Multi-mode grid-connected control method for flexible interconnection micro-grid group

Abstract

The invention relates to the technical field of micro-grid group control, and discloses a multi-mode grid-connected control method for a flexible interconnection micro-grid group, which comprises the following steps: enabling a flexible interconnection switch to be in a to-be-used state, enabling each micro-grid subnet to be connected into a power distribution network through a common connection point, and realizing an independent grid-connected mode of the micro-grid group; each micro-grid subnet is connected into the power distribution network through a common connection point, and a part of the micro-grid subnets are mutually connected through flexible interconnection switches, so that a local interconnection grid-connected mode of the micro-grid group is realized; each micro-grid subnet is connected into the power distribution network through a common connection point, and all the micro-grid subnets are mutually connected through a flexible interconnection switch, so that an all-interconnection grid-connected mode of the micro-grid group is realized; parts of the micro-grid subnets are connected into the power distribution network through the common connection points and connected with one another through the flexible interconnection switches, the micro-grid subnets which are not connected into the power distribution network through the common connection points are connected into ports of the flexible interconnection switches and then indirectly connected into the power distribution network, and the indirect grid-connected mode of the micro-grid group is achieved. According to the invention, the grid-connected mode of the flexible interconnection micro-grid group and the control problem thereof are solved.

Description

Technical Field

[0001] The present invention relates to the field of microgrid group control technology, and in particular to a multi-mode grid-connected control method for a flexible interconnected microgrid group. Background Art

[0002] In traditional microgrid groups, each microgrid subgrid is connected to each other through busbars or circuit breakers. The microgrid subgrids have a great influence on each other, which will cause power quality problems to be transmitted within the group, which is not conducive to the stability of the microgrid group and its high anti-interference ability. As an emerging interconnection solution, multi-port flexible switches have become a new choice for solving the interconnection of microgrid groups due to their advantages such as good disturbance isolation, strong regulation ability and fast fault recovery. However, for isolated microgrid groups, the stability of their voltage and frequency is one of the prerequisites for the safe and stable operation of the microgrid group. Therefore, for microgrid subgrids using constant power control, voltage and frequency support must be provided. The virtual synchronization port provides voltage and frequency support for the microgrids connected to it by using voltage and frequency control in multi-port flexible switches, thereby improving the group control and group regulation of the microgrid group. However, due to the complexity of the microgrid group structure, it presents diversity in grid connection. This diversity not only affects the energy flow and supply and demand balance within the microgrid group and the control strategy of its converter, but also has an important impact on the stability and power quality of the entire power system. Therefore, it is particularly important to study the different grid-connected modes of microgrids and the corresponding control methods. Summary of the invention

[0003] The purpose of the present invention is to solve the grid-connected mode and control problems of a flexible interconnected microgrid group and to provide a multi-mode grid-connected control method for a flexible interconnected microgrid group.

[0004] In order to achieve the above-mentioned object of the invention, the embodiment of the present invention provides the following technical solutions:

[0005] A multi-mode grid-connected control method for a flexible interconnected microgrid group comprises the following steps:

[0006] The flexible interconnection switch is placed in a standby state, and each microgrid subnet is connected to the distribution network through a public connection point to realize the independent grid-connected mode of the microgrid group;

[0007] Each microgrid subnet is connected to the distribution network through a public connection point, and some of the microgrid subnets are connected to each other through flexible interconnection switches to achieve a local interconnection and grid-connected mode of the microgrid group;

[0008] Each microgrid subnet is connected to the distribution network through a public connection point, and all microgrid subnets are interconnected through flexible interconnection switches to achieve a fully interconnected and grid-connected mode for the microgrid group;

[0009] Some microgrid subnets are connected to the distribution network through a public connection point and are connected to each other through flexible interconnection switches. Microgrid subnets that are not connected to the distribution network through a public connection point are connected to the flexible interconnection switch port and then indirectly connected to the distribution network, realizing the indirect grid-connected mode of the microgrid group.

[0010] Compared with the prior art, the present invention has the following beneficial effects: the present invention interconnects microgrid subgrids through flexible interconnection switches, and the microgrid subgrids can be connected to the distribution network through a public connection point, realizing a multi-mode grid-connected mode of microgrid group independent grid-connected mode, microgrid group partial interconnected grid-connected mode, microgrid group full interconnected grid-connected mode, and microgrid group indirect grid-connected mode, solving the grid-connected mode and control problems of flexible interconnected microgrid groups. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0012] Figure 1 This is a schematic diagram of the independent grid-connected mode of the microgrid group of the present invention;

[0013] Figure 2 It is a schematic diagram of the local interconnection and grid-connection mode of the microgrid group of the present invention;

[0014] Figure 3 This is a schematic diagram of the interconnected and grid-connected mode of the microgrid group of the present invention;

[0015] Figure 4 It is a schematic diagram of the indirect grid-connected mode of the microgrid group of the present invention;

[0016] Figure 5 This is a schematic diagram of voltage pre-synchronization control of the present invention. DETAILED DESCRIPTION

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work belong to the scope of protection of the present invention.

[0018] It should be noted that similar numbers and letters represent similar items in the following figures, so once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. At the same time, in the description of the present invention, the terms "first", "second", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance, or implying any such actual relationship or order between these entities or operations. In addition, the terms "connected", "connected", etc. can be directly connected between elements, or indirectly connected via other elements.

[0019] Embodiment 1:

[0020] The present invention is achieved through the following technical solutions: Figure 1 As shown, a multi-mode grid-connected system for a flexible interconnected microgrid group is connected to a distribution network bus, and includes: a flexible interconnection switch, a plurality of microgrid subnets, each microgrid subnet is connected to the distribution network bus through a common connection point, and the flexible interconnection switch realizes the connection between the microgrid subnets. The flexible interconnection switch refers to a power electronic converter used as a connection port for microgrid interconnection, which can realize the conversion from AC to DC to AC, and has multiple ports to connect different microgrid subnets, and these ports share the intermediate DC link.

[0021] Due to the complexity of the structure of the microgrid group, there are many ways to connect to the grid. For the various ways to connect to the grid, there should be different inverter control strategies and microgrid group power control strategies. In detail, this solution uses flexible interconnection in the microgrid group to connect the microgrid subnets to the grid through a variety of different modes. The various modes include independent grid-connected mode of the microgrid group, partial interconnected grid-connected mode of the microgrid group, fully interconnected grid-connected mode of the microgrid group, and indirect grid-connected mode of the microgrid group.

[0022] See also Figure 1 , the independent grid-connected mode of the microgrid group means that each microgrid subnet is directly connected to the distribution network bus through its public connection point, and each microgrid subnet adopts the PQ control strategy (i.e., active power and reactive power control strategy) to control the power between the microgrid subnet and the connected distribution network, and the voltage of the microgrid subnet is supported by the connected distribution network. In this mode, there is no interconnection port between the microgrid subnets, that is, the flexible interconnection switch is in a standby state. Each microgrid subnet is connected to the AC bus through the public connection point of the microgrid group, and can decide whether to enter the operating state according to its own state. Its operating conditions do not affect other microgrid subnets, so it has high independence. In addition, the grid-connected mode of this mode is relatively simple, and does not require a more complex control structure. It only needs to switch the control strategy of the grid-connected microgrid subnet to connect to the grid. However, in contrast, when the power between the microgrid subnets is mutually beneficial, the microgrid subnet with surplus power needs to merge the mutual benefit power into the distribution network bus, and then dispatch it to the microgrid subnet that needs mutual benefit, that is, the microgrid subnet with power shortage through the distribution network.

[0023] Assume that the output power of the distribution network is P G , the excess or shortage power of each microgrid subnet is P1, P2, ..., P n , where a positive power value indicates excess power and a negative power value indicates a power shortage, then the following formula is satisfied:

[0024] P G -(P1+P2+...+P n )=0

[0025] Where n is the number of microgrid subnets.

[0026] See also Figure 2 The local interconnection mode of microgrid group refers to that after some microgrid subnets are connected to the grid through the public connection point, they are still connected to each other through the flexible interconnection switch, while the remaining microgrid subnets are only connected to the grid through the public connection point. The bus voltage of the microgrid subnet connected to the grid is supported by the distribution network, and the power between the microgrid subnet and the distribution network is controlled by the PQ control strategy. In addition to controlling the power between the microgrid subnet and the distribution network through the PQ control strategy, the microgrid subnets connected to each other through the flexible interconnection switch can also use the PQ control strategy on the port of the flexible interconnection switch to control the power between the interconnected microgrid subnets, realize power mutual assistance, and the power scheduling is relatively flexible.

[0027] See also Figure 3 The fully interconnected grid-connected mode of the microgrid group means that all microgrid subgrids are connected to the grid at their respective public connection points, and then interconnected through flexible interconnection switches. The flexible interconnection switches use a fixed DC voltage control strategy to control the power between different interconnected microgrid subgrids.

[0028] The PQ control strategy of the microgrid subnet refers to the direct control of active power and reactive power in the microgrid group, which is generally set in the control loop of the main converter inside the microgrid subnet. The PQ control adopted by the port of the flexible interconnection switch refers to the direct control of active power and reactive power in the control loop of the flexible interconnection switch to control the power flow in the flexible interconnection switch, thereby controlling the power flow between the interconnected microgrid subnets. For the microgrid group after grid connection, all microgrid subnets can achieve power mutual assistance with the distribution network. At the same time, the microgrid subnets can also achieve power mutual assistance through the flexible interconnection switch.

[0029] See also Figure 4The indirect grid-connected mode of the microgrid group refers to that some microgrid subnets are connected to each other through flexible interconnection switches, and then directly connected to the distribution network through a common connection point. The remaining microgrid subnets are connected to each other through flexible interconnection switches and then indirectly connected to the distribution network, instead of being directly connected to the distribution network through a common connection point. In this mode, the flexible interconnection switch provides a virtual synchronization port to provide voltage support for the indirectly connected microgrid subnets. The control of the virtual synchronization port refers to the virtual synchronization port control strategy set in the flexible interconnection switch, which simulates the characteristics of the synchronous generator, so that the connection port of the flexible interconnection switch has a voltage characteristic similar to that of the synchronous generator, and provides voltage support for the busbar of the microgrid subnet that is not directly connected to the grid. At the same time, it can control the power between the microgrid subnet and the flexible interconnection switch to achieve the purpose of power control between the microgrid subnets.

[0030] See also Figure 5 ,When the indirect connection mode of the microgrid group is adopted, it is necessary to add a pre-synchronization link in the ,control system of the flexible interconnection switch, which adjusts the voltage of the ,port of the flexible interconnection switch when the microgrid subgrid is connected to the grid. Figure 5 Medium V g is the distribution network voltage, ω is the distribution network angular frequency, ω m The grid-connected inverter outputs an angular frequency, and the voltage and frequency of the grid-connected inverter are compensated by the voltage difference ΔV and frequency difference Δω between the distribution network and the grid-connected inverter, so that the voltage and frequency of the grid-connected inverter gradually track the voltage of the distribution network. When the port voltage is consistent with the distribution network voltage, the microgrid subnet is connected to the distribution network, and the power control between the microgrid subnets is realized by combining the PQ control strategy. In this mode, when the microgrid subnet connected to the distribution network needs power mutual assistance, the PQ control strategy inside the microgrid subnet can be used to adjust the power within the microgrid subnet to achieve power mutual assistance with the distribution network; it can also achieve power mutual assistance with the indirectly interconnected microgrid subnet through the virtual synchronization port, that is, the connection port provided by the flexible interconnection switch can play a role in bidirectional power control.

[0031] Based on the above system, this scheme also proposes a multi-mode grid-connected control method for a flexible interconnected microgrid group, including the following steps:

[0032] Step 1: put the flexible interconnection switch in a standby state, and each microgrid subnet is connected to the distribution network through a public connection point to realize the independent grid-connected mode of the microgrid group.

[0033] In the independent grid-connected mode of the microgrid group, each microgrid subnet adopts the PQ control strategy to control the power between the microgrid subnet and the distribution network, and the distribution network provides voltage and frequency support for the microgrid subnet through the grid-connected interface. After the microgrid group is connected to the grid, if there is a power shortage in any microgrid subnet, the shortage power will be provided by the distribution network; if there is a power surplus in any microgrid subnet, the surplus power will be connected to the distribution network.

[0034] In step 2, each microgrid subnet is connected to the distribution network through a public connection point, and some of the microgrid subnets are connected to each other through flexible interconnection switches to realize the local interconnection and integration mode of the microgrid group.

[0035] In the local interconnection mode of the microgrid group, the flexible interconnection switch adopts the PQ control strategy to control the power between different interconnected microgrid subgrids, and the microgrid subgrid independently connected to the distribution network adopts the PQ control strategy to control the power between the microgrid subgrid and the distribution network.

[0036] The independently grid-connected microgrid subnet is provided with voltage support by the distribution network, and the power between the microgrid subnet and the distribution network is controlled by the PQ control strategy; in addition to controlling the power between the microgrid subnet and the distribution network by the PQ control strategy, the microgrid subnet connected by the flexible interconnection switch can also use PQ control on the port of the flexible interconnection switch to control the power between the interconnected microgrid subnets, thereby realizing power mutual assistance and relatively flexible power scheduling.

[0037] In step 3, each microgrid subnet is connected to the distribution network through a public connection point, and all microgrid subnets are interconnected through flexible interconnection switches to achieve a fully interconnected and grid-connected mode for the microgrid group.

[0038] In the fully interconnected mode of the microgrid group, the flexible interconnection switch adopts a constant DC voltage control strategy to control the power between different interconnected microgrid subgrids.

[0039] Each microgrid subnet can also control the power between each microgrid subnet and the distribution network through the PQ control strategy; for the flexible interconnection switch connecting the microgrid subnet, set the fixed DC voltage control to maintain the stability of the DC side voltage in the flexible interconnection switch. In this grid-connected mode, all microgrid subnets can obtain voltage and frequency support from the distribution network, and can directly obtain power mutual assistance from the distribution network, so the stability is strong in terms of voltage and frequency support and power mutual assistance; at the same time, the power between each microgrid subnet can flow and mutual assistance, which enhances the degree of power control of the microgrid group.

[0040] Step 4: Some microgrid subnets are connected to the distribution network through a public connection point and are connected to each other through flexible interconnection switches. Microgrid subnets that are not connected to the distribution network through a public connection point are connected to the flexible interconnection switch port and then indirectly connected to the distribution network, realizing the indirect grid connection mode of the microgrid group.

[0041] In the indirect grid-connected mode of the microgrid group, the flexible interconnection switch provides a virtual synchronization port to provide voltage support for the microgrid subgrid indirectly connected to the distribution network.

[0042] It should be noted that the four steps of this solution have no order of execution and will not be executed simultaneously. The step numbers are only used to distinguish different modes.

[0043] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A multi-mode grid-connected control method for a flexible interconnected microgrid group, characterized in that: The following steps are involved: The flexible interconnection switch is placed in a standby state, and each microgrid subnet is connected to the distribution network through a public connection point to realize the independent grid-connected mode of the microgrid group; Each microgrid subnet is connected to the distribution network through a public connection point, and some of the microgrid subnets are connected to each other through flexible interconnection switches to achieve a local interconnection and grid-connected mode of the microgrid group; Each microgrid subnet is connected to the distribution network through a public connection point, and all microgrid subnets are interconnected through flexible interconnection switches to achieve a fully interconnected and grid-connected mode for the microgrid group; Some microgrid subnets are connected to the distribution network through a public connection point and are connected to each other through flexible interconnection switches. Microgrid subnets that are not connected to the distribution network through a public connection point are connected to the flexible interconnection switch port and then indirectly connected to the distribution network, realizing the indirect grid-connected mode of the microgrid group.

2. The multi-mode grid-connected control method for a flexible interconnected microgrid group according to claim 1, characterized in that: In the independent grid-connected mode of the microgrid group, each microgrid subgrid adopts a PQ control strategy to control the power between the microgrid subgrid and the distribution network, and the distribution network provides voltage and frequency support for the microgrid subgrid through the grid-connected interface; after the microgrid group is connected to the grid, if any microgrid subgrid has a power shortage, the shortage power is provided by the distribution network; if any microgrid subgrid has a power surplus, the surplus power is integrated into the distribution network.

3. The multi-mode grid-connected control method for a flexible interconnected microgrid group according to claim 1, characterized in that: In the local interconnection mode of the microgrid group, in addition to controlling the power between the microgrid subgrid and the distribution network through the PQ control strategy, the microgrid subgrid connected by the flexible interconnection switch can also adopt the PQ control strategy on the port of the flexible interconnection switch to control the power between the interconnected microgrid subgrids; the microgrid subgrid independently connected to the distribution network adopts the PQ control strategy to control the power between the microgrid subgrid and the distribution network, and the voltage support of the independently connected microgrid subgrid is provided by the distribution network.

4. The multi-mode grid-connected control method for a flexible interconnected microgrid group according to claim 1, characterized in that: In the indirect grid-connected mode of the microgrid group, the flexible interconnection switch provides a virtual synchronization port to provide voltage support for the microgrid subgrid indirectly connected to the distribution network.

5. The multi-mode grid-connected control method for a flexible interconnected microgrid group according to claim 1, characterized in that: In the fully interconnected grid-connected mode of the microgrid group, the flexible switch uses constant DC voltage control to control the power between different interconnected microgrid subgrids. Each microgrid subgrid can also control the power between each microgrid subgrid and the distribution network through the PQ control strategy.

6. The multi-mode grid-connected control method for a flexible interconnected microgrid group according to any one of claims 2 to 5, characterized in that: The PQ control strategy refers to the direct control of active power and reactive power adopted in the microgrid group, which is set in the control loop of the main converter inside the microgrid subnet.

7. The multi-mode grid-connected control method for a flexible interconnected microgrid group according to any one of claims 1 to 5, characterized in that: The flexible interconnection switch refers to a switch that uses a power electronic converter as a connection port for microgrid interconnection to achieve conversion from AC to DC to AC. It has multiple ports to connect different microgrid subnets, and these ports share a DC link.

8. The multi-mode grid-connected control method for a flexible interconnected microgrid group according to any one of claims 3 to 5, characterized in that: The PQ control strategy of the flexible interconnected switch refers to setting up direct control of active power and reactive power in the control loop of the flexible interconnected switch to control the power flow in the flexible interconnected switch, thereby controlling the power flow between the interconnected microgrid subgrids.

9. The multi-mode grid-connected control method for a flexible interconnected microgrid group according to claim 5, characterized in that: The virtual synchronous port refers to a virtual synchronous port control strategy set in the flexible interconnection switch, which simulates the characteristics of the synchronous generator so that the connection port of the flexible interconnection switch has the voltage characteristics of the synchronous generator, provides voltage support for the busbar of the microgrid subgrid that is not directly incorporated into the distribution network, and controls the power between the microgrid subgrid and the flexible interconnection switch to achieve the purpose of power control between the microgrid subgrids.

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