Impedance modeling method and impedance participation factor analysis method suitable for direct current micro-grid comprising communication network

Abstract

The invention discloses a system overall impedance modeling method and an impedance participation factor analysis method suitable for a direct current micro-grid comprising a communication network. The method comprises the following steps: dividing a system into a line side and an equipment side; adopting small signal modeling or impedance measurement to obtain impedance / admittance matrixes of ports on two sides, and constructing a system overall impedance model; calculating closed-loop system poles to evaluate stability; by disturbing equipment control parameters and combining the port impedance change rate and the whole system admittance matrix, participation factors of the equipment parameters on closed-loop poles are calculated. According to the invention, the system does not need to be divided into a source subsystem and a load subsystem, and the application range of the impedance modeling and participation factor analysis method is expanded to a micro-grid system comprising a communication network and electrical connection from mutually independent subsystems with interconnected electrical ports. And the participation factors of the internal parameters of the equipment for the closed-loop poles of the system can be obtained by using the integral admittance matrix of the system.

Description

Technical Field

[0001] The present invention belongs to the field of power system analysis, and particularly relates to an impedance modeling method and an impedance participation factor analysis method applicable to a DC microgrid including a communication network. Background Art

[0002] With the development of renewable energy and power electronics technology, a new energy grid system characterized by a high proportion of new energy determines that power electronic equipment will be widely used in the power system, and the power system tends to be power electronic. In a power system dominated by traditional synchronous generators, the huge rotor part of the synchronous generator plays a significant role. When the power system is subjected to interference, the rotor absorbs / releases energy to maintain the energy balance inside the system. However, with the high degree of power electronics in the power system, the network structure and characteristics of the power system have changed greatly, seriously threatening the safe and stable operation of the new generation of power systems. When power electronic equipment largely replaces electromechanical energy conversion equipment represented by synchronous motors, the power system has undergone a qualitative change at the physical level, manifested as a reduction in the physical entity of the rotor, resulting in a significant decrease in the stored kinetic energy and a deterioration in the system's anti-disturbance ability. Therefore, the stability analysis and instability principle analysis of the new power system are of great significance for the safe and stable operation of the power system.

[0003] A DC microgrid is a small power system networking form using a DC bus, which can be used for the efficient utilization of distributed power sources. It generally includes DC distributed power sources such as photovoltaic panels, fuel cells, and energy storage devices, as well as DC loads such as electric vehicles. The DC microgrid can supply power to local loads independently, that is, operate in island mode; it can also operate in grid-connected mode and exchange energy with the grid. The DC microgrid can flexibly switch between the two modes and has no problems of reactive power loss and phase mismatch. Compared with traditional large power grids, the DC microgrid is more suitable for the access and local consumption of distributed new energy. In addition, in recent years, DC microgrids have also been widely used in multi-electric aircraft and ship power systems.

[0004] A DC microgrid can not only operate in parallel with the external power grid, but also enter the island mode and use distributed power sources to support local loads. The island microgrid generally adopts a hierarchical control structure: the primary control usually uses a peer-to-peer control mode based on the droop control method, introducing a virtual impedance to achieve the parallel operation of distributed power sources. However, due to the differences in line impedance and the fact that the virtual impedance will cause the output voltage to be lower than the reference value, the current distribution is inaccurate and the bus voltage is low. Therefore, it is necessary to introduce secondary control to compensate the reference value of the primary control. At present, there are mainly two implementation forms of secondary control: centralized and distributed. In the centralized secondary control, the central controller uniformly collects and processes information and issues instructions through the communication network. Therefore, the stable operation of the system depends on the reliability of the central controller and the communication line. In the distributed control, the controllers of adjacent distributed power sources exchange information through the communication link, estimate the global information of the system through a certain iterative algorithm (such as the consensus algorithm), and then adjust the system indexes. In the distributed control, each unit exchanges information through a distributed communication network, so it can avoid the influence of single-point failures and has high reliability; at the same time, each unit can estimate the global information through distributed communication, so it can achieve the global control goal. Therefore, at present, the distributed control has become the main implementation way of the secondary control of the DC microgrid.

[0005] The participation factor theory is a theory based on the small-signal model of the system, which describes the quantitative relationship between system parameters and the closed-loop poles characterizing system stability. Based on the analysis results of the participation factor, the optimization of equipment parameters can be realized and the stability of the system can be improved. Most of the traditional participation factor theories are based on two types: the state space model and the nodal admittance matrix of the system. Among them, the exact relationship between each variable and the closed-loop poles of the system can be obtained by using the state space model. However, for large-scale power systems, a detailed state space model cannot be obtained, and it is not widely used in practical engineering; the participation factors of each node device in the system without a communication network can be obtained by using the nodal admittance matrix, but it is difficult to be applied to the DC microgrid system containing a communication network. In summary, there are still certain gaps in the impedance modeling and participation factor theory for the system containing a communication network. Summary of the Invention

[0006] The object of the present invention is to address the deficiencies of the above-mentioned existing technologies and propose an impedance modeling and participation factor analysis method applicable to a DC microgrid including a communication network. The original impedance model is improved: the traditional impedance analysis method is only applicable to a system with only electrical connections and no communication network between devices. The impedance modeling method proposed by the present invention establishes the impedance of the device side and the electrical line side respectively, calculates the overall admittance matrix of the system and the closed-loop poles of the system, and then determines the stability of the system. Based on the overall admittance matrix of the system and the key oscillation modes of the system, the participation factors of each control parameter corresponding to this oscillation mode are calculated. This method globally models the impedance of the device side including the communication network, where the diagonal elements represent the impedance of a single device itself, and the off-diagonal elements represent the mutual influence brought by the communication network between different devices, thus solving the problem of difficult impedance establishment for a system including a communication network. In addition to the impedance of the device side, the impedance of the electrical line network is established, thus realizing a unified theory for impedance modeling and participation factor calculation of a DC microgrid system including a communication network. Compared with the traditional impedance analysis method, the present invention is applicable to a DC microgrid system including a communication network.

[0007] The technical solution of the present invention is as follows:

[0008] An impedance modeling and participation factor analysis method applicable to a DC microgrid including a communication network, the method comprising the following steps:

[0009] S1, divide the DC microgrid system including a communication network into two parts, one part is the line side, including the electrical line network between each converter; the other part is the device side, including all distributed power sources and their local controllers interconnected through the communication network;

[0010] S2, use the small-signal modeling or impedance measurement method to obtain the port impedance / admittance matrix of the device side and the line side respectively, and calculate the overall impedance model of the system based on the impedance / admittance matrix of the device side and the line side.

[0011] S3, based on the overall impedance model of the system, calculate the closed-loop poles of the system and judge the system stability.

[0012] S4, under the condition of system stability, apply a perturbation to a certain parameter of a certain device, and calculate the participation factor of this parameter in a certain oscillation mode in combination with the overall impedance model of the system.

[0013] The specific steps are as follows:

[0014] Divide the system into the device side and the electrical line side, and derive the impedance / admittance models of each sub-part through system small-signal modeling, that is, Y ap and Z net . For the device network, a small voltage perturbation signal can also be injected, that is, and measure the current disturbance signals of each port to obtain the device admittance matrix. On the other hand, the electrical network impedance matrix can be defined as Z net (s), which contains the specific information of the electrical network topology, lines and loads. The diagonal elements of Y ap (s) represent the admittance information of each converter itself, and the non-zero off-diagonal elements represent the mutual influence between different converters brought by the communication network. If there is no communication network in the system, then Y ap (s) is a diagonal matrix. The above definitions contain the physical meaning of the multi-port impedance / admittance modeling method used in this chapter.

[0015] Based on the device admittance matrix and the line impedance matrix, the overall system admittance matrix can be obtained as follows:

[0016] Y sys = Y ap (I + Z net Y ap ) -1

[0017] At the same time, due to the duality of impedance and admittance, Z net and Y ap can be expressed by Y net and Z ap respectively as follows:

[0018] Z ap = Y ap -1 , Z net = Y net -1

[0019] Therefore, the overall system admittance matrix can also be expressed as:

[0020] Y sys = (I + Y net Z ap ) -1 Y net

[0021] For the proposed overall system impedance modeling method including a communication network, the system stability can be judged by the poles of the system impedance or admittance matrix. The specific system stability criterion is as follows:

[0022] Assume that each distributed power source is stable when working independently. On this premise, when and only when all the poles of Y sys are located in the left half plane, that is, when the zeros of det(I + Y net Z ap ) are located in the left half plane, the studied system is stable.

[0023] The overall system admittance matrix Y sys is a multiple input multiple output (MIMO) system. Each diagonal element in the matrix describes the closed-loop dynamic characteristics of the corresponding node. Specifically, the overall system admittance matrix Y sys can be expanded as follows:

[0024]

[0025] where all elements in Y sys have the same poles, so any of them can be used to judge the stability of the entire system.

[0026] For the overall system admittance matrix Y sys , the participation factor of a certain parameter ρ with respect to a certain closed-loop pole λ is shown as follows:

[0027]

[0028] where,

[0029]

[0030] where <,> represents the Frobenius inner product of two matrices, and Res λ G ρ denotes the residue matrix of G ρ at the pole λ (taking the residue of each element in the matrix), and the superscript * represents the conjugate transpose of the residue matrix.

[0031] Considering the influence of the parameter ρ on the device network matrix, the partial derivative of the device impedance matrix with respect to a certain parameter is shown as follows:

[0032]

[0033] Similarly, considering the change of the line network parameter L, the corresponding impedance participation factor for Z net can be calculated as follows:

[0034]

[0035] where,

[0036]

[0037] This participation factor accurately reflects the influence of the change rate of the parameter on the system closed-loop pole λ. Using this method can locate the key parameters affecting the system stability and guide how to optimize and adjust the parameters. To illustrate the differences between the present invention and the traditional state-space method and the traditional impedance method, Figure 1Summarize the calculation methods of different types of participation factors, including the calculation of traditional state - space participation factors, the calculation of traditional impedance participation factors, and the calculation of the overall impedance participation factors of the system including the communication network.

[0038] The analysis method proposed by the present invention is based on the impedance mathematical model of a DC micro - grid including a communication network. When using the impedance participation factor analysis method, first, it is necessary to obtain the closed - loop mathematical model of the whole system, so as to determine the stability and stability margin of the system according to the closed - loop poles of the system. When there are closed - loop poles in the right - half plane or closed - loop poles close to the imaginary axis in the system, these closed - loop poles are identified as the analysis objects of the participation factor theory. Secondly, after clarifying the studied closed - loop poles, it is necessary to further adjust the control parameters, operating parameters, circuit parameters, etc. of the equipment to improve the stability of the system. By obtaining the change value of the impedance at the equipment port after parameter perturbation, the participation factor of the parameter with respect to the closed - loop pole is accurately calculated. If the real part of the participation factor is positive, it means that increasing this parameter will move the closed - loop pole of the system to the right - half plane, making the system stability worse. On the contrary, if the real part of the participation factor is negative, it means that reducing this parameter will improve the stability of the system. In summary, the impedance participation factor analysis theory proposed by the present invention can provide a quantitative analysis tool for optimizing the stability of a DC micro - grid system including a communication network. Brief Description of the Drawings

[0039] Figure 1 It is a summary block diagram of the characteristic - root participation factor analysis method under system - parameter changes;

[0040] Figure 2 It is the topology of a DC micro - grid system including a communication network;

[0041] Figure 3 They are different typical oscillation modes of the DC micro - grid;

[0042] Figure 4 It is the analysis result of the impedance participation factor of the dominant mode of the two - layer control of the DC micro - grid;

[0043] Figure 5 It is the time - domain waveform diagram of the output current of each converter under the load step change;

[0044] Figure 6 It is the flow chart of the impedance modeling method and the impedance participation factor analysis method applicable to the DC micro - grid including a communication network in the present invention. Detailed Embodiments

[0045] To make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be described in more detail below with reference to the accompanying drawings and embodiments. It must be noted that the embodiments of the present invention described below are only used to further explain the present invention and are not used to limit the present invention. Based on the embodiments of the present invention, any other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0046] Meanwhile, in the following description, for the purpose of explanation, many specific details are set forth to provide a thorough understanding of the embodiments of the present invention. However, for those skilled in the art, the present invention may be implemented without these specific details or in the specific manners described herein.

[0047] An impedance modeling method applicable to a DC microgrid including a communication network, as Figure 6 shown, includes:

[0048] S1. System segmentation: Divide the DC microgrid system including the communication network into a line side (electrical line network) and a device side (distributed power sources and their controllers interconnected through the communication network), so as to separately consider the effects of electrical connections and the communication network on the system impedance.

[0049] S2. Establishment of impedance / admittance matrix:

[0050] Device side: Use small-signal modeling or impedance measurement methods to obtain the device admittance matrix (Y ap ). The diagonal elements of this matrix represent the admittances of the respective devices, and the non-diagonal elements represent the mutual influences between devices caused by the communication network.

[0051] Line side: Define the electrical network impedance matrix (Z net ), which contains the topology, lines and load information of the electrical network.

[0052] S3. Establishment of the overall system admittance matrix:

[0053] Combine the device admittance matrix and the line impedance matrix to calculate the overall system admittance matrix (Y sys ).

[0054] An impedance participation factor analysis method applicable to a DC microgrid including a communication network, includes:

[0055] S11. System stability judgment:

[0056] Judge the stability of the system by analyzing the poles of the overall system admittance matrix. The system is considered stable if and only if all poles are located in the left half of the complex plane.

[0057] S12. Calculation of participation factors:

[0058] Under the condition of system stability, a small perturbation is applied to the control parameters of a certain device. Combining with the overall impedance model of the system, the participation factor of this parameter in a certain oscillation mode is calculated.

[0059] The DC microgrid system considered in this embodiment is as Figure 2 shown, including six distributed power sources and corresponding local loads. The distributed power sources are interconnected through an electrical network and a communication network. Based on the overall impedance model of the DC microgrid system including the communication network, the influencing factors of a certain oscillation mode of the system can be analyzed through the numerical calculation of the participation factor. When it is observed that the system has underdamping or oscillation phenomena at a certain frequency, the analysis method proposed in this section is used to optimize the system stability. For the studied DC microgrid system including the communication network, mathematical modeling or frequency sweep measurement methods are used to obtain the device impedance matrix and line admittance matrix of the system. Calculate the closed-loop poles of the overall admittance matrix of the system, and pay special attention to the poles whose imaginary parts are near the oscillation frequency of the system. Conduct impedance participation factor analysis on the corresponding poles, calculate the participation factors of each control parameter for this eigenvalue, so as to judge the influence of each control parameter on the change trend of the eigenvalue, and provide guidance for the system parameter design. By adjusting the corresponding control parameters, the position of the system poles is changed, thereby increasing the damping under a certain oscillation mode of the system and improving the system stability.

[0060] Mathematical modeling is carried out on a 6-node DC microgrid system including a communication network, and the overall admittance model of the system is calculated according to the device impedance matrix and line admittance matrix of the system to obtain the pole distribution of the system. Among them, the poles relatively close to the imaginary axis have a greater impact on the stability of the system. Select three representative clusters of poles for participation factor calculation. According to the relative magnitudes of the participation factors, the oscillation modes can be divided into those dominated by first-layer control, second-layer control, and communication algorithms, as Figure 3 shown. Taking the oscillation mode dominated by second-layer control as an example, Figure 4 shows the participation factors of specific parameters in the 85Hz oscillation mode. Among them, f primary_v represents the first-layer voltage control bandwidth, T com represents the communication delay in the communication network, ε represents the step size of the communication algorithm, f secondary_v and f secondary_i represent the bandwidths of the voltage and current controllers in the second-layer control respectively. According to the calculation results of the impedance participation factor, the influence of the change of control parameters on the movement trend of the system eigenvalues can be predicted. As Figure 4 shown, the real part of the impedance participation factor of f secondary_i is positive, indicating that increasing f secondary_i will cause the closed-loop pole (85Hz) corresponding to the concerned oscillation mode to move to the right, thereby reducing the system stability. At this time, the second-layer current control bandwidth should be reduced; in addition, fsecondary_i The imaginary part of the impedance participation factor is positive, indicating that increasing f secondary_i will cause the closed-loop poles (85 Hz) corresponding to the concerned oscillation mode to move upward, and the oscillation frequency increases.

[0061] In a DC microgrid containing a communication network, to verify the effectiveness of the impedance participation factor calculation results, corresponding time-domain verification was carried out. The distributed two-layer control was enabled at 0.4 s, and the system was loaded and unloaded at 1 s and 1.5 s respectively. The output currents of each converter are as Figure 5 shown. Figure 5 (a) is the output current waveform under the initial system parameters. There is an 85-Hz oscillation in the system during the transient process. It can be seen from the time-domain simulation that the system oscillation frequency is consistent with the frequency where the poles of the overall system impedance model ( Figure 4 ) are located. According to the impedance participation factor calculation results, Figure 5 (b) and (c) are the time-domain waveforms after adjusting the two-layer control current loop bandwidth in the forward and reverse directions respectively. It can be seen from Figure 5 (b) that according to the impedance participation factor calculation results, after reducing the two-layer current control bandwidth from 110 Hz to 80 Hz, the system oscillation weakens, the stability is improved, and the oscillation frequency decreases to 72 Hz, verifying the Figure 4 calculation results. On the other hand, as shown in Figure 5 (c), when the two-layer current control bandwidth is continuously increased to 115 Hz, the system oscillation intensifies, and the oscillation frequency increases to 95.4 Hz, and the damping effect of the system becomes worse. In summary, this embodiment demonstrates the usage method of the theory involved in the present invention, and the obtained results verify the correctness of the theory.

[0062] Although the content of the present invention has been introduced in detail through the above embodiments, it should be recognized that the above description should not be considered as a limitation of the present invention. After those skilled in the art read the above content, various modifications and substitutions to the present invention will be obvious. Therefore, the protection scope of the present invention should be defined by the appended claims.

Claims

1. An impedance modeling method for a DC microgrid including a communication network, characterized in that: The method comprises the following steps: Step 1. Divide the DC microgrid system including the communication network into two parts: the line side and the device side, wherein the line side includes the electrical line network between each converter, and the device side includes all distributed power sources and their local controllers interconnected by the communication link; Step 2. Use small signal modeling or impedance measurement methods to obtain the device side admittance matrix Y ap , Equipment side impedance matrix Y net , line side admittance matrix Z ap , line side impedance matrix Z net ; Step 3. Based on the device side and line side admittance / impedance matrices, calculate the system overall admittance matrix Y sys , the formula is as follows: Y sys = Y ap (I + Z net Y ap ) -1 or Y sys = (I + Y net Z ap ) -1 Y net .

2. An impedance participation factor analysis method for a DC microgrid including a communication network, characterized in that: The method comprises the following steps: Step 11. Using the system overall impedance modeling method as claimed in claim 1, obtain an overall impedance model of the DC microgrid system; Step 12: Analyze the poles of the overall admittance matrix of the system to determine the stability of the system; Step 13. When the system is stable, a disturbance is applied to the control parameters of a certain device, and the participation factor of the corresponding parameter of the device for a certain closed-loop pole is calculated by combining the port impedance change rate and the admittance matrix of the whole system.

3. The impedance participation factor analysis method for a DC microgrid including a communication network according to claim 1, characterized in that: The overall admittance matrix Y of the system in step 12. sys , expanded as follows: Among them, Y sys All elements in have the same extreme points, that is, any element can be used to judge the stability of the entire system. ij represents the admittance from node j to node i, that is, the mutual influence between devices caused by the communication network; the diagonal element Y ii It represents the self-admittance of the corresponding node, that is, the interaction between the node and other nodes in the network and its own dynamic characteristics.

4. The impedance participation factor analysis method for a DC microgrid including a communication network according to claim 1, characterized in that: The participation factors in step 13. are: For the system overall admittance matrix Y sys , the participation factor of a certain parameter ρ to a certain closed-loop pole λ is as follows: in, Among them, <,> represents the Frobenius inner product of two matrices, Res λ G ρ Represents G ρ The residue matrix at the extreme point λ (residues are taken for each element in the matrix), superscript * represents the conjugate transpose of the residue matrix.

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