A method for determining the stability of a multi-voltage-level DC distribution system containing multiple DC microgrids

Abstract

A method for determining the stability of a multi-voltage-level DC distribution system containing multiple DC microgrids is disclosed. The method first confirms that the equivalent admittance of each microgrid subsystem has no right-half-plane poles based on the specific control mode of the interconnecting converter and the equivalent open-loop gain of each microgrid operating independently. Second, based on the two-port network matrix of the interconnecting converter, the multi-voltage-level DC distribution system containing multiple DC microgrids is simplified to a single-bus DC system containing only the distribution bus. The stability of the entire system is then determined using the equivalent impedance ratio. Under the premise that each unit within the system can operate stably independently, the stability of the entire system is guaranteed when the equivalent open-loop gain of each microgrid operating independently and the equivalent impedance ratio on the distribution bus side both satisfy the Nyquist criterion. This invention utilizes the equivalent open-loop gain of each DC microgrid operating independently and the equivalent impedance ratio of the system on the distribution bus side to analyze the stability of a multi-voltage-level DC distribution system containing multiple DC microgrids. It has good flexibility and applicability, and can provide a reliable theoretical basis for the design of multi-voltage-level DC distribution systems.

Description

Technical Field

[0001] This invention relates to a method for determining the stability of a multi-voltage-level DC power distribution system, belonging to the field of power transmission and distribution technology. Background Technology

[0002] With the development of new energy power generation technologies, the proportion of distributed renewable energy (such as photovoltaics and wind power) and DC loads (such as communication equipment and electric vehicle charging piles) is growing rapidly. It is estimated that by 2050, the proportion of new energy power generation will reach about 60%. Compared with traditional AC distribution networks, DC distribution networks, with their advantages of strong controllability, high power supply reliability, high power quality, and large power supply capacity, will become an effective means of accommodating large-scale distributed energy and loads with DC characteristics.

[0003] As the penetration rate of distributed energy in microgrids increases, the equilibrium of renewable energy consumption within a single DC microgrid is easily disrupted, making it possible for multiple DC microgrids to coexist in a DC distribution network. While networking multiple DC microgrids under a DC distribution network can solve the problem of local consumption of intermittent renewable energy within each microgrid to the greatest extent, the high proportion of power electronic equipment and renewable energy in the system makes the stability problem of the DC distribution network increasingly prominent.

[0004] For DC distribution systems with multiple voltage levels, existing stability analysis methods can be used to determine the stability of the system. However, for DC distribution systems with multiple DC microgrids or multiple microgrid clusters, existing analysis methods have problems such as large computational load, narrow application scope, and difficulty in implementation. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for determining the stability of a multi-voltage-level DC power distribution system containing multiple DC microgrids. This method features reliable determination results, high flexibility, and wide applicability, and can provide a basis for parameter design of each unit controller in the system.

[0006] The problem described in this invention is solved by the following technical solution:

[0007] A method for determining the stability of a multi-voltage-level DC distribution system containing multiple DC microgrids is provided. The method first confirms that the equivalent admittance of each microgrid subsystem has no right-half-plane poles based on the specific control mode of the interconnecting converter and the equivalent open-loop gain of each microgrid operating independently. Second, based on the two-port network matrix of the interconnecting converter, the multi-voltage-level DC distribution system containing multiple DC microgrids is simplified to a single-bus DC system containing only the distribution bus. The stability of the entire system is then determined using the equivalent impedance ratio. Under the premise that each unit within the system can operate stably independently, and only when the equivalent open-loop gain of each microgrid operating independently and the equivalent impedance ratio on the distribution bus side both satisfy the Nyquist criterion, the stability of the entire system can be guaranteed.

[0008] The method for determining the stability of a multi-voltage-level DC distribution system containing multiple DC microgrids, as described above, involves the following steps:

[0009] a. For various types of generation units and load units in DC power distribution systems, based on the output characteristics of their interface converters on the DC bus side, each unit is divided into two categories: voltage-controlled and power-controlled. Among them, voltage-controlled units mainly include generation units that maintain a constant bus voltage and AC / DC systems, which can be equivalent to the Thevenin equivalent circuit with series output impedance of an ideal voltage source. Power-controlled units mainly include generation units operating in power mode and various load units, which can be equivalent to the Norton equivalent circuit with parallel input admittance of an ideal current source.

[0010] b. For the interconnecting converter (IC) between each DC microgrid and the DC bus of the distribution network, the interconnecting converter is equivalent to the corresponding two-port network according to the control method adopted. The input side of the two-port network is defined as the DC distribution network side and the output side is the microgrid bus side. Based on this, the small-signal model of each DC microgrid is obtained, and the equivalent open-loop gain of each DC microgrid side is obtained on this basis.

[0011] c. Based on the small-signal model of each DC microgrid and using Kirchhoff's laws to convert each DC microgrid into the form of ideal current source parallel input impedance, and combining the equivalent circuit of each unit, the small-signal model of the multi-voltage level DC distribution network containing multiple DC microgrids on the distribution bus side is obtained, and the equivalent impedance ratio of the whole system on the distribution bus side is obtained accordingly.

[0012] d. Determining the stability of the system: Under the premise that all units in the entire DC distribution system can operate stably on their own, if the equivalent open-loop gain of each DC microgrid and the equivalent impedance ratio of the system on the distribution bus side satisfy the Nyquist criterion, then the multi-voltage level DC distribution system containing multiple DC microgrids is determined to have met the stability requirements; otherwise, the multi-voltage level DC distribution system containing multiple DC microgrids is determined to have not met the stability requirements.

[0013] The method described above for determining the stability of a multi-voltage-level DC distribution system containing multiple DC microgrids uses the equivalent open-loop gain of each DC microgrid side, which is obtained based on the small-signal model of each DC microgrid and Kirchhoff's laws.

[0014] The method described above for determining the stability of a multi-voltage-level DC distribution system containing multiple DC microgrids uses the equivalent impedance ratio of the system at the distribution bus side, which is obtained through a small-signal model of the system at the distribution bus side and by an impedance matching criterion.

[0015] Beneficial effects

[0016] This invention utilizes the equivalent open-loop gain of each DC microgrid operating independently and the equivalent impedance ratio of the entire system at the distribution network bus to determine the stability of a multi-voltage-level DC distribution system containing multiple DC microgrids. Existing techniques for multi-voltage-level DC distribution systems with multiple DC microgrids require deriving specific expressions for the small-signal components of each DC bus voltage based on the small-signal model of the entire system to obtain the equivalent loop gain, resulting in a large computational burden. In contrast, the method described in this invention only requires calculating the equivalent open-loop gain of each DC microgrid operating independently and the equivalent impedance ratio at the distribution network side to determine the system's stability. This method offers excellent flexibility and applicability, providing a reliable theoretical basis for the design of multi-voltage-level DC distribution systems. Attached Figure Description

[0017] The invention will now be described in further detail with reference to the accompanying drawings.

[0018] Figure 1 It is a topology for a multi-voltage-level DC power distribution system containing multiple DC microgrids;

[0019] Figure 2 This is a small-signal model of the DC distribution system ICk in voltage control mode for the k# DC microgrid;

[0020] Figure 3 This is a small-signal model of the DC distribution system ICk in power control mode for the k# DC microgrid.

[0021] Figure 4 This is a small-signal model of a DC power distribution system on the distribution bus side.

[0022] The labels in the diagram and the symbols in the text are as follows: N represents the total number of DC microgrids in a multi-voltage-level DC distribution system, and ICk represents the interconnecting converters in the k# (k=1, 2, ..., N) DC microgrids. This represents the small-signal component of the k# DC microgrid bus voltage. This refers to the small-signal component of the distribution network side bus voltage in a DC power distribution system. , , , These are the small-signal components of the input-side current and voltage, and the output-side current and voltage of ICk under voltage control mode. , In voltage control mode, ICk represents the input admittance on the input side and the output impedance on the output side. , , , These are the small-signal components of the input-side current and voltage, and the output-side current and voltage of ICk in power control mode. , Let ICk be the input admittance on the input side and the output admittance on the output side in power control mode. M1k and M2k are the total number of voltage-controlled and power-controlled cells in microgrid k#. , Let be the small-signal component of the equivalent voltage source and output current of the x-th (x=1, 2, ..., M1k) voltage control unit in the k# DC microgrid. Let x be the output impedance of the x-th voltage control unit in the k# DC microgrid. , Let be the small-signal component of the equivalent current source and input current of the y-th (y=1, 2, ..., M2k) power control unit in the k# DC microgrid. Let N1 be the input admittance of the y-th current control unit in the k# DC microgrid, and N2 be the total number of voltage-controlled and power-controlled units on the distribution network side. , This refers to the small-signal component of the output current and the output impedance of the i-th (i=1, 2, ..., N1) voltage control unit on the distribution network bus side. , The small-signal component and input admittance of the input current of the j-th (j=1,2, ..., N2) power control unit on the distribution network bus side are given. For the small-signal component of the input current of the k# DC microgrid subsystem, The equivalent admittance of the k# DC microgrid subsystem is given. Detailed Implementation

[0023] This invention addresses the shortcomings of existing technologies by proposing a method for determining the stability of a multi-voltage-level DC distribution system containing multiple DC microgrids. First, based on the specific control method of the interconnecting converter and utilizing the equivalent open-loop gain of each microgrid operating independently, this invention confirms that the equivalent admittance of each microgrid subsystem has no right-half-plane poles. Second, based on the two-port network matrix of the interconnecting converter, the multi-voltage-level DC distribution system is simplified to a single-bus DC system containing only the distribution bus. The stability of the distribution bus-side subsystem is then determined using the equivalent impedance ratio. Under the premise that each unit within the system can operate stably independently, the entire system can be determined to be stable when both the equivalent open-loop gain of each microgrid operating independently and the equivalent impedance ratio on the distribution bus side satisfy the Nyquist criterion.

[0024] The specific steps for determining system stability are as follows:

[0025] a. For various types of generation units and load units in a DC power distribution system, each unit can be classified into two categories based on the output characteristics of its interface converter on the DC bus side: voltage-controlled and power-controlled. Voltage-controlled units mainly include generation units that maintain a constant bus voltage and AC / DC systems, which can be equivalently represented by the Thevenin equivalent circuit with series output impedance of an ideal voltage source. Power-controlled units mainly include generation units operating in power mode and various load units, which can be equivalently represented by the Norton equivalent circuit with parallel input admittance of an ideal current source.

[0026] b. For the interconnection converter between each DC microgrid and the DC bus of the distribution network, the interconnection converter is equivalent to the corresponding two-port network according to the control method adopted. The input side of the two-port network is defined as the DC distribution network side and the output side is the microgrid bus side. Based on this, the small-signal model of each DC microgrid is obtained, and the equivalent open-loop gain of each DC microgrid side is obtained on this basis.

[0027] c. Based on the small-signal model of each DC microgrid and using Kirchhoff's laws, each DC microgrid can be equivalent to an ideal current source in parallel input impedance. Combining the equivalent circuit of each unit, the small-signal model of a multi-voltage level DC distribution network containing multiple DC microgrids on the distribution bus side can be obtained, and the equivalent impedance ratio of the entire system on the distribution bus side can be obtained from this model.

[0028] d. Determining the stability of the system: Under the premise that all units in the entire DC distribution system can operate stably on their own, if the equivalent open-loop gain of each DC microgrid and the equivalent impedance ratio of the system on the distribution bus side satisfy the Nyquist criterion, then the multi-voltage level DC distribution system containing multiple DC microgrids is determined to have met the stability requirements; otherwise, the multi-voltage level DC distribution system containing multiple DC microgrids is determined to have not met the stability requirements.

[0029] In step b, the equivalent open-loop gain of each DC microgrid side is obtained based on the small-signal model of each DC microgrid and Kirchhoff's laws.

[0030] In step c, the equivalent impedance ratio of the system on the distribution network bus side is obtained by using the small-signal model of the system on the distribution network bus side and by the impedance matching criterion.

[0031] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0032] See Figure 1 , Figure 1 The multi-voltage level DC power distribution system containing multiple DC microgrids includes: N DC microgrids connected in parallel to the same DC bus via interconnecting converters. Each DC bus integrates energy storage, photovoltaics, and DC loads. Each DC microgrid can operate in parallel with the grid to achieve power mutual assistance, or it can operate independently with complete decoupling. ICk (k=1,2,...,N) can switch between different control modes according to the system operation mode.

[0033] a. Based on the control modes of the distributed generation units and load unit converters in the system, they are divided into two categories: voltage-controlled units and current-controlled units, and their corresponding small-signal models are obtained.

[0034] b. Based on the equivalent two-port network of the interconnect converter and combined with step a, the small-signal models of the k# DC microgrid in voltage control mode and power control mode of the interconnect converter can be obtained respectively, such as Figure 2 , Figure 3 As shown.

[0035] When ICk is controlled by a constant voltage, the input voltage, output current, and small-signal components of the input current and output voltage of ICk are selected as the input and output variables of its two-port network, respectively. The equivalent two-port network is shown in the following equation.

[0036]

[0037] In the formula, This represents the small-signal component of the input voltage of ICk. This represents the small-signal component of the input current of ICk. This represents the small-signal component of the output voltage of ICk. This represents the small-signal component of the output current of ICk. This refers to the closed-loop input admittance on the input side. The output closed-loop impedance is the output impedance. This is the closed-loop reverse current gain. The closed-loop voltage gain is given; the transfer function expression in the matrix is ​​as follows:

[0038] ; ; ;

[0039] ;

[0040] When ICk uses constant power control, the small-signal components of the input-side voltage, output-side voltage, input-side current, and output-side current of ICk are selected as the input and output variables of its two-port network, respectively. The equivalent two-port network is shown in the following equation.

[0041]

[0042] In the formula, This represents the small-signal component of the input voltage of ICk. This represents the small-signal component of the input current of ICk. This represents the small-signal component of the output voltage of ICk. This represents the small-signal component of the output current of ICk. This refers to the closed-loop input admittance on the input side. The closed-loop input admittance is the output side. This is the closed-loop transfer admittance from the input side to the output side. The output-to-input closed-loop transfer admittance is given; the transfer function expression in the matrix is ​​as follows:

[0043] ; ; ;

[0044] ;

[0045] c. By Figure 2 The equivalent input admittance of ICk on the distribution network side of the k# DC microgrid in voltage control mode can be obtained as follows:

[0046]

[0047]

[0048] according to Figure 2 Combining Kirchhoff's laws, we can obtain the small-signal component of the bus voltage when the k# DC microgrid is running alone in voltage control mode:

[0049]

[0050] definition:

[0051]

[0052] By comparison as well as From the expressions, we can see that both formulas contain Therefore, under the premise that each unit in the k# micronet and ICk can operate stably and independently, in order to ensure... There are no poles in the right half-plane; it is only necessary to ensure that the k# microgrid can operate stably on its own. There are no poles in the right half-plane.

[0053] right Simplify the expression:

[0054]

[0055] make:

[0056]

[0057] After simplification From the expression, assuming that each unit and ICk in the k# microgrid subsystem can operate stably independently, the equivalent open-loop gain of ICk when the k# DC microgrid operates independently in voltage control mode is... Satisfying the Nyquist criterion, i.e. Since the Nyquist curve does not encircle the point (-1,0), the k# DC microgrid subsystem can operate stably on its own, thus ensuring... There are no poles in the right half-plane.

[0058] d. by Figure 3 The equivalent input admittance of the k# DC microgrid on the distribution bus side in power control mode and the small-signal component of the bus voltage when the k# DC microgrid is running alone are obtained as follows:

[0059]

[0060]

[0061] In the formula:

[0062]

[0063] Depend on and From the expression, we can see that This is precisely the equivalent open-loop gain of ICk when the k# DC microgrid operates independently in power control mode. Therefore, assuming that each unit and ICk within the k# microgrid subsystem can operate stably and independently, if Satisfying the Nyquist criterion, i.e. The Nyquist curve does not encircle the point (-1,0). Under power control mode, the k# DC microgrid subsystem can operate stably independently. At this time, There are no poles in the right half-plane.

[0064] e. Based on the port characteristics of the ICk input side under different control modes, the k# DC microgrid can be equivalently represented as an ideal current source with parallel input admittance on the distribution bus side. Therefore, the small-signal model of a multi-voltage-level DC distribution system containing multiple DC microgrids on the distribution bus side is as follows: Figure 4 As shown.

[0065] Depend on Figure 4 Using the impedance matching criterion, the equivalent impedance ratio of a multi-voltage-level DC distribution system containing multiple DC microgrids on the distribution bus side can be obtained as follows:

[0066]

[0067]

[0068] Therefore, assuming that each unit on the distribution bus side can operate stably and independently, if all DC microgrids can also operate stably and independently, when When the Nyquist criterion is satisfied, i.e. The Nyquist curve does not encircle the point (-1,0), and the entire DC system can operate stably.

[0069] f. Based on the above steps, the conditions for stable operation of a multi-voltage-level DC distribution system containing multiple DC microgrids can be obtained: Under the premise that each unit in the entire DC distribution system can operate stably independently, if... And the equivalent open-loop gain T of each DC microgrid when operating independently Lk The Nyquist curves for (k=1,2,...,N) do not encircle the point (-1,0), indicating stable operation of the DC power distribution system; otherwise, it is unstable. Here, T Lk The expression is shown in the following formula:

[0070]

[0071] In summary, compared with existing technologies, the method proposed in this invention for determining the stability of multi-voltage-level DC distribution systems containing multiple DC microgrids utilizes the equivalent open-loop gain of each microgrid subsystem operating independently to confirm that its equivalent admittance has no right-half-plane poles. Then, it uses the equivalent impedance ratio of the entire system at the distribution bus side to determine the stability of the entire system. The system can operate stably if and only if the equivalent open-loop gain of each microgrid subsystem and the equivalent impedance ratio at the distribution bus side satisfy the Nyquist criterion. This method has excellent flexibility and applicability, and can provide a reliable theoretical basis for the design of multi-voltage-level DC distribution systems.

Claims

1. A method for determining the stability of a multi-voltage level DC power distribution system comprising a multi-DC microgrid, characterized in that, The method firstly confirms that the equivalent admittance of each microgrid subsystem has no right-half-plane pole according to the specific control mode of the interconnected converter and by using the equivalent open-loop gain when each microgrid independently operates; secondly, the multi-voltage-level DC distribution system containing multiple DC microgrids is simplified into a single-bus DC system containing only distribution network buses according to the two-port network matrix of the interconnected converter, and the stability of the entire system is determined by using the equivalent impedance ratio, and on the premise that each unit in the system can independently and stably operate, the entire system can be guaranteed to be stable only when the equivalent open-loop gain of each microgrid when independently operating and the equivalent impedance ratio at the distribution network bus side both satisfy the Nyquist criterion; The specific operation is as follows: The equivalent input admittance of the k# DC microgrid at the distribution network side in the voltage control mode of the DC distribution system ICk is: ； ； The bus voltage small signal component of the k# DC microgrid when independently operating in the voltage control mode of ICk is: ； Definition: ； By comparing the expressions of and , it can be found that both expressions contain , thus, under the premise that each unit and ICk in the k# microgrid can operate stably independently, to ensure no right half-plane pole, it is only required to ensure that the k# microgrid can operate stably independently, i.e. no right half-plane pole; Simplify the expression for ​ ； Let: ； From the simplified expression , it can be seen that, assuming that each unit and ICk in the k# microgrid subsystem can be stably operated independently, if the equivalent open-loop gain of ICk in the voltage control mode when the k# DC microgrid is operated independently is , the Nyquist curve of does not enclose the (-1, 0) point, and the k# DC microgrid subsystem can be stably operated independently. By using the impedance matching criterion, the equivalent impedance ratio of the multi-voltage-level DC distribution system containing multiple DC microgrids at the distribution network bus side is: ； ； ； Therefore, under the premise that each unit on the distribution bus side can operate stably independently, if all the DC microgrids can also operate stably independently, when the Nyquist criterion is met, i.e. the Nyquist curve of the (-1, 0) point does not surround the entire DC system, the entire DC system can operate stably.

2. The method for determining the stability of the multi-voltage-level DC power distribution system containing multi-DC microgrids according to claim 1, characterized in that, The method further includes the following steps: a. For multiple different types of power generation units and load units in the DC distribution system, each unit can be divided into voltage control type and power control type according to the output characteristics of the interface converter at the DC bus side, wherein the voltage control type unit mainly includes the power generation unit maintaining the bus voltage constant and the AC / DC alternating current system, which can be equivalent to the Thevenin equivalent circuit of an ideal voltage source in series with output impedance; the power control type unit mainly includes the power generation unit operating in the power mode and various load units, which can be equivalent to the Norton equivalent circuit of an ideal current source in parallel with input admittance; b. For the interconnected converter between each DC microgrid and the distribution network DC bus, the interconnected converter is equivalent to the corresponding two-port network according to the control mode adopted, the input side of the two-port network is defined as the DC distribution network side, and the output side is defined as the microgrid bus side, thereby obtaining the small signal model of each DC microgrid, and the equivalent open-loop gain of each DC microgrid side is obtained on this basis; c. According to the small signal model of each DC microgrid and by using Kirchhoff's law, each DC microgrid can be equivalent to an ideal current source in parallel with input impedance, and by combining the equivalent circuit of each unit, the small signal model of the multi-voltage-level DC distribution network containing multiple DC microgrids at the distribution network bus side can be obtained, and the equivalent impedance ratio of the entire system at the distribution network bus side is obtained; d. Determine the stability of the system: on the premise that all units in the entire DC distribution system can independently and stably operate, if the equivalent open-loop gain of each DC microgrid and the equivalent impedance ratio of the system at the distribution network bus side both satisfy the Nyquist criterion, it is determined that the multi-voltage-level DC distribution system containing multiple DC microgrids meets the stability requirement, otherwise, it is determined that the multi-voltage-level DC distribution system containing multiple DC microgrids does not meet the stability requirement.

3. The method for determining the stability of the multi-voltage-level DC power distribution system containing multi-DC microgrids according to claim 1, characterized in that, The equivalent open-loop gain of each DC microgrid side is obtained according to the small signal model of each DC microgrid and by using Kirchhoff's law.

4. The method for determining stability of a multi-voltage-level DC power distribution system containing multi-DC microgrids according to claim 1, characterized in that, The equivalent impedance ratio of the system at the distribution network bus side is obtained through the small signal model of the system at the distribution network bus side and by using the impedance matching criterion.

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