Isolated microgrid decentralized secondary control method based on generalized washout filter and adaptive virtual impedance

Abstract

The present application relates to the island micro-grid dispersed secondary control method based on generalized washout filter and adaptive virtual impedance, compared with prior art, the defects of poor power sharing ability, large frequency deviation and strong dependence on communication existing in traditional control method under line impedance mismatch of island micro-grid are solved.The present application comprises the following steps: establishing the micro-grid droop control model based on generalized washout filter; obtaining the steady-state output power error of micro-grid distributed power supply; generating adaptive virtual impedance based on active mode switching; carrying out island micro-grid dispersed secondary control.The present application obtains active power error based on dynamic phasor model, then generates adaptive virtual impedance, and sets the constraint condition that the virtual impedance increment is zero, which can effectively ensure the smoothness of micro-grid point of common coupling voltage.

Description

Technical Field

[0001] This invention relates to the field of hierarchical control technology for islanded microgrids, specifically to a distributed secondary control method for islanded microgrids based on a generalized scour filter and adaptive virtual impedance. Background Technology

[0002] Microgrids (MGs), composed of distributed generation (DG), energy storage, and loads, can utilize renewable energy more rationally and effectively, reducing environmental pollution. Microgrids operate in both islanded and grid-connected modes, offering flexibility and stability in control. In islanded mode, microgrids use hierarchical control—primary and secondary—to coordinate the provision of active and reactive power from different types of DGs to local loads, achieving power sharing and maintaining voltage and frequency stability. To ensure power sharing by each DG even without communication, primary control in the hierarchical control typically employs droop control, adjusting frequency and voltage amplitude through droop control for active power frequency regulation and reactive power voltage regulation. While droop control enables active power sharing due to frequency being a global variable, the existence of line impedance differences leads to suboptimal reactive power sharing performance. To address the issue of poor reactive power sharing caused by line impedance mismatch, the main measure adopted is the introduction of virtual impedance control.

[0003] The voltage amplitude and frequency deviations that occur in steady-state droop control can be addressed through secondary control (SC). Traditional methods use improved secondary control to restore the voltage amplitude and frequency to their rated values; however, these methods use low-bandwidth communication lines, resulting in control signal delays that affect the effectiveness of secondary control.

[0004] Therefore, how to achieve power sharing and frequency recovery among all DG units in an islanded microgrid under conditions of no communication is a key technical problem that urgently needs to be solved in the field of hierarchical control technology for islanded microgrids. This is of great significance for improving power supply reliability and promoting the development of renewable energy. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies in islanded microgrids under line impedance mismatch, such as poor power sharing capability, large frequency deviation, and strong dependence on communication, when using traditional control methods. This invention provides a distributed secondary control method for islanded microgrids based on a generalized washout filter (GWF) and adaptive virtual impedance to solve these problems.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A distributed secondary control method for islanded microgrids based on generalized scour filters and adaptive virtual impedance includes the following steps:

[0008] A microgrid droop control model based on a generalized scour filter is established;

[0009] Obtain the steady-state output power error of distributed power sources in microgrids;

[0010] Generate adaptive virtual impedance based on active mode switching;

[0011] Decentralized secondary control of isolated microgrids.

[0012] The establishment of the microgrid droop control model based on the generalized scour filter involves designing the microgrid droop control model based on the generalized scour filter using the droop control equations, including the following steps:

[0013] Suppose that the two DGs are connected to a common coupling point PCC, and the line impedances are mismatched, then:

[0014] DG i The output active power and reactive power are expressed as follows:

[0015]

[0016] Where: P i Q represents the active power output of the i-th DG; i V represents the reactive power output of the i-th DG; i DG i Output voltage amplitude; V L It is the PCC voltage amplitude; δ i The output voltage V of the i-th DG i and PCC voltage V L The phase angle difference between them; R i X is the resistance of the i-th line impedance; i It is the inductance of the impedance of the i-th line;

[0017] Assume δ i The value is extremely small, and sinδ is present. i ≈δ i ,cosδ i ≈1, simplifying equations (1) and (2) to:

[0018]

[0019] Among them, active power P i and phase angle difference δ i Proportional to reactive power Q i With output voltage amplitude V i related;

[0020] Calculate DG using the droop control equation i The frequency and amplitude of the output voltage are expressed as follows:

[0021] ω i =ω0-m i (P i -P 0,i (5)

[0022] V i =V0-n i (Q i -Q 0,i (6)

[0023] Where: ω i V represents the output angular frequency of the i-th DG; i The output voltage of the i-th DG is represented by ω0 and V0, respectively, which are the nominal angular frequency and the nominal output voltage amplitude. i and n i These are the droop coefficients of the i-th DG frequency droop controller and voltage droop controller, respectively; P i Q represents the output power of the i-th DG; i P is the output power of the i-th DG; 0,i and Q 0,i These are the nominal active power and nominal reactive power of the i-th DG, respectively;

[0024] A microgrid droop control model based on a generalized scour filter is established, and its expression is as follows:

[0025] ω i =ω0-m i ·G GWF (s)(P i -P 0,i (7)

[0026] V i =V0-n i ·G GWF (s)(Q i -Q 0,i (8)

[0027] Among them: G GWF (s) represents the transfer function of the generalized scour filter;

[0028]

[0029] Where: ω l It is the cutoff frequency of the low-pass filter; ω h The cutoff frequency of the scouring filter is s, where s represents the complex frequency domain.

[0030] The method for obtaining the steady-state output power error of the microgrid's distributed generation (DG) is as follows: The DG output power transfer function based on the generalized scour filter droop control is derived using the dynamic vector method. The final value theorem is then applied to obtain the steady-state output power of the DG based on the generalized scour filter droop control, thus obtaining the steady-state output power error of the microgrid's DG. This includes the following steps:

[0031] Derivation of Distributed Generator (DG) Based on Dynamic Phasor Model i For i = 1 and 2, the output active power and reactive power are:

[0032]

[0033] Where: P i Q represents the active power output of the i-th DG; i V represents the reactive power output of the i-th DG; i V represents the output voltage amplitude of the i-th DG; L It is the PCC voltage amplitude; δ i The output voltage V of the i-th DG i and PCC voltage V L Phase difference between them; R i X is the resistance of the i-th line impedance; i is the inductance of the i-th line impedance, and s represents the complex frequency domain;

[0034] Ignoring line losses, we have:

[0035]

[0036] Where: P L With Q L The active and reactive power of the load;

[0037] Linearizing equations (10) and (11) and applying the final value theorem, the steady-state output power of the DG based on the droop control of the generalized scour filter is obtained as follows:

[0038]

[0039] Where: P ss i Q represents the steady-state active power output of the i-th DG; ss i Let X1 be the steady-state reactive power output of the i-th DG; X2 be the line inductances of the two transmission lines, respectively; P is the steady-state reactive power output of the i-th DG; X1 and X2 are ... L With Q L To represent the active and reactive power of the load respectively; m1 and m2 are the droop coefficients of the two DG frequency droop controllers respectively; V L It is the PCC voltage amplitude; ω hω is the cutoff frequency of the flush filter; l It is the cutoff frequency of the low-pass filter;

[0040] To achieve precise power sharing, the droop factor relationship of multiple parallel DGs is as follows:

[0041]

[0042] Where: P i * , Let m be the expected active and reactive power of the i-th DG. i and n i These are the droop coefficients of the i-th DG frequency droop controller and voltage droop controller, respectively.

[0043] Obtain the steady-state output power error of the microgrid DG:

[0044] Define the active power sharing error as P erri The reactive power sharing error is Q. erri And there are:

[0045]

[0046]

[0047] Where: P erri Let Q be the active power sharing error of the i-th DG. erri P represents the reactive power sharing error of the i-th DG. i Q represents the output power of the i-th DG; i Let i be the output power of the i-th DG;

[0048] Extending to islanded microgrids with multiple distributed generation (DG) units operating in parallel, the active power sharing error ΔP of the DGs is based on generalized scour filter droop control. erri for:

[0049]

[0050] Where: ΔP erri The active power sharing error of the i-th DG is: m ∑ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ V represents the sum of the impedances of all lines; L It is the voltage amplitude of the PCC line.

[0051] The method for generating adaptive virtual impedance based on active mode switching involves: operating the controller in a droop control mode based on a generalized scour filter, sampling the power value, then switching to the droop control mode to obtain the power sharing error, and adaptively generating virtual impedance using the derived virtual impedance generation formula; including the following steps:

[0052] The virtual impedance is designed as follows:

[0053]

[0054] Where: X vi The virtual impedance designed for the i-th DG; X represents the expected value of the impedance of the i-th DG line; i The actual line impedance of the i-th DG;

[0055] If we set the constraint that the total increment of virtual impedance is 0, while keeping the total line impedance constant, then the following condition is satisfied:

[0056]

[0057] From equations (19) and (20), we get:

[0058]

[0059] in: X is the sum of the expected values ​​of the line impedance; ∑ This is the sum of the actual line impedances;

[0060] A sufficient condition for achieving a reasonable and even power distribution when k distributed generators of different capacities are connected in parallel is:

[0061]

[0062] in: m represents the expected value of the impedance of the i-th DG line; i Let be the droop coefficient of the i-th DG frequency droop controller;

[0063] Substituting equations (19) and (22) into equation (21), we get:

[0064]

[0065] Where: X vi For the generated virtual impedance; m ∑ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ This represents the sum of the impedances of all lines; m i X is the droop coefficient of the i-th DG frequency droop controller; i The actual line impedance of the i-th DG;

[0066] Substituting equation (18) into equation (23) generates a virtual impedance:

[0067]

[0068] Where: X vi For the generated virtual impedance; m ∑ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ ΔP represents the sum of the impedances of all lines; erri V represents the active power sharing error of the i-th DG; L It is the voltage amplitude of the PCC line;

[0069] For droop control based on a generalized scour filter, after active mode switching, it is switched to droop control to generate ΔP. erri That is:

[0070]

[0071] Where: P i P is the output power of the i-th DG; i * P is the expected active power of the i-th DG; G.ave and P D.ave These represent the steady-state values ​​of active power in the droop control mode based on a generalized scour filter and the traditional droop mode, respectively; P L The active power of the load;

[0072] Taking into account the influence of line resistance, a compensation coefficient K is introduced to generate an adaptive virtual impedance X that takes into account the influence of line resistance. vi And there are:

[0073]

[0074] Where: K is the compensation coefficient; X vi For the generated virtual impedance; m ∑ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ ΔP represents the sum of the impedances of all lines; erri V represents the active power sharing error of the i-th DG; L It is the voltage amplitude of the PCC line.

[0075] The decentralized secondary control of the islanded microgrid is as follows: each DG unit samples and records its active power based on droop control using a generalized scour filter; then the controller is switched to droop control mode, and after stabilization, the active power value at this point is recorded as the desired value P. D,aveThe proposed adaptive virtual impedance control is activated to enter a new steady state; then the active power control mode is switched to the droop control mode of the generalized scour filter to achieve unbiased frequency control, thus ultimately realizing power sharing and frequency recovery among all DGs; including the following steps:

[0076] Based on active mode switching, the active power sharing error ΔP is obtained. erri Then generate the adaptive virtual impedance X v The d-axis and q-axis reference voltages of the DG controlled by adaptive virtual impedance are:

[0077]

[0078] In the formula: and These are the d-axis reference voltage and the q-axis reference voltage, respectively; V d With V q These are the actual voltages along the d-axis and q-axis, respectively; X v For the generated virtual impedance; I od and I oq This refers to the dq-axis output current of the DG;

[0079] and The voltage outer loop reference value is used in the voltage-current dual closed-loop system. Both the voltage outer loop and the current inner loop use PI control to track the frequency and voltage reference value.

[0080] Define the controller's operating mode and basic events;

[0081] Decentralized secondary control of islanded microgrids is achieved based on droop control of generalized scour filters and adaptive virtual impedance.

[0082] The definition of the controller's operating mode and basic events includes the following steps:

[0083] Setting F v =1 indicates that adaptive virtual impedance control is enabled; F G =1 indicates that DG is operating in droop control mode based on a generalized scour filter, F G =0 indicates that DG is operating in the traditional droop control mode; F t =1 indicates that the islanded microgrid is in transient operation, F t =0 indicates that the islanded microgrid is in steady-state operation;

[0084] The rate of change of active power and the rate of change of reactive power The input is fed into a low-pass filter with a cutoff frequency of 10. or Output after low-pass filtering;

[0085] If the defined threshold ξ is exceeded PQ If so, the islanded microgrid is determined to be in transient operation, i.e., F t =1; and The outputs after low-pass filtering are all less than the threshold, indicating that the islanded microgrid is in steady-state operation, i.e., F t =0;

[0086] Event A is set to record the current local output active power P. G,ave This value is recorded by the sample-and-hold module, and the active power control mode is switched to the traditional droop control mode. G =0;

[0087] Event B is defined as recording and maintaining the current active power P. D,ave Enable the proposed adaptive virtual impedance control;

[0088] Event C is defined as switching the active power control mode to a droop control mode based on a generalized scour filter, F G =1.

[0089] The decentralized secondary control of an islanded microgrid based on droop control and adaptive virtual impedance using a generalized scour filter includes the following steps:

[0090] When a load change occurs in an isolated microgrid, the system enters a transient state, i.e., F. t When = 1, each distributed generator set operates in a droop control mode based on a scour filter to achieve decentralized secondary control;

[0091] Once the system reaches steady state, i.e., F... t =0, triggering event A, the sample-and-hold module records the active power P. G,ave Furthermore, the active power control mode is switched to droop control mode, i.e., F G =0, the reactive power control mode continues to use the generalized scour filter-based mode;

[0092] Once the active power is detected to be stable, the current active power value is recorded as the expected value P. D,ave Event B is triggered, enabling the proposed adaptive virtual impedance control, i.e., F. v =1, waiting for the system to re-enter a steady state;

[0093] Once the system is detected to have re-entered steady state, event C is triggered, switching the active power control mode to a droop control mode based on a generalized scour filter, i.e., F. G =1, achieving unbiased power and frequency control, and ultimately realizing droop control based on a generalized scour filter and distributed secondary control of an islanded microgrid based on adaptive virtual impedance.

[0094] Beneficial effects

[0095] This invention presents a distributed secondary control method for islanded microgrids based on a generalized scour filter and adaptive virtual impedance. Compared with existing technologies, this method obtains the active power error based on a dynamic phasor model, generates an adaptive virtual impedance, and sets a constraint condition that the virtual impedance increment is zero. This effectively ensures the stability of the voltage at the common coupling point of the microgrid. Furthermore, combining the advantages of traditional droop control and droop control based on a generalized scour filter, it innovatively proposes an active mode switching method to achieve power sharing and frequency recovery in islanded microgrids without communication requirements. This invention has the advantages of requiring no communication support, high reliability, and strong stability. Attached Figure Description

[0096] Figure 1 This is a sequence diagram of the method of the present invention;

[0097] Figure 2 A single-line diagram of an islanded microgrid with two DGs;

[0098] Figure 3 The flowchart shows the distributed secondary control process of an islanded microgrid DG without communication, based on droop control and adaptive virtual impedance control using GWF.

[0099] Figure 4 Diagram of an islanded microgrid with three distributed generation (DG) systems;

[0100] Figure 5 The image shows the simulation results. Detailed Implementation

[0101] To provide a better understanding of the structural features and effects achieved by the present invention, a detailed description is provided below, accompanied by preferred embodiments and accompanying drawings:

[0102] like Figure 1 As shown, the distributed secondary control method for isolated microgrids based on a generalized scouring filter and adaptive virtual impedance, as described in this invention, overcomes the limitations of traditional secondary control methods that rely on communication conditions. By employing an active mode switching method, it innovatively proposes a distributed secondary control method for isolated microgrids, achieving power sharing and frequency recovery in isolated microgrids without the need for communication. This effectively solves the problems of poor power sharing capability, large frequency deviation, and strong dependence on communication when using traditional control methods in isolated microgrids with line impedance mismatch. Specifically, it includes the following steps:

[0103] The first step is to establish a microgrid droop control model based on a generalized scour filter. The active and reactive power outputs of the distributed generation (DG) are derived, and the results are simplified by incorporating actual line impedance to obtain the traditional droop control equations. Then, the microgrid droop control model based on the generalized scour filter is established. The specific steps are as follows:

[0104] (1) Derivation of the traditional droop control equation: such as Figure 2 As shown, the analysis is carried out taking the example of two DG units connected to a common PCC point and the line impedance mismatch. The active power and reactive power output by DG i (i=1,2) can be expressed as:

[0105]

[0106] Where: P i Q represents the output power of the i-th DG; i V is the output power of the i-th DG; i DG i Output voltage amplitude; V L It is the voltage amplitude of the PCC line; δ i The output voltage V of the i-th DG i and PCC line voltage V L The phase angle difference between them; R i X is the resistance of the i-th line impedance; i It is the inductance of the impedance of the i-th line;

[0107] Assume δ i The value is extremely small, and sinδ is present. i ≈δ i ,cosδ i ≈1, simplifying equations (1) and (2) to:

[0108]

[0109] Among them, active power and phase angle difference δ i The reactive power is directly proportional to the output voltage amplitude V. i related;

[0110] The frequency and amplitude of the output voltage are calculated using the droop control equation, which is expressed as follows:

[0111] ω i =ω0-m i (P i -P 0,i (5)

[0112] V i =V0-n i (Q i -Q 0,i (6)

[0113] Where: ω i V represents the output angular frequency of the i-th DG; i The output voltage of the i-th DG is represented by ω0 and V0, respectively, which are the nominal angular frequency and the nominal output voltage amplitude. i and n i These are the droop coefficients of the i-th DG frequency droop controller and voltage droop controller, respectively; P i Q represents the output power of the i-th DG; i P is the output power of the i-th DG; 0,i and Q 0,i These are the nominal active power and reactive power of the i-th DG, respectively.

[0114] (2) A microgrid droop control model based on a generalized scour filter is established, and its expression is as follows:

[0115] ω i =ω0-m i ·G GWF (s)(P i -P 0,i (7)

[0116] V i =V0-n i ·G GWF (s)(Q i -Q 0,i (8)

[0117] Among them: G GWF (s) represents the transfer function of the generalized scour filter;

[0118]

[0119] Where: ω l It is the cutoff frequency of the low-pass filter; ω h The cutoff frequency of the scouring filter is s, where s represents the complex frequency domain.

[0120] The second step is to obtain the steady-state output power error of the microgrid's distributed generators (DGs): Based on a dynamic phasor model, the active and reactive power outputs of the distributed generators are derived, linearized, and then the final value theorem is used to obtain their steady-state output power. This method cleverly utilizes the characteristic of droop control's natural distribution of active power to obtain the expected output power of each DG, ultimately yielding the steady-state output power error of each DG. This method can be extended to multiple DGs, demonstrating strong universality.

[0121] The specific steps to obtain the steady-state output power error of a microgrid's distributed generation (DG) are as follows:

[0122] (1) Deriving Distributed Generator (DG) Based on Dynamic Phasor Model i For i = 1 and 2, the output active and reactive power is:

[0123]

[0124] Where: P i Q represents the output power of the i-th DG; i V is the output power of the i-th DG; i V represents the output voltage amplitude of the i-th DG; L It is the voltage amplitude of the PCC line; δ i The output voltage V of the i-th DG i and PCC line voltage V L The phase angle difference between them; R i X is the resistance of the i-th line impedance; i is the inductance of the i-th line impedance, and s represents the complex frequency domain;

[0125] Ignoring line losses, we have:

[0126]

[0127] Where: P L With Q L The active and reactive power of the load;

[0128] (2) Linearizing equations (10) and (11) and applying the final value theorem, the steady-state output power of the DG based on the droop control of the generalized scour filter is obtained as follows:

[0129]

[0130] Where: P ss i Q represents the steady-state active power output of the i-th DG; ss i Let X1 be the steady-state reactive power output of the i-th DG; X2 be the line inductances of the two transmission lines, respectively; P is the steady-state reactive power output of the i-th DG; X1 and X2 are ... L With Q L To represent the active and reactive power of the load respectively; m1 and m2 are the droop coefficients of the two DG frequency droop controllers and voltage droop controllers, respectively; V L It is the amplitude of the PCC line voltage; ω h ω is the cutoff frequency of the flush filter; l It is the cutoff frequency of the low-pass filter;

[0131] To achieve precise power sharing, the droop factor relationship of multiple parallel DGs is as follows:

[0132]

[0133] Where: P i * , The expected active and reactive power of the i-th DG; m i and n i These are the droop coefficients of the i-th DG frequency droop controller and voltage droop controller, respectively.

[0134] (3) Obtain the steady-state output power error of the microgrid DG:

[0135] Define the active power sharing error as P erri The reactive power sharing error is Q. erri And there are:

[0136]

[0137] Where: P erri Let Q be the active power sharing error of the i-th DG. erri P represents the reactive power sharing error of the i-th DG. i Q represents the output power of the i-th DG; i Let i be the output power of the i-th DG;

[0138] Extending to islanded microgrids with multiple distributed generation (DG) units operating in parallel, the active power sharing error ΔP of the DGs is based on generalized scour filter droop control. erri for:

[0139]

[0140] Where: ΔP erri The active power sharing error of the i-th DG is: m ∑ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ V represents the sum of the impedances of all lines; L It is the voltage amplitude of the PCC line.

[0141] The third step involves generating an adaptive virtual impedance based on active mode switching. This invention proposes setting a constraint condition of zero virtual impedance increment to improve the line impedance distribution without increasing the voltage drop and without affecting the voltage at the common coupling point. Furthermore, compared with traditional virtual impedance generation methods, this method has the advantage of not requiring communication conditions, and has higher reliability and stronger scalability.

[0142] The process of generating adaptive virtual impedance based on active mode switching includes the following steps:

[0143] (1) Design the virtual impedance as follows:

[0144]

[0145] Where: X vi The virtual impedance designed for the i-th DG; X represents the expected value of the impedance of the i-th DG line; i The actual line impedance of the i-th DG;

[0146] If we set the constraint that the total increment of virtual impedance is 0, while keeping the total line impedance constant, then the following condition is satisfied:

[0147]

[0148] From equations (19) and (20), we get:

[0149]

[0150] in: X is the sum of the expected values ​​of the line impedance; ∑ This is the sum of the actual line impedances;

[0151] (2) The sufficient condition for achieving a reasonable and even power distribution when k DGs of different capacities are connected in parallel is:

[0152]

[0153] in: m represents the expected value of the impedance of the i-th DG line; i Let be the droop coefficient of the i-th DG frequency droop controller;

[0154] Substituting equations (19) and (22) into equation (21), we get:

[0155]

[0156] Where: X vi For the generated virtual impedance; m ∑ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ This represents the sum of the impedances of all lines; m i X is the droop coefficient of the i-th DG frequency droop controller; i The actual line impedance of the i-th DG;

[0157] Substituting equation (18) into equation (23) generates a virtual impedance:

[0158]

[0159] Where: X vi For the generated virtual impedance; m Σ X represents the sum of the droop coefficients of each DG frequency droop controller; ΣΔP represents the sum of the impedances of all lines; erri V represents the active power sharing error of the i-th DG; L It is the voltage amplitude of the PCC line;

[0160] (3) For droop control based on a generalized scour filter, after active mode switching, it is switched to droop control to generate ΔP. erri That is:

[0161]

[0162] Where: P i P is the output power of the i-th DG; i * P is the expected active power of the i-th DG; G.ave and P D.ave These represent the steady-state values ​​of active power in the droop control mode based on a generalized scour filter and the traditional droop mode, respectively; P L The active power of the load;

[0163] Taking into account the influence of line resistance, a compensation coefficient K is introduced to generate an adaptive virtual impedance X that takes into account the influence of line resistance. vi And there are:

[0164]

[0165] Where: K is the compensation coefficient; X vi For the generated virtual impedance; m Σ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ ΔP represents the sum of the impedances of all lines; erri V represents the active power sharing error of the i-th DG; L It is the voltage amplitude of the PCC line.

[0166] The fourth step involves decentralized secondary control of the islanded microgrid. This involves using generated adaptive virtual impedance to compensate for impedance mismatches in the microgrid lines, and employing active mode switching to achieve power equalization and voltage / frequency restoration. Specifically, this includes the following steps:

[0167] (1) Based on active mode switching, obtain the active power sharing error ΔP erri Then generate the adaptive virtual impedance X v The d-axis and q-axis reference voltages of the DG controlled by adaptive virtual impedance are:

[0168]

[0169] In the formula: and These are the d-axis reference voltage and the q-axis reference voltage, respectively; Vd With V q These are the actual voltages along the d-axis and q-axis, respectively; X v For the generated virtual impedance; I od and I oq This refers to the dq-axis output current of the DG;

[0170] V d * With V q * The voltage outer loop reference value is used in the voltage-current dual closed-loop system. Both the voltage outer loop and the current inner loop use PI control to track the frequency and voltage reference value.

[0171] (2) Define the controller's operating mode and basic events;

[0172] Defining the controller's operating mode and basic events includes the following steps:

[0173] A1) Set F v =1 indicates that adaptive virtual impedance control is enabled; F G =1 indicates that DG is operating in droop control mode based on a generalized scour filter, F G =0 indicates that DG is operating in the traditional droop control mode; F t =1 indicates that the islanded microgrid is in transient operation, F t =0 indicates that the islanded microgrid is in steady-state operation;

[0174] The rate of change of active power and the rate of change of reactive power The input is fed into a low-pass filter with a cutoff frequency of 10. or Output after low-pass filtering;

[0175] If the defined threshold ξ is exceeded PQ If so, the islanded microgrid is determined to be in transient operation, i.e., F t =1; and The outputs after low-pass filtering are all less than the threshold, indicating that the islanded microgrid is in steady-state operation, i.e., F t =0;

[0176] A2) Set event A to record the current local output active power P. G,ave This value is recorded by the sample-and-hold module, and the active power control mode is switched to the traditional droop control mode. G =0;

[0177] Event B is defined as recording and maintaining the current active power P. D,ave Enable the proposed adaptive virtual impedance control;

[0178] Event C is defined as switching the active power control mode to a droop control mode based on a generalized scour filter, F G =1.

[0179] (3) Distributed secondary control of an islanded microgrid based on droop control and adaptive virtual impedance using a generalized scour filter. The process of distributed secondary control of an islanded microgrid based on droop control and adaptive virtual impedance is as follows: Figure 3 As shown, the specific steps include:

[0180] B1) When a load change occurs in an islanded microgrid, the system enters a transient state, i.e., F t When = 1, each distributed generator set operates in a droop control mode based on a scour filter to achieve decentralized secondary control;

[0181] B2) After monitoring that the system has reached steady state, i.e., F t =0, triggering event A, the sample-and-hold module records the active power P. G,ave Furthermore, the active power control mode is switched to droop control mode, i.e., F G =0, the reactive power control mode continues to use the generalized scour filter-based mode;

[0182] B3) After the active power is detected to be stable, the active power value at this time is recorded as the expected value P. D,ave Event B is triggered, enabling the proposed adaptive virtual impedance control, i.e., F. v =1, waiting for the system to re-enter a steady state;

[0183] B4) After detecting that the system has re-entered steady state, event C is triggered, switching the active power control mode to the droop control mode based on the generalized scour filter, i.e., F. G =1, achieving unbiased power and frequency control, and ultimately realizing distributed secondary control of islanded microgrids based on generalized scour filters and adaptive virtual impedance.

[0184] To verify the correctness of the proposed control, a system was established based on MatLab / Simulink software, as follows: Figure 4 The simulation model of an islanded microgrid with three distributed generation (DG) is shown in Table 1. The main parameters of the islanded microgrid are shown in Table 1.

[0185] Table 1. Parameters of an islanded microgrid where two DG units are connected in parallel to supply power to a common load.

[0186]

[0187] Simulation results are as follows Figure 5As shown, when all DG units in an isolated microgrid have the same rated power, the system first operates in GWF droop control mode, and the power increases rapidly. At t=1.1s, the system enters steady state and actively switches to droop control mode, achieving accurate sharing of active power. At t=1.82s, the proposed virtual impedance (AVI) control is triggered, improving the reactive power sharing capability. At t=2.75s, the proposed control actively switches back to GWF droop control mode, achieving unbiased frequency control. Figure 5 (a) and (b) clearly demonstrate that, under the proposed control, the isolated microgrid DG, through active mode switching, compensates for the impact of line impedance mismatch on power sharing performance, achieving a significant improvement in active and reactive power sharing performance; Figure 5 As shown in (c), when the load changes, the frequency fluctuation is small, and the frequency can be restored to the nominal value, realizing distributed secondary control; Figure 5 (d) shows the output voltage of each DG, and it can be seen that the voltage can be effectively restored using the control method proposed in this invention.

[0188] Simulation results show that the proposed method for communication-free secondary control of islanded microgrids based on generalized scouring filters and adaptive virtual impedance effectively solves the problems of poor power sharing capability, large frequency deviation, and strong dependence on communication in islanded microgrids under line impedance mismatch when using traditional control methods.

[0189] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A distributed secondary control method for islanded microgrids based on a generalized scour filter and adaptive virtual impedance, characterized in that, Includes the following steps: 11) Establish a microgrid droop control model based on a generalized scour filter; 12) Obtain the steady-state output power error of the distributed generation in the microgrid; 13) Generate adaptive virtual impedance based on active mode switching; The adaptive virtual impedance based on active mode switching is as follows: the controller is run in the droop control mode based on the generalized scour filter, the power value is sampled, and then the controller is switched to the traditional droop control mode to obtain the power sharing error. The virtual impedance is then generated adaptively using the derived virtual impedance generation formula. 14) Implement distributed secondary control for the islanded microgrid; the distributed secondary control for the islanded microgrid is as follows: each DG unit samples and records the active power based on the droop control of the generalized scour filter; then the controller is switched to the traditional droop control mode, and after stabilization, the active power value at this time is recorded as the expected value P. D,ave The proposed adaptive virtual impedance control is enabled to enter a new steady state; then the active power control mode is switched to the droop control mode of the generalized scour filter to achieve unbiased frequency control, that is, to finally realize power sharing and frequency recovery of each DG.

2. The distributed secondary control method for islanded microgrids based on a generalized scour filter and adaptive virtual impedance as described in claim 1, characterized in that, The establishment of the microgrid droop control model based on the generalized scour filter involves designing the microgrid droop control model based on the generalized scour filter using the droop control equations, including the following steps: 21) Suppose that two DGs are connected to a common coupling point PCC, and the line impedances are mismatched, then: DG i The output active power and reactive power are expressed as follows: Where: P i Q represents the active power output of the i-th DG; i V represents the reactive power output of the i-th DG; i DG i Output voltage amplitude; V L It is the PCC voltage amplitude; δ i The output voltage V of the i-th DG i and PCC voltage V L The phase angle difference between them; R i X is the resistance of the i-th line impedance; i It is the inductance of the impedance of the i-th line; Assume δ i The value is extremely small, and sinδ is present. i ≈δ i ,cosδ i ≈1, simplifying equations (1) and (2) to: Among them, active power P i and phase angle difference δ i Proportional to reactive power Q i With output voltage amplitude V i related; Calculate DG using the droop control equation i The frequency and amplitude of the output voltage are expressed as follows: oh i =ω0-m i (P i -P 0,i ) (5) V i =V0-n i (Q i -Q 0,i ) (6) Where: ω i V represents the output angular frequency of the i-th DG; i The output voltage of the i-th DG is represented by ω0 and V0, respectively, which are the nominal angular frequency and the nominal output voltage amplitude. i and n i These are the droop coefficients of the i-th DG frequency droop controller and voltage droop controller, respectively; P i Q represents the output power of the i-th DG; i P is the output power of the i-th DG; 0,i and Q 0,i These are the nominal active power and nominal reactive power of the i-th DG, respectively; 22) Establish a microgrid droop control model based on a generalized scour filter, the expression of which is as follows: oh i =ω0-m i ·G GWF (s)(P i -P 0,i ) (7) V i =V0-n i ·G GWF (s)(Q i -Q 0,i ) (8) Among them: G GWF (s) represents the transfer function of the generalized scour filter; Where: ω l It is the cutoff frequency of the low-pass filter; ω h The cutoff frequency of the scouring filter is s, where s represents the complex frequency domain.

3. The distributed secondary control method for islanded microgrids based on a generalized scour filter and adaptive virtual impedance as described in claim 1, characterized in that, The method for obtaining the steady-state output power error of the microgrid's distributed generation (DG) is as follows: The DG output power transfer function based on the generalized scour filter droop control is derived using the dynamic vector method. The final value theorem is then applied to obtain the steady-state output power of the DG based on the generalized scour filter droop control, thus obtaining the steady-state output power error of the microgrid's DG. This includes the following steps: 31) Derive Distributed Generator (DG) based on dynamic phasor model i For i = 1 and 2, the output active power and reactive power are: Where: P i Q represents the active power output of the i-th DG; i V represents the reactive power output of the i-th DG; i V represents the output voltage amplitude of the i-th DG; L It is the PCC voltage amplitude; δ i The output voltage V of the i-th DG i and PCC voltage V L Phase difference between them; R i X is the resistance of the i-th line impedance; i is the inductance of the i-th line impedance, and s represents the complex frequency domain; Ignoring line losses, we have: Where: P L With Q L The active and reactive power of the load; 32) Linearizing equations (10) and (11) and applying the final value theorem, the steady-state output power of the DG based on the droop control of the generalized scour filter is obtained as follows: Where: P ss i Q represents the steady-state active power output of the i-th DG; ss i Let X1 be the steady-state reactive power output of the i-th DG; X2 be the line inductances of the two transmission lines, respectively; P is the steady-state reactive power output of the i-th DG; X1 and X2 are ... L With Q L To represent the active and reactive power of the load respectively; m1 and m2 are the droop coefficients of the two DG frequency droop controllers respectively; V L It is the PCC voltage amplitude; ω h ω is the cutoff frequency of the scouting filter; l It is the cutoff frequency of the low-pass filter; To achieve precise power sharing, the droop factor relationship of multiple parallel DGs is as follows: Where: P i * , Let m be the expected active and reactive power of the i-th DG. i and n i These are the droop coefficients of the i-th DG frequency droop controller and voltage droop controller, respectively. 33) Obtain the steady-state output power error of the microgrid DG: Define the active power sharing error as P erri The reactive power sharing error is Q. erri And there are: Where: P erri Let Q be the active power sharing error of the i-th DG. erri P represents the reactive power sharing error of the i-th DG. i Q represents the output power of the i-th DG; i Let i be the output power of the i-th DG; Extending to islanded microgrids with multiple distributed generation (DG) units operating in parallel, the active power sharing error ΔP of the DGs is based on generalized scour filter droop control. erri for: Where: ΔP erri The active power sharing error of the i-th DG is: m ∑ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ V represents the sum of the impedances of all lines; L It is the voltage amplitude of the PCC line.

4. The distributed secondary control method for islanded microgrids based on a generalized scour filter and adaptive virtual impedance as described in claim 2, characterized in that, The process of generating adaptive virtual impedance based on active mode switching includes the following steps: 41) Design the virtual impedance as follows: Where: X vi The virtual impedance designed for the i-th DG; X represents the expected value of the impedance of the i-th DG line; i The actual line impedance of the i-th DG; If we set the constraint that the total increment of virtual impedance is 0, while keeping the total line impedance constant, then the following condition is satisfied: From equations (19) and (20), we get: in: X is the sum of the expected values ​​of the line impedance; ∑ This is the sum of the actual line impedances; 42) The sufficient condition for achieving a reasonable and even power distribution when k drain generators of different capacities are connected in parallel is: in: m represents the expected value of the impedance of the i-th DG line; i Let be the droop coefficient of the i-th DG frequency droop controller; Substituting equations (19) and (22) into equation (21), we get: Where: X vi For the generated virtual impedance; m ∑ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ This represents the sum of the impedances of all lines; m i X is the droop coefficient of the i-th DG frequency droop controller; i The actual line impedance of the i-th DG; Substituting equation (18) into equation (23) generates a virtual impedance: Where: X vi For the generated virtual impedance; m ∑ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ ΔP represents the sum of the impedances of all lines; erri V represents the active power sharing error of the i-th DG; L It is the voltage amplitude of the PCC line; 43) For droop control based on a generalized scour filter, after active mode switching, it is switched to traditional droop control to generate ΔP. erri That is: Where: P i P is the output power of the i-th DG; i * P is the expected active power of the i-th DG; G.ave and P D.ave These represent the steady-state values ​​of active power in the droop control mode based on a generalized scour filter and the traditional droop mode, respectively; P L The active power of the load; Taking into account the influence of line resistance, a compensation coefficient K is introduced to generate an adaptive virtual impedance X that takes into account the influence of line resistance. vi And there are: Where: K is the compensation coefficient; X vi For the generated virtual impedance; m ∑ X represents the sum of the droop coefficients of each DG frequency droop controller; ∑ ΔP represents the sum of the impedances of all lines; erri V represents the active power sharing error of the i-th DG; L It is the voltage amplitude of the PCC line.

5. The distributed secondary control method for islanded microgrids based on a generalized scour filter and adaptive virtual impedance as described in claim 1, characterized in that, The decentralized secondary control of the islanded microgrid includes the following steps: 51) Based on active mode switching, obtain the active power sharing error ΔP. erri Then generate the adaptive virtual impedance X v The d-axis and q-axis reference voltages of the DG controlled by adaptive virtual impedance are: In the formula: and These are the d-axis reference voltage and the q-axis reference voltage, respectively; V d With V q These are the actual voltages along the d-axis and q-axis, respectively; X v For the generated virtual impedance; I od and I oq This refers to the dq-axis output current of the DG; and The voltage outer loop reference value is used in the voltage-current dual closed-loop system. Both the voltage outer loop and the current inner loop use PI control to track the frequency and voltage reference value. 52) Define the controller's operating modes and basic events; 53) Decentralized secondary control of islanded microgrids is realized based on droop control of generalized scour filter and adaptive virtual impedance.

6. The distributed secondary control method for islanded microgrids based on a generalized scour filter and adaptive virtual impedance as described in claim 5, characterized in that, The definition of the controller's operating mode and basic events includes the following steps: 61) Set F v =1 indicates that adaptive virtual impedance control is enabled; F G =1 indicates that DG is operating in droop control mode based on a generalized scour filter, F G =0 indicates that DG is operating in the traditional droop control mode; F t =1 indicates that the islanded microgrid is in transient operation, F t =0 indicates that the islanded microgrid is in steady-state operation; The rate of change of active power and the rate of change of reactive power The input is fed into a low-pass filter with a cutoff frequency of 10. or Output after low-pass filtering; If the defined threshold ξ is exceeded PQ If so, the islanded microgrid is determined to be in transient operation, i.e., F t =1; and The outputs after low-pass filtering are all less than the threshold, indicating that the islanded microgrid is in steady-state operation, i.e., F t =0; 62) Set event A to record the current local output active power P G,ave This value is recorded by the sample-and-hold module, and the active power control mode is switched to the traditional droop control mode. G =0; Event B is defined as recording and maintaining the current active power P. D,ave Enable the proposed adaptive virtual impedance control; Event C is defined as switching the active power control mode to a droop control mode based on a generalized scour filter, F G =1.

7. The distributed secondary control method for islanded microgrids based on a generalized scour filter and adaptive virtual impedance as described in claim 5, characterized in that, The decentralized secondary control of an islanded microgrid based on droop control and adaptive virtual impedance using a generalized scour filter includes the following steps: 71) When a load change occurs in an islanded microgrid, the system enters a transient state, i.e., F t When =1, each distributed generator set operates in a droop control mode based on a generalized scour filter to achieve decentralized secondary control; 72) After monitoring that the system has reached steady state, i.e., F t =0, triggering event A, the sample-and-hold module records the active power P. G,ave Furthermore, the active power control mode will be switched to the traditional droop control mode, i.e., F G =0, the reactive power control mode continues to use the generalized scour filter-based mode; 73) After the active power is detected to be stable, the active power value at this time is recorded as the expected value P. D,ave Event B is triggered, enabling the proposed adaptive virtual impedance control, i.e., F. v =1, waiting for the system to re-enter a steady state; 74) After the system is detected to have re-entered steady state, event C is triggered, switching the active power control mode to the droop control mode based on the generalized scour filter, i.e., F. G =1, achieving unbiased power and frequency control, and ultimately realizing droop control based on a generalized scour filter and distributed secondary control of an islanded microgrid based on adaptive virtual impedance.

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