Virtual impedance droop control method based on corrected output power

Abstract

The application provides a virtual impedance droop control method based on corrected output power, and belongs to the technical field of inverter control, and the method comprises the following steps: S1, signal acquisition and calculation; S2, virtual impedance voltage correction and corrected power calculation; S3, voltage reference value generation; and S4, voltage and current inner loop control. The application realizes accurate load power sharing by taking into account the virtual impedance loss through the voltage correction link.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of inverter control, in particular to a virtual impedance droop control method based on modified output power. BACKGROUND

[0002] The control strategy of the inverter can be divided into two kinds of communication-dependent control strategy and communication-independent control strategy, wherein the communication-independent control strategy is usually based on the droop characteristics of the inverter, that is, droop control. Droop control is first used for the control of parallel uninterruptible power supply (UPS). In order to reduce the cost and improve the reliability and complexity of the system, it is often necessary to avoid introducing communication lines in the parallel inverter power control. Based on the droop control, the parallel inverter can be realized without information interaction, and the reliability is high, and the plug-and-play of the inverter and the load is easy to realize. Therefore, the droop control is widely used in the control of the inverter. When using the droop control, by simulating the characteristics of the synchronous generator, the inverter can achieve natural current sharing through its external characteristics, thereby avoiding information interaction between the inverter units.

[0003] The droop control can be divided into a power outer loop and a voltage and current inner loop. The power outer loop calculates the output power of the inverter, and then determines the reference value of the output voltage of the inverter according to the droop equation; the voltage and current inner loop makes the inverter output voltage quickly track the reference value of the inverter output voltage in the dq coordinate system through the corresponding controller. Through the voltage and current double-loop control, the output voltage of the inverter can quickly track the reference value of the inverter output voltage in the dq coordinate system, which is the process of the droop control.

[0004] In order to improve the effect of reactive power distribution of the parallel inverter in the case of line impedance mismatch, one of the common methods is to add a virtual impedance link in the control of the inverter to change the equivalent output impedance of the inverter. The existing virtual impedance droop control improves the power distribution under line impedance mismatch by introducing a virtual impedance, but the internal potential calculation deviation and power distribution error are caused due to the neglect of the virtual impedance power loss. SUMMARY

[0005] The present application provides a virtual impedance droop control method based on modified output power, which realizes accurate load power sharing by taking into account the virtual impedance loss through a voltage modification link.

[0006] To achieve the above purpose, the present application adopts the following technical scheme:

[0007] The virtual impedance droop control method based on modified output power comprises:

[0008] S1. Signal acquisition and calculation; the output voltage signal and the output current signal of the inverter are acquired, coordinate transformation processing is performed, and the output voltage dq axis component and the output current dq axis component are obtained;

[0009] S2. Virtual impedance voltage correction and corrected power calculation; obtain preset virtual resistance and virtual inductance parameters, calculate dq-axis components of virtual impedance voltage drop based on output current dq-axis components and virtual impedance parameters; perform voltage correction processing through output voltage dq-axis components and virtual impedance voltage drop dq-axis components to obtain internal potential dq-axis components; calculate corrected active power and corrected reactive power based on internal potential dq-axis components and output current dq-axis components;

[0010] S3. Voltage reference value generation; compare corrected active power and corrected reactive power with rated active power and rated reactive power, and generate inverter output voltage reference value through droop control algorithm processing;

[0011] S4. Voltage and current inner loop control; compare inverter output voltage reference value with actual output voltage dq-axis components, and generate pulse width modulation signal through voltage and current double loop controller processing to control inverter switching device action.

[0012] In the specification, the virtual impedance droop control method based on corrected output power further comprises: S5. Parallel system power distribution verification; in the parallel connection scenario of multiple inverters, monitor the corrected active power and corrected reactive power of each inverter, and verify the balance of load power distribution among the inverters.

[0013] In the specification, the coordinate transformation processing in S1 is specifically: first, convert three-phase output voltage signals and three-phase output current signals into signals in the αβ coordinate system through Clark transformation, and then convert them into output voltage dq-axis components and output current dq-axis components in the dq coordinate system through Park transformation.

[0014] In the specification, the virtual resistance and virtual inductance parameters in S2 are obtained by: according to the rated capacity of the inverter and the line impedance matching requirement, the values of the virtual resistance and the virtual inductance are preset to match the equivalent output impedance of the inverter with the capacity of the inverter. The output impedance of the inverter is composed of its virtual impedance and line impedance, and by setting the size of its virtual impedance, the output impedance of the inverter can be inversely proportional to its capacity (i.e. matched).

[0015] In the specification, the calculation of dq-axis components of virtual impedance voltage drop in S2 is specifically: based on the equivalent impedance model of virtual impedance, combine the output current dq-axis components and the system angular frequency to calculate the dq-axis components of virtual impedance voltage drop.

[0016] In the specification, the voltage correction process in S2 is specifically: adding the d-axis voltage and the q-axis voltage in the output voltage dq-axis component to the d-axis component and the q-axis component of the virtual impedance voltage drop respectively to obtain the d-axis internal potential and the q-axis internal potential in the internal potential dq-axis component.

[0017] In the specification, the droop control algorithm in S3 is specifically: adjusting the frequency and the amplitude of the inverter output voltage reference value through the active droop coefficient and the reactive droop coefficient according to the deviation of the corrected active power from the rated active power and the deviation of the corrected reactive power from the rated reactive power.

[0018] In the specification, the voltage and current double-loop controller in S4 includes a voltage outer loop controller and a current inner loop controller: the voltage outer loop controller performs proportional integral adjustment on the deviation of the output voltage reference value from the actual output voltage dq-axis component to generate a current inner loop reference value; and the current inner loop controller performs proportional integral adjustment on the deviation of the current inner loop reference value from the output current dq-axis component to generate a pulse width modulation signal.

[0019] In the specification, the control of the action of the inverter switching device in S4 is specifically: inputting the generated pulse width modulation signal into the driving circuit of the inverter to control the conduction and the turn-off of the switching device, so that the inverter output voltage tracks the output voltage reference value.

[0020] In the specification, the calculation method of the corrected active power and the corrected reactive power is:

[0021] ;

[0022] is the corrected active power, is the corrected reactive power, are the d-axis and q-axis components of the internal potential respectively, are the d-axis and q-axis components of the inverter output current respectively.

[0023] In summary, the present application has at least the following beneficial effects:

[0024] The present application introduces a virtual impedance droop control strategy for correcting the output power, effectively solves the problem that the virtual impedance power loss is ignored in the prior art, and significantly improves the power distribution accuracy and system stability of the parallel inverter. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0026] Figure 1 This is a schematic diagram of the virtual impedance droop control method based on modified output power involved in this invention.

[0027] Figure 2 This is a schematic diagram of the droop control inverter involved in this invention.

[0028] Figure 3 This is a schematic diagram of the equivalent model of the droop control inverter involved in this invention.

[0029] Figure 4 This is a schematic diagram of the voltage and current inner loop control block diagram involved in this invention.

[0030] Figure 5 This is a schematic diagram of the droop control block diagram after introducing virtual impedance involved in this invention.

[0031] Figure 6a This is a schematic diagram of the equivalent model of the inverter before the introduction of virtual impedance involved in this invention.

[0032] Figure 6b This is a schematic diagram of the equivalent model of the inverter after introducing virtual impedance involved in this invention.

[0033] Figure 7 This is a schematic diagram of the block diagram of the virtual impedance droop control inverter based on modified output power involved in this invention.

[0034] Figure 8 This is a schematic diagram of the simulation circuit involved in this invention.

[0035] Figure 9a The inverter output power in the simulation results of the existing virtual impedance droop control involved in this invention. A schematic diagram.

[0036] Figure 9b The line impedance power loss in the simulation results of the existing virtual impedance droop control involved in this invention. A schematic diagram.

[0037] Figure 9c The power distribution results in the simulation results of the existing virtual impedance droop control involved in this invention. A schematic diagram.

[0038] Figure 9d The inverter output power in the simulation results of the existing virtual impedance droop control involved in this invention. A schematic diagram.

[0039] Figure 9eFig. 1 is a schematic diagram of line impedance power loss in simulation results of the existing virtual impedance droop control involved in the present application.

[0040] Figure 9f Fig. 2 is a schematic diagram of power distribution results in simulation results of the existing virtual impedance droop control involved in the present application.

[0041] Figure 10a Fig. 3 is a schematic diagram of modified output power in simulation results of the virtual impedance droop control based on modified output power involved in the present application.

[0042] Figure 10b Fig. 4 is a schematic diagram of output impedance power loss in simulation results of the virtual impedance droop control based on modified output power involved in the present application.

[0043] Figure 10c Fig. 5 is a schematic diagram of power distribution results in simulation results of the virtual impedance droop control based on modified output power involved in the present application.

[0044] Figure 10d Fig. 6 is a schematic diagram of modified output power in simulation results of the virtual impedance droop control based on modified output power involved in the present application.

[0045] Figure 10e Fig. 7 is a schematic diagram of output impedance power loss in simulation results of the virtual impedance droop control based on modified output power involved in the present application.

[0046] Figure 10f Fig. 8 is a schematic diagram of power distribution results in simulation results of the virtual impedance droop control based on modified output power involved in the present application.

[0047] Figure 11a Fig. 9 is a schematic diagram of A-phase loop current simulation waveform when the existing virtual impedance droop control is adopted.

[0048] Figure 11b Fig. 10 is a schematic diagram of A-phase loop current simulation waveform when the virtual impedance droop control based on modified output power is adopted. DETAILED DESCRIPTION

[0049] ​​​​​​​​In the following, only certain exemplary embodiments are simply described. As those skilled in the art can recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the embodiments of the present application. Therefore, the drawings and description are to be regarded as being illustrative in nature rather than restrictive.

[0050] The following disclosure provides many different embodiments, or examples, for implementing different structures of the embodiments of the present application. For the purpose of simplicity and clarity, the descriptions of the specific examples in the following text are described. Of course, they are only examples, and the purpose is not to limit the embodiments of the present application. In addition, the embodiments of the present application can refer to the same reference numerals and / or reference letters in different examples, and such repetition is for the purpose of simplification and clarity, which does not indicate the relationship between the various embodiments and / or settings discussed.

[0051] The embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0052] As Figure 1 shown, the embodiments provide a virtual impedance droop control method based on corrected output power, including:

[0053] S1. Signal acquisition and calculation; acquire inverter output voltage signal and output current signal, perform coordinate transformation processing, obtain output voltage dq axis component and output current dq axis component;

[0054] S2. Virtual impedance voltage correction and corrected power calculation; obtain the preset virtual resistance and virtual inductance parameters, calculate the dq axis component of the virtual impedance voltage drop according to the output current dq axis component and the virtual impedance parameters; through the output voltage dq axis component and the dq axis component of the virtual impedance voltage drop, voltage correction processing is performed to obtain the internal potential dq axis component; according to the internal potential dq axis component and the output current dq axis component, the corrected active power and the corrected reactive power are calculated;

[0055] S3. Voltage reference value generation; compare the corrected active power, the corrected reactive power with the rated active power, the rated reactive power, and generate the inverter output voltage reference value through the droop control algorithm processing;

[0056] S4. Voltage and current inner loop control; compare the inverter output voltage reference value with the actual output voltage dq axis component, and generate the pulse width modulation (PWM) signal through the voltage and current double loop controller processing to control the inverter switching device action;

[0057] S5. Parallel system power distribution verification; in the parallel connection scene of multiple inverters, monitor the corrected active power and the corrected reactive power of each inverter, and verify the balance of the load power distribution among the inverters.

[0058] In some embodiments, the virtual impedance droop control method based on the corrected output power further comprises: S5. verifying in parallel with the system power distribution; in the parallel connection scenario of multiple inverters, monitoring the corrected active power and the corrected reactive power of each inverter, and verifying the balance of the load power distribution among the inverters.

[0059] In some embodiments, the coordinate transformation process in S1 is specifically: first converting the three-phase output voltage signal and the three-phase output current signal into signals in the αβ coordinate system through Clark transformation, and then converting the output voltage dq-axis components and the output current dq-axis components in the dq coordinate system through Park transformation.

[0060] In some embodiments, the virtual resistance and virtual inductance parameters in S2 are obtained by: according to the rated capacity of the inverter and the line impedance matching requirement, the values of the virtual resistance and the virtual inductance are pre-set to match the equivalent output impedance of the inverter with the capacity of the inverter. The output impedance of the inverter is composed of its virtual impedance and line impedance, and by setting the size of the virtual impedance, the output impedance of the inverter can be inversely proportional to its capacity (i.e. matched).

[0061] In some embodiments, the dq-axis component of the virtual impedance voltage drop in S2 is specifically: based on the equivalent impedance model of the virtual impedance, the dq-axis component of the virtual impedance voltage drop is calculated in combination with the output current dq-axis component and the system angular frequency.

[0062] In some embodiments, the voltage correction process in S2 is specifically: adding the d-axis voltage and the q-axis voltage in the output voltage dq-axis component to the d-axis component and the q-axis component of the virtual impedance voltage drop, respectively, to obtain the d-axis internal potential and the q-axis internal potential in the internal potential dq-axis component.

[0063] In some embodiments, the droop control algorithm in S3 is specifically: according to the deviation of the corrected active power from the rated active power and the deviation of the corrected reactive power from the rated reactive power, the frequency and amplitude of the inverter output voltage reference value are adjusted through the active droop coefficient and the reactive droop coefficient.

[0064] In some embodiments, the voltage and current double-loop controller in S4 includes a voltage outer loop controller and a current inner loop controller: the voltage outer loop controller proportionally integrates the deviation between the output voltage reference value and the actual output voltage dq-axis component to generate a current inner loop reference value; the current inner loop controller proportionally integrates the deviation between the current inner loop reference value and the output current dq-axis component to generate a pulse width modulation signal.

[0065] In some embodiments, the controlling the action of the inverter switching device in S4 specifically comprises: inputting the generated pulse width modulation signal into a driving circuit of the inverter, controlling the turn-on and turn-off of the switching device, and making the inverter output voltage track the output voltage reference value.

[0066] In some embodiments, the method for calculating the corrected active power and the corrected reactive power is:

[0067] ;

[0068] is the corrected active power, is the corrected reactive power, are the d-axis and q-axis components of the internal electromotive force, respectively, are the d-axis and q-axis components of the inverter output current, respectively.

[0069] In this specification, the subscript ref represents a reference value, the subscript o represents an output value, bold represents a vector or a matrix, for example, the three-phase output voltage of the inverter is represented as , and the subscript dq represents a variable obtained by converting a three-phase variable into a dq coordinate system, for example, the inverter three-phase output current reference value

[0070] is converted into the dq coordinate system to obtain . The superscript T represents the transpose of a vector or a matrix, which is used to convert a row vector into a column vector.

[0071] The technical idea of the present application is as follows:

[0072] In the power outer loop, the calculation method of the output power is:

[0073] ;

[0074] wherein, and are the instantaneous active power and the instantaneous reactive power output by the inverter, respectively, are the instantaneous active power and the instantaneous reactive power output by the inverter, respectively, the dq-axis components of the inverter output voltage and the output current, which are obtained by coordinate

[0075] transforming the inverter output voltage and the output current; are the d-axis and q-axis components of , respectively, are the d-axis and q-axis components of , respectively;

[0076] The droop equation is:

[0077] ;

[0078] in, For Laplace variables, This is the reference value for angular frequency.

[0079] and These are the rated angular frequency and rated voltage, respectively. and These are the inverter's rated active power and rated reactive power, respectively. These are the active power droop factor and reactive power droop factor of the inverter, respectively. This is the cutoff frequency of the low-pass filter. That is, the inverter output voltage reference value in the dq coordinate system. for The d-axis and q-axis components.

[0080] Through dual-loop voltage and current control, the inverter's output voltage It can track quickly The above describes the process of droop control, and its flowchart is as follows. Figure 2 As shown, the inverter is connected to the Point of Common Coupling (PCC) via an LC filter and lines. Figure 2 middle and These are the filter inductor and filter capacitor, respectively, and the inverter output voltage. That is, the voltage across the filter capacitor.

[0081] The equivalent model of a droop-controlled inverter is as follows: Figure 3 As shown. Among them These represent the active and reactive power injected into the PCC of the inverter, respectively. These represent the active and reactive power losses along the line impedance, respectively. and These are the active and reactive power outputs of the inverter, respectively. Therefore, we have and Droop control is based on the power output of the inverter. and The reference amplitude and frequency of the inverter output voltage are calculated by the droop equation (Equation (2)) and the inverter output voltage is controlled to track the reference value.

[0082] Regarding voltage and current inner loop control: Voltage and current inner loop control is used to control the inverter output voltage (i.e., capacitor voltage), so that the output voltage... Fast Tracking Specifically, its block diagram is as follows: Figure 4 As shown. In steady state, .

[0083] Figure 3 This can be extended to the case of multiple droop-controlled inverters in parallel, in which case the n inverters share the load at the PCC (i.e. the load at the PCC is equal to the sum of the power output by all inverters minus the power loss on all line impedances). However, large differences in output voltage between parallel droop-controlled inverters can result in circulating currents, leading to reduced efficiency, equipment heating, and in extreme cases, system instability. Therefore, it is necessary to study the power distribution strategy for parallel droop-controlled inverters.

[0084] In order to achieve reasonable distribution of load power, the droop coefficient in equation (2) should generally be inversely proportional to the capacity of the inverter, i.e.

[0085] ;

[0086] where subscript i represents the i-th inverter, and subscript j represents the j-th inverter, and subscript k represents the k-th inverter. or represents the parameter of the i-th or j-th inverter, respectively, are the active droop coefficients of the i-th and j-th inverters, respectively, are the rated active powers of the i-th and j-th inverters, respectively, are the reactive droop coefficients of the i-th and j-th inverters, respectively, are the rated reactive powers of the i-th and j-th inverters. In addition, reasonable distribution of load power also requires that the line impedance of the inverter be matched, i.e. the capacity of the inverter is inversely proportional to the size of the corresponding line impedance: ; where,

[0087] respectively, are the line resistances of the i-th and j-th inverters,

[0088] ;

[0089] where, 、 respectively, are the line reactances of the i-th and j-th inverters.

[0090] ​​​​​​​However, the condition of formula (4) is usually difficult to meet, at this time, although the active power output by the inverter is proportional to its capacity, the distribution of the reactive power is greatly affected. In order to improve the effect of the reactive power distribution of the parallel inverters in the case of mismatched line impedance, one of the common methods is to add a virtual impedance link in the control of the inverter to change the equivalent output impedance of the inverter. However, the existing droop control with virtual impedance (hereinafter referred to as "virtual impedance droop control") ignores the power loss of the virtual impedance, so that the accurate sharing of the inverter power cannot be realized. The present application is aimed at the shortcoming of the existing virtual impedance droop control, and proposes a corresponding improvement method.

[0091] The droop control inverter can be equivalent to a controlled voltage source, and the voltage frequency and amplitude thereof satisfy formula (2). When a plurality of parallel inverters are connected in parallel, only when the line impedance satisfies formula (4), the power sharing can be realized. In order to improve the power distribution effect when formula (4) is not satisfied, a common method is to introduce a virtual impedance.

[0092] The most similar prior art implementation scheme to the present application is the virtual impedance droop control. Originally, the reference value of the output voltage of the droop control inverter is directly obtained by the droop equation (formula (2)); and if a virtual impedance of a certain size is introduced, the voltage drop on the virtual impedance needs to be calculated in real time, and the voltage drop is subtracted from the original output voltage reference value as a new reference value.

[0093] For convenience of discussion, hereinafter, the "voltage calculated by the droop equation" is referred to as "internal potential", denoted by the letter . As mentioned above, after the introduction of the virtual impedance, the droop equation (formula (2)) no longer directly obtains the output voltage reference value, but obtains the internal potential reference value:

[0094] ;

[0095] wherein, is the internal potential in the dq coordinate system, are the d-axis and q-axis components of , respectively;

[0096] The internal potential reference value minus the voltage drop of the virtual impedance obtains the output voltage reference value:

[0097] ;

[0098] wherein, and are the virtual resistance and virtual inductance, respectively, and are the dq-axis components of the voltage drop of the virtual impedance.

[0099] In formula (6), and The expression is derived as follows:

[0100] If the size of the introduced virtual impedance is , the voltage on the virtual impedance is formula (7):

[0101] ;

[0102] Transformed into the dq coordinate system, formula (8) is obtained:

[0103] ;

[0104] Wherein:

[0105] ;

[0106] The virtual impedance in the dq coordinate system, since the system reaches a steady state, is the direct current, so the differential term in formula (9) contributes little, and the differential term in formula (9) can be ignored, so formula (9) is simplified to formula (10):

[0107] ;

[0108] Therefore, formula (11) is obtained:

[0109] ;

[0110] The block diagram of the virtual impedance droop control is shown in Figure 5 , and a virtual impedance link is added compared to Figure 2 .

[0111] The inverter adopting the traditional droop control directly outputs the voltage obtained from formula (2); and after the introduction of the virtual impedance, the voltage obtained by the droop curve (i.e. the internal potential of the inverter ) is not directly output, but is subtracted by the voltage drop on the virtual impedance (an artificially calculated voltage drop) and then used as the output voltage of the inverter . Therefore, the output voltage of the inverter after the introduction of the virtual impedance is reduced by the voltage on the virtual impedance compared to before the introduction, which is equivalent to that the inverter is additionally connected in series with a virtual impedance. Therefore, is not a real existing voltage output by the inverter, but only exists in the control process (which also leads to the problems of the existing method described in the present application); that is, the output voltage of the inverter is still (the voltage on the filter capacitor), and is equal to minus the voltage drop of the virtual impedance.

[0112] After introducing the virtual impedance, the output impedance of the inverter is composed of the line impedance and the virtual impedance The matching condition of the inverter output impedance is changed from equation (4) to equation (12) after introducing the virtual impedance:

[0113] ;

[0114] wherein and are the virtual resistance of the first and the second inverter respectively, and are the virtual inductance of the first and the second inverter respectively. By introducing the virtual impedance, equation (12) is established, which can improve the power distribution effect of the inverter. According to equation (12), the size of the virtual impedance needs to be known in advance. If the size of the line impedance of the first

[0115] inverter is unknown, the estimation method is as shown in equation (13):

[0116] wherein and are the d-axis and q-axis components of the line impedance voltage drop of the first inverter respectively, and are the d-axis and q-axis components of the output voltage of the first inverter respectively, is the output voltage angular frequency of the first inverter. Equation (13) is derived as follows:

[0117] For the line impedance , by analogy with equations (7)-(11), the expression of the voltage drop in the dq coordinate system can be written as:

[0118] ;

[0119] Equation (13) can be solved by equation (14). In addition, and can be obtained by subtracting the PCC voltage from the inverter output voltage, that is:

[0120] ;

[0121] wherein and are the d-axis and q-axis components of the output voltage of the first inverter respectively, and are the d-axis and q-axis components of the voltage at the PCC in the dq coordinate system of the inverter, respectively. Combining equation (15) and equation (13), the line impedance of the inverter can be estimated, and then the size of the virtual impedance can be set.

[0122] However, the prior art has the disadvantage of ignoring the power consumed by the virtual impedance, so the virtual impedance introduced cannot be completely equivalent to the actual impedance. For example, for an inverter with a line impedance of , if part of the line impedance is replaced by a virtual impedance, its equivalent model changes from Figure 6a to Figure 6b , where Figure 6b is the virtual impedance element. It can be noted that since the output power of the inverter is calculated by measuring its output voltage and output current, after replacing

[0123] with a virtual impedance, the power loss on will not be included in the output power and (contrasting the positions of and Figure 6a in Figure 6b and ); and because the internal potential of the inverter is determined by the output power through the droop equation ( ), the neglect of the power loss of the virtual impedance will cause the deviation of the internal potential of the inverter, thereby affecting the effect of power distribution, i.e. the virtual impedance introduced by the prior art cannot be completely equivalent to the actual impedance. It can also be seen that in order to make the virtual impedance completely equivalent to the actual impedance, the corrected power Figure 5 and instead of and should be used for control, as in equation (16). is the active power corresponding to the corrected power (i.e. active power correction power), is the reactive power corresponding to the corrected power (i.e. reactive power correction power).

[0124] ;

[0125] The prior art envisages a virtual impedance in series with the inverter, and the output voltage of the inverter (i.e. the internal potential ​​) to simulate the virtual impedance; however, if the inverter is connected in series with an actually existing impedance, the impedance will not only affect the output voltage of the inverter due to voltage drop, but also affect the output voltage of the inverter due to power consumption. Therefore, the prior art only considers the effect of voltage drop on the virtual impedance on the output voltage of the inverter, but ignores the fact that the power consumed by the virtual impedance should also affect the output voltage of the inverter. Therefore, the virtual impedance introduced by the prior art cannot be completely equivalent to the actually existing impedance, and ultimately the power of the inverter cannot be completely divided.

[0126] The neglect of the power loss of the virtual impedance in the prior art virtual impedance droop control will affect the accuracy of the inverter power distribution, so the present application improves the virtual impedance droop control by adding a voltage correction link, and the calculated corrected output power takes into account the power loss of the virtual impedance, thereby overcoming the shortcomings of the prior art.

[0127] The defects of the prior art are that the inverter output power and neglects the power loss of the virtual impedance and Therefore, the present application replaces and with corrected power and , as shown in equation (17):

[0128] ;

[0129] and can be calculated in the dq coordinate system by a method similar to equation (1):

[0130] ;

[0131] are the voltage drops of the virtual impedance in the dq coordinate system, the expressions of which have been derived (equation (11)). Substituting equation (18) into equation (17) gives:

[0132] ;

[0133] Since equation (1) is also calculated using , substituting equation (1) into equation (19) can further simplify:

[0134] ;

[0135] It can be seen that the calculation of the corrected power only needs to replace in equation (1) with and Therefore, from the perspective of simplifying control, it can be started by... and Calculated and (Voltage correction) Recalculate power, instead of calculating and and and Then add them together. From equation (11), and The calculation method is as follows:

[0136] ;

[0137] The power calculation method was subsequently revised as follows:

[0138] ;

[0139] Therefore, the virtual impedance droop control strategy based on modified output power proposed in this invention is as follows: Figure 7 As shown. Compared to existing droop control that introduces virtual impedance, the strategy proposed in this invention adds a voltage correction step before calculating the inverter output power, so that the calculated corrected output power includes the power consumed by the virtual impedance. It is worth mentioning that the strategy proposed in this invention still requires the line impedance to be known, and if the line impedance is unknown or its magnitude changes, its magnitude needs to be calculated according to equation (13).

[0140] The effectiveness of the proposed improvement strategy is demonstrated by comparing the performance of existing virtual impedance droop control and the virtual impedance droop control based on modified output power proposed in this invention through simulations on the MATLAB / Simulink platform. The two inverters are set to have equal capacity, and the simulation circuits and parameters are as follows: Figure 8 As shown in Table 1, it can be observed that the sum of the line impedance and the virtual impedance of the two inverters is equal at this time. During simulation time The active and reactive loads stepped to 4kW and 1.5kvar, respectively.

[0141] Table 1. Simulation Parameters

[0142]

[0143] When using existing virtual impedance droop control, the simulation results are as follows: Figure 9a , Figure 9b , Figure 9c , Figure 9d , Figure 9e , Figure 9fThe load voltage is lower than the rated value and the load is also less than the rated value due to the droop characteristics and voltage drop on the line impedance. As mentioned above, the difference between the inverter output power and the power loss on the line impedance under the existing strategy is the load power allocated by the inverter. Since the frequencies of the output voltages of the two inverters are the same when the system reaches steady state and the droop curves of the two inverters are also the same, the active output powers of the two inverters are the same; however, the active load powers allocated by the two inverters are not equal due to the different active power consumed on the line impedance. In addition, the output voltage of inverter 1 is too high due to the difference in the line impedance, which results in that the reactive load power allocated by inverter 1 is more than that of inverter 2, so the existing strategy cannot achieve the power sharing of the inverters.

[0144] When the virtual impedance droop control based on the modified output power is adopted, the simulation results are as shown in Figure 10a 、 Figure 10b 、 Figure 10c 、 Figure 10d 、 Figure 10e 、 Figure 10f As mentioned above, under the improved strategy, the difference between the modified output power and the power loss on the output impedance is the load power allocated by the inverter. It can be known from Figure 10a to Figure 10f that the modified output powers of the two inverters can remain consistent and the power losses on the output impedances of the inverters are also the same when the system reaches steady state, so the accurate allocation of the load power is achieved, and the simulation results are also consistent with the above analysis.

[0145] In addition, the inverter A phase current simulation waveforms under the existing strategy and the improved strategy are as shown in Figure 11a and Figure 11b It can be seen that the strategy proposed in the present application achieves the accurate allocation of the inverter power, thereby effectively suppressing the size of the inverter inter-loop current.

[0146] The elimination of the loop current can be understood as that the elimination of the loop current and the power sharing are equivalent. Specifically as follows:

[0147] The power injected by the i-th inverter into the point of common coupling (PCC) is:

[0148] ;

[0149] The power calculation here is carried out in the dq coordinate system (indicated by subscript D or Q) of the PCC voltage, and are the active and reactive powers injected by the i-th inverter into the PCC, is the d-axis component of the PCC voltage in its dq coordinate system, and are the active and reactive powers injected by the i-th inverter into the PCC, is the d-axis component of the PCC voltage in its dq coordinate system, The d and q axis components of the output currents of the inverters in the dq frame of the PCC voltage, where they affect the active and reactive power, respectively.

[0150] If the two inverters have the same capacity, the power injected into the PCC by them should be equal, which means that the d and q axis components of the output currents of the inverters in the dq frame of the PCC voltage should be equal, i.e. and , which means that the output currents of the two inverters should be equal, so that the power injected into the PCC by them is equal, which indirectly eliminates the circulating current. If the capacity of inverter 1 is twice that of inverter 2, the power injected into the PCC by inverter 1 should be twice that of inverter 2, which is equivalent to controlling the output current of inverter 1 to be twice that of inverter 2, which also eliminates the (modified) circulating current. In summary, by controlling the power injected into the PCC by the inverters, the circulating current between the inverters can be naturally eliminated without measuring the circulating current.

[0151] The simulation proves that the method herein can achieve the complete equalization of the power injected into the PCC by the inverters, i.e. the output currents of the two inverters are completely equal, i.e. the circulating current is equal to 0 (in the case of the two inverters with the same capacity discussed herein).

[0152] The setting and adjustment method of the virtual impedance is illustrated below.

[0153] Taking the parameters used in the simulation herein as an example, it is known in advance that the line resistance and line inductance of inverter 1 are and , the line resistance and line inductance of inverter 2 are and , and the capacities of the two inverters are equal, so according to formula (12), after setting the virtual impedance, the two inverters should satisfy:

[0154] ;

[0155] Here, the virtual impedance shown in Table 1 is taken, i.e. the virtual resistance and inductance of inverter 1 are 0 and , respectively, and the virtual resistance and inductance of inverter 2 are and , respectively, so that the output resistance of the two inverters is , and the output inductance is .

[0156] ;

[0157] The simulation has proved that the power injected by the two inverters into the PCC is exactly the same at this time, that is, there is no circulating current. On the contrary, if there is circulating current or power distribution error, it is only because the output impedance does not match as expected due to the error of the line impedance parameter.

[0158] Real-time comparison of the consistency of the modified active power and the modified reactive power of each inverter, if the difference exceeds the preset threshold, it is determined that the power distribution is uneven;

[0159] Monitor the circulating current between each inverter, if the effective value of the circulating current exceeds the preset threshold, the virtual impedance parameter adjustment mechanism is triggered;

[0160] If the preset threshold of power distribution difference is set to 5%, the virtual impedance adjustment mechanism is started when the difference between the active or reactive power injected by the two inverters into the PCC exceeds 5% of the rated power. At this time, the source of the power distribution error can only be that the line impedance has an error or changes. For example, re-measure the line impedance according to formula (13), find that the line impedance of inverter 2 has no error, but the line resistance and inductance of inverter 1 are actually and Instead of and , you can reset the virtual resistance and virtual inductance of inverter 1 to and , respectively, so as to ensure that the output resistance of the two inverters is , and the output inductance is , thereby ensuring power sharing. Of course, the output impedance can also be set to other values, but it should not be too small (may cause instability) or too large (cause PCC voltage to be excessively reduced).

[0161] ;

[0162] Similarly, if the capacity of inverter 2 is twice that of inverter 1, the output impedance of the latter should be set to twice that of the former (for example, the former is , , and the latter is , ), and the virtual impedance size should be set by subtracting the line impedance from the output impedance.

[0163] In summary, it has been proved by simulation that the method proposed here can achieve complete power sharing (i.e. eliminate circulating current) in the case of output impedance matching. If there is error, it can only be because the known line impedance parameters do not match the actual ones, for example, there is error or the wiring has changed. In practice, the line impedance generally does not change, and how to reduce the error in line impedance measurement is beyond the scope of this discussion, and the invention still takes the known line impedance as a prerequisite.

[0164] The above-described embodiments are intended to illustrate the present application, and are not intended to limit the present application, so the change of example values or the replacement of equivalent elements should still belong to the scope of the present application.

[0165] From the above detailed description, it can be seen by those skilled in the art that the present application can achieve the above-mentioned purposes, and has met the requirements of the Patent Law.

[0166] Although the preferred embodiments of the present application have been described, those skilled in the art can make further changes and modifications to these embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including all the preferred embodiments and all the changes and modifications falling within the scope of the present application. The above description is only the preferred embodiments of the present application and is not intended to limit the present application. It should be noted that any modification, equivalent replacement and improvement made within the spirit and principle of the present application should be included in the protection scope of the present application.

[0167] It should be noted that the above description of the flow is only for example and illustration, and does not limit the scope of the present application. Those skilled in the art can make various modifications and changes to the flow under the guidance of the present application. However, these modifications and changes are still within the scope of the present application.

Claims

1. A method of virtual impedance droop control based on modified output power, characterized in that, Comprise: S1. Signal acquisition and calculation; Acquire inverter output voltage signal and output current signal, perform coordinate transformation processing to obtain output voltage dq axis component and output current dq axis component; S2. Virtual impedance voltage correction and corrected power calculation; obtain preset virtual resistance and virtual inductance parameters, calculate virtual impedance voltage drop dq axis component according to output current dq axis component and virtual impedance parameters; perform voltage correction processing through output voltage dq axis component and virtual impedance voltage drop dq axis component to obtain internal potential dq axis component; According to the internal potential dq axis component and the output current dq axis component, the corrected active power and the corrected reactive power are calculated; S3. Voltage reference value generation; Compare the corrected active power, the corrected reactive power with the rated active power, the rated reactive power, and generate the inverter output voltage reference value through the droop control algorithm processing; S4. Voltage and current inner loop control; compare the inverter output voltage reference value with the actual output voltage dq axis component, and generate the pulse width modulation signal through the voltage and current double loop controller processing to control the inverter switch device action.

2. The modified output power based virtual impedance droop control method according to claim 1, wherein Also include: S5. Parallel system power distribution verification; In the parallel inverter scene, monitor the corrected active power and the corrected reactive power of each inverter, and verify the balance of load power distribution among inverters.

3. The modified output power based virtual impedance droop control method of claim 1, wherein, The coordinate transformation processing in S1 is specifically: first convert the three-phase output voltage signal and the three-phase output current signal into signals in the αβ coordinate system through Clark transformation, and then convert them into output voltage dq axis component and output current dq axis component in the dq coordinate system through Park transformation.

4. The modified output power based virtual impedance droop control method of claim 1, wherein, The virtual resistance and virtual inductance parameters in S2 are obtained by: according to the rated capacity of the inverter and the line impedance matching requirement, the values of the virtual resistance and the virtual inductance are preset to match the equivalent output impedance of the inverter with the capacity of the inverter.

5. The modified output power based virtual impedance droop control method of claim 1, wherein, The calculation of the dq axis component of the virtual impedance voltage drop in S2 is specifically: based on the equivalent impedance model of the virtual impedance, the dq axis component of the virtual impedance voltage drop is calculated combined with the output current dq axis component and the system angular frequency.

6. The modified output power based virtual impedance droop control method of claim 1, wherein, The voltage correction processing in S2 is specifically: add the d-axis voltage and the q-axis voltage in the output voltage dq axis component to the d-axis component and the q-axis component of the virtual impedance voltage drop respectively to obtain the d-axis internal potential and the q-axis internal potential in the internal potential dq axis component.

7. The modified output power based virtual impedance droop control method of claim 1, wherein, The droop control algorithm in S3 is specifically: according to the deviation of the corrected active power and the rated active power, the deviation of the corrected reactive power and the rated reactive power, the frequency and amplitude of the inverter output voltage reference value are adjusted through the active droop coefficient and the reactive droop coefficient.

8. The modified output power based virtual impedance droop control method of claim 1, wherein, The voltage and current double loop controller in S4 includes a voltage outer loop controller and a current inner loop controller: the voltage outer loop controller performs proportional integral adjustment on the deviation of the output voltage reference value and the actual output voltage dq axis component to generate a current inner loop reference value; the current inner loop controller performs proportional integral adjustment on the deviation of the current inner loop reference value and the output current dq axis component to generate a pulse width modulation signal.

9. The modified output power based virtual impedance droop control method of claim 1, wherein, The control of the action of the inverter switching device in the S4 is specifically: inputting the generated pulse width modulation signal into a driving circuit of the inverter, controlling the turn-on and turn-off of the switching device, and making the inverter output voltage track the output voltage reference value.

10. The modified output power based virtual impedance droop control method of claim 1, wherein, The calculation method of the corrected active power and the corrected reactive power is: ； for correcting the active power, for correcting the reactive power, are the d- and q-axis components of the internal voltage, are the d- and q-axis components of the inverter output current.

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