A control method for parallel operation of inverters with adaptive reactive power sharing

Abstract

The present invention provides an inverter parallel operation control method with adaptive reactive power sharing, which belongs to the field of inverter parallel operation control technology. The method includes: introducing virtual impedance to match the line impedance with the inverter capacity, adjusting the inverter output voltage in the form of virtual voltage drop together with the voltage reference value generated by droop control, and performing adaptive virtual voltage drop correction. The present invention introduces an adaptive reactive power sharing control method in reactive power droop control, and through the voltage drop correction term, the virtual voltage drop repair is adaptively adjusted in real time according to the reactive load change, so that the reactive power sharing accuracy is not affected by the line impedance change.

Description

Technical Field

[0001] The invention relates to the technical field of inverter parallel operation control, and in particular to an inverter parallel operation control method with adaptive reactive power sharing. Background Art

[0002] Under the vision of dual carbon goals, the installed capacity of new energy power generation such as photovoltaic and wind power has increased rapidly. The access of this low-inertia, random system to the power grid will have a huge impact on the power grid. The use of new energy to form a microgrid through inverters is an effective way to reduce the impact on the power grid. The parallel operation of inverters can improve output capacity, economy and reliability. Therefore, parallel control of inverters is an important control method for microgrid operation.

[0003] The most commonly used parallel inverter control strategy is droop control, which has the characteristics of plug-and-play, easy expansion and independent operation without the need for communication interconnection lines. However, due to the influence of line impedance properties and differences in the location of distributed power sources, the impedance of each branch in parallel is different, resulting in uneven reactive power distribution of the microgrid inverter under droop control. Uneven reactive power distribution will not only cause the inverter to heat up and reduce the operating efficiency of the inverter, but also damage the service life of the inverter, threatening the safe operation of the microgrid.

[0004] In order to solve the problem of uneven reactive power distribution in droop control, virtual impedance is often introduced to adjust the line impedance, which plays two main roles: first, it changes the line impedance properties of the microgrid and improves its inductive component, which helps to improve the power decoupling effect of droop control in the microgrid; second, it can make the equivalent line impedance after the introduction of virtual impedance inversely proportional to the inverter capacity through reasonable parameter settings, thereby realizing reactive power sharing of parallel inverters. The introduction of virtual impedance is a fixed value, and the reactive power sharing effect is not good when the line impedance changes. Therefore, a variety of adaptive virtual impedances have been studied. An adaptive virtual impedance strategy in the existing literature can adjust the size of the virtual impedance in real time according to the change of line impedance, which improves the reactive power sharing accuracy of the virtual impedance when the equivalent impedance changes, but the introduction of adaptive virtual impedance will cause different degrees of bus voltage drop. In order to reduce the bus voltage drop caused by virtual impedance, a virtual complex impedance control strategy in the existing literature is used, and both the virtual inductance and the virtual negative resistance are designed as adaptive forms. The role of the adaptive inductance is to ensure reactive power sharing, and the virtual negative resistance is to eliminate the resistive component in the line impedance, which improves the flexibility of the virtual impedance method. However, this method involves two kinds of adaptive virtual impedances in the adjustment. In particular, unreasonable setting of virtual negative resistance can easily cause system instability, and the parameter design is too complicated. Summary of the invention

[0005] The present invention provides an inverter parallel operation control method with adaptive reactive power sharing to solve one or more of the above problems.

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

[0007] An inverter parallel operation control method with adaptive reactive power sharing, comprising:

[0008] Introducing virtual impedance to match line impedance with inverter capacity, adjusting inverter output voltage in the form of virtual voltage drop together with voltage reference value generated by droop control, and performing adaptive virtual voltage drop correction;

[0009] Among them, the reactive power ratio of parallel inverters is expressed as follows:

[0010]

[0011] Considering the line impedance change scenario, the adaptive virtual voltage drop correction item meets the following conditions:

[0012]

[0013] In the formula and represent the adaptive virtual voltage drop compensation items of the first and second inverters respectively; ΔX1 and ΔX2 represent the line impedance changes of the first and second inverters respectively; Q1 and Q2 represent the reactive power output by the first and second inverters respectively; X1 and X2 represent the line impedance of the first and second inverters respectively; U pcc represents the bus voltage amplitude; U1 and U2 represent the output voltage amplitudes of the first and second inverters respectively; Q ref1 and Q ref2 Respectively represent the reactive power set for the first and second inverters;

[0014] The improved reactive power droop equation after introducing the adaptive virtual voltage drop correction term is expressed as:

[0015]

[0016] In the formula represents the theoretical value of reactive power sharing of the i-th inverter; k vi represents the virtual voltage drop correction coefficient of the i-th inverter; U i_droop represents the inverter droop control voltage amplitude reference value in the dq coordinate system; It indicates that the d-axis voltage reference value is obtained after droop calculation; is the reference value of the d-axis voltage amplitude of the inverter droop control; ω is the angular frequency; L v represents the virtual inductance parameter; i gq is the line q-axis current; is the reference value of the q-axis voltage amplitude for inverter droop control; igd is the line d-axis current; n is the reactive power droop coefficient; U n Indicates the grid voltage rating for droop control; Q ref i represents the reactive setting power of the droop control of the i-th inverter; s represents the Laplace operator;

[0017] Then the small signal model under the first-order dynamic phasor model is expressed as:

[0018]

[0019] Where ΔQ i is the output reactive power change of the i-th inverter; ΔU i is the output voltage amplitude change of the i-th inverter; L i represents the circuit inductance between the i-th inverter and the common bus; ΔU n is the change in the rated value of the grid voltage; λ is the reactive power regulation coefficient; R i The resistance representing the line impedance between the ith inverter and the common load; ω c Represents the cutoff frequency of the low-pass filter;

[0020] Then the small signal block diagram of the reactive power control loop of the i-th inverter is obtained after the introduction of the adaptive virtual voltage drop correction term, and its corresponding transfer function is expressed as:

[0021]

[0022] The subscript i is omitted, and the parameters are as follows:

[0023]

[0024] According to the transfer function, the virtual pressure drop correction coefficient k is obtained v Root locus diagram of the reactive loop when changes.

[0025] In this specification, the virtual pressure drop correction factor k is considered v The impact on the dynamic process of the reactive loop, the small signal model of the virtual voltage drop correction term of the first inverter is expressed as:

[0026]

[0027] The inverter output reactive power after the virtual voltage drop correction term is introduced into the first-order low-pass filter is expressed as:

[0028]

[0029] The small signal model of the inverter output reactive power is expressed as:

[0030]

[0031] Where U 1n Indicates the output voltage amplitude of the inverter using droop control;

[0032] According to the small signal model of the virtual voltage drop correction term of the first inverter and the small signal model of the inverter output reactive power, the transfer function block diagram after the introduction of the virtual voltage drop correction term is obtained, and the corresponding transfer function is:

[0033]

[0034] ω n is the natural oscillation frequency, and ζ is the system damping coefficient.

[0035] In this specification, an adaptive amplitude compensation term is applied to the inverter output voltage to achieve error-free regulation of the bus voltage amplitude without affecting the reactive power sharing accuracy, so as to eliminate the bus voltage drop caused by virtual impedance. The relationship between the output reactive power and the output voltage of the parallel inverter is expressed as:

[0036]

[0037] Since adjusting the virtual impedance will affect the power decoupling effect of the inverter, in order to eliminate the bus voltage drop caused by the virtual impedance, an amplitude-invariant compensation term is applied to the output voltage U. The bus voltage amplitude after compensation should satisfy:

[0038]

[0039] Indicates the amplitude indifference compensation term, U * pcc Indicates the bus voltage reference value;

[0040] The reactive power droop equation after the introduction of the amplitude difference compensation term is:

[0041]

[0042] Where k ui represents the amplitude compensation coefficient of the i-th inverter;

[0043] Then, the improved reactive loop control block diagram after introducing the adaptive virtual voltage drop correction term and the amplitude difference compensation term is obtained. The bus voltage amplitude U pcc The change of will drive the inverter output voltage U to change, and U will react to adjust U pcc , until it stabilizes at the rated value U * pcc .

[0044] In this manual, the virtual pressure drop correction factor k v1 The value of is 0.8.

[0045] In this specification, the amplitude compensation coefficient k u The value of is 20.

[0046] In this manual, the process of obtaining the expression of the reactive power ratio of parallel inverters is as follows:

[0047] The output power of the two inverters is:

[0048]

[0049] Where U i represents the output voltage amplitude of the i-th inverter; θ i represents the output voltage phase angle of the i-th inverter; U pcc Indicates the bus voltage amplitude; α i represents the impedance angle of the i-th line, satisfying α i =arctan(X i / R i );R i , X i denote the resistance and reactance of the line impedance between the i-th inverter and the common load respectively;

[0050] At high voltage levels, the line impedance is approximately inductive, satisfying α≈90°. During steady-state operation, the inverter output power angle is small, with sinθ≈θ and cosθ≈1. The above formula is simplified to:

[0051]

[0052] When the line impedance is inductive, the droop control is expressed as:

[0053]

[0054] Where f and f n Respectively represent the droop control frequency reference value and the power frequency; U and U n They represent the voltage amplitude reference value of droop control and the rated value of the grid voltage respectively; m and n represent the active and reactive droop coefficients respectively; P ref and Q ref Indicates the set active power and reactive power; P and Q indicate the active power and reactive power output by the inverter;

[0055] The voltage reference value is obtained by calculating the active and reactive power droop and

[0056] The power transfer characteristics of the inverter based on droop control are:

[0057]

[0058] Where s represents the Laplace operator.

[0059] In this manual, reactive power sharing should meet the following conditions:

[0060]

[0061] Among them U i 、n i , Q ref i , X i They respectively represent the voltage amplitude reference value of the droop control of the i-th inverter, the reactive power droop coefficient, the reactive power setting power, and the reactance of the line impedance between the inverter and the common load.

[0062] In this specification, the virtual impedance introduced when achieving reactive power sharing should satisfy the following relationship:

[0063]

[0064] Where, X i Indicates line reactance; X vi Represents the introduced virtual impedance.

[0065] In this specification, in the dq coordinate system, the reactive loop control equation based on virtual impedance is expressed as:

[0066]

[0067] Where U i_droop represents the inverter droop control voltage amplitude reference value in the dq coordinate system; It indicates that the d-axis voltage reference value is obtained after droop calculation; is the reference value of the d-axis voltage amplitude of the inverter droop control; ω is the angular frequency; L v represents the virtual inductance parameter; i gq is the line q-axis current; is the reference value of the q-axis voltage amplitude for inverter droop control; i gd is the line d-axis current.

[0068] The embodiments of this specification can at least achieve the following beneficial effects:

[0069] To solve the problem that the accuracy of reactive power sharing decreases when the line impedance changes due to virtual impedance, an adaptive reactive power sharing control method is introduced in reactive power droop control. Through the voltage drop correction term, the virtual voltage drop is adaptively adjusted in real time according to the change of reactive load, so that the accuracy of reactive power sharing is not affected by the change of line impedance.

[0070] In order to further solve the problem of different degrees of bus voltage drop caused by introducing adaptive virtual impedance, an adaptive bus voltage amplitude compensation method is proposed to achieve zero-difference adjustment of bus voltage amplitude while ensuring the accuracy of reactive power sharing, thereby improving the bus voltage quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0072] Figure 1 It is a schematic block diagram of a small signal of a reactive power control loop after introducing an adaptive virtual voltage drop correction term involved in the present invention.

[0073] Figure 2 It is a schematic diagram of the equivalent structure of the parallel inverter involved in the present invention.

[0074] Figure 3 It is a schematic block diagram of the traditional droop control involved in the present invention.

[0075] Figure 4 It is an equivalent schematic diagram of the parallel inverter with virtual impedance involved in the present invention.

[0076] Figure 5 is the different virtual pressure drop correction coefficients k involved in the present invention v Schematic diagram of the root locus of the reactive loop.

[0077] Figure 6 It is a schematic block diagram of the transfer function after the adaptive virtual voltage drop correction term is introduced in the present invention.

[0078] Figure 7 is the different virtual pressure drop correction coefficients k involved in the present invention v1 Schematic diagram of the unit step response under .

[0079] Figure 8 It is a schematic block diagram of the improved reactive loop control after introducing the adaptive virtual voltage drop correction term and the amplitude difference compensation term involved in the present invention.

[0080] Fig. 9 It is a schematic diagram of the example structure involved in the present invention.

[0081] Fig.10 It is a schematic diagram of the reactive power waveform of the example involved in the present invention.

[0082] Fig.11 It is a schematic diagram of the bus voltage amplitude of the example involved in the present invention. DETAILED DESCRIPTION

[0083] In the following, only some exemplary embodiments are briefly described. As those skilled in the art will appreciate, the described embodiments may be modified in various ways without departing from the spirit or scope of the embodiments of the present invention. Therefore, the drawings and descriptions are considered to be exemplary and non-restrictive in nature.

[0084] The disclosure below provides many different embodiments or examples to implement different structures of the embodiments of the present invention. In order to simplify the disclosure of the embodiments of the present invention, the parts and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the embodiments of the present invention. In addition, the embodiments of the present invention can repeat reference numbers and / or reference letters in different examples, and this repetition is for the purpose of simplification and clarity, which itself does not indicate the relationship between the various embodiments and / or settings discussed.

[0085] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0086] The embodiments of this specification provide an inverter parallel operation control method with adaptive reactive power sharing, the specific contents of which are as follows.

[0087] 1. Microgrid distributed power generation inverter parallel operation structure diagram

[0088] Affected by the location distribution of new energy sources and the capacity of a single inverter, multiple inverters are often used in parallel to provide power to the load. For ease of analysis, two inverters are selected. The inverter parallel model structure diagram is as follows Figure 2 As shown, DG1 and DG2 represent the equivalent amplitude of the LC filter inverter, U i (i=1,2), the phase angle is θ i (i=1,2) ideal voltage source, P i , Q i (i=1,2) represent the active power and reactive power output by the i-th inverter respectively; R i +jX i (i=1,2) represent the line impedance between the ith inverter and the common load; P and Q represent the active and reactive power consumed by the common load respectively; U pcc Indicates the bus voltage amplitude; Z load For public load.

[0089] according to Figure 2 It can be seen that the output power of the two inverters is:

[0090]

[0091] Where Pi represents the active power output by the i-th inverter; Q i represents the reactive power output by the i-th inverter; U i represents the output voltage amplitude of the i-th inverter; θ i represents the output voltage phase angle of the i-th inverter; α i represents the impedance angle of the i-th line, satisfying α i =arctan(X i / R i );R i , X i represent the resistance and reactance of the line impedance between the i-th inverter and the common load respectively.

[0092] At high voltage levels, the line impedance is approximately inductive, satisfying α≈90°. During steady-state operation, the inverter output power angle is small, and sinθ≈θ, cosθ≈1. Then equation (1) can be simplified to:

[0093]

[0094] Formula (2) shows that when the line impedance is inductive, the active power output of the inverter is mainly related to the output voltage power angle θ, while the reactive power output of the inverter is mainly related to the output voltage amplitude U. Therefore, by controlling the power angle θ and the amplitude U respectively, the active power and reactive power can be adjusted independently. However, in practical applications, it is difficult to directly control the power angle θ of the output voltage. The power angle δ is often indirectly controlled by controlling the angular frequency ω or frequency f of the output voltage. When the line impedance is inductive, the droop control is expressed as:

[0095]

[0096] Where f and f n Respectively represent the droop control frequency reference value and the power frequency; U and U n They represent the voltage amplitude reference value of droop control and the rated value of the grid voltage respectively; m and n represent the active and reactive droop coefficients respectively; P ref and Q ref Indicates the set active power and reactive power; P and Q indicate the active power and reactive power output by the inverter.

[0097] According to formula (3), the traditional droop control block diagram can be obtained as follows: Figure 3 To reduce the noise interference in measuring active and reactive power, the active and reactive power are first processed by low-pass filtering. Figure 3 Medium c Represents the cutoff frequency of the low-pass filter, and then the voltage reference value is obtained through active and reactive droop calculations and

[0098] According to formula (3) and Figure 3 The inverter power transfer characteristics based on droop control can be obtained:

[0099]

[0100] Where s represents the Laplace operator; P(s) represents the active power output by the inverter in the frequency domain; Q(s) represents the reactive power output by the inverter in the frequency domain.

[0101] The frequency is a global variable. When the system is running stably, the output voltage frequency of each inverter is equal. When the system runs to a steady state, the differential term is 0. According to formula (4), the active power output of the inverter is only related to the active power droop coefficient m. Therefore, the active power droop coefficient is inversely proportional to the active capacity of the inverter, which can achieve natural active power sharing and is not affected by line impedance.

[0102] According to formula (4), the reactive power sharing should meet the following conditions:

[0103]

[0104] Among them U i 、n i , Q ref i , X i They respectively represent the voltage amplitude reference value of the droop control of the i-th inverter, the reactive power droop coefficient, the reactive power setting power, and the reactance of the line impedance between the inverter and the common load.

[0105] According to formula (5), the reactive power output of the inverter is not only related to the reactive droop coefficient n, but also needs to satisfy the line impedance is inversely proportional to the reactive capacity to achieve reactive power sharing. However, in actual working conditions, due to the different locations of distributed power sources, the line impedance is difficult to match the inverter capacity, so it is difficult for parallel inverters based on droop control to achieve reactive power sharing.

[0106] 2. Traditional virtual impedance reactive power sharing strategy

[0107] From the above analysis, we can know that the line impedance X is related to the reactive power sharing accuracy. Some scholars have proposed to control the line impedance ratio between inverters through virtual impedance, thereby improving the reactive power sharing accuracy. The role of virtual impedance here is not only to increase the inductive component of line impedance to achieve decoupling control, but more importantly, to make the line impedance inversely proportional to the inverter capacity. After the introduction of virtual impedance, it can be equivalent to a part of the line impedance, such as Figure 4 shown.

[0108] According to formula (5), the virtual impedance introduced when achieving reactive power sharing should satisfy the following relationship:

[0109]

[0110] Where, X i Indicates line reactance; X vi Represents the introduced virtual impedance.

[0111] In the dq coordinate system, the reactive loop control equation based on virtual impedance is expressed as:

[0112]

[0113] Where U i_droop represents the inverter droop control voltage amplitude reference value in the dq coordinate system; It indicates that the d-axis voltage reference value is obtained after droop calculation; It indicates that the q-axis voltage reference value is obtained after droop calculation; is the reference value of the d-axis voltage amplitude of the inverter droop control; ω is the angular frequency; L v represents the virtual inductance parameter; i gq is the line q-axis current; is the reference value of the q-axis voltage amplitude for inverter droop control; i gd is the line d-axis current.

[0114] This method requires knowing the line impedance information in advance. The reactive power sharing accuracy is greatly affected by the line impedance measurement accuracy. In addition, changes in line impedance will make the set fixed virtual impedance unable to match the inverter capacity, thereby affecting the reactive power sharing accuracy under the virtual impedance strategy.

[0115] Although the existing improved virtual impedance strategy can adjust the virtual impedance size according to the line impedance change to achieve adaptive reactive power sharing, the adaptive virtual impedance may reduce the virtual impedance size during the adjustment process, thereby affecting the power decoupling control effect. In addition, whether it is fixed virtual impedance or adaptive virtual impedance, it will cause bus voltage drop when achieving reactive power sharing, seriously affecting the bus voltage quality.

[0116] 3. Improved virtual impedance strategy with adaptive reactive power sharing and voltage compensation functions

[0117] 3.1 Design of adaptive reactive power sharing link

[0118] Introducing virtual impedance X vIt can match the line impedance with the inverter capacity and improve the accuracy of reactive power sharing, but the accuracy is easily affected by the change of equivalent impedance. The essence of virtual impedance is to adjust the inverter output voltage together with the voltage reference value generated by droop control in the form of virtual voltage drop. To this end, this manual adds an adaptive virtual voltage drop correction link on the basis of traditional virtual impedance, and proposes an adaptive reactive power sharing strategy based on reactive power deviation drive. Compared with the traditional reactive power sharing strategy based on virtual impedance, the reactive power sharing effect of this strategy is not affected by local load switching and line impedance changes, and the control is simpler.

[0119] According to formula (4), the reactive power ratio of parallel inverters can be expressed as:

[0120]

[0121] Considering the line impedance change scenario, the adaptive virtual voltage drop correction item should meet the following conditions:

[0122]

[0123] In the formula and They represent the adaptive virtual voltage drop compensation items of inverters 1 and 2 respectively; ΔX1 and ΔX2 represent the line impedance changes respectively.

[0124] The improved reactive power droop equation after introducing the adaptive virtual voltage drop correction term is expressed as:

[0125]

[0126] Where Q i represents the actual value of reactive power sharing of the i-th inverter; represents the adaptive virtual voltage drop compensation item of the i-th inverter; k represents the index value at which the summation starts; represents the theoretical value of reactive power sharing of the i-th inverter; k vi Represents the virtual voltage drop correction coefficient of the i-th inverter.

[0127] According to formula (10), the virtual voltage drop correction coefficient will affect the system stability and the dynamic performance of the reactive power sharing process. Therefore, k v Combining equation (1) and equation (10), we can get the small signal model under the first-order dynamic phasor model, which is expressed as:

[0128]

[0129] Where ΔQ i is the output reactive power change of the i-th inverter; ΔU i is the output voltage amplitude change of the i-th inverter; L irepresents the circuit inductance between the i-th inverter and the common bus; ΔU n The change in the rated value of the grid voltage; Q refk is the reactive power setting of droop control of k inverters; i is the reactive power regulation coefficient of the i-th inverter; λ is the reactive power regulation coefficient.

[0130] According to formula (11), the small signal block diagram of the reactive power control loop of the i-th inverter after introducing the adaptive virtual voltage drop correction term can be obtained, as follows: Figure 1 As shown, for the convenience of analysis, the subscript i is omitted.

[0131] according to Figure 1 The corresponding transfer function can be expressed as:

[0132]

[0133] The specific parameters in the formula are:

[0134]

[0135] b1, b0, A, B, C, D, and E are all transfer function coefficients and have no specific meaning. Their main purpose is to simplify the expression. R represents the resistance of the line impedance between the i-th inverter and the common load. kv is the virtual voltage drop correction factor.

[0136] According to formula (12), the virtual pressure drop correction coefficient k can be obtained: v The root locus diagram of the reactive loop when it changes, such as Figure 5 As shown, the arrow represents k v The direction in which the pole position moves when it increases. Analysis Figure 5 It can be seen that as the virtual pressure drop correction factor k v As the real part of the pole changes from negative to positive, the excessive virtual voltage drop correction factor k v It will cause system instability.

[0137] In addition to considering stability, it is also necessary to consider the virtual pressure drop correction factor k v Impact on the dynamic process of the reactive loop. Taking two inverters in parallel as an example, the small signal model of the virtual voltage drop correction term of inverter 1 can be expressed as:

[0138]

[0139] represents the change of the adaptive virtual voltage drop compensation item of the first inverter; λ1 represents the reactive power regulation coefficient of the first inverter; k v1 is the virtual voltage drop correction coefficient of the first inverter; ΔQ1 represents the reactive power change of the first inverter.

[0140] In the first-order low-pass filter stage, the inverter output reactive power after introducing the virtual voltage drop correction term can be expressed as:

[0141]

[0142] Where X represents the line impedance of the inverter; U 1n The grid voltage rating for the droop control of the first inverter; Represents the adaptive virtual voltage drop compensation item of the inverter.

[0143] The small signal model of formula (14) can be expressed as:

[0144]

[0145] ΔQ1 represents the reactive power change of the first inverter; Indicates the change in the adaptive virtual voltage drop compensation item of the first inverter.

[0146] According to equations (13) and (15), the transfer function block diagram after the introduction of the virtual voltage drop correction term can be obtained, as shown in Figure 6 shown.

[0147] according to Figure 6 The corresponding transfer function can be expressed as:

[0148]

[0149] Equation (16) represents a second-order system, whose damping coefficient and natural oscillation frequency are:

[0150]

[0151] According to formula (17), it can be seen that a larger virtual pressure drop correction coefficient k is selected v1 It can improve the dynamic response speed of the reactive loop, but k v1 If the value is too large, the system damping coefficient ζ will be reduced, affecting the system stability. Different virtual pressure drop correction coefficients k v1 The unit step response under Figure 7 As shown, with k v1 As the voltage increases, the system dynamic response time is accelerated, but the system damping gradually changes from an over-damped state to an under-damped state. In order to take into account the dynamic response speed and stability of the reactive loop, the virtual voltage drop correction coefficient k selected in this manual is v1 The value of is 0.8.

[0152] 3.2 Design of adaptive bus voltage amplitude compensation link

[0153] It is worth noting that the introduction of virtual voltage drop correction items can improve the reactive power sharing accuracy of parallel inverters in line impedance change scenarios, but it will also cause different degrees of bus voltage drop. To this end, this specification proposes an adaptive bus voltage amplitude compensation method, which applies an adaptive amplitude compensation item to the inverter output voltage, achieves zero-difference adjustment of the bus voltage amplitude without affecting the reactive power sharing accuracy, and eliminates the bus voltage drop caused by virtual impedance.

[0154] According to formula (2), the relationship between the output reactive power and output voltage of the parallel inverter can be obtained, which is expressed as:

[0155]

[0156] U is the output voltage; Q is the inverter output reactive power.

[0157] According to formula (18), it can be known that the influence on bus voltage U pcc The variables include the inverter output voltage U and the line impedance X. Since adjusting the virtual impedance will affect the power decoupling effect of the inverter, in order to eliminate the bus voltage drop caused by the virtual impedance, this manual applies an amplitude-invariant compensation item to the output voltage U. The bus voltage amplitude after compensation should meet the following requirements:

[0158]

[0159] In the formula Indicates the amplitude indifference compensation term, U * pcc It represents the bus voltage reference value after the introduction of the zero-difference compensation term.

[0160] The reactive power droop equation after the introduction of the amplitude difference compensation term is:

[0161]

[0162] Where k ui represents the amplitude compensation coefficient of the i-th inverter; represents the amplitude invariant compensation term of the i-th inverter; It indicates that the d-axis voltage reference value is obtained by reactive droop control calculation after the amplitude difference compensation term is introduced; It indicates that the q-axis voltage reference value is obtained by reactive droop control calculation after the amplitude zero-difference compensation term is introduced.

[0163] According to equations (18) and (20), the improved reactive loop control block diagram after introducing the adaptive virtual voltage drop correction term and the amplitude difference compensation term can be obtained, as shown in Figure 8 Bus voltage amplitude U pcc The change of will drive the inverter output voltage U to change, and U will react to adjust U pcc , until it stabilizes at the rated value U *pcc .

[0164] According to formula (20), the amplitude compensation coefficient k u The value of will affect the dynamic process of the reactive power control loop. The method of selecting the virtual voltage drop correction coefficient is also adopted. The influence of the parameters on the system stability and dynamic performance is compromised. Finally, the amplitude compensation coefficient k is taken u =20.

[0165] In order to verify the effectiveness of the improved virtual impedance strategy proposed in this manual, the parallel operation conditions of two inverters of the same capacity under the scenarios of nonlinear load access and line impedance change were simulated on the Matlab / Simulink platform. The main simulation parameters are shown in Table 1.

[0166] Table 1 Main parameters of droop control

[0167]

[0168]

[0169] Example: Verification of reactive power sharing and voltage compensation effect under line impedance change scenario

[0170] This manual simulates the operating conditions of two parallel inverters of the same capacity when the line impedance changes, and compares the traditional virtual impedance strategy, the adaptive virtual impedance strategy and the strategy proposed in this manual. The model structure is as follows: Fig. 9 As shown, where L f and C f Respectively represent the inductance and capacitance in the LC filter, Z1 and Z2 represent the line impedance. The reactive power and bus voltage amplitude waveforms under the three strategies are shown as follows: Fig.10 and Fig.11 shown. Fig.10 (a) describes the reactive power curve under the traditional virtual impedance strategy; Fig.10 (b) describes the reactive power curve under the adaptive virtual impedance strategy; Fig.10 (c) describes the reactive power curve of the strategy proposed in the present invention.

[0171] according to Fig.10 It can be seen that the traditional virtual impedance strategy has a good reactive power sharing effect during t=0-2s, but the equivalent line impedance changes after the local load is connected at t=2s, so the reactive power sharing effect is not ideal. The adaptive virtual impedance strategy and the strategy proposed in this manual can always adjust the inverter output reactive power in real time when the line impedance changes, and the reactive power sharing accuracy is not affected by the line impedance change. Fig.11It can be seen that the bus voltage amplitude under the traditional virtual impedance strategy and the adaptive virtual impedance strategy deviates far from the rated value, and there is a bus voltage drop. The strategy proposed in this specification always maintains the bus voltage amplitude near the rated value under the scenarios of line impedance change and reactive load change, effectively compensating the bus voltage amplitude and improving the voltage quality of the island microgrid.

[0172] In summary, the strategy proposed in this specification can achieve adaptive reactive power sharing of parallel inverters while adjusting the bus voltage amplitude without difference, so that it is always maintained at the rated value.

[0173] The above-described embodiments are used to illustrate the present invention, not to limit the present invention, so changes in the exemplified values ​​or replacement of equivalent elements should still fall within the scope of the present invention.

[0174] From the above detailed description, it can be understood by those skilled in the art that the present invention can indeed achieve the aforementioned objectives and is in compliance with the provisions of the Patent Law.

[0175] Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they are aware of the basic creative concepts. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention. The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. It should be pointed out that any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0176] It should be noted that the above description of the relevant process is only for example and explanation, and does not limit the scope of application of this specification. For those skilled in the art, various modifications and changes can be made to the process under the guidance of this specification. However, these modifications and changes are still within the scope of this specification.

[0177] The basic concepts have been described above. Obviously, for those of ordinary skill in the art who have read this application, the above invention disclosure is only for example and does not constitute a limitation of this application. Although not explicitly stated here, those of ordinary skill in the art may make various modifications, improvements and amendments to this application. Such modifications, improvements and amendments are suggested in this application, so such modifications, improvements and amendments still belong to the spirit and scope of the exemplary embodiments of this application.

[0178] At the same time, the present application uses specific words to describe the embodiments of the present application. For example, "one embodiment", "an embodiment", and / or "some embodiments" refer to a certain feature, structure or characteristic related to at least one embodiment of the present application. Therefore, it should be emphasized and noted that "one embodiment" or "an embodiment" or "an alternative embodiment" mentioned twice or more in different positions in this specification does not necessarily refer to the same embodiment. In addition, some features, structures or characteristics in one or more embodiments of the present application can be appropriately combined.

[0179] In addition, it will be appreciated by those skilled in the art that various aspects of the present application may be illustrated and described by a number of patentable categories or situations, including any new and useful combination of processes, machines, products or substances, or any new and useful improvements thereto. Therefore, various aspects of the present application may be implemented entirely by hardware, entirely by software (including firmware, resident software, microcode, etc.), or by a combination of hardware and software. The above hardware or software may all be referred to as "units", "modules" or "systems". In addition, various aspects of the present application may take the form of a computer program product embodied in one or more computer-readable media, wherein computer-readable program code is contained therein.

[0180] The computer program code required for the operation of each part of the application can be written in any one or more programming languages, including object-oriented programming languages ​​such as Java, Scala, Smalltalk, Eiffel, JADE, Emerald, C++, C#, VB.NET, Python, etc., such as C programming language, VisualBasic, Fortran2103, Perl, COBOL2102, PHP, ABAP, such as Python, Ruby and Groovy dynamic programming language or other programming languages. The program code can be run completely on the user's computer, or run on the user's computer as an independent software package, or run partly on the user's computer and partly on the remote computer, or run completely on the remote computer or server. In the latter case, the remote computer can be connected to the user's computer through any network form, such as a local area network (LAN) or a wide area network (WAN), or connected to an external computer (e.g., via the Internet), or in a cloud computing environment, or used as a service such as software as a service (SaaS).

[0181] In addition, unless explicitly stated in the claims, the order of the processing elements and sequences described in this application, the use of alphanumeric characters, or the use of other names are not intended to limit the order of the processes and methods of this application. Although the above disclosure discusses some invention embodiments that are currently considered useful through various examples, it should be understood that such details are only for illustrative purposes, and the attached claims are not limited to the disclosed embodiments. On the contrary, the claims are intended to cover all modifications and equivalent combinations that are consistent with the essence and scope of the embodiments of this application. For example, although the implementation of the various components described above can be embodied in a hardware device, it can also be implemented as a pure software solution, for example, installation on an existing server or mobile device.

[0182] Similarly, it should be noted that in order to simplify the description disclosed in this application and thus help understand one or more embodiments of the invention, in the above description of the embodiments of the application, multiple features are sometimes combined into one embodiment, figure or description thereof. However, this method of the application should not be interpreted as reflecting the intention that the claimed object requires more features than those explicitly stated in each claim. On the contrary, the subject of the invention should have fewer features than the above single embodiment.

Claims

1. An inverter parallel operation control method with adaptive reactive power sharing, characterized in that: include: Introducing virtual impedance to match line impedance with inverter capacity, adjusting inverter output voltage in the form of virtual voltage drop together with voltage reference value generated by droop control, and performing adaptive virtual voltage drop correction; Among them, the reactive power ratio of parallel inverters is expressed as follows: Considering the line impedance change scenario, the adaptive virtual voltage drop correction item meets the following conditions: In the formula and represent the adaptive virtual voltage drop compensation items of the first and second inverters respectively; ΔX1 and ΔX2 represent the line impedance changes of the first and second inverters respectively; Q1 and Q2 represent the reactive power output by the first and second inverters respectively; X1 and X2 represent the line impedance of the first and second inverters respectively; U pcc represents the bus voltage amplitude; U1 and U2 represent the output voltage amplitudes of the first and second inverters respectively; Q ref1 and Q ref2 Respectively represent the reactive power set for the first and second inverters; The improved reactive power droop equation after introducing the adaptive virtual voltage drop correction term is expressed as: Where Q i represents the actual value of reactive power sharing of the i-th inverter; represents the adaptive virtual voltage drop compensation item of the i-th inverter; k represents the index value at which the summation starts; represents the theoretical value of reactive power sharing of the i-th inverter; k vi represents the virtual voltage drop correction coefficient of the i-th inverter; U i_droop represents the inverter droop control voltage amplitude reference value in the dq coordinate system; It indicates that the d-axis voltage reference value is obtained after droop calculation; It indicates that the q-axis voltage reference value is obtained after droop calculation; is the reference value of the d-axis voltage amplitude of the inverter droop control; ω is the angular frequency; L v represents the virtual inductance parameter; i gq is the line q-axis current; is the reference value of the q-axis voltage amplitude for inverter droop control; i gd is the line d-axis current; n is the reactive power droop coefficient; U n Indicates the grid voltage rating for droop control; Q refi represents the reactive setting power of the droop control of the i-th inverter; s represents the Laplace operator; Then the small signal model under the first-order dynamic phasor model is expressed as: Where ΔQ i is the output reactive power change of the i-th inverter; ΔU i is the output voltage amplitude change of the i-th inverter; L i represents the circuit inductance between the i-th inverter and the common bus; ΔU n is the change in the rated value of the grid voltage; λ is the reactive power regulation coefficient; Q refk is the reactive power setting of droop control of k inverters; i is the reactive power regulation coefficient of the i-th inverter; R i The resistance representing the line impedance between the ith inverter and the common load; ω c Represents the cutoff frequency of the low-pass filter; Then the small signal block diagram of the reactive power control loop of the i-th inverter is obtained after the introduction of the adaptive virtual voltage drop correction term, and its corresponding transfer function is expressed as: The subscript i is omitted, and the parameters are as follows: b1, b0, A, B, C, D, and E are all transfer function coefficients; R represents the resistance of the line impedance between the i-th inverter and the common load; k v is the virtual pressure drop correction coefficient; the virtual pressure drop correction coefficient k is obtained according to the transfer function v Root locus diagram of the reactive loop when changes.

2. The inverter parallel operation control method with adaptive reactive power sharing according to claim 1, characterized in that: Considering the virtual pressure drop correction factor k v The impact on the dynamic process of the reactive loop, the small signal model of the virtual voltage drop correction term of the first inverter is expressed as: represents the change of the adaptive virtual voltage drop compensation item of the first inverter; λ1 represents the reactive power regulation coefficient of the first inverter; k v1 is the virtual voltage drop correction factor of the first inverter; ΔQ1 represents the reactive power change of the first inverter; The inverter output reactive power after the virtual voltage drop correction term is introduced into the first-order low-pass filter is expressed as: X represents the line impedance of the inverter; U 1n The grid voltage rating for the droop control of the first inverter; represents the adaptive virtual voltage drop compensation item of the first inverter; The small signal model of the inverter output reactive power is expressed as: ΔQ1 represents the reactive power change of the first inverter; Indicates the change in the adaptive virtual voltage drop compensation item of the first inverter; According to the small signal model of the virtual voltage drop correction term of the first inverter and the small signal model of the inverter output reactive power, the transfer function block diagram after the introduction of the virtual voltage drop correction term is obtained, and the corresponding transfer function is: ω n is the natural oscillation frequency, and ζ is the system damping coefficient.

3. The inverter parallel operation control method with adaptive reactive power sharing according to claim 2, characterized in that: An adaptive amplitude compensation term is applied to the inverter output voltage to achieve error-free regulation of the bus voltage amplitude without affecting the reactive power sharing accuracy, so as to eliminate the bus voltage drop caused by virtual impedance. The relationship between the output reactive power and the output voltage of the parallel inverter is expressed as: U is the output voltage; Q is the inverter output reactive power; Since adjusting the virtual impedance will affect the power decoupling effect of the inverter, in order to eliminate the bus voltage drop caused by the virtual impedance, an amplitude-invariant compensation term is applied to the output voltage U. The bus voltage amplitude after compensation should satisfy: Indicates the amplitude indifference compensation term, U * pcc Indicates the bus voltage reference value; The reactive power droop equation after the introduction of the amplitude difference compensation term is: Where k ui represents the amplitude compensation coefficient of the i-th inverter; represents the amplitude invariant compensation term of the i-th inverter; It indicates that the d-axis voltage reference value is obtained by reactive droop control calculation after the amplitude difference compensation term is introduced; It indicates that the q-axis voltage reference value is obtained by reactive droop control calculation after the amplitude difference compensation term is introduced; Then, the improved reactive loop control block diagram after introducing the adaptive virtual voltage drop correction term and the amplitude difference compensation term is obtained. The bus voltage amplitude U pcc The change of will drive the inverter output voltage U to change, and U will react to adjust U pcc , until it stabilizes at the rated value U * pcc .

4. The inverter parallel operation control method with adaptive reactive power sharing according to claim 3, characterized in that: Virtual pressure drop correction factor k v1 The value of is 0.

8.

5. The inverter parallel operation control method with adaptive reactive power sharing according to claim 4, characterized in that: Amplitude compensation coefficient k u The value of is 20.

6. The inverter parallel operation control method with adaptive reactive power sharing according to claim 5, characterized in that: The process of obtaining the expression of the reactive power ratio of parallel inverters is as follows: The output power of the two inverters is: Where P i represents the active power output by the i-th inverter; Q i represents the reactive power output by the i-th inverter; U i represents the output voltage amplitude of the i-th inverter; θ i represents the output voltage phase angle of the i-th inverter; α i represents the impedance angle of the i-th line, satisfying α i =arctan(X i / R i );R i , X i denote the resistance and reactance of the line impedance between the i-th inverter and the common load respectively; At high voltage levels, the line impedance is approximately inductive, satisfying α≈90°. During steady-state operation, the inverter output power angle is small, with sinθ≈θ and cosθ≈1. The above formula is simplified to: When the line impedance is inductive, the droop control is expressed as: Where f and f n Respectively represent the droop control frequency reference value and the power frequency; U and U n They represent the voltage amplitude reference value of droop control and the grid voltage rated value respectively; m, n represent active and reactive droop coefficients respectively; P ref and Q ref Indicates the set active power and reactive power; P and Q indicate the active power and reactive power output by the inverter; The voltage reference value is obtained by calculating the active and reactive power droop and The power transfer characteristics of the inverter based on droop control are: Where P(s) represents the active power output by the inverter in the frequency domain; Q(s) represents the reactive power output by the inverter in the frequency domain; s represents the Laplace operator.

7. The inverter parallel operation control method with adaptive reactive power sharing according to claim 6, characterized in that: Reactive power sharing should meet the following conditions: Among them U i 、n i , Q refi , X i They respectively represent the voltage amplitude reference value of the droop control of the i-th inverter, the reactive power droop coefficient, the reactive power setting power, and the reactance of the line impedance between the inverter and the common load.

8. The inverter parallel operation control method with adaptive reactive power sharing according to claim 7, characterized in that: The virtual impedance introduced when achieving reactive power sharing should satisfy the following relationship: Where, X i Indicates line reactance; X vi Represents the introduced virtual impedance.

9. The inverter parallel operation control method with adaptive reactive power sharing according to claim 8, characterized in that: In the dq coordinate system, the reactive loop control equation based on virtual impedance is expressed as: Where U i_droop represents the inverter droop control voltage amplitude reference value in the dq coordinate system; It indicates that the d-axis voltage reference value is obtained after droop calculation; The d-axis voltage amplitude reference value for inverter droop control; ω is the angular frequency; L v represents the virtual inductance parameter; i gq is the line q-axis current; is the reference value of the q-axis voltage amplitude for inverter droop control; i gd is the line d-axis current.

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