Neutral-point potential balance method considering safety margin, energy storage inverter control method and device

Abstract

The invention discloses a neutral-point potential balancing method considering safety margin and an energy storage inverter control method and device, and the method comprises the steps: deducing the relation between the neutral-point potential and the output current of an inverter through analyzing the influence of a three-level inverter output voltage vector on the neutral-point potential; by detecting the change of the output current and the neutral-point potential, the switching state capable of effectively balancing the neutral-point potential can be screened out. In the model prediction control, the balance control of the neutral-point potential can be realized by using the specific switch states, and the neutral-point potential does not need to be considered by introducing an additional weight item into a value function, so that the influence of the fluctuation of the neutral-point potential on the output current is remarkably reduced. Meanwhile, the inertia characteristic of the direct-current side capacitor is considered, and the concept of midpoint potential safety margin is provided for avoiding the situation that the direct-current side capacitor is damaged due to the fact that the midpoint potential is too large. In different safety margins, different switching states are adopted, and dual control over the waveform quality of the neutral-point potential and the output current is achieved.

Description

Technical Field

[0001] The present invention belongs to the field of energy storage inverter control, and in particular relates to a midpoint potential balancing method considering a safety margin, and an energy storage inverter control method and device. 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. The research on three-level inverters is mainly divided into the following two directions: one is to optimize and innovate the topology of three-level inverters at the hardware level, mainly including midpoint clamped type, flying capacitor type and cascaded H-bridge type; the other is to improve the software algorithm, among which the research on model predictive control is one of the important directions.

[0003] NPC three-level inverters have been widely used in medium-high voltage and high-power applications in recent years, such as motor drive, renewable energy grid connection, and industrial automation, due to their simple topology, high utilization of power devices, and relatively easy control strategy. However, the DC side voltage is composed of two sets of equivalent capacitors in series, which makes the midpoint potential susceptible to the imbalance of the upper and lower bus capacitor voltages during operation, resulting in the problem of midpoint potential drift. The imbalance of the midpoint potential will not only weaken the symmetry and waveform quality of the inverter output voltage, increase the harmonic content, but also increase the thermal stress of the power devices and reduce the stability of the system operation. If this imbalance cannot be effectively suppressed in a timely manner, it may even cause the entire voltage of the DC bus to be concentrated on a certain capacitor, exceeding the withstand voltage range of the capacitor, and eventually causing capacitor damage or failure of the entire system. Therefore, how to suppress the imbalance of the midpoint potential has become one of the key issues in the research of NPC three-level inverters.

[0004] In order to solve the problem of unbalanced midpoint potential, many scholars have proposed various solutions from different angles. In the field of model predictive control, a common method is to introduce the midpoint potential into the value function and assign it a certain weight coefficient, and calculate the modulation waveform through rolling optimization, so as to reduce the fluctuation of the midpoint potential while stabilizing the output reference current. However, since the midpoint potential is included in the calculation, this method inevitably affects the control of other output quantities (such as output voltage and current), resulting in an increase in the harmonic content of the output voltage and current waveforms. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a midpoint potential balancing method considering safety margin, a control method and device for energy storage inverter. By analyzing the influence of the output voltage vector of the three-level inverter on the midpoint potential, the relationship between the midpoint potential and the output current is derived, and based on the relationship, the switch state that can balance the midpoint potential is screened. In model predictive control, these switch states are used to achieve the balance of the midpoint potential, without adding additional weights to the value function, reducing the influence of the midpoint potential fluctuation on the output current, and considering the inertial characteristics of the DC side capacitor, the concept of midpoint potential safety margin is proposed, and the appropriate switch state is selected according to different margins, while taking into account the balance of the midpoint potential and the quality of the output current waveform, to avoid the problem of capacitor damage caused by excessive midpoint potential.

[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 midpoint potential balancing considering a safety margin, the method comprising:

[0008] Set the critical voltage value of the midpoint potential safety margin to no more than 100V, and define 0V-u′ o is the first safety margin, u′ o and above is the second safety margin, u′ o is the critical voltage value of the midpoint potential safety margin;

[0009] The three-phase reference current and midpoint potential are fuzzy processed by using the triangular membership function and the trapezoidal membership function, and a rule table is formulated in combination with the fluctuation degree of the set safety margin critical voltage and the corresponding weight coefficient; the weight coefficient is obtained by the reference current and voltage calculated by the membership function;

[0010] After the weight coefficient is normalized, the value of the safety margin is calculated according to the midpoint potential safety margin critical voltage;

[0011] The midpoint potential is controlled according to the calculated value of the safety margin to achieve midpoint potential balance.

[0012] In one embodiment, the value of the safety margin is:

[0013]

[0014] Among them, u′ onew is the critical voltage value of the midpoint potential safety margin after adjustment, u′ oref is the standard value of the critical voltage of the midpoint potential safety margin, Δu′ o is the critical voltage change value of the midpoint potential safety margin, ω[i] is the weight coefficient under different fuzzy rules, δu′ o[i] is the fluctuation degree of the critical voltage of the safety margin under different fuzzy rules.

[0015] In one embodiment, the midpoint potential is controlled according to the calculated safety margin value to achieve midpoint potential balance, and the specific method includes:

[0016] Determine the safety margin of the midpoint potential and select different switch states within different safety margins;

[0017] When the absolute value of the midpoint potential is within the first safety margin, the current sum corresponding to all redundant small vectors is calculated according to the relationship between the midpoint potential and the switch state; the appropriate redundant small vector is selected as the switch state by combining the current sum of each redundant small vector and the corresponding equivalent three-phase current and the current state of the midpoint potential;

[0018] When the absolute value of the midpoint potential is within the second safety margin, the components of the inverter output voltage in the two-phase stationary coordinate system at the current moment are calculated according to the topological structure model of the NPC type energy storage inverter, and the sector in which the components are located is determined according to the phase and magnitude relationship of the components, and a small vector that reduces the midpoint potential is selected in the sector as the switching state;

[0019] In a second aspect, the present invention provides a method for controlling a midpoint potential balanced energy storage inverter, the method comprising:

[0020] Establish the topological structure model of NPC type energy storage inverter;

[0021] In order to achieve equal voltages between the upper and lower bus capacitors, the DC side voltage state equation of the energy storage inverter is obtained by taking the DC side upper and lower bus capacitors C1 and C2 as state variables, and the relationship between the midpoint potential and the upper and lower bus currents is obtained;

[0022] According to the relationship between the midpoint potential and the upper and lower bus currents, the relationship between the three-phase current on the grid side of the inverter and the bus current on the DC side is obtained, and the relationship between the midpoint potential and the three-phase current is derived;

[0023] The relationship between the midpoint potential and the switch state is: when the selected switch state makes the current sum positive, the switch state can reduce the midpoint potential at the next moment; when the selected switch state makes the current sum negative, the switch state can increase the midpoint potential at the next moment;

[0024] The forward Euler formula is used to discretize the relationship between the midpoint potential and the three-phase current, and the relationship between the midpoint potential and the switch state is obtained;

[0025] The above-mentioned midpoint potential balance method considering the safety margin is adopted to determine the safety margin of the midpoint potential, and different switch states are selected within different safety margins;

[0026] The value of the value function of the selected switch state is calculated through the rolling optimized value function, and the switch state with the smallest value function is selected as the modulation wave output.

[0027] In one implementation, the calculation of the components of the inverter output voltage at the current moment in the two-phase stationary coordinate system is:

[0028]

[0029] Among them, u αref (k),u βref (k) is the reference value of the inverter output voltage in the two-phase stationary coordinate system at the current moment, 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, i 1αref (k), i 1βref (k) is the reference value of the inverter output current sampled at the current moment in the two-phase stationary coordinate system, i 1αref (k-1), i 1βref (k-1) is the reference value of the inverter output current sampled at the previous moment in the two-phase stationary coordinate system.

[0030] In one embodiment, the relationship between the midpoint potential and the three-phase current is derived based on the relationship between the three-phase current on the grid side of the inverter and the bus current on the DC side, including:

[0031] In the three-level energy storage inverter, the switch state function of each phase bridge arm is defined as S a , S b , S c :

[0032]

[0033] Among them, V x1 、V x2 、V x3 、V x4 Respectively represent the four switching devices of the inverter x (x = a, b, c) phase bridge arm;

[0034] According to the topological structure of the NPC inverter, the relationship between the three-phase current on the grid side and the bus voltage on the DC side is:

[0035]

[0036] Substitute the obtained relationship between the three-phase current on the grid side and the bus current on the DC side 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 on the grid side is:

[0037]

[0038] Among them, S a , S b , S c is the switching state of each phase bridge arm, u o is the midpoint potential, i 1a 、i 1b 、i 1c is the three-phase current value of the inverter grid side.

[0039] In one embodiment, all small vectors are divided into six basic redundant small vectors:

[0040] V min1 、V min2 、V min3 、V min4 、V min5 、V min6 ; Then divide these six small vectors into positive small vectors and negative small vectors; Among them,

[0041] V3 is V min1 The positive small vector of V4 is V min1 The negative small vector of 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 A small negative vector.

[0042] In one implementation, the conditions for selecting a suitable redundant small vector as the switch state by combining the current of each redundant small vector and the corresponding equivalent three-phase current and the current state of the midpoint potential are as follows:

[0043]

[0044] Among them, i 1a 、i 1b 、i 1c is the three-phase current value of the inverter grid side, u o is the midpoint potential.

[0045] In one embodiment, the value function of the rolling optimization is:

[0046] J=[i 1αref (k+1)-i 1α (k+1)] 2 +[i 1βref (k+1)-i 1β (k+1)] 2

[0047] Among them, i 1αref (k+1), i 1βref (k+1) is the reference value of the inverter output current in the two-phase stationary coordinate system at the next moment, i 1α (k+1), i 1β (k+1) is the predicted value of the inverter output current in the two-phase stationary coordinate system at the next moment.

[0048] In a third aspect, the present invention provides a midpoint potential balanced energy storage inverter control device, the device comprising:

[0049] Topology model module, used to establish the topology model of NPC type energy storage inverter;

[0050] The DC side voltage state equation module is used to achieve equal voltages of the upper and lower bus capacitors, obtain the DC side voltage state equation of the energy storage inverter with the DC side upper and lower bus capacitors C1 and C2 as state variables, and derive the relationship between the midpoint potential and the upper and lower bus currents;

[0051] A relational module is used to obtain the relationship between the three-phase current on the grid side of the inverter and the bus current on the DC side according to the relationship between the midpoint potential and the upper and lower bus currents, and to derive the relational expression between the midpoint potential and the three-phase current;

[0052] 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 using the forward Euler formula, and to derive the relationship between the midpoint potential and the switch state;

[0053] A midpoint potential balancing module considering safety margin is used to execute the above-mentioned midpoint potential balancing method considering safety margin, determine the safety margin of the midpoint potential, and select different switch states within different safety margins;

[0054] The determining of the safety margin of the midpoint potential and selecting different switch states within different safety margins include:

[0055] When the absolute value of the midpoint potential is within the first safety margin, the current sum corresponding to all redundant small vectors is calculated according to the relationship between the midpoint potential and the switch state; the appropriate redundant small vector is selected as the switch state by combining the current sum of each redundant small vector and the corresponding equivalent three-phase current and the current state of the midpoint potential;

[0056] When the absolute value of the midpoint potential is within the second safety margin, the components of the inverter output voltage in the two-phase stationary coordinate system at the current moment are calculated according to the topological structure model of the NPC type energy storage inverter, and the sector in which the components are located is determined according to the phase and magnitude relationship of the components, and a small vector that reduces the midpoint potential is selected in the sector as the switching state;

[0057] The output module is used to calculate the value of the value function of the selected switch state through the rolling optimized value function, and select the switch state with the smallest value function as the modulation wave output.

[0058] Compared with the prior art, the beneficial effects are:

[0059] The present invention screens out appropriate switch states in advance, and these specific switch states can be used to achieve balanced control of the midpoint potential. There is no need to introduce additional weight terms in the value function to consider the midpoint potential, thereby significantly reducing the impact of midpoint potential fluctuations on the output current. At the same time, taking into account the inertial characteristics of the DC side capacitor, in order to avoid damage to the DC side capacitor due to excessive midpoint potential, the concept of midpoint potential safety margin is proposed. Within different safety margins, different switch states are used to achieve dual control of the midpoint potential and the waveform quality of the output current, avoiding the problem of excessive midpoint potential due to ensuring the quality of the current waveform. The present invention has a simple structure, a small amount of calculation, and is easy to implement in hardware. It can not only ensure fast calculations, but also take into account the accuracy of the results. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] 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.

[0061] Figure 1 A flow chart of a method for controlling a midpoint potential balanced energy storage inverter provided by an embodiment of the present invention;

[0062] Figure 2 A topological structure diagram of an energy storage inverter provided in an embodiment of the present invention;

[0063] Figure 3 A three-level inverter switch state and space voltage vector diagram provided in an embodiment of the present invention;

[0064] Figure 4 A simulated waveform diagram of the midpoint potential at a reference current value of 350A provided in the embodiment of the present invention for the predictive control of the weighted coefficient model;

[0065] Figure 5 A simulation waveform diagram of mid-point potential at a reference current value of 350A for the mid-point potential balance model predictive control taking into account the safety margin provided in an embodiment of the present invention;

[0066] Figure 6 A simulation waveform diagram of the output current of phase A of the midpoint potential balance model predictive control considering the safety margin provided in an embodiment of the present invention;

[0067] Figure 7 It is a spectrum analysis diagram of the output current waveform at a reference current value of 350A provided in the embodiment of the present invention with a weighted coefficient model predictive control;

[0068] Figure 8 It is a spectrum analysis diagram of the output current waveform at a reference current value of 350A of the midpoint potential balance model predictive control considering the safety margin provided in an embodiment of the present invention;

[0069] Fig. 9 A midpoint potential simulation waveform diagram of the midpoint potential balance model predictive control considering the safety margin provided in an embodiment of the present invention at a reference current value of 150A;

[0070] Fig.10 It is a spectrum analysis diagram of the output current waveform at a reference current value of 150A provided in the midpoint potential balance model predictive control taking into account the safety margin provided in the embodiment of the present invention;

[0071] 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

[0072] 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.

[0073] Due to the inertia of the DC side capacitor, the adjustment of the mid-point potential is delayed; at the same time, in order to avoid the situation in which, during rolling optimization, the switching state that can balance the mid-point potential is not selected in order to ensure the quality of the output current waveform, causing the mid-point potential to exceed the rated voltage of the DC side capacitor, resulting in capacitor damage and further causing safety problems, the present invention proposes the concept of a mid-point potential safety margin.

[0074] The present disclosure provides a method for midpoint potential balancing considering a safety margin, the method comprising:

[0075] Step 1: Set the critical voltage value of the midpoint potential safety margin to no more than 100V, and define 0V-u′ o is the first safety margin, u′ o and above is the second safety margin, u′ o It is the critical voltage value of the midpoint potential safety margin.

[0076] According to the DC side voltage under actual working conditions and the rated voltage of the upper and lower bus capacitors on the DC side, select a suitable midpoint potential safety margin critical voltage value. Assuming that the DC side voltage is 760V, the rated voltage of the upper and lower bus capacitors is 450V, and considering that each capacitor is given a 20V deviation, the given safety margin critical voltage value cannot exceed 100V. At the same time, in order to ensure the quality of the output current waveform, define 0V-u′ o is the first safety margin, u′ o and above is the second safety margin, u′ o It is the critical voltage value of the midpoint potential safety margin, in V.

[0077] Step 2: Use the triangle membership function and trapezoidal membership function to perform fuzzy processing on the three-phase reference current and midpoint potential, and formulate a rule table in combination with the fluctuation degree of the set safety margin critical voltage and the corresponding weight coefficient; the weight coefficient is obtained by the reference current and voltage calculated by the membership function. The rule table is as shown in Table 1 below:

[0078] Table 1

[0079]

[0080]

[0081] In the table, i ref_low 、i ref_med 、i ref_high is the reference current related value calculated by the membership function, u o_low 、u o_med 、u o_high It is the reference current related value calculated by the membership function.

[0082] Step 3: After the weight coefficient is normalized, the value of the safety margin is calculated according to the midpoint potential safety margin critical voltage.

[0083]

[0084] Among them, ε is a very small value, which can be 10 -6 , which is used to avoid the denominator being 0; finally, the value of the safety margin is calculated by the following formula:

[0085]

[0086] Among them, u′ onew is the critical voltage value of the midpoint potential safety margin after adjustment, u′ oref is the standard value of the critical voltage of the midpoint potential safety margin, Δu′ o is the critical voltage change value of the midpoint potential safety margin.

[0087] Step 4: Control the midpoint potential according to the calculated safety margin value to achieve midpoint potential balance.

[0088] Specifically, it is necessary to first determine which safety margin the midpoint potential is within, use different switch states within different safety margin ranges, and finally use these switch states for rolling optimization to output the modulated wave. That is:

[0089] When the absolute value of the midpoint potential is within the first safety margin, the relationship between the midpoint potential and the output current can be derived by analyzing the influence of the output voltage vector of the three-level inverter on the midpoint potential. Based on this relationship, the switching state that can balance the midpoint potential can be screened out, and the value function can be used for rolling optimization to obtain the most appropriate switching state.

[0090] When the absolute value of the midpoint potential is within the second safety margin, we can first determine in which small sector the inverter output voltage reference value is located, then select a small vector that can reduce the midpoint potential, and only use these switching states to substitute into the value function for rolling optimization to obtain the most appropriate switching state.

[0091] like Figure 1 As shown, the present invention proposes a method for controlling an energy storage inverter with midpoint potential balance without weight coefficient considering safety margin, and the method comprises the following steps:

[0092] Step S100: Establishing a topological structure model of an NPC type energy storage inverter.

[0093] refer to Figure 2 The topological structure diagram shown in the figure shows that the energy storage inverter topological structure model is as follows:

[0094]

[0095] Among them, R, L, and C are line resistance, filter inductor, and filter capacitor, respectively. 1α 、i 1β is the output current of the converter side, i 2ε 、i 2β is the grid-side output current, u 1α 、u 1β is the output voltage on the converter side, u 2α 、u 2βis the filter capacitor voltage, e α 、e β Represents the three-phase electromotive force of the power grid.

[0096] Step S200: To achieve equal voltages between the upper and lower bus capacitors, the DC side voltage state equation of the energy storage inverter is obtained using the DC side upper and lower bus capacitors C1 and C2 as state variables, and the relationship between the midpoint potential and the upper and lower bus currents is obtained.

[0097] Furthermore, the DC side voltage state equation of the energy storage inverter is:

[0098]

[0099] Among them, C1 and C2 are the upper and lower bus capacitance values ​​and C1=C2, u C1 、u C2 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 battery after being boosted by the DC / DC module, i p 、i n is the upper and lower bus current.

[0100] Midpoint potential u o The relationship between the upper and lower bus capacitor voltages is as follows:

[0101] u o =u C1 -u C2

[0102] Furthermore, the current of the upper and lower bus capacitor branches is substituted into the relationship between the midpoint potential and the upper and lower bus capacitor voltages, and the relationship between the midpoint potential and the upper and lower bus currents is obtained as follows:

[0103]

[0104] Step S300: The relationship between the three-phase current on the grid side of the inverter and the bus current on the DC side is obtained based on the relationship between the midpoint potential and the upper and lower bus currents, and a relationship between the midpoint potential and the three-phase current is derived.

[0105] Specifically, 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:

[0106] In the three-level energy storage inverter, the switch state function of each phase bridge arm is defined as S a , S b , S c :

[0107]

[0108] Among them, V x1 、V x2 、V x3 、V x4 They represent the four switching devices of the inverter x (x=a, b, c) phase bridge arm respectively.

[0109] According to the topological structure of the NPC inverter, the relationship between the three-phase current on the grid side and the bus voltage on the DC side can be obtained as follows:

[0110]

[0111] Furthermore, the obtained relationship between the three-phase current on the grid side and the bus current on the DC side is substituted 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 on the grid side is:

[0112]

[0113] Among them, S a , S b , S c is the switching state of each phase bridge arm, u o is the midpoint potential, i 1a 、i 1b 、i 1c is the three-phase current value of the inverter grid side.

[0114] Step S400: discretize the relationship between the midpoint potential and the three-phase current using the forward Euler formula, and obtain the relationship between the midpoint potential and the switch state.

[0115] Specifically, for finite set model predictive control, assuming that its sampling period is T s , using the forward Euler formula By discretizing the relationship between the midpoint potential and the three-phase current, we can obtain the following relationship between the switch state and the midpoint potential:

[0116]

[0117] Among them, u o (k+1) is the midpoint potential value at the next sampling moment, u o (k) is the midpoint potential value at the current sampling moment, i 1a (k), i 1b (k), i 1c (k) represent the three-phase current values ​​at the current sampling moment, i 1s (k) represents the sum of the switching state currents.

[0118] Depend on Figure 3As 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 1s The equivalent relationship with the three-phase current is shown in Table 2:

[0119] Table 2

[0120] Positive small vector <![CDATA[S x (x=a,b,c)]]> <![CDATA[i 1s ]]> Negative small vector <![CDATA[S x (x=a,b,c)]]> <![CDATA[i 1s ]]> <![CDATA[V3]]> 1，0，0 <![CDATA[i 1a ]]> <![CDATA[V4]]> 0，-1，-1 <![CDATA[-i 1a ]]> <![CDATA[V5]]> 1，1，0 <![CDATA[-i 1c ]]> <![CDATA[V6]]> 0，0，-1 <![CDATA[i 1c ]]> <![CDATA[V7]]> 0，1，0 <![CDATA[i 1b ]]> <![CDATA[V8]]> -1，0，-1 <![CDATA[-i 1b ]]> <![CDATA[V9]]> 0，1，1 <![CDATA[-i 1a ]]> <![CDATA[V 10 ]]> -1，0，0 <![CDATA[i 1a ]]> <![CDATA[V 11 ]]> 0，0，1 <![CDATA[i 1c ]]> <![CDATA[V 12 ]]> -1，-1，0 <![CDATA[-i 1c ]]> <![CDATA[V 13 ]]> 1，0，1 <![CDATA[-i 1b ]]> <![CDATA[V 14 ]]> 0，-1，0 <![CDATA[i 1b ]]>

[0121] Furthermore, the following relationship can be derived from the above table: when the selected switch state makes the current sum positive, the switch state can reduce the midpoint potential at the next moment; when the selected switch state makes the current sum negative, the switch state can increase the midpoint potential at the next moment.

[0122] 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 current and the midpoint potential in each switching state.

[0123] Step S500: adopting a midpoint potential balancing method taking into account a safety margin to determine the safety margin of the midpoint potential, and selecting different switch states within different safety margins.

[0124] Determine the safety margin of the midpoint potential and select different switch states within different safety margins, including:

[0125] When the absolute value of the midpoint potential is within the first safety margin, the current sum corresponding to all redundant small vectors is calculated according to the relationship between the midpoint potential and the switch state; the appropriate redundant small vector is selected as the switch state by combining the current sum of each redundant small vector and the corresponding equivalent three-phase current and the current state of the midpoint potential.

[0126] Specifically, according to the relationship between the midpoint potential and the switch state, the current and i corresponding to all redundant small vectors are calculated. 1s . The currents and corresponding to all redundant small vectors are analyzed through the relationship between the midpoint potential and the switch state shown in Table 2. Combined with the currents and corresponding equivalent three-phase currents of each redundant small vector and the current state of the midpoint potential, a suitable redundant small vector is selected to control the midpoint potential of the inverter DC side.

[0127] Further, all small vectors are divided into six basic redundant small vectors V min1 、V min2 、V min3 、V min4 、V min5 、V min6 Then, these six small vectors are divided into positive small vectors and negative small vectors; among them, V3 is V min1 The positive small vector of V4 is V min1 The negative small vector of V5 is Vmin2 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 A small negative vector.

[0128] With the basic redundant small vector V min1 For example, according to Table 1, when V min1 When a small positive vector is selected as the switch state, i 1s (k) = i 1a (k), if i 1a (k)>0, the midpoint potential decreases. 1a (k)<0, the midpoint potential increases; when V min1 When a negative small vector is selected as the switch state, i 1s (k) = -i 1a (k), if i 1a (k)>0, the midpoint potential increases. 1a If (k) < 0, the midpoint potential decreases.

[0129] Furthermore, it can be obtained that the conditions for selecting redundant small vectors based on the current state of the equivalent three-phase current and the midpoint potential are as follows:

[0130]

[0131] Among them, i 1a 、i 1b 、i 1c is the three-phase current value of the inverter grid side, u o is the midpoint potential.

[0132] When the absolute value of the midpoint potential is within the second safety margin, the components of the inverter output voltage in the two-phase stationary coordinate system at the current moment are calculated according to the topological structure model of the NPC type energy storage inverter, and the sector in which the components are located is determined according to the phase and magnitude relationship of the components, and a small vector that reduces the midpoint potential is selected in the sector as the switching state;

[0133] Specifically, the NPC inverter topology model is used to calculate the components of the inverter output voltage in the two-phase stationary coordinate system at the current moment.

[0134] The formula for calculating the components of the inverter output voltage in the two-phase stationary coordinate system at the current moment is as follows:

[0135]

[0136] Among them, u αref (k),u βref (k) is the reference value of the inverter output voltage in the two-phase stationary coordinate system at the current moment, 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, i 1αref (k), i 1βref (k) is the reference value of the inverter output current sampled at the current moment in the two-phase stationary coordinate system, i 1αref (k-1), i 1βref (k-1) is the reference value of the inverter output current sampled at the previous moment in the two-phase stationary coordinate system.

[0137] The inverter output voltage reference value in the two-phase stationary coordinate system is calculated to obtain its components on the α-axis and β-axis. The phase and magnitude relationship of these two components can be used to determine which small sector it is in, and then the corresponding optional switching state can be obtained.

[0138] This step determines within which safety margin the midpoint potential is located, adopts different switch states within different safety margin ranges, and finally uses these switch states for rolling optimization to output a modulated wave.

[0139] It should be noted that for a detailed description of the midpoint potential balancing method considering the safety margin, please refer to the description of the same or similar parts mentioned above, which will not be repeated here.

[0140] Step S600: Calculate the value of the cost function of the selected switch state through the rolling optimized cost function, and select the switch state with the smallest cost function as the modulation wave output.

[0141] Furthermore, the value function of rolling optimization is:

[0142] J=[i 1αref (k+1)-i 1α (k+1)] 2 +[i 1βref (k+1)-i 1β (k+1)] 2

[0143] Among them, i1αref (k+1), i 1βref (k+1) is the reference value of the inverter output current in the two-phase stationary coordinate system at the next moment, i 1α (k+1), i 1β (k+1) is the predicted value of the inverter output current in the two-phase stationary coordinate system at the next moment.

[0144] 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.

[0145] The present invention is verified by building a simulation in the Matlab / Simulink platform, and the relevant parameters are shown in Table 3. Under these parameters, the energy storage inverter simulation of the weighted coefficient model predictive control with a reference current value of 350A and the midpoint potential balance model predictive control without a weighted coefficient considering a safety margin, and the energy storage inverter simulation of the midpoint potential balance model predictive control without a weighted coefficient considering a safety margin with a reference current value of 150A are carried out.

[0146] Table 3

[0147]

[0148]

[0149] Based on Matlab / Simulink simulation, we can get the following Figure 4-Figure 10 result. Figure 6 It is a simulated waveform diagram of the output current of phase a under the predictive control of the midpoint potential balance model without weight coefficient and considering the safety margin with a reference current value of 350A. It can be seen from the figure that the present invention can realize the control of the three-level energy storage inverter.

[0150] Figure 4 and Figure 5 They are respectively the mid-point potential waveform diagrams of the weighted coefficient model predictive control under the reference current value of 350A and the mid-point potential balance model predictive control considering the safety margin. It can be seen intuitively that the control method adopted by the present invention can effectively reduce the fluctuation amplitude of the mid-point potential.

[0151] Figure 7 and Figure 8They are output current waveform spectrum analysis diagrams of the weighted coefficient model predictive control under a reference current value of 350A and the mid-point potential balance model predictive control considering a safety margin. It can be seen from the diagram that the THD value of the traditional method is 0.55%, and the THD value of the method adopted in the present invention is 0.69%, which proves that the inverter output current obtained by the method adopted in the present invention meets the relevant requirements and can achieve effective control of the mid-point potential balance at the expense of a little waveform quality.

[0152] Fig. 9 and Fig.10 They are the midpoint potential simulation waveform diagram and output current waveform spectrum analysis diagram of the midpoint potential balance model predictive control considering the safety margin when the reference current value is 150A. By comparing with the graph when the reference current value is 350A, it can be seen intuitively that when the reference current value is small, the midpoint potential is also small, and there will be no over-limit situation, and the THD value of the output current also meets the relevant requirements.

[0153] The following is an embodiment of a midpoint potential balanced energy storage inverter control device of the present invention, which can be used to execute an embodiment of a midpoint potential balanced energy storage inverter control method of the present invention. For details not disclosed in the embodiment of a midpoint potential balanced energy storage inverter control device of the present invention, please refer to the embodiment of a midpoint potential balanced energy storage inverter control method of the present invention.

[0154] In one embodiment, a midpoint potential balanced energy storage inverter control device is provided, the device comprising:

[0155] Topology model module, used to establish the topology model of NPC type energy storage inverter;

[0156] The DC side voltage state equation module is used to achieve equal voltages of the upper and lower bus capacitors, obtain the DC side voltage state equation of the energy storage inverter with the DC side upper and lower bus capacitors C1 and C2 as state variables, and derive the relationship between the midpoint potential and the upper and lower bus currents;

[0157] A relational module is used to obtain the relationship between the three-phase current on the grid side of the inverter and the bus current on the DC side according to the relationship between the midpoint potential and the upper and lower bus currents, and to derive the relational expression between the midpoint potential and the three-phase current;

[0158] 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 using the forward Euler formula, and to derive the relationship between the midpoint potential and the switch state;

[0159] A midpoint potential balancing module considering safety margin is used to execute a midpoint potential balancing method considering safety margin, determine the safety margin of the midpoint potential, and select different switch states within different safety margins;

[0160] The determining of the safety margin of the midpoint potential and selecting different switch states within different safety margins include:

[0161] When the absolute value of the midpoint potential is within the first safety margin, the current sum corresponding to all redundant small vectors is calculated according to the relationship between the midpoint potential and the switch state; the appropriate redundant small vector is selected as the switch state by combining the current sum of each redundant small vector and the corresponding equivalent three-phase current and the current state of the midpoint potential;

[0162] When the absolute value of the midpoint potential is within the second safety margin, the components of the inverter output voltage in the two-phase stationary coordinate system at the current moment are calculated according to the topological structure model of the NPC type energy storage inverter, and the sector in which the components are located is determined according to the phase and magnitude relationship of the components, and a small vector that reduces the midpoint potential is selected in the sector as the switching state;

[0163] The output module is used to calculate the value of the value function of the selected switch state through the rolling optimized value function, and select the switch state with the smallest value function as the modulation wave output.

[0164] It should be noted that, for a detailed description of the midpoint potential balancing method considering the safety margin, please refer to the description of the same or similar parts mentioned above, which will not be repeated here.

[0165] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0166] 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.

[0167] 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 midpoint potential balancing method considering safety margin, characterized in that: The method comprises: Set the critical voltage value of the midpoint potential safety margin to no more than 100V, and define 0V-u o ′ is the first safety margin, u o ′ and above is the second safety margin, u o ′ is the critical voltage value of the midpoint potential safety margin; The three-phase reference current and midpoint potential are fuzzy processed by using the triangular membership function and the trapezoidal membership function, and a rule table is formulated in combination with the fluctuation degree of the set safety margin critical voltage and the corresponding weight coefficient; the weight coefficient is obtained by the reference current and voltage calculated by the membership function; After the weight coefficient is normalized, the value of the safety margin is calculated according to the midpoint potential safety margin critical voltage; The midpoint potential is controlled according to the calculated value of the safety margin to achieve midpoint potential balance.

2. The midpoint potential balancing method considering safety margin according to claim 1 is characterized in that: The value of the safety margin is: Among them, u o ' new is the critical voltage value of the midpoint potential safety margin after adjustment, u o ' ref is the standard value of the critical voltage of the midpoint potential safety margin, Δu′ o is the critical voltage change value of the midpoint potential safety margin, ω[i] is the weight coefficient under different fuzzy rules, δu′ o [i] is the fluctuation degree of the critical voltage of the safety margin under different fuzzy rules.

3. The midpoint potential balancing method considering safety margin according to claim 1, characterized in that: The midpoint potential is controlled according to the calculated safety margin value to achieve midpoint potential balance, and the specific method includes: Determine the safety margin of the midpoint potential and select different switch states within different safety margins; When the absolute value of the midpoint potential is within the first safety margin, the current sum corresponding to all redundant small vectors is calculated according to the relationship between the midpoint potential and the switch state; the appropriate redundant small vector is selected as the switch state by combining the current sum of each redundant small vector and the corresponding equivalent three-phase current and the current state of the midpoint potential; When the absolute value of the midpoint potential is within the second safety margin, the components of the inverter output voltage in the two-phase stationary coordinate system at the current moment are calculated according to the NPC energy storage inverter topology model, and the sector in which the components are located is determined according to the phase and magnitude relationship of the components, and a small vector that reduces the midpoint potential is selected in the sector as the switching state.

4. A midpoint potential balanced energy storage inverter control method, characterized in that: The method comprises: Establish the topological structure model of NPC type energy storage inverter; In order to achieve equal voltages between the upper and lower bus capacitors, the DC side voltage state equation of the energy storage inverter is obtained by taking the DC side upper and lower bus capacitors C1 and C2 as state variables, and the relationship between the midpoint potential and the upper and lower bus currents is obtained; According to the relationship between the midpoint potential and the upper and lower bus currents, the relationship between the three-phase current on the grid side of the inverter and the bus current on the DC side is obtained, and the relationship between the midpoint potential and the three-phase current is derived; The forward Euler formula is used to discretize the relationship between the midpoint potential and the three-phase current, and the relationship between the midpoint potential and the switch state is obtained; The relationship between the midpoint potential and the switch state is: when the selected switch state makes the current sum positive, the switch state can reduce the midpoint potential at the next moment; when the selected switch state makes the current sum negative, the switch state can increase the midpoint potential at the next moment; Adopting the midpoint potential balancing method considering the safety margin as described in any one of claims 1 to 3, determining the safety margin of the midpoint potential, and selecting different switch states within different safety margins; The value of the value function of the selected switch state is calculated through the rolling optimized value function, and the switch state with the smallest value function is selected as the modulation wave output.

5. The method for controlling a midpoint potential balanced energy storage inverter according to claim 4, characterized in that: The calculation of the components of the inverter output voltage at the current moment in the two-phase stationary coordinate system is: Among them, u αref (k) and u βref (k) is the reference value of the inverter output voltage in the two-phase stationary coordinate system at the current moment, 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, i 1αref (k), i 1βref (k) is the reference value of the inverter output current sampled at the current moment in the two-phase stationary coordinate system, i 1αref (k-1), i 1βref (k-1) is the reference value of the inverter output current sampled at the previous moment in the two-phase stationary coordinate system.

6. The method for controlling a midpoint potential balanced energy storage inverter according to claim 4, characterized in that: The relationship between the midpoint potential and the three-phase current is derived based on the relationship between the three-phase current on the grid side of the inverter and the bus current on the DC side, including: In the three-level energy storage inverter, the switch state function of each phase bridge arm is defined as S a , S b , S c : Among them, V x1 、V x2 、V x3 、V x4 They represent the four switching devices of the inverter x (x = a, b, c) phase bridge arm respectively; According to the topological structure of the NPC inverter, the relationship between the three-phase current on the grid side and the bus voltage on the DC side is: Substitute the obtained relationship between the three-phase current on the grid side and the bus current on the DC side 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 on the grid side 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 1a 、i 1b 、i 1c is the three-phase current value of the inverter grid side.

7. The method for controlling a midpoint potential balanced energy storage inverter according to claim 6, characterized in that: Divide all small vectors into six basic redundant small vectors V min1 、V min2 、V min3 、V min4 、V min5 、V min6 ; Then divide these six small vectors into positive small vectors and negative small vectors; Among them, V3 is V min1 The positive small vector of V4 is V min1 The negative small vector of 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 A small negative vector.

8. The method for controlling a midpoint potential balanced energy storage inverter according to claim 7, characterized in that: The conditions for selecting a suitable redundant small vector as the switch state by combining the current of each redundant small vector and the corresponding equivalent three-phase current and the current state of the midpoint potential are as follows: Among them, i 1a 、i 1b 、i 1c is the three-phase current value of the inverter grid side, u o is the midpoint potential.

9. The method for controlling a midpoint potential balanced energy storage inverter according to claim 4 or 8, characterized in that: The value function of the rolling optimization is: J=[i 1αref (k+1)-i 1α (k+1)] 2 +[i 1βref (k+1)-i 1β (k+1)] 2 Among them, i 1αref (k+1), i 1βref (k+1) is the reference value of the inverter output current in the two-phase stationary coordinate system at the next moment, i 1α (k+1), i 1β (k+1) is the predicted value of the inverter output current in the two-phase stationary coordinate system at the next moment.

10. A midpoint potential balanced energy storage inverter control device, characterized in that: The device comprises: Topology model module, used to establish the topology model of NPC type energy storage inverter; The DC side voltage state equation module is used to achieve equal voltages of the upper and lower bus capacitors, obtain the DC side voltage state equation of the energy storage inverter with the DC side upper and lower bus capacitors C1 and C2 as state variables, and derive the relationship between the midpoint potential and the upper and lower bus currents; A relational module is used to obtain the relationship between the three-phase current on the grid side of the inverter and the bus current on the DC side according to the relationship between the midpoint potential and the upper and lower bus currents, and to derive the relational expression between the midpoint potential and the three-phase current; 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 using the forward Euler formula, and to derive the relationship between the midpoint potential and the switch state; A midpoint potential balancing module considering safety margin, used to execute the midpoint potential balancing method considering safety margin as described in any one of claims 1-2, determine the safety margin of the midpoint potential, and select different switch states within different safety margins; The determining of the safety margin of the midpoint potential and selecting different switch states within different safety margins include: When the absolute value of the midpoint potential is within the first safety margin, the current sum corresponding to all redundant small vectors is calculated according to the relationship between the midpoint potential and the switch state; the appropriate redundant small vector is selected as the switch state by combining the current sum of each redundant small vector and the corresponding equivalent three-phase current and the current state of the midpoint potential; When the absolute value of the midpoint potential is within the second safety margin, the components of the inverter output voltage in the two-phase stationary coordinate system at the current moment are calculated according to the topological structure model of the NPC type energy storage inverter, and the sector in which the components are located is determined according to the phase and magnitude relationship of the components, and a small vector that reduces the midpoint potential is selected in the sector as the switching state; The output module is used to calculate the value of the value function of the selected switch state through the rolling optimized value function, and select the switch state with the smallest value function as the modulation wave output.

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