Medium-voltage flexible interconnection control method and device based on virtual synchronous generator

Abstract

The invention discloses a medium-voltage flexible interconnection control method and device based on a virtual synchronous generator, and the method comprises the steps: carrying out the virtual synchronous generator control of a network building type MMC of a medium-voltage flexible interconnection system, so as to calculate a three-phase current reference value, superposing the three-phase current reference value with a preset unbalance compensation current and a preset harmonic current, and obtaining a three-phase current reference value; and a total output reference current is obtained, a three-phase circulation value and a three-phase sub-module capacitor voltage value are measured and input to a pre-established current prediction control model, and a sub-module switching state is output and finally acts on the network construction type MMC. Therefore, through the control of the virtual synchronous generator, the medium-voltage flexible interconnection system has the voltage and frequency regulation characteristics of a synchronous machine, and then through the model prediction current control, the rapid tracking of output power current and compensation current, the internal circulation suppression and the multi-target control of sub-module capacitance voltage sharing can be better realized; and the output current dynamic response capability and the output current quality are improved.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and more specifically, to a medium-voltage flexible interconnection control method and device based on a virtual synchronous generator. Background Art

[0002] In today's society, the demand for electricity continues to climb, and the environmental pressure is also increasing day by day. The power system is developing towards a direction with a high proportion of new energy and a high proportion of power electronic devices. The new energy power generation such as wind power and photovoltaic power generation has grown rapidly. At the same time, power electronic equipment has become a key part of the power system, and the modular multilevel converter (MMC) is a key component among them. The traditional power grid form dominated by synchronous generators has changed.

[0003] At present, power electronic converters mostly adopt a grid-following control mode. Most of the existing flexible interconnection control strategies rely on the voltage and frequency of the power grid and cannot provide voltage and frequency support for the power grid by themselves, making it difficult to independently support the power grid system. Although the grid-forming converter using droop control can provide active support for frequency and voltage, the droop control target is mainly power synchronization. It not only requires an additional complex control loop to suppress the internal circulating current of the MMC and the equal voltage of the sub-modules, but also the droop control mainly targets the output power of the converter and cannot quickly track the current components such as three-phase unbalance suppression.

[0004] Based on this, how to better quickly track the output power current and compensation current, effectively control the suppression of the internal circulating current and the equal voltage of the sub-modules is an issue that needs attention. Summary of the Invention

[0005] In view of the above problems, the present application provides a medium-voltage flexible interconnection control method and device based on a virtual synchronous generator, so that the virtual synchronous generator can achieve better current tracking and dynamic response through the voltage and frequency regulation characteristics of the synchronous machine, thereby effectively controlling the suppression of the internal circulating current and the equal voltage of the sub-modules, and improving the dynamic response ability of the output current and the output current quality.

[0006] To achieve the above object, the following specific solutions are proposed:

[0007] A medium-voltage flexible interconnection control method based on a virtual synchronous generator, comprising:

[0008] Performing virtual synchronous generator control 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;

[0009] Superimposing the three-phase current reference values with a preset unbalance compensation current and a preset harmonic current to obtain a total output reference current;

[0010] Measure the three-phase circulating current measurement values and the three-phase sub-module capacitor voltage values of the network-forming MMC, and input the total output reference current, the three-phase circulating current measurement values, and the three-phase sub-module capacitor voltage values into a pre-established current prediction control model to output the sub-module switching states corresponding to each phase arm of the network-forming MMC;

[0011] Apply the sub-module switching states corresponding to each phase arm to the network-forming MMC.

[0012] Optionally, performing virtual synchronous generator control 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 includes:

[0013] Obtain the three-phase grid-connected voltages and the three-phase grid-connected currents of the network-forming MMC of the medium-voltage flexible interconnection system, and calculate the active output power and the reactive output power of the network-forming MMC according to the three-phase grid-connected voltages and the three-phase grid-connected currents;

[0014] Based on the active output power and its reference value, and the reactive output power and its reference value, perform virtual synchronous generator control to obtain the phase angle output by the network-forming MMC;

[0015] Determine the d-axis current reference value output by the network-forming MMC according to the reference value of the active output power, and determine the q-axis current reference value output by the network-forming MMC according to the reference value of the reactive output power;

[0016] Through the phase angle, transform the d-axis current reference value and the q-axis current reference value to obtain the three-phase current reference values output by the network-forming MMC.

[0017] Optionally, calculating the active output power and the reactive output power of the network-forming MMC according to the three-phase grid-connected voltages and the three-phase grid-connected currents includes:

[0018] Use the following formula to calculate the active output power and the reactive output power of the network-forming MMC:

[0019]

[0020] 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 a-phase voltage of the three-phase grid-connected voltages, is the b-phase voltage of the three-phase grid-connected voltages, is the c-phase voltage of the three-phase grid-connected voltages, is the a-phase current of the three-phase current at the grid connection point, is the b-phase current of the three-phase current at the grid connection point, is the c-phase current of the three-phase current at the grid connection point.

[0021] Optionally, determining the reference value of the d-axis current output by the grid-forming MMC according to the reference value of the active output power, and determining the reference value of the q-axis current output by the grid-forming MMC according to the reference value of the reactive output power, includes:

[0022] Using the following formula to determine the reference value of the d-axis current output by the grid-forming MMC:

[0023]

[0024] where, is the reference value of the d-axis current output by the grid-forming MMC, is the effective value of the phase voltage, is the reference value of the active output power;

[0025] Using the following formula to determine the reference value of the q-axis current output by the grid-forming MMC:

[0026]

[0027] where, is the reference value of the q-axis current output by the grid-forming MMC, is the reference value of the reactive output power.

[0028] 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:

[0029] 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:

[0030]

[0031] where, 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.

[0032] Optionally, the establishment process of the current prediction control model includes:

[0033] Obtain the current prediction training data of the grid-forming MMC, where the current prediction training data includes the three-phase output current of the distribution network where the grid-forming MMC is located, the three-phase circulating current data of the grid-forming MMC, the three-phase sub-module capacitor voltage data of the grid-forming MMC, the upper-bridge-arm current and voltage data of the grid-forming MMC, and the lower-bridge-arm current and voltage data of the grid-forming MMC;

[0034] Train a current prediction control model through the current prediction training data, and the current prediction control model is:

[0035]

[0036] where k is the current moment, k + 1 is the next moment, and j is the phase sequence number. is the predicted value of the current output by the j-th phase of the grid-forming MMC at the next moment. is the output current of the j-th phase of the distribution network. is the measured voltage of the j-th phase of the distribution network at the current moment. is the output voltage of the j-th phase of the distribution network. is the sampling period. is the filter inductor of the AC bus connected to the distribution network. is the predicted value of the circulating current of the j-th phase of the grid-forming MMC at the next moment. is the measured value of the circulating current of the j-th phase of the grid-forming MMC. is the upper-bridge-arm inductor of the grid-forming MMC. is the lower-bridge-arm inductor of the grid-forming MMC. is the measured value of the DC voltage of the j-th phase of the grid-forming MMC. is the measured value of the upper-bridge-arm voltage of the grid-forming MMC. is the measured value of the lower-bridge-arm voltage of the grid-forming MMC. is the loop voltage output by the j-th phase of the grid-forming MMC. is the predicted value of the capacitor voltage of the I-th sub-module of the x-th bridge arm of the j-th phase of the grid-forming MMC at the next moment. is the measured value of the capacitor voltage of the I-th sub-module of the x-th bridge arm of the j-th phase of the grid-forming MMC. is the switching constant, S = 1 indicates that the sub-module is conducting, and S = 0 indicates that the sub-module is off. is the measured value of the current of the x-th bridge arm of the j-th phase of the grid-forming MMC.

[0037] Optionally, the objective function of the current prediction control model is:

[0038]

[0039] Among them, is the output current objective function, is the reference value of the output current of the j-th phase, is the circulating current suppression objective function, is the AC-side output power of the distribution network, is the capacitor voltage equalization objective function of the I-th sub-module of the x-th bridge arm, is the total number of sub-modules of each bridge arm.

[0040] 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;

[0041] Each phase of each grid-forming MMC includes two bridge arms, and each bridge arm includes a bridge arm inductor and multiple sub-modules;

[0042] Each phase of each grid-forming MMC is connected to the corresponding AC bus through a filter inductor.

[0043] A medium-voltage flexible interconnection control device based on a virtual synchronous generator includes:

[0044] A virtual synchronous generator control unit for performing virtual synchronous generator control on the grid-forming MMC of the medium-voltage flexible interconnection system to calculate the reference values of the three-phase currents output by the grid-forming MMC;

[0045] A current superposition unit for superposing the three-phase current reference values with a preset unbalance compensation current and a preset harmonic current to obtain a total output reference current;

[0046] A current model prediction unit for measuring the three-phase circulating current measurement values and the three-phase sub-module capacitor voltage values of the grid-forming MMC, and inputting the total output reference current, the three-phase circulating current measurement values, and the three-phase sub-module capacitor voltage values into a pre-established current prediction control model to output the sub-module switch states corresponding to each phase bridge arm of the grid-forming MMC;

[0047] A switch state application unit for applying the sub-module switch states corresponding to each phase bridge arm to the grid-forming MMC.

[0048] Optionally, the virtual synchronous generator control unit includes:

[0049] An output power calculation unit for obtaining the three-phase voltage and three-phase current at the grid connection point of the grid-forming MMC in the medium-voltage flexible interconnected system, and calculating the active output power and reactive output power of the grid-forming MMC according to the three-phase voltage and three-phase current at the grid connection point;

[0050] A phase angle determination unit for performing virtual synchronous generator control 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;

[0051] A current reference value determination unit for determining the d-axis current reference value output by the grid-forming MMC according to the reference value of the active output power, and determining the q-axis current reference value output by the grid-forming MMC according to the reference value of the reactive output power;

[0052] A dq-axis current inverse transformation unit for transforming 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.

[0053] Optionally, the output power calculation unit includes:

[0054] An output power calculation subunit for calculating the active output power and reactive output power of the grid-forming MMC by using the following formula:

[0055]

[0056] 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 a-phase voltage of the three-phase voltage at the grid connection point, is the b-phase voltage of the three-phase voltage at the grid connection point, is the c-phase voltage of the three-phase voltage at the grid connection point, is the a-phase current of the three-phase current at the grid connection point, is the b-phase current of the three-phase current at the grid connection point, is the c-phase current of the three-phase current at the grid connection point.

[0057] Optionally, the current reference value determination unit includes:

[0058] A d-axis current reference value calculation unit for determining the d-axis current reference value output by the grid-forming MMC by using the following formula:

[0059]

[0060] where, is the reference value of the d-axis current output by the grid-forming MMC, is the effective value of the phase voltage, is the reference value of the active output power;

[0061] The q-axis current reference value calculation unit is used to determine the reference value of the q-axis current output by the grid-forming MMC by using the following formula:

[0062]

[0063] where, is the reference value of the q-axis current output by the grid-forming MMC, is the reference value of the reactive output power.

[0064] Optionally, the dq-axis current inverse transformation unit includes:

[0065] The dq-axis current inverse transformation sub-unit is used to transform the reference value of the d-axis current and the reference value of the q-axis current through the phase angle by using the following formula to obtain the reference value of the three-phase current output by the grid-forming MMC:

[0066]

[0067] where, 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.

[0068] Optionally, the device further includes:

[0069] The training data acquisition unit is used to acquire the current prediction training data of the grid-forming MMC, and the current prediction training data includes the three-phase output current of the distribution network where the grid-forming MMC is located, the three-phase circulating current data of the grid-forming MMC, the three-phase sub-module capacitor voltage data of the grid-forming MMC, the upper bridge arm current and voltage data of the grid-forming MMC, and the lower bridge arm current and voltage data of the grid-forming MMC;

[0070] The model training unit is used to train the current prediction control model through the current prediction training data, and the current prediction control model is:

[0071]

[0072] where k is the current moment, k + 1 is the next moment, and j is the phase number, is the predicted value of the current output by the j-th phase of the grid-forming MMC at the next moment, is the output current of the j-th phase of the distribution network, is the measured voltage of the j-th phase of the distribution network at the current moment, is the output voltage of the j-th phase of the distribution network, is the sampling period, is the filtering inductor connected to the AC bus of the distribution network, is the predicted value of the circulating current of the j-th phase of the grid-forming MMC at the next moment, is the measured value of the circulating current of the j-th phase of the grid-forming MMC, is the upper arm inductor of the grid-forming MMC, is the lower arm inductor of the grid-forming MMC, is the measured value of the DC voltage of the j-th phase of the grid-forming MMC, is the measured value of the upper arm voltage of the grid-forming MMC, is the measured value of the lower arm voltage of the grid-forming MMC, is the loop voltage output by the j-th phase of the grid-forming MMC, is the predicted value of the capacitor voltage of the I-th sub-module of the x-th arm of the j-th phase of the grid-forming MMC at the next moment, is the measured value of the capacitor voltage of the I-th sub-module of the x-th arm of the j-th phase of the grid-forming MMC, is the switching constant, S = 1 indicates that the sub-module is turned on, and S = 0 indicates that the sub-module is turned off, is the measured value of the current of the x-th arm of the j-th phase of the grid-forming MMC.

[0073] With the above technical solutions, the present application controls the grid-forming MMC of the medium-voltage flexible interconnection system through virtual synchronous generator control to calculate the three-phase current reference values output by the grid-forming MMC, superimpose the three-phase current reference values with the preset unbalanced compensation current and the preset harmonic current to obtain the total output reference current, measure the three-phase circulating current measurement values and the three-phase sub-module capacitor voltage values of the grid-forming MMC, and input the total output reference current, the three-phase circulating current measurement values and the three-phase sub-module capacitor voltage values into a pre-established current prediction control model to output the sub-module switching states corresponding to each phase arm of the grid-forming MMC, and apply the sub-module switching states corresponding to each phase arm to the grid-forming MMC. Thus, through virtual synchronous generator control, the inertia and damping characteristics of the synchronous motor are simulated, and the output power is controlled, so that the medium-voltage flexible interconnection system has the voltage and frequency regulation characteristics of the synchronous machine. Furthermore, through model predictive current control, it is possible to better achieve multi-objective control of rapid tracking of output power current and compensation current, internal circulating current suppression, and equal voltage sharing of sub-module capacitors, and improve the dynamic response ability of the output current and the quality of the output current. Description of the Drawings

[0074] 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 illustrating the preferred embodiments and are not considered to be a limitation of the present application. Moreover, throughout the drawings, the same reference numerals are used to represent the same components. In the drawings:

[0075] Figure 1 FIG. is a schematic flowchart of a process for implementing medium-voltage flexible interconnection control based on a virtual synchronous generator provided by an embodiment of the present application;

[0076] Figure 2 FIG. is a schematic structural diagram of a medium-voltage flexible interconnection system provided by an embodiment of the present application;

[0077] Figure 3 FIG. is a schematic structural diagram of a grid-forming MMC provided by an embodiment of the present application;

[0078] Figure 4 FIG. is a general control block diagram of medium-voltage flexible interconnection control based on virtual synchronous generator control provided by an embodiment of the present application;

[0079] Figure 5 FIG. is a schematic flowchart of a process for implementing virtual synchronous generator control by a grid-forming MMC of a medium-voltage flexible interconnection system provided by an embodiment of the present application;

[0080] Figure 6 FIG. is a control strategy block diagram of a virtual synchronous generator control strategy provided by an embodiment of the present application;

[0081] Figure 7 FIG. is a control strategy block diagram of a control strategy of a current prediction control model provided by an embodiment of the present application;

[0082] Figure 8 FIG. is a schematic structural diagram of a device for implementing medium-voltage flexible interconnection control based on a virtual synchronous generator provided by an embodiment of the present application. Detailed Embodiments

[0083] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts shall fall within the protection scope of the present application.

[0084] 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.

[0085] Next, in conjunction with Figure 1As described above, the medium-voltage flexible interconnection control method based on a virtual synchronous generator of the present application may include the following steps:

[0086] Step S110: Perform virtual synchronous generator control 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.

[0087] Specifically, through virtual synchronous generator control, the medium-voltage flexible interconnection system can obtain the angular frequency, phase angle, and power current reference values of the grid connection point voltage and current.

[0088] Among them, as Figure 2 shown, the medium-voltage flexible interconnection system may include two network-forming MMCs with the same structure, a first AC bus, a second AC bus, and a DC bus. The two network-forming MMCs are connected by the DC bus, and the DC sides of the two network-forming MMCs can be connected in a "back-to-back" manner through the DC bus. One of the network-forming MMCs is connected to the first AC bus, and the other network-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.

[0089] Among them, each phase of each network-forming MMC includes two bridge arms, and each bridge arm includes a bridge arm inductor and multiple sub-modules. Each phase of each network-forming MMC is connected to the AC bus through a filter inductor.

[0090] As Figure 3 shown, each phase of the network-forming MMC consists of upper and lower bridge arms, and each bridge arm consists of X cascaded half-bridge sub-modules and 1 bridge arm inductor L mj composed. Here, m represents the phase in which 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. Each phase of the network-forming MMC is connected to the AC bus through a filter inductor L sm The upper end point of the upper bridge arm of each phase is connected to the P node. The lower end point of the lower bridge arm of each phase is connected to the N node.

[0091] Among them, the bridge arm inductor of the network-forming MMC is 2 mH, the number of half-bridge sub-modules X of each bridge arm is 10, and the rated value of the sub-module capacitor voltage is 2000 V.

[0092] Step S120: Superimpose the three-phase current reference values with the preset unbalance compensation current and the preset harmonic current to obtain the total output reference current.

[0093] Step S130: Measure the three-phase circulating current measurement values and the three-phase sub-module capacitor voltage values of the grid-forming MMC, and input the total output reference current, the three-phase circulating current measurement values, and the three-phase sub-module capacitor voltage values into a pre-established current prediction control model to output the sub-module switching states corresponding to each phase arm of the grid-forming MMC.

[0094] Step S140: Apply the sub-module switching states corresponding to each phase arm to the grid-forming MMC.

[0095] Specifically, referring to Figure 4 , in the virtual synchronous generator (VSG) power control module, by detecting the output voltage U om (k) and the output current i gm (k) of the grid-forming MMC at the grid connection point, the active output power and the reactive output power can be obtained through instantaneous power calculation. Furthermore, the phase angle θ can be obtained after VSG control. At the same time, from the reference value of the active output power and the reference value of the reactive output power, the d-axis current reference value i d * and the q-axis current reference value i q * are calculated. The dq-axis current reference values are inverse-transformed to obtain the three-phase current reference value i pm * . The three-phase current reference value i pm * is superimposed with the preset unbalance compensation current and the preset harmonic current i hm * to obtain the total output reference current. Finally, in the model predictive current control (MPCC) module, the total output reference current i gm * is controlled by model predictive current to output the three-phase switching signals S a 、S b 、S c and apply them to the grid-forming MMC.

[0096] The medium-voltage flexible interconnection control method based on a virtual synchronous generator provided in this embodiment controls the grid-forming MMC of the medium-voltage flexible interconnection system through virtual synchronous generator control to calculate the three-phase current reference values output by the grid-forming MMC. The three-phase current reference values are superimposed with a preset unbalance compensation current and a preset harmonic current to obtain a total output reference current. The three-phase circulating current measurement values and the three-phase sub-module capacitor voltage values of the grid-forming MMC are measured, and the total output reference current, the three-phase circulating current measurement values, and the three-phase sub-module capacitor voltage values are input into a pre-established current predictive control model to output the sub-module switching states corresponding to each phase bridge arm of the grid-forming MMC. The sub-module switching states corresponding to each phase bridge arm are applied to the grid-forming MMC. Thus, through virtual synchronous generator control, the inertia and damping characteristics of a synchronous motor are simulated to control the output power, enabling the medium-voltage flexible interconnection system to have the voltage and frequency regulation characteristics of a synchronous machine. Furthermore, through model predictive current control, multi-objective control of rapid tracking of output power current and compensation current, internal circulating current suppression, and equal voltage sharing of sub-module capacitors can be better achieved, improving the dynamic response ability of the output current and the quality of the output current.

[0097] In some embodiments of this application, the process of step S110, performing virtual synchronous generator control 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, is introduced as follows. Figure 5 As shown, this process may include:

[0098] Step S111: Obtain the three-phase grid connection point voltage and the three-phase grid connection point current of the grid-forming MMC of the medium-voltage flexible interconnection system, and calculate the active output power and the reactive output power of the grid-forming MMC according to the three-phase grid connection point voltage and the three-phase grid connection point current.

[0099] Specifically, in the virtual synchronous generator control strategy, the relationships between active power, reactive power, inertia, frequency, and voltage in the virtual synchronous motor are shown in the following formula:

[0100]

[0101] Where is the instantaneous active power, is the instantaneous reactive power, is the required active power reference value for output, is the required reactive power reference value for output, is the active droop coefficient, = 500 Nm s / rad, is the angular frequency of the virtual synchronous generator, is the rated angular frequency, = 100π, is the effective value of the output voltage, is the effective value of the phase voltage, = 5.77 kV, is the virtual moment of inertia, = 0.057 kg m 2 , is the inertia coefficient, = 7.1 A s.

[0102] Among them, the output voltage U of the grid-forming MMC at the grid connection point can be detected om (k), and the current i gm (k), where m = a, b, c. Then, the active output power and reactive output power of the grid-forming MMC are calculated using the following formula:

[0103]

[0104] 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 phase a voltage of the three-phase voltage at the grid connection point, is the phase b voltage of the three-phase voltage at the grid connection point, is the phase c voltage of the three-phase voltage at the grid connection point, is the phase a current of the three-phase current at the grid connection point, is the phase b current of the three-phase current at the grid connection point, is the phase c current of the three-phase current at the grid connection point.

[0105] Step S112: Based on the active output power and its reference value, and the reactive output power and its reference value, perform virtual synchronous generator control to obtain the phase angle output by the grid-forming MMC.

[0106] 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 of the output of the grid-forming MMC and the amplitude E of the modulation wave can be obtained.

[0107] Step S113: Determine the reference value of the d-axis current output by the grid-forming MMC according to the reference value of the active output power, and determine the reference value of the q-axis current output by the grid-forming MMC according to the reference value of the reactive output power.

[0108] Specifically, according to the instantaneous power control theory, the reference value of the d-axis current output by the grid-forming MMC can be determined using the following formula:

[0109]

[0110] Where, is the reference value of the d-axis current output by the grid-forming MMC, is the effective value of the phase voltage, is the reference value of the active output power;

[0111] Using the following formula, determine the reference value of the q-axis current output by the grid-forming MMC:

[0112]

[0113] Where, is the reference value of the q-axis current output by the grid-forming MMC, is the reference value of the reactive output power.

[0114] Step S114: Through the phase angle, transform the reference value of the d-axis current and the reference value of the q-axis current to obtain the reference value of the three-phase current output by the grid-forming MMC.

[0115] Specifically, through the Park-Clarke inverse transformation, the reference value of the three-phase current required for the MMC output power can be obtained. Specifically, the following formula can be used 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:

[0116]

[0117] Where, 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.

[0118] In some embodiments of the present application, the establishment process of the current prediction control model mentioned in the foregoing embodiments is introduced. This establishment process may include:

[0119] S1: Obtain the current prediction training data of the grid-forming MMC.

[0120] Specifically, the current prediction training data may include the three-phase output current of the distribution network where the network-forming MMC is located, the three-phase circulating current data of the network-forming MMC, the three-phase sub-module capacitor voltage data of the network-forming MMC, the upper-bridge-arm current and voltage data of the network-forming MMC, and the lower-bridge-arm current and voltage data of the network-forming MMC.

[0121] S2. Train a current prediction control model using the current prediction training data.

[0122] Specifically, the control objectives of the model predictive current control strategy consist of three parts, namely current tracking, internal circulating current suppression, and sub-module capacitor voltage equalization.

[0123] Among them, the current prediction control model is:

[0124]

[0125] Among them, k is the current moment, k + 1 is the next moment, and j is the phase sequence number. is the predicted value of the current output by the j-th phase of the network-forming MMC at the next moment. is the output current of the j-th phase of the distribution network. is the measured voltage of the j-th phase of the distribution network at the current moment. is the output voltage of the j-th phase of the distribution network. is the sampling period. is the filter inductor of the AC bus connected to the distribution network. is the predicted value of the circulating current of the j-th phase of the network-forming MMC at the next moment. is the measured value of the circulating current of the j-th phase of the network-forming MMC. is the upper-bridge-arm inductor of the network-forming MMC. is the lower-bridge-arm inductor of the network-forming MMC. is the measured value of the DC voltage of the j-th phase of the network-forming MMC. is the measured value of the upper-bridge-arm voltage of the network-forming MMC. is the measured value of the lower-bridge-arm voltage of the network-forming MMC. is the loop voltage output by the j-th phase of the network-forming MMC. is the predicted value of the capacitor voltage of the I-th sub-module of the x-th bridge arm of the j-th phase of the network-forming MMC at the next moment. is the measured value of the capacitor voltage of the I-th sub-module of the x-th bridge arm of the j-th phase of the network-forming MMC. is the switching constant, S = 1 indicates that the sub-module is conducting, and S = 0 indicates that the sub-module is off. is the measured value of the current of the x-th bridge arm of the j-th phase of the network-forming MMC.

[0126] Furthermore, the objective function of the current prediction control model can be:

[0127]

[0128] where is the output current objective function, is the reference value of the output current of the j-th phase, is the circulating current suppression objective function, is the AC-side output power of the distribution network, is the capacitor voltage equalization objective function of the I-th sub-module of the x-th bridge arm, is the total number of sub-modules of each bridge arm.

[0129] Specifically, the minimum value of the objective function can be calculated , to determine the voltage level of the output voltage U oa required by the network-forming MMC, calculate the minimum value of the objective function , to determine the number of sub-modules when the circulating current is minimized, number the sub-modules and calculate the objective function , according to sort the upper and lower bridge arm sub-modules, and select the sub-modules with the capacitor voltage closest according to the number of sub-modules to be turned on.

[0130] The control strategy block diagram of the current prediction control model is as shown in Figure 7 . k represents the input at the current moment, and k + 1 represents the result of predicting the next moment. is the three-phase output current of the distribution network at the current moment, is the three-phase output voltage of the distribution network at present, is the capacitor voltage value of the i-th sub-module of the x-th bridge arm of the three phases of the network-forming MMC at the current moment, is the three-phase output current reference value, S a , S b , S c represent the three-phase switches respectively.

[0131] Next, the device for implementing medium-voltage flexible interconnection control based on a virtual synchronous generator provided in the embodiments of the present application will be described. The device for implementing medium-voltage flexible interconnection control based on a virtual synchronous generator described below can be correspondingly referred to the method for implementing medium-voltage flexible interconnection control based on a virtual synchronous generator described above.

[0132] See Figure 8 , Figure 8 which is a schematic structural diagram of a device for implementing medium-voltage flexible interconnection control based on a virtual synchronous generator disclosed in the embodiments of the present application.​

[0133] As Figure 8 shown, the device may include:

[0134] A virtual synchronous generator control unit 11 for performing virtual synchronous generator control on the network-forming MMC of the medium-voltage flexible interconnected system to calculate the three-phase current reference values output by the network-forming MMC;

[0135] A current superposition unit 12 for superposing the three-phase current reference values with a preset unbalance compensation current and a preset harmonic current to obtain a total output reference current;

[0136] A current model prediction unit 13 for measuring the three-phase circulating current measurement values and the three-phase sub-module capacitor voltage values of the network-forming MMC, and inputting the total output reference current, the three-phase circulating current measurement values, and the three-phase sub-module capacitor voltage values into a pre-established current prediction control model to output the sub-module switch states corresponding to each phase bridge arm of the network-forming MMC;

[0137] A switch state application unit 14 for applying the sub-module switch states corresponding to each phase bridge arm to the network-forming MMC.

[0138] Optionally, the virtual synchronous generator control unit includes:

[0139] An output power calculation unit for obtaining the grid-connected three-phase voltage and the grid-connected three-phase current of the network-forming MMC of the medium-voltage flexible interconnected system, and calculating the active output power and the reactive output power of the network-forming MMC according to the grid-connected three-phase voltage and the grid-connected three-phase current;

[0140] A phase angle determination unit for performing virtual synchronous generator control 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 network-forming MMC;

[0141] A current reference value determination unit for determining the d-axis current reference value output by the network-forming MMC according to the reference value of the active output power, and determining the q-axis current reference value output by the network-forming MMC according to the reference value of the reactive output power;

[0142] A dq-axis current inverse transformation unit for transforming 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.

[0143] Optionally, the output power calculation unit includes:

[0144] An output power calculation subunit, configured to calculate the active output power and the reactive output power of the grid-forming MMC by using the following formula:

[0145]

[0146] 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 a-phase voltage of the three-phase voltage at the grid connection point, is the b-phase voltage of the three-phase voltage at the grid connection point, is the c-phase voltage of the three-phase voltage at the grid connection point, is the a-phase current of the three-phase current at the grid connection point, is the b-phase current of the three-phase current at the grid connection point, is the c-phase current of the three-phase current at the grid connection point.

[0147] Optionally, the current reference value determination unit includes:

[0148] A d-axis current reference value calculation unit, configured to determine the d-axis current reference value output by the grid-forming MMC by using the following formula:

[0149]

[0150] where, is the d-axis current reference value output by the grid-forming MMC, is the effective value of the phase voltage, is the reference value of the active output power;

[0151] A q-axis current reference value calculation unit, configured to determine the q-axis current reference value output by the grid-forming MMC by using the following formula:

[0152]

[0153] where, is the q-axis current reference value output by the grid-forming MMC, is the reference value of the reactive output power.

[0154] Optionally, the dq-axis current inverse transformation unit includes:

[0155] A dq-axis current inverse transformation subunit, 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 value output by the grid-forming MMC:

[0156]

[0157] Among them, is the reference value of the a-phase current output by the network-forming MMC, is the reference value of the b-phase current output by the network-forming MMC, is the reference value of the c-phase current output by the network-forming MMC, is the phase angle.

[0158] Optionally, the device further includes:

[0159] 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 of the distribution network where the network-forming MMC is located, three-phase circulating current data of the network-forming MMC, three-phase sub-module capacitor voltage data of the network-forming MMC, upper-bridge-arm current and voltage data of the network-forming MMC, and lower-bridge-arm current and voltage data of the network-forming MMC;

[0160] 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:

[0161]

[0162] where k is the current moment, k + 1 is the next moment, and j is the serial number of the phase, is the predicted value of the current output by the j-th phase of the network-forming MMC at the next moment, is the output current of the j-th phase of the distribution network, is the measured voltage of the j-th phase of the distribution network at the current moment, is the output voltage of the j-th phase of the distribution network, is the sampling period, is the filter inductor of the AC bus connected to the distribution network, is the predicted value of the circulating current of the j-th phase of the network-forming MMC at the next moment, is the measured value of the circulating current of the j-th phase of the network-forming MMC, is the upper-bridge-arm inductor of the network-forming MMC, is the lower-bridge-arm inductor of the network-forming MMC, is the measured value of the DC voltage of the j-th phase of the network-forming MMC, is the measured value of the upper-bridge-arm voltage of the network-forming MMC, is the measured value of the lower-bridge-arm voltage of the network-forming MMC, is the loop voltage output by the j-th phase of the network-forming MMC, is the predicted value of the capacitor voltage of the I-th sub-module of the x-th bridge arm of the j-th phase of the network-forming MMC at the next moment, The measured value of the capacitor voltage of the I-th sub-module of the x-th arm of the j-th phase of the network-forming MMC is the switching constant, where S = 1 indicates that the sub-module is turned on, and S = 0 indicates that the sub-module is turned off is the measured value of the current of the x-th arm of the j-th phase of the network-forming MMC

[0163] Optionally, the objective function of the current prediction control model is

[0164]

[0165] where is the output current objective function is the reference value of the output current of the j-th phase is the circulating current suppression objective function is the AC-side output power of the distribution network is the capacitor voltage equalization objective function of the I-th sub-module of the x-th arm is the total number of sub-modules of each arm

[0166] Optionally, the medium-voltage flexible interconnection system includes two network-forming MMCs with the same structure, a first AC bus, a second AC bus, and a DC bus. The two network-forming MMCs are connected through the DC bus. One of the network-forming MMCs is connected to the first AC bus, and the other network-forming MMC is connected to the second AC bus;

[0167] Each phase of each network-forming MMC includes two arms, and each arm includes an arm inductor and multiple sub-modules;

[0168] Each phase of each network-forming MMC is connected to the corresponding AC bus through a filter inductor

[0169] Finally, it should also 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 such 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

[0170] 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.

[0171] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A medium voltage flexible interconnection control method based on virtual synchronous generator, characterized in that: include: Performing virtual synchronous generator control on a grid-forming MMC of a medium voltage flexible interconnection system to calculate a three-phase current reference value output by the grid-forming MMC; Superimposing the three-phase current reference value with a preset unbalanced compensation current and a preset harmonic current to obtain a total output reference current; Measure the three-phase circulating current measurement value and the three-phase sub-module capacitor voltage value of the meshed MMC, input the total output reference current, the three-phase circulating current measurement value and the three-phase sub-module capacitor voltage value into a pre-established current prediction control model, and output the sub-module 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 virtual synchronous generator control on the grid-forming MMC of the medium voltage flexible interconnection system to calculate the three-phase current reference value output by the grid-forming MMC includes: Obtaining the three-phase voltage and the three-phase current of the grid-connected point of the grid-connected MMC of the medium voltage flexible interconnection system, and calculating the active output power and the reactive output power of the grid-connected MMC according to the three-phase voltage and the three-phase current of the grid-connected point; Based on the active output power and its reference value, as well as the reactive output power and its reference value, a virtual synchronous generator control is performed to obtain a phase angle output by the grid-forming MMC; Determine a d-axis current reference value output by the grid-forming MMC according to the reference value of the active output power, and determine a q-axis current reference value output by the grid-forming MMC according to the reference value of the reactive output power; 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 According to the three-phase voltage at the grid connection point and the three-phase current at the grid connection point, the active output power and the reactive output power of the grid-connected MMC are calculated, 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 a-phase voltage of the three-phase voltage at the grid connection point, is the b-phase voltage of the three-phase voltage at the grid connection point, is the c-phase voltage of the three-phase voltage at the grid connection point, is the a-phase current of the three-phase current at the grid connection point, is the b-phase current of the three-phase current at the grid connection point, is the c-phase current of the three-phase current at the grid connection point.

4. The method according to claim 2, characterized in that: Determining a d-axis current reference value output by the grid-forming MMC according to the reference value of the active output power, and determining a q-axis current reference value output by the grid-forming MMC according to the reference value of the reactive output power, comprising: The d-axis current reference value output by the meshed MMC is determined using the following formula: in, is the d-axis current reference value output by the meshed MMC, is the effective value of the phase voltage, is the reference value of the active output power; The q-axis current reference value output by the grid-type MMC is determined using the following formula: in, is the q-axis current reference value output by the meshed MMC, is the reference value of the reactive output power.

5. The method according to claim 4, 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.

6. 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, the current prediction training data including the three-phase output current of the distribution network where the meshed MMC is located, the three-phase circulating current data of the meshed MMC, the three-phase submodule capacitor voltage data of the meshed MMC, the upper bridge arm current and voltage data of the meshed MMC, and the lower bridge arm current and voltage data of the meshed MMC; 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, j is the sequence number of the phase, is the predicted value of the current output by the jth phase of the grid-type MMC at the next moment, is the j-th phase output current of the distribution network, is the measured voltage of the distribution network at the current moment of the jth phase, is the output voltage of the j-th phase of the distribution network, is the sampling period, is the filter inductance connected to the AC busbar of the distribution network, is the predicted value of the circulation at the next moment of the jth phase of the grid-type MMC, is the measured value of the circulation of the jth phase of the grid-type MMC, is the upper arm inductance of the meshed MMC, is the lower arm inductor of the meshed MMC, is the measured DC voltage value of the jth phase of the grid-type MMC, is the measured value of the upper bridge arm voltage of the meshed MMC, is the measured value of the lower bridge arm voltage of the grid-type MMC, is the loop voltage of the j-th phase output of the meshed MMC, is the predicted value of the capacitor voltage of the I-th submodule of the x-th bridge arm of the j-th phase of the meshed MMC at the next moment, is the capacitance voltage measurement value of the I-th submodule of the x-th bridge arm of the j-th phase of the meshed MMC, is the switching constant, S=1 means the submodule is turned on, S=0 means the submodule is turned off, is the current measurement value of the xth bridge arm of the jth phase of the grid-type MMC.

7. The method according to claim 6, characterized in that The objective function of the current predictive control model is: in, is the output current objective function, is the j-th phase output current reference value, is the objective function of circulation suppression, is the AC side output power of the distribution network, is the capacitor voltage balancing objective function of the I-th submodule of the x-th bridge arm, is the total number of submodules in each bridge arm.

8. 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.

9. A medium voltage flexible interconnection control device based on virtual synchronous generator, characterized in that: include: A virtual synchronous generator control unit, used to perform virtual synchronous generator control on a meshing type MMC of a medium voltage flexible interconnection system, so as to calculate a three-phase current reference value output by the meshing type MMC; A current superposition unit, used for superimposing the three-phase current reference value with a preset unbalanced compensation current and a preset harmonic current to obtain a total output reference current; A current model prediction unit, used to measure the three-phase circulating current measurement value and the three-phase sub-module capacitor voltage value of the meshed MMC, and input the total output reference current, the three-phase circulating current measurement value and the three-phase sub-module capacitor voltage value into a pre-established current prediction control model, and output the sub-module switch state corresponding to each phase bridge arm of the meshed MMC; The switch state application unit is used to apply the switch state of the submodule corresponding to each phase bridge arm to the meshed MMC.

10. The device according to claim 9, characterized in that The virtual synchronous generator control unit comprises: An output power calculation unit, used to obtain the three-phase voltage and the three-phase current of the grid-connected point of the grid-connected MMC of the medium voltage flexible interconnection system, and calculate the active output power and reactive output power of the grid-connected MMC according to the three-phase voltage and the three-phase current of the grid-connected point; A phase angle determination unit, configured to perform virtual synchronous generator control based on the active output power and its reference value, and the reactive output power and its reference value, to obtain a phase angle output by the grid-forming MMC; A current reference value determination unit, configured to determine a d-axis current reference value output by the grid-forming MMC according to the reference value of the active output power, and to determine a q-axis current reference value output by the grid-forming MMC according to the reference value of the reactive output power; The 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.

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