Active power sharing control method for ring-type flexible interconnected microgrid based on back-to-back converters

Abstract

The present invention discloses an active power sharing control method for a ring-shaped flexible interconnected microgrid group based on back-to-back converters, and the steps include: 1. constructing a topological structure of a ring-shaped flexible interconnected microgrid group including N microgrid units and N back-to-back converters connected end to end; 2. The central controller of the microgrid group collects the load data and active power sharing ratio data of each microgrid unit at a fixed sampling time interval ΔT; 3. The central controller compares the active power sharing ratio of each microgrid unit at the τth sampling moment and obtains the microgrid unit corresponding to the minimum value; 4. According to the consistent target of the active power sharing ratio of the microgrid group and the unique solution condition of the power constraint equation group, a unique solution power balance constraint equation group is constructed, and the transmission power instruction of the back-to-back converter is solved to achieve the active power sharing control target. The present invention can realize the sharing of the active power output of all distributed power sources in the microgrid group according to the capacity ratio, thereby ensuring the safe and stable operation of the isolated ring-shaped flexible interconnected microgrid group.

Description

Technical Field

[0001] The present invention relates to the field of hierarchical collaborative recovery control of flexible interconnected microgrids, and in particular to an active power sharing control method for a ring-shaped flexible interconnected microgrid of back-to-back converters. Background Art

[0002] Interconnecting multiple geographically adjacent microgrids (MG) to form a microgrid cluster (MGC) is one of the effective ways to absorb distributed generation (DG), and it is also an effective way to improve the reliability of power supply at the end of remote mountainous areas, island areas and even urban distribution networks. At the same time, as an emerging interconnection solution, the flexible multi-state switch (FMSS) of the back-to-back converter (BTBC) has become a new choice for solving the problem of multi-microgrid interconnection integration due to its advantages such as good disturbance isolation, strong regulation ability and fast fault recovery. However, considering the nonlinear controlled source characteristics of the back-to-back converter equipment, compared with the conventional impedance interconnected microgrid group, the microgrid units of the flexible interconnected microgrid no longer follow the power transmission law of the impedance line, and the grid operation frequency degenerates from the global to the local variable, and different microgrid units can operate asynchronously at different frequencies. Under the influence of asynchronous interconnection characteristics, effective "frequency signal" transmission cannot be formed between microgrid units. The frequency disturbance smoothing task of each microgrid unit can only be undertaken by the power generation equipment in the area alone. The goal of evenly distributing the active power output of distributed power sources at the cluster level according to the capacity ratio is difficult to achieve.

[0003] Under the action of the traditional droop control method, each microgrid unit can automatically achieve the active power sharing control target of each distributed power source, but without the reasonable given back-to-back converter device transmission instructions, the active power sharing ratios between different microgrid units are not the same, that is, the load rate levels of each microgrid unit are different. This may cause some small-capacity microgrid units to experience transient instability under the condition of a sudden increase in load, seriously threatening the stability of the flexible interconnected microgrid group. Therefore, how to effectively calculate the transmission power instructions of each back-to-back converter and ensure that the output active power of each distributed power source in the flexible interconnected microgrid group can still be accurately distributed according to the droop coefficient (maximum capacity) under load changes is a problem that technicians in this field urgently need to solve. Summary of the invention

[0004] In view of the problems existing in the active power sharing control of the existing back-to-back converter flexible interconnected microgrid group, the present invention proposes an active power sharing control method for a ring-shaped flexible interconnected microgrid group based on back-to-back converters, in order to achieve disturbance sharing and output balance among the microgrid units, and finally achieve the goal of consistent active power sharing ratio of all distributed power sources in the microgrid group according to capacity, thereby ensuring the safe and stable operation of the isolated flexible interconnected microgrid group.

[0005] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme:

[0006] The active power sharing control method of a ring-shaped flexible interconnected microgrid group based on back-to-back converters of the present invention is also characterized in that it includes the following steps:

[0007] S1: N microgrid units in the ring-shaped flexible interconnected microgrid group are respectively connected to N back-to-back converters and then connected end to end to form a ring structure; the microgrid units are used as points and the back-to-back converters are used as edges to construct a microgrid group interconnection topology connectivity graph;

[0008] S2: The central controller of the ring-shaped flexible interconnected microgrid group collects the load data P of the load-side transformer in the k-th microgrid unit at the τ-th sampling time at a fixed sampling time interval ΔT. Load,k (τ) and the active power sharing ratio of the leader distributed generation α k,pin (τ), 1 <k<N;

[0009] S3: The central controller compares the active power sharing ratio values ​​of each microgrid unit at the τth sampling time, and obtains the microgrid unit corresponding to the minimum active power sharing ratio, which is numbered as min; 1≤min≤N;

[0010] S4: Active power sharing control of the ring-shaped flexible interconnected microgrid group:

[0011] S41, using formula (1) to calculate the transmission power instruction of the back-to-back converter corresponding to the min-th microgrid unit at the τ-th sampling time

[0012]

[0013] In formula (1), represents the transmission capacity limit of the min-th microgrid unit, represents the preset transmittable capacity of the min-th microgrid unit at the τ-th sampling time, r represents the adjustable proportional coefficient of the transmission instruction, It represents the local optimal transfer capacity of the back-to-back converter corresponding to the min-th microgrid unit at the τ-th sampling time, and:

[0014]

[0015] In formula (2), is the equivalent droop coefficient of the kth microgrid unit; is the equivalent droop coefficient of the lth microgrid unit; P LD,k (τ) is the active power of the kth microgrid unit load and line loss at the τth sampling moment; P LD,l (τ) is the active power of the lth microgrid unit load and line loss at the τth sampling time; N min is the set of interconnected microgrid units at both ends of the min-th back-to-back converter;

[0016] S42, taking the uniform active power sharing ratio of distributed power sources in all microgrid units as the control target, constructing a power balance constraint equation group, and based on the transmission power instruction, using formula (3) to obtain the unique solution power balance constraint equation group of the ring flexible interconnected microgrid group;

[0017]

[0018] In formula (3), is the inverse vector of the equivalent droop coefficient of each microgrid unit; M MGC ={m k,q} N×N is the association matrix between edges and points in the microgrid interconnection topology connectivity graph; m k,q It represents the correlation coefficient between the kth microgrid unit and the qth back-to-back converter, and the positive direction is the active power transmission from the constant DC voltage control side converter to the constant power control side converter in the back-to-back converter, then the direction from the back-to-back converter to the microgrid unit is 1, the direction from the microgrid unit to the back-to-back converter is -1, and the irrelevant direction is 0; a transfer power command vector calculated for each back-to-back converter; represents the transmission power instruction of the Nth microgrid unit at the τth sampling time, is the power vector composed of load and line loss, and: P LD,k (τ) = P load,k (τ)+P loss,k (τ); P load,k (t) is the total load of the kth microgrid unit; P loss,k (t) is the transmission loss of the kth microgrid unit; e min is an N-dimensional unit column vector, and the minth element is 1 and the other elements are 0;

[0019] S43. Solve equation (3) to obtain the transmission power instructions of the remaining back-to-back converters at the τth sampling moment, thereby realizing active power sharing control of all distributed power sources in the ring flexible interconnected microgrid group.

[0020] The active power sharing control method of the ring-shaped flexible interconnected microgrid group based on back-to-back converters of the present invention is also characterized in that the power balance constraint equation group in step S42 is obtained according to the following steps:

[0021] Sa: Using equation (4), we can get the basic power balance constraint equation inside the kth microgrid unit at any time t:

[0022]

[0023] In formula (4), P c,q (t) is the active power transmitted by the qth back-to-back converter at time t; P k,i (t) is the output active power of the i-th distributed generation in the k-th microgrid unit at time t;

[0024] Sb: According to the output of each distributed power source in the kth microgrid unit under droop control, the active power sharing relationship shown in formula (5) is satisfied, so the overall active power output P of the kth microgrid unit at time t is obtained using formula (6): MG,k (t):

[0025]

[0026]

[0027] In formula (5), Pin is the number of the leader distributed generation in the kth microgrid unit, α k,Pin (t) is the active power sharing ratio of the leader distributed generation within the kth microgrid unit; and is the active droop coefficient of the leader distributed generation within the kth microgrid unit; n k represents the number of distributed generation in the kth microgrid unit;

[0028] In formula (6), represents the active droop coefficient of the i-th distributed generation in the k-th microgrid unit;

[0029] Sc: Using equation (7), we can get the active power balance equation of the kth microgrid unit at time t:

[0030]

[0031] Sd: The control target of the ring flexible interconnected microgrid at the τth sampling time is obtained using formula (8):

[0032] α 1,Pin (τ) = α 2,Pin (τ)=...=α N,Pin(τ)=α0(τ) (8)

[0033] In formula (8), α0(τ) is the convergence value of the active power sharing ratio; α N,Pin (τ) is the active droop coefficient of the leader distributed generation of the Nth microgrid unit at the τth sampling moment;

[0034] Se: Using equation (9), we can get the power balance constraint equation of the kth microgrid unit at the τth sampling time:

[0035]

[0036] In formula (9), is the active power transmission instruction value to be calculated for the qth back-to-back converter at the τth sampling time; P load,k (τ) is the total load of the kth microgrid unit at the τth sampling time; P loss,k (τ) is the transmission loss of the kth microgrid unit at the τth sampling time;

[0037] Sf: Use equation (10) to construct a linear equation system considering the power balance constraints of N microgrid units:

[0038]

[0039] Sg: Matrix form of power balance constraint constructed using equation (11):

[0040]

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] 1) In view of the problems existing in the active power sharing control of the existing back-to-back converter flexible interconnected microgrid group, the present invention designs a new centralized-distributed hybrid hierarchical collaborative control communication network architecture, derives and proposes an active power sharing control method for the flexible interconnected microgrid group based on the power balance equation group, and overcomes the problems that other methods cannot achieve differential active power sharing control at the cluster level or rely on energy storage power sources to achieve control. Under this control method, the active power sharing ratio of all distributed power sources in the ring-shaped flexible interconnected microgrid group is consistent, which effectively improves the disturbance sharing capability and safe and stable operation level of the microgrid group.

[0043] 2) Compared with other methods, the improved active power sharing control method proposed in the present invention does not need to add energy storage power supply on the DC side of the back-to-back converter, nor does it change the underlying control method of the distributed power supply and back-to-back converter in the microgrid group. At the same time, the method proposed in this application only needs to solve N-order non-homogeneous linear equations, with a small amount of collected data, a small amount of solution work, and simple operation, which effectively ensures the real-time performance of the control algorithm and is easy to promote in engineering practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a flow chart of the active power sharing control method for a ring-shaped flexible interconnected microgrid group of back-to-back converters according to the present invention;

[0045] Figure 2 This is a schematic diagram of the structure of a microgrid group system based on a back-to-back converter ring flexible interconnection according to the present invention;

[0046] Figure 3 This is a case diagram of a four-back-to-back converter ring-shaped flexible interconnected microgrid system of the present invention;

[0047] Figure 4 A hierarchical collaborative control architecture diagram of the flexible interconnected microgrid system of the present invention;

[0048] Figure 5 It is a parameter diagram of system components and controllers of the flexible interconnected microgrid group of the present invention;

[0049] Figure 6 It is a dynamic response diagram of the active power sharing ratio of the ring-shaped flexible interconnected microgrid group under light load access of the present invention;

[0050] Figure 7 This is a dynamic response diagram of the active power sharing ratio of the ring-shaped flexible interconnected microgrid group under heavy load access of the present invention. DETAILED DESCRIPTION

[0051] In this embodiment, a method for active power sharing control of a ring-shaped flexible interconnected microgrid group based on back-to-back converters is provided. Figure 1 As shown, the following steps are included:

[0052] S1: Connect the N microgrid units in the ring-shaped flexible interconnected microgrid group to the N back-to-back converters respectively, and then connect them end to end to form a ring structure; take the microgrid units as points and the back-to-back converters as edges to construct a microgrid group interconnection topology connectivity graph. The detailed topology and connectivity relationship of the ring-shaped flexible interconnected microgrid group implemented by this method can be shown as follows: Figure 2 As shown in Figure 1, it consists of N AC microgrid units and N back-to-back converters between each unit, where N ≥ 3. The kth AC microgrid unit contains n k A distributed power source using droop control and an equivalent constant impedance equivalent load connected to the common connection bus, the distributed power source and the load are connected through a conventional AC line. In the AC / DC / AC converter control structure of the conventional back-to-back converter type flexible multi-state switch, one end of the converter adopts a constant DC voltage control (V dc Q control), maintaining the DC side voltage V dc The other end uses constant power control (PQ control) to control the transmission power of the back-to-back converter.

[0053] In the above network topology and control mode settings, for the convenience of explaining the problem, Figure 3 The microgrid group with four microgrid units and four back-to-back converter devices connected in a ring-shaped manner is taken as an example to illustrate the sequence numbering problem of the microgrid group. In this embodiment, the positive direction of the transmission power of the back-to-back converter device is defined as V dc The Q control side converter transmits to the PQ control side converter, and the corresponding sequence numbering method can be written as:<MG1,MG2> →BTBC1,<MG2,MG3> →BTBC2,<MG3,MG4> →BTBC3,<MG4,MG1> →BTBC4, as follows Figure 3 shown.

[0054] S2: In this embodiment, the ring-shaped flexible interconnected microgrid group adopts a hierarchical control architecture. Figure 4 As shown in FIG. 1 , a hierarchical collaborative control architecture diagram of a common four-microgrid unit and four-back-to-back converter flexible interconnected microgrid group is shown, which is divided into two layers, namely, 1) microgrid unit layer; 2) cluster layer.

[0055] Each distributed power source in the above microgrid unit layer adopts conventional droop control, as shown in formula (1):

[0056]

[0057] In formula (1): V k,i (t) and ω k,i (t) are the voltage amplitude and frequency of the distributed power supply output respectively; because the d-axis voltage orientation is adopted, we have Existence. k,i and Q k,i is the average active and reactive power after passing through the low-pass filter; and are active and reactive droop coefficients, respectively, and their magnitudes are usually related to the maximum active and reactive capacity of each distributed generation, i.e. and Inversely proportional relationship.

[0058] The cluster layer adopts a centralized control method to calculate the transmission power command of each back-to-back converter, and uses the microgrid central controller to collect the load data P of the load-side transformer in the kth microgrid unit at the τth sampling time at a fixed sampling time interval ΔT. Load,k (τ) and the active power sharing ratio of the leader distributed generation α k,pin (τ), 1 <k<N。

[0059] S3: The central controller compares the active power sharing ratio values ​​of each microgrid unit at the τth sampling moment, and obtains the microgrid unit corresponding to the minimum active power sharing ratio, which is numbered as min; 1≤min≤N.

[0060] S4: The active power sharing control of the ring flexible interconnected microgrid group is divided into two control objectives:

[0061] 1) Control objective 1: To realize active power equalization control of each distributed power source in each microgrid unit, that is, each distributed power source in each microgrid unit outputs power in a balanced manner according to its capacity (droop coefficient).

[0062]

[0063] In formula (2): It represents the active output ratio of the i-th distributed generation according to the droop coefficient.

[0064] 2) Control objective 2: Active power equalization control among microgrid units at the cluster level, that is, coordinating the power transmission of each back-to-back converter to achieve balanced output among different microgrid units according to their capacity.

[0065]

[0066] In formula (3): represents the overall active output ratio of microgrid unit k according to its equivalent droop coefficient, is the equivalent droop coefficient of microgrid unit k, satisfying the relationship in, is the active droop coefficient of each distributed generation in microgrid unit k. Combining objective 2, another expression of objective 2 can be derived as shown in formula (4):

[0067]

[0068] That is, all distributed power sources in the flexible interconnected microgrid system achieve the active power sharing control target.

[0069] According to the traditional droop control method, when the load changes dynamically, the distributed power sources in each microgrid unit can dynamically adjust the output active power through frequency changes, realize frequency recovery and automatically achieve control target 1. However, since the back-to-back converter relies on the given transmission power command, control target 2 cannot be automatically achieved without reasonable calculation. Therefore, it is necessary to solve the transmission power command of the back-to-back converter that meets the condition of consistent active power sharing ratio. The transmission power command of each back-to-back converter is analyzed in detail below.

[0070] According to the energy conservation relationship within the microgrid unit, the basic power balance constraint equation within the kth microgrid unit at any time t can be obtained as shown in formula (5):

[0071]

[0072] In formula (5), P c,q(t) is the active power transmitted by the qth back-to-back converter at time t; P k,i (t) is the output active power of the i-th distributed generation in the k-th microgrid unit at time t;

[0073] Sb: According to the output of each distributed power source in the kth microgrid unit under the droop control of formula (1), the active power sharing relationship shown in formula (6) is satisfied, so the overall active power output P of the kth microgrid unit at time t is obtained by formula (7): MG,k (t):

[0074]

[0075]

[0076] In formula (6), Pin is the number of the leader distributed generation in the kth microgrid unit, α k,Pin (t) is the active power sharing ratio of the leader distributed generation within the kth microgrid unit; and is the active droop coefficient of the leader distributed generation within the kth microgrid unit; n k represents the number of distributed generation in the kth microgrid unit;

[0077] In formula (7), It represents the active power droop coefficient of the i-th distributed generation in the k-th microgrid unit.

[0078] Substituting the relationship in equation (7) into equation (5), we can obtain the active power balance equation of the kth microgrid unit at time t, as shown in equation (8):

[0079]

[0080] In order to achieve the active power sharing control target at the cluster layer, the control target of the ring flexible interconnected microgrid at the τth sampling time is obtained by using formula (9):

[0081] α 1,Pin (τ) = α 2,Pin (τ)=...=α N,Pin (τ)=α0(τ) (9)

[0082] In formula (9), α0(τ) is the convergence value of the active power sharing ratio, which is unknown and needs to be determined; N,Pin (τ) is the active droop coefficient of the leader distributed generation of the Nth microgrid unit at the τth sampling moment;

[0083] Substituting the control target of formula (9) into formula (8), we can obtain the power balance constraint equation of the kth microgrid unit at the τth sampling time:

[0084]

[0085] In formula (10), is the active power transmission instruction value to be calculated for the qth back-to-back converter at the τth sampling time; P load,k (τ) is the total load of the kth microgrid unit at the τth sampling time; P loss,k (τ) is the transmission loss of the kth microgrid unit at the τth sampling time.

[0086] Obviously, the N microgrid units in the microgrid group all have the power balance relationship shown in formula (10), so it is written as a linear equation system of power balance constraints:

[0087]

[0088] The matrix form of power balance constraint is constructed using formula (12):

[0089]

[0090] In formula (12), is the inverse vector of the equivalent droop coefficient of each microgrid unit; M MGC ={m k,q} N×N is the association matrix between edges and points in the microgrid interconnection topology connectivity graph; m k,q It represents the correlation coefficient between the kth microgrid unit and the qth back-to-back converter, and the positive direction is the active power transmission from the constant DC voltage control side converter to the constant power control side converter in the back-to-back converter, then the direction from the back-to-back converter to the microgrid unit is 1, the direction from the microgrid unit to the back-to-back converter is -1, and the irrelevant direction is 0; is the power vector composed of load and line loss, and: P LD,k (τ) = P load,k (τ)+P loss,k (τ); P load,k (t) is the total load of the kth microgrid unit; P loss,k (t) is the transmission loss of the kth microgrid unit.

[0091] Observing the above equation, there are N equation relationships and N+1 unknown quantities. According to the characteristics of solving non-homogeneous linear equations, the above control equations are underdetermined and there are infinite sets of non-zero solutions. A conventional idea is to use the optimization algorithm to design constraints and optimization objectives to find the minimum norm solution, but the convergence time of the optimization algorithm is uncertain and the real-time performance is difficult to guarantee. Another idea is to pre-calculate a back-to-back converter transmission instruction and convert it into a problem of determining a feasible solution. The technical solution of this method adopts the latter idea.

[0092] First, use equation (13) to calculate the transmission power command of the back-to-back converter corresponding to the min-th microgrid unit at the τ-th sampling time:

[0093]

[0094] In formula (13), represents the transmission capacity limit of the min-th microgrid unit, represents the preset transmittable capacity of the min-th microgrid unit at the τ-th sampling time, r represents the adjustable proportional coefficient of the transmission instruction, It represents the local optimal transfer capacity of the back-to-back converter corresponding to the min-th microgrid unit at the τ-th sampling time, and:

[0095]

[0096] In formula (14), is the equivalent droop coefficient of the kth microgrid unit; is the equivalent droop coefficient of the lth microgrid unit; P LD,k (τ) is the active power of the kth microgrid unit load and line loss at the τth sampling moment; P LD,l (τ) is the active power of the lth microgrid unit load and line loss at the τth sampling time; N min is the set of interconnected microgrid units at both ends of the min-th back-to-back converter.

[0097] Formula (14) assumes that there are only microgrid units k and l and back-to-back converter min, and it is the local optimal active power that should be transmitted to balance the active power sharing ratio of microgrid units k and l. Formula (13) is based on formula (14), adding the adjustable proportional coefficient r and the upper limit of transmission capacity.

[0098] Substituting the pre-transmission power instruction of formula (13) into the power balance constraint equation group of formula (12), a new power balance constraint equation group shown in formula (15) is obtained. Formula (15) satisfies the unique solution condition of the equation group;

[0099]

[0100] In formula (15), a transfer power command vector calculated for each back-to-back converter; represents the transmission power instruction of the Nth microgrid unit at the τth sampling time, e min is an N-dimensional unit column vector whose min element is 1 and the other elements are 0.

[0101] Solving equation (15), we can obtain the transmission power instructions of the remaining back-to-back converters at the τth sampling time, thereby realizing the active power sharing control of all distributed power sources in the ring flexible interconnected microgrid. The back-to-back converter transmission instructions obtained by solving the above steps can realize the active power sharing control target of each distributed power source according to the capacity ratio at the global level of the ring flexible interconnected microgrid.

[0102] In order to verify the effectiveness of the hierarchical control method proposed in this technical solution, a system as described above is built in Matlab / Simulink. Figure 3 The simulation model shown in Figure 1 has a corresponding communication link as shown in Figure 2. Figure 4 Using the 0-1 weight method, the adjacency matrix of each microgrid unit is expressed as shown in formula (16):

[0103]

[0104] The corresponding leader node connection matrix is ​​shown in formula (17):

[0105]

[0106] In the microgrid group system studied by the technical scheme of this method, each microgrid unit adopts a multi-machine parallel connection mode with a single busbar structure. The distributed power supply in each unit is modeled as an ideal inverter-type distributed power supply without considering the fluctuation on the DC side, and the load is set as a constant impedance load. The double-ended converters of each back-to-back converter device adopt constant DC voltage control and constant power control respectively, and the positive direction of power is always defined as the flow from the constant DC voltage control side to the constant power control side. According to the association relationship between the back-to-back converter (edge) and the microgrid unit (point), the association matrix of the microgrid group system is obtained as shown in formula (18):

[0107]

[0108] In the example, the components and control parameters of the flexible interconnected microgrid system at the initial time are as follows: Figure 5 shown.

[0109] The active power sharing control method for the flexible interconnected microgrid group system proposed in the technical solution of this method is essentially to collect the load and active power sharing ratio status of each microgrid unit on the basis of the independent droop control architecture of each microgrid unit, and calculate the transmission power instructions of each back-to-back converter device in real time to realize active power sharing control at the cluster level.

[0110] To verify the effectiveness of the control scheme, frequency and voltage droop control are used as primary control in the microgrid unit control, and controllers for frequency, voltage recovery and active power sharing are designed in the secondary control; at the cluster level, a centralized control method is used to adjust the transmission instructions of back-to-back converter devices in real time to achieve active power sharing control of the entire network. To this end, two working conditions are set to verify the effectiveness of the technical scheme proposed by this method.

[0111] Condition 1: System dynamic response under light load access (unsaturated);

[0112] Therefore, the simulation example of working condition 1 is designed as follows: from 0 to 1s, the system only uses droop control; at 1s, the secondary recovery control of the microgrid unit layer is put into operation; at 3s, the cluster layer active power sharing control is put into operation; at 5s, the light load LOAD is connected to the microgrid unit 1. 1,2 , capacity is 6kW+j6kVar; 8s, light load LOAD 1,2 Exit operation; 10s later, LOAD in microgrid unit 3 3,1 The load is reduced by 7kW+j7kVar; the simulation ends after 15s. Among them, the key parameters are r=0 and ΔT=0.01.

[0113] observe Figure 6 It can be seen that in the 0-1s period, the active power sharing ratio of each microgrid unit is different in the microgrid group under the action of droop control. In the 1-3s period, each distributed power source in each microgrid unit applies secondary recovery control. Figure 6 It shows that the active power sharing control at the cluster level has not been achieved. In the 3-5s period, the active power sharing control at the cluster level is further applied, and it can be observed that the active power sharing ratios of each microgrid unit have tended to be consistent, and the active power sharing control at the cluster level has been achieved. In the 5-10s period, the continuous switching of loads in microgrid units 1 and 3 has no significant impact on the overall frequency / voltage recovery and active power sharing control effect of the system, which shows the effectiveness of the control method proposed in the technical solution of this method.

[0114] Condition 2: Dynamic response of the system under heavy load access (saturated);

[0115] Similar to the scenario 1, the settings are exactly the same from 0 to 5 seconds. At 5 seconds, the microgrid unit 1 is connected to a heavy load LOAD. 1,3 , the capacity is 30kW+j30kVar; because the back-to-back converter 1 enters the transmission command and enters saturation, the system automatically clears the transmission command control at 9s and lasts for 2s; at 8s, the heavy load LOAD 1,3 Exit operation, at the same time; 10s, LOAD in microgrid unit 3 3,1 The load is reduced by 7kW+j7kVar; the simulation ends after 15s. Among them, the key parameters are r=0 and ΔT=0.01.

[0116] Different from the light load access situation, it can be observed that after the heavy load is accessed at 5s, Figure 7 The back-to-back converter 1 device first reaches reverse saturation -10kW. At this time, the active power sharing ratio is used to judge that the active power sharing control at the cluster layer can no longer be achieved. After 1s in the saturation stage, the central controller of the microgrid group clears the transmission instructions of each back-to-back converter and restarts at 8s. At this time, the load 1,3 After exiting operation, the active power sharing control of the cluster layer can be realized in the 8-15s period, and the frequency and voltage of the system remain stable in the entire dynamic stage. It can be seen that in actual engineering applications, the capacity of the back-to-back converter should be set reasonably to ensure that its adjustable range can meet the active power sharing control requirements of the cluster layer.

Claims

1. A method for active power sharing control of a ring-shaped flexible interconnected microgrid group based on back-to-back converters, characterized in that: The following steps are involved: S1: N microgrid units in the ring-shaped flexible interconnected microgrid group are respectively connected to N back-to-back converters and then connected end to end to form a ring structure; the microgrid units are used as points and the back-to-back converters are used as edges to construct a microgrid group interconnection topology connectivity graph; S2: The central controller of the ring-shaped flexible interconnected microgrid group collects the load data P of the load-side transformer in the k-th microgrid unit at the τ-th sampling time at a fixed sampling time interval ΔT. Load,k (τ) and the active power sharing ratio of the leader distributed generation α k,pin (τ), 1 <k<N; S3: the central controller compares the active power sharing ratio values ​​of each microgrid unit at the τth sampling time, and obtains the microgrid unit corresponding to the minimum active power sharing ratio, which is numbered as min; 1 ≤ min ≤ N; S4: Active power sharing control of the ring-shaped flexible interconnected microgrid group: S41, using formula (1) to calculate the transmission power instruction of the back-to-back converter corresponding to the min-th microgrid unit at the τ-th sampling time : (1) In formula (1), represents the transmission capacity limit of the min-th microgrid unit, represents the preset transmittable capacity of the min-th microgrid unit at the τ-th sampling time, Indicates the adjustable proportional coefficient of the transmission instruction. It represents the local optimal transfer capacity of the back-to-back converter corresponding to the min-th microgrid unit at the τ-th sampling time, and: (2) In formula (2), is the equivalent droop coefficient of the kth microgrid unit; is the equivalent droop coefficient of the lth microgrid unit; is the active power of the kth microgrid unit load and line loss at the τth sampling moment; is the active power of the lth microgrid unit load and line loss at the τth sampling moment; is the set of interconnected microgrid units at both ends of the min-th back-to-back converter; S42, taking the uniform active power sharing ratio of distributed power sources in all microgrid units as the control target, constructing a power balance constraint equation group, and based on the transmission power instruction, using formula (3) to obtain the unique solution power balance constraint equation group of the ring flexible interconnected microgrid group; (3) In formula (3), is the inverse vector of the equivalent droop coefficient of each microgrid unit; It is the association matrix between edges and points in the microgrid interconnection topology connectivity graph; It represents the correlation coefficient between the kth microgrid unit and the qth back-to-back converter, and the positive direction is the active power transmission from the constant DC voltage control side converter to the constant power control side converter in the back-to-back converter, then the direction from the back-to-back converter to the microgrid unit is 1, the direction from the microgrid unit to the back-to-back converter is -1, and the irrelevant direction is 0; a transfer power command vector calculated for each back-to-back converter; represents the transmission power instruction of the Nth microgrid unit at the τth sampling time, is the power vector composed of load and line loss, and: ;P load,k (t) is the total load of the kth microgrid unit; P loss,k (t) is the transmission loss of the kth microgrid unit; e min is an N-dimensional unit column vector, and satisfies that the min element is 1 and the other elements are 0; α0(τ) is the convergence value of the active power sharing ratio; S43. Solve equation (3) to obtain the transmission power instructions of the remaining back-to-back converters at the τth sampling moment, thereby realizing active power sharing control of all distributed power sources in the ring flexible interconnected microgrid group.

2. The active power sharing control method of a ring-shaped flexible interconnected microgrid group based on back-to-back converters according to claim 1 is characterized in that: The power balance constraint equations in step S42 are obtained according to the following steps: Sa: Using equation (4), we can get the basic power balance constraint equation inside the kth microgrid unit at any time t: (4) In formula (4), P c,q (t) is the active power transmitted by the qth back-to-back converter at time t; P k,i (t) is the output active power of the i-th distributed generation in the k-th microgrid unit at time t; Sb: According to the output of each distributed power source in the kth microgrid unit under droop control, the active power sharing relationship shown in formula (5) is satisfied, so the overall active power output P of the kth microgrid unit at time t is obtained using formula (6): MG,k (t): (5) (6) In formula (5), Pin is the number of the leader distributed generation in the kth microgrid unit, is the active power sharing ratio of the leader distributed generation within the kth microgrid unit; and ; is the active droop coefficient of the leader distributed generation within the kth microgrid unit; represents the number of distributed generation in the kth microgrid unit; In formula (6), , represents the active droop coefficient of the i-th distributed generation in the k-th microgrid unit; Sc: Using equation (7), we can get the active power balance equation of the kth microgrid unit at time t: (7) Sd: The control target of the ring flexible interconnected microgrid at the τth sampling time is obtained using formula (8): (8) In formula (8), α0(τ) is the convergence value of the active power sharing ratio; is the active droop coefficient of the leader distributed generation of the Nth microgrid unit at the τth sampling moment; Se: Using equation (9), we can get the power balance constraint equation of the kth microgrid unit at the τth sampling time: (9) In formula (9), is the active power transmission instruction value to be calculated for the qth back-to-back converter at the τth sampling time; is the total load of the kth microgrid unit at the τth sampling time; is the transmission loss of the kth microgrid unit at the τth sampling time; Sf: Use equation (10) to construct a linear equation system considering the power balance constraints of N microgrid units: (10) Sg: Matrix form of power balance constraint constructed using equation (11): (11)。

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