A multi-inverter parallel control method based on adaptive virtual impedance

Abstract

The application provides a multi-inverter parallel control method based on adaptive virtual impedance, which comprises the following steps: step 1, establishing a droop control equation under multi-inverter parallel connection; step 2, analyzing the condition of active power sharing under multi-inverter parallel connection; and step 3, constructing an improved droop control model based on adaptive virtual impedance. The technical scheme of the application overcomes the problems of uneven active power sharing and parallel current generation caused by different line impedances when multi-inverters are connected in parallel in the prior art.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic inverter control, and in particular to a multi-inverter parallel control method based on adaptive virtual impedance. Background Art

[0002] With the continuous development of renewable energy power generation technologies, multi-inverter paralleling has been widely researched. Multi-inverter parallel systems, due to their flexible structure, plug-and-play functionality, and scalable capacity, have become a research hotspot in the renewable energy power generation field. However, multi-inverter parallel systems suffer from problems such as uneven active power distribution and poor stability due to the varying line impedances caused by the uncertain geographical locations of the different inverters.

[0003] To address these issues, some researchers have proposed an inverter parallel control strategy that introduces a virtual inductor. This scheme can reduce power inequality and suppress parallel circulating currents. However, if the virtual inductor is too small, the power sharing effect is insufficient, while if it is too large, the inverter output voltage quality and stability will decrease. When the virtual inductor is connected to a low-voltage line, due to the large resistive component in the line impedance, the virtual inductor has limited improvement in power sharing and can cause the output voltage to drop beyond the line. Other researchers have proposed a multi-inverter parallel control strategy in which a central controller collects power information. This scheme can achieve power sharing and eliminate parallel circulating currents. However, this scheme places high demands on the central controller and communication network, and relies on real-time transmission of output power between multiple inverters, which deprives the multi-inverter parallel system of plug-and-play features and structural flexibility, and reduces system reliability. Therefore, there is a need for a multi-inverter parallel control method that can share active power, eliminate parallel circulating currents, and improve system stability when multiple inverters are connected in parallel. Summary of the Invention

[0004] The main purpose of the present invention is to provide a multi-inverter parallel control method based on adaptive virtual impedance to solve the problems of uneven active power distribution and parallel circulating current generated by different line impedances when multiple inverters are connected in parallel in the prior art.

[0005] To achieve the above object, the present invention provides a multi-inverter parallel control method based on adaptive virtual impedance, comprising the following steps:

[0006] Step 1: Establish the droop control equation of multiple inverters in parallel;

[0007] Step 2: Analyze the conditions for active power sharing of multiple inverters in parallel;

[0008] Step 3: Construct an improved droop control model based on adaptive virtual impedance.

[0009] Furthermore, a multi-inverter parallel model is established based on the relationship between the output active power, output voltage, and common connection point voltage when multiple inverters are connected in parallel. The droop control equation for the multi-inverter reverse parallel connection under the resistive line impedance is established as follows:

[0010]

[0011] Among them, P and Q are the active power and reactive power output by the inverter, respectively, U and U * are the inverter output voltage amplitude and voltage given value respectively; f, f * are the frequency and frequency set value of the output voltage when the inverter is running; m and n are the active-voltage droop coefficient and reactive-frequency droop coefficient respectively.

[0012] Furthermore, based on the multi-inverter parallel model, the output active power sharing error relationship when two inverters are connected in parallel with droop control is established as follows:

[0013]

[0014] Among them, U i 、U j are the output voltages of the i-th and j-th inverters, R Li 、R Lj are the equivalent resistances from the ith and jth inverters to the common connection point, P i 、P j are the rated powers of the i-th and j-th inverters, respectively, m i 、m j are the active power-voltage droop coefficients of the i-th inverter and the j-th inverter respectively.

[0015] Furthermore, when the numerator of the output active power sharing error ΔP is equal to zero, the active power of multiple inverters in parallel can be shared equally, that is:

[0016]

[0017] The analysis shows that the condition for active power equalization of multiple inverters in parallel is that the ratio of the equivalent line impedance of each inverter when multiple inverters are connected in parallel is equal to the ratio of the product of the active power-voltage droop coefficient of each inverter and the output voltage.

[0018] Furthermore, step 3 includes the following specific steps:

[0019] Step 31: Introduce virtual impedance to change the equivalent impedance of each parallel line. When the ratio of the equivalent line impedance of each inverter is equal to the ratio of the product of the active power-voltage droop coefficient of each inverter and the output voltage, the active power can be evenly distributed according to the rated power.

[0020] Step 32: Establish an adaptive virtual impedance model as follows:

[0021] Z V =R v +jX v

[0022] Among them, R V 、X V They are virtual resistance and virtual inductance respectively. The virtual inductor takes a given negative value. The virtual resistance is an adaptive virtual resistance, which consists of two parts, one is a given positive value, and the other is a variable positive value.

[0023] Step 33: Build a multi-inverter parallel simulation model to verify whether the improved droop control method with the introduction of virtual impedance can achieve equal distribution of the output active power according to the rated capacity when multiple inverters are connected in parallel.

[0024] Furthermore, the virtual resistance expression in the adaptive virtual impedance model is:

[0025]

[0026] Among them, R vi is the virtual resistance of the ith parallel inverter, R set For a given positive virtual resistance, w S1,i is the ratio of the rated capacity of the first parallel inverter to the i-th parallel inverter, k v is the virtual resistance coefficient, i odi is the d-axis component of the output current of the i-th parallel inverter.

[0027] Furthermore, the inverter with the smallest rated capacity among the parallel inverters is inverter No. 1, and its fixed virtual resistance R set The value is 0.1~0.4Ω, and the virtual resistance coefficient k v The value range is 0.2 to 0.6.

[0028] The present invention has the following advantages:

[0029] 1. The output current information in the adaptive virtual impedance model comes from the local inverter, eliminating the need for communication between inverters. This ensures that the active power of parallel inverters is evenly distributed according to the rated power, while also improving the flexibility and reliability of the system.

[0030] 2. The multi-inverter parallel control method based on adaptive virtual impedance of the present invention can realize global communication-free multi-inverter parallel output power equal distribution according to rated capacity, suppress the circulating current of the multi-inverter parallel, and ensure the stability of the multi-inverter parallel system. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work. In the drawings:

[0032] Figure 1 A control structure diagram of a multi-inverter parallel control method based on adaptive virtual impedance according to the present invention is shown;

[0033] Figure 2 1 is a control structure diagram of a droop controller of a multi-inverter parallel control method based on adaptive virtual impedance according to the present invention;

[0034] Figure 3 The invention shows a droop control circuit model of a multi-inverter parallel control method based on adaptive virtual impedance according to the present invention;

[0035] Figure 4 1 is a diagram showing an adaptive virtual impedance control structure of a multi-inverter parallel control method based on adaptive virtual impedance according to the present invention;

[0036] Figure 5 This is a waveform diagram of the active power output of multiple inverters in parallel when the rated power of the inverters is the same and adaptive virtual impedance is not introduced;

[0037] Figure 6 This is a waveform diagram of the output phase current of multiple inverters connected in parallel when the rated power of the inverters is the same and adaptive virtual impedance is not introduced;

[0038] Figure 7 The output active power waveform of multiple inverters after introducing the multi-inverter parallel active power sharing control strategy based on adaptive virtual impedance when the rated power of the inverters is the same;

[0039] Figure 8 Output phase current waveforms of multiple inverters after introducing the multi-inverter parallel active power sharing control strategy based on adaptive virtual impedance when the rated power of the inverters is the same;

[0040] Figure 9 This is the waveform of the active power output of multiple inverters in parallel when the ratio of the rated powers of the two inverters is 1:2 and adaptive virtual impedance is not introduced;

[0041] Figure 10 The output active power waveform of multiple inverters after the active power sharing control strategy of multiple inverters in parallel based on adaptive virtual impedance is introduced when the ratio of the rated powers of the two inverters is 1:2. DETAILED DESCRIPTION

[0042] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0043] like Figure 1 The multi-inverter parallel control method based on adaptive virtual impedance includes the following steps:

[0044] Step 1: Establish the droop control equation of multiple inverters in parallel;

[0045] Step 2: Analyze the conditions for active power sharing of multiple inverters in parallel;

[0046] Step 3: Construct an improved droop control model based on adaptive virtual impedance.

[0047] Specifically, the multi-inverter parallel control method based on adaptive virtual impedance mainly includes a multi-inverter parallel droop control method and an output current-based adaptive virtual impedance control method.

[0048] Specifically, the multi-inverter parallel droop control method includes the following aspects:

[0049] Since multi-inverter parallel lines are generally low-voltage, with voltage levels typically ranging from 380V to 1kV, the line impedance is primarily resistive. Assuming the inductive portion of the line impedance is sufficiently small and negligible, the inverter output power expression for a resistive line impedance is:

[0050]

[0051] Among them, P and Q are the active power and reactive power output by the inverter respectively, U is the output voltage of the inverter, and U pcc is the voltage at the common connection point of the inverter, is the inverter output voltage phase, R L is the line resistance.

[0052] Specifically, a multi-inverter parallel model is established based on the relationship between the output active power, output voltage and common connection point voltage when multiple inverters are connected in parallel, and a droop control equation for multi-inverter reverse parallel connection under resistive line impedance is established.

[0053] From formula (1), we can see that the active power P and reactive power Q output by the inverter are respectively proportional to the inverter output voltage U and the output voltage phase It is a linear relationship, and the frequency f can be used to replace the inverter output voltage phase Therefore, the droop control equation of multiple inverters in parallel when the low-voltage line impedance is resistive can be deduced as follows:

[0054]

[0055] Among them, U, U * are the inverter output voltage amplitude and voltage given value respectively; f, f * are the frequency and frequency given value of the output voltage when the inverter is running; m and n are the active-voltage droop coefficient and reactive-frequency droop coefficient respectively.

[0056] The control structure diagram of the multi-inverter parallel droop controller is as follows: Figure 2 shown.

[0057] Specifically, according to Figure 3 The multi-inverter parallel droop control circuit model in the figure shows that the relationship between the output power, output voltage and bus voltage of the parallel inverter is:

[0058]

[0059] Among them, U i is the output voltage of the i-th inverter, R Li is the equivalent resistance from the ith inverter to the common connection point, P i * is the rated power of the i-th inverter, m i is the active power-voltage droop coefficient of the i-th inverter, U * is the reference voltage, U PCC is the voltage at the common connection point.

[0060] Specifically, ideally, when multiple inverters are connected in parallel, the output power of each inverter is evenly divided according to the active power droop coefficient. According to formula (3), the output active power sharing error relationship when two inverters are connected in parallel can be obtained as follows:

[0061]

[0062] Among them, U i 、U j are the output voltages of the i-th and j-th inverters, R Li 、R Lj are the equivalent resistances from the ith and jth inverters to the common connection point, P i 、P j are the rated powers of the i-th and j-th inverters, respectively, m i 、m j are the active power-voltage droop coefficients of the i-th inverter and the j-th inverter respectively.

[0063] As can be seen from formula (4), when the molecular part of formula (4) △P is equal to zero, the active power of the multi-inverter parallel connection can be evenly divided, that is:

[0064]

[0065] Therefore, the condition for the active power of the multi-inverter parallel connection to be evenly divided is that the ratio of the equivalent line impedance of each inverter when the multi-inverters are connected in parallel and the product of the active-voltage droop coefficient of each inverter and the output voltage are equal.

[0066] Specifically, the adaptive virtual impedance control method based on the output current comprises the following aspects:

[0067] In reality, the equivalent line impedance of each parallel inverter is uncontrollable, so the condition for the active power of the multi-inverter parallel connection to be evenly divided is difficult to achieve. Therefore, when the multi-inverters are connected in parallel, the output active power cannot be evenly divided according to the rated capacity. When the virtual impedance is introduced, the virtual impedance is equivalent to changing the equivalent impedance of each parallel line, and when the ratio of the equivalent line impedance of each inverter and the product of the active-voltage droop coefficient of each inverter and the output voltage are equal, the active power can be evenly divided according to the rated power.

[0068] The adaptive virtual impedance model in the application is:

[0069] Z V =R v +jX v (6)

[0070] Wherein, R V , X V are virtual resistance and virtual inductance respectively. The virtual inductance is given a negative value, and the negative value of the virtual inductance part can offset the influence of the line inductance on the equivalent impedance and make the equivalent output impedance of the inverter resistive, so that the system meets the requirements of the multi-inverter parallel connection. The virtual resistance is an adaptive virtual resistance, which is composed of two parts, one part is a given positive value, and the other part is a variable positive value. The adaptive virtual resistance can suppress circulating current, improve power division accuracy, balance line impedance difference, and achieve active power division of the multi-inverter parallel connection.

[0071] Specifically, the adaptive virtual resistance expression proposed is:

[0072]

[0073] Wherein, R vi is the virtual resistance of the i th parallel inverter, R set is a given positive value virtual resistance, w S1,i is the ratio of the rated capacity of the first parallel inverter to the i th parallel inverter, k v is a virtual resistance coefficient, and i odiis the d-axis component of the output current of the ith parallel inverter. Moreover, because the introduction of an excessively large virtual resistance will lead to system instability and output voltage drop, it is assumed that the inverter with the smallest rated capacity among the parallel inverters is inverter No. 1, and its fixed virtual resistance R set The value is 0.1~0.4Ω, and the virtual resistance coefficient k v The value range is 0.2 to 0.6. The virtual resistance of other parallel inverters is multiplied by the ratio of the rated capacity of the first parallel inverter to the i-th parallel inverter, w S1,i Therefore, the value is no greater than the virtual resistance of inverter No. 1.

[0074] The output current information in the proposed adaptive virtual impedance model comes from the local inverter, and there is no need for communication between the inverters. Therefore, the active power of the parallel inverters is evenly distributed according to the rated power while improving the flexibility and reliability of the system.

[0075] In order to illustrate the active power sharing effect of adaptive virtual impedance, i=2, w S1,i =1, R L1 >R L2 , that is, the rated capacity of the parallel inverters is the same, and the line impedance of inverter No. 1 is greater than the line impedance of inverter No. 2. This is demonstrated by taking an example. At this time, the active power sharing error can be simplified to:

[0076]

[0077] The left-hand side of △P' is similar to △P in equation (4). From the numerator, it is equivalent to adding the same virtual resistance given value to the line impedance, which is equivalent to reducing the line impedance deviation. In addition, the denominator adds an adaptive virtual impedance proportional to the output current. Therefore, the left-hand side is smaller than the average sharing error of the active power when the adaptive virtual impedance is not introduced.

[0078] The relationship between the inverter's output power and the voltage and current in the dq rotating coordinate system is:

[0079]

[0080] Among them, v od and v oq are the d-axis component and q-axis component of the output voltage of the parallel inverter respectively. From formula (9), we can see that the d-axis output current i od It is proportional to the output power P and inversely proportional to the d-axis output voltage. Therefore, when the line impedance R Li <R Lj When the output power Pi of the parallel inverter is greater than Pj, the output voltage U i j , v iod <v jod ​So i odi i odj So, U i m i k v i odj -U j m j k v i odi <0, at this time the right is negative, thus making △P' < △P, play a role in eliminating the error of active power sharing, thus the multi-inverter parallel active power sharing control strategy based on adaptive virtual impedance can realize the multi-inverter output active power sharing.

[0081] The adaptive virtual impedance control structure is shown in Figure 4 .

[0082] The Matlab / Simulink software simulation is used to build a multi-inverter parallel simulation model, two examples are established to verify whether the improved droop control strategy with virtual impedance has good control effect, and whether the output active power of the multi-inverter parallel can be shared according to the rated capacity when the rated capacity of the parallel inverters is the same and when the rated capacity is different.

[0083] In the two examples established, the rated capacity of the parallel inverters in the first example is the same, and the ratio of the rated capacity of the inverters in the second example is 1:2, and other conditions are the same. The simulation time is 1.5s, and the load 1 and the load 2 are both rated power 3KW active load. At the initial moment, the parallel inverters supply power to the load 1 through the common connection point at the same time, at 0.5s, the common connection point is connected to the load 2, and the parallel inverters supply power to the load 1 and the load 2 at the same time, at 1s, the load 1 is cut off, and the two parallel inverters supply power to the load 2 at the same time.

[0084] As can be seen from Figure 5 : when the adaptive virtual impedance is not introduced, the inverter with smaller line impedance outputs more active power, and the inverter with larger line impedance outputs less active power, and the active power cannot be shared, and as the load increases, the degree of uneven distribution of the output active power of the multi-inverter parallel also increases;

[0085] As can be seen from Figure 6 : when the adaptive virtual impedance is not introduced, when the multi-inverter is parallel, because the active power cannot be shared, there is a large deviation in the output current of the parallel inverters, so the circulating current in the system is large, which affects the stability of the system;

[0086] As can be seen from Figure 7It can be seen from the figure that after the introduction of the multi-inverter parallel active power sharing control strategy based on adaptive virtual impedance, the active power sharing degree of the multi-inverter parallel connection is significantly increased, and as the load power increases, the active power sharing degree of the multi-inverter parallel connection will also increase. Therefore, the multi-inverter parallel active power sharing control strategy based on adaptive virtual impedance can achieve active power sharing;

[0087] from Figure 8 It can be seen that after introducing the multi-inverter parallel active power sharing control strategy based on adaptive virtual impedance, the output current deviation is significantly reduced when multiple inverters are connected in parallel, and the circulating current suppression capability and system stability are improved;

[0088] from Figure 9 It can be seen from the figure that when adaptive virtual impedance is not introduced, when the rated capacities of the two inverters are different, the line impedance does not meet the equalization condition, and the inverters cannot achieve equal distribution of active power according to the rated capacity. Therefore, the parallel inverters may be overloaded, affecting the system stability.

[0089] from Figure 10 It can be seen from the figure that after introducing the multi-inverter parallel active power sharing control strategy based on adaptive virtual impedance, when the rated capacities of the two inverters are different, the active power of the multi-inverter parallel connection can be shared according to the rated capacity. In addition, as the load increases, the power sharing accuracy will be further improved, thereby improving the stability and reliability of the system.

[0090] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the scope of protection of the present invention.

Claims

1. A multi-inverter parallel control method based on adaptive virtual impedance, characterized in that: The steps include: Step 1: Establish the droop control equation of multi-inverter parallel connection; Step 2: Analyze the conditions for active power sharing of multiple inverters in parallel; Step 3: Construct an improved droop control model based on adaptive virtual impedance; According to the multi-inverter parallel model, the output active power sharing error relationship when two inverters are connected in parallel with droop control is established as follows: Among them, U i 、U j are the output voltages of the i-th and j-th inverters, R Li 、R Lj are the equivalent resistances from the ith and jth inverters to the common connection point, P i 、P j are the rated powers of the i-th and j-th inverters, respectively, m i 、m j are the active power-voltage droop coefficients of the i-th inverter and the j-th inverter respectively; When the numerator of the output active power sharing error △P is equal to zero, the active power of multiple inverters in parallel can be shared equally, that is: The analysis shows that the condition for active power equalization of multiple inverters in parallel is that the ratio of the equivalent line impedance of each inverter when multiple inverters are connected in parallel is equal to the ratio of the product of the active power-voltage droop coefficient of each inverter and the output voltage.

2. The method for controlling multiple inverters in parallel based on adaptive virtual impedance according to claim 1, wherein: According to the relationship between the output active power, output voltage and common connection point voltage when multiple inverters are connected in parallel, a multi-inverter parallel model is established. The droop control equation of multi-inverter reverse parallel connection under the resistive line impedance is established as follows: Among them, P and Q are the active power and reactive power output by the inverter, respectively, U and U * are the inverter output voltage amplitude and voltage given value respectively; f, f * are the frequency and frequency set value of the output voltage when the inverter is running; m and n are the active-voltage droop coefficient and reactive-frequency droop coefficient respectively.

3. The method for controlling multiple inverters in parallel based on adaptive virtual impedance according to claim 1, wherein: Step 3 includes the following specific steps: Step 31: Introduce virtual impedance to change the equivalent impedance of each parallel line. When the ratio of the equivalent line impedance of each inverter is equal to the ratio of the product of the active power-voltage droop coefficient of each inverter and the output voltage, the active power can be evenly distributed according to the rated power. Step 32: Establish an adaptive virtual impedance model as follows: Z V =R v +jX v Among them, R V 、X V They are virtual resistance and virtual inductance respectively. The virtual inductance takes a given negative value. The virtual resistance is an adaptive virtual resistance consisting of two parts, one of which is a given positive value and the other is a variable positive value. Step 33: Build a multi-inverter parallel simulation model to verify whether the improved droop control method with the introduction of virtual impedance can achieve equal distribution of the output active power according to the rated capacity when multiple inverters are connected in parallel.

4. The method for controlling multiple inverters in parallel based on adaptive virtual impedance according to claim 3, wherein: The virtual resistance expression in the adaptive virtual impedance model is: Among them, R vi is the virtual resistance of the ith parallel inverter, R set For a given positive virtual resistance, w S1,i is the ratio of the rated capacity of the first parallel inverter to the i-th parallel inverter, k v is the virtual resistance coefficient, i odi is the d-axis component of the output current of the i-th parallel inverter.

5. The method for controlling multiple inverters in parallel based on adaptive virtual impedance according to claim 4, characterized in that: The inverter with the smallest rated capacity among the parallel inverters is inverter No. 1, and its fixed virtual resistance R set The value is 0.1~0.4Ω, and the virtual resistance coefficient k v The value range is 0.2 to 0.6.

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