A multi-converter independent network control method

Abstract

The present application discloses a method for independent network control of multiple converters, which belongs to the field of converter control technology. The method includes determining the output frequency of the converter based on the active link of the network control; determining the internal potential of the converter based on the reactive link of the network control; determining the current command value based on the grid connection point voltage, the output frequency of the converter, and the internal potential of the converter through the virtual impedance link; the current controller determines a three-phase reference wave based on the output frequency of the converter and the current command value, and the three-phase reference wave is used to achieve coordinated operation between the converters without relying on communication when multiple converters are running in parallel. The present application uses virtual impedance to convert the internal potential and frequency output by the network control link into a reference value for the current controller, which is used to achieve coordinated operation between converters without relying on communication when multiple converters are running in parallel.

Description

Technical Field

[0001] The present application belongs to the technical field of converter control, and specifically relates to a method for controlling independent networking of multiple converters. Background Art

[0002] Grid-type converters often use voltage controllers to convert the phase and internal potential amplitude control instructions output by the grid control link into current control instructions. This current controller accurately controls the output current in steady state and suppresses fault current in the event of a fault. When multiple grid-type converters operate in parallel, the electrical distance between the output voltages of each converter is close, and control conflicts may arise between the voltage controllers of different converters. To address this issue, existing technologies often use a method in which the voltage controller of one converter dominates the voltage control at the grid connection point, and the other converters follow the current control instructions of this converter. This method couples the converter control systems, and a failure in the control system of a single converter can disrupt system stability. Summary of the Invention

[0003] Purpose of the invention: This application develops a multi-converter independent network control method, which uses virtual impedance to convert the internal potential and frequency output by the network control link into reference values ​​for the current controller, and is used to achieve coordinated operation between converters without relying on communication when multiple converters are operated in parallel.

[0004] Technical Solution: An embodiment of the present application provides a multi-converter independent grid control method, which is applied to a power system. The power system includes a grid control active link, a grid control reactive link, multiple converters operating in parallel, a current controller, and a virtual impedance link. The method includes:

[0005] Determining the output frequency of the converter based on the active link of the network control;

[0006] Determining the internal potential of the converter based on the reactive link of the network construction control;

[0007] Determining a current command value based on the grid connection point voltage, the output frequency of the converter, and the internal potential of the converter through the virtual impedance link;

[0008] The current controller determines a three-phase reference wave based on the output frequency of the converter and the current command value. The three-phase reference wave is used to achieve coordinated operation between the converters without relying on communication when multiple converters operate in parallel.

[0009] In some embodiments, the active link of network control includes a frequency control module, and determining the output frequency of the converter based on the active link of network control includes:

[0010] Obtaining the active power output by the converter;

[0011] Determining the difference between the active power output by the converter and the active target value as the active power difference;

[0012] Based on the active frequency droop curve, the frequency control module determines the output frequency corresponding to the active target value when the active power difference is 0 as the output frequency of the converter; the active frequency droop curve is a curve of the corresponding relationship between the active target value and the output frequency of the converter.

[0013] In some embodiments, determining the active frequency droop curve includes:

[0014] A point on the active power frequency droop curve is determined based on the active power command value and the frequency command value, and the active power frequency droop curve is determined according to a set slope.

[0015] In some embodiments, the active power control link of the network construction further includes an active power frequency droop module; and the determination of the active power target value includes:

[0016] The active frequency droop module determines the active power target value based on a frequency command value, an active power command value, and an output frequency of the converter.

[0017] In some embodiments, the method further comprises:

[0018] determining that the active power of each converter fluctuates, and each converter adjusts the output frequency of the converter according to the active power target value;

[0019] After each of the converters adjusts the output frequency of the respective converters, the active power target value is updated according to the active power frequency droop curve;

[0020] Each of the converters adjusts its output frequency according to the updated active power target value until the output frequencies of the converters are consistent, so that the power system returns to a stable state, and the active power of the converters is coordinated and controlled without relying on communication.

[0021] In some embodiments, the network control reactive link includes an internal potential control module, and determining the internal potential of the converter based on the network control reactive link includes:

[0022] Obtaining reactive power output by the converter and grid connection point voltage;

[0023] Determine the difference between the reactive power output by the converter and the reactive target value as a reactive power difference;

[0024] Based on the reactive voltage droop curve, the internal potential control module determines the grid connection point voltage corresponding to the reactive target value when the reactive power difference is 0 as the internal potential of the converter; the reactive voltage droop curve is a curve of the corresponding relationship between the reactive target value and the grid connection point voltage.

[0025] In some embodiments, determining the reactive voltage droop curve includes:

[0026] A point on a reactive voltage droop curve is determined based on the voltage command value and the reactive power command value, and the reactive voltage droop curve is determined according to a set slope.

[0027] In some embodiments, the network control reactive link further includes a reactive voltage droop module, and the determination of the reactive target value includes:

[0028] The reactive voltage droop module determines the reactive power target value based on the grid connection point voltage, the voltage command value, and the reactive power command value.

[0029] In some embodiments, the method further comprises:

[0030] Determining that the reactive power of each converter fluctuates and the grid connection point voltage fluctuates, and each converter adjusts its internal potential according to the reactive power target value;

[0031] After each of the converters adjusts the internal potential of the respective converters, the reactive power target value is updated according to the reactive voltage droop curve;

[0032] Each of the converters adjusts its internal potential according to the updated reactive power target value until the internal potentials of the converters are consistent, so that the power system returns to a stable state, and the reactive power of the converters is coordinated and controlled without relying on communication.

[0033] In some embodiments, determining the current command value based on the grid connection point voltage, the output frequency of the converter, and the internal potential of the converter through the virtual impedance link includes:

[0034] Determining a d-axis component and a q-axis component of the grid connection point voltage based on an output frequency of the converter;

[0035] Determine the difference between the d-axis component of the grid connection point voltage and the internal potential of the converter as the d-axis component of the virtual voltage difference, and determine the q-axis component of the grid connection point voltage as the q-axis component of the virtual voltage difference;

[0036] The d-axis component of the virtual voltage difference and the q-axis component of the virtual voltage difference are input into the virtual impedance link, and the virtual current generated after the virtual voltage difference is applied to the virtual impedance is simulated according to the circuit principle. After the virtual current passes through the current limiting link, the d-axis component of the current command value and the q-axis component of the current command value are determined.

[0037] In some embodiments, the virtual impedance link may be a fixed value virtual impedance link or a variable value virtual impedance link;

[0038] The fixed value virtual impedance link keeps the virtual impedance value unchanged during the fault period;

[0039] The variable value virtual impedance link adjusts the value of the virtual impedance according to the active current target value and the reactive current target value during the fault period; the active current target value and the reactive current target value are determined by the reactive current response curve during the fault period, the maximum short-circuit current tolerance value and the active power before the fault.

[0040] In some embodiments, when the virtual impedance link adopts a variable value virtual impedance link, the active current target value is determined as the d-axis component of the virtual current, and the reactive current target value is determined as the q-axis component of the virtual current; the method further includes:

[0041] determining a complex form of the virtual voltage difference according to a d-axis component of the virtual voltage difference and a q-axis component of the virtual voltage difference, wherein the d-axis component is a real part of the complex number and the q-axis component is an imaginary part of the complex number;

[0042] Determining a complex form of the virtual current according to a d-axis component of the virtual current and a q-axis component of the virtual current, wherein the d-axis component is the real part of the complex number and the q-axis component is the imaginary part of the complex number;

[0043] dividing the complex form of the virtual voltage difference by the complex form of the virtual current to determine the complex form of the virtual impedance;

[0044] The virtual resistance value of the virtual impedance link is determined according to the real part of the complex form of the virtual impedance, and the virtual reactance value of the virtual impedance link is determined according to the imaginary part of the complex form of the virtual impedance.

[0045] In some embodiments, simulating a virtual current generated by applying a virtual voltage difference to a virtual impedance according to circuit principles includes:

[0046] determining a complex form of the virtual current by dividing the complex form of the virtual voltage difference by the complex form of the virtual impedance;

[0047] A d-axis component of the virtual current is determined according to a real part of the complex form of the virtual current, and a q-axis component of the virtual current is determined according to an imaginary part of the complex form of the virtual current.

[0048] In some embodiments, the current controller determines a three-phase reference wave based on the output frequency of the converter and the current command value, including:

[0049] determining a d-axis component of an output current of the converter and a q-axis component of an output current of the converter based on an output frequency of the converter;

[0050] The current controller adjusts the reference wave d-axis component of the converter output voltage and the reference wave q-axis component of the converter output voltage so that the d-axis component of the converter output current is equal to the d-axis component of the current command value, and the q-axis component of the converter output current is equal to the q-axis component of the current command value;

[0051] The three-phase reference wave for controlling the inverter is determined by dividing the reference wave d-axis component of the inverter output voltage and the reference wave q-axis component of the inverter output voltage based on the output frequency of the inverter.

[0052] Beneficial effect: The embodiment of the present application provides a method for independent networking control of multiple converters. This method calculates the reference value of the current controller by using a virtual impedance link, and converts the internal potential and frequency output by the networking control link into a current reference value without using a voltage controller, thereby avoiding the problem of voltage control conflicts when multiple converters are operated in parallel; each converter is independently controlled and can operate stably without relying on communication between controllers. During a fault, the current reference value is limited, and the current controller is used to suppress the rise of the fault current. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0054] Figure 1 Schematic diagram of the independent control structure of multiple converters according to an embodiment of the present application.

[0055] Figure 2 This is an overall flow chart of the multi-converter independent networking control method according to an embodiment of the present application.

[0056] Figure 3 Schematic diagram of a multi-converter decoupling control architecture according to an embodiment of the present application.

[0057] Figure 4 This is a control block diagram of the virtual impedance link in an embodiment of the present application.

[0058] Figure 5 This is a control block diagram of the active link of network control in the embodiment of the present application.

[0059] Figure 6 It is a schematic diagram of the active power-frequency droop control principle in an embodiment of the present application.

[0060] Figure 7 This is a control block diagram of the reactive link of network control in the embodiment of the present application.

[0061] Figure 8 It is a schematic diagram of the reactive power-voltage droop control principle in an embodiment of the present application.

[0062] Figure 9 It is a schematic diagram of the virtual impedance calculation process in an embodiment of the present application. DETAILED DESCRIPTION

[0063] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the embodiments described are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative efforts are within the scope of protection of this application.

[0064] It should be understood that although the terms first, second, etc. may be used herein to describe various components, these components should not be limited by these terms. These terms are used to distinguish one component from another. Thus, the first component discussed below could be referred to as the second component without departing from the teachings of the present invention. As used herein, the term "and / or" includes any one and all combinations of one or more of the associated listed items.

[0065] Those skilled in the art will appreciate that the drawings are merely schematic diagrams of exemplary embodiments and may not be to scale. The modules or processes in the drawings are not necessarily required to implement the present application and therefore cannot be used to limit the scope of protection of the present application.

[0066] See also Figure 1 , Figure 1 The multi-converter independent control structure of the embodiment of the present application is illustrated. Figure 1As shown, the power system has multiple converters running in parallel. The converter control systems in each converter are independent of each other. Each converter independently controls the output frequency, internal potential, current reference value and reference wave of the converter output voltage according to the measured electrical quantity. When the control system of a converter fails, it will not affect the operation of the control systems of other converters, thereby realizing decoupling between converters. There is no need to set up an upper-level unified coordination controller or establish communication between different converters for coordinated control, so as to ensure the stable operation of the power system.

[0067] See also Figure 2 and Figure 3 , Figure 2 The overall process of the multi-converter independent network control method according to the embodiment of the present application is illustrated; Figure 3 The multi-converter decoupling control architecture of the embodiment of the present application is illustrated. The multi-converter decoupling control architecture is used to achieve independent control of each converter when multiple converters are running in parallel, resolve the control conflict problem between multiple converters, replace the traditional follower control method, and avoid system stability problems caused by mutual coupling between controllers. The multi-converter independent network control method of the embodiment of the present application is applied to the power system, which includes a network control active link, a network control reactive link, multiple converters running in parallel, a current controller, and a virtual impedance link. The method specifically includes the following steps:

[0068] Step 101: Determine the output frequency of the converter based on the active link of the network control.

[0069] The active power control link collects electrical quantities such as voltage, current, active power, or DC voltage, and uses the rotor motion equation and virtual oscillator calculation method to determine the converter output phase θ, thereby achieving phase synchronization between the converter and the system. It should be noted that the active power control link includes a frequency control module and an active frequency droop module. The frequency control module can adopt first-order rotor motion equation control, second-order rotor motion equation control, or lead-lag link control. The active frequency droop module is used to receive the frequency command value, active power command value, and the converter output frequency.

[0070] In some embodiments, see Figure 5 , Figure 5 The active power link of network construction control is shown in the figure. The specific steps for determining the output frequency of the converter based on the active power link of network construction control are as follows:

[0071] Step 201: Obtaining the active power output by the converter;

[0072] Step 202: determining the difference between the active power output by the converter and the active power target value as the active power difference;

[0073] Step 203: The frequency control module determines the output frequency corresponding to the active power target value when the active power difference is 0 as the output frequency of the converter based on the active power frequency droop curve; the active power frequency droop curve is a curve showing the corresponding relationship between the active power target value and the output frequency of the converter.

[0074] Among them, when multiple converters are operated in parallel, the inconsistent frequencies of different converters will cause the power angle to swing, and the active power will generate unstable circulation between the converters. In order to achieve frequency coordination between multiple converters, the current active power target value is determined according to the active power command value and the frequency generated by the current frequency synchronization link according to the active frequency droop curve. The frequency control module adjusts the frequency according to the active power target value and the collected active power to make the active power equal to the active power target value. While achieving phase synchronization between the converter and the system, active coordinated control between multiple converters is realized.

[0075] In some embodiments, the active frequency droop curve is determined by determining a point on the active frequency droop curve based on the active power command value and the frequency command value, and determining the active frequency droop curve according to a set slope.

[0076] In some embodiments, the active power target value is determined by an active frequency droop module based on a frequency command value, an active power command value, and an output frequency of the converter.

[0077] In some embodiments, see Figure 6 , Figure 6 The active power-frequency droop control principle is shown. ref and frequency command value f ref , determining an active power-frequency droop curve, and achieving coordinated control of active power when multiple converters are operated in parallel. Specifically, when multiple converters are operated in parallel, the output frequencies of each converter are consistent in a stable state. It is determined that the active power of each converter has fluctuated, and each converter adjusts its output frequency according to the active power target value. After adjusting its output frequency, each converter updates its active power target value according to the active power frequency droop curve. Each converter adjusts its output frequency according to the updated active power target value. The above process is repeated until the output frequencies of each converter are consistent, so that the power system returns to a stable state, and the active power of the converters is coordinated and controlled without relying on communication.

[0078] In an example, take two converters running in parallel as an example. When the two converters run in parallel, the initial active powers are P1 and P2 respectively. At this time, the frequencies of the two converters are controlled to f1 and f2 respectively according to the active power-frequency droop curve. Due to the inconsistent frequencies, the phase inconsistency of the two converters will cause further changes in power. When the active power changes from P1 and P2 to the active power instruction value P refWhen the two converters are close to each other, the frequencies f1 and f2 of the two converters are simultaneously directed to the frequency command value f ref When f1 and f2 are close to the frequency command value f ref When they are equal, the system enters a stable state and active power coordination is completed between the converters.

[0079] Step 102: Determine the internal potential of the converter based on the reactive link of the network control.

[0080] Among them, the reactive link of the network control collects electrical quantities such as voltage, current or reactive power, and uses calculation methods such as virtual excitation control to determine the internal potential d-axis control instruction E dref and internal potential q-axis control command E qref It should be noted that the reactive power control link for grid connection includes an internal potential control module and a reactive voltage droop module. The internal potential control module can adopt virtual excitation control, PI control, or PR control. The reactive voltage droop module is used to receive the grid connection point voltage, voltage command value, and reactive power command value.

[0081] In some embodiments, see Figure 7 , Figure 7 The following diagram illustrates the reactive power control link of the network. The specific steps for determining the internal potential of the converter based on the reactive power control link of the network are as follows:

[0082] Step 301: Obtain reactive power output by the converter and grid connection point voltage;

[0083] Step 302: Determine the difference between the reactive power output by the converter and the reactive power target value as the reactive power difference;

[0084] Step 303: Based on the reactive voltage droop curve, the internal potential control module determines the grid connection point voltage corresponding to the reactive power target value when the reactive power difference is 0 as the internal potential of the converter; the reactive voltage droop curve is a curve showing the corresponding relationship between the reactive power target value and the grid connection point voltage.

[0085] Among them, when multiple converters are operated in parallel, the inconsistent voltage amplitudes of different converters will cause unstable circulating current of reactive power between the converters. In order to achieve voltage amplitude coordination among multiple converters, the current reactive target value is determined by the reactive voltage droop curve according to the reactive power command value and the current voltage amplitude. The internal potential control module adjusts the internal potential according to the reactive target value and the collected reactive power to make the reactive power equal to the reactive target value. While achieving voltage stability of the converter and the system, reactive coordinated control among multiple converters is realized.

[0086] In some embodiments, the reactive frequency droop curve is determined by determining a point on the reactive voltage droop curve based on the voltage command value and the reactive power command value, and determining the reactive voltage droop curve according to a set slope.

[0087] In some embodiments, the reactive power target value is determined by the reactive voltage droop module based on the grid connection point voltage, the voltage command value, and the reactive power command value.

[0088] In some embodiments, see Figure 8 , Figure 8 The reactive power-voltage droop control principle is shown. According to the reactive power command value Q ref And the voltage command value U ref , determine the reactive power-voltage droop curve. Specifically, when multiple converters are operated in parallel, the voltage at each converter's grid connection point remains stable under steady state. If the reactive power of each converter fluctuates, the grid connection point voltage will fluctuate synchronously. Each converter adjusts its internal potential according to the reactive power target value. After adjusting its internal potential, each converter updates its reactive power target value according to the reactive voltage droop curve. Each converter adjusts its internal potential according to the updated reactive power target value. The above process is repeated until the internal potentials of each converter are consistent and the grid connection point voltages of each converter return to stability, so that the power system returns to a stable state and the reactive power of the converters is coordinated and controlled without relying on communication.

[0089] In an example, take two converters running in parallel as an example. When the two converters run in parallel, the initial reactive power is Q1 and Q2 respectively. At this time, the grid connection point voltages of the two converters are controlled to U1 and U2 respectively according to the reactive-voltage droop curve. Due to the inconsistency of the grid connection point voltages, the reactive circulating current generated between the two converters will cause further changes in power. When the reactive power changes from Q1 and Q2 to the reactive command value P ref When the two converters are close to each other, the grid connection voltages U1 and U2 of the two converters are simultaneously directed to the voltage command value U ef When U1 and U2 are close to the voltage command value U ref When they are equal, the system enters a stable state and reactive power coordination is completed between the converters.

[0090] Step 103: Determine the current command value based on the grid connection point voltage, the output frequency of the converter, and the internal potential of the converter through the virtual impedance link.

[0091] The virtual impedance link can adopt a fixed-value virtual impedance link or a variable-value virtual impedance link. The fixed-value virtual impedance link maintains the virtual impedance value unchanged during the fault. The variable-value virtual impedance link adjusts the virtual impedance value according to the active current target value and reactive current target value during the fault. The active current target value and reactive current target value are determined by the reactive current response curve during the fault period, the maximum short-circuit current withstand value, and the active power before the fault.

[0092] It should be noted that during a fault, the expected reactive current value corresponding to the current grid-connection point voltage is determined based on the grid-connection point voltage and the reactive current response curve, serving as the reactive current target value. The maximum active current tolerance value is determined by taking the square root of the difference between the square of the maximum short-circuit current tolerance value and the square of the reactive current target value. The expected active current value is determined by dividing the pre-fault active power by the virtual voltage difference. The target active current value is determined by taking the maximum value between the maximum active current tolerance value and the expected active current value. The reactive current response curve determines the expected reactive current values ​​corresponding to different grid-connection point voltage values ​​during the fault period based on design requirements, forming a reactive current response curve. The maximum short-circuit current tolerance value is determined based on the overcurrent capacity of the equipment during a short circuit.

[0093] In some embodiments, see Figure 9 , Figure 9 The virtual impedance calculation process is shown in Figure 1. The virtual impedance calculation process steps are as follows:

[0094] Step 410: Calculate the active current target value and reactive current target value based on the collected grid connection point voltage, pre-fault active power, maximum active current tolerance value, and reactive current response curve. These values ​​are used to calculate the virtual resistance value and virtual inductance value of the virtual impedance link. During a fault, the system voltage drops, and the converter needs to provide reactive current to the system to support the system voltage. After providing the desired reactive current to the system, the active power of the fault is maintained within the maximum active current tolerance value to ensure active power output. This embodiment adjusts the current reference value of the current controller through the virtual impedance link to provide the desired active current and reactive current to the system during the fault.

[0095] In some embodiments, step 410 specifically includes the following steps:

[0096] Step 411: Collect the grid connection point voltage and pre-fault active power. It should be noted that the grid connection point voltage includes the voltage amplitude, the grid connection point voltage d-axis component, and the grid connection point voltage q-axis component.

[0097] Step 412: Determine the reactive current target value based on the grid connection point voltage and reactive current response curve. It should be noted that in the embodiment of the present application, the reactive current impact curve represents the expected reactive current value under different voltages. The reactive current target value at the current moment is determined based on the grid connection point voltage amplitude collected during the fault period and the reactive current response curve.

[0098] When a fault occurs, the converter can control the output reactive current value according to the degree of system voltage drop to meet the system's requirements for the reactive support capacity of the grid-type converter.

[0099] Step 413: Calculate the active current target value based on the active power before the fault and the grid voltage. The calculation method is to divide the active power before the fault by the grid voltage amplitude.

[0100] Step 414: Determine whether the target active current value is greater than the maximum active current tolerance value. It should be noted that when the target active current value is less than the maximum active current tolerance value, the target active current value remains unchanged. When the target active current value is greater than the maximum active current tolerance value, the target active current value is set to the maximum active current tolerance value, limiting the current flowing through the converter to not exceed the tolerance capacity of the device.

[0101] Step 420: Based on the d-axis component U of the grid-connected point voltage d and the q-axis component U of the grid-connected point voltage q and internal potential E and active current target value I pref and reactive current target value I qref Calculate the virtual resistance value R v and virtual inductance L v Active current target value I pref As the d-axis component of the virtual current, the reactive current target value I qref As the q-axis component of the virtual current, the complex form of the virtual current is determined according to the d-axis component of the virtual current and the q-axis component of the virtual current, the difference between the d-axis component of the grid connection point voltage and the internal potential is taken as the d-axis component of the virtual voltage difference, the difference between the q-axis component of the grid connection point voltage and 0 is taken as the q-axis component of the virtual voltage difference, and the complex form of the virtual voltage difference is determined according to the d-axis component of the virtual voltage difference and the q-axis component of the virtual voltage difference, wherein the d-axis component is the real part of the complex number and the q-axis component is the imaginary part of the complex number; wherein the d-axis component is the real part of the complex number and the q-axis component is the imaginary part of the complex number; the complex form of the virtual voltage difference is divided by the complex form of the virtual current to determine the complex form of the virtual impedance; the virtual resistance value of the virtual impedance link is determined according to the real part of the complex form of the virtual impedance, and the virtual reactance value of the virtual impedance link is determined according to the imaginary part of the complex form of the virtual impedance. The calculation method can be expressed as:

[0102]

[0103] In formula (1), R v is the virtual resistance value, j is the imaginary unit, w is the output frequency of the converter, L v is the virtual inductance value, U d is the d-axis component of the grid-connected point voltage, U q is the q-axis component of the grid-connected point voltage, E dref is the internal potential d-axis control instruction, E qref is the internal potential q-axis control instruction, I dref is the d-axis component of the current control command, I qrefis the q-axis component of the current control command.

[0104] The calculated virtual resistance value R v and virtual inductance L v It is used to calculate the current command value at this moment, and the above process is repeated at the next moment based on the collected grid connection point voltage and pre-fault active power.

[0105] In some embodiments, the current command value is determined by determining the d-axis component and q-axis component of the grid-connected point voltage based on the output frequency of the converter; determining the difference between the d-axis component of the grid-connected point voltage and the internal potential of the converter as the d-axis component of the virtual voltage difference, and determining the q-axis component of the grid-connected point voltage as the q-axis component of the virtual voltage difference; inputting the d-axis component of the virtual voltage difference and the q-axis component of the virtual voltage difference into the virtual impedance link, simulating the virtual current generated after the virtual voltage difference is applied to the virtual impedance according to the circuit principle, and determining the d-axis component of the current command value and the q-axis component of the current command value after the virtual current passes through the current limiting link.

[0106] It should be noted that, according to the circuit principle, the virtual current generated after the virtual voltage difference is applied to the virtual impedance is expressed as follows: the complex form of the virtual voltage difference is divided by the complex form of the virtual impedance to determine the complex form of the virtual current; the d-axis component of the virtual current is determined according to the real part of the complex form of the virtual current, and the q-axis component of the virtual current is determined according to the imaginary part of the complex form of the virtual current.

[0107] It should be noted that the current limiting link includes an upper current limit value and a lower current limit value, and the d-axis and q-axis components of the current reference value serve as inputs to the current limiting link. When the input to the current limiting link is greater than the upper current limit value, the output of the current limiting link is the upper current limit value; when the input to the current limiting link is less than the lower current limit value, the output of the current limiting link is the lower current limit value; when the input to the current limiting link is greater than the lower current limit value and less than the upper current limit value, the output of the current limiting link is the input to the current limiting link. During a fault, the d-axis and q-axis components of the current command value are limited to limit the fault current.

[0108] In some embodiments, when the virtual impedance link adopts a variable value virtual impedance link, the active current target value is determined as the d-axis component of the virtual current, and the reactive current target value is determined as the q-axis component of the virtual current. Step 103 also includes the following steps:

[0109] determining a complex form of the virtual voltage difference according to a d-axis component of the virtual voltage difference and a q-axis component of the virtual voltage difference, wherein the d-axis component is a real part of the complex number and the q-axis component is an imaginary part of the complex number;

[0110] Determine the complex form of the virtual current according to the d-axis component of the virtual current and the q-axis component of the virtual current, wherein the d-axis component is the real part of the complex number and the q-axis component is the imaginary part of the complex number;

[0111] The complex form of the virtual voltage difference is divided by the complex form of the virtual current to determine the complex form of the virtual impedance;

[0112] The virtual resistance value of the virtual impedance link is determined according to the real part of the complex form of the virtual impedance, and the virtual reactance value of the virtual impedance link is determined according to the imaginary part of the complex form of the virtual impedance.

[0113] In some embodiments, the virtual impedance link is based on the converter output phase θ to the three-phase voltage U sabc Perform dq transformation to calculate the d-axis component U of the grid connection point voltage sd and the q-axis component of the grid-connected point voltage U sq , according to the complex form of the grid voltage U sd +j Usq and the complex form of the internal potential E dref +j Eqref , calculate the voltage drop on the virtual impedance ΔU=U sd -E dref +j Usq -E qref , according to the relationship between the voltage drop and current on the impedance in the circuit principle, calculate the current flowing through the virtual impedance R v +jX v Virtual current ΔU / (R v +jX v ), after being limited by the limiter link, it is used as the d-axis component I of the current control command dref and the current control command q-axis component I qref The limiting link can adopt amplitude limiting, vector limiting and low voltage current limiting to limit the increase of current command value during fault period.

[0114] Step 104: The current controller determines a three-phase reference wave based on the output frequency and current command value of the converter. The three-phase reference wave is used to achieve coordinated operation between converters without relying on communication when multiple converters operate in parallel.

[0115] In some embodiments, the d-axis component of the output current of the converter and the q-axis component of the output current of the converter are determined based on the output frequency of the converter; the current controller adjusts the reference wave d-axis component of the converter output voltage and the reference wave q-axis component of the converter output voltage so that the d-axis component of the output current of the converter is equal to the d-axis component of the current command value, and the q-axis component of the output current of the converter is equal to the q-axis component of the current command value; the reference wave d-axis component of the converter output voltage and the reference wave q-axis component of the converter output voltage are adjusted based on the output frequency of the converter to determine the three-phase reference wave for converter control.

[0116] Specifically, the current controller controls the three-phase current I according to the converter output phase θ. sabc Perform dq transformation to obtain the current d-axis component I sd and the q-axis component of the current I sq , the current command value I dref and I qref With current I sd and I sq The difference is 0 as the control target, and the reference wave d-axis component U is calculated through closed-loop feedback control. cdref and U cqref And calculate the three-phase reference wave U through dq inverse transformation cabc , used for converter control.

[0117] See also Figure 4 , Figure 4 This is the control block diagram of the virtual impedance link. The current command value is calculated based on the grid connection point voltage and internal potential. This is used to replace the voltage controller when multiple converters are operating in parallel, preventing control conflicts between multiple voltage controllers.

[0118] The traditional voltage closed-loop controller, which calculates current control commands, uses a zero-difference regulation control method. However, when multiple converters operate in parallel, their voltages influence each other due to their close electrical distance. Factors such as voltage measurement errors make it difficult to coordinate the voltage control targets of different converters, leading to control conflicts. Typically, one converter serves as the master converter, controlling the voltage and calculating the current command value. The other converters follow the master's calculated command value. This method relies on data exchange between converter controllers and the coupling between control systems. A failure in the control system of the master converter can affect the operation of other converters. The virtual impedance link allows for differential voltage regulation without control conflicts. Data exchange between converter controllers is unnecessary, eliminating coupling between controllers and improving system stability.

[0119] In some embodiments, the virtual impedance includes a virtual resistor R v and virtual inductance L v, used to simulate the voltage-current relationship across an impedance in an actual circuit and convert voltage command values ​​into current command values. In this embodiment, the calculation of positive-sequence current command values ​​and negative-sequence current command values ​​by positive-sequence decomposition is used as an example. Optionally, command values ​​such as full-sequence current, zero-sequence current, and current amplitude can also be calculated.

[0120] Specifically, according to the positive sequence voltage d-axis command value E dref_pos and the positive sequence voltage q-axis command value E qref_pos The measured positive sequence voltage d-axis component U sd_pos and the measured positive sequence voltage q-axis component U sq_pos Calculate the positive sequence voltage drop on the virtual impedance and calculate the positive sequence current d-axis command value I according to the circuit principle dref_pos and the positive sequence current q-axis command value I qref_pos , the calculation method can be expressed as:

[0121]

[0122] In formula (2), I dref_pos is the positive sequence current d-axis command value, R v is a virtual resistor, s represents the complex variable of Laplace transform, ω1 is the rated frequency, L v is the virtual inductance, U sd_pos is the d-axis component of the positive sequence voltage, E qref_pos is the positive sequence voltage q-axis command value, I qref_pos is the positive sequence current q-axis command value, U sq_pos is the measured positive sequence voltage q-axis component, E qref_pos is the positive sequence voltage q-axis command value.

[0123] In the embodiment of the present application, according to the negative sequence voltage d-axis command value E dref_neg and negative sequence voltage q-axis command value E qref_neg The measured negative sequence voltage d-axis component U sd_neg and the measured negative sequence voltage q-axis component U sq_neg Calculate the negative sequence voltage drop on the virtual impedance and calculate the negative sequence current d-axis command value I according to the circuit principle dref_neg and the negative sequence current q-axis command value I qref_neg , the calculation method can be expressed as:

[0124]

[0125] In the above embodiments, the description of each embodiment has its own focus. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0126] The above is a detailed introduction to the multi-converter independent networking control method provided in the embodiments of the present application, and specific examples are used to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the technical solutions and core ideas of the present application; ordinary technicians in this field should understand that: they can still modify the technical solutions recorded in the aforementioned embodiments, or replace some of the technical features therein with equivalents; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A multi-converter independent network control method, characterized in that: Applied to a power system, the power system includes a network control active link, a network control reactive link, multiple converters operating in parallel, a current controller, and a virtual impedance link. The method includes: Determining the output frequency of the converter based on the active link of the network control; The internal potential of the converter is determined based on the network control reactive link; the network control reactive link includes an internal potential control module, and the internal potential of the converter is determined based on the network control reactive link, including: Obtaining reactive power output by the converter and grid connection point voltage; Determine the difference between the reactive power output by the converter and the reactive target value as a reactive power difference; Based on the reactive voltage droop curve, the internal potential control module determines the grid connection point voltage corresponding to the reactive power target value when the reactive power difference is 0 as the internal potential of the converter; the reactive voltage droop curve is a curve of the corresponding relationship between the reactive power target value and the grid connection point voltage; Determining a current command value based on the grid connection point voltage, the output frequency of the converter, and the internal potential of the converter through the virtual impedance link; The current controller determines a three-phase reference wave based on the output frequency of the converter and the current command value. The three-phase reference wave is used to achieve coordinated operation between the converters without relying on communication when multiple converters operate in parallel.

2. A multi-converter independent network control method according to claim 1, characterized in that: The active link of network control includes a frequency control module, which determines the output frequency of the converter based on the active link of network control, including: Obtaining the active power output by the converter; Determining the difference between the active power output by the converter and the active target value as the active power difference; Based on the active frequency droop curve, the frequency control module determines the output frequency corresponding to the active target value when the active power difference is 0 as the output frequency of the converter; the active frequency droop curve is a curve of the corresponding relationship between the active target value and the output frequency of the converter.

3. A multi-converter independent network control method according to claim 2, characterized in that: Determining the active frequency droop curve includes: A point on the active power frequency droop curve is determined based on the active power command value and the frequency command value, and the active power frequency droop curve is determined according to a set slope.

4. A multi-converter independent network control method according to claim 2, characterized in that: The active link of network control also includes an active frequency droop module; Determining the active power target value includes: The active frequency droop module determines the active power target value based on a frequency command value, an active power command value, and an output frequency of the converter.

5. A multi-converter independent network control method according to claim 4, characterized in that: The method further comprises: determining that the active power of each converter fluctuates, and each converter adjusts the output frequency of the converter according to the active power target value; After each of the converters adjusts the output frequency of the respective converters, the active power target value is updated according to the active power frequency droop curve; Each of the converters adjusts its output frequency according to the updated active power target value until the output frequencies of the converters are consistent, so that the power system returns to a stable state, and the active power of the converters is coordinated and controlled without relying on communication.

6. The multi-converter independent network control method according to claim 1, characterized in that: Determining the reactive voltage droop curve includes: A point on a reactive voltage droop curve is determined based on the voltage command value and the reactive power command value, and the reactive voltage droop curve is determined according to a set slope.

7. The multi-converter independent network control method according to claim 1, characterized in that: The network construction and reactive power control link also includes a reactive voltage droop module, and the determination of the reactive power target value includes: The reactive voltage droop module determines the reactive power target value based on the grid connection point voltage, the voltage command value, and the reactive power command value.

8. A multi-converter independent network control method according to claim 7, characterized in that: The method further comprises: Determining that the reactive power of each converter fluctuates and the grid connection point voltage fluctuates, and each converter adjusts its internal potential according to the reactive power target value; After each of the converters adjusts the internal potential of the respective converters, the reactive power target value is updated according to the reactive voltage droop curve; Each of the converters adjusts its internal potential according to the updated reactive power target value until the internal potentials of the converters are consistent, so that the power system returns to a stable state, and the reactive power of the converters is coordinated and controlled without relying on communication.

9. The multi-converter independent network control method according to claim 1, characterized in that: Determining a current command value based on the grid connection point voltage, the output frequency of the converter, and the internal potential of the converter through the virtual impedance link includes: Determining a d-axis component and a q-axis component of the grid connection point voltage based on an output frequency of the converter; Determine the difference between the d-axis component of the grid connection point voltage and the internal potential of the converter as the d-axis component of the virtual voltage difference, and determine the q-axis component of the grid connection point voltage as the q-axis component of the virtual voltage difference; The d-axis component of the virtual voltage difference and the q-axis component of the virtual voltage difference are input into the virtual impedance link, and the virtual current generated after the virtual voltage difference is applied to the virtual impedance is simulated according to the circuit principle. After the virtual current passes through the current limiting link, the d-axis component of the current command value and the q-axis component of the current command value are determined.

10. The multi-converter independent network control method according to claim 9, characterized in that: The virtual impedance link may be a fixed value virtual impedance link or a variable value virtual impedance link; The fixed value virtual impedance link keeps the virtual impedance value unchanged during the fault period; The variable value virtual impedance link adjusts the value of the virtual impedance according to the active current target value and the reactive current target value during the fault period; the active current target value and the reactive current target value are determined by the reactive current response curve during the fault period, the maximum short-circuit current tolerance value and the active power before the fault.

11. A multi-converter independent network control method according to claim 10, characterized in that: When the virtual impedance link adopts a variable value virtual impedance link, the active current target value is determined as the d-axis component of the virtual current, and the reactive current target value is determined as the q-axis component of the virtual current; the method further includes: determining a complex form of the virtual voltage difference according to a d-axis component of the virtual voltage difference and a q-axis component of the virtual voltage difference, wherein the d-axis component is a real part of the complex number and the q-axis component is an imaginary part of the complex number; Determining a complex form of the virtual current according to a d-axis component of the virtual current and a q-axis component of the virtual current, wherein the d-axis component is the real part of the complex number and the q-axis component is the imaginary part of the complex number; dividing the complex form of the virtual voltage difference by the complex form of the virtual current to determine the complex form of the virtual impedance; The virtual resistance value of the virtual impedance link is determined according to the real part of the complex form of the virtual impedance, and the virtual reactance value of the virtual impedance link is determined according to the imaginary part of the complex form of the virtual impedance.

12. A multi-converter independent network control method according to claim 11, characterized in that: According to the circuit principle, the virtual current generated by the virtual voltage difference applied to the virtual impedance is simulated, including: determining a complex form of the virtual current by dividing the complex form of the virtual voltage difference by the complex form of the virtual impedance; A d-axis component of the virtual current is determined according to a real part of the complex form of the virtual current, and a q-axis component of the virtual current is determined according to an imaginary part of the complex form of the virtual current.

13. The multi-converter independent network control method according to claim 1, characterized in that: The current controller determines a three-phase reference wave based on the output frequency of the converter and the current command value, including: determining a d-axis component of an output current of the converter and a q-axis component of an output current of the converter based on an output frequency of the converter; The current controller adjusts the reference wave d-axis component of the converter output voltage and the reference wave q-axis component of the converter output voltage so that the d-axis component of the converter output current is equal to the d-axis component of the current command value, and the q-axis component of the converter output current is equal to the q-axis component of the current command value; The three-phase reference wave for controlling the inverter is determined by dividing the reference wave d-axis component of the inverter output voltage and the reference wave q-axis component of the inverter output voltage based on the output frequency of the inverter.

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