A decentralized hybrid secondary frequency modulation control method for AC microgrids

Abstract

The present invention discloses a decentralized distributed hybrid secondary frequency modulation control method for an AC microgrid cluster, comprising the steps of: S1, establishing an AC microgrid cluster model, where the AC microgrid cluster model includes K microgrid clusters and operates in an island mode. Among them, the k-th microgrid cluster contains n k distributed energy sources, k = 1,..., K, and n k is a positive integer; S2, obtaining the output frequency of the i-th distributed energy source in the k-th microgrid cluster based on the droop equation, i = 1,..., n k ; S3, obtaining the frequency deviation and the cooperative control error parameter of each distributed energy source; S4, constructing a secondary controller based on leaky integration; S5, constructing a frequency modulation control model based on the droop equation and the secondary controller, and using the frequency modulation control model to achieve the frequency recovery and power distribution of the corresponding distributed energy source. This method can quickly achieve the frequency recovery and power distribution of the AC microgrid cluster, with low communication cost and wide application range.

Description

Technical Field

[0001] The present invention belongs to the technical field of microgrid control, and in particular relates to a decentralized hybrid secondary frequency modulation control method for an AC microgrid group. Background Art

[0002] In view of the increasing shortage of traditional energy and the growth of social electricity consumption, the development of new energy sources such as photovoltaic and wind energy has become the path of global energy development. The new power supply method of microgrid (MG) combined with load, energy storage elements and control unit has effectively alleviated the impact of large-scale distributed energy on the large power grid with its flexible and efficient characteristics, and has become a research hotspot.

[0003] Microgrids can be divided into AC microgrids and DC microgrids according to the current type. There are two main operating modes, namely, parallel operation with the large grid and island operation. When the microgrid is in the grid-connected stage, when the power supply energy is excess, the excess energy is fed back to the large grid; when the power supply energy is insufficient to meet the power supply demand, energy can be absorbed from the large grid. When the microgrid is in the island stage, it independently supplies power to its own load, optimizes the power supply quality, and relieves the power supply pressure of the large grid. For the islanded AC microgrid, its control unit is mainly divided into three controls: the primary control realizes power distribution through droop control, the secondary control is mainly to eliminate the voltage and frequency deviation caused by the primary control, and the third is to optimize the system power supply and energy distribution. In the secondary control, centralized, decentralized and distributed are three common control strategies. The centralized control is to control all distributed energy (distributed generation, DG) through a central controller; the decentralized control is centered on a single DG, and the secondary controller collects local information for independent control; the distributed control is a decentralized strategy, and the controller controls through local information and neighbor information, which is efficient and reliable.

[0004] The existing secondary control method of AC microgrid cluster mainly builds a two-layer secondary control to realize the two-layer communication of distributed energy within the cluster and the distributed energy proxy between clusters. It not only has a large communication volume and high communication cost, but also requires more information for control, and it is easy to ignore the situation of a single distributed energy within the cluster. Therefore, an efficient, universal and low-communication cost secondary control method is proposed. Summary of the invention

[0005] The purpose of the present invention is to address the above problems and propose a decentralized distributed hybrid secondary frequency modulation control method for an AC microgrid cluster, which can quickly realize frequency recovery and power distribution of an AC microgrid cluster, has low communication cost and a wide range of applications.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] The present invention proposes a method for controlling a distributed hybrid secondary frequency modulation of an AC microgrid, comprising the following steps:

[0008] S1. Establish an AC microgrid cluster model. The AC microgrid cluster model includes K microgrid clusters and is in an island operation mode. The kth microgrid cluster contains n k Distributed energy sources, k = 1, ..., K, n k is a positive integer;

[0009] S2. Obtain the output frequency of the i-th distributed energy in the k-th microgrid cluster based on the droop equation, which is calculated as follows:

[0010]

[0011] In the formula, is the time constant of the filter of the i-th distributed energy source in the k-th microgrid cluster, w k,i The derivative of w k,i is the output frequency of the i-th distributed energy in the k-th microgrid cluster, w d is the expected frequency of the AC microgrid model, is the droop gain coefficient of the i-th distributed energy in the k-th microgrid cluster, P k,i is the active power of the i-th distributed energy source in the k-th microgrid cluster, is the expected active power of the i-th distributed energy source in the k-th microgrid cluster, i=1,……,n k ;

[0012] S3. Obtain the frequency deviation and coordinated control error parameters of each distributed energy source, where:

[0013] Frequency deviation of the i-th distributed energy resource in the k-th microgrid cluster The calculation is as follows:

[0014]

[0015] Coordinated control error of the i-th distributed energy source in the k-th microgrid cluster The calculation is as follows:

[0016]

[0017] In the formula, u k,o is the control input signal of the oth distributed energy source in the kth microgrid cluster, N k,i is the neighboring distributed energy resource set of the i-th distributed energy resource in the k-th microgrid cluster, u k,iis the control input signal of the i-th distributed energy source in the k-th microgrid cluster;

[0018] S4. Construct a quadratic controller based on leaky integral, which is expressed as follows:

[0019]

[0020] In the formula, σ k,i ,η k,i ∈R + is the gain coefficient of the secondary controller, α k,i is the proxy coefficient of the i-th distributed energy in the k-th microgrid cluster. When the i-th distributed energy is a proxy distributed energy, α k,i =1, when the i-th distributed energy source is not a proxy distributed energy source, α k,i =0, the proxy distributed energy receives the AC bus voltage signal, represents the leaky integral term;

[0021] S5. A frequency modulation control model is constructed based on the droop equation and the quadratic controller. The frequency modulation control model is used to realize the frequency recovery and power distribution of the corresponding distributed energy. The frequency modulation control model is expressed as follows:

[0022]

[0023] Preferably, each microgrid cluster has a proxy distributed energy source, the proxy distributed energy sources do not communicate with each other, and the distributed energy sources in each microgrid cluster communicate with each other in a one-way manner.

[0024] Preferably, the first distributed energy source of each microgrid cluster is a proxy distributed energy source.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The method models the AC microgrid cluster, obtains the frequency deviation term and the collaborative control error term, and constructs a new secondary controller to achieve frequency recovery and power distribution of the AC microgrid cluster. Compared with the existing technology, it can not only solve the frequency deviation problem caused by droop in the primary control, so that the frequency is kept within the error range and synchronized to the reference value, but also solve the situation of a single distributed energy in the AC microgrid cluster, and realize the function of reducing communication costs, that is, the proxy distributed energy in the microgrid communicates with other distributed energy in a one-way manner, and there is no need for communication between the proxy distributed energy. The microgrid only needs one-way information communication between neighbors to achieve control. It can not only achieve frequency recovery and power distribution of the entire AC microgrid cluster, but also reduce a lot of communication. The proportion of power distribution can be achieved by adjusting the parameters of the secondary controller, which can greatly reduce the communication cost and improve the economic benefit. It can still be well applied when the topology structure of the AC microgrid cluster or the number of distributed energy changes, such as pure distributed, pure distributed, and distributed and mixed cases. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a flow chart of the decentralized hybrid secondary frequency modulation control method of AC microgrid group of the present invention;

[0028] Figure 2 The physical connection structure and communication topology diagram of the AC microgrid group model of the present invention;

[0029] Figure 3 It is a structural block diagram of the frequency modulation control model of the i-th distributed energy in the k-th microgrid cluster of the present invention;

[0030] Figure 4 This is a comparison diagram of the output active power before and after the method of the present application is adopted in an embodiment of the present invention;

[0031] Figure 5 This is a comparison diagram of output frequencies before and after the method of the present application is used in an embodiment of the present invention;

[0032] Figure 6 This is a comparison diagram of the active power output before and after the amount of distributed energy is changed in an embodiment of the present invention;

[0033] Figure 7 This is a frequency comparison diagram of the output before and after the amount of distributed energy is changed in an embodiment of the present invention. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0035] It should be noted that when a component is referred to as being "connected" to another component, it may be directly connected to the other component or there may be an intermediate component. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0036] The present invention first samples the operating data of distributed energy in an AC microgrid cluster to obtain its current frequency value, including output frequency and expected frequency, and further obtains the frequency deviation as a leaky integral link for controlling the distributed energy. The control input signal of the distributed energy is then transmitted to other distributed energy sources in the corresponding microgrid cluster, thereby achieving frequency compensation and power distribution.

[0037] like Figure 1-7 As shown, a distributed hybrid secondary frequency modulation control method for an AC microgrid group includes the following steps:

[0038] S1. Establish an AC microgrid cluster model. The AC microgrid cluster model includes K microgrid clusters and is in an island operation mode. The kth microgrid cluster contains n k Distributed energy sources, k = 1, ..., K, n k is a positive integer;

[0039] S2. Obtain the output frequency of the i-th distributed energy in the k-th microgrid cluster based on the droop equation, which is calculated as follows:

[0040]

[0041] In the formula, is the time constant of the filter of the i-th distributed energy source in the k-th microgrid cluster, w k,i The derivative of w k,i is the output frequency of the i-th distributed energy in the k-th microgrid cluster, w d is the expected frequency of the AC microgrid model, is the droop gain coefficient of the i-th distributed energy in the k-th microgrid cluster, P k,i is the active power of the i-th distributed energy source in the k-th microgrid cluster, is the expected active power of the i-th distributed energy source in the k-th microgrid cluster, i=1,……,n k ;

[0042] S3. Obtain the frequency deviation and coordinated control error parameters of each distributed energy source, where:

[0043] Frequency deviation of the i-th distributed energy resource in the k-th microgrid cluster The calculation is as follows:

[0044]

[0045] Coordinated control error of the i-th distributed energy source in the k-th microgrid cluster The calculation is as follows:

[0046]

[0047] In the formula, u k,o is the control input signal of the oth distributed energy source in the kth microgrid cluster, N k,i is the neighboring distributed energy resource set of the i-th distributed energy resource in the k-th microgrid cluster, u k,i is the control input signal of the i-th distributed energy source in the k-th microgrid cluster;

[0048] S4. Construct a quadratic controller based on leaky integral, which is expressed as follows:

[0049]

[0050] In the formula, σ k,i ,η k,i ∈R + is the gain coefficient of the secondary controller, α k,i is the proxy coefficient of the i-th distributed energy in the k-th microgrid cluster. When the i-th distributed energy is a proxy distributed energy, α k,i =1, when the i-th distributed energy source is not a proxy distributed energy source, α k,i =0, the proxy distributed energy receives the AC bus voltage signal, represents the leaky integral term;

[0051] S5. A frequency modulation control model is constructed based on the droop equation and the quadratic controller. The frequency modulation control model is used to realize the frequency recovery and power distribution of the corresponding distributed energy. The frequency modulation control model is expressed as follows:

[0052]

[0053] In one embodiment, each microgrid cluster has a proxy distributed energy source, the proxy distributed energy sources do not communicate with each other, and the distributed energy sources in each microgrid cluster communicate with each other in a one-way manner.

[0054] In one embodiment, the first distributed energy source of each microgrid cluster is a proxy distributed energy source. It should be noted that the proxy distributed energy source can also be adjusted according to actual needs.

[0055] In order to intuitively verify the effect of the method proposed in the present invention, the experimental results are respectively described below through examples.

[0056] The islanded AC microgrid model of the present invention includes three 220V, 50Hz microgrid clusters, which are composed of 2, 1, and 3 distributed energy sources (DG) respectively. In the case of decentralized and distributed hybrid, the first distributed energy source in the first microgrid cluster is recorded as DG. 1,1 , and the same is true for the others. It should be noted that the same applies to pure decentralized and pure distributed cases. Pure decentralized means that each distributed energy source is a microgrid cluster alone, and pure distributed means that there are more than two distributed energy sources in each microgrid cluster. The physical topology structure and corresponding communication topology structure of the AC microgrid cluster model are as follows: Figure 2 As shown, Figure 2 (a) is the physical topology diagram, where loads 1-6 correspond to DG 1,1 , DG 1,2 , DG 2,1 , DG 3,1 , DG 3,2 , DG 3,3 The load is connected by transmission lines, such as DG 1,1 , DG 1,2 , DG 2,1 , DG 3,3 , DG 3,2 , DG 3,1 Each pair is connected by a transmission line. Figure 2 (b) is the communication topology diagram. Only the distributed energy in the microgrid cluster communicates, and the direction is proxy distributed energy → ordinary distributed energy. There is no communication between microgrid clusters. Compared with the existing methods, the communication cost is greatly reduced. The experimental platform used is mainly Matlab / Simulink for modeling and simulation. Table 1 gives the main electrical parameters and controller (primary controller and secondary controller) parameters. α is the proxy coefficient, σ and η are the gain coefficients of the secondary controller. Figure 3 In the above example, the primary controller is the formula established based on the droop equation in step S2, and the local information refers to w k,i , neighbor information refers to u k,o .

[0057] Table 1

[0058]

[0059] Figure 4 and 5 The experimental waveforms obtained before and after the AC microgrid model is controlled by the method proposed in this application (divided by the start position of the secondary controller) for normal operation of the AC microgrid group model. When the secondary controller is started, the given condition satisfies α 1，1 =α 2，1 =α 3，1 =1,α 1，2 =α 3，2 =α 3，3 = 0. Figure 5 As shown, due to the droop function in the primary control, the output frequency of the distributed energy in each microgrid cluster cannot reach the reference value. By adopting the method proposed in this application, the output frequency of the 6 distributed energy DGs in the 3 microgrid clusters can better reach the reference value. Figure 4 As shown, the output active power can be distributed proportionally according to the controller parameters and the droop coefficient.

[0060] After each microgrid cluster is connected to the method proposed in this application and runs stably for a period of time, the number of distributed energy DGs in the microgrid cluster is changed, such as Figure 6 As shown in the figure, at 40 seconds, the second DG (DG 3,2 ) is disconnected, and the active power of the other five DGs increases to varying degrees. However, when the AC microgrid cluster model becomes stable, the active power output of the remaining five DGs reaches the ratio required by the controller parameters and the droop coefficient again. At 50 seconds, the second DG in the third cluster is reconnected to the microgrid cluster. After a short adjustment, the active power ratio of the six DGs reaches the original ratio again. Figure 7 As shown, except for a short adjustment time, the expected frequency of the AC microgrid cluster model always reaches the benchmark value.

[0061] It can be seen from the above experimental results that the decentralized hybrid secondary frequency modulation control method for an AC microgrid cluster proposed in the present invention can not only realize the frequency recovery and power distribution of the entire AC microgrid cluster, but also significantly reduce the communication cost according to the communication topology. The power distribution ratio can be achieved by adjusting the parameters of the secondary controller, thereby effectively improving the economic benefits of the microgrid.

[0062] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0063] The above-described embodiments only express the more specific and detailed embodiments described in this application, but they cannot be understood as limiting the scope of the patent application. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of this application, which all belong to the protection scope of this application. Therefore, the protection scope of the patent application shall be based on the attached claims.

Claims

1. A distributed hybrid secondary frequency modulation control method for an AC microgrid group, characterized by: The AC microgrid group decentralized hybrid secondary frequency modulation control method comprises the following steps: S1. Establish an AC microgrid cluster model, wherein the AC microgrid cluster model includes K microgrid clusters and is in an island operation mode, wherein the kth microgrid cluster contains n k Distributed energy sources, k = 1, ..., K, n k is a positive integer; S2. Obtain the output frequency of the i-th distributed energy in the k-th microgrid cluster based on the droop equation, which is calculated as follows: In the formula, is the time constant of the filter of the i-th distributed energy source in the k-th microgrid cluster, w k,i The derivative of w k,i is the output frequency of the i-th distributed energy in the k-th microgrid cluster, w d is the expected frequency of the AC microgrid model, is the droop gain coefficient of the i-th distributed energy in the k-th microgrid cluster, P k,i is the active power of the i-th distributed energy source in the k-th microgrid cluster, is the expected active power of the i-th distributed energy source in the k-th microgrid cluster, i=1,……,n k ; S3. Obtain the frequency deviation and coordinated control error parameters of each distributed energy source, where: Frequency deviation of the i-th distributed energy resource in the k-th microgrid cluster The calculation is as follows: Coordinated control error of the i-th distributed energy source in the k-th microgrid cluster The calculation is as follows: In the formula, u k,o is the control input signal of the oth distributed energy source in the kth microgrid cluster, N k,i is the neighboring distributed energy resource set of the i-th distributed energy resource in the k-th microgrid cluster, u k,i is the control input signal of the i-th distributed energy source in the k-th microgrid cluster; S4. Construct a quadratic controller based on leaky integral, which is expressed as follows: In the formula, σ k,i ,η k,i ∈R + is the gain coefficient of the secondary controller, α k,i is the proxy coefficient of the i-th distributed energy in the k-th microgrid cluster. When the i-th distributed energy is a proxy distributed energy, α k,i =1, when the i-th distributed energy source is not a proxy distributed energy source, α k,i =0, the proxy distributed energy receives the AC bus voltage signal, represents the leaky integral term; S5. A frequency modulation control model is constructed based on the droop equation and the quadratic controller, and the frequency modulation control model is used to realize the frequency recovery and power distribution of the corresponding distributed energy. The frequency modulation control model is expressed as follows:

2. The AC microgrid distributed hybrid secondary frequency modulation control method according to claim 1, characterized in that: Each of the microgrid clusters has a proxy distributed energy source, the proxy distributed energy sources do not communicate with each other, and the distributed energy sources in each of the microgrid clusters communicate with each other in a one-way manner.

3. The AC microgrid distributed hybrid secondary frequency modulation control method according to claim 2, characterized in that: The first distributed energy source of each of the microgrid clusters is a proxy distributed energy source.

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