for rolling optimization, and output the modulation wave.

[0116] Specifically, adjacent vectors of the sector where the voltage is located are selected, and the value functions are calculated respectively for rolling optimization, because small vectors that can suppress the change of midpoint potential have been selected in the process of screening redundant small vectors.

[0117] Furthermore, the calculation formula of the value function can be simplified as follows:

[0118] J=|i αref (k+1)-i α (k+1)|+|i βref (k+1)-i β (k+1)| (16)

[0119] Among them, i αref (k+1), i βref (k+1) is the reference value of the inverter output current in the two-phase stationary coordinate system at the next moment, i α (k+1), i β (k+1) is the predicted value of the inverter output current in the two-phase stationary coordinate system at the next moment.

[0120] By calculating the value of the cost function J under each adjacent vector switching state in the sector, the switching state with the smallest cost function is selected as the modulation wave output through comparison to control the operation of the inverter.

[0121] In an optional embodiment, after selecting a correct redundant small vector to control the midpoint potential of the inverter DC side, the method further includes:

[0122] The virtual middle vector is synthesized by using the basic small vector and the middle vector to construct a new space voltage vector and eliminate the midpoint potential difference generated by the basic vector.

[0123] The virtual vector synthesis formula is as follows:

[0124] V 16mew =1 / 3(V4+V5+V 16 ) (17)

[0125] Among them, V 16new is the constructed virtual middle vector, V4 and V5 are basic small vectors, V 16 is the basic mean vector.

[0126] For finite set model predictive control, in order to solve the problem that the influence of the midpoint potential in the model predictive control vector cannot be controlled, the present invention uses the basic small vectors that can offset each other in the midpoint current and the mid vector to synthesize a virtual mid vector and construct a new space voltage vector. 16 The relationship between the switching state and the midpoint current is that in the case of three-phase balance, the midpoint current i o =i a +i b +i c = 0, the synthesized virtual center vector completely eliminates the midpoint potential difference generated by the basic vector, which can effectively improve the model prediction midpoint potential control effect. All virtual center vectors constitute the vector as shown in Table 2:

[0127] Table 2

[0128] Virtual Medium Vector Composition vector Vector action order <![CDATA[V 16new ]]> <![CDATA[V4,V5,V 16 ]]> <![CDATA[V4-V 16 -V5-V 16 -V4]]> <![CDATA[V 18new ]]> <![CDATA[V5,V8,V 18 ]]> <![CDATA[V8-V 18 -V5-V 18 -V8]]> <![CDATA[V 20new ]]> <![CDATA[V8,V9,V 20 ]]> <![CDATA[V8-V 20 -V9-V 20 -V8]]> <![CDATA[V 22new ]]> <![CDATA[V9,V 12 ,In 22 ]]> <![CDATA[V 12 -V 22 -V9-V 22 -V 12 ]]> <![CDATA[V 24new ]]> <![CDATA[V 12 ,V 13 ,V 24 ]]> <![CDATA[V 12 -V 24 -V 13 -V 24 -V 12 ]]> <![CDATA[V 26new ]]> <![CDATA[V 13 ,V4,V 26 ]]> <![CDATA[V4-V 26 -V 13 -V 26 -V4]]>

[0129] A simulation model was built according to the data in Table 3. Before 0.13 s, the control method adopted the traditional midpoint potential control with weight coefficient. After 0.13 s, the control mode was switched to the midpoint potential control method without weight coefficient of the present invention for comparative analysis.

[0130] Table 3

[0131] Electrical symbols describe value <![CDATA[e a ,And b ,And c ]]> Grid phase voltage effective value 100V f Grid voltage frequency 50HZ <![CDATA[f s ]]> Sampling frequency 20kHz <![CDATA[C1]]> DC side capacitor 1080μF <![CDATA[C2]]> DC side lower capacitor 1080μF R AC side filter resistor 0.05Ω L AC side filter inductor 20mH Mag Reference current amplitude 10A

[0132] The method of the present invention is based on an NPC inverter topology structure, analyzes the influence of a three-level inverter output voltage vector on a midpoint potential, derives the relationship between the midpoint potential and the inverter output current, and screens out a switch state that can effectively reduce the midpoint potential by detecting the values ​​of the current and the midpoint potential; in model predictive control, the midpoint potential balance can be controlled by using a specific switch state, and there is no need to add a weight to a value function to calculate the midpoint potential, which greatly reduces the influence of the midpoint potential on the output current in the model predictive control.

[0133] Figure 5The midpoint potential waveform is obtained by building a simulation model based on the data in Table 3. It can be seen intuitively that the midpoint potential of the NPC inverter can be stably controlled within ±0.8V before and after the control algorithm is switched, indicating that the control method of the present invention can achieve midpoint potential control without a weight coefficient.

[0134] Figure 7 This is the output current simulation waveform when the upper and lower bus capacitances are equal. It can be seen that the inverter output currents of the two control methods are both stable AC currents. The output current waveform spectrum analysis of the traditional control method gives the following results: Figure 8 As shown, the current amplitude is 10A, the current THD = 0.80%, and after 0.13s, it is switched to the midpoint potential control method without weight coefficient. The output current waveform spectrum analysis of this control method obtains the following results Fig. 9 As shown, the current amplitude is 10A, THD = 0.72%. By comparing the current THD values ​​before and after the switching control method, it can be concluded that the midpoint potential control method without weight coefficient effectively reduces the influence of the midpoint potential on the output current waveform during model predictive control.

[0135] Figure 4 The present invention improves the three-level inverter space voltage vector diagram based on the virtual vector, calculates the new sector by the three-level inverter space voltage vector diagram to perform model predictive control, and the midpoint potential waveform obtained by simulation is as follows: Figure 6 As shown in the figure, it can be seen intuitively that the midpoint potential prediction control method based on virtual vector without weight coefficient can stably control the midpoint potential of NPC inverter within ±0.4V. The results of output current waveform spectrum analysis are as follows Fig.10 As shown, the current amplitude is 10A, and the current THD = 0.73%. By comparing the current THD values ​​before and after the switching control method, it can be concluded that the weightless midpoint potential control method based on the virtual vector effectively reduces the midpoint potential during model predictive control, while having little effect on the output current quality.

[0136] In one embodiment, a non-weighted coefficient midpoint potential control device for an NPC type inverter is shown, the device comprising:

[0137] Topology model module, used to establish NPC inverter topology model;

[0138] The DC side voltage state equation module is used to make the upper and lower bus capacitors C1 and C2 equal, and take the DC side capacitors C1 and C2 as state variables to obtain the DC side voltage state equation of the NPC inverter;

[0139] The relational module is used to obtain the relational expression between the midpoint potential and the three-phase current by analyzing the relationship between the collected three-phase current and the DC side bus current;

[0140] The module for the relationship between the midpoint potential and the switch state is used to discretize the relationship between the midpoint potential and the three-phase current according to the forward Euler formula, and summarize the relationship between the midpoint potential and the switch state;

[0141] The calculation module 1 is used to calculate the current sum corresponding to all redundant small vectors according to the relationship between the midpoint potential and the switch state;

[0142] A redundant small vector selection control module is used to select a correct redundant small vector to control the midpoint potential of the inverter DC side according to the current corresponding to each redundant small vector and the current state of the equivalent three-phase current and the midpoint potential;

[0143] The calculation module 2 is used to calculate the value of the inverter output voltage at the current moment in the two-phase stationary coordinate system according to the NPC inverter topology structure model;

[0144] The optimization module is used to determine the sector where the inverter is located according to the voltage value output by the inverter at the current moment, select the switching state of the adjacent voltage vector for rolling optimization, and output the modulation wave.

[0145] It should be noted that the above-mentioned embodiment provides an NPC type inverter midpoint potential control device without weight coefficient, when executing an NPC type inverter midpoint potential control method without weight coefficient, only uses the division of the above-mentioned functional modules as an example. In actual applications, the above-mentioned functions can be assigned to different functional modules as needed, that is, the internal structure of the device is divided into different functional modules to complete all or part of the functions described above. In addition, the above-mentioned embodiment provides an NPC type inverter midpoint potential control device without weight coefficient and an embodiment of an NPC type inverter midpoint potential control method without weight coefficient. The embodiment and implementation process thereof are detailed in an NPC type inverter midpoint potential control method without weight coefficient, which will not be repeated here.

[0146] In one embodiment, a computer-readable storage medium is provided, on which computer program instructions are stored. When the instructions are executed by a processor, the above-mentioned NPC type inverter midpoint potential control device without weight coefficient is implemented.

[0147] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0148] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any slight modification, equivalent substitution and improvement made to the above embodiment based on the technical essence of the present invention should be included in the protection scope of the technical solution of the present invention.

Claims

1. A method for controlling the midpoint potential of an NPC inverter without weight coefficient, characterized by: The method comprises the following steps: Establish NPC inverter topology model; Make the upper and lower bus capacitances C1 and C2 equal, and use the DC side capacitances C1 and C2 as state variables to derive the DC side voltage state equation of the NPC inverter. By analyzing the relationship between the collected three-phase current and the DC side bus current, the relationship between the midpoint potential and the three-phase current is obtained; The relationship between the midpoint potential and the three-phase current is discretized according to the forward Euler formula, and the relationship between the midpoint potential and the switch state is summarized; According to the relationship between the midpoint potential and the switch state, the current sum corresponding to all redundant small vectors is calculated; According to the current corresponding to each redundant small vector, the equivalent three-phase current and the current state of the midpoint potential, the correct redundant small vector is selected to control the midpoint potential of the inverter DC side; According to the NPC inverter topology model, calculate the value of the inverter output voltage at the current moment in the two-phase stationary coordinate system; The sector in which the inverter is located is determined based on the voltage value output by the inverter at the current moment, and the switching state of the adjacent voltage vectors is selected for rolling optimization to output the modulated wave.

2. The method for controlling the midpoint potential of an NPC inverter without weight coefficient based on a virtual vector according to claim 1, wherein: The DC side voltage state equation of the NPC inverter is: Among them, C1 and C2 are upper and lower bus capacitors, u dc1 、u dc2 is the voltage value of the upper and lower bus capacitors, i C1 、i C2 is the current flowing through the upper and lower busbar capacitor branches, i dc is the output current of the DC voltage source, i p 、i n is the upper and lower bus current; Midpoint potential u o and the upper and lower bus capacitor voltage u dc1 、u dc2 The relationship is as follows: in o =in dc1 -in dc2 Substituting the currents of the upper and lower bus capacitor branches into the relationship between the midpoint potential and the upper and lower bus capacitor voltages, the relationship between the midpoint potential and the bus current is obtained as follows:

3. The method for controlling the midpoint potential of an NPC inverter without weight coefficient based on a virtual vector according to claim 2, wherein: The relationship between the collected three-phase current and the DC side bus current is analyzed to obtain the relationship between the midpoint potential and the three-phase current, including: In the three-level inverter, the switching function of each phase bridge arm is defined as s a 、s b 、s c : Among them, V a1 、V a2 、V a3 、V a4 They represent the four switching devices of the inverter A phase bridge arm. V b1 、V b2 、V b3 、V b4 They represent the four switching devices of the inverter B phase bridge arm. V c1 、V c2 、V c3 、V c4 They represent the four switching devices of the inverter C-phase bridge arm; According to the NPC inverter topology, the relationship between the grid-side three-phase current and the DC-side bus current is: Substitute the obtained relationship between the three-phase current and the DC side bus current into the relationship between the midpoint potential and the bus current. At this time, the relationship between the midpoint potential and the three-phase current is: Among them, s a 、s b 、s c is the switching state of each phase bridge arm, u o is the midpoint potential, i a 、i b 、i c is the three-phase current value of the inverter grid side.

4. The method for controlling the midpoint potential of an NPC inverter without weight coefficient based on a virtual vector according to claim 3, wherein: The relationship between the midpoint potential and the three-phase current is discretized according to the forward Euler formula, and the relationship between the midpoint potential and the switch state is summarized, including: Assume that the sampling period of model predictive control is T s , according to the forward Euler formula The relationship between the midpoint potential and the three-phase current is discretized, and the relationship between the midpoint potential and the switch state is as follows: Among them, u o (k+1) is the midpoint potential value at the next sampling moment, u o (k) is the midpoint potential value i at the current sampling moment a (k), i b (k), i c (k) represents the three-phase current value at the current sampling moment, i s (k) represents the sum of the switching state currents; According to the relationship between the midpoint potential and the switch state, it can be concluded that when the selected switch state makes the current sum positive, the switch state can reduce the midpoint potential at the next moment; if the currently selected switch state makes the current sum negative, the switch state can increase the midpoint potential at the next moment.

5. The method for controlling the midpoint potential of an NPC inverter without weight coefficient based on a virtual vector according to claim 4, wherein: According to the current and i corresponding to each redundant small vector s The current state of the equivalent three-phase current and the midpoint potential is used to select the correct redundant small vector to control the midpoint potential of the inverter DC side, including: Divide all small vectors into six basic redundant small vectors V min1 、V min2 、V min3 、V min4 、V min5 、V min6 ; Among them, V3 is V min1 A small positive vector, V4 is V min1 The negative small vector, V5 is V min2 A small positive vector, V6 is V min2 The negative small vector, V7 is V min3 The positive small vector of V8 is V min3 The negative small vector, V9 is V min4 The small positive vector, V 10 V min4 The negative small vector, V 11 V min5 The small positive vector, V 12 V min5 The negative small vector, V 13 V min6 The small positive vector, V 14 V min6 The negative small vector of The conditions for selecting redundant small vectors based on three-phase current and midpoint potential are:

6. The method for controlling the midpoint potential of an NPC inverter without weight coefficient based on a virtual vector according to claim 1, wherein: The value of the inverter output voltage at the current moment in the two-phase stationary coordinate system is: Among them, u oαref (k),u oβref (k) is the value of the inverter output voltage in the two-phase stationary coordinate system at the current moment, i αref (k), i βref (k) is the value of the inverter output current sampled at the current moment in the two-phase stationary coordinate system, i αref (k-1), i βref (k-1) is the value of the inverter output current sampled at the last moment in the two-phase stationary coordinate system, e α (k), e β (k) is the value of the grid-side voltage sampled at the current moment in the two-phase stationary coordinate system.

7. The method for controlling the midpoint potential of an NPC inverter without weight coefficient based on a virtual vector according to claim 1, characterized in that: The value function of rolling optimization is: J=|i αref (k+1)-i α (k+1)|+|i βref (k+1)-i β (k+1)| Among them, i αref (k+1), i βref (k+1) is the reference value of the inverter output current in the two-phase stationary coordinate system at the next moment, i α (k+1), i β (k+1) is the predicted value of the inverter output current at the next moment in the two-phase stationary coordinate system.

8. The method for controlling the midpoint potential of an NPC inverter without weight coefficient based on a virtual vector according to claim 1, wherein: After selecting the correct redundant small vector to control the midpoint potential of the inverter DC side, the method further includes: synthesizing a virtual midpoint vector by using the basic small vector and the midpoint vector to construct a new space voltage vector and eliminate the midpoint potential difference generated by the basic midpoint vector; The virtual vector synthesis formula is as follows: Among them, V 16new is the constructed virtual middle vector, V4 and V5 are basic small vectors, V 16 is the basic mean vector.

9. An NPC type inverter midpoint potential control device without weight coefficient, characterized in that: The device comprises: Topology model module, used to establish the topology model of NPC inverter; The DC side voltage state equation module is used to make the upper and lower bus capacitors C1 and C2 equal, and use the DC side capacitors C1 and C2 as state variables to derive the DC side voltage state equation of the NPC inverter; The relational module is used to obtain the relational expression between the midpoint potential and the three-phase current by analyzing the relationship between the collected three-phase current and the DC bus current; The midpoint potential and switch state relationship module is used to discretize the relationship between the midpoint potential and the three-phase current according to the forward Euler formula, and summarize the relationship between the midpoint potential and the switch state; Calculation module 1, used to calculate the current sum corresponding to all redundant small vectors based on the relationship between the midpoint potential and the switch state; The redundant small vector selection control module is used to select the correct redundant small vector to control the midpoint potential of the inverter DC side according to the current corresponding to each redundant small vector and the equivalent three-phase current and the current state of the midpoint potential; The second calculation module is used to calculate the value of the inverter output voltage at the current moment in the two-phase stationary coordinate system according to the NPC inverter topology model; The optimization module is used to determine the sector in which the inverter is located according to the voltage value output by the inverter at the current moment, select the switching state of adjacent voltage vectors for rolling optimization, and output the modulation wave.

10. A computer-readable storage medium, characterized in that Computer program instructions are stored thereon, and when the instructions are executed by a processor, the method for controlling the midpoint potential of an NPC type inverter without a weight coefficient as described in any one of claims 1 to 8 is implemented.

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