Network construction type MMC control method and device based on droop control with low-pass filter

Abstract

The invention discloses a network construction type MMC control method and device based on droop control with a low-pass filter, and the method comprises the steps: carrying out the droop control with the low-pass filter on a network construction type MMC of a medium-voltage flexible interconnection system, so as to calculate a three-phase current reference value outputted by the network construction type MMC, superposing the three-phase current reference value with a three-phase unbalanced current and a harmonic compensation current, and obtaining a three-phase current reference value; and obtaining a comprehensive three-phase reference current, converting the comprehensive three-phase reference current into a dq-axis reference current, inputting the dq-axis reference current and the dq-axis voltage into a pre-established current prediction control model, and outputting a sub-module switch state corresponding to each phase of bridge arm of the network construction type MMC so as to act on the network construction type MMC. Therefore, after power synchronization is realized through droop control with a low-pass filter, virtual inertia can be generated under large disturbance, accurate and rapid response of the network-forming type MMC is realized, then model prediction current control is applied to a current inner loop, the dynamic response speed of the system can be further increased, and the operation stability of the system is improved.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and more specifically, to a grid-forming MMC control method and device based on droop control with a low-pass filter. Background Art

[0002] In modern power systems, power electronic converters, as the core devices for realizing power conversion, play an indispensable role in key links such as power production, power transmission, and power application. Therefore, the research on converters is of great significance. Compared with traditional multilevel converters, the modular multilevel converter (MMC) has been widely used in fields such as high-voltage direct current transmission and flexible alternating current transmission due to its significant advantages such as high modularity, strong scalability, and low device losses.

[0003] However, the grid-forming control of MMC involves the coordinated control of multiple sub-modules, and the design complexity of its control system is extremely high. It requires extremely precise modulation strategies and feedback control mechanisms to ensure the stable operation of the system and good dynamic response performance.

[0004] Currently, to achieve the grid-forming control of MMC, the commonly used method is to use PI controllers in both the inner loop and the outer loop. However, this method will result in a long adjustment time, and it is difficult to determine the PI parameters, leading to low dynamic response performance of the system and reducing the operating stability of the system.

[0005] Based on this, how to control the MMC in the system to improve the dynamic response performance of the system and the operating stability of the system is an issue that needs attention. Summary of the Invention

[0006] In view of the above problems, the present application provides a grid-forming MMC control method and device based on droop control with a low-pass filter to improve the dynamic response performance of the system and the operating stability of the system.

[0007] To achieve the above object, the following specific solutions are proposed:

[0008] A grid-forming MMC control method based on droop control with a low-pass filter includes:

[0009] Performing droop control with a low-pass filter on the grid-forming MMC of the medium-voltage flexible interconnected system to calculate the three-phase current reference values output by the grid-forming MMC;

[0010] Superposing the three-phase current reference values with a preset three-phase unbalanced reference current and a preset harmonic compensation reference current to obtain a comprehensive three-phase reference current, and transforming the comprehensive three-phase reference current into a dq-axis comprehensive reference current;

[0011] Measure the three-phase voltage output by the network-forming MMC on the AC side, and convert the three-phase voltage into dq-axis voltage;

[0012] Input the dq-axis combined reference current and the dq-axis voltage into a pre-established current prediction control model, and output the sub-module switching state corresponding to each phase arm of the network-forming MMC;

[0013] Apply the sub-module switching state corresponding to each phase arm to the network-forming MMC.

[0014] Optionally, perform droop control with a low-pass filter on the network-forming MMC of the medium-voltage flexible interconnection system to calculate the three-phase current reference values output by the network-forming MMC, including:

[0015] Detect the three-phase voltage and three-phase current output by the network-forming MMC of the medium-voltage flexible interconnection system at the grid connection point, transform the three-phase voltage to obtain the d-axis voltage and q-axis voltage, and transform the three-phase current to obtain the d-axis current and q-axis current;

[0016] Calculate the active output power and reactive output power of the network-forming MMC according to the d-axis voltage, the q-axis voltage, the d-axis current, and the q-axis current;

[0017] Based on the active output power and its reference value, and the reactive output power and its reference value, perform droop control with a low-pass filter to obtain the phase angle and the d-axis voltage reference value output by the network-forming MMC;

[0018] Determine the voltage vector amplitude through the active output power and the reactive output power, and determine the q-axis voltage reference value output by the network-forming MMC according to the voltage vector amplitude and the d-axis voltage reference value;

[0019] Calculate the d-axis current reference value and the q-axis current reference value according to the d-axis voltage reference value and the q-axis voltage reference value;

[0020] Transform the d-axis current reference value and the q-axis current reference value through the phase angle to obtain the three-phase current reference values output by the network-forming MMC.

[0021] Optionally, calculate the active output power and reactive output power of the network-forming MMC according to the d-axis voltage, the q-axis voltage, the d-axis current, and the q-axis current, including:

[0022] Calculate the active output power and reactive output power of the network-forming MMC using the following formula:

[0023]

[0024] where \(k\) is the current moment, is the active output power of the grid-forming MMC, is the reactive output power of the grid-forming MMC, is the \(d\)-axis voltage, is the \(q\)-axis voltage, is the \(d\)-axis current, is the \(q\)-axis current.

[0025] Optionally, determining the reference value of the \(q\)-axis voltage output by the grid-forming MMC according to the amplitude of the voltage vector and the reference value of the \(d\)-axis voltage includes:

[0026] Using the following formula to determine the reference value of the \(q\)-axis voltage output by the grid-forming MMC:

[0027]

[0028] where, is the reference value of the \(q\)-axis voltage output by the grid-forming MMC, is the effective value of the phase voltage, , is the amplitude of the voltage vector, is the reference value of the \(d\)-axis voltage.

[0029] Optionally, calculating the reference value of the \(d\)-axis current and the reference value of the \(q\)-axis current according to the reference value of the \(d\)-axis voltage and the reference value of the \(q\)-axis voltage includes:

[0030] Using the following formula to calculate the reference value of the \(d\)-axis current and the reference value of the \(q\)-axis current:

[0031]

[0032] where, is the reference value of the \(d\)-axis current, is the reference value of the \(q\)-axis current, is the reference value of the active output power, is the reference value of the reactive output power.

[0033] Optionally, transforming the reference value of the \(d\)-axis current and the reference value of the \(q\)-axis current through the phase angle to obtain the reference value of the three-phase current output by the grid-forming MMC includes:

[0034] Using the following formula to transform the reference value of the \(d\)-axis current and the reference value of the \(q\)-axis current through the phase angle to obtain the reference value of the three-phase current output by the grid-forming MMC:

[0035]

[0036] Among them, is the reference value of the a-phase current output by the grid-forming MMC, is the reference value of the b-phase current output by the grid-forming MMC, is the reference value of the c-phase current output by the grid-forming MMC, is the phase angle.

[0037] Optionally, the establishment process of the current prediction control model includes:

[0038] Obtain the current prediction training data of the grid-forming MMC, where the current prediction training data includes the three-phase output current and three-phase output voltage output by the grid-forming MMC on the AC side;

[0039] Convert the three-phase output current into a d-axis output current and a q-axis output current, and convert the three-phase output voltage into a d-axis output voltage and a q-axis output voltage;

[0040] Train a current prediction control model through the current prediction training data. The current prediction control model is:

[0041]

[0042] Among them, k is the current moment, k + 1 is the next moment, and k - 1 is the previous moment. is the predicted value of the d-axis current output by the grid-forming MMC at the next moment, is the d-axis output current, is the q-axis output current, is the sampled value of the d-axis voltage output by the grid-forming MMC at the previous moment, is the d-axis voltage increment output by the grid-forming MMC on the AC side, is the predicted value of the q-axis current output by the grid-forming MMC at the next moment, is the sampled value of the q-axis voltage output by the grid-forming MMC at the previous moment, is the q-axis voltage increment output by the grid-forming MMC on the AC side, is the first coefficient, , is the sampling coefficient, is the sampling period, is the second coefficient, , is the current angular frequency of the medium-voltage flexible interconnection system, is the third coefficient, , is the arm inductance of the grid-forming MMC;

[0043] Perform feedback correction on the current prediction control model to obtain the current prediction control model after feedback correction. The process of the feedback correction is as follows:

[0044] .

[0045] Among them, is the predicted value of the d-axis current after correction, is the predicted value of the q-axis current after correction, is the correction factor of the d-axis current error, is the correction factor of the q-axis current error.

[0046] Optionally, the objective function of the current prediction control model is:

[0047]

[0048] Among them, is the objective function, is the weight coefficient of the d-axis current error, is the weight coefficient of the q-axis current error, is the weight coefficient of the d-axis control voltage increment error, is the weight coefficient of the q-axis control voltage increment error, is the reference value of the d-axis output current, is the reference value of the q-axis output current.

[0049] Optionally, the medium-voltage flexible interconnection system includes two grid-forming MMCs with the same structure, a first AC bus, a second AC bus, and a DC bus. The two grid-forming MMCs are connected through the DC bus. One of the grid-forming MMCs is connected to the first AC bus, and the other grid-forming MMC is connected to the second AC bus;

[0050] Each phase of each grid-forming MMC includes two bridge arms, and each bridge arm includes a bridge arm inductor and multiple sub-modules;

[0051] Each phase of each grid-forming MMC is connected to the corresponding AC bus through a filter inductor.

[0052] A grid-forming MMC control device based on droop control with a low-pass filter includes:

[0053] A droop control unit with a low-pass filter is used to perform droop control with a low-pass filter on the grid-forming MMC of the medium-voltage flexible interconnection system to calculate the three-phase current reference values output by the grid-forming MMC;

[0054] A current superposition unit, configured to superpose the three-phase current reference values with a preset three-phase unbalance reference current and a preset harmonic compensation reference current to obtain a comprehensive three-phase reference current, and transform the comprehensive three-phase reference current into a dq-axis comprehensive reference current;

[0055] A voltage conversion unit, configured to measure the three-phase voltage output by the grid-forming MMC on the AC side and transform the three-phase voltage into dq-axis voltages;

[0056] A model output unit, configured to input the dq-axis comprehensive reference current and the dq-axis voltages into a pre-established current prediction control model and output the sub-module switching states corresponding to each phase bridge arm of the grid-forming MMC;

[0057] A sub-module switching state acting unit, configured to actuate the sub-module switching states corresponding to each phase bridge arm on the grid-forming MMC.

[0058] Optionally, the droop control unit with a low-pass filter includes:

[0059] A three-phase current conversion unit, configured to detect the three-phase voltage and three-phase current output by the grid-forming MMC of the medium-voltage flexible interconnected system at the grid connection point, transform the three-phase voltage to obtain a d-axis voltage and a q-axis voltage, and transform the three-phase current to obtain a d-axis current and a q-axis current;

[0060] An output power calculation unit, configured to calculate the active output power and reactive output power of the grid-forming MMC according to the d-axis voltage, the q-axis voltage, the d-axis current, and the q-axis current;

[0061] A phase reference value determination unit, configured to perform droop control with a low-pass filter based on the active output power and its reference value, and the reactive output power and its reference value to obtain the phase angle output by the grid-forming MMC and the d-axis voltage reference value;

[0062] A q-axis voltage reference value determination unit, configured to determine the voltage vector amplitude through the active output power and the reactive output power, and determine the q-axis voltage reference value output by the grid-forming MMC according to the voltage vector amplitude and the d-axis voltage reference value;

[0063] A current reference value calculation unit, configured to calculate a d-axis current reference value and a q-axis current reference value according to the d-axis voltage reference value and the q-axis voltage reference value;

[0064] A dq-axis current inverse transformation unit, configured to transform the d-axis current reference value and the q-axis current reference value through the phase angle to obtain the three-phase current reference values output by the grid-forming MMC.

[0065] Optionally, the output power calculation unit includes:

[0066] An output power calculation subunit, configured to calculate the active output power and reactive output power of the network-forming MMC by using the following formula:

[0067]

[0068] where k is the current moment, is the active output power of the network-forming MMC, is the reactive output power of the network-forming MMC, is the d-axis voltage, is the q-axis voltage, is the d-axis current, is the q-axis current.

[0069] Optionally, the q-axis voltage reference value determination unit includes:

[0070] A q-axis voltage reference value determination subunit, configured to determine the q-axis voltage reference value output by the network-forming MMC by using the following formula:

[0071]

[0072] where, is the q-axis voltage reference value output by the network-forming MMC, is the effective value of the phase voltage, , is the amplitude of the voltage vector, is the d-axis voltage reference value.

[0073] Optionally, the current reference value calculation unit includes:

[0074] A current reference value calculation subunit, configured to calculate the d-axis current reference value and q-axis current reference value by using the following formula:

[0075]

[0076] where, is the d-axis current reference value, is the q-axis current reference value, is the reference value of the active output power, is the reference value of the reactive output power.

[0077] Optionally, the dq-axis current inverse transformation unit includes:

[0078] The dq-axis current inverse transformation sub-unit is used to transform the d-axis current reference value and the q-axis current reference value through the phase angle according to the following formula to obtain the three-phase current reference values output by the network-forming MMC:

[0079]

[0080] Wherein, is the a-phase current reference value output by the network-forming MMC, is the b-phase current reference value output by the network-forming MMC, is the c-phase current reference value output by the network-forming MMC, is the phase angle.

[0081] Optionally, the device further includes:

[0082] A training data acquisition unit, configured to acquire current prediction training data of the network-forming MMC, where the current prediction training data includes three-phase output currents and three-phase output voltages output by the network-forming MMC on the AC side;

[0083] A dq-axis voltage and current conversion unit, configured to convert the three-phase output currents into a d-axis output current and a q-axis output current, and convert the three-phase output voltages into a d-axis output voltage and a q-axis output voltage;

[0084] A model training unit, configured to train a current prediction control model through the current prediction training data, and the current prediction control model is:

[0085]

[0086] Wherein, is the predicted value of the d-axis current output by the network-forming MMC at the next moment, is the d-axis output current, is the q-axis output current, is the sampled value of the d-axis voltage output by the network-forming MMC at the previous moment, is the increment of the d-axis voltage output by the network-forming MMC on the AC side, is the predicted value of the q-axis current output by the network-forming MMC at the next moment, is the sampled value of the q-axis voltage output by the network-forming MMC at the previous moment, is the increment of the q-axis voltage output by the network-forming MMC on the AC side, is the first coefficient, , is the sampling coefficient, is the sampling period, is the second coefficient, , is the current angular frequency of the medium-voltage flexible interconnection system, is the third coefficient, , is the arm inductor of the grid-forming MMC;

[0087] The model feedback correction unit is used to perform feedback correction on the current prediction control model to obtain the feedback-corrected current prediction control model. The process of the feedback correction is as follows:

[0088]

[0089] where k is the current moment, k + 1 is the next moment, and k - 1 is the previous moment, is the predicted value of the corrected d-axis current, is the predicted value of the corrected q-axis current, is the d-axis current error correction factor, is the q-axis current error correction factor.

[0090] By means of the above technical solution, in this application, the grid-forming MMC of the medium-voltage flexible interconnection system is subjected to droop control with a low-pass filter to calculate the three-phase current reference values output by the grid-forming MMC, and the three-phase current reference values are superimposed with the preset three-phase unbalanced reference current and the preset harmonic compensation reference current to obtain the comprehensive three-phase reference current. The comprehensive three-phase reference current is transformed into the dq-axis comprehensive reference current, the three-phase voltage output by the grid-forming MMC on the AC side is measured, and the three-phase voltage is transformed into the dq-axis voltage. The dq-axis comprehensive reference current and the dq-axis voltage are input into the pre-established current prediction control model to output the sub-module switching states corresponding to each phase arm of the grid-forming MMC, and the sub-module switching states corresponding to each phase arm are applied to the grid-forming MMC. It can be seen that after power synchronization is achieved through droop control with a low-pass filter, virtual inertia can be generated under large disturbances to achieve precise and rapid response of the grid-forming MMC. Furthermore, applying model predictive current control to the current inner loop can further accelerate the system dynamic response speed and improve the system operation stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0091] By reading the following detailed description of the preferred embodiments, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of this application. Moreover, throughout the drawings, the same reference numerals are used to represent the same components. In the drawings:

[0092] Figure 1 is a schematic flow chart for implementing the control of the grid-forming MMC based on droop control with a low-pass filter provided by an embodiment of this application;

[0093] Figure 2 It is a schematic structural diagram of a medium-voltage flexible interconnection system provided by an embodiment of the present application;

[0094] Figure 3 It is a schematic structural diagram of a network-forming MMC provided by an embodiment of the present application;

[0095] Figure 4 It is a general control block diagram of network-forming MMC control based on droop control with a low-pass filter provided by an embodiment of the present application;

[0096] Figure 5 It is a schematic flow diagram of realizing droop control with a low-pass filter for the network-forming MMC of the medium-voltage flexible interconnection system provided by an embodiment of the present application;

[0097] Figure 6 It is a droop control strategy block diagram with a low-pass filter provided by an embodiment of the present application;

[0098] Figure 7 It is a schematic diagram of the control process of the sub-module switch state provided by an embodiment of the present application;

[0099] Figure 8 It is a schematic structural diagram of a device for realizing network-forming MMC control based on droop control with a low-pass filter provided by an embodiment of the present application. Detailed implementation manners

[0100] Next, in combination with the drawings in the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0101] The solution of the present application can be implemented based on a terminal with data processing capabilities, and the terminal can be a computer, the cloud, a server, etc.

[0102] Next, in combination with Figure 1 As described above, the medium-voltage flexible interconnection control method of the network-forming MMC of the present application may include the following steps:

[0103] Step S110: Perform droop control with a low-pass filter on the network-forming MMC of the medium-voltage flexible interconnection system to calculate the three-phase current reference values output by the network-forming MMC.

[0104] Specifically, power synchronization can be achieved through droop control with a low-pass filter, and the angular frequency, phase angle, and power current reference values of the medium-voltage flexible interconnection system can be obtained.

[0105] Among them, as Figure 2 shown, the medium-voltage flexible interconnection system may include two grid-forming MMCs with the same structure, a first AC bus, a second AC bus, and a DC bus. The two grid-forming MMCs are connected through the DC bus, and the DC sides of the two grid-forming MMCs can be connected in a "back-to-back" manner through the DC bus. One of the grid-forming MMCs is connected to the first AC bus, and the other grid-forming MMC is connected to the second AC bus. Both AC buses can be 10 kV AC buses, and the voltage of the DC bus can be 10 kV.

[0106] Among them, each phase of each grid-forming MMC includes two bridge arms, and each bridge arm includes a bridge arm inductor and multiple sub-modules. Each phase of each grid-forming MMC is connected to the AC bus through a filter inductor.

[0107] As Figure 3 shown, each phase of the grid-forming MMC consists of upper and lower bridge arms. Each bridge arm consists of X cascaded half-bridge sub-modules and 1 bridge arm inductor L. Here, m represents the phase where the MMC converter is connected to the AC bus, m = a, b, c respectively represent the three phases, j represents the upper and lower bridge arms, j = 1 represents the upper bridge arm, and j = 2 represents the lower bridge arm. The upper end point of each phase's upper bridge arm is connected to the P node. The lower end point of each phase's lower bridge arm is connected to the N node.

[0108] Among them, the DC bus voltage is 20 kV, the bridge arm inductor of the grid-forming MMC is 2 mH, the number X of half-bridge sub-modules in each bridge arm is 10, and the rated value of the sub-module capacitor voltage is 2000 V.

[0109] Step S120: Superimpose the three-phase current reference values with the preset three-phase unbalanced reference current and the preset harmonic compensation reference current to obtain a comprehensive three-phase reference current, and transform the comprehensive three-phase reference current into a dq-axis comprehensive reference current.

[0110] Step S130: Measure the three-phase voltages output by the grid-forming MMC on the AC side, and transform the three-phase voltages into dq-axis voltages.

[0111] Step S140: Input the dq-axis comprehensive reference current and the dq-axis voltage into a pre-established current prediction control model, and output the sub-module switching states corresponding to each phase's bridge arm of the grid-forming MMC.

[0112] Step S150: Apply the sub-module switching states corresponding to each phase's bridge arm to the grid-forming MMC.

[0113] Specifically, refer to Figure 4, in the droop control module, power synchronization can be achieved through droop control to obtain the phase angle θ, and further perform three-phase current-voltage conversion to obtain the dq-axis current i dq (k) and voltage U dq (k). The output power of the grid-forming MMC is obtained by calculating the power through the dq-axis current and voltage. Further, according to the instantaneous power control theory, after reactive power control, the reference value of the dq-axis output voltage U dq * is obtained, and the reference value of the dq-axis current i dq * is calculated through the voltage loop according to the instantaneous power. The reference value of the dq-axis current is inverse-transformed to obtain the reference value of the three-phase current i pm * . The reference value of the three-phase current i pm * is superimposed with the preset three-phase unbalanced reference current and the preset harmonic compensation reference current i hm * to obtain the comprehensive three-phase reference current. In the model predictive current control module (MPCC) module, the comprehensive three-phase reference current is converted into the dq-axis comprehensive reference current i gpd *, and the dq-axis comprehensive reference current i gpd * is controlled by model predictive current to output the three-phase switching signals S a 、S b 、S c and act on the grid-forming MMC.

[0114] For the grid-forming MMC control method based on droop control with a low-pass filter provided in this embodiment, by performing droop control with a low-pass filter on the grid-forming MMC of the medium-voltage flexible interconnection system to calculate the reference value of the three-phase current output by the grid-forming MMC, superimposing the reference value of the three-phase current with the preset three-phase unbalanced reference current and the preset harmonic compensation reference current to obtain the comprehensive three-phase reference current, and converting the comprehensive three-phase reference current into the dq-axis comprehensive reference current, measuring the three-phase voltage output by the grid-forming MMC on the AC side, and converting the three-phase voltage into the dq-axis voltage, inputting the dq-axis comprehensive reference current and the dq-axis voltage into a pre-established current prediction control model, outputting the sub-module switching state corresponding to each phase bridge arm of the grid-forming MMC, and acting the sub-module switching state corresponding to each phase bridge arm on the grid-forming MMC. It can be seen that after power synchronization is achieved through droop control with a low-pass filter, virtual inertia can be generated under large disturbances to achieve precise and rapid response of the grid-forming MMC. Furthermore, applying model predictive current control to the current inner loop can further accelerate the system dynamic response speed and improve the system operation stability.

[0115] In some embodiments of the present application, the process of performing droop control with a low-pass filter on the network-forming MMC of the medium-voltage flexible interconnection system to calculate the three-phase current reference values output by the network-forming MMC is introduced as follows Figure 5 As shown, this process may include:

[0116] Step S111: Detect the three-phase voltage and three-phase current output by the network-forming MMC of the medium-voltage flexible interconnection system at the grid connection point, and transform the three-phase voltage to obtain the d-axis voltage and q-axis voltage, and transform the three-phase current to obtain the d-axis current and q-axis current.

[0117] Specifically, the droop control strategy can be composed of an active loop and a reactive loop to respectively achieve active-frequency and reactive-voltage regulation. The active-frequency and reactive-voltage relationships are shown in the following equations:

[0118]

[0119] Wherein, is the instantaneous active power, is the instantaneous reactive power, is the required output active power reference value, is the required output reactive power reference value, is the active droop coefficient, =6.28*10 6 , is the second droop coefficient, =5.75*10 -4 , is the output angular frequency of the network-forming MMC, is the rated angular frequency of the distribution network, =100π, is the d-axis output voltage reference value, is the effective value of the phase voltage, =5.77kV.

[0120] Among them, the three-phase current and voltage can be transformed into dq-axis current i om (k), current i gm (k), m = a, b, c, by detecting the output voltage U gd (k) and current i gq (k) of the network-forming MMC at the grid connection point, to obtain dq-axis current i od (k) and i oq (k), as well as dq-axis voltage U

[0121] Step S112: Calculate the active output power and reactive output power of the grid-forming MMC according to the d-axis voltage, q-axis voltage, d-axis current, and q-axis current.

[0122] Specifically, the active output power and reactive output power of the grid-forming MMC can be calculated using the following formula:

[0123]

[0124] where k is the current moment, is the active output power of the grid-forming MMC, is the reactive output power of the grid-forming MMC, is the d-axis voltage, is the q-axis voltage, is the d-axis current, is the q-axis current.

[0125] Step S113: Based on the active output power and its reference value, as well as the reactive output power and its reference value, perform droop control with a low-pass filter to obtain the phase angle output by the grid-forming MMC and the reference value of the d-axis voltage.

[0126] Specifically, as Figure 6 shown, by inputting the instantaneous active and reactive powers P(k), Q(k) and their reference values P * and Q * the phase angle θ output by the grid-forming MMC can be obtained, and the reference value U d * of the d-axis output voltage.

[0127] It can be understood that in order to reduce the high-frequency noise that may exist in the output frequency signal, a low-pass filter is connected in series in the active synchronization loop. Among them, ω APC is the cut-off frequency of the low-pass filter.

[0128] Step S114: Determine the voltage vector amplitude through the active output power and reactive output power, and determine the reference value of the q-axis voltage output by the grid-forming MMC according to the voltage vector amplitude and the reference value of the d-axis voltage.

[0129] Specifically, according to the instantaneous power control theory, given the active and reactive power reference values P* and Q * , the reference value U d * of the d-axis output voltage, U q * can be determined through geometric relationships. The magnitude |U| of the voltage vector should be equal to the effective value U n of the phase voltage:

[0130]

[0131] Furthermore, the q-axis voltage reference value output by the grid-forming MMC can be determined using the following formula:

[0132]

[0133] where is the q-axis voltage reference value output by the grid-forming MMC, is the effective value of the phase voltage, , is the amplitude of the voltage vector, is the d-axis voltage reference value.

[0134] Step S115: Calculate the d-axis current reference value and the q-axis current reference value according to the d-axis voltage reference value and the q-axis voltage reference value.

[0135] Specifically, the current reference value to be output can be inversely deduced according to the instantaneous power calculation. Specifically, the following formula is used to calculate the d-axis current reference value and the q-axis current reference value:

[0136]

[0137] where is the d-axis current reference value, is the q-axis current reference value, is the reference value of the active output power, is the reference value of the reactive output power.

[0138] Step S116: Transform the d-axis current reference value and the q-axis current reference value through the phase angle to obtain the three-phase current reference value output by the grid-forming MMC.

[0139] Specifically, the three-phase current reference value required for the MMC output power can be obtained through the Park-Clarke inverse transformation. Specifically, the following formula is used to transform the d-axis current reference value and the q-axis current reference value through the phase angle to obtain the three-phase current reference value output by the grid-forming MMC:

[0140]

[0141] where is the a-phase current reference value output by the grid-forming MMC, is the b-phase current reference value output by the grid-forming MMC, is the c-phase current reference value output by the grid-forming MMC, is the phase angle.

[0142] In some embodiments of the present application, the establishment process of the current prediction control model mentioned in the foregoing embodiments is introduced. The establishment process may include:

[0143] S1. Obtain the current prediction training data of the network-forming MMC.

[0144] Specifically, the current prediction training data includes the three-phase output current and three-phase output voltage output by the network-forming MMC on the AC side.

[0145] S2. Convert the three-phase output current into a d-axis output current and a q-axis output current, and convert the three-phase output voltage into a d-axis output voltage and a q-axis output voltage.

[0146] S3. Train a current prediction control model through the current prediction training data.

[0147] Specifically, the training process of the current prediction control model may include three stages: model establishment, model feedback correction, and rolling optimization.

[0148] Among them, the initially established current prediction control model is:

[0149]

[0150] where k is the current moment, k + 1 is the next moment, and k - 1 is the previous moment. is the predicted value of the d-axis current output by the network-forming MMC at the next moment. is the d-axis output current. is the q-axis output current. is the sampled value of the d-axis voltage output by the network-forming MMC at the previous moment. is the d-axis voltage increment output by the network-forming MMC on the AC side. is the predicted value of the q-axis current output by the network-forming MMC at the next moment. is the sampled value of the q-axis voltage output by the network-forming MMC at the previous moment. is the q-axis voltage increment output by the network-forming MMC on the AC side. is the first coefficient. , is the sampling coefficient. is the sampling period. is the second coefficient. , is the current angular frequency of the medium-voltage flexible interconnection system. is the third coefficient. , is the arm inductance of the network-forming MMC.

[0151] Further, perform feedback correction on the current prediction control model to obtain the current prediction control model after feedback correction.

[0152] Specifically, the process of feedback correction can be as follows:

[0153] .

[0154] Among them, is the predicted value of the corrected d-axis current, is the predicted value of the corrected q-axis current, is the d-axis current error correction factor, is the q-axis current error correction factor;

[0155] Further, establish a control objective function regarding the measured value and reference value of the output current to complete rolling optimization.

[0156] Specifically, the objective function of the current prediction control model can be:

[0157]

[0158] Among them, is the objective function, is the d-axis current error weight coefficient, is the q-axis current error weight coefficient, is the d-axis control voltage increment error weight coefficient, is the q-axis control voltage increment error weight coefficient, is the d-axis output current reference value, is the q-axis output current reference value.

[0159] It can be understood that after the current prediction control model is trained, the switching states of the sub-modules of each phase bridge arm can be controlled according to the objective function and in combination with the Nearest Level Modulation (NLM) equal voltage sharing algorithm. The specific control process can include:

[0160] S1. For each phase bridge arm, determine the number n of sub-modules to be put into the phase bridge arm according to the objective function.

[0161] S2. If the total number N of sub-modules in each phase bridge arm is 0, then cut off all the sub-modules of the phase bridge arm.

[0162] S3. If the number n of sub-modules to be put into each phase bridge arm is equal to the total number N of sub-modules of the bridge arm, then put all the sub-modules of the phase bridge arm into operation.

[0163] S4. If the total number N of sub-modules in each phase leg is not zero, and the number n of sub-modules to be inserted is not equal to or less than the total number N of sub-modules in the phase leg, then sort the sub-modules to be inserted according to their capacitor voltages.

[0164] S5. If the output current value of the sub-modules in each phase leg is greater than 0, then insert the n sub-modules with lower voltages in the phase leg; otherwise, insert the n sub-modules with higher voltages in the phase leg.

[0165] Specifically, the application of the NLM voltage equalization algorithm to the control process of the sub-module switching state can refer to Figure 7 , I sm is the output current value of the sub-modules in each phase leg.

[0166] Next, the device for implementing the grid-forming MMC control based on droop control with a low-pass filter provided in the embodiments of the present application will be described. The device for implementing the grid-forming MMC control based on droop control with a low-pass filter described below can be correspondingly referred to the method for implementing the grid-forming MMC control based on droop control with a low-pass filter described above.

[0167] See Figure 8 , Figure 8 which is a schematic structural diagram of a device for implementing the grid-forming MMC control based on droop control with a low-pass filter disclosed in the embodiments of the present application.

[0168] As Figure 8 shown, the device may include:

[0169] The droop control unit 11 with a low-pass filter is configured to perform droop control with a low-pass filter on the grid-forming MMC of the medium-voltage flexible interconnection system to calculate the three-phase current reference values output by the grid-forming MMC;

[0170] The current superposition unit 12 is configured to superpose the three-phase current reference values with a preset three-phase unbalanced reference current and a preset harmonic compensation reference current to obtain a comprehensive three-phase reference current, and transform the comprehensive three-phase reference current into a dq-axis comprehensive reference current;

[0171] The voltage conversion unit 13 is configured to measure the three-phase voltages output by the grid-forming MMC on the AC side and transform the three-phase voltages into dq-axis voltages;

[0172] The model output unit 14 is configured to input the dq-axis comprehensive reference current and the dq-axis voltages into a pre-established current predictive control model and output the sub-module switching states corresponding to each phase leg of the grid-forming MMC;

[0173] The sub-module switch state acting unit 15 is used to actuate the sub-module switch states corresponding to each phase arm on the grid-forming MMC.

[0174] Optionally, the droop control unit with a low-pass filter includes:

[0175] A three-phase current conversion unit is used to detect the three-phase voltage and three-phase current output by the grid-forming MMC of the medium-voltage flexible interconnected system, and convert the three-phase voltage to obtain the d-axis voltage and q-axis voltage, and convert the three-phase current to obtain the d-axis current and q-axis current;

[0176] An output power calculation unit is used to calculate the active output power and reactive output power of the grid-forming MMC according to the d-axis voltage, the q-axis voltage, the d-axis current, and the q-axis current;

[0177] A phase reference value determination unit is used to perform droop control with a low-pass filter based on the active output power and its reference value, and the reactive output power and its reference value, to obtain the phase angle output by the grid-forming MMC and the d-axis voltage reference value;

[0178] A q-axis voltage reference value determination unit is used to determine the voltage vector amplitude through the active output power and the reactive output power, and determine the q-axis voltage reference value output by the grid-forming MMC according to the voltage vector amplitude and the d-axis voltage reference value;

[0179] A current reference value calculation unit is used to calculate the d-axis current reference value and the q-axis current reference value according to the d-axis voltage reference value and the q-axis voltage reference value;

[0180] A dq-axis current inverse conversion unit is used to transform the d-axis current reference value and the q-axis current reference value through the phase angle to obtain the three-phase current reference value output by the grid-forming MMC.

[0181] Optionally, the output power calculation unit includes:

[0182] An output power calculation sub-unit is used to calculate the active output power and reactive output power of the grid-forming MMC by using the following formula:

[0183]

[0184] where k is the current moment, is the active output power of the grid-forming MMC, is the reactive output power of the grid-forming MMC, is the d-axis voltage, is the q-axis voltage, is the d-axis current, is the q-axis current.

[0185] Optionally, the q-axis voltage reference value determination unit includes:

[0186] The q-axis voltage reference value determination subunit is configured to determine the q-axis voltage reference value output by the network-forming MMC by using the following formula:

[0187]

[0188] where is the q-axis voltage reference value output by the network-forming MMC, is the effective value of the phase voltage, , is the amplitude of the voltage vector, is the d-axis voltage reference value.

[0189] Optionally, the current reference value calculation unit includes:

[0190] The current reference value calculation subunit is configured to calculate the d-axis current reference value and the q-axis current reference value by using the following formula:

[0191]

[0192] where is the d-axis current reference value, is the q-axis current reference value, is the reference value of the active output power, is the reference value of the reactive output power.

[0193] Optionally, the dq-axis current inverse transformation unit includes:

[0194] The dq-axis current inverse transformation subunit is configured to transform the d-axis current reference value and the q-axis current reference value through the phase angle by using the following formula to obtain the three-phase current reference values output by the network-forming MMC:

[0195]

[0196] where is the a-phase current reference value output by the network-forming MMC, is the b-phase current reference value output by the network-forming MMC, is the c-phase current reference value output by the network-forming MMC, is the phase angle.

[0197] Optionally, the device further includes:

[0198] A training data acquisition unit for acquiring current prediction training data of the grid-forming MMC, where the current prediction training data includes three-phase output currents and three-phase output voltages output by the grid-forming MMC on the AC side;

[0199] A dq-axis voltage and current conversion unit for converting the three-phase output currents into a d-axis output current and a q-axis output current, and converting the three-phase output voltages into a d-axis output voltage and a q-axis output voltage;

[0200] A model training unit for training a current prediction control model through the current prediction training data, and the current prediction control model is:

[0201]

[0202] where, is the predicted value of the d-axis current output by the grid-forming MMC at the next moment, is the d-axis output current, is the q-axis output current, is the sampled value of the d-axis voltage output by the grid-forming MMC at the previous moment, is the d-axis voltage increment output by the grid-forming MMC on the AC side, is the predicted value of the q-axis current output by the grid-forming MMC at the next moment, is the sampled value of the q-axis voltage output by the grid-forming MMC at the previous moment, is the q-axis voltage increment output by the grid-forming MMC on the AC side, is the first coefficient, , is the sampling coefficient, is the sampling period, is the second coefficient, , is the current angular frequency of the medium-voltage flexible interconnection system, is the third coefficient, , is the arm inductance of the grid-forming MMC;

[0203] A model feedback correction unit for performing feedback correction on the current prediction control model to obtain a feedback-corrected current prediction control model, and the process of the feedback correction is:

[0204]

[0205] where k is the current moment, k + 1 is the next moment, and k - 1 is the previous moment, is the corrected predicted value of the d-axis current, is the corrected predicted value of the q-axis current, is the d-axis current error correction factor, is the q-axis current error correction factor.

[0206] Optionally, the objective function of the current prediction control model is:

[0207]

[0208] wherein, is the objective function, is the d-axis current error weight coefficient, is the q-axis current error weight coefficient, is the d-axis control voltage increment error weight coefficient, is the q-axis control voltage increment error weight coefficient, is the d-axis output current reference value, is the q-axis output current reference value.

[0209] Optionally, the medium-voltage flexible interconnection system includes two grid-forming MMCs with the same structure, a first AC bus, a second AC bus, and a DC bus. The two grid-forming MMCs are connected through the DC bus, one of the grid-forming MMCs is connected to the first AC bus, and the other grid-forming MMC is connected to the second AC bus;

[0210] Each phase of each grid-forming MMC includes two arms, and each arm includes an arm inductor and multiple sub-modules;

[0211] Each phase of each grid-forming MMC is connected to the corresponding AC bus through a filter inductor.

[0212] Finally, it should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements, but also other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising an..." does not exclude the existence of additional identical elements in the process, method, article or device comprising the element.

[0213] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. The various embodiments can be combined as needed, and the same or similar parts can be referred to each other.

[0214] The foregoing description of the disclosed embodiments enables those skilled in the art to practice or use the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A control method for a meshed MMC based on droop control with a low-pass filter, characterized in that: include: Performing droop control with a low-pass filter on a meshed MMC of a medium voltage flexible interconnection system to calculate a three-phase current reference value output by the meshed MMC; The three-phase current reference value is superimposed with a preset three-phase unbalanced reference current and a preset harmonic compensation reference current to obtain a comprehensive three-phase reference current, and the comprehensive three-phase reference current is converted into a dq-axis comprehensive reference current; Measuring the three-phase voltage outputted by the grid-forming MMC on the AC side, and converting the three-phase voltage into dq axis voltage; Inputting the dq axis comprehensive reference current and the dq axis voltage into a pre-established current prediction control model, and outputting the submodule switch state corresponding to each phase bridge arm of the meshed MMC; The switch state of the submodule corresponding to each phase bridge arm is applied to the meshed MMC.

2. The method according to claim 1, characterized in that The method of performing droop control with a low-pass filter on the meshed MMC of the medium voltage flexible interconnection system to calculate the three-phase current reference value output by the meshed MMC includes: Detecting the three-phase voltage and three-phase current outputted by the grid-connected MMC of the medium voltage flexible interconnection system at the grid connection point, and transforming the three-phase voltage to obtain the d-axis voltage and the q-axis voltage, and transforming the three-phase current to obtain the d-axis current and the q-axis current; Calculating the active output power and reactive output power of the grid-forming MMC according to the d-axis voltage, the q-axis voltage, the d-axis current, and the q-axis current; Based on the active output power and its reference value, and the reactive output power and its reference value, a droop control with a low-pass filter is performed to obtain a phase angle and a d-axis voltage reference value output by the grid-forming MMC; Determine a voltage vector amplitude through the active output power and the reactive output power, and determine a q-axis voltage reference value output by the grid-forming MMC according to the voltage vector amplitude and the d-axis voltage reference value; Calculating a d-axis current reference value and a q-axis current reference value according to the d-axis voltage reference value and the q-axis voltage reference value; The d-axis current reference value and the q-axis current reference value are transformed by the phase angle to obtain the three-phase current reference value output by the meshed MMC.

3. The method according to claim 2, characterized in that Calculating the active output power and reactive output power of the grid-forming MMC according to the d-axis voltage, the q-axis voltage, the d-axis current, and the q-axis current, including: The active output power and reactive output power of the grid-forming MMC are calculated using the following formula: Among them, k is the current time, is the active output power of the grid-forming MMC, is the reactive output power of the grid-type MMC, is the d-axis voltage, is the q-axis voltage, is the d-axis current, is the q-axis current.

4. The method according to claim 2, characterized in that Determining a q-axis voltage reference value output by the meshed MMC according to the voltage vector amplitude and the d-axis voltage reference value includes: The q-axis voltage reference value output by the meshed MMC is determined using the following formula: in, is the q-axis voltage reference value output by the meshed MMC, is the effective value of the phase voltage, , is the voltage vector magnitude, is the d-axis voltage reference value.

5. The method according to claim 4, characterized in that Calculating a d-axis current reference value and a q-axis current reference value according to the d-axis voltage reference value and the q-axis voltage reference value includes: Use the following formula to calculate the d-axis current reference value and the q-axis current reference value: in, is the d-axis current reference value, is the q-axis current reference value, is the reference value of the active output power, is the reference value of the reactive output power.

6. The method according to claim 5, characterized in that The d-axis current reference value and the q-axis current reference value are transformed by the phase angle to obtain a three-phase current reference value output by the grid-forming MMC, including: The d-axis current reference value and the q-axis current reference value are transformed by the phase angle using the following formula to obtain the three-phase current reference value output by the grid-forming MMC: in, is the a-phase current reference value output by the grid-type MMC, is the b-phase current reference value output by the grid-type MMC, is the c-phase current reference value output by the grid-type MMC, is the phase angle.

7. The method according to claim 1, characterized in that The process of establishing the current prediction control model includes: Acquire current prediction training data of the meshed MMC, wherein the current prediction training data includes three-phase output current and three-phase output voltage output by the meshed MMC on the AC side; Converting the three-phase output current into a d-axis output current and a q-axis output current, and converting the three-phase output voltage into a d-axis output voltage and a q-axis output voltage; The current prediction control model is obtained by training the current prediction training data, and the current prediction control model is: Among them, k is the current moment, k+1 is the next moment, and k-1 is the previous moment. is the predicted value of the d-axis current output by the meshed MMC at the next moment, is the d-axis output current, is the q-axis output current, is the d-axis voltage sampling value output by the meshed MMC at the previous moment, is the d-axis voltage increment output by the meshed MMC on the AC side, is the predicted value of the q-axis current output by the meshed MMC at the next moment, is the q-axis voltage sampling value output by the meshed MMC at the previous moment, is the q-axis voltage increment output by the grid-type MMC on the AC side, is the first coefficient, , is the sampling coefficient, is the sampling period, is the second coefficient, , is the current angular frequency of the medium voltage flexible interconnection system, is the third coefficient, , is the bridge arm inductance of the meshed MMC; Feedback correction is performed on the current prediction control model to obtain a current prediction control model after feedback correction. The feedback correction process is as follows: in, is the corrected d-axis current prediction value, is the corrected q-axis current prediction value, is the d-axis current error correction factor, is the q-axis current error correction factor.

8. The method according to claim 7, characterized in that The objective function of the current predictive control model is: in, is the objective function, is the d-axis current error weight coefficient, is the q-axis current error weight coefficient, is the d-axis control voltage increment error weight coefficient, is the q-axis control voltage increment error weight coefficient, is the d-axis output current reference value, It is the reference value of q-axis output current.

9. The method according to claim 1, characterized in that: The medium voltage flexible interconnection system comprises two meshing MMCs of the same structure, a first AC bus, a second AC bus and a DC bus, wherein the two meshing MMCs are connected via the DC bus, wherein one of the meshing MMCs is connected to the first AC bus, and the other meshing MMC is connected to the second AC bus; Each phase of each grid-type MMC includes two bridge arms, and each bridge arm includes a bridge arm inductor and multiple submodules; Each phase of each grid-type MMC is connected to the corresponding AC bus through a filter inductor.

10. A meshed MMC control device based on droop control with a low-pass filter, characterized in that: include: A droop control unit with a low-pass filter, used to perform droop control with a low-pass filter on a meshed MMC of a medium-voltage flexible interconnection system, so as to calculate a three-phase current reference value output by the meshed MMC; A current superposition unit, used for superimposing the three-phase current reference value with a preset three-phase unbalanced reference current and a preset harmonic compensation reference current to obtain a comprehensive three-phase reference current, and converting the comprehensive three-phase reference current into a dq-axis comprehensive reference current; A voltage conversion unit, used to measure the three-phase voltage output by the meshed MMC on the AC side and convert the three-phase voltage into dq axis voltage; A model output unit, used to input the dq axis comprehensive reference current and the dq axis voltage into a pre-established current prediction control model, and output the submodule switch state corresponding to each phase bridge arm of the meshed MMC; The submodule switch state action unit is used to apply the submodule switch state corresponding to each phase bridge arm to the meshed MMC.

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