Distributed secondary control method for convergence of voltage frequency of island alternating-current micro-grid within preset time

Abstract

The invention provides a distributed secondary control method for convergence of voltage frequency of an island alternating-current micro-grid within preset time, relates to the technical field of micro-grids, and describes a communication network among distributed power supplies in the island alternating-current micro-grid based on a graph theory. Based on an output frequency equation and a voltage amplitude equation of droop control in a primary control layer of the distributed power supply, and in combination with a communication network diagram, a frequency controller and an active power controller in a secondary control layer are respectively designed; the frequency of the distributed power supply is recovered to the rated value within the preset time, and meanwhile, the target of distributing the active power according to the proportion is achieved. And designing a voltage controller in the secondary control layer to achieve the target that the output voltage of the distributed power supply is recovered to a rated value within a preset time, so that the minimum upper bound of the voltage / frequency convergence time of the island alternating current micro-grid can be preset within any physical allowable range to adapt to different application requirements.

Description

Technical Field

[0001] The present invention relates to the technical field of microgrids, and more specifically, to a distributed secondary control method for an island AC microgrid whose voltage and frequency converge within a predetermined time. Background Art

[0002] Microgrid is a small autonomous power system composed of multiple distributed generation (DG), energy storage, load and related monitoring and protection devices. It can realize self-control and management, and has two operation modes: grid-connected operation and island operation. When the microgrid is in grid-connected operation, its dynamic characteristics are mainly determined by the main grid; when it is in island operation, due to the loss of frequency and voltage support from the main grid, it needs to rely on the autonomous control of its own distributed generation to stabilize the system frequency and voltage and maintain power balance, which also makes the control strategy of island microgrid a difficult point to study.

[0003] In order to achieve efficient and flexible control of isolated microgrids, a hierarchical control method consisting of primary control, secondary control, and tertiary control has been widely used. Among them, the main goal of primary control is to maintain system frequency, voltage stability, and supply and demand power balance, and droop control is generally used. However, primary control based on droop control is essentially differential control, which will cause voltage and frequency to deviate from their rated values; secondary control focuses on the dynamic operation control of the microgrid, which is used to restore the deviation caused by droop control; the main function of tertiary control is the economic operation and energy management of the microgrid. Secondary control is a key link to ensure the flexibility, stability, and power quality of the microgrid. Therefore, the secondary control of isolated microgrids has received extensive attention from researchers in the field of microgrid technology at home and abroad.

[0004] Traditional secondary control of isolated microgrids alleviates the steady-state deviation of voltage and frequency in a centralized manner. The centralized control strategy relies on the central processor to collect and process a large amount of DG data, which requires a complex communication network topology and is prone to single point failures. The system has poor scalability and is difficult to meet the "plug and play" control requirements of DG. In order to overcome the drawbacks of centralized control, distributed cooperative control based on the consistency of multi-agent system (MAS) is introduced to solve the secondary control of isolated microgrids. References "BIDRAM A, DAVOUDI A, LEWIS FL, et al. Secondary control of microgrids based on distributed cooperative control of multi-agent systems[J]. IET Generation, Transmission&Distribution, 2013, 7(8): 822-831" and "SIMPSON-PORCO JW, SHAFIEE Q, F, et al. Secondary frequency and voltage control of islanded microgrids via distributed averaging [J]. IEEE Transactions on Industrial Electronics, 2015, 62 (11): 7025-7038” uses feedback linearization technology to transform the secondary control into a first-order system tracking synchronization problem, and then designs a distributed secondary control strategy based on the leader-follower consensus algorithm. The literature “Zhou Ye, Wang Keyou, Li Guojie, et al. Distributed hierarchical control strategy of microgrids based on multi-agent consensus algorithm [J]. Automation of Electric Power Systems, 2017, 41 (11): 142-149” designs a distributed secondary control strategy based on the average consensus algorithm. The above distributed secondary control strategy can only achieve asymptotic time convergence.

[0005] In practical applications, there is an inherent coupling between active and reactive power in the isolated microgrid, that is, voltage and frequency will affect each other. Shortening the frequency recovery and active power ratio distribution time can reduce the inherent frequency and voltage coupling when the line impedance is uneven to a certain extent, which plays a vital role in improving the power quality of the AC isolated microgrid. To this end, scholars have studied the finite time secondary control strategy of the isolated microgrid that can effectively shorten the convergence time. Although the distributed finite time control strategy can shorten the convergence time of the controlled state to a certain extent, the convergence time of the finite time secondary control strategy is often related to the initial value of the controlled state. In other words, the size of this time range ultimately depends on the initial value of the controlled state at that time, which is random and difficult to adjust. Since the initial conditions may not be obtained in advance, it makes it difficult to preset the setting time offline, and it is impossible to achieve the coordinated control of multiple distributed power sources in the isolated AC microgrid, which is not conducive to the improvement of the power quality of the AC isolated microgrid.

[0006] In order to obtain a fixed convergence time range, scholars have studied fixed-time control. The paper "NI J, LIU L, LIU C, et al. Fixed-time leader-following consensus for second-order multiagent systems with input delay[J]. IEEE Transactions on Industrial Electronics, 2017, 64(11): 8635-864" proposed a distributed leader-follower fixed-time consensus algorithm for multi-agent systems, which can ensure that the convergence time is independent of the initial state. The literature "Chen Gang, Li Zhiyong, Wei Mengli. Distributed fixed-time quadratic coordinated control for isolated microgrids [J]. Control and Decision, 2019, 34(01): 205-212." and "ZHAO D, ZHANG C, SUN Y, et al. Distributed robust frequency restoration and active power sharing for autonomous microgrids with event-triggered strategy [J]. IEEE Transactions on Smart Grid, 2021, 12(5): 3819-3834." applied the distributed fixed-time control method to isolated microgrids to improve their dynamic performance. Although the convergence time under fixed-time control is independent of the initial value of the controlled state, it often has a complex relationship with multiple system control parameters, making it difficult to directly obtain the upper bound of the convergence time, and the convergence estimate time is usually conservative.

[0007] In the hierarchical control of microgrids, the time scale between layers is very important. If the rapid convergence of the microgrid can be achieved based on the pre-determined convergence time, the dynamic performance of the entire microgrid system will be further improved and the power safety of sensitive loads will be guaranteed. Summary of the invention

[0008] In order to solve the problem that traditional distributed secondary control cannot pre-set the voltage and frequency convergence time of an isolated island microgrid, the present invention proposes a distributed secondary control method for the voltage and frequency of an isolated AC microgrid to converge within a preset time, so that the minimum upper bound of the voltage and frequency convergence time of the isolated AC microgrid can be pre-set within any physically allowed range to adapt to different application requirements, and achieve the control goal of proportional distribution of active power while achieving the voltage and frequency recovery within the preset time.

[0009] In order to achieve the above technical effects, the technical solution of the present invention is as follows:

[0010] A distributed secondary control method for island AC microgrid voltage frequency convergence within a predetermined time, comprising the following steps:

[0011] Describe the communication network between distributed generation in island AC microgrid based on graph theory;

[0012] Acquire a basic control structure of a distributed power source, wherein the basic control structure includes a primary control layer and a secondary control layer, and the primary control layer includes droop control;

[0013] Construct the output frequency equation and voltage amplitude equation of droop control in the primary control layer;

[0014] Based on the communication network and output frequency equation between distributed power sources in the isolated AC microgrid described by graph theory, the frequency controller and active power controller in the secondary control layer are designed respectively to achieve the goal of restoring the frequency of the distributed power sources to the rated value within the predetermined time and achieving the goal of proportional distribution of active power;

[0015] Based on the communication network and voltage amplitude equation between distributed power sources in an islanded AC microgrid described by graph theory, the voltage controller in the secondary control layer is designed to achieve the goal of restoring the output voltage of the distributed power sources to the rated value within a predetermined time.

[0016] Preferably, the isolated AC microgrid includes N distributed power sources DG, and the communication graph of the N distributed power sources DG is expressed as: G = (V, E, A), where V = {υ1,υ2,…,υ N} is the set of distributed generation DG nodes in the isolated AC microgrid, E = {e ij =(υ i ,υ j):υ i ,υ j ∈V} represents the communication connection set between distributed generation DGs, A=[a ij ]∈R N×N is a non-negative weighted adjacency matrix;

[0017] If DG i Can receive DG j information, then it is called DG j It's DG i The neighbor node of j ,υ i )∈E and a ij >0, otherwise a ij =0; the neighbor node set of node i is represented by N i ={ j ∈V:(υ i ,υ j )∈E}, if for any i,j∈V, there is a ij =a ji , then the graph G is an undirected graph; if there is a path between any two nodes in the undirected graph G, then the graph G is a connected graph; the degree matrix D of the undirected graph G is D = diag (d i ) represents the diagonal matrix formed by the degree of node i. The Laplacian matrix of graph G is expressed as: L = D-Aa;

[0018] In the leader-follower case, define the diagonal matrix B = diag{b 10 ,b 20 ,…,b N0}, where b i0 Indicates DG i The connection weight between DG i can receive the leader's information, then b i0 >0, otherwise, b i0 = 0. If at least one DG can receive the reference value of the virtual leader, a graph M is formed, M = L + B, and M is a connected graph.

[0019] Preferably, any distributed power source DG in the isolated AC microgrid is composed of a DC voltage source, a voltage source inverter, an LC filter and a basic transmission line, the output end of the DC voltage source is connected to the input end of the voltage source inverter, the output end of the voltage source inverter is connected to the input end of the LC filter, and the output end of the LC filter is connected to the basic transmission line;

[0020] The primary control layer includes power calculation, low-pass filter, droop control, voltage loop control, and current loop control. The primary control layer collects the output current i of the voltage source inverter. Labci , the output current i of the LC filterOabci And the output voltage v of the LC filter Oabci , where the output current i of the LC filter is Oabci And the output voltage v of the LC filter Oabci After dq transformation, it enters power calculation to achieve decoupling of active power and reactive power, and obtains DG i The instantaneous active power output p i (t) Instantaneous reactive power q i (t), and then the instantaneous active power p is filtered out by a low-pass filter i (t) Instantaneous reactive power q i The high-frequency harmonics contained in (t) are used to obtain the average active power P i (t) and average reactive power Q i (t), average active power P i (t) and average reactive power Q i (t) as the input of droop control, generating output frequency equation and voltage amplitude equation in droop control;

[0021] Droop control causes the DG output voltage and frequency to deviate, and the secondary control layer is introduced to generate the frequency reference value ω ni and voltage reference V ni Correct the deviation, and the new reference value generated by the secondary control layer is applied to the droop control to generate the reference value for the voltage loop control and In voltage loop control, generate reference value for current loop control and The output current i of the voltage source inverter Labci After dq transformation and The current errors are calculated respectively, and finally the output of the voltage source inverter is adjusted by sinusoidal pulse width modulation.

[0022] Preferably, the output frequency equation and the voltage amplitude equation are respectively:

[0023] ω i (t) = ω ni (t)-m i P i (t)

[0024] V i (t) = V ni (t)-n i Q i (t)

[0025] Among them, ω i (t) and V i (t) are the output frequency and voltage amplitude respectively; ω ni (t) and Vni (t) are the reference frequency and voltage amplitude of the primary control layer respectively; m i and n i DG i The active power droop coefficient and reactive power droop coefficient.

[0026] According to the above technical means, the droop control output frequency equation and voltage amplitude equation can be used to simulate the characteristics of the synchronous generator to maintain the stability of the microgrid voltage and frequency and the reasonable allocation of DG capacity in the microgrid.

[0027] Preferably, the frequency of the distributed power source is restored to the rated value within a predetermined time while achieving the goal of distributing active power in proportion to each other, satisfying the expression:

[0028]

[0029] Among them, T ω Indicates the scheduled time; ref Indicates the reference frequency.

[0030] Preferably, the frequency controller u in the designed secondary control layer is ωi The expression is:

[0031] u ωi =-c1sig(φ i (t)) 1+α -c2sig(φ i (t)) 1-α -βsign(φ i (t))

[0032] The active power controller u in the designed secondary control layer pi The expression is:

[0033] u pi =-c1sig(v i (t)) 1+α -c2sig(v i (t)) 1-α -βsign(v i (t))

[0034] in,

[0035] 0<α<1,β>0 is the control gain, and the intermediate

[0036] Variable φ i (t) and θ i (t) are defined as:

[0037]

[0038] Among them, sig() represents the sign function.

[0039] If the communication network graph between DGs in the isolated AC microgrid is an undirected connected graph, and at least one DG can receive the reference frequency signal, when the control gain and the upper bound of the disturbance satisfy: β ≥ D, then the frequency controller u designed by the above means is ωi and active power controller u pi In the pre-set time T P The frequency is restored to the rated value while the active power is proportionally distributed. The minimum upper bound of the convergence time is taken as an explicit parameter (i.e., the inverse of the control gain), which can be predetermined within any physically allowed range according to application requirements, is easy to adjust, and is independent of the initial conditions.

[0040] Preferably, the output voltage of the distributed power source is restored to a rated value within a predetermined time to meet the expression:

[0041]

[0042] Among them, T ω Indicates the scheduled time; V ref Indicates the reference voltage.

[0043] Preferably, the voltage controller expression in the designed secondary control layer is:

[0044] u vi =-ε1sig(η i (t)) 1+μ -ε2sig(η i (t)) 1-μ -γsign(η i (t))

[0045] Among them, 0<μ<1,γ>0 is the control gain, sig() represents the sign function;

[0046] And the intermediate variable η i (t) is defined as:

[0047]

[0048] If the communication network between DGs in the isolated AC microgrid is an undirected connected graph, and at least one DG can receive the reference voltage signal, when the relationship between the control gain and the upper bound of the disturbance satisfies: γ ≥ D, the isolated AC microgrid can be controlled within the predetermined time T under the voltage controller designed according to the above technical means. vThe voltage recovery is realized within a certain range, and the minimum upper bound of the convergence time is taken as an explicit parameter (i.e., the inverse of the control gain), which can be predetermined within any physically allowed range according to application requirements, is easy to adjust, and is independent of the initial conditions.

[0049] Preferably, the distributed secondary control method is applicable to load change scenarios of isolated AC microgrids.

[0050] Preferably, the distributed secondary control method is applicable to a plug-and-play scenario of distributed power sources in an isolated AC microgrid.

[0051] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0052] The present invention proposes a distributed secondary control method for the voltage and frequency of an isolated AC microgrid to converge within a predetermined time. The communication network between distributed power sources in the isolated AC microgrid is described based on graph theory. Based on the output frequency equation and voltage amplitude equation of the droop control in the primary control layer of the distributed power source, the frequency controller and active power controller in the secondary control layer are designed respectively in combination with the communication network diagram to achieve the goal of restoring the frequency of the distributed power source to the rated value within a predetermined time and achieving the goal of allocating active power in proportion. The voltage controller in the secondary control layer is designed to achieve the goal of restoring the output voltage of the distributed power source to the rated value within a predetermined time, so that the minimum upper bound of the voltage and frequency convergence time of the isolated AC microgrid can be pre-set within any physically allowed range to meet different application requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 A flow chart showing a distributed secondary control method for island AC microgrid voltage and frequency convergence within a predetermined time proposed by an embodiment of the present invention;

[0054] Figure 2 A DG control structure diagram based on a voltage source inverter proposed in an embodiment of the present invention is shown;

[0055] Figure 3 A communication topology diagram of an isolated AC microgrid proposed in an embodiment of the present invention is shown;

[0056] Figure 4 A graph showing output frequency changes of four DGs in a load change scenario of an isolated AC microgrid proposed in an embodiment of the present invention;

[0057] Figure 5 A diagram showing a change in the ratio of active power of four DGs in a load change scenario of an isolated AC microgrid proposed in an embodiment of the present invention;

[0058] Figure 6 A diagram showing changes in output voltage amplitudes of four DGs in a load change scenario of an isolated island AC microgrid proposed in an embodiment of the present invention;

[0059] Figure 7 A graph showing the output frequency variation of five DGs in a plug-and-play scenario of an isolated AC microgrid distributed power source proposed in an embodiment of the present invention;

[0060] Figure 8 A graph showing a change in the ratio of active power of five DGs in a plug-and-play scenario of an isolated AC microgrid distributed power source proposed in an embodiment of the present invention;

[0061] Fig. 9 A graph showing the output voltage amplitude variation of five DGs in a plug-and-play scenario of an isolated AC microgrid distributed power source proposed in an embodiment of the present invention;

[0062] Fig.10 A graph showing a change in output frequency of the third DG when the method proposed in the embodiment of the present invention is compared with the existing distributed progressive time control strategy, the existing distributed finite time quadratic control strategy, and the existing distributed fixed time quadratic control strategy;

[0063] Fig.11 A graph showing a change in the output voltage amplitude of the third DG, comparing the method proposed in an embodiment of the present invention with the existing distributed progressive time control strategy, the existing distributed finite time quadratic control strategy, and the existing distributed fixed time quadratic control strategy. DETAILED DESCRIPTION

[0064] The drawings are for illustrative purposes only and should not be construed as limiting the present patent;

[0065] In order to better illustrate the present embodiment, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the actual size;

[0066] It is understandable to those skilled in the art that descriptions of certain well-known contents in the drawings may be omitted.

[0067] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0068] The positional relationships described in the drawings are only for illustrative purposes and should not be construed as limiting the present patent;

[0069] Example 1

[0070] like Figure 1 As shown, this embodiment proposes a distributed secondary control method for island AC microgrid voltage frequency convergence within a predetermined time, including the following steps:

[0071] S1: Describe the communication network between distributed generation in islanded AC microgrid based on graph theory;

[0072] S2: obtaining a basic control structure of a distributed power source, wherein the basic control structure includes a primary control layer and a secondary control layer, wherein the primary control layer includes droop control;

[0073] S3: construct the output frequency equation and voltage amplitude equation of droop control in the primary control layer;

[0074] S4: Based on the communication network and output frequency equation between distributed power sources in the isolated AC microgrid described by graph theory, the frequency controller and active power controller in the secondary control layer are designed respectively to achieve the goal of restoring the frequency of the distributed power source to the rated value within the predetermined time and achieving the goal of proportional distribution of active power;

[0075] S5: Based on the communication network and voltage amplitude equation between distributed power sources in the isolated AC microgrid described by graph theory, the voltage controller in the secondary control layer is designed to achieve the goal of restoring the output voltage of the distributed power source to the rated value within a predetermined time.

[0076] It should be noted that this embodiment adopts the specific execution order of S1 to S5. In actual implementation, the execution order of S4 and S5 is not limited, and they can also be executed simultaneously.

[0077] Among them, the communication network diagram between distributed power sources in the isolated AC microgrid described in S1, in this embodiment, it is assumed that the isolated AC microgrid includes N distributed power sources DG, and the communication diagram of N distributed power sources DG is expressed as: G = (V, E, A), where V = {υ1,υ2,…,υ N} is the set of distributed generation DG nodes in the isolated AC microgrid, E = {e ij =(υ i ,υ j ):υ i ,υ j ∈V} represents the communication connection set between distributed generation DGs, A=[a ij ]∈R N×N is a non-negative weighted adjacency matrix;

[0078] If DG i Can receive DG j information, then it is called DG j It's DG i The neighbor node of j ,υ i )∈E and a ij >0, otherwise a ij =0; the neighbor node set of node i is represented by N i ={ j ∈V:(υ i ,υ j)∈E}, if for any i,j∈V, there is a ij =a ji , then the graph G is an undirected graph; if there is a path between any two nodes in the undirected graph G, then the graph G is a connected graph; the degree matrix D of the undirected graph G is D = diag (d i ) represents the diagonal matrix formed by the degree of node i. The Laplacian matrix of graph G is expressed as: L = D-Aa;

[0079] In the leader-follower case, define the diagonal matrix B = diag{b 10 ,b 20 ,…,b N0}, where b i0 Indicates DG i The connection weight between DG i can receive the leader's information, then b i0 >0, otherwise, b i0 = 0. If at least one DG can receive the reference value of the virtual leader, a graph M is formed, M = L + B, and M is a connected graph.

[0080] In this embodiment, an AC microgrid system consisting of N DGs is considered. When it operates in island mode, the DGs are usually connected to the microgrid via a voltage source inverter. Any distributed power source DG in an island AC microgrid consists of a DC voltage source, a voltage source inverter, an LC filter, and a basic transmission line. For the i-th DG, see Figure 2 The output end of the DC voltage source is connected to the input end of the voltage source inverter, the output end of the voltage source inverter is connected to the input end of the LC filter, and the output end of the LC filter is connected to the basic transmission line.

[0081] like Figure 2 As shown, in this embodiment, the primary control layer includes power calculation, low-pass filter, droop control, voltage loop control, and current loop control. The primary control layer collects the output current i of the voltage source inverter. Labci , the output current i of the LC filter Oabci And the output voltage v of the LC filter Oabci , where the output current i of the LC filter is Oabci And the output voltage v of the LC filter Oabci After dq transformation, it enters power calculation to achieve decoupling of active power and reactive power, and obtains DG i The instantaneous active power output p i (t) Instantaneous reactive power q i (t), the expression is:

[0082]

[0083] Among them, v odi (t),v oqi (t) and i odi (t),i oqi (t) are the components of voltage and current on the d-axis and q-axis in the dq coordinate system, respectively.

[0084] Then the instantaneous active power p is filtered out by a low-pass filter i (t) Instantaneous reactive power q i The high-frequency harmonics contained in (t) are used to obtain the average active power P i (t) and average reactive power Q i (t), the expression is:

[0085]

[0086] Among them, ω ci is the filter cutoff frequency.

[0087] Average active power P i (t) and average reactive power Q i (t) As the input of droop control, the droop control output frequency equation and voltage amplitude equation can be used to simulate the characteristics of synchronous generators to maintain the stability of microgrid voltage and frequency and the reasonable allocation of DG capacity in the microgrid. The output frequency equation and voltage amplitude equation are generated in the droop control; the output frequency equation and voltage amplitude equation are respectively:

[0088] ω i (t) = ω ni (t)-m i P i (t)

[0089] V i (t) = V ni (t)-n i Q i (t)

[0090] Among them, ω i (t) and V i (t) are the output frequency and voltage amplitude respectively; ω ni (t) and V ni (t) are the reference frequency and voltage amplitude of the primary control layer respectively; m i and n i DG i The active power droop coefficient and reactive power droop coefficient.

[0091] Since the droop control causes the DG output voltage and frequency to deviate, a secondary control layer is introduced to generate a frequency reference value ω ni and voltage reference Vni Correct the deviation, and the new reference value generated by the secondary control layer is applied to the droop control to generate the reference value for the voltage loop control and In voltage loop control, generate reference value for current loop control and The output current i of the voltage source inverter Labci After dq transformation and The current errors are calculated respectively, and finally the output of the voltage source inverter is adjusted by sinusoidal pulse width modulation (SPWM).

[0092] In an island AC microgrid, due to line impedance mismatch, there is a contradiction between achieving inverter output voltage recovery and reactive power proportional distribution at the same time. Therefore, this embodiment designs the following goals:

[0093] (1) The frequency of the distributed generation is restored to the rated value within the predetermined time while achieving the goal of active power distribution in proportion. The expression is:

[0094]

[0095] Among them, T ω Indicates the scheduled time; ref Indicates the reference frequency.

[0096] (2) The output voltage of the distributed power source is restored to the rated value within a predetermined time. The expression is:

[0097]

[0098] Among them, T ω Indicates the scheduled time; V ref Indicates the reference voltage.

[0099] In order to realize frequency recovery and active power proportional distribution, the first-order derivative of the output frequency equation is obtained to obtain the frequency ω i Dynamic characteristics of (t):

[0100]

[0101] Among them, ωi (t) is the introduced frequency controller, and the active power controller is defined as:

[0102]

[0103] Therefore, the reference frequency ω of primary control ni (t) is calculated as follows:

[0104] ω ni (t) = v(u ωi (t)+upi (t))dt

[0105] ω ni (t) = v(u ωi (t)+u pi (t))dt

[0106] However, in practical applications, microgrid controllers are usually affected by external disturbances. Therefore, the reference frequency ω of the primary control ni (t) is calculated as follows:

[0107] ω ni (t) = v(u ωi (t)+u pi (t)+d i (t))dt

[0108] Where, d i (t) represents external disturbance. In this embodiment, it is assumed that d i (t) is uniformly bounded, that is: d i (t)≤D.

[0109] If the communication network graph between DGs in the isolated AC microgrid is an undirected connected graph, and at least one DG can receive the reference frequency signal, when the control gain and the upper bound of the disturbance satisfy: β ≥ D, then the frequency controller u designed by the above means is ωi and active power controller u pi In the pre-set time T P The frequency is restored to the rated value while the active power is proportionally distributed. The minimum upper bound of the convergence time is taken as an explicit parameter (i.e., the inverse of the control gain), which can be predetermined within any physically allowed range according to application requirements, is easy to adjust, and is independent of the initial conditions.

[0110] In order to achieve the convergence of the isolated AC microgrid within the predetermined time, the frequency and active power controllers u are designed based on the electrical information of the DG itself and the electrical information exchange of its adjacent DGs. ωi and u pi .

[0111] The frequency controller u in the designed secondary control layer ωi The expression is:

[0112] u ωi =-c1sig(φ i (t)) 1+α -c2sig(φ i (t)) 1-α -βsign(φ i (t))

[0113] The active power controller u in the designed secondary control layer pi The expression is:

[0114] u pi =-c1sig(θ i (t)) 1+α -c2sig(θ i (t)) 1-α -βsign(θ i (t))

[0115] in,

[0116] 0<α<1,β>0 is the control gain, and the intermediate variable φ i (t) and θ i (t) are defined as:

[0117]

[0118] Among them, sig() represents the sign function.

[0119] The following example demonstrates the effectiveness of the above design. Before the proof, the following lemma is given.

[0120] Lemma 1: Consider the following system:

[0121]

[0122] In the formula, x(t)=[x1(t),x2(t),…,x N (t)] T ,f(·):R n →R n is a nonlinear vector function, and the origin is the equilibrium point of the system. If there exists a positive definite function And there is So that the following is true:

[0123]

[0124] In the formula, if the constant T>0, 0<α<1, the origin of the system is stable within the preset time T.

[0125] Lemma 2: If x1,x2,…,x N ≥0, 0<ρ≤1, q>1, then

[0126]

[0127] Lemma 3: For an undirected connected graph G, its Laplacian matrix L is semi-positive definite and symmetric, 0 is an eigenvalue of the matrix L, and 1N is its corresponding eigenvector. The second smallest eigenvalue of L λ2(L)>0 if Then x T Lx ≥ λ2(L)x T x. In addition, if there is b i0 >0, then L+B is positive definite, its minimum eigenvalue λ1(L+B)>0, and x T (L+B)x≥λ1(L+B)x T x is established.

[0128] Lemma 4: If |y| represents the absolute value of a real number y, then

[0129]

[0130] Define sig(y) α =sign(y)|y| a , and sign(y) is the sign function.

[0131] Define the frequency deviation variable as: e ωi (t) = ω i (t)-ω ref , i∈{1,…,N}, then the intermediate variable φ i (t) can be written as:

[0132] Considering the influence of external disturbance, ωi (t) The derivative is: In order to achieve the predetermined goals of frequency recovery and active power proportional allocation, the following Lyapunov function is selected:

[0133]

[0134] Where E ω (t) = [e ω1 (t),e ω2 (t),…,e ωN (t)] T ,P(t)=[m1P1(t),m2P2(t),…,m N P N (t)] T From Lemma 3, we know that both L and M are semi-positive matrices, and it is easy to verify that V1(t)≥0 is a reasonable Lyapunov function. ω (t) The derivative is:

[0135]

[0136] According to Lemma 2, we can get:

[0137]

[0138] According to Lemma 3:

[0139]

[0140] therefore:

[0141] According to Lemma 1, V1(t) at time T ω tends to 0, and when V1(t)=0, E ω = 0 and P(t)∈span(1 n ).

[0142] In order to realize voltage amplitude recovery, the voltage output equation is differentiated to obtain the voltage amplitude dynamic characteristics:

[0143]

[0144] Among them, u vi (t) is the voltage controller. Considering the influence of external disturbance, the reference voltage V of the primary control of the system is ni (t) is calculated as follows:

[0145]

[0146] In order to achieve system convergence within the predetermined time, the voltage controller u is designed based on the interaction of DG's own electrical information and its neighbor DG's electrical information. vi , if the communication network between DGs in the isolated AC microgrid is an undirected connected graph, and at least one DG can receive the reference voltage signal, when the relationship between the control gain and the upper bound of the disturbance satisfies: γ ≥ D, the isolated AC microgrid can be controlled within the predetermined time T under the voltage controller designed according to the above technical means. v The voltage recovery is realized within 10 ...

[0147] u vi =-ε1sig(η i (t)) 1+μ -ε2sig(η i (t)) 1-μ -γsign(η i (t))

[0148] Among them, 0<μ<1,γ>0 is the control gain, sig() represents the sign function;

[0149] And the intermediate variable ηi (t) is defined as:

[0150]

[0151] The following example demonstrates the effectiveness of the above design. In this proof, Lemma 1 to Lemma 3 are also used.

[0152] Define the voltage deviation variable as: e vi (t) = V i (t)-V ref , i∈{1,…,N}, select the following Lyapunov function:

[0153]

[0154] Where E v (t) = [e v1 (t),e v2 (t),…,e vN (t)] T , from Lemma 3, we can see that M is a positive definite matrix, so V2(t) is positive definite.

[0155] Using an analysis similar to the previous proof, we can obtain:

[0156]

[0157] According to Lemma 1, V2(t) at time T v tends to 0, and when V2(t)=0, there is a voltage deviation e vi (t)=0.

[0158] Example 2

[0159] This example verifies the effectiveness of the proposed scheduled time secondary control strategy by building a 311V / 50Hz three-phase AC island microgrid test system in MATLAB / Simulink. Figure 3 The communication topology of the isolated AC microgrid shown in Figure 1 includes 4 DGs, 3 power lines and 4 loads, and it is assumed that DG1 can obtain reference information. The simulation parameters and control parameters of each DG in the microgrid test system are shown in Table 1. The control strategy gain parameters in the simulation are set to α=μ=0.4, β=γ=1, T P =T v = 1. The time-based external disturbance is set to d i (t) = 0.1 + 0.2cos(200t + 2).

[0160] Table 1 shows the relevant parameter settings of the four DGs.

[0161]

[0162] The distributed secondary control method proposed in this embodiment is applicable to load change scenarios of isolated AC microgrids.

[0163] For the scenario of load change, this embodiment is divided into four stages. Stage 1: At t = 2s, only the primary control (droop control) is put into operation, and load #4 is not connected to the microgrid. Stage 2: The proposed distributed secondary control method is put into operation at t = 2s. Stage 3: Load #4 is connected to the microgrid at t = 5s. Stage 4: Load #3 is disconnected from the microgrid at t = 8s.

[0164] Figure 4 A diagram showing output frequency changes of four DGs in a load change scenario of an isolated AC microgrid according to an embodiment of the present invention. Figure 5 A diagram showing a change in the ratio of active power of four DGs in a load change scenario of an isolated AC microgrid proposed in an embodiment of the present invention. Figure 6 A diagram showing changes in output voltage amplitudes of four DGs in a load change scenario of an isolated AC microgrid according to an embodiment of the present invention.

[0165] Depend on Figure 4 and Figure 6 It can be seen that when only droop control is used in stage 1, the DG output frequency and output voltage amplitude both deviate from the rated value. Figures 4 to 6 It shows that after the method proposed in the present invention is put into use in stage 2, the DG output frequency and output voltage amplitude are restored to the rated value within a preset 1 second. No matter how the subsequent load fluctuates, the method proposed in the present invention can achieve frequency and voltage recovery within 1 second. Figure 5 It is shown that the active power proportional distribution can also be achieved within 1 second. Therefore, the distributed secondary control method proposed in the present invention can realize the proportional distribution of active power while the frequency and voltage are restored within a preset time, and is not affected by external disturbances.

[0166] The distributed secondary control method proposed in this embodiment is applicable to the plug-and-play scenario of distributed power sources of isolated AC microgrids.

[0167] Due to the intermittent nature of distributed DG, plug-and-play is a key performance that needs to be met during the operation of the microgrid. The plug-and-play capability of the proposed control strategy will be verified through this example. The simulation phase one is the same as the aforementioned load change scenario. All four loads are connected to the microgrid, and the proposed distributed secondary control method is started at the same time. Phase two: Assume that the additional DG#5 (with the same parameters as DG#4) is suddenly connected to the microgrid at t=3s, and phase three: DG#3 suddenly exits the microgrid at t=6s. Once the DG is authorized to connect or exit the microgrid, the corresponding communication link will be added or deleted. The plug-and-play simulation test results are shown in the figure. Figure 7 to Figure 9 shown.

[0168] Figure 7 A graph showing output frequency changes of five DGs in a plug-and-play scenario of an isolated AC microgrid distributed power source proposed in an embodiment of the present invention. Figure 8 A graph showing the change in ratio of active power of five DGs in a plug-and-play scenario of an isolated AC microgrid distributed power source proposed in an embodiment of the present invention. Fig. 9 A graph showing changes in output voltage amplitudes of five DGs in a plug-and-play scenario of an isolated AC microgrid distributed power source according to an embodiment of the present invention.

[0169] Depend on Figure 7 and Fig. 9 It can be seen that when DG#5 suddenly connects or DG#3 suddenly exits the isolated AC microgrid, the frequency and voltage fluctuate slightly. Under the distributed secondary control method proposed in the present invention, the frequency and voltage return to the reference value within a predetermined time of 1s. Figure 8 It can be seen that whether DG#5 is connected or DG#3 is removed, the active power can be distributed proportionally. Therefore, the distributed secondary control method proposed in the present invention can meet the "plug and play" operation requirements of the island AC microgrid system.

[0170] Example 3

[0171] In order to verify the superiority of the proposed predetermined time distributed control strategy, the same system parameters as those in Example 2 are used. The method proposed in this embodiment is compared with the existing distributed progressive time control strategy, the existing distributed finite time quadratic control strategy, and the existing distributed fixed time quadratic control strategy, and the output frequency and voltage amplitude of the third DG under different control strategies are simulated. The simulated control parameters are consistent with the parameters required in the above-mentioned existing strategies. The output frequency and voltage amplitude change diagrams are respectively shown in Fig.10 and Fig.11 ,from Fig.10 and Fig.11 It can be seen that the secondary distributed control method proposed in the present invention has a faster convergence speed in both frequency and voltage than asymptotic time control, and is comparable to the convergence speeds of fixed time control and time-limited control. Among them, under fixed time control, the time for the system frequency and voltage amplitude to converge to their rated values ​​is 0.97s and 0.96s respectively, however, the estimated minimum lower bounds of the convergence time are 5.10s and 7.66s respectively, which are several times the actual convergence time. It can be seen that the estimated setting time of fixed time control is very conservative. Under scheduled time control, the convergence time is set to 1s, and the system frequency and voltage amplitude return to their rated values ​​within 0.87s and 0.84s respectively, which are very close to the preset values. Therefore, scheduled time control has lower conservatism in convergence time estimation. At the same time Fig.10 It is shown that under the progressive time control strategy, the frequency of the microgrid will deviate from the rated frequency of 50Hz after being stabilized by the disturbance, and the proposed predetermined time control is just stabilized at 50Hz. Therefore, the secondary distributed control method proposed in the present invention has good anti-disturbance ability.

[0172] In summary, in order to realize the coordinated control of multiple distributed power sources in an isolated AC microgrid, based on the multi-agent consistency theory and the predetermined time convergence theory, the advantages of the distributed secondary control method proposed in this embodiment are summarized as follows:

[0173] (1) Under the distributed secondary control method proposed in this invention, each DG in the isolated AC microgrid only needs to exchange electrical information with adjacent DGs through sparse communication, and the secondary control target can be achieved without centralized collection of electrical information. At the same time, the stability of the strategy is proved by using Lyapunov theory.

[0174] (2) The minimum upper bound of the convergence time of the distributed quadratic control method proposed in the present invention can be predetermined as an explicit parameter (i.e., the inverse of the control gain) within any physically permissible range according to application requirements, is easy to adjust, and is independent of the initial conditions of the system.

[0175] (3) Theoretical analysis and simulation tests have verified that the proposed control strategy can achieve rapid convergence of the system based on a predetermined convergence time. Compared with the existing finite-time / fixed-time distributed quadratic control strategy, the convergence estimation time of the distributed quadratic control method proposed in the present invention is less conservative.

[0176] The embodiments are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A distributed secondary control method for island AC microgrid voltage frequency convergence within a predetermined time, characterized in that: The following steps are involved: Describe the communication network between distributed generation in island AC microgrid based on graph theory; Acquire a basic control structure of a distributed power source, wherein the basic control structure includes a primary control layer and a secondary control layer, and the primary control layer includes droop control; Construct the output frequency equation and voltage amplitude equation of droop control in the primary control layer; Based on the communication network and output frequency equation between distributed power sources in the isolated AC microgrid described by graph theory, the frequency controller and active power controller in the secondary control layer are designed respectively to achieve the goal of restoring the frequency of the distributed power sources to the rated value within the predetermined time and achieving the goal of proportional distribution of active power; Based on graph theory, the communication network and voltage amplitude equation between distributed power sources in isolated AC microgrid are described, and the voltage controller in the secondary control layer is designed to achieve the goal of restoring the output voltage of distributed power sources to the rated value within a predetermined time.

2. The distributed control method for island AC microgrid voltage frequency convergence within a predetermined time according to claim 1, characterized in that: Assume that the island AC microgrid includes N distributed power sources DG, and the communication graph of N distributed power sources DG is expressed as: G = (V, E, A), Where V = {υ1,υ2,…,υ N } is the set of distributed generation DG nodes in the isolated AC microgrid, E = {e ij =(υ i ,υ j ):υ i ,υ j ∈V} represents the communication connection set between distributed generation DGs, A=[a ij ]∈R N×N is a non-negative weighted adjacency matrix; If DG i Can receive DG j information, then it is called DG j It's DG i The neighbor node of j ,υ i )∈E and a ij >0, otherwise a ij =0; the neighbor node set of node i is represented by N i ={ j ∈V:(υ i ,υ j )∈E}, if for any i,j∈V, there is a ij =a ji , then the graph G is an undirected graph; if there is a path between any two nodes in the undirected graph G, then the graph G is a connected graph; the degree matrix D of the undirected graph G is D = diag (d i ) represents the diagonal matrix formed by the degree of node i. The Laplacian matrix of graph G is expressed as: L = D-Aa; In the leader-follower case, define the diagonal matrix B = diag{b 10 ,b 20 ,…,b N0 }, where b i0 Indicates DG i The connection weight between DG and the leader. i can receive the leader's information, then b i0 >0, otherwise, b i0 = 0. If at least one DG can receive the reference value of the virtual leader, a graph M is formed, M = L + B, and M is a connected graph.

3. The distributed control method for island AC microgrid voltage frequency convergence within a predetermined time according to claim 2, characterized in that: Any distributed power source DG in the isolated AC microgrid consists of a DC voltage source, a voltage source inverter, an LC filter and a basic transmission line. The output end of the DC voltage source is connected to the input end of the voltage source inverter, the output end of the voltage source inverter is connected to the input end of the LC filter, and the output end of the LC filter is connected to the basic transmission line. The primary control layer includes power calculation, low-pass filter, droop control, voltage loop control, and current loop control. The primary control layer collects the output current i of the voltage source inverter. Labci , the output current i of the LC filter Oabci And the output voltage v of the LC filter Oabci , where the output current i of the LC filter is Oabci And the output voltage v of the LC filter Oabci After dq transformation, it enters power calculation to achieve decoupling of active power and reactive power, and obtains DG i The instantaneous active power output p i (t) Instantaneous reactive power q i (t), and then the instantaneous active power p is filtered out by a low-pass filter i (t) Instantaneous reactive power q i The high-frequency harmonics contained in (t) are used to obtain the average active power P i (t) and average reactive power Q i (t), average active power P i (t) and average reactive power Q i (t) as the input of droop control, generating output frequency equation and voltage amplitude equation in droop control; Droop control causes the DG output voltage and frequency to deviate, and the secondary control layer is introduced to generate the frequency reference value ω ni and voltage reference V ni Correct the deviation, and the new reference value generated by the secondary control layer is applied to the droop control to generate the reference value for the voltage loop control and In voltage loop control, generate reference value for current loop control and The output current i of the voltage source inverter Labci After dq transformation and The current errors are calculated respectively, and finally the output of the voltage source inverter is adjusted by sinusoidal pulse width modulation.

4. The distributed control method for island AC microgrid voltage frequency convergence within a predetermined time according to claim 3, characterized in that: The output frequency equation and voltage amplitude equation are: ω i (t)=ω ni (t)-m i P i (t) V i (t)=V ni (t)-n i Q i (t) Among them, ω i (t) and V i (t) are the output frequency and voltage amplitude respectively; ω ni (t) and V ni (t) are the reference frequency and voltage amplitude of the primary control layer respectively; m i and n i DG i The active power droop coefficient and reactive power droop coefficient.

5. The distributed control method for island AC microgrid voltage frequency convergence within a predetermined time according to claim 4, characterized in that: The frequency of the distributed generation is restored to the rated value within the predetermined time while achieving the goal of distributing active power in proportion to the column, satisfying the expression: Among them, T ω Indicates the scheduled time; ref Indicates the reference frequency.

6. The distributed control method for island AC microgrid voltage frequency convergence within a predetermined time according to claim 5, characterized in that: The frequency controller u in the designed secondary control layer ωi The expression is: you ωi =-c1sig(φ i (t)) 1+α -c2sig(φ i (t)) 1-α -βsign(φ i (t)) The active power controller u in the designed secondary control layer pi The expression is: you pi =-c1sig(θ i (t)) 1+α -c2sig(θ i (t)) 1-α -βsign(θ i (t)) in, 0<α<1,β>0 is the control gain, and the intermediate variable φ i (t) and v i (t) are defined as: Among them, sig() represents the sign function.

7. The distributed control method for island AC microgrid voltage frequency convergence within a predetermined time according to claim 4, characterized in that: The output voltage of the distributed generation is restored to the rated value within a predetermined time to meet the expression: Among them, T ω Indicates the scheduled time; V ref Indicates the reference voltage.

8. The distributed control method for island AC microgrid voltage frequency convergence within a predetermined time according to claim 7, characterized in that: The voltage controller expression in the designed secondary control layer is: you vi =-ε1sig(η i (t)) 1+μ -ε2sig(η i (t)) 1-μ -γsign(η i (t)) Among them, 0<μ<1,γ>0 is the control gain, sig() represents the sign function; And the intermediate variable η i (t) is defined as:

9. The distributed secondary control method for island AC microgrid voltage frequency convergence within a predetermined time according to claim 1, characterized in that: The distributed secondary control method is suitable for load change scenarios of isolated AC microgrids.

10. The distributed secondary control method for island AC microgrid voltage frequency convergence within a predetermined time according to claim 1, characterized in that: The distributed secondary control method is suitable for the plug-and-play scenario of distributed power sources in an isolated AC microgrid.

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