High-order impedance modeling and stability analysis method suitable for distributed DC microgrid

Abstract

The invention provides a high-order impedance modeling and stability analysis method suitable for a distributed DC micro-grid, and the method comprises the steps: building a third-order impedance model of an AC / DC interface converter unit in the DC micro-grid based on a droop control principle and a port power conservation principle, and enabling main diagonal elements of a third-order impedance matrix to comprise dq AC impedance and DC impedance, the secondary diagonal element comprises an alternating current coupling term and an alternating current-direct current coupling term; establishing an alternating-current grid-side impedance model in the direct-current micro-grid; establishing a direct-current grid-side impedance model in the direct-current micro-grid; combining the alternating current network side impedance model and the direct current network side impedance model to obtain a three-order impedance matrix model of the network side; and integrating the three-order impedance model of the AC / DC interface converter unit and the three-order impedance matrix model of the grid side to obtain a high-order impedance model of the DC micro-grid. The method improves the accuracy of the model, reduces the neglect of the system characteristics, and deeply reveals the coupling relation between the AC side and the DC side in the DC micro-grid.

Description

Technical Field

[0001] The present invention relates to the field of DC microgrid systems supplied by distributed power sources, and in particular to a high-order impedance modeling method suitable for distributed DC microgrids and a high-order MIMO system stability analysis method. Background Art

[0002] A microgrid is a local power system composed of distributed power sources, energy storage devices and power loads. It can operate independently or in conjunction with the grid and is characterized by flexibility and high efficiency.

[0003] It can enhance the reliability and energy efficiency of the power grid while supporting the integration and use of renewable energy, and is particularly suitable for scenarios such as remote areas and industrial parks. Compared with AC microgrids, DC microgrids have several significant advantages. First, DC microgrids eliminate frequency synchronization problems, simplify the power conversion link, and reduce energy losses. Secondly, DC systems can more efficiently connect directly with renewable energy sources such as solar energy and wind energy and energy storage systems, thereby improving energy utilization efficiency. In addition, the control structure of DC microgrids is relatively simple, which can achieve faster response speeds and more precise power control, and is particularly suitable for scenarios such as electric vehicles, multi-electric / all-electric aircraft, etc. that require stable voltage power supply.

[0004] The salient feature of a DC microgrid is decentralization, that is, the system contains multiple distributed power sources, and each power generation channel independently supplies power to the diverse loads in the microgrid. Due to its distributed structure, a reasonable control strategy is required to ensure balanced power distribution among each power generation channel and stabilize the voltage level of the DC bus. Among the centralized, distributed, and decentralized control strategies, decentralized droop control is most suitable for the control needs of a DC microgrid. The advantage of this strategy is that there is no need for local information exchange between power generation units. It only requires the introduction of a droop control module in each local controller to achieve balanced distribution of multi-source power and ensure the flexibility and stability of system operation.

[0005] Due to the application of droop control, system nonlinearity and complex dynamic characteristics are introduced into DC microgrids. In order to ensure the reliability and efficiency of the system, comprehensive and accurate modeling and analysis must be carried out. Typical DC microgrid power sources include generators, photovoltaic systems, energy storage devices and other distributed energy sources. The modeling methods for DC power sources such as photovoltaics and batteries are relatively mature, but for generators, since most generators output in the form of AC, they need to be converted through AC / DC interface rectifiers to supply the DC bus, and the modeling process is relatively complicated and therefore not perfect.

[0006] At present, the modeling of AC / DC power conversion units mainly adopts the small signal impedance modeling method, which equates the system characteristics to the DC side through the power conservation relationship and establishes a first-order impedance model in the form of a transfer function. However, this modeling method has certain limitations. First, the traditional model usually only considers the d-axis characteristics of active power control and ignores the q-axis characteristics of reactive power control. Secondly, although the first-order impedance model simplifies the complexity of the system, it cannot capture the coupling relationship between the AC dq axis and it is difficult to reflect the coupling dynamics between AC and DC. The second-order dq impedance model and the positive and negative sequence impedance modeling method focus on revealing the AC characteristics, which are more suitable for inverter systems and are not applicable to the impedance modeling of DC microgrids. At the same time, the traditional modeling method usually unifies the AC or DC characteristics of the system to one side for processing. Although this method can obtain a complete system model, there is a significant problem with this unified model: whether on the source side, the load side, or when any disturbance occurs in the power converter, the model may lose accuracy, resulting in the model no longer being applicable, and the entire system needs to be remodeled. This limitation greatly increases the complexity and maintenance cost of the model, especially in microgrids, where frequent load changes or multi-source access make it difficult for this unified modeling method to meet actual needs. From the above problems, it can be seen that the modeling of DC microgrids needs to seek a balance between maintaining simplicity and improving accuracy to better reflect the actual operating status of the system. Summary of the invention

[0007] In view of the defects in the prior art, the object of the present invention is to provide a high-order impedance modeling method and a system stability analysis method suitable for distributed DC microgrids.

[0008] According to one aspect of the present invention, a high-order impedance modeling method applicable to a distributed DC microgrid is provided, comprising:

[0009] Based on the droop control principle and the port power conservation principle, a third-order impedance model of the AC / DC interface converter unit in the DC microgrid is established. The main diagonal elements of the third-order impedance matrix of the third-order impedance model include dq AC impedance and DC impedance, and the secondary diagonal elements include AC coupling terms and AC / DC coupling terms.

[0010] Establish the AC grid side impedance model in the DC microgrid;

[0011] Establish the DC grid side impedance model in DC microgrid;

[0012] Combining the AC grid-side impedance model and the DC grid-side impedance model to obtain a third-order impedance matrix model of the grid side;

[0013] The third-order impedance model of the AC / DC interface converter unit and the third-order impedance matrix model of the grid side are integrated to obtain a high-order impedance model of the DC microgrid.

[0014] Preferably, the third-order impedance model of the AC / DC interface converter unit in the DC microgrid is established based on the port power conservation principle and the droop control principle, including:

[0015] The droop control principle is:

[0016]

[0017] Among them, v dc represents the actual measured voltage value on the DC bus side, k represents the droop gain, and the superscript "ref" represents the reference value in the controller. and Respectively represent the reference values ​​of DC current and voltage;

[0018] The port power conservation principle is:

[0019]

[0020] Among them, i dc Indicates the DC side current value, v d and v q Respectively represent the d-axis and q-axis voltages on the AC side, i d and i q They represent the d-axis and q-axis currents on the AC side respectively;

[0021] The final form of the high-order impedance matrix of the AC / DC interface converter unit obtained by high-order impedance modeling is:

[0022]

[0023] Where Y represents the system admittance, and the bold font represents the matrix form; Y dq It is a 2*2 matrix, Y dqdc It is a 2*1 order matrix, and the two admittance matrices constitute the upper half of the high-order impedance matrix; Y dq The main diagonal elements of dd and Y qq , respectively represent the AC d-axis characteristics and q-axis characteristics of the AC / DC interface converter unit, and the diagonal element Y dq and Y qd Respectively represent the dq AC coupling characteristics of the AC / DC interface converter unit; Y dcdq It is a 1*2 matrix, Y dc is a 1*1 matrix, Y dqdc and Y dcdq Both represent the AC and DC coupling characteristics of the AC / DC interface converter unit.

[0024] Preferably, the establishing of the AC grid-side impedance model in the DC microgrid includes:

[0025] Analyze the status of the AC grid side: a permanent magnet synchronous generator is used as the power source, and there is line impedance when the AC power is output to the AC / DC interface converter unit;

[0026] Based on the AC grid side state, the AC grid side impedance model is established as:

[0027]

[0028] is the line impedance, is the generator stator impedance, ω e is the motor electrical angular velocity, R ac =R c +R s ,L ac =L c +L s； R c and L c Represents line resistance and inductance respectively, R s and L s They represent the stator resistance and inductance of the permanent magnet synchronous generator respectively.

[0029] Preferably, the step of establishing a DC grid-side impedance model in a DC microgrid includes:

[0030] Analyze the DC grid side status: the DC grid side impedance includes the local capacitance of the distributed generation unit, the line impedance, the high-voltage DC bus capacitance and the load; the load includes the constant power load CPL;

[0031] Based on the DC grid side state, a first-order DC grid side impedance model is established as:

[0032]

[0033] Among them, Z CPL Represents the power load impedance, Z cable Represents the line impedance, C x Represents the local capacitance of the distributed generation unit, the subscript "x" represents the number of different distributed generation units, R c and L c Indicates the DC line resistance, P L Indicates the power of the constant power load, C bus Indicates the busbar capacitance, C x Represents the local capacitance of the distributed generation unit, V bus Indicates the DC bus voltage.

[0034] Preferably, the merging of the AC grid-side impedance model and the DC grid-side impedance model to obtain a grid-side third-order impedance matrix model includes:

[0035]

[0036] Preferably, the third-order impedance model of the AC / DC interface converter unit and the third-order impedance matrix model of the grid-side model are integrated to obtain a high-order impedance model of the DC microgrid, specifically:

[0037] Combine the AC / DC interface converter unit model and the grid-side impedance model in the DC microgrid to obtain:

[0038]

[0039] Among them, I3 represents the third-order identity matrix, v dB and Δv qB represents the voltage of the permanent magnet synchronous generator, Δu d and Δu q is the voltage at the interface converter port; Δu dc =[Δu d ,Δu q ].

[0040] According to a second aspect of the present invention, there is provided a system stability analysis method, comprising:

[0041] A high-order impedance model of a DC microgrid obtained according to a high-order impedance modeling method suitable for distributed DC microgrids;

[0042] Based on the high-order impedance model, and according to the loop gain principle of the system and the generalized Nyquist criterion GNC, the system stability is judged.

[0043] Preferably, judging the system stability based on the high-order impedance model and according to the loop gain principle and the generalized Nyquist criterion GNC of the system includes:

[0044] According to the third-order impedance model of the AC / DC interface converter unit in the high-order impedance model and the third-order impedance matrix of the grid-side model, the following formula is obtained:

[0045]

[0046] Where Δv dB and Δv qB represents the voltage of the permanent magnet synchronous generator, Δu d and Δu q is the interface converter port voltage.

[0047] I3 is the third-order identity matrix;

[0048] The system small loop gain is defined as:

[0049]

[0050] Draw a Nyquist curve diagram of the path gain L(s);

[0051] Using the generalized Nyquist criterion GNC, when the Nyquist curve shows that the system surrounds the point (-1, j0), the system is unstable; if the system does not surround the point (-1, j0), the system is stable.

[0052] According to a third aspect of the present invention, there is provided a terminal, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor can be used to execute the method described in the preceding claims when executing the program.

[0053] According to a fourth aspect of the present invention, there is provided a computer-readable storage medium having a computer program stored thereon, which can be used to execute the method when executed by a processor.

[0054] Compared with the prior art, the embodiments of the present invention have at least one of the following beneficial effects:

[0055] The high-order impedance modeling method suitable for distributed DC microgrids implemented in the present invention expands the order of the model on the basis of traditional impedance modeling, and upgrades the traditional first-order impedance model to a third-order matrix form, thereby improving the accuracy of the model and reducing the neglect of system characteristics. In this model, the main diagonal elements of the converter and control impedance matrix respectively characterize the AC characteristics and DC characteristics of the system, while the side diagonal elements reveal the AC coupling characteristics and the AC-DC coupling characteristics, and deeply reveal the coupling relationship between the AC side and the DC side in the DC microgrid.

[0056] At the same time, the grid-side impedance is also modeled and integrated in the same third-order matrix form, which makes the overall system have good segmentation characteristics and makes the modeling of the converter unit and the grid side independent of each other, thereby ensuring that when a disturbance occurs in a system unit, only the impedance model related to the unit is affected, significantly reducing the complexity and cost of model maintenance.

[0057] The system stability analysis method in the implementation of the present invention performs stability analysis on the entire system by applying the generalized Nyquist stability criterion, thereby further verifying the validity of the model and the stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0059] Figure 1A DC microgrid system and its control schematic diagram in a specific embodiment of the present invention;

[0060] Figure 2 A comparison diagram of a theoretical Bode diagram and a simulation verification diagram in a specific embodiment of the present invention;

[0061] Figure 3 A generalized Nyquist curve diagram obtained by stability analysis under selected working conditions in a specific embodiment of the present invention;

[0062] Figure 4 This is a simulation experiment diagram under selected working conditions in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0063] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0064] In one embodiment of the present invention, a high-order impedance modeling method applicable to a distributed DC microgrid is provided, comprising:

[0065] S1, based on the droop control principle and the port power conservation principle, a third-order impedance model of the AC / DC interface converter unit in the DC microgrid is established. The main diagonal elements of the third-order impedance matrix of the third-order impedance model include dq AC impedance and DC impedance, and the secondary diagonal elements include AC coupling terms and AC / DC coupling terms;

[0066] S2, establish the AC grid side impedance model in the DC microgrid;

[0067] S3, establishing a DC grid-side impedance model in a DC microgrid;

[0068] S4, combining the AC grid-side impedance model and the DC grid-side impedance model to obtain a third-order impedance matrix model of the grid side;

[0069] S5, integrating the third-order impedance model of the AC / DC interface converter unit and the third-order impedance matrix model of the grid side to obtain a high-order impedance model of the DC microgrid.

[0070] On the basis of the traditional impedance model, by increasing the model order to the third-order matrix form, the accuracy is improved and the neglect of system characteristics is reduced. In the model, the main diagonal elements of the converter and control impedance matrix represent the AC and DC characteristics respectively, and the side diagonal elements reveal the AC and AC-DC coupling characteristics, and deeply analyze the coupling relationship between the AC side and the DC side in the DC microgrid. At the same time, the grid side impedance is also modeled using a third-order matrix, which realizes the independence of the converter unit and the grid side modeling, ensuring that the system unit disturbance only affects the relevant impedance model, reducing the complexity and cost of model maintenance.

[0071] The order of establishing the third-order impedance model of the AC / DC interface converter unit, the AC grid side impedance model, and the DC grid side impedance model is not limited. Therefore, in some other embodiments, the implementation steps may be S2, S3, S4, S1, S5; or S3, S2, S4, S1, S5.

[0072] In order to further improve the problem of ignoring the q-axis characteristics of reactive power control and deeply reveal the coupling relationship between the AC side and the DC side in the DC microgrid, in a preferred embodiment of the present invention, a high-order impedance modeling is performed on the interface AC / DC interface converter unit and its control in the DC microgrid, mainly through the port power conservation principle and the droop control principle. The main steps are as follows:

[0073] S1.1, considering the input and output power conservation of the ports on both sides of the interface converter, can be expressed as follows:

[0074]

[0075] v dc Indicates the actual measured voltage value on the DC bus side, i dc Indicates the DC side current value, i d and i q They represent the d-axis and q-axis currents on the AC side respectively;

[0076] By performing small signal modeling on the above equation, the following equation can be obtained:

[0077]

[0078] in: Δi dq =[Δi d Δi q ] T ,Δu dq =[Δu d Δu q ] T , Δu dc =[Δu d ,Δu c ];

[0079] Where U do and U qo Represents the AC voltage of the d-axis and q-axis respectively, I do and I qo The d-axis and q-axis AC currents are shown in Figure 2. The superscript “T” indicates the transpose of the matrix, and the subscript “o” indicates the steady-state value. d and i q They represent the d-axis and q-axis currents on the AC side, u d and u q is the port voltage of the interface converter, and “Δ” represents the small signal variable.

[0080] The power conservation formula after small signal modeling is rewritten as follows:

[0081]

[0082] S1.2, from the interface converter control, it can be known that there is the following relationship between the modulation ratio and the controller output voltage value:

[0083]

[0084] Among them, m d and m q They represent the d-axis and q-axis modulation ratios of the controller output respectively. The superscript “*” represents the controller output value, not the actual circuit value. and Respectively represent the dq voltage value output by the controller, U dc is the DC voltage;

[0085] The relationship between the modulation ratio and the controller output voltage value (4) is converted into the following small signal form:

[0086]

[0087] Considering that the controller current inner loop outputs a voltage control signal to generate a modulation signal, and there is a dq-axis decoupling feedforward compensation term inside the current inner loop, it can be written as the following small signal form:

[0088]

[0089] Among them, G in and G de They represent the current inner loop PI controller and feedforward coupling compensation respectively, which are specifically expressed as:

[0090]

[0091] In the formula, k pi and k iiThey represent the proportional term and integral term of the current inner loop PI controller, ω e represents the generator angular velocity, L s Represents the stator inductance of the motor, s is a complex variable, representing the frequency in the frequency domain. In the transfer function of the controller, s is used to describe the response characteristics of the system to input signals of different frequencies.

[0092] S1.3 considers the voltage outer loop PI controller and distributed droop control, which can be expressed as the following small signal form:

[0093]

[0094] In the formula, k po and k io They represent the proportional term and integral term of the voltage outer loop PI controller respectively.

[0095] In order to simplify the control structure and ensure the maximum power factor, the reactive power d-axis current is selected to be 0. Of course, in other implementations, the reactive power can also be selected as any reference value, and the relevant high-order impedance model can be obtained.

[0096] The voltage outer loop output formula is organized into the following matrix form:

[0097]

[0098] The specific form of the matrix is:

[0099]

[0100] S1.4, integrate all the above controller internal formulas (3)(5)(9) to obtain the following formula:

[0101]

[0102] Where I represents the second-order identity matrix diag(1,1).

[0103] S1.5, we can get the expression of some elements of the converter admittance matrix by sorting:

[0104]

[0105] Among them, Y dq It is a 2*2 matrix, Y dqdc It is a 2*1 order matrix, and the two admittance matrices constitute the upper half of the high-order impedance matrix. dq The main diagonal elements of dd、 Y qq , respectively represent the AC d-axis characteristics and q-axis characteristics of the interface converter part, and the side diagonal element Y dq、 Y qdThey represent the dq AC coupling characteristics of the interface converter part respectively.

[0106] S1.6, substitute the above admittance matrix formula (12) back into the converter port power conservation formula (1), and integrate the formulas to obtain the remaining interface converter admittance matrix elements:

[0107]

[0108] Among them, Y dcdq It is a 1*2 matrix, Y dc is a 1*1 matrix. dqdc and Y dcdq Both represent the AC and DC coupling characteristics of the interface converter.

[0109] At this point, the high-order admittance model of the interface converter has been completely established, and after integration, we can get:

[0110]

[0111] Among them, Y ddc , Y qdc , Y dcd , Y dcq Both represent the internal AC and DC coupling characteristics of the interface converter.

[0112] The small signal admittance matrix can show the internal AC and DC characteristics of the converter:

[0113]

[0114] In the above embodiment, the AC side and DC side impedances are integrated and established into a unified third-order matrix impedance model by using the power conservation principle in the microgrid through a small signal impedance modeling method, thereby obtaining an impedance model including AC terms, DC terms, and AC / DC coupling terms. This method expands the order of the impedance model, deeply reveals the coupling relationship between the AC side and the DC side in the DC microgrid, and improves the problem of neglecting certain system characteristics in the traditional AC / DC impedance model during the modeling process.

[0115] In order to make the whole system have good segmentation characteristics, in one embodiment of the present invention, the grid-side impedance is also modeled and integrated in the same third-order matrix form. Specifically, the following steps can be adopted:

[0116] S2, establish the AC grid side impedance model in the DC microgrid;

[0117] The permanent magnet synchronous generator PMSG is selected as the power supply side. The electrical small signal model of this type of motor can be expressed as:

[0118]

[0119] Among them, R s represents the stator resistance of the permanent magnet synchronous generator. Considering the surface mounted PMSG (s-PMSG), the quadrature axis and direct axis inductance are the same, and L d =L q =L s ,ω e is the motor electrical angular velocity.

[0120] Write the above formula in matrix form:

[0121]

[0122] The impedance in the formula is specifically expressed as:

[0123]

[0124] At the same time, considering the existence of line resistance on the AC grid side, the impedance on the AC grid side can be written as:

[0125]

[0126] Among them, R ac =R c +R s ,L ac =L c +L s Where R c and L c Represents line resistance and inductance, R s and L s They represent the stator resistance and inductance of the permanent magnet synchronous generator respectively. Furthermore, if we consider that the line resistance is smaller than the stator resistance and inductance of the generator and can be ignored, we can assume that

[0127] S3, establishing a DC grid-side impedance model in a DC microgrid;

[0128] For the DC bus capacitor and DC load, consider a typical constant power load (CPL), which can represent many loads in a DC microgrid, such as batteries and other energy storage units that need to be connected to a DC / DC converter for bidirectional power flow. The small signal model of the constant power load can be expressed as:

[0129]

[0130] Considering the local voltage stabilization filter capacitor of the distributed generation unit and the total DC bus capacitance, and considering the line impedance from the distributed power supply output to the DC bus, the small signal impedance on the DC side can be expressed as:

[0131]

[0132] The subscript "x" represents different distributed generation unit numbers, R c and L c Indicates the DC line resistance, P L Indicates the power of the constant power load, C bus Indicates the busbar capacitance, C x Represents the local capacitance of the distributed generation unit, V bus Indicates the DC bus voltage.

[0133] S4, combining the AC grid-side impedance model and the DC grid-side impedance model to obtain a third-order impedance matrix model of the grid side.

[0134]

[0135] The AC grid-side impedance model is 2*2 order, the DC grid-side impedance model is 1 order, and the combined grid-side impedance is 3*3, that is, the grid-side impedance is also modeled and integrated in the same third-order matrix form, so that the overall has good segmentation characteristics, and the converter unit and the grid-side modeling are independent of each other, thereby ensuring that when a system unit is disturbed, only the impedance model related to the unit is affected, significantly reducing the complexity and cost of model maintenance.

[0136] Continuing from the above embodiment, in a preferred embodiment, S4 integrates the third-order impedance model of the AC / DC interface converter unit and the third-order impedance matrix model of the grid-side model to obtain a high-order impedance model of the DC microgrid.

[0137] Through the interface converter admittance matrix and grid-side impedance obtained above, the following formula can be obtained:

[0138]

[0139] Where Δv dB and Δv qB represents the voltage of the permanent magnet synchronous generator, Δu d and Δu q is the port voltage of the interface converter. I3 is the third-order unit matrix.

[0140] Based on the same inventive concept, in another embodiment of the present invention, a system stability analysis method is provided, and its main steps are as follows:

[0141] Step 1, obtaining a high-order impedance model of a DC microgrid according to the high-order impedance modeling method applicable to a distributed DC microgrid in the above embodiment;

[0142] Step 2: Based on the high-order impedance model and in accordance with the loop gain principle of the system and the generalized Nyquist criterion GNC, determine the system stability.

[0143] Further, in a preferred embodiment, the system stability is judged according to the loop gain principle of the system and the generalized Nyquist criterion GNC, and the following steps can be specifically adopted:

[0144] Step 2.1, according to the third-order impedance model of the AC / DC interface converter unit in the high-order impedance model and the third-order impedance matrix of the grid-side model, obtain the following formula:

[0145]

[0146] Where Δv dB and Δv qB represents the voltage of the permanent magnet synchronous generator, Δu d and Δu q is the interface converter port voltage.

[0147] I3 is the third-order identity matrix;

[0148] Step 2.2, define the system small loop gain as:

[0149]

[0150] Step 2.3, drawing a Nyquist curve diagram of the path gain L(s);

[0151] Step 2.4, using the generalized Nyquist criterion GNC, when the Nyquist curve shows that the system surrounds the point (-1, j0), the system is unstable; if the system does not surround the point (-1, j0), the system is stable.

[0152] In order to verify the feasibility and effectiveness of the method in the above embodiment, a simulation experiment is carried out in a specific embodiment of the present invention. Specifically, the system is powered by a permanent magnet synchronous generator (PMSG), and its structure is as follows: Figure 1 The control loop adopts droop control, and the entire DC microgrid system includes a generator, an AC / DC power converter, a DC bus, and a constant power load.

[0153] The simulation parameters are shown in Table 1.

[0154] Table 1 System parameters used in simulation

[0155]

[0156] In order to verify the accuracy of the proposed high-order impedance model (because the grid-side impedances are all definite system parameters and do not need to be verified. Therefore, the essence of the high-order impedance model here refers to the AC / DC interface converter unit model), a simulation sweep verification was carried out. Select multiple different disturbance frequency points, and inject three independent voltage disturbances into the AC / DC power converter port at each frequency point: positive-sequence disturbance on the AC side, negative-sequence disturbance on the AC side, and sinusoidal disturbance on the DC side. Under each set of disturbances, the voltage and current data of the system on the AC d-axis, q-axis and DC side are recorded respectively. A total of nine sets of voltage and nine sets of current data are obtained at each disturbance frequency point. Subsequently, the amplitude and phase angle corresponding to each frequency point are obtained through admittance calculation. The multiple sets of amplitudes and phase angles obtained are plotted on the Bode diagram and compared with the theoretical values ​​of the high-order impedance model proposed in the embodiment of the present invention, as shown in FIG. Figure 2 The system parameters used in the simulation verification are shown in Table 1, and the constant power load is selected as 10kW.

[0157] After verifying the correctness of the obtained high-order impedance model, the impedance model of the AC grid side and the DC bus side and the converter admittance matrix model were further combined to perform system loop gain analysis to evaluate the stability of the overall system. The DC microgrid system under the above parameters was simulated using the generalized Nyquist criterion to verify the consistency and accuracy of the model and simulation results. Figure 3 As shown in the figure. In the simulation, a constant power load is selected starting from 20kW and gradually increasing to 50kW at a rate of 10kW every 0.5s. Under the three operating conditions of 20kW to 40kW, the DC bus voltage remains stable. Due to the "virtual impedance" effect in droop control, as the DC voltage in the system gradually increases, the DC bus voltage drops slightly.

[0158] In the generalized Nyquist plot, the system curves under the three working conditions of 20kW-40kW do not surround the point (-1, j0), indicating that the system is still in a stable state. However, as the constant power load increases, the system gradually approaches the unstable area. When the load reaches 50kW, the generalized Nyquist curve surrounds the point (-1, j0), indicating that the system enters an unstable state under this condition. The corresponding simulation results are shown in Figure 2. Figure 4 As shown, the system oscillates under a load of 50 kW and loses stability. The simulation results are consistent with the theoretical results, further proving the accuracy and effectiveness of the high-order impedance modeling method in the above embodiment.

[0159] It can be seen from this embodiment that the high-order impedance modeling method for DC microgrids proposed in the embodiment of the present invention significantly improves the accuracy of the model by expanding the order of system impedance, avoiding the neglect of some system characteristics during the simplification process. In addition, the stability analysis of the high-order impedance model of the system is carried out using the generalized Nyquist criterion, and the theoretical analysis results are highly consistent with the simulation verification, which fully proves the accuracy and effectiveness of the modeling method.

[0160] Based on the same inventive concept, in other embodiments of the present invention, a terminal is provided, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor can be used to execute the above-mentioned method or to run the above-mentioned system when executing the program.

[0161] Optionally, the memory is used to store programs; the memory may include volatile memory (English: volatile memory), such as random-access memory (English: random-access memory, abbreviated: RAM), such as static random-access memory (English: static random-access memory, abbreviated: SRAM), double data rate synchronous dynamic random access memory (English: Double Data Rate Synchronous Dynamic Random Access Memory, abbreviated: DDR SDRAM), etc.; the memory may also include non-volatile memory (English: non-volatile memory), such as flash memory (English: flash memory). The memory is used to store computer programs (such as applications, functional modules, etc. that implement the above method), computer instructions, etc., and the above-mentioned computer programs, computer instructions, etc. can be partitioned and stored in one or more memories.

[0162] The processor is used to execute the computer program stored in the memory to implement the various steps in the method involved in the above embodiment. For details, please refer to the relevant description in the above method embodiment.

[0163] The processor and the memory may be independent structures or integrated structures. When the processor and the memory are independent structures, the memory and the processor may be coupled and connected via a bus.

[0164] Based on the same inventive concept, in other embodiments of the present invention, a computer-readable storage medium is provided, on which a computer program is stored. When the program is executed by a processor, it can be used to execute the above method, or run the above system.

[0165] Among them, computer-readable media include computer storage media and communication media, wherein the communication media include any media that facilitates the transmission of computer programs from one place to another. The storage medium can be any available medium that can be accessed by a general or special-purpose computer. An exemplary storage medium is coupled to the processor so that the processor can read information from the storage medium and write information to the storage medium. Of course, the storage medium can also be an integral part of the processor. The processor and the storage medium can be located in an ASIC. In addition, the ASIC can be located in a user device. Of course, the processor and the storage medium can also be present in a communication device as discrete components.

[0166] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented in one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that include computer-usable program code.

[0167] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0168] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0169] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0170] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A high-order impedance modeling method suitable for distributed DC microgrids, characterized in that: include: Based on the droop control principle and the port power conservation principle, a third-order impedance model of the AC / DC interface converter unit in the DC microgrid is established. The main diagonal elements of the third-order impedance matrix of the third-order impedance model include dq AC impedance and DC impedance, and the secondary diagonal elements include AC coupling terms and AC / DC coupling terms. Establish the AC grid side impedance model in the DC microgrid; Establish the DC grid side impedance model in DC microgrid; Combining the AC grid-side impedance model and the DC grid-side impedance model to obtain a third-order impedance matrix model of the grid side; The third-order impedance model of the AC / DC interface converter unit and the third-order impedance matrix model of the grid side are integrated to obtain a high-order impedance model of the DC microgrid.

2. A high-order impedance modeling method suitable for distributed DC microgrid according to claim 1, characterized in that: The third-order impedance model of the AC / DC interface converter unit in the DC microgrid is established based on the port power conservation principle and the droop control principle, including: The droop control principle is: Among them, v dc represents the actual measured voltage value on the DC bus side, k represents the droop gain, and the superscript "ref" represents the reference value in the controller. and Respectively represent the reference values ​​of DC current and voltage; The port power conservation principle is: Among them, i dc Indicates the DC side current value, v d and v q Respectively represent the d-axis and q-axis voltages on the AC side, i d and i q They represent the d-axis and q-axis currents on the AC side respectively; The final form of the high-order impedance matrix of the AC / DC interface converter unit obtained by high-order impedance modeling is: Where Y represents the system admittance, and the bold font represents the matrix form; Y dq It is a 2*2 matrix, Y dqdc It is a 2*1 order matrix, and the two admittance matrices constitute the upper half of the high-order impedance matrix; Y dq The main diagonal elements of dd and Y qq , respectively represent the AC d-axis characteristics and q-axis characteristics of the AC / DC interface converter unit, and the diagonal element Y dq and Y qd Respectively represent the dq AC coupling characteristics of the AC / DC interface converter unit; Y dcdq It is a 1*2 matrix, Y dc is a 1*1 matrix, Y dqdc and Y dcdq Both represent the AC and DC coupling characteristics of the AC / DC interface converter unit.

3. A high-order impedance modeling method suitable for distributed DC microgrid according to claim 1, characterized in that: The step of establishing an AC grid-side impedance model in a DC microgrid includes: Analyze the status of the AC grid side: a permanent magnet synchronous generator is used as the power source, and there is line impedance when the AC power is output to the AC / DC interface converter unit; Based on the AC grid side state, the AC grid side impedance model is established as: is the line impedance, is the generator stator impedance, ω e is the motor electrical angular velocity, R ac =R c +R s ,L ac =L c +L s； R c and L c Represents line resistance and inductance respectively, R s and L s They represent the stator resistance and inductance of the permanent magnet synchronous generator respectively.

4. A high-order impedance modeling method suitable for distributed DC microgrid according to claim 3, characterized in that: The step of establishing a DC grid-side impedance model in a DC microgrid includes: Analyze the DC grid side status: the DC grid side impedance includes the local capacitance of the distributed generation unit, the line impedance, the high-voltage DC bus capacitance and the load; the load includes the constant power load CPL; Based on the DC grid side state, a first-order DC grid side impedance model is established as: Among them, Z CPL Represents the power load impedance, Z cable Represents the line impedance, C x Represents the local capacitance of the distributed generation unit, the subscript "x" represents the number of different distributed generation units, R c and L c Indicates the DC line resistance, P L Indicates the power of the constant power load, C bus Indicates the busbar capacitance, C x Represents the local capacitance of the distributed generation unit, V bus Indicates the DC bus voltage.

5. A high-order impedance modeling method suitable for distributed DC microgrid according to claim 4, characterized in that: The combining the AC grid-side impedance model and the DC grid-side impedance model to obtain a grid-side third-order impedance matrix model includes:

6. The high-order impedance modeling method applicable to a distributed DC microgrid according to claim 5, characterized in that: The third-order impedance model of the AC / DC interface converter unit is integrated with the third-order impedance matrix model of the grid side to obtain a high-order impedance model of the DC microgrid, specifically: Combine the AC / DC interface converter unit model and the grid-side impedance model in the DC microgrid to obtain: Among them, I3 represents the third-order identity matrix, v dB and Δv qB represents the voltage of the permanent magnet synchronous generator, Δu d and Δu q is the voltage at the interface converter port; Δu dc =[Δu d ,Δu q ].

7. A system stability analysis method, characterized in that: include: A high-order impedance model of a DC microgrid obtained by the high-order impedance modeling method applicable to a distributed DC microgrid according to any one of claims 1 to 6; Based on the high-order impedance model, and according to the loop gain principle of the system and the generalized Nyquist criterion GNC, the system stability is judged.

8. A system stability analysis method according to claim 7, characterized in that: The method of judging the stability of the system based on the high-order impedance model and according to the loop gain principle of the system and the generalized Nyquist criterion GNC includes: According to the third-order impedance model of the AC / DC interface converter unit in the high-order impedance model and the third-order impedance matrix on the grid side, the following formula is obtained: Where Δv dB and Δv qB represents the voltage of the permanent magnet synchronous generator, Δu d and Δu q is the port voltage of the interface converter; I3 is the third-order unit matrix; The system small loop gain is defined as: Draw a Nyquist curve diagram of the path gain L(s); Using the generalized Nyquist criterion GNC, when the Nyquist curve shows that the system surrounds the point (-1, j0), the system is unstable; if the system does not surround the point (-1, j0), the system is stable.

9. A terminal comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the program, it can be used to perform the method according to any one of claims 1 to 8.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, it can be used to execute the method according to any one of claims 1 to 8.

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