A droop control method and device for an inverter

Abstract

The present application relates to the field of inverter control. The present application provides a droop control method and device for an inverter. By setting the droop control loop to be represented by the negative number of the ratio of the unit matrix to the voltage outer loop transfer function, the current output impedance of the inverter is updated to the target output impedance according to the voltage closed-loop transfer function and the droop coefficient of the droop control loop, so that the output impedance is adjusted to be controlled and adjusted by the droop coefficient, eliminating the influence of the control loop parameters and the output power on the closed-loop output impedance, reducing the adjustment conflict of the output impedance when the inverter realizes dynamic performance and the power sharing capability between inverters, and improving the flexibility of adjusting the output impedance.

Description

Technical Field

[0001] The present application relates to the field of inverter control, and in particular to a droop control method and device for an inverter. Background Art

[0002] An inverter is a converter that converts direct current (DC) into alternating current (AC). In the field of power electronics, inverters play an important industrial role. For example, in renewable energy, such as solar photovoltaic power and wind farms, inverters convert DC power generated by solar or wind energy into AC power, which can then be connected to the grid or used to power local loads.

[0003] When an inverter processes DC power far exceeding its conversion capacity, multiple inverters can be connected in parallel to output AC power. To improve the stability of the output AC power when multiple inverters are connected in parallel, the inverters should have good dynamic performance and power sharing capabilities between the inverters.

[0004] The dynamic performance of the inverter and the power sharing capability between inverters are both related to the output impedance of the inverter. In order to share power equally among the parallel inverters, the output impedance of the inverter needs to be increased. However, improving the dynamic performance of the inverter requires reducing the output impedance of the inverter, which leads to conflicts when adjusting the output impedance of the inverter, and it is impossible to achieve the dynamic performance of the inverter and the power sharing capability between the inverters at the same time. Summary of the Invention

[0005] In order to solve the problem of output impedance conflict when the inverter realizes dynamic performance and power sharing capability between inverters.

[0006] In a first aspect, some embodiments of the present application provide a droop control method for an inverter, including:

[0007] Obtain the delay transfer function, the current inner loop transfer function, and the duty cycle to inductor current transfer function;

[0008] Performing a product calculation on the delay transfer function, the current inner-loop transfer function, and the duty cycle-to-inductor current transfer function to obtain a current open-loop transfer function;

[0009] Calculating a current closed-loop transfer function based on the identity matrix and the current open-loop transfer function;

[0010] Performing a product calculation on the current closed-loop transfer function, the voltage outer-loop transfer function, and the transfer function from the inductor current to the output voltage to obtain a voltage open-loop transfer function;

[0011] Calculating a voltage closed-loop transfer function based on the identity matrix and the voltage open-loop transfer function;

[0012] Setting the droop control loop of the inverter to a target representation, wherein the target representation is a negative representation of the ratio of a unit matrix to the voltage outer loop transfer function;

[0013] The current output impedance of the inverter is updated to a target output impedance according to the voltage closed-loop transfer function and the droop coefficient of the droop control loop through the target representation.

[0014] In some embodiments, before the step of setting the droop control loop to the target representation mode, the method further comprises:

[0015] Calculating a voltage difference between a first voltage reference value and a second voltage reference value; wherein the first voltage reference value is a voltage reference value after the inverter performs droop control, and the second voltage reference value is a voltage reference value before the inverter performs droop control;

[0016] calculating a target ratio of the voltage difference to the output current of the inverter;

[0017] The droop coefficient and the target ratio are added to obtain the droop control loop.

[0018] In some embodiments, the voltage outer loop transfer function can be expressed by the following formula:

[0019] ;

[0020] in, is the voltage outer loop transfer function, is the ratio of the voltage outer loop of the inverter, is the integral parameter of the voltage outer loop of the inverter, s is the Laplace variable.

[0021] In some embodiments, the current inner loop transfer function can be expressed by the following formula:

[0022] ;

[0023] in, is the current inner loop transfer function, is the ratio of the current inner loop of the inverter, is the integral parameter of the current inner loop of the inverter.

[0024] In some embodiments, the step of setting the droop control loop to a target representation includes:

[0025] Obtaining a current output impedance of the inverter; the current output impedance is obtained by calculating a sum of a first impedance result and a second impedance result, where the first impedance result is obtained by calculating a product of an open-loop output impedance and a difference between the unit matrix and the voltage closed-loop transfer function; and the second impedance result is obtained by calculating a sum of a ratio of the droop coefficient, the droop control loop, and a transfer function from the output current to the inductor current to the voltage outer-loop transfer function, and then calculating a product of the sum and the voltage closed-loop transfer function.

[0026] A target representation of the droop control loop is calculated based on the first impedance result and the second impedance result.

[0027] In some embodiments, before the step of obtaining the current output impedance of the inverter, the method further includes:

[0028] Obtaining a transfer function from output current to output voltage, a transfer function from output current to inductor current, and a transfer function from inductor current to output voltage;

[0029] Calculate the product of the transfer function from the output current to the inductor current and the transfer function from the inductor current to the output voltage;

[0030] The open-loop output impedance of the inverter is obtained by calculating the difference between a negative output current-to-output voltage transfer function and a product of a negative output current-to-inductor current transfer function and a negative output voltage transfer function.

[0031] In some embodiments, the step of calculating the current closed-loop transfer function based on the identity matrix and the current open-loop transfer function includes:

[0032] Calculating an addition result of the unit matrix and the current open-loop transfer function;

[0033] The current closed-loop transfer function is obtained by calculating a ratio of the current open-loop transfer function to a sum of the unit matrix and the current open-loop transfer function.

[0034] In some embodiments, the step of calculating the voltage closed-loop transfer function based on the identity matrix and the voltage open-loop transfer function includes:

[0035] Calculating an addition result of the unit matrix and the voltage open-loop transfer function;

[0036] The voltage closed-loop transfer function is obtained by calculating a ratio of the voltage open-loop transfer function to a sum of the unit matrix and the voltage open-loop transfer function.

[0037] In some embodiments, the delay transfer function can be expressed by the following formula:

[0038] ;

[0039] in, is the delay transfer function, To control the delay time.

[0040] In a second aspect, some embodiments of the present application provide a droop control device for an inverter, including a control module, wherein the control module is configured to:

[0041] Obtain the delay transfer function, the current inner loop transfer function, and the duty cycle to inductor current transfer function;

[0042] Performing a product calculation on the delay transfer function, the current inner-loop transfer function, and the duty cycle-to-inductor current transfer function to obtain a current open-loop transfer function;

[0043] Calculating a current closed-loop transfer function based on the identity matrix and the current open-loop transfer function;

[0044] Performing a product calculation on the current closed-loop transfer function, the voltage outer-loop transfer function, and the transfer function from the inductor current to the output voltage to obtain a voltage open-loop transfer function;

[0045] Calculating a voltage closed-loop transfer function based on the identity matrix and the voltage open-loop transfer function;

[0046] Setting the droop control loop of the inverter to a target representation, wherein the target representation is a negative representation of the ratio of a unit matrix to the voltage outer loop transfer function;

[0047] The current output impedance of the inverter is updated to a target output impedance according to the voltage closed-loop transfer function and the droop coefficient of the droop control loop through the target representation.

[0048] It can be seen from the above technical solution that the present application provides a droop control method and device for an inverter, which sets the droop control loop to be represented by the negative number of the ratio of the unit matrix to the voltage outer loop transfer function, and updates the current output impedance of the inverter to the target output impedance according to the voltage closed-loop transfer function and the droop coefficient of the droop control loop, so that the output impedance is adjusted to be controlled and adjusted by the droop coefficient, eliminating the influence of the control loop parameters and output power on the closed-loop output impedance, reducing the adjustment conflict of the output impedance when the inverter realizes dynamic performance and power sharing capability between inverters, and improving the flexibility of adjusting the output impedance. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the technical solution of the present application, the following is a brief introduction to the drawings required for use in the embodiments. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0050] Figure 1 This is a schematic diagram of two inverters working in parallel in an embodiment of the present application;

[0051] Figure 2 A flowchart of a droop control method for an inverter according to an embodiment of the present application;

[0052] Figure 3 This is a diagram of the inverter droop control structure provided in an embodiment of the present application;

[0053] Figure 4 A load voltage fluctuation diagram of the inverter provided in an embodiment of the present application without performing droop control;

[0054] Figure 5 This is a diagram of unloaded voltage fluctuation when the inverter provided in an embodiment of the present application does not perform droop control;

[0055] Figure 6 A load voltage fluctuation diagram of the inverter performing droop control provided in an embodiment of the present application;

[0056] Figure 7 This is a diagram of unloaded voltage fluctuation when the inverter according to an embodiment of the present application performs droop control. DETAILED DESCRIPTION

[0057] In order to make the purpose and implementation of this application clearer, the exemplary implementation of this application will be clearly and completely described below in conjunction with the drawings in the exemplary embodiments of this application. Obviously, the described exemplary embodiments are only part of the embodiments of this application, not all of the embodiments.

[0058] It should be noted that the brief descriptions of terms in this application are only for the purpose of facilitating the understanding of the embodiments described below, and are not intended to limit the embodiments of this application. Unless otherwise specified, these terms should be understood according to their ordinary and usual meanings.

[0059] In this specification and the accompanying drawings, the terms "first," "second," "third," etc. are used to distinguish similar or similar objects or entities, and are not necessarily intended to limit a particular order or precedence, unless otherwise noted. It should be understood that the terms used in this manner are interchangeable under appropriate circumstances.

[0060] The terms "comprise," "include," and "have," and any variations thereof, are intended to cover but not exclude inclusion; for example, a product or device comprising a list of components is not necessarily limited to all the components expressly listed but may include other components not expressly listed or inherent to such product or device.

[0061] A three-phase inverter is a converter that converts direct current (DC) to alternating current (AC) for connection to the grid or to power local loads. For example, in renewable energy generation systems, such as solar photovoltaic (PV) power plants, a three-phase inverter converts the DC power generated by solar energy into AC power, which is then output to the load. Another example is a wind farm, where a three-phase inverter converts the DC power generated by wind energy into AC power. In these application scenarios, varying environmental conditions and load performance require the three-phase inverter to accurately and stably output AC power. However, due to varying application scenarios, the same inverter cannot be used for current conversion. Therefore, multiple three-phase inverters may be required, operating in parallel. For example, the first three-phase inverter can convert the DC power converted from solar energy into AC power, while the second three-phase inverter can convert the DC power generated by wind energy from the wind farm into AC power. The first three-phase inverter can operate in parallel with the second three-phase inverter to simultaneously convert DC power from different energy sources into AC power suitable for the load.

[0062] In addition, as the magnitude of current conversion increases, when the inverter processes DC power that is far greater than the inverter's conversion capacity, multiple inverters can also be connected in parallel to output AC power. Figure 1 Schematic diagram showing two inverters working in parallel in an embodiment of the present application. Figure 1 , including inverter 1 and inverter 2 connected in parallel, wherein the structures and working principles of inverter 1 and inverter 2 are the same. Inverter 1 is taken as an example for explanation. The connection structure and working principle of inverter 2 can refer to the structure and working principle of inverter 1. V ac To input DC power to inverter 1, switches S1, S2, S3, and S4 form an H-bridge structure, converting the DC power into AC power suitable for the load. During the positive half-cycle, S1 and S4 conduct, and the current flows from P → S1 → L → load → S4 → N, resulting in a positive output voltage. During the negative half-cycle, S2 and S3 conduct, and the current flows from P → S2 → L → load → S3 → N, resulting in a negative output voltage.

[0063] P is the positive pole of the DC input terminal, N is the negative pole of the DC input terminal, V P The voltage of the positive terminal of the DC input, V Nis the voltage of the negative pole of the DC input terminal, and O is the neutral point. A, B, C and N are the A-phase high-voltage line, B-phase high-voltage line, C-phase high-voltage line and ground line of the three-phase inverter respectively. After the DC power enters the three high-voltage lines, it passes through the LC filter composed of capacitors and inductors. i La is the inductive current of the A-phase high-voltage line, i Lb is the inductive current of the B-phase high-voltage line, i Lc is the inductive current of the C-phase high-voltage line, i LN is the ground inductor current. i ca is the capacitive current filtered by the A-phase high-voltage line, i cb is the capacitive current filtered by the A-phase high-voltage line, i cc The capacitive current filtered out by the A-phase high-voltage line is the output current of the A-phase high-voltage line. i a , the output current of the B-phase high-voltage line i b , output current of phase C high voltage line i c , to provide AC power to the load.

[0064] To improve the stability of AC output when multiple inverters are connected in parallel, three-phase inverters must have good dynamic performance and power balancing between inverters. Dynamic performance means reducing voltage fluctuations across the three-phase inverters when the load increases or decreases, thereby improving the stability of the output current across the three-phase inverters. Power balancing ensures that the power distributed between the three-phase inverters operating in parallel is equal during current conversion.

[0065] However, there's a conflict between dynamic performance and the ability to share power between inverters. To achieve power sharing between each three-phase inverter, it's necessary to increase the output impedance of the three-phase inverter, such as by adjusting virtual impedance or inserting an inductor at the inverter's output. Improving the dynamic performance of a three-phase inverter requires reducing its output impedance, which creates a conflict when adjusting the output impedance. It's impossible to achieve both the desired dynamic performance and power sharing between inverters.

[0066] To address the conflicting issue of output impedance adjustment when achieving dynamic performance and power sharing between inverters, some embodiments of the present application provide a droop control method for an inverter, applied to a three-phase inverter. For ease of description, the three-phase inverter will be referred to as the inverter below. The inverter can execute the droop control method via an internal control module. In this embodiment, the inverter's control module is the executing entity, and other electrical components in the inverter can perform other current conversion functions. Figure 2 Flowchart of the inverter droop control method provided in the embodiment of the present application. Figure 2 , the method comprising:

[0067] S100: Obtaining a delay transfer function, a current inner loop transfer function, and a duty cycle to inductor current transfer function.

[0068] In the embodiments of the present application, the output impedance of the inverter is affected by the inverter output power and control loop parameters when achieving dynamic performance and power sharing capability between inverters. The output power is used to maintain the inverter's power sharing capability, and the dynamic performance is used to maintain the stability of the inverter's output voltage when the load is increased or decreased. The inverter's control loop includes a droop control loop, a current inner loop, and a voltage outer loop. The droop control loop parameters of the droop control loop, the current inner loop parameters of the current inner loop, and the voltage outer loop parameters of the voltage outer loop are all control loop parameters.

[0069] When an inverter responds to a control signal for current conversion, there is a certain delay. This delay occurs between the time the inverter receives the control signal and the time the inverter begins converting current in response to the control signal. Therefore, a delay transfer function is required.

[0070] The delay transfer function can be expressed as follows:

[0071] ;

[0072] in, is the delay transfer function, To control the delay time, the droop coefficient can be adjusted through the delay transfer function to offset the effect of the delay time on the current conversion in the subsequent droop control process.

[0073] In order to describe the dynamic performance of the inverter, the following transfer function relationship can be obtained when the inverter is in working state through the physical working relationship of the inverter:

[0074] ;

[0075] in, is the inductor current of the inverter, is the output current of the inverter, d is the output duty cycle of the inverter, is the transfer function from output current to inductor current, is the transfer function from duty cycle to inductor current.

[0076] ;

[0077] in, is the output voltage, is the transfer function from inductor current to output voltage, is the transfer function from output current to output voltage. is the small signal component of the variable, for example, is the small signal component of the inductor current, is the small signal component of the output current, is the small signal component of the output duty cycle, is the small signal component of the output voltage. In the above transfer function relationship, the transfer function from duty cycle to inductor current can be obtained, that is, It should be noted that the transfer functions in the embodiments of the present application are all based on the Laplace variable s Calculated as a variable.

[0078] The current inner loop transfer function, that is, the transfer function of the current inner loop PI controller (proportional-integral controller), can be calculated through the current inner loop. The current inner loop can be expressed as follows:

[0079] ;

[0080] in, is the current reference value, that is, the current value of the inverter before droop control is performed. is the current inner loop transfer function. According to the formula of the current inner loop and the small signal component of the output duty cycle , the current inner loop transfer function can be calculated , the current inner loop transfer function It can be expressed as follows:

[0081] ;

[0082] in, is the ratio of the current inner loop of the inverter, is the integral parameter of the inverter's current inner loop, as well as These are all current inner loop parameters.

[0083] S200: Perform product calculation on the delay transfer function, the current inner-loop transfer function, and the duty cycle to inductor current transfer function to obtain a current open-loop transfer function.

[0084] After obtaining the delay transfer function, the current inner loop transfer function, and the duty cycle to inductor current transfer function, the control module can calculate the current open loop transfer function , the current open-loop transfer function can be expressed as:

[0085] ;

[0086] Therefore, the control module may perform a product calculation on the delay transfer function, the current inner-loop transfer function, and the duty cycle to inductor current transfer function to obtain the current open-loop transfer function.

[0087] S300: Calculating a current closed-loop transfer function according to a unit matrix and the current open-loop transfer function.

[0088] The current open-loop transfer function is the basis for designing the current closed-loop transfer function. By analyzing the open-loop characteristics, the inverter compensator parameters, such as proportional gain and integration time, can be determined to optimize the closed-loop dynamic response.

[0089] In this embodiment, the control module may calculate the sum of a unit matrix and a current open-loop transfer function. The unit matrix is a conventional matrix in mathematics. After obtaining the sum, the control module may calculate the ratio of the current open-loop transfer function to the sum of the unit matrix and the current open-loop transfer function to obtain the current closed-loop transfer function.

[0090] In this embodiment, based on the generation relationship between the current open-loop transfer function and the current closed-loop transfer function, the current closed-loop transfer function can be calculated using the current open-loop transfer function and the identity matrix, as shown in the following formula:

[0091] ;

[0092] in, is the current open-loop transfer function, E is the identity matrix.

[0093] S400: Perform product calculation on the current closed-loop transfer function, the voltage outer-loop transfer function, and the transfer function from the inductor current to the output voltage to obtain a voltage open-loop transfer function.

[0094] The voltage open-loop transfer function can be defined as follows:

[0095] ;

[0096] in, is the voltage open-loop transfer function, is the voltage outer loop transfer function, that is, the transfer function of the voltage outer loop PI controller. The voltage outer loop transfer function can be expressed as follows:

[0097] ;

[0098] in, is the ratio of the voltage outer loop, The integral parameter of the voltage outer loop. The ratio and integral parameter of the voltage outer loop are both parameters of the voltage outer loop. Therefore, it is necessary to adjust the droop coefficient of the droop control loop in subsequent steps to eliminate the influence of the voltage outer loop parameters on the output impedance. The voltage outer loop transfer function can be derived from the voltage outer loop, and the voltage outer loop can be expressed as follows:

[0099] ;

[0100] in, Indicates the reference value of the inductor current.

[0101] S500: Calculating a voltage closed-loop transfer function according to a unit matrix and the voltage open-loop transfer function.

[0102] The voltage open-loop transfer function is the basis for designing the voltage closed-loop transfer function. In this embodiment, based on the generative relationship between the voltage open-loop transfer function and the voltage closed-loop transfer function, the control module can calculate the sum of the identity matrix and the voltage open-loop transfer function. After calculating the sum, the ratio of the voltage open-loop transfer function to the sum of the identity matrix and the voltage open-loop transfer function can be calculated to obtain the voltage closed-loop transfer function. The voltage closed-loop transfer function can be calculated from the voltage open-loop transfer function and the identity matrix as follows:

[0103] ;

[0104] in, is the voltage closed-loop transfer function.

[0105] S600: Set the droop control loop to target representation mode.

[0106] In order to decouple the output impedance from the inverter's output power and the inverter's control loop parameters, it is necessary to reset the target representation of the droop control loop. To this end, the control module can first set the final representation of the output impedance as follows:

[0107] ;

[0108] in, The final expression of the inverter's output impedance is: is the droop coefficient. Since the voltage closed-loop transfer function is a fixed function formula, when the output impedance is finally expressed, when the output impedance is controlled solely by the droop coefficient, that is, the inverter can determine the output impedance of the inverter by adjusting the droop function, decoupling the output impedance from the inverter's output power and the inverter's control loop parameters, thereby avoiding the influence of power and control loop parameters on the output impedance.

[0109] Based on the final representation of the output impedance, the target representation of the droop control loop can be reversely derived through the calculation formula of the output impedance.

[0110] To this end, the control module needs to first calculate the current output impedance of the inverter. The calculation formula for the current output impedance can be expressed as follows:

[0111] ;

[0112] in, is the current output impedance of the inverter, is the droop control loop. When the negative number of the ratio of the unit matrix to the voltage outer loop transfer function is expressed, the voltage outer loop transfer function including the control loop parameters can be eliminated by calculation. , the transfer function from output current to inductor current and The impact of The inverter's open-loop output impedance, i.e., the inverter's output impedance when current control is not in effect, is simplified to eliminate the effect of control parameters on the output impedance, thereby increasing the flexibility of output impedance adjustment.

[0113] In some embodiments, the droop control loop can be expressed as:

[0114] ;

[0115] in, is the first voltage reference value, i.e., the first voltage reference value after the inverter performs droop control, is the set second voltage reference value, i.e. the voltage reference value of the inverter before droop control, is the droop coefficient.

[0116] Through derivation, the control module can calculate the voltage difference between a first voltage reference value and a second voltage reference value. The first voltage reference value is the voltage difference after the inverter executes droop control, and the second voltage reference value is the voltage reference before the inverter executes droop control. After calculating the voltage difference, the target ratio of the voltage difference to the output current is calculated. The droop coefficient is then added to the target ratio to obtain the droop control loop, i.e., the current representation of the droop control loop. By substituting the current representation of the droop control loop into the calculation formula for the current output impedance, the target representation of the droop control loop can be calculated.

[0117] Among them, after substituting the final expression of the output impedance into the calculation formula of the current output impedance, the following formula can be derived:

[0118] ;

[0119] Then the transfer function from inductor current to output voltage is and the transfer function of output current to output voltage After substituting, we can get the target expression of the droop control loop. That is, the target expression of the droop control loop is as follows:

[0120] ;

[0121] When the target representation of the droop control loop is the negative of the ratio of the unit matrix to the voltage outer loop transfer function, the output impedance of the inverter can be controlled independently by the droop coefficient. , and The control loop parameters in the corresponding transfer function cannot affect the output impedance. Therefore, the inverter can adjust the output impedance by changing the droop coefficient alone to balance the power sharing and dynamic adjustment capabilities of the inverters when working in parallel, thereby improving the application flexibility of the inverter.

[0122] The current output impedance can be obtained by calculating the sum of a first impedance result and a second impedance result. The control module can first calculate the difference between the unit matrix and the voltage closed-loop transfer function, and then calculate the product of the open-loop output impedance and the difference between the unit matrix and the voltage closed-loop transfer function to obtain the first impedance result. The control module can calculate the sum of the droop coefficient, the droop control loop, and the ratio of the transfer function from the output current to the inductor current to the voltage outer loop transfer function, and then calculate the product of the sum and the voltage closed-loop transfer function to obtain the second impedance result.

[0123] The current output impedance is obtained by adding the first impedance result and the second impedance result, and the target representation of the droop control loop is reversely calculated using the current output impedance and the final representation of the output impedance set in step S600.

[0124] In some embodiments, the open-loop output impedance of the inverter can be obtained by obtaining the transfer function of the output current to the output voltage, the transfer function of the output current to the inductor current, and the transfer function of the inductor current to the output voltage based on the functional relationship in step S100. The control module can first calculate the product of the transfer function of the output current to the inductor current and the transfer function of the inductor current to the output voltage, and then calculate the difference between the negative output current to output voltage transfer function and the above product to obtain the open-loop output impedance of the inverter. The open-loop output impedance of the inverter can be expressed by the following formula:

[0125] ;

[0126] S700: Update the current output impedance of the inverter to a target output impedance according to the voltage closed-loop transfer function and the droop coefficient of the droop control loop through the target representation.

[0127] By using the target representation of the droop control loop, the inverter's output impedance can be controlled based on the voltage closed-loop transfer function and the droop coefficient of the droop control loop to update the inverter's current output impedance to the target output impedance. During the target output impedance update process, the control module can adjust the droop coefficient based on the dynamic performance requirements corresponding to the inverter parallel connection and the power sharing accuracy requirements to configure a target output impedance suitable for the current operating conditions within the bandwidth range. This resolves the output impedance adjustment conflict when achieving the inverter's dynamic performance and the power sharing capability between inverters.

[0128] Figure 3 This is a control flow chart of the inverter parallel operation provided in the embodiment of the present application. Figure 3 , wherein inverter 1 and inverter 2 are in parallel relationship and both execute control strategies in the dq coordinate system. First, the control structure and connection relationship of inverter 1 are described. , are the voltage and current in the d coordinate system, , They are the voltage and current in the q coordinate system respectively. After the voltage and current in the d coordinate system and the voltage and current in the q coordinate system are input into inverter 1, the frequency w The power P obtained through power calculation is input into the Pf-Droop module. The Pf-Droop module is an active power-frequency droop control module used to adjust the power sharing capability of inverter 1 and inverter 2.

[0129] After passing through the Pf-Droop module, the power P and the voltage of the three-phase high-voltage line The coordinate transformation module is input together to perform coordinate transformation through s / 1 to convert DC into AC, and the voltage part after coordinate transformation, that is, and After passing through the abc / Dq module, that is, the Park conversion module, the Park conversion is obtained. and , Parker converted and Input voltage outer loop. The current part after coordinate conversion is the same as the current of three-phase high voltage Input the Park conversion module together to perform Park conversion to obtain and , in the and Input current into the inner loop to output to the load.

[0130] The inverter 1 also includes a QV-Droop module and a first dynamic control module connected in parallel with the Pf-Droop module. The QV-Droop module is a reactive-voltage droop control module. The QV-Droop module and the first dynamic control module can respectively perform dynamic adjustment capabilities. Since the two are connected in parallel, they do not affect each other when performing dynamic adjustment capabilities. The first dynamic control module includes a d-axis coordinate system droop control loop arranged in parallel. and virtual impedance , where the virtual impedance The dynamic adjustment capability of inverter 1 can be adjusted to droop control loop That is, the output impedance of the inverter 1 is adjusted to achieve dynamic adjustment and power sharing capabilities at the same time. The inputs of the QV-Droop module and the first dynamic control module are , thereby calculating the voltage reference value in the d coordinate system And through the voltage outer loop, according to the voltage reference value , output the current reference value in the d coordinate system to the current inner loop The inverter 1 is also provided with a second dynamic control module connected in series with the voltage outer loop, i.e., a dynamic control module in the q coordinate system. The second dynamic control module also includes a droop control loop provided in parallel. and virtual impedance .

[0131] It should be noted that this application only performs dynamic adjustment on the active part, that is, the output impedance in the d coordinate system. Therefore, the droop control loop in the second dynamic control module There is no adjustment effect on the output impedance of the inverter. The second dynamic control module input is , the output of the current inner loop is the current reference value .

[0132] The connection structure and control principle of inverter 2 are the same as those of inverter 1, that is, the input of inverter 2 is , , , .in, , are the voltage and current in the d coordinate system, , They are the voltage and current in the q coordinate system respectively. After the voltage and current in the d coordinate system and the voltage and current in the q coordinate system are input into inverter 2, the power P and frequency are calculated by power operation. w Enter the Pf-Droop module. The Pf-Droop module is an active power-frequency droop control module used to adjust the power sharing capability of inverter 1 and inverter 2.

[0133] Power P and voltage of three-phase high-voltage line The coordinate transformation module is input together to perform coordinate transformation through s / 1 to convert DC into AC, and the voltage part after coordinate transformation, that is, and After the Park conversion module, the converted and , the converted and Input voltage outer loop. The current part after coordinate conversion is the same as the current of three-phase high voltage Input the Park conversion module together to perform Park conversion to obtain the converted and , that is, converted into alternating current, and then and Input current into the inner loop to output to the load.

[0134] The inverter 2 also includes a QV-Droop module and a first dynamic control module connected in parallel with the Pf-Droop module. The QV-Droop module is a reactive-voltage droop control module. The QV-Droop module and the first dynamic control module can respectively perform dynamic adjustment without affecting each other. The first dynamic control module includes a droop control loop arranged in parallel. and virtual impedance , where the virtual impedance The dynamic adjustment capability of inverter 2 can be adjusted to droop control loop The output impedance of the inverter 2 can be adjusted to achieve dynamic adjustment and power sharing capabilities at the same time. The inputs of the QV-Droop module and the first dynamic control module are , thereby calculating the voltage reference value in the d coordinate system And through the voltage outer loop, according to the voltage reference value , output the current reference value in the d coordinate system to the current inner loop The inverter 2 is also provided with a second dynamic control module connected in series with the voltage outer loop, i.e., a dynamic control module in the q coordinate system. The second dynamic control module also includes a droop control loop provided in parallel. and virtual impedance .

[0135] It should be noted that the inverter 2 only performs dynamic adjustment on the active part, that is, the output impedance in the d coordinate system. Therefore, the droop control loop in the second dynamic control module There is no adjustment effect on the output impedance of inverter 2. The second dynamic control module input is , the output of the current inner loop is the current reference value .

[0136] In order to achieve power sharing, inverter 2 is equipped with a phase-locked loop (PLL), which can obtain the voltage of the three-phase high-voltage line input by inverter 1 from the Pf-Droop module of inverter 1. v c,abc,1 , and inside the phase-locked loop, the inverter 2 pairs v c,abc,1 Perform the Park transform, which outputs and and through the PI controller according to and Calculate the phase angle of inverter 2 , and performs current conversion at the phase angle of inverter 1. When inverter 2 maintains the same phase angle as inverter 1, that is, and the phase angle in inverter 1 When the values are the same, the allocated power can be kept the same, thus improving the consistency of power allocation.

[0137] It should be noted that the embodiment of the present application only uses two inverters working in parallel as an example. In actual application, the number of inverters can be increased according to the DC power situation. The working principles and connection relationships of the parallel inverters can refer to the above-mentioned embodiments, and this application will not go into details. This application does not impose specific restrictions on the number of parallel inverters.

[0138] Figure 4 This is a loading voltage fluctuation diagram of the inverter provided in an embodiment of the present application without executing the droop control method. Figure 5 The inverter provided in the embodiment of the present application does not implement the droop control method. Figure 4 and Figure 5 It can be seen that before the droop control method disclosed in the embodiment of the present application is executed, the fluctuating voltage of the inverter is 22.3V when the voltage is loaded, and the fluctuating voltage is 23.5V when the voltage is unloaded. Figure 6 A voltage load fluctuation diagram of the inverter performing the droop control method provided in an embodiment of the present application. Figure 7 The voltage unloading fluctuation diagram of the inverter performing the droop control method provided in the embodiment of the present application. Figure 6 and Figure 7 It can be seen that after the droop control method disclosed in the embodiment of the present application is not executed, the fluctuating voltage of the inverter when the voltage is loaded is 8.8V, and the fluctuating voltage when the voltage is unloaded is 14.6V. From the experimental data, it can be seen that the droop control method provided by the present application can effectively reduce the voltage fluctuation amplitude of the inverter when the voltage is loaded and the voltage is unloaded, and improve the dynamic performance of the inverter.

[0139] Some embodiments of the present application further provide a droop control device for an inverter, which is applied to the inverter and is used to perform a droop control method for the inverter. The device includes a control module that can be connected to a dynamic control module in the inverter to perform calculations on a droop control loop in the dynamic control module. The control module is configured to:

[0140] S100: Obtaining a delay transfer function, a current inner loop transfer function, and a duty cycle to inductor current transfer function.

[0141] S200: Perform product calculation on the delay transfer function, the current inner-loop transfer function, and the duty cycle to inductor current transfer function to obtain a current open-loop transfer function.

[0142] S300: Calculating a current closed-loop transfer function according to a unit matrix and the current open-loop transfer function.

[0143] S400: Perform product calculation on the current closed-loop transfer function, the voltage outer-loop transfer function, and the transfer function from the inductor current to the output voltage to obtain a voltage open-loop transfer function.

[0144] S500: Calculating a voltage closed-loop transfer function according to a unit matrix and the voltage open-loop transfer function.

[0145] S600: Set the droop control loop to target representation mode.

[0146] The target representation is a negative representation of the ratio of the unit matrix to the voltage outer loop transfer function.

[0147] S700: Update the current output impedance of the inverter to a target output impedance according to the voltage closed-loop transfer function and the droop coefficient of the droop control loop through the target representation.

[0148] It can be seen from the above technical solution that the present application provides a droop control method and device for an inverter, which sets the droop control loop to be represented by the negative number of the ratio of the unit matrix to the voltage outer loop transfer function, and updates the current output impedance of the inverter to the target output impedance according to the voltage closed-loop transfer function and the droop coefficient of the droop control loop, so that the output impedance is adjusted to be controlled and adjusted by the droop coefficient, eliminating the influence of the control loop parameters and output power on the closed-loop output impedance, reducing the adjustment conflict of the output impedance when the inverter realizes dynamic performance and power sharing capability between inverters, and improving the flexibility of adjusting the output impedance.

[0149] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

[0150] For ease of explanation, the above description has been presented in conjunction with specific embodiments. However, the above exemplary discussion is not intended to be exhaustive or to limit the embodiments to the specific forms disclosed above. Based on the above teachings, various modifications and variations are possible. The above embodiments have been selected and described to better explain the present disclosure, thereby enabling those skilled in the art to better utilize the embodiments.

Claims

1. A droop control method for an inverter, characterized in that: Applied to inverters, including: Obtain the delay transfer function, the current inner loop transfer function, and the duty cycle to inductor current transfer function; Performing a product calculation on the delay transfer function, the current inner-loop transfer function, and the duty cycle-to-inductor current transfer function to obtain a current open-loop transfer function; Calculating a current closed-loop transfer function based on the identity matrix and the current open-loop transfer function; Performing a product calculation on the current closed-loop transfer function, the voltage outer-loop transfer function, and the transfer function from the inductor current to the output voltage to obtain a voltage open-loop transfer function; Calculating a voltage closed-loop transfer function based on the identity matrix and the voltage open-loop transfer function; Get the final representation of the preset output impedance and the current output impedance; The target representation of the droop control loop is derived based on the final representation of the output impedance and the current output impedance. The final representation of the output impedance is expressed by the following formula: ； in, The final expression of the inverter's output impedance is: is the droop coefficient, is the voltage closed-loop transfer function; Among them, the droop control loop is expressed as: ； in, is a first voltage reference value after the inverter performs droop control, is a second voltage reference value before the inverter performs droop control, is the output current of the inverter, It is the droop control loop; The current output impedance is expressed by the following formula: ； in, is the current output impedance of the inverter, is the open-loop output impedance of the inverter, E is the identity matrix, is the voltage outer loop transfer function, is the transfer function from output current to inductor current; Substituting the final representation of the output impedance into the calculation formula of the current output impedance, the derivation formula of the droop control loop is obtained: Wherein, the derivation formula is expressed as: ； The transfer function from inductor current to output voltage is and the transfer function of output current to output voltage Substituting the derived formula into the target expression of the droop control loop, The target expression of the droop control loop is: ； When the target representation of the droop control loop is a negative representation of the ratio of the unit matrix to the voltage outer loop transfer function, the output impedance of the inverter is controlled by the droop coefficient; When the droop control loop is in a target representation mode, the current output impedance of the inverter is updated to a target output impedance according to the voltage closed-loop transfer function and the droop coefficient of the droop control loop.

2. The droop control method of the inverter according to claim 1, characterized in that: The method further comprises: Calculating a voltage difference between a first voltage reference value and a second voltage reference value; wherein the first voltage reference value is a voltage reference value after the inverter performs droop control, and the second voltage reference value is a voltage reference value before the inverter performs droop control; calculating a target ratio of the voltage difference to an output current of the inverter; The droop coefficient and the target ratio are added to obtain the droop control loop.

3. The droop control method of the inverter according to claim 1, characterized in that: The voltage outer loop transfer function is expressed by the following formula: ； in, is the voltage outer loop transfer function, is the proportional coefficient of the voltage outer loop of the inverter, is the integral parameter of the voltage outer loop of the inverter, s is the Laplace variable.

4. The droop control method of the inverter according to claim 3, characterized in that: The current inner loop transfer function is expressed by the following formula: ； in, is the current inner loop transfer function, is the proportional coefficient of the current inner loop of the inverter, is the integral parameter of the current inner loop of the inverter.

5. The droop control method of the inverter according to claim 1, characterized in that: The steps for obtaining the current output impedance include: calculating a sum of a first impedance result and a second impedance result, where the first impedance result is obtained by calculating a product of an open-loop output impedance and a difference between the unit matrix and the voltage closed-loop transfer function, and the second impedance result is obtained by calculating a ratio of a transfer function from an output current to an inductor current to the voltage outer-loop transfer function, then calculating a sum of the droop coefficient, the droop control loop, and the ratio, and then calculating a product of the sum and the voltage closed-loop transfer function; The current output impedance is obtained according to the sum of the first impedance result and the second impedance result.

6. The droop control method of the inverter according to claim 5, characterized in that: The method further comprises: Obtaining a transfer function from output current to output voltage, a transfer function from output current to inductor current, and a transfer function from inductor current to output voltage; Calculate the product of the transfer function from the output current to the inductor current and the transfer function from the inductor current to the output voltage; Calculate the transfer function from output current to output voltage in negative form; The difference between the transfer function from the output current to the output voltage in a negative form and the product result is calculated to obtain the open-loop output impedance of the inverter.

7. The droop control method of the inverter according to claim 1, characterized in that: The step of calculating the current closed-loop transfer function based on the unit matrix and the current open-loop transfer function comprises: Calculating an addition result of the unit matrix and the current open-loop transfer function; The current closed-loop transfer function is obtained by calculating a ratio of the current open-loop transfer function to a sum of the unit matrix and the current open-loop transfer function.

8. The droop control method of the inverter according to claim 1, characterized in that: The step of calculating the voltage closed-loop transfer function based on the identity matrix and the voltage open-loop transfer function comprises: Calculating an addition result of the unit matrix and the voltage open-loop transfer function; The voltage closed-loop transfer function is obtained by calculating a ratio of the voltage open-loop transfer function to a sum of the unit matrix and the voltage open-loop transfer function.

9. The droop control method of the inverter according to claim 1, characterized in that: The delay transfer function is expressed by the following formula: ； in, is the delay transfer function, To control the delay time, s is the Laplace variable.

10. A droop control device for an inverter, characterized in that: The invention comprises a control module, wherein the control module is configured to: Obtain the delay transfer function, the current inner loop transfer function, and the duty cycle to inductor current transfer function; Performing a product calculation on the delay transfer function, the current inner-loop transfer function, and the duty cycle-to-inductor current transfer function to obtain a current open-loop transfer function; Calculating a current closed-loop transfer function based on the identity matrix and the current open-loop transfer function; Performing a product calculation on the current closed-loop transfer function, the voltage outer-loop transfer function, and the transfer function from the inductor current to the output voltage to obtain a voltage open-loop transfer function; Calculating a voltage closed-loop transfer function based on the identity matrix and the voltage open-loop transfer function; Get the final representation of the preset output impedance and the current output impedance; The target representation of the droop control loop is derived based on the final representation of the output impedance and the current output impedance. The final representation of the output impedance is expressed by the following formula: ； in, The final expression of the inverter's output impedance is: is the droop coefficient, is the voltage closed-loop transfer function; Among them, the droop control loop is expressed as: ； in, is a first voltage reference value after the inverter performs droop control, is a second voltage reference value before the inverter performs droop control, is the output current of the inverter, It is the droop control loop; The current output impedance is expressed by the following formula: ； in, is the current output impedance of the inverter, is the open-loop output impedance of the inverter, E is the identity matrix, is the voltage outer loop transfer function, is the transfer function from output current to inductor current; Substituting the final representation of the output impedance into the calculation formula of the current output impedance, the derivation formula of the droop control loop is obtained: Wherein, the derivation formula is expressed as: ； The transfer function from inductor current to output voltage is and the transfer function of output current to output voltage Substituting the derived formula into the target expression of the droop control loop, The target expression of the droop control loop is: ； When the target representation of the droop control loop is a negative representation of the ratio of the unit matrix to the voltage outer loop transfer function, the output impedance of the inverter is controlled by the droop coefficient; When the droop control loop is in a target representation mode, the current output impedance of the inverter is updated to a target output impedance according to the voltage closed-loop transfer function and the droop coefficient of the droop control loop.

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