Distributed secondary control method for ac-dc hybrid microgrid based on dynamic consistency

Abstract

The application provides a kind of AC / DC hybrid microgrid distributed secondary control method based on dynamic consistency, which adds sparse communication network on the basis of original microgrid control architecture, and distributed power supply and neighbor node carry out information interaction, and based on dynamic consistency theory, the interactive information is directly used for the correction of power reference in droop control. Interconnected converter only relies on local information to participate in secondary control, uses DC side information to realize active power mutual aid between AC / DC microgrid, and uses AC side information to provide reactive power support to AC subnetwork. Finally, under the cooperative control of interconnected converter and distributed power supply, the control objectives of voltage and frequency recovery and full-network distributed power supply power sharing are completed. The application simplifies the communication network topology, optimizes the plug and play function, fully utilizes the remaining capacity of interconnected converter, strengthens the mutual support ability of both sides of subnetwork, and improves the robustness of AC / DC hybrid microgrid system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of AC / DC hybrid microgrid control, and particularly relates to a distributed secondary control method for AC / DC hybrid microgrid based on dynamic consistency. BACKGROUND

[0002] In modern power systems, microgrids not only provide an energy interface for distributed power sources, but also improve the reliability of traditional power systems in extreme environments. However, with the rapid development of new energy technologies and distributed power generation, the proportion of DC-type distributed power sources and loads gradually increases. When connected to an AC power grid, energy conversion devices are needed for conversion, which undoubtedly increases the cost and reduces the efficiency. The emergence of AC / DC hybrid microgrids solves the above problems. This hybrid microgrid is composed of an AC subgrid, a DC subgrid, and an interconnection converter connecting the two subgrids. It combines the advantages of AC microgrids and DC microgrids, and is suitable for a wider variety of distributed power sources and loads, making it flexible to access the system, reducing the power conversion link, and improving the reliability and economy of microgrid power supply. However, the complex network structure has higher requirements for control strategies, especially the coordinated control between the interconnection converter and the distributed power source.

[0003] Currently, most microgrid control strategies use a hierarchical control structure: primary control layer, secondary control layer, and tertiary control layer. The primary control layer determines the output characteristics of individual distributed power sources, the secondary control layer is responsible for frequency and voltage recovery as well as power distribution, and the tertiary control layer is responsible for optimization control and economic operation. Among them, the implementation of secondary control can be divided into centralized control, decentralized control, and distributed control. Distributed control is based on the decentralized idea, and by introducing global variables into the control link through communication between distributed power sources, the system has strong robustness while meeting the plug-and-play function of distributed power sources, becoming the mainstream implementation method of the secondary control layer.

[0004] In recent years, the control strategy based on distributed consensus algorithm has become the focus of scholars' research, and a large number of research work has been carried out in the single power supply mode microgrid. However, the application of this technology in AC / DC hybrid microgrid is not common, and the main difficulty is the selection of interconnection converter control strategy and the determination of communication topology structure between AC / DC subnets. On the basis of meeting the control requirements of single subnet, the voltage support and power sharing between subnets also need to be considered. At present, foreign scholars Enrique Espina González et al. applied distributed consensus algorithm to AC / DC hybrid microgrid to improve the power sharing accuracy of distributed power in the whole network, but the strategy requires that there is a communication link between the distributed power and the interconnection converter, which will make the communication network more complex, and the communication delay and more communication variables may cause stability problems. In addition, since the interconnection converter has a large capacity redundancy under normal circumstances, its reactive power support capability for AC subnet needs to be explored. SUMMARY

[0005] In order to overcome the problems existing in the prior art, the purpose of the present application is to provide a kind of AC / DC hybrid microgrid distributed secondary control method based on dynamic consensus, which solves the voltage and frequency support and recovery problem of AC / DC hybrid microgrid, realizes the power sharing of distributed power in the whole network. This method is still feasible and effective under special circumstances such as load fluctuation and communication failure, meets the plug and play function of distributed power, not only improves the robustness and power supply reliability of AC / DC hybrid microgrid system, but also improves the system full stability and economic operation level.

[0006] In order to achieve the above technical purpose, the technical scheme adopted by the present application is as follows:

[0007] The AC / DC hybrid microgrid distributed secondary control method based on dynamic consensus introduces dynamic consensus theory into the secondary control of AC / DC hybrid microgrid system. The interconnection converter adjusts the power flow on both sides of AC subnet and DC subnet through local information, provides reactive power support for AC subnet, and the distributed power relies on sparse communication network for secondary control. The interconnection converter and the distributed power are cooperatively controlled to realize the power sharing of distributed power in the whole network.

[0008] The AC-DC hybrid micro-grid system is composed of an AC sub-network, a DC sub-network, an interconnection converter and a sparse communication network; wherein the interconnection converter connects the AC sub-network and the DC sub-network, the AC sub-network is internally connected with AC distributed power and load, and the DC sub-network is internally connected with DC distributed power and load; the sparse communication network is composed of secondary controllers of all network distributed power, the interconnection converter does not participate in communication, the nodes in the AC sub-network and the DC sub-network perform inter-neighbor communication, and there is at least one communication link between the AC sub-network and the DC sub-network, and the secondary controller is responsible for collecting and sending local information and receiving neighbor node information.

[0009] The distributed secondary control method specifically comprises the following steps:

[0010] Step 1), establishing an AC-DC hybrid micro-grid system, obtaining AC-DC hybrid micro-grid structure parameters and rated parameters of each distributed power and interconnection converter;

[0011] Step 2), judging the proximity relationship of each distributed power according to the structure of the AC-DC hybrid micro-grid system, determining the communication network structure of the adjacent distributed power in the AC sub-network and the DC sub-network and the communication relationship of the distributed power between the AC sub-network and the DC sub-network;

[0012] Step 3), determining the AC micro-source control mode according to formula (1) and formula (2), the AC micro-source adopts a primary control strategy of droop characteristics to support the frequency and voltage of the AC sub-network, and the distributed secondary control adjusts the state variables ψ i and χ i to realize the frequency and voltage recovery and power sharing of the AC sub-network; wherein the power sharing specifically refers to the active power being distributed according to the capacity in the distributed power of the whole network, and the reactive power being proportionally distributed according to the capacity in the distributed power of the AC sub-network;

[0013]

[0014]

[0015] In the formula, ω i and u i are the output frequency and output voltage of the i th AC micro-source; ω ref and U ref are the reference values of the output frequency and output voltage; n p and n q are the active-frequency and reactive-voltage droop control coefficients; P i * and are the unit values of the active power and reactive power output by the i th AC micro-source; with respectively represent the reference value of active power and reactive power output by the i-th AC micro-source; ψ i and χ i respectively represent the state variable of active secondary control and reactive secondary control; τ i and κ i are control parameters of distributed secondary control; α i and β i are frequency and voltage recovery coefficients respectively; N ac and N dc respectively represent the total number of AC micro-sources and DC micro-sources in the whole network; a ik and b ik respectively represent the communication coefficient of active secondary control and reactive secondary control; P k * with represent the per-unit value of active power and reactive power output by the k-th distributed power source obtained through communication;

[0016] Step 4), determine the DC micro-source control mode according to formula (3), the DC micro-source adopts the primary control strategy of droop characteristic to support the DC voltage of DC sub-network, and the distributed secondary control adjusts the state variable ζ j to realize the DC voltage recovery and power sharing of DC sub-network; wherein, the power sharing specifically refers to the proportional distribution of active power in the whole network distributed power source according to its capacity;

[0017]

[0018] In the formula, u dcj is the output DC voltage of the j-th DC micro-source; U dcref is the output DC voltage reference value; n dc represents the active-DC voltage droop control coefficient; represents the per-unit value of active power output by the j-th DC micro-source; represents the active power reference value output by the j-th DC micro-source; ζ j represents the state variable of active secondary control; ò j is the control parameter of distributed secondary control; γ j is the DC voltage recovery coefficient; c jk represents the communication coefficient of active secondary control; P k * represents the per-unit value of active power output by the k-th distributed power source obtained through communication;

[0019] Step 5), determine the interconnection converter control mode according to formula (4), the interconnection converter participates in active and reactive power secondary control by relying on local AC / DC information respectively, and the specific content comprises that: in the active power secondary control, the interconnection converter adopts fixed DC voltage control on the d-axis, and only relies on the local information on the DC side to adjust the active power flow between the AC sub-network and the DC sub-network; when the AC sub-network undertakes more load, the interconnection converter transmits the active power from the DC sub-network to the AC sub-network, and vice versa; in the reactive power secondary control, the interconnection converter adopts reactive power-voltage droop control on the q-axis, and only relies on the local information on the AC side to participate in the reactive power support of the AC sub-network; when the AC sub-network voltage drops, the interconnection converter outputs reactive power to support the AC voltage, and vice versa, absorbs reactive power to reduce the AC voltage;

[0020]

[0021] In the formula, is the DC side output voltage of the interconnection converter; is the DC side rated voltage of the interconnection converter; is the reactive power output reference value of the interconnection converter; represents that the interconnection converter outputs reactive power; n ic is the reactive power support droop control coefficient; is the average value of the effective value of the AC voltage; is the AC side rated voltage of the interconnection converter.

[0022] Compared with the prior art, the present application has the following advantages:

[0023] 1) On the basis of realizing the above technical purpose, the communication network topology is simplified, the interconnection converter and the distributed power supply do not need to communicate, the plug and play function is optimized, the power supply reliability is improved, and the operation cost is reduced;

[0024] 2) The interconnection converter participates in active and reactive power secondary control at the same time, adjusts the active power flow of the two sides of the sub-network, and also participates in the reactive power adjustment of the AC side, fully utilizes the remaining capacity of the interconnection converter, strengthens the mutual support ability of the two sides of the sub-network, and improves the robustness of the AC / DC hybrid micro-grid system. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 It is a method flowchart of the present application;

[0026] Figure 2 It is a communication network structure diagram of the AC / DC hybrid micro-grid system;

[0027] Figure 3 It is a distributed power supply control block diagram of the whole network;

[0028] Figure 4Control block diagram of interconnection converter;

[0029] Fig. 5 (a) is an active power curve of the distributed power supply when the AC load changes;

[0030] Fig. 5 (b) is an active power curve of the interconnection converter when the AC load changes;

[0031] Fig. 6 (a) is an active power curve of the distributed power supply when the DC load changes;

[0032] Fig. 6 (b) is an active power curve of the interconnection converter when the DC load changes;

[0033] Fig. 7 (a) is a reactive power curve of the distributed power supply when the reactive load changes;

[0034] Fig. 7 (b) is a reactive power curve of the interconnection converter when the reactive load changes;

[0035] Figure 8 Fig. 8 is a power curve when a single-point communication failure occurs;

[0036] Figure 9 Fig. 9 is a power curve when the plug-and-play function of the distributed power supply is verified. DETAILED DESCRIPTION

[0037] In order to make the technical solutions of the present application clearer and more complete, the present application will be described in detail below in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0038] The specific embodiment process is shown in the accompanying Figure 1 The steps are as follows:

[0039] Step 1), a hybrid AC / DC microgrid system is established, the structure of which is shown in the accompanying Figure 2 The structure parameters of the hybrid AC / DC microgrid and the rated parameters of the grid-connected converter are obtained, and the parameters are shown in Table 1. As shown in the accompanying Figure 2 The hybrid AC / DC microgrid system is composed of an AC subnetwork, a DC subnetwork, an interconnection converter, and a sparse communication network; the interconnection converter connects the AC subnetwork and the DC subnetwork, the AC subnetwork is internally connected with an AC-type distributed power supply and a load, and the DC subnetwork is internally connected with a DC-type distributed power supply and a load; the sparse communication network is composed of secondary controllers of all distributed power supplies in the network, the interconnection converter does not participate in communication, the nodes in the AC subnetwork and the DC subnetwork perform inter-neighbor communication, and there is at least one communication link between the AC subnetwork and the DC subnetwork, and the secondary controllers are responsible for collecting and sending local information and receiving neighbor node information.

[0040] Table 1 Microgrid structure parameters

[0041]

[0042] Step 2): Based on the AC / DC hybrid microgrid system structure, determine the proximity relationships of each distributed power source, identify the communication network structure of adjacent distributed power sources within the AC and DC subgrids, and determine the communication relationships between distributed power sources between the AC and DC subgrids. The communication network topology is shown in the attached figure. Figure 2 As shown.

[0043] Step 3): Determine the control mode of the AC micro-source according to equations (1) and (2), and its control block diagram is attached. Figure 3 As shown. The AC micro-source employs a droop-characteristic primary control strategy to support the AC subgrid frequency and voltage, while the distributed secondary control adjusts the state variable ψ using power information from adjacent distributed power sources. i and χ i To achieve frequency and voltage restoration and power equalization in the AC subgrid; wherein, the power equalization specifically refers to the distribution of active power in the distributed power sources of the entire network according to capacity, and the proportional distribution of reactive power in the distributed power sources of the AC subgrid according to capacity.

[0044]

[0045]

[0046] In the formula, ω i with u i These represent the output frequency and output voltage of the i-th AC micro-source, respectively; ω ref with U ref These are the reference values ​​for the output frequency and output voltage, respectively; n p With n q These represent the active power-frequency and reactive power-voltage droop control coefficients, respectively; P i * and These represent the per-unit values ​​of the active and reactive power output by the i-th AC micro-source, respectively. and ψ represents the reference values ​​of the active power and reactive power output of the i-th AC micro-source, respectively; i and χ i τ represents the state variables of active power secondary control and reactive power secondary control, respectively; i and κ i For distributed secondary control, the control parameters are α. i and β i These are the frequency and voltage recovery coefficients, respectively; N ac and N dc These represent the total number of AC and DC micro-sources in the entire network, respectively; a ik and b ikP represents the communication coefficients for active power secondary control and reactive power secondary control, respectively; k * and This represents the per-unit values ​​of the active and reactive power output of the k-th distributed power source obtained through communication; the control parameters are shown in Table 2.

[0047] Table 2 Microgrid Control Parameters

[0048]

[0049]

[0050] Step 4), determine the DC micro-source control mode according to equation (3), and its control block diagram is attached. Figure 3 As shown. The DC micro-source employs a droop-characteristic primary control strategy to support the DC subgrid voltage, and the distributed secondary control adjusts the state variable ζ using power information from adjacent distributed power sources. j To achieve DC voltage recovery and power equalization in the DC subgrid; wherein, the power equalization specifically refers to the proportional distribution of active power among the distributed power sources in the entire grid according to their capacity;

[0051]

[0052] In the formula, u dcj U is the output DC voltage of the j-th DC micro-source; dcref This is the reference value for the output DC voltage; n dc This represents the active-DC voltage droop control coefficient; This represents the per-unit value of the active power output by the j-th DC micro-source; ζ represents the reference value of the active power output of the j-th DC micro-source; j Represents the state variables of active secondary control; j For distributed secondary control, the control parameters are γ; j c is the DC voltage recovery coefficient; jk P represents the communication coefficient for active secondary control; k * This represents the per-unit value of the active power output of the kth distributed power source obtained through communication; the control parameters are shown in Table 2.

[0053] Step 5), determine the control mode of the interconnected converter according to equation (4), and its control block diagram is attached. Figure 4The interconnection converters participate in active and reactive power secondary control respectively by local AC / DC information, and the specific content includes: in the active power secondary control, the interconnection converters adopt constant DC voltage control on the d-axis, and only rely on local information on the DC side to adjust the active power flow between the AC sub-network and the DC sub-network; when the AC sub-network undertakes more load, the interconnection converter transmits active power from the DC sub-network to the AC sub-network, and vice versa; in the reactive power secondary control, the interconnection converters adopt reactive power-voltage droop control on the q-axis, and only rely on local information on the AC side to participate in the reactive power support of the AC sub-network; when the AC sub-network voltage drops, the interconnection converter outputs reactive power to support the AC voltage, and vice versa;

[0054]

[0055] wherein, is the output voltage of the DC side of the interconnection converter; is the rated voltage of the DC side of the interconnection converter; is the reactive power output reference value of the interconnection converter; represents that the interconnection converter outputs reactive power; n ic is the reactive power support droop control coefficient; is the average value of the effective value of the AC voltage; is the rated voltage of the AC side of the interconnection converter. The control parameters are shown in Table 2.

[0056] The embodiment verifies the feasibility of the control strategy under different working conditions:

[0057] Fig. 5(a) is an active power curve of the distributed power supply when the AC load changes, and Fig. 5(b) is an active power curve of the interconnection converter when the AC load changes. As can be seen from the figures, after adopting the control strategy of the application, the three distributed power supplies achieve active power sharing, and can quickly coordinate and restore the power sharing state when the AC side load increases and decreases subsequently. The interconnection converter can dynamically adjust the active power according to the local information, which is specifically manifested as increasing the transmission power from the DC side to the AC side when the AC load increases, or as reducing the transmission power from the AC side to the DC side, and vice versa, thereby assisting the distributed power supply to complete power sharing.

[0058] Fig. 6(a) is a graph of active power curve of the distributed power supply when the DC load changes, and Fig. 6(b) is a graph of active power curve of the interconnection converter when the DC load changes. As shown in the figures, when the DC load increases or decreases, the control strategy of the present application can still ensure that the distributed power supply is in the power sharing state. Unlike when the AC load changes, since the interconnection converter uses the constant DC voltage control in the process, it relies on the DC side information, so when the DC load changes, its power support action is faster, but it can still remain stable in the subsequent control and dynamically adjust the active power according to the load.

[0059] Fig. 7(a) is a graph of reactive power curve of the distributed power supply when the reactive load changes, and Fig. 7(b) is a graph of reactive power curve of the interconnection converter when the reactive load changes. As shown in the figures, when the reactive load increases or decreases, the strategy can still ensure that the AC micro source maintains the reactive power sharing. In addition, the interconnection converter will also dynamically adjust the reactive power according to the local AC side information, which is manifested as follows: when the AC side reactive load increases and causes the voltage to drop, the interconnection converter outputs the reactive power to support the AC voltage, and vice versa.

[0060] Figure 8 Fig. 8 is a power curve graph when a single-point communication failure fault occurs. As shown in the figure, after the three distributed power supplies complete power sharing, DG2 is disconnected to simulate a communication failure fault, and then the load is increased. It can be seen that the remaining two distributed power supplies can still achieve the power sharing state, while DG2 can only maintain the droop control due to the lack of information interaction, but it can continue to participate in power sharing after the subsequent communication is restored. This implementation case shows that a small-area communication failure will not affect the stability of the control system.

[0061] Figure 9 Fig. 9 is a power curve graph of the plug-and-play of the distributed power supply. As shown in the figure, at t=10s, DG2 is cut off and the communication is disconnected, and the remaining two distributed power supplies can still maintain the power sharing; at t=35s, DG2 is connected, and at this time, only the droop control is used to output the power; at t=45s, DG2 is connected to the communication network, so that the power sharing is realized. This implementation case shows that the control strategy of the present application can meet the plug-and-play function of the distributed power supply, and the investment and cutting-off are flexible, and the system stability can still be ensured in the process, which has good power supply reliability and system robustness.

[0062] The technical solutions of the present application are described in detail above with reference to the drawings, but are not a limitation on the protection scope of the present application. Various modifications or variations made by those skilled in the art on the basis of the technical solutions of the present application without creative labor are still within the protection scope of the present application.

Claims

1. A distributed secondary control method for AC / DC hybrid microgrid based on dynamic consistency, characterized in that: The dynamic consistency theory is introduced into the secondary control of the AC / DC hybrid microgrid system. The interconnected converter adjusts the power flow between the AC subgrid and the DC subgrid through local information, provides reactive power support for the AC subgrid, and the distributed power sources are controlled by the sparse communication network. The interconnected converter and the distributed power sources are cooperatively controlled to achieve power sharing of the distributed power sources in the whole network. The AC / DC hybrid microgrid system is composed of an AC subgrid, a DC subgrid, an interconnected converter and a sparse communication network. The interconnected converter is connected to the AC subgrid and the DC subgrid. The AC subgrid is connected to AC distributed power sources and loads, and the DC subgrid is connected to DC distributed power sources and loads. The sparse communication network is composed of secondary controllers of the distributed power sources in the whole network. The interconnected converter does not participate in communication. The nodes in the AC subgrid and the DC subgrid communicate with each other. There is at least one communication link between the AC subgrid and the DC subgrid. The secondary controller is responsible for collecting and sending local information and receiving neighbor node information. The method comprises the following steps: Step 1), establishing an AC / DC hybrid microgrid system, obtaining the structure parameters of the AC / DC hybrid microgrid and the rated parameters of each distributed power source and the interconnected converter; Step 2), determining the proximity relationship of each distributed power source according to the structure of the AC / DC hybrid microgrid system, and determining the communication network structure of the adjacent distributed power sources in the AC subgrid and the DC subgrid and the communication relationship of the distributed power sources between the AC subgrid and the DC subgrid; Step 3), determining the AC micro-source control mode according to formula (1) and formula (2), the AC micro-source adopting a primary control strategy of droop characteristics to support the frequency and voltage of the AC sub-network, and a distributed secondary control adjusting state variables ψ and χ by power information of adjacent distributed power sources to achieve frequency and voltage recovery and power sharing of the AC sub-network i and χ i ; wherein the power sharing specifically refers to active power being distributed among the distributed power sources in the whole network according to capacity, and reactive power being proportionally distributed among the distributed power sources in the AC sub-network according to capacity. ωi i and u i are the output frequency and output voltage of the ith AC micro-source, respectively; ω ref and U ref are the reference values of the output frequency and output voltage, respectively; n p and n q represent the active-frequency and reactive-voltage droop control coefficients, respectively; P i * and represent the per-unit value of the active power and reactive power output by the ith AC micro-source, respectively; and represent the reference values of the active power and reactive power output by the ith AC micro-source, respectively; ψ i and χ i represent the state variables of the active secondary control and reactive secondary control, respectively; τ i and κ i are the control parameters of the distributed secondary control; α i and β i are the frequency and voltage recovery coefficients, respectively; N ac and N dc represent the total number of AC micro-sources and DC micro-sources in the whole network, respectively; a ik and b ik represent the communication coefficients of the active secondary control and reactive secondary control, respectively; P k * and represent the per-unit value of the active power and reactive power output by the kth distributed power source obtained through communication. Step 4), determining a DC micro-source control mode according to formula (3), the DC micro-source adopting a primary control strategy of droop characteristics to support a DC sub-network DC voltage, and a distributed secondary control adjusting a state variable ζ through power information of adjacent distributed power sources j to realize DC voltage recovery and power sharing of the DC sub-network; wherein the power sharing specifically refers to proportional distribution of active power among the distributed power sources in the whole network according to their capacities. In the formula, u dcj is the output DC voltage of the jth DC micro-source; U dcref is the output DC voltage reference value; n dc represents the active-DC voltage droop control coefficient; represents the active power unit value output by the jth DC micro-source; represents the active power reference value output by the jth DC micro-source; ζ j represents the state variable of active secondary control; ∈ j is the control parameter of distributed secondary control; γ j is the DC voltage recovery coefficient; c jk represents the communication coefficient of active secondary control; represents the active power unit value output by the kth distributed power source obtained by communication; Step 5), determining the control mode of the interconnected converter according to formula (4). The interconnected converter cooperatively participates in active and reactive secondary control by relying on local AC / DC information. The specific content includes: in the active secondary control, the interconnected converter adopts constant DC voltage control on the d-axis and only relies on local information on the DC side to adjust the active power flow between the AC subgrid and the DC subgrid. When the AC subgrid bears more loads, the interconnected converter transmits active power from the DC subgrid to the AC subgrid, and vice versa. In the reactive secondary control, the interconnected converter adopts reactive-voltage droop control on the q-axis and only relies on local information on the AC side to participate in the reactive power support of the AC subgrid. When the AC subgrid voltage decreases, the interconnected converter outputs reactive power to support the AC voltage, and vice versa. wherein, is the output voltage of the DC side of the interconnection converter; is the rated voltage of the DC side of the interconnection converter; is the reactive power output reference value of the interconnection converter; represents the reactive power sent by the interconnection converter; n ic is the droop control coefficient of the reactive power support; is the average value of the effective value of the AC voltage; is the rated voltage of the AC side of the interconnection converter.

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