Parallel converter circulating current suppression control method and system under alternating-current and direct-current series-parallel micro-grid

Abstract

The invention discloses a parallel converter circulating current suppression control method and system under an AC-DC hybrid microgrid, and the method comprises the steps: obtaining a reference current based on a DC bus voltage actual value and a reference value; the main converter obtains a corresponding future current prediction value and a neutral point voltage deviation prediction value through system circuit parameters; grouping the switching states of the converter based on the common-mode voltage value corresponding to each switching state; constructing a cost function based on the reference current, traversing candidate switch states, and selecting the switch state which enables the cost function to be minimum as the switch state of the next moment; traversing and selecting a switch enabling the cost function to be minimum as the switching state of the next moment from the switching states of the same group as the vector of the main converter by the slave converter; and controlling the master converter and the slave converter based on the obtained switching state at the next moment. According to the invention, circulation-free parallel operation can be realized, and the operation efficiency of the parallel system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of circulating current suppression of parallel converters, and particularly to a control method and system for suppressing circulating current of parallel converters in an AC-DC hybrid microgrid. Background Art

[0002] The statements in this part only provide background technical information related to the present invention and do not necessarily constitute prior art.

[0003] In an AC-DC hybrid microgrid system, in order to improve the system capacity, three-level converters operating in parallel are usually used. However, zero-sequence circulating current often occurs between parallel three-level converters due to reasons such as incomplete parameter consistency, inconsistent dead-time of switching devices, and inconsistent carrier phases, seriously affecting the stable operation of the system.

[0004] Model predictive control has great advantages in the control of high-power converters due to its good dynamic performance and easy control of multiple objectives. Traditional model predictive current control for suppressing zero-sequence circulating current of parallel three-level converters only uses switching states with zero common-mode voltage to suppress zero-sequence circulating current. Although it can reduce zero-sequence circulating current, it does not fully utilize the advantage of more three-level switching states, and reduces the utilization rate of the DC bus voltage, prone to problems such as overmodulation and reduced control performance. Summary of the Invention

[0005] To solve the above problems, the present invention proposes a control method and system for suppressing circulating current of parallel converters in an AC-DC hybrid microgrid. While using one controller, by adjusting the converter priority, the main converter makes switch selection first, and the slave converter makes switch selection later, so as to achieve the suppression of zero-sequence circulating current.

[0006] In some embodiments, the following technical solutions are adopted:

[0007] A control method for suppressing circulating current of parallel converters in an AC-DC hybrid microgrid includes:

[0008] Based on the actual value and reference value of the DC bus voltage, obtain the reference current and

[0009] The main converter obtains the corresponding future current prediction value through the system circuit parameters and the predicted value of the neutral point voltage deviation

[0010] Based on the common-mode voltage value corresponding to each switching state, group the switching states of the converter; based on the reference current and Construct a cost function, traverse the candidate switch states, and select the switch state that minimizes the cost function as the switch state at the next moment;

[0011] From the switch states of the converter in the same group as the main converter vector, traverse and select the switch that minimizes the cost function as the switch state at the next moment;

[0012] Control the main converter and the slave converter based on the obtained switch state at the next moment.

[0013] Among them, the main converter obtains the predicted value of the current at the corresponding future moment through the system circuit parameters Specifically:

[0014]

[0015] Among them, R g is the grid-side resistance, L g is the grid-side inductance, i α (k), i β (k) are the current values at the current moment respectively, T s is the control period, e α (k + 1), e β (k + 1) are the grid voltages at the k + 1 moment respectively, v α (k + 1), v β (k + 1) are the output voltages of the converter at the k + 1 moment respectively.

[0016] Furthermore, the main converter obtains the predicted value of the neutral point voltage deviation at the corresponding future moment through the system circuit parameters. Specifically:

[0017]

[0018] Among them, i a , i b , i c are the three-phase grid-side currents, F a , F b , F c are the three-phase switching functions, V o (k) is the current neutral point voltage value, and C is the DC bus capacitor value.

[0019] In some other embodiments, the following technical solution is adopted:

[0020] A circulating current suppression control system for parallel converters in an AC-DC hybrid microgrid, comprising:

[0021] A reference current calculation module, configured to obtain a reference current based on the actual value and the reference value of the DC bus voltage and

[0022] A parameter prediction module, configured to obtain corresponding future current prediction values and neutral point voltage deviation prediction values for the main converter through system circuit parameters. And the neutral point voltage deviation prediction value

[0023] A converter control module, configured to group the switching states of the converter; construct a cost function based on the reference current and...; traverse the candidate switching states, and select the switching state that minimizes the cost function as the switching state of the main converter at the next moment; from the switching vectors of the converter in the same group as the vector of the main converter, traverse and select the switching vector that minimizes the cost function as the switching state of the slave converter at the next moment; control the main converter and the slave converter based on the obtained switching state at the next moment. And ...construct a cost function, traverse the candidate switching states, and select the switching state that minimizes the cost function as the switching state of the main converter at the next moment; from the switching vectors of the converter in the same group as the vector of the main converter, traverse and select the switching vector that minimizes the cost function as the switching state of the slave converter at the next moment; control the main converter and the slave converter based on the obtained switching state at the next moment. ...minimized as the switching state of the slave converter at the next moment; control the main converter and the slave converter based on the obtained switching state at the next moment.

[0024] In some other embodiments, the following technical solution is adopted:

[0025] A terminal device, which includes a processor and a memory. The processor is used to implement instructions; the memory is used to store multiple instructions, and the instructions are adapted to be loaded and executed by the processor to perform the above-mentioned control method for suppressing circulating current of parallel converters in an AC-DC hybrid microgrid.

[0026] In some other embodiments, the following technical solution is adopted:

[0027] A computer-readable storage medium, in which multiple instructions are stored, and the instructions are adapted to be loaded and executed by the processor of the terminal device to perform the above-mentioned control method for suppressing circulating current of parallel converters in an AC-DC hybrid microgrid.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] (1) By analyzing the influence of the switching states of the three-level converter on the circulating current, the switching states of the converter are grouped, and at the same time, the priorities of the converters are divided. The main converter first selects the switching state, and the slave converter traverses the optimal switching state in the same group on the basis of the main converter selecting the switching state; by setting the master-slave converters, with the main converter having priority and the slave converter cooperating with the main converter, the non-circulating parallel operation is realized, thereby improving the operation efficiency of the parallel system.

[0030] Other features and additional advantages of the present invention will be partially given in the following description, partially will become obvious from the following description, or will be understood through the practice of this aspect. Description of the Drawings

[0031] Figure 1This is the topology diagram of the parallel NPC three-level converter in the embodiment of the present invention;

[0032] Figure 2 These are the 27 switching states of the three-level converter in the embodiment of the present invention;

[0033] Figure 3 This is the schematic diagram of the circulating current suppression control process of the parallel converter under the AC-DC hybrid microgrid in the embodiment of the present invention. Detailed implementation manners

[0034] It should be noted that the following detailed description is exemplary and is intended to provide further illustration of the present application. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs.

[0035] It should be noted that the terms used herein are only for describing specific implementation manners and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0036] Embodiment 1

[0037] In one or more embodiments, a method for suppressing circulating current of a parallel converter under an AC-DC hybrid microgrid is disclosed. By analyzing the influence of each switching state of the three-level converter on the circulating current, the switching states of the converter are grouped, and at the same time, the priorities of the converters are divided. The main converter first selects the switching state, and the slave converter traverses the optimal switching state in the same group on the basis of the switching state selected by the main converter, and acts on the converter through a single controller.

[0038] The core point of this embodiment is to propose an efficient predictive control method considering zero-sequence circulating current for a three-level power converter system in parallel operation, so as to ensure that the converters in parallel operation maintain high performance under complex working conditions. Taking the NPC (neutral point clamped) three-level converter as an example, the parallel three-level NPC converter model, the generation mechanism of parallel zero-sequence circulating current, and the model predictive control method for suppressing zero-sequence circulating current proposed in this embodiment will be introduced respectively.

[0039] The topology of the parallel NPC three-level converter is as Figure 1 shown. This converter system includes a three-phase voltage source (v ga , v gb , v gc ), a grid-side filter inductor (L n), the grid-side equivalent resistance (R n ), the DC bus capacitor (C n1 , C n2 )(n = 1, 2, …). V dcn1 and V dcn2 are the upper and lower capacitor voltages of the DC bus of the nth converter in parallel, respectively. i an , i bn , i cn is the load current of converter n. v anN , v bnN , v cnN are the three-phase output voltages of converter n, and i z is the zero-sequence circulating current flowing before the parallel converters. By controlling the conduction state of the IGBT, the three-level converter can output three levels: high level (P), zero level (O), and low level (N), and the corresponding voltage values are 0,

[0040] The mechanism of the zero-sequence circulating current generation of the parallel three-level converter will be introduced below. Figure 1 is the topology of two parallel three-level converters. The definition of the zero-sequence circulating current is:

[0041]

[0042] In the three-level converter, since each phase switch has 3 switching states P, O, N, the total number of combinations of the three-phase switches is 3 3 = 27 combinations, and 27 voltage vectors are output, as shown in Figure 2 . The common-mode voltage value corresponding to each switching vector can be calculated by , where u a , u b , u c are the three-phase voltages.

[0043] Grouping them using the common-mode voltage values of each switching vector, they can be divided into 7 groups, namely:

[0044] The first group: PPP.

[0045] The second group: NNN.

[0046] The third group: ONN, NON, NNO.

[0047] The fourth group: PPO, POP, OPP.

[0048] The fifth group: PPN, PNP, NPP, POO, OPO, OOP.

[0049] The sixth group: NNP, NPN, PNN, NOO, ONO, OON.

[0050] Group 7: OOO, PON, OPN, NPO, NOP, ONP, PNO.

[0051] The common - mode voltage values are respectively: Vdc / 2, -Vdc / 2, Vdc / 3, -Vdc / 3, Vdc / 6, -Vdc / 6, 0.

[0052] For Figure 1 Write the KVL equation for the parallel topology:

[0053]

[0054] In the formula V dcx2 , V dc , V ox are respectively the capacitor voltage at the lower end of the DC side of the converter x, the DC - bus voltage, and the neutral - point voltage.

[0055] According to the above formula, it can be sorted out that:

[0056]

[0057] That is, the zero - sequence circulating current is due to the difference in the common - mode voltage Δv cmv , and the difference in the neutral - point voltage Δv o generated.

[0058] The advantage of the converter model - predictive control algorithm considering circulating - current suppression proposed in this embodiment is that a main converter preferentially traverses the switching states. The selection of the switching state of the main converter is not restricted, and the candidate combinations are 27. After the main converter selects the switching state, the slave converter makes a selection. At this time, the candidate switching states of the slave converter need to be in the same group as the switching state selected by the main converter. In this way, the difference in the common - mode voltage generated by the switching states of the two converters can be made zero, that is, the generation of the zero - sequence circulating current is suppressed.

[0059] Combined with Figure 3 , the method of this embodiment specifically includes the following process:

[0060] (1) Sample to obtain the actual value of the DC - bus voltage V dc = V dc1 + V dc2 , and pass it through a PI controller with the DC - bus voltage reference value to obtain the reference currents and That is, the reference currents and

[0061] (2) The main converter obtains the predicted current value at the corresponding future moment through the system circuit parameters and the predicted neutral point voltage deviation value Specifically:

[0062]

[0063]

[0064] Among them, R g is the grid-side resistance, L g is the grid-side inductance, i α (k), i β (k) are the current values at the current moment respectively, T s is the control period, e α (k + 1), e β (k + 1) are the grid voltages at the moment of k + 1 respectively, v α (k + 1), v β (k + 1) are the output voltages of the converter at the moment of k + 1 respectively. i a , i b , i c are the three-phase grid-side currents, F a , F b , F c are the three-phase switching functions, V o (k) is the current neutral point voltage value, and C is the DC bus capacitance value.

[0065] (3) Group the switching states of the converter; the specific grouping method has been described before, and it is divided into 7 groups in total. Based on the reference current and construct a cost function, traverse the candidate switching states, and select the switching state that makes the cost function the smallest as the switching state at the next moment;

[0066] Specifically, the cost function is:

[0067]

[0068] The switching state with the smallest value is used as the switching state S x1 (x ∈ {a, b, c}), where the switching state can be 1, 0 or -1.

[0069] (4) From the switching vectors in the same group as the main converter vector of the converter, traverse and select the switching vector that makes the cost function the smallest as the switching state at the next moment;

[0070] Select the switching vector in the same group as the main converter vector from the converters. That is, assume that the main converter selects the vector NNP as the switching state. The slave converter traverses and selects the switching vector that minimizes the cost function in the sixth group where NNP is located as the switching state for the next moment.

[0071] (5) Control the main converter and the slave converter based on the obtained switching state for the next moment.

[0072] The method of this embodiment realizes the circulation-free parallel operation by setting the master-slave converters, with the main converter taking priority and the slave converter cooperating with the main converter, thereby improving the operation efficiency of the parallel system.

[0073] Embodiment 2

[0074] In one or more embodiments, a control system for suppressing circulating current in parallel converters in an AC-DC hybrid microgrid is disclosed, including:

[0075] A reference current calculation module for obtaining a reference current based on the actual value and the reference value of the DC bus voltage and

[0076] A parameter prediction module for the main converter to obtain the corresponding predicted future current value and the predicted neutral point voltage deviation value

[0077] A converter control module for grouping the switching states of the converter; constructing a cost function based on the reference current and traversing the candidate switching states, and selecting the switching state that minimizes the cost function as the switching state of the main converter for the next moment; the slave converter traverses and selects the switching vector that minimizes the cost function in the switching vectors in the same group as the main converter vector as the switching state of the slave converter for the next moment; controlling the main converter and the slave converter based on the obtained switching state for the next moment.

[0078] The specific implementation manners of the above modules are the same as those in Embodiment 1 and will not be elaborated here.

[0079] Embodiment 3

[0080] In one or more embodiments, a terminal device is disclosed, including a server. The server includes a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the program, it implements the method for suppressing circulating current in parallel converters in an AC-DC hybrid microgrid in Embodiment 1. For the sake of brevity, it will not be elaborated here.

[0081] It should be understood that in this embodiment, the processor may be a central processing unit (CPU), and the processor may also be other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or any conventional processor, etc.

[0082] The memory may include a read-only memory and a random access memory, and provide instructions and data to the processor. A part of the memory may also include a non-volatile random access memory. For example, the memory may also store information about the device type.

[0083] In the implementation process, each step of the above method may be completed by the integrated logic circuit in the hardware of the processor or the instructions in the form of software.

[0084] Embodiment 4

[0085] In one or more embodiments, a computer-readable storage medium is disclosed, in which multiple instructions are stored, and the instructions are adapted to be loaded and executed by the processor of the terminal device to perform the circulating current suppression control method of the parallel converters under the AC-DC hybrid microgrid described in Embodiment 1.

[0086] Although the specific implementation manners of the present invention are described above in conjunction with the drawings, it is not a limitation to the protection scope of the present invention. Those skilled in the art should understand that based on the technical solutions of the present invention, various modifications or deformations that can be made by those skilled in the art without creative efforts are still within the protection scope of the present invention.

Claims

1. A control method for suppressing circulating current of parallel converters in an AC / DC hybrid microgrid, characterized in that Including: Based on the actual value and the reference value of the DC bus voltage, a reference current is obtained and The main converter obtains the corresponding predicted future current value through the system circuit parameters and the predicted neutral point voltage deviation value Group the switching states of the converter based on the common-mode voltage values corresponding to each switching state; based on the reference current and Construct a cost function, traverse the candidate switching states, and select the switching state that minimizes the cost function as the switching state for the next moment; Among the switching states of the converter in the same group as the main converter vector, traverse and select the switch that minimizes the cost function as the switching state for the next moment; Controlling the main converter and the slave converter based on the obtained switching state at the next moment.

2. The circulating current suppression control method for a parallel converter under an AC-DC hybrid microgrid according to claim 1, characterized in that, The main converter obtains the predicted value of the current at the corresponding future moment through the system circuit parameters Specifically: Among them, R g is the grid-side resistance, L g is the grid-side inductance, i α (k), i β (k) are the current values at the current moment, T s is the control period, e α (k + 1), e β (k + 1) are the grid voltages at the moment of k + 1, v α (k + 1), v β (k + 1) are the output voltages of the converter at the moment of k + 1.

3. A control method for suppressing circulating current of parallel inverters in an AC-DC hybrid microgrid according to claim 1, characterized in that The main converter obtains the predicted value of the corresponding future neutral point voltage deviation through the system circuit parameters, specifically: where, i a , i b , i c are the three-phase grid-side currents, F a , F b , F c are the three-phase switching functions, V o (k) is the current neutral point voltage value, and C is the DC bus capacitance value.

4. A method for suppressing circulating current of a parallel converter under an AC / DC hybrid microgrid as described in claim 1, characterized in that Grouping the switching states of the converter based on the common-mode voltage values corresponding to each switching state, with a total of 7 groups, specifically: The first group: PPP; The second group: NNN; The third group: ONN, NON, NNO; The fourth group: PPO, POP, OPP; The fifth group: PPN, PNP, NPP, POO, OPO, OOP; The sixth group: NNP, NPN, PNN, NOO, ONO, OON; The seventh group: OOO, PON, OPN, NPO, NOP, ONP, PNO; The corresponding common-mode voltage values are: Vdc / 2, -Vdc / 2, Vdc / 3, -Vdc / 3, Vdc / 6, -Vdc / 6, 0.

5. A circulating current suppression control method for a parallel converter under an AC / DC hybrid microgrid as described in claim 1, characterized in that, The common-mode voltage value u corresponding to each switch state cmv is as follows: Among them, u a , u b , u c are three-phase voltages.

6. A control method for suppressing circulating current of a parallel converter under an AC / DC hybrid microgrid according to claim 1, characterized in that Based on the reference current and construct a cost function, specifically as follows: Among them, λ dc is the weight coefficient, which is used to determine the importance of control; is the predicted value of the neutral point voltage.

7. A method for suppressing circulating current in a parallel converter under an AC / DC hybrid microgrid as described in claim 1, characterized in that Obtain the three-phase current and calculate the zero-sequence circulating current i z , specifically as follows: Among them, R1 and R2 are respectively the equivalent resistances on the grid side, L1 and L2 are respectively the filtering inductors on the grid side, and Δv cmv is the difference in the common-mode voltages of the switching vectors of the main converter and the slave converter, and Δv o is the difference in the neutral point voltages of the main converter and the slave converter.

8. A circulating current suppression control system for a parallel converter under an AC / DC hybrid microgrid, characterized in that, Including: A reference current calculation module, configured to obtain a reference current based on an actual value and a reference value of a DC bus voltage and The parameter prediction module is used for the main converter to obtain the corresponding predicted future current value through the system circuit parameters and the predicted value of the neutral point voltage deviation A converter control module is used to group the switching states of the converter; based on the reference current and construct a cost function, traverse the candidate switching states, and select the switching state that minimizes the cost function as the switching state of the main converter at the next moment; The slave converter traverses and selects, from the switching vectors in the same group as the main converter vector, the switching vector that minimizes the cost function as the switching state of the slave converter at the next moment; Controlling the main converter and the slave converter based on the obtained switching state at the next moment.

9. A terminal device, comprising a processor and a memory, the processor being configured to implement instructions; the memory being configured to store a plurality of instructions, characterized in that, The instruction is suitable for being loaded and executed by a processor to perform the circulating current suppression control method for parallel converters in an AC-DC hybrid microgrid according to any one of claims 1-7.

10. A computer-readable storage medium storing multiple instructions, characterized in that, The instruction is suitable for being loaded and executed by a processor of a terminal device to perform the circulating current suppression control method for parallel converters in an AC-DC hybrid microgrid according to any one of claims 1-7.

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