Consider the safety margin midpoint potential balance method, energy storage inverter control method and device

Abstract

The application discloses a midpoint potential balance method considering safety margin, a control method and device of an energy storage inverter. The method analyzes the influence of three-level inverter output voltage vector on the midpoint potential, and deduces the relationship between the midpoint potential and the inverter output current. By detecting the changes of the output current and the midpoint potential, the switching states that can effectively balance the midpoint potential can be screened out. In model predictive control, the balance control of the midpoint potential can be realized by using the specific switching states, and an additional weight term does not need to be introduced into the value function to consider the midpoint potential, so that the influence of the midpoint potential fluctuation on the output current is significantly reduced. Meanwhile, the inertia characteristics of the DC side capacitor are considered, and the concept of midpoint potential safety margin is proposed to avoid damage of the DC side capacitor caused by excessive midpoint potential. In different safety margins, different switching states are adopted to realize the double control of the waveform quality of the midpoint potential and the output current.

Description

Technical Field

[0001] This invention belongs to the field of energy storage inverter control, and particularly relates to a method for balancing the midpoint potential with consideration of safety margin, an energy storage inverter control method and device. Background Technology

[0002] In recent years, three-level inverters have been widely used in high-power applications such as motor drives and grid-connected converters. Research on three-level inverters mainly focuses on two directions: first, at the hardware level, optimizing and innovating the topology of three-level inverters, including midpoint clamp type, flying capacitor type, and cascaded H-bridge type; second, improving software algorithms, with model predictive control being one of the important directions.

[0003] NPC-type three-level inverters have been widely used in medium- and high-voltage, high-power applications in recent years, such as motor drives, renewable energy grid connection, and industrial automation, due to their simple topology, high power device utilization, and relatively easy-to-implement control strategies. However, the characteristic of their DC-side voltage being composed of two sets of equivalent capacitors connected in series makes the midpoint potential susceptible to voltage imbalance between the upper and lower bus capacitors during operation, leading to midpoint potential drift. Midpoint potential imbalance not only weakens the symmetry and waveform quality of the inverter's output voltage and increases harmonic content, but may also exacerbate thermal stress on power devices, reducing system stability. If this imbalance cannot be effectively suppressed in a timely manner, it may even cause the entire DC bus voltage to concentrate on a single capacitor, exceeding the capacitor's withstand voltage range, ultimately leading to capacitor damage or system failure. Therefore, suppressing midpoint potential imbalance has become one of the key issues in NPC-type three-level inverter research.

[0004] To address the problem of midpoint potential imbalance, many scholars have proposed various solutions from different perspectives. In the field of model predictive control, a common approach is to incorporate the midpoint potential into the value function and assign it a certain weighting coefficient. The modulation waveform is then calculated through rolling optimization, thereby reducing midpoint potential fluctuations 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), leading to an increase in the harmonic content of the output voltage and current waveforms. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a method for balancing the midpoint potential with a safety margin, a control method for an energy storage inverter, and a device. By analyzing the influence of the output voltage vector of a three-level inverter on the midpoint potential, the relationship between the midpoint potential and the output current is derived, and based on this relationship, switching states that can balance the midpoint potential are selected. In model predictive control, these switching states are used to achieve midpoint potential balance without adding extra weights to the value function, reducing the impact of midpoint potential fluctuations on the output current. Simultaneously, considering the inertial characteristics of the DC-side capacitor, the concept of a midpoint potential safety margin is proposed. Appropriate switching states are selected based on different margins, balancing midpoint potential balance and output current waveform quality, thus avoiding capacitor damage caused by excessively high midpoint potential.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a method for balancing the midpoint potential considering a safety margin, the method comprising:

[0008] The critical voltage value for the safety margin of the midpoint potential is set to not exceed 100V, and 0V-u′ is defined. o For the first safety margin, u′ o The above constitutes the second safety margin, u′ o It is the critical voltage value for the safety margin of the midpoint potential;

[0009] The three-phase reference current and midpoint potential are fuzzified using triangular and trapezoidal membership functions, and a rule table is formulated by combining the fluctuation degree of the set safety margin critical voltage and the corresponding weight coefficients; the weight coefficients are obtained by calculating the reference current and voltage using the membership functions.

[0010] After normalizing the weighting coefficients, the safety margin value is calculated based on the critical voltage of the midpoint potential safety margin.

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

[0012] In one implementation, the safety margin is defined as follows:

[0013]

[0014] Where, u′ onew To adjust the critical voltage value of the midpoint potential safety margin, u′ oref The critical voltage standard value for the midpoint potential safety margin is Δu′. o δu′ represents the critical voltage change value for the midpoint potential safety margin, ω[i] is the weighting coefficient under different fuzzy rules, and δu′ is the value of the change in the midpoint potential safety margin. o[i] represents the degree of fluctuation of the safety margin critical voltage under different fuzzy rules.

[0015] In one implementation, controlling the midpoint potential based on the calculated safety margin value to achieve midpoint potential balance includes the following specific methods:

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

[0017] When the absolute value of the midpoint potential is within the first safety margin, calculate the sum of currents corresponding to all redundant small vectors based on the relationship between the midpoint potential and the switching state; combine the current sum of each redundant small vector with the corresponding equivalent three-phase current and the current state of the midpoint potential, and select a suitable redundant small vector as the switching state.

[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 topology model of the NPC type energy storage inverter. The sector in which the component is located is determined according to the phase and magnitude relationship of the component. Within the sector, the small vector that reduces the midpoint potential is selected as the switching state.

[0019] Secondly, the present invention provides a control method for an energy storage inverter with neutral point potential balance, the method comprising:

[0020] Establish a topology model for an NPC-type energy storage inverter;

[0021] 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 upper and lower bus capacitors C1 and C2 on the DC side as state variables, and the relationship between the midpoint potential and the upper and lower bus currents is derived.

[0022] Based on the relationship between the midpoint potential and the upper and lower bus currents, the relationship between the three-phase current on the grid side and the DC bus current of the inverter is derived, and the relationship between the midpoint potential and the three-phase current is also derived.

[0023] The relationship between the midpoint potential and the switch state is as follows: when the selected switch state makes the sum of currents positive, the switch state can decrease the midpoint potential at the next moment; when the selected switch state makes the sum of currents negative, the switch state can increase the midpoint potential at the next moment.

[0024] The relationship between the midpoint potential and the three-phase current is discretized using the forward Euler formula, and the relationship between the midpoint potential and the switching state is obtained.

[0025] Using the above-mentioned method of considering the safety margin and balancing the midpoint potential, the safety margin of the midpoint potential is determined, and different switching states are selected within different safety margins.

[0026] The value of the value function of the selected switching state is calculated by rolling optimization, and the switching state with the smallest value function is selected as the modulated wave output.

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

[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 time step in the two-phase stationary coordinate system.

[0030] In one embodiment, deriving the relationship between the neutral point potential and the three-phase current based on the relationship between the three-phase current on the grid side of the inverter and the DC bus current includes:

[0031] In a three-level energy storage inverter, the switching state functions of each phase arm are defined as S... a S b S c :

[0032]

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

[0034] Based on the topology of the NPC inverter, the relationship between the three-phase current on the grid side and the DC bus voltage is as follows:

[0035]

[0036] Substituting the obtained relationship between the three-phase current on the grid side and the DC bus current into the relationship between the neutral point potential and the bus current, the relationship between the neutral point potential and the three-phase current on the grid side is now:

[0037]

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

[0039] In one implementation, 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, V4 is V min1 The negative small vector, V5 is V min2 The positive small vector, V6 is V min2 The negative small vector, V7 is V min3 The positive smallest vector, V8 is V min3 The negative small vector, V9 is V min4 The positive small vector, V 10 For V min4 The negative small vector, V 11 For V min5 The positive small vector, V 12 For V min5 The negative small vector, V 13 For V min6 The positive small vector, V 14 For V min6 The negative small vector.

[0042] In one implementation, the condition for selecting a suitable redundant small vector as the switching state by combining the current state of each redundant small vector current and the corresponding equivalent three-phase current and the neutral point potential is as follows:

[0043]

[0044] Among them, i 1a i 1b i 1c The inverter's three-phase current value on the grid side, u o It is the midpoint potential.

[0045] In one implementation, 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] Thirdly, the present invention provides a control device for an energy storage inverter with balanced midpoint potential, the device comprising:

[0049] The topology model module is used to build the topology model of the NPC type energy storage inverter.

[0050] The DC-side voltage state equation module is used to 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 in order to achieve equal voltage between the upper and lower bus capacitors, and to derive the relationship between the midpoint potential and the upper and lower bus currents.

[0051] The relational module is used to derive the relationship between the three-phase current on the grid side and the DC bus current of the inverter based on the relationship between the midpoint potential and the upper and lower bus currents, and to derive the relational formula between the midpoint potential and the three-phase current.

[0052] The module for the relationship between midpoint potential and switch state is used to discretize the relationship between midpoint potential and three-phase current using the forward Euler formula, and to derive the relationship between midpoint potential and switch state.

[0053] The 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 switching states within different safety margins.

[0054] The determination of the safety margin of the midpoint potential, and the selection of different switching states within different safety margins, includes:

[0055] When the absolute value of the midpoint potential is within the first safety margin, calculate the sum of currents corresponding to all redundant small vectors based on the relationship between the midpoint potential and the switching state; combine the current sum of each redundant small vector with the corresponding equivalent three-phase current and the current state of the midpoint potential, and select a suitable redundant small vector as the switching state.

[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 topology model of the NPC type energy storage inverter. The sector in which the component is located is determined according to the phase and magnitude relationship of the component. Within the sector, the small vector that reduces the midpoint potential is selected 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 optimization value function, and select the switch state with the minimum value function as the modulated wave output.

[0058] Compared with existing technologies, the beneficial effects are:

[0059] This invention pre-selects suitable switching states, enabling balanced control of the midpoint potential without introducing additional weighting terms into the value function, thus significantly reducing the impact of midpoint potential fluctuations on the output current. Simultaneously, considering the inertial characteristics of the DC-side capacitor, a midpoint potential safety margin concept is proposed to prevent damage to the DC-side capacitor due to excessive midpoint potential. Within different safety margins, different switching states are employed to achieve dual control over the waveform quality of both the midpoint potential and the output current, avoiding the problem of excessively high midpoint potential caused by prioritizing current waveform quality. This invention features a simple structure, low computational complexity, and ease of hardware implementation, ensuring both rapid computation and accurate results. Attached Figure Description

[0060] The accompanying drawings, as part of this invention, are provided to further illustrate the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention, but do not constitute an undue limitation thereof. Clearly, the drawings described below are merely some embodiments, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0061] Figure 1 This is a flowchart of a midpoint potential balancing energy storage inverter control method provided in an embodiment of the present invention;

[0062] Figure 2 This is a topology diagram of the energy storage inverter provided in an embodiment of the present invention;

[0063] Figure 3 This is a vector diagram of the switching states and space voltage of a three-level inverter provided in an embodiment of the present invention;

[0064] Figure 4 This is a simulation waveform of the midpoint potential under a reference current of 350A provided in an embodiment of the present invention, based on a weighted coefficient model predictive control.

[0065] Figure 5 The simulation waveform of the midpoint potential under a reference current of 350A is provided in the embodiment of the present invention for midpoint potential balance model predictive control considering safety margin.

[0066] Figure 6 This is a simulation waveform of the output current of phase a in the midpoint potential balance model predictive control considering safety margin, provided in an embodiment of the present invention.

[0067] Figure 7 This is a spectrum analysis diagram of the output current waveform under a reference current value of 350A for a weighted coefficient model predictive control provided in an embodiment of the present invention.

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

[0069] Figure 9 The simulation waveform of the midpoint potential under a reference current of 150A is provided in the embodiment of the present invention for midpoint potential balance model predictive control considering safety margin.

[0070] Figure 10 The output current waveform spectrum analysis diagram of the midpoint potential balance model predictive control considering safety margin provided in the embodiment of the present invention under a reference current value of 150A.

[0071] It should be noted that these accompanying drawings and textual descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art by referring to specific embodiments. Detailed Implementation

[0072] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0073] Because of the inertia of the DC-side capacitor, the adjustment of the midpoint potential is delayed. At the same time, in order to ensure the quality of the output current waveform during rolling optimization, a switching state that can balance the midpoint potential is not selected, which would cause the midpoint potential to exceed the rated voltage of the DC-side capacitor and lead to capacitor damage, thereby causing safety problems, this invention proposes the concept of midpoint potential safety margin.

[0074] This disclosure provides a method for balancing midpoint potential considering safety margins, the method comprising:

[0075] Step 1: Set the critical voltage value for the safety margin of the midpoint potential to not exceed 100V, and define 0V-u′. o For the first safety margin, u′ o The above constitutes the second safety margin, u′ o It is the critical voltage value for the safety margin of the midpoint potential.

[0076] Based on the actual operating conditions of the DC-side voltage and the rated voltages of the upper and lower DC-side bus capacitors, a suitable critical voltage value for the midpoint potential safety margin is selected. Assuming the DC-side voltage is 760V and the rated voltages of both the upper and lower bus capacitors are 450V, and considering a 20V deviation for each capacitor, the given critical voltage value for the safety margin cannot exceed 100V. Simultaneously, to ensure the quality of the output current waveform, 0V-u′ is defined. o For the first safety margin, u′ o The above constitutes the second safety margin, u′ o It is the critical voltage value for the safety margin of the midpoint potential, and the unit is V.

[0077] Step 2: Fuzzyenization of the three-phase reference current and midpoint potential is performed using triangular and trapezoidal membership functions. A rule table is then developed based on the fluctuation level of the set safety margin critical voltage and corresponding weighting coefficients. These weighting coefficients are obtained from the reference current and voltage calculated using the membership functions. The resulting rule table is shown in Table 1 below.

[0078] Table 1

[0079]

[0080]

[0081] In the table, i ref_low i ref_med i ref_high The reference current correlation value, u, is calculated through the membership function. o_low u o_med u o_high The reference current correlation value is calculated using the membership function.

[0082] Step 3: After normalizing the weighting coefficients, calculate the safety margin value based on the critical voltage of the midpoint potential safety margin.

[0083]

[0084] Here, ε is a very small value, which can be 10. -6 This is used to avoid the denominator being 0; finally, the safety margin value is calculated using the following formula:

[0085]

[0086] Where, u′ onew To adjust the critical voltage value of the midpoint potential safety margin, u′ oref The critical voltage standard value for the midpoint potential safety margin is Δu′. o This represents the critical voltage change value for the safety margin of the midpoint potential.

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

[0088] Specifically, the first step is to determine the safety margin of the midpoint potential. Different switching states are used within different safety margins. Finally, these switching states are used 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 states that can balance the midpoint potential can be selected, and the most suitable switching state can be obtained by rolling optimization using the value function.

[0090] When the absolute value of the midpoint potential is within the second safety margin, the inverter output voltage reference value can be determined first, and then a small vector that can reduce the midpoint potential can be selected. Only these switching states are used to perform rolling optimization in the value function to obtain the most suitable switching state.

[0091] like Figure 1 As shown, this invention proposes a control method for an energy storage inverter with unweighted coefficient midpoint potential balance considering safety margin. This method includes the following steps:

[0092] Step S100: Establish the topology model of the NPC type energy storage inverter.

[0093] refer to Figure 2 The topology diagram shown is used to construct the energy storage inverter topology model as follows:

[0094]

[0095] Where R, L, and C are the line resistance, filter inductor, and filter capacitor, respectively, i 1α i 1β i is the output current on the converter side. 2ε i 2β For the grid-side output current, u 1α u 1β The output voltage on the converter side, u 2α u 2βe is the voltage across the filter capacitor. α e β It represents the three-phase electromotive force of the power grid.

[0096] Step S200: 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 with the 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 derived.

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

[0098]

[0099] Where C1 and C2 are the capacitance values ​​of the upper and lower busbars, and C1 = C2, u C1 u C2 i represents the voltage values ​​of the upper and lower bus capacitors. C1 i C2 i is the current flowing through the upper and lower bus capacitor branches. dc i is the output current of the battery after being boosted by the DC / DC module. p i n The currents are for the upper and lower busbars.

[0100] Midpoint potential u o The relationship with the voltage of the upper and lower bus capacitors is shown below:

[0101] u o =u C1 -u C2

[0102] Furthermore, substituting the currents of the upper and lower bus capacitor branches into the relationship between the midpoint potential and the voltages of the upper and lower bus capacitors, we obtain the relationship between the midpoint potential and the currents of the upper and lower buses as follows:

[0103]

[0104] Step S300: Based on the relationship between the neutral point potential and the upper and lower bus currents, the relationship between the three-phase current on the grid side of the inverter and the DC bus current is obtained, and the relationship between the neutral point 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 neutral point potential and the three-phase current is derived, including:

[0106] In a three-level energy storage inverter, the switching state functions of each phase arm are defined as S... a S b S c :

[0107]

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

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

[0110]

[0111] Furthermore, substituting the obtained relationship between the three-phase current on the grid side and the DC bus current into the relationship between the neutral point potential and the bus current, the relationship between the neutral point potential and the three-phase current on the grid side is now:

[0112]

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

[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 switching state.

[0115] Specifically, for finite set model predictive control, assuming its sampling period is T s Using the forward Euler formula Discretizing the relationship between the midpoint potential and the three-phase current yields the following relationship between the switching state and the midpoint potential:

[0116]

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

[0118] Depend on Figure 3As shown, only the small voltage vector contains redundant vectors, proving that selecting the switching state of the small vector can effectively suppress the neutral point potential imbalance. The current and i corresponding to the switching state of each small vector three-phase bridge arm are shown. 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, based on the table above, the following relationship can be derived: when the selected switch state makes the sum of currents positive, the switch state can lower the midpoint potential at the next moment; when the selected switch state makes the sum of currents negative, the switch state can raise 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 sign of the three-phase current and the midpoint potential under each switching state.

[0123] Step S500: Using the method of considering the safety margin and midpoint potential balance, determine the safety margin of the midpoint potential, and select different switching states within different safety margins.

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

[0125] When the absolute value of the midpoint potential is within the first safety margin, calculate the sum of currents corresponding to all redundant small vectors based on the relationship between the midpoint potential and the switching state; combine the current sum of each redundant small vector with the corresponding equivalent three-phase current and the current state of the midpoint potential, and select a suitable redundant small vector as the switching state.

[0126] Specifically, based on the relationship between the midpoint potential and the switching state, calculate the current and i corresponding to all redundant small vectors. 1s By analyzing the relationship between the midpoint potential and the switching state shown in Table 2, the currents corresponding to all redundant small vectors are analyzed. Combining the currents of each redundant small vector and the corresponding equivalent three-phase currents with the current state of the midpoint potential, a suitable redundant small vector is selected to control the DC-side midpoint potential of the inverter.

[0127] Furthermore, 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 and negative small vectors; where V3 is V min1 The positive small vector, V4 is V min1 The negative small vector, V5 is Vmin2 The positive small vector, V6 is V min2 The negative small vector, V7 is V min3 The positive smallest vector, V8 is V min3 The negative small vector, V9 is V min4 The positive small vector, V 10 For V min4 The negative small vector, V 11 For V min5 The positive small vector, V 12 For V min5 The negative small vector, V 13 For V min6 The positive small vector, V 14 For V min6 The negative small vector.

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

[0129] Furthermore, the conditions for selecting the redundant small vector 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 The inverter's three-phase current value on the grid side, u o It 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 topology model of the NPC type energy storage inverter. The sector in which the component is located is determined according to the phase and magnitude relationship of the component. Within the sector, the small vector that reduces the midpoint potential is selected as the switching state.

[0133] Specifically, using the NPC inverter topology model, the components of the inverter output voltage in the two-phase stationary coordinate system at the current moment are calculated.

[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 time step in the two-phase stationary coordinate system.

[0137] By using the inverter output voltage reference value obtained from the calculation in the two-phase stationary coordinate system, its components on the α-axis and β-axis can be obtained. By the phase and magnitude relationship of these two components, it can be determined which small sector it is in, and then the corresponding selectable switching state can be obtained.

[0138] This step determines the safety margin of the midpoint potential, uses different switching states within different safety margins, and finally uses these switching states for rolling optimization to output the modulated wave.

[0139] It should be noted that a detailed description of the method for balancing the midpoint potential with consideration of safety margin can be found in the aforementioned sections of the same or similar descriptions, and will not be repeated here.

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

[0141] Furthermore, the value function for 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] The inverter's operation is controlled by calculating the value function J of each adjacent vector switching state in the sector, comparing and selecting the switching state with the smallest value function as the modulation wave output.

[0145] This invention was verified through simulations built on the Matlab / Simulink platform, with relevant parameters shown in Table 3. Simulations were performed on an energy storage inverter with weighted coefficient model predictive control (with a reference current of 350A) and unweighted coefficient midpoint potential balance model predictive control (considering safety margin), as well as an energy storage inverter with unweighted coefficient midpoint potential balance model predictive control (considering safety margin) with a reference current of 150A.

[0146] Table 3

[0147]

[0148]

[0149] Simulations based on Matlab / Simulink can yield results such as Figures 4-10 The result. Figure 6 The figure shows the simulated waveform of the output current of phase a under the predictive control of the unweighted coefficient midpoint potential balance model with safety margin at 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 The figures show the midpoint potential waveforms for a weighted coefficient model predictive control with a reference current of 350A and a midpoint potential balance model predictive control considering safety margin. It can be clearly seen that the control method adopted in this invention can effectively reduce the fluctuation amplitude of the midpoint potential.

[0151] Figure 7 and Figure 8The figures show the output current waveform spectrum analysis of the weighted coefficient model predictive control and the midpoint potential balance model predictive control considering the safety margin, respectively, with a reference current of 350A. It can be seen from the figures that the THD value of the traditional method is 0.55%, while the THD value of the method adopted in this invention is 0.69%. This proves that the inverter output current obtained by the method adopted in this invention meets the relevant requirements and can achieve effective control of the midpoint potential balance by sacrificing some waveform quality.

[0152] Figure 9 and Figure 10 These are the simulation waveforms of the midpoint potential and the spectrum analysis of the output current waveform under the midpoint potential balance model predictive control with a reference current of 150A, taking into account the safety margin. Comparing the graphs with the reference current of 350A, it can be clearly seen that the midpoint potential is also smaller when the reference current is smaller, and there will be no over-limit situation. In addition, the THD value of the output current also meets the relevant requirements.

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

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

[0155] The topology model module is used to build the topology model of the NPC type energy storage inverter.

[0156] The DC-side voltage state equation module is used to 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 in order to achieve equal voltage between the upper and lower bus capacitors, and to derive the relationship between the midpoint potential and the upper and lower bus currents.

[0157] The relational module is used to derive the relationship between the three-phase current on the grid side and the DC bus current of the inverter based on the relationship between the midpoint potential and the upper and lower bus currents, and to derive the relational formula between the midpoint potential and the three-phase current.

[0158] The module for the relationship between midpoint potential and switch state is used to discretize the relationship between midpoint potential and three-phase current using the forward Euler formula, and to derive the relationship between midpoint potential and switch state.

[0159] The midpoint potential balancing module, which considers safety margins, is used to execute the midpoint potential balancing method that considers safety margins, determine the safety margin of the midpoint potential, and select different switching states within different safety margins.

[0160] The determination of the safety margin of the midpoint potential, and the selection of different switching states within different safety margins, includes:

[0161] When the absolute value of the midpoint potential is within the first safety margin, calculate the sum of currents corresponding to all redundant small vectors based on the relationship between the midpoint potential and the switching state; combine the current sum of each redundant small vector with the corresponding equivalent three-phase current and the current state of the midpoint potential, and select a suitable redundant small vector as the switching state.

[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 topology model of the NPC type energy storage inverter. The sector in which the component is located is determined according to the phase and magnitude relationship of the component. Within the sector, the small vector that reduces the midpoint potential is selected 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 optimization value function, and select the switch state with the minimum value function as the modulated wave output.

[0164] It should be noted that for a detailed description of the method for balancing the midpoint potential with consideration of safety margin, please refer to the descriptions in the same or similar sections above, and will not be repeated here.

[0165] The technical features of the above embodiments can be combined in any way. For the sake of brevity, 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 present 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 its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0167] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any minor modifications, equivalent substitutions, and improvements made to the above embodiments based on the technical essence of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for balancing midpoint potential considering safety margin, characterized in that: The method includes: The critical voltage value for the safety margin of the midpoint potential is set to not exceed 100V, and 0V~ is defined. For the first safety margin, The above constitutes the second safety margin. It is the critical voltage value for the safety margin of the midpoint potential; The three-phase reference current and midpoint potential are fuzzified using triangular and trapezoidal membership functions, and a rule table is formulated by combining the fluctuation degree of the set safety margin critical voltage and the corresponding weight coefficients; the weight coefficients are obtained by calculating the reference current and voltage using the membership functions. After normalizing the weighting coefficients, the safety margin value is calculated based on the standard value of the critical voltage for the midpoint potential safety margin. The midpoint potential is controlled based on the calculated safety margin value to achieve midpoint potential balance; The method for controlling the midpoint potential based on the calculated safety margin value to achieve midpoint potential balance includes: Determine the safety margin of the midpoint potential and select different switching states within different safety margins; When the absolute value of the midpoint potential is within the first safety margin, calculate the sum of currents corresponding to all redundant small vectors based on the relationship between the midpoint potential and the switching state; combine the current sum of each redundant small vector with the corresponding equivalent three-phase current and the current state of the midpoint potential, and select a suitable redundant small vector as the switching state. 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 topology model of the NPC type energy storage inverter. The sector in which the component is located is determined according to the phase and magnitude relationship of the component. Within the sector, the small vector that reduces the midpoint potential is selected as the switching state.

2. The method for balancing the midpoint potential considering safety margin according to claim 1, characterized in that: The value of the safety margin is: in, To adjust the critical voltage value for the safety margin of the midpoint potential after adjustment, This is the standard value of the critical voltage for the safety margin of the midpoint potential. This represents the critical voltage change value for the safety margin of the midpoint potential. These are the weighting coefficients under different fuzzy rules. This refers to the degree of fluctuation in the safety margin critical voltage under different fuzzy rules.

3. A control method for an energy storage inverter with balanced midpoint potential, characterized in that: The method includes: Establish a topology model for an NPC-type energy storage inverter; To ensure equal voltage across the upper and lower bus capacitors, the DC side upper and lower bus capacitors are used as the reference. The state equation of the DC side voltage of the energy storage inverter is obtained by taking the state variables as the state variables, and the relationship between the midpoint potential and the upper and lower bus currents is derived. Based on the relationship between the midpoint potential and the upper and lower bus currents, the relationship between the three-phase current on the grid side and the DC bus current of the inverter is derived, and the relationship between the midpoint potential and the three-phase current is also derived. The relationship between the midpoint potential and the three-phase current is discretized using the forward Euler formula, and the relationship between the midpoint potential and the switching state is obtained. The relationship between the midpoint potential and the switch state is as follows: when the selected switch state makes the sum of currents positive, the switch state can decrease the midpoint potential at the next moment; when the selected switch state makes the sum of currents negative, the switch state can increase the midpoint potential at the next moment. Using the midpoint potential balance method considering safety margin as described in any one of claims 1-2, the safety margin of the midpoint potential is determined, and different switching states are selected within different safety margins. The value of the value function of the selected switching state is calculated by rolling optimization, and the switching state with the smallest value function is selected as the modulated wave output.

4. The energy storage inverter control method for midpoint potential balance according to claim 3, characterized in that: The components of the inverter output voltage in the two-phase stationary coordinate system at the current moment are calculated as follows: in, This is the reference value of the inverter output voltage in the two-phase stationary coordinate system at the current moment. It is the value of the grid-side voltage sampled at the current moment in a two-phase stationary coordinate system. It is the reference value of the inverter output current sampled at the current moment in the two-phase stationary coordinate system. This is the reference value of the inverter output current sampled at the previous moment in a two-phase stationary coordinate system, where R represents the line resistance; L represents the filter inductance; and T represents the reference value. s Indicates the sampling period.

5. The energy storage inverter control method for midpoint potential balance according to claim 3, characterized in that: Based on the relationship between the three-phase current on the grid side and the DC bus current of the inverter, the relationship between the neutral point potential and the three-phase current is derived, including: In a three-level energy storage inverter, the switching state functions of each phase arm are defined as follows: : in, They represent inverters Four switching devices in the phase bridge arm; Based on the topology of the NPC inverter, the relationship between the three-phase current on the grid side and the DC bus voltage is as follows: Substituting the obtained relationship between the three-phase current on the grid side and the DC bus current into the relationship between the neutral point potential and the bus current, the relationship between the neutral point potential and the three-phase current on the grid side is now: in, For the switching state of each phase arm, It is the midpoint potential. This represents the three-phase current value on the grid side of the inverter.

6. The energy storage inverter control method for midpoint potential balance according to claim 5, characterized in that: All small vectors are divided into six basic redundant small vectors. ; These six small vectors are then divided into positive and negative small vectors; among them, for small positive vectors, for The negative small vector, for small positive vectors, for The negative small vector, for small positive vectors, for The negative small vector, for small positive vectors, for The negative small vector, for small positive vectors, for The negative small vector, for small positive vectors, for The negative small vector.

7. The energy storage inverter control method for midpoint potential balance according to claim 6, characterized in that: Based on the current state of each redundant small vector current and the corresponding equivalent three-phase current and neutral point potential, the following conditions are used to select appropriate redundant small vectors as the switching state conditions: in, These are the three-phase current values ​​on the grid side of the inverter. It is the midpoint potential.

8. The energy storage inverter control method for midpoint potential balance according to claim 3 or 7, characterized in that: The value function of the rolling optimization is: in, This is the reference value of the inverter output current in the two-phase stationary coordinate system at the next moment. It is the predicted value of the inverter output current in the two-phase stationary coordinate system at the next moment.

9. A control device for an energy storage inverter with balanced midpoint potential, characterized in that: The device includes: The topology model module is used to build the topology model of the NPC type energy storage inverter. The DC-side voltage state equation module is used to achieve equal voltages between the upper and lower bus capacitors on the DC side. The state equation of the DC side voltage of the energy storage inverter is obtained by taking the state variables as the state variables, and the relationship between the midpoint potential and the upper and lower bus currents is derived. The relational module is used to derive the relationship between the three-phase current on the grid side and the DC bus current of the inverter based on the relationship between the midpoint potential and the upper and lower bus currents, and to derive the relational formula between the midpoint potential and the three-phase current. The module for the relationship between midpoint potential and switch state is used to discretize the relationship between midpoint potential and three-phase current using the forward Euler formula, and to derive the relationship between midpoint potential and switch state. A midpoint potential balancing module considering safety margin is 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 switching states within different safety margins. The determination of the safety margin of the midpoint potential, and the selection of different switching states within different safety margins, includes: When the absolute value of the midpoint potential is within the first safety margin, calculate the sum of currents corresponding to all redundant small vectors based on the relationship between the midpoint potential and the switching state; combine the current sum of each redundant small vector with the corresponding equivalent three-phase current and the current state of the midpoint potential, and select a suitable redundant small vector as the switching state. 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 topology model of the NPC type energy storage inverter. The sector in which the component is located is determined according to the phase and magnitude relationship of the component. Within the sector, the small vector that reduces the midpoint potential is selected 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 optimization value function, and select the switch state with the minimum value function as the modulated wave output.

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