A method for deriving three-dimensional asymptotic stability region of AC / DC hybrid microgrid system

Abstract

The present invention discloses a method for deriving a three-dimensional asymptotic stability domain of an AC / DC hybrid microgrid system. First, an energy function model of the microgrid system under the condition of charging and discharging of an energy storage unit is obtained according to the topology and control strategy of the AC / DC hybrid microgrid system; according to the obtained energy function model, a corresponding Lyapunov function P* is established using a hybrid potential function method, and the first-order derivative dP* / dt of the Lyapunov function P* with respect to time is obtained; then, according to the second stability theorem of Lyapunov, a region that satisfies the conditions that the Lyapunov function P* is greater than zero and the first-order derivative dP* / dt of the Lyapunov function P* with respect to time is less than zero is obtained, and this region is the asymptotic stability domain. The method can effectively derive the asymptotic stability domain of the system, and obtain the influence trend of parameter changes on the size of the asymptotic stability domain, providing design criteria for system parameter optimization design.

Description

Technical Field

[0001] The present invention relates to the technical field of AC / DC hybrid microgrid systems, and in particular to a method for deriving a three-dimensional asymptotic stability domain of an AC / DC hybrid microgrid system. Background Art

[0002] The microgrid system is a micro-integrated energy system composed of distributed power sources such as wind power and photovoltaic power, as well as various loads, energy storage devices and multiple types of converters. According to the different system voltages, microgrids can be divided into three categories: AC type, DC type and AC / DC hybrid type. Among them, AC / DC hybrid microgrid has become the preferred microgrid mode for the development and utilization of distributed new energy because it has the advantages of both AC microgrid and DC microgrid. With the continuous development of new energy power generation such as wind power and solar energy, more and more intermittent and unstable power sources appear in microgrid systems, and the demand for energy storage systems is becoming more and more intense. The energy storage system can not only solve the problems caused by the randomness and uncontrollability of the output of new energy power generation itself, and reduce the impact of new energy output changes on the power grid, but also store electricity when the power is sufficient, and release electricity when the load is peak, which plays a role in peak shaving and valley filling and reducing the system's standby demand. The combination of energy storage and new energy power generation is the main form of future energy development. The energy storage unit can be regarded as the inertial link of the microgrid system, which can enhance the system's anti-interference and stability.

[0003] However, in an AC / DC hybrid microgrid system, most loads are connected to the busbar through a closed-loop controlled converter, which can be regarded as a constant power load with a negative impedance characteristic. When disturbed, it is equivalent to adding positive feedback, which can easily cause system instability. For an AC / DC hybrid microgrid system that includes both energy storage units and constant power loads, how to quantitatively determine whether it can work stably and how to obtain the asymptotic stability domain of the system near the steady-state equilibrium operating point are urgent problems that need to be solved. Summary of the invention

[0004] The purpose of the present invention is to provide a method for deriving a three-dimensional asymptotic stability domain of an AC / DC hybrid microgrid system. The method can obtain the asymptotic stability domain of the system and the influence trend of parameter changes on the size of the asymptotic stability domain, thereby providing design criteria for system parameter optimization design.

[0005] The objective of the present invention is achieved through the following technical solutions:

[0006] A method for deriving a three-dimensional asymptotic stability region of an AC / DC hybrid microgrid system, the method comprising:

[0007] Step 1: According to the topology and control strategy of the AC / DC hybrid microgrid system, an energy function model of the microgrid system under the condition of charging and discharging of the energy storage unit is obtained;

[0008] Step 2: According to the obtained energy function model, establish the corresponding Lyapunov function P*, and obtain the first-order derivative dP* / dt of the Lyapunov function P* with respect to time;

[0009] Step 3: According to Lyapunov's second stability theorem, a region is obtained where both the Lyapunov function P* is greater than zero and the first-order derivative dP* / dt of the Lyapunov function P* with respect to time is less than zero. This region is the asymptotic stability domain of the AC / DC hybrid microgrid system.

[0010] It can be seen from the technical solution provided by the present invention that the above method can effectively derive the asymptotic stability domain of the system and obtain the influence trend of parameter changes on the size of the asymptotic stability domain, thereby providing design criteria for system parameter optimization design. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative work.

[0012] Figure 1 A schematic flow chart of a method for deriving a three-dimensional asymptotic stability region of an AC / DC hybrid microgrid system provided in an embodiment of the present invention;

[0013] Figure 2 A typical topology diagram of an AC / DC hybrid microgrid system according to an embodiment of the present invention;

[0014] Figure 3 A schematic diagram of a control strategy of a bidirectional Buck-Boost converter according to an embodiment of the present invention;

[0015] Figure 4 A schematic diagram of a DC / AC converter control strategy for an energy storage unit according to an embodiment of the present invention;

[0016] Figure 5 This is a schematic diagram of the PQ control of the energy storage unit bidirectional DC / AC converter according to an embodiment of the present invention;

[0017] Figure 6 This is a schematic diagram of an equivalent topology of a microgrid system in which the energy storage unit is in a discharge mode according to an embodiment of the present invention;

[0018] Figure 7 This is a schematic diagram of an equivalent topology of a microgrid system in which the energy storage unit is in a charging mode according to an embodiment of the present invention;

[0019] Figure 8A schematic diagram of solving the asymptotic stability domain using Lyapunov's second stability theorem according to an embodiment of the present invention;

[0020] Fig. 9 It is a schematic diagram of the asymptotic stability region of the microgrid system in the charging mode of the example given in the present invention;

[0021] Fig.10 This is a schematic diagram of the asymptotic stability region of the microgrid system under the discharge mode of the example given in the present invention. DETAILED DESCRIPTION

[0022] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments, which does not constitute a limitation of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] like Figure 1 The figure is a flow chart of a method for deriving a three-dimensional asymptotic stability region of an AC / DC hybrid microgrid system provided by an embodiment of the present invention, the method comprising:

[0024] Step 1: According to the topology and control strategy of the AC / DC hybrid microgrid system, an energy function model of the microgrid system under the condition of charging and discharging of the energy storage unit is obtained;

[0025] In this step, if Figure 2 The figure shows a typical topological diagram of the AC / DC hybrid microgrid system described in the embodiment of the present invention, including a photovoltaic power generation unit on the AC side, an energy storage unit on the DC side, and a constant power load. Among them, the loads of closed-loop control can all be regarded as constant power loads, which are common on the bus side of each level. Because of their negative impedance characteristics, this type of load will have a serious impact on the stability of the system. The battery of the energy storage unit is connected to the AC bus through a bidirectional Buck-Boost converter cascaded with a bidirectional DC / AC converter. The energy storage unit belongs to the inertia link of the system, and will offset the adverse effects of the constant power load when discharging. The interaction between the two will affect the stability of the entire system.

[0026] In the specific implementation, Figure 3 FIG. 1 is a schematic diagram of a control strategy of a bidirectional Buck-Boost converter according to an embodiment of the present invention, which adopts a control method of a DC voltage outer loop and a current inner loop to maintain a constant DC bus voltage and realize automatic switching of battery charge and discharge, wherein U ref is the DC side voltage reference value, U dc is the actual voltage on the DC side, I ref is the current reference value output after voltage outer loop regulation, I bis the actual value of DC current.

[0027] like Figure 4 The figure shows a schematic diagram of the control strategy of the energy storage unit DC / AC converter according to an embodiment of the present invention. The energy storage unit bidirectional DC / AC converter adopts droop control, which mainly includes power control, current and voltage dual-loop modules. The power control module includes three parts: instantaneous power calculation link, droop characteristic control link, and reference voltage control link; wherein, U abc is the actual value of the AC voltage, I abc is the actual value of the AC current, p and q are the active power and reactive power obtained by the instantaneous power calculation module, U dref and U qref are the d-axis and q-axis voltage reference values ​​obtained after droop control, i od and i oq They are the actual values ​​of the d-axis and q-axis currents obtained by park transformation of the AC current measurement.

[0028] like Figure 5 FIG. 1 is a schematic diagram of a bidirectional DC / AC converter PQ control of an energy storage unit according to an embodiment of the present invention, wherein V a 、V b 、V c For AC voltage measurement, P ref and Q ref They are respectively the active power set value and reactive power set value of the photovoltaic power generation unit, u d and u q They are the actual values ​​of the d-axis and q-axis voltages obtained by park transformation of the AC side voltage, and u sd and u sq are the voltage reference values ​​of the d-axis and q-axis respectively.

[0029] According to the topology and control strategy of the AC / DC hybrid microgrid system analyzed above, in the discharge mode of the energy storage unit, the bidirectional Buck-Boost converter of the energy storage unit adopts voltage and current dual closed-loop control, so the battery, Buck-Boost converter and control unit in the discharge mode can be equivalent to a controlled current source, and its discharge power is P b , P is the power consumed by the AC constant power load; the photovoltaic module is equivalent to a current source, and the power is P G ; L s , C s are filter inductance and capacitance respectively; v s is the filter capacitor C s Voltage across the two ends; R s is the equivalent resistance of the circuit; v d is the DC bus voltage; v dc is the battery side voltage; C dcis the voltage stabilizing capacitor on the battery side; i2 is the current filtering inductor L s i3 is the output current of the DC / AC converter;

[0030] Thus, the equivalent topology of the microgrid system with the energy storage unit in the discharge mode is obtained, as follows: Figure 6 The figure shows an equivalent topology diagram of a microgrid system in which the energy storage unit is in a discharge mode according to an embodiment of the present invention, and the energy function model in the discharge mode of the energy storage unit is obtained as follows:

[0031]

[0032] By verifying the model shown in formula (1), we can get:

[0033]

[0034] It can be seen from formula (2) that when the energy storage unit is in the discharge mode, the model obtained by formula (1) is correct.

[0035] When the energy storage unit is in charging mode, the battery, Buck-Boost converter and its control unit are equivalent to a resistor R, thereby obtaining the equivalent topology of the microgrid system when the energy storage unit is in charging mode, as shown in Figure 7 The figure shows an equivalent topology diagram of a microgrid system in which the energy storage unit is in a charging mode according to an embodiment of the present invention, and the energy function model of the energy storage unit in the charging mode is obtained as follows:

[0036]

[0037] By verifying the model shown in formula (3), we can get:

[0038]

[0039] It can be seen from formula (4) that when the energy storage unit is in charging mode, the model obtained by formula (3) is correct.

[0040] Step 2: According to the obtained energy function model, establish the corresponding Lyapunov function P*, and obtain the first-order derivative dP* / dt of the Lyapunov function P* with respect to time;

[0041] In this step, when the energy storage unit is in the discharge mode, the Lyapunov function P* corresponding to the energy storage unit in the discharge mode is obtained according to the energy function model shown in formula (1) and is expressed as:

[0042]

[0043] In formula (5), it satisfies:

[0044]

[0045] The first-order derivative dP* / dt of the Lyapunov function with respect to time shown in formula (5) is expressed as:

[0046]

[0047] And in formula (7), it satisfies:

[0048]

[0049] When the energy storage unit is in charging mode, the Lyapunov function P* corresponding to the energy storage unit charging mode is obtained according to the energy function model shown in formula (3) as follows:

[0050]

[0051] In formula (9), it satisfies:

[0052]

[0053] The first-order derivative dP* / dt of the Lyapunov function with respect to time shown in formula (9) is expressed as:

[0054]

[0055] In formula (11), it satisfies:

[0056]

[0057] Step 3: According to Lyapunov's second stability theorem, a region is obtained where both the Lyapunov function P* is greater than zero and the first-order derivative dP* / dt of the Lyapunov function P* with respect to time is less than zero. This region is the asymptotic stability domain of the AC / DC hybrid microgrid system.

[0058] In this step, Lyapunov's second stability theorem is first explained as follows:

[0059] like Figure 8 The figure shows a schematic diagram of solving the asymptotically stable domain using Lyapunov's second stability theorem according to an embodiment of the present invention. If the equilibrium state Xe is stable in the sense of Lyapunov, any solution starting from the sphere S(γ) will not only not exceed the sphere S(γ) when t→∞, but will also converge to the sphere S(δ), then such equilibrium state is asymptotically stable. Where Xe is a stable point in the state space, and the spheres S(γ) and S(δ) are different regions in the state space.

[0060] Considering the autonomous system Where f(x) is continuous, V(x) has a continuous first-order partial derivative with respect to x, and in the neighborhood around the equilibrium point, the following conditions are satisfied simultaneously: (1) V(x) is locally positive definite, (2) There exists a negative definite region; then the system is asymptotically stable at the equilibrium point, and this neighborhood is the asymptotically stable region of the system. Where V(x) is the Lyapunov function constructed by the autonomous system.

[0061] Based on the above Lyapunov second stability theorem, a region is obtained in which both the Lyapunov function P* is greater than zero and the first-order derivative dP* / dt of the Lyapunov function P* with respect to time is less than zero. This region is the asymptotic stability domain of the AC / DC hybrid microgrid system.

[0062] In the specific implementation, the asymptotic stability region of the AC / DC hybrid microgrid system under the energy storage unit discharge mode is expressed as:

[0063]

[0064] The asymptotic stability region of the AC / DC hybrid microgrid system in the energy storage unit charging mode is expressed as:

[0065]

[0066] In addition, after obtaining the asymptotic stability domain of the AC / DC hybrid microgrid system, the key parameters affecting the large signal stability of the system can be found by comparing the obtained asymptotic stability domains, and the influence trend of parameter changes on the size of the asymptotic stability domain can be obtained, providing a basis for the system parameter optimization design.

[0067] For example, this embodiment builds Figure 2 The AC / DC hybrid microgrid system shown in the figure has system parameters as shown in Table 1:

[0068] Table 1 Microgrid system simulation parameters

[0069]

[0070] According to the parameters shown in Table 1 and formula (5), the Lyapunov function of the microgrid system under the energy storage unit discharge mode can be calculated. The function is based on the AC bus voltage V s , DC bus voltage v dc and the AC side inductor current i2 are variables, and the Lyapunov function of the microgrid system has an area greater than zero during discharge.

[0071] Similarly, the Lyapunov function of the microgrid system under the energy storage unit charging mode can be obtained. When the energy storage unit is charging, the Lyapunov function of the microgrid system has a region greater than zero;

[0072] According to the parameters shown in Table 1 and formula (7), the first-order derivative of the Lyapunov function of the microgrid system with respect to time in the energy storage unit discharge mode can also be calculated. When the energy storage unit is discharged, the first-order derivative of the Lyapunov function of the microgrid system with respect to time exists in a region less than zero;

[0073] Similarly, a four-dimensional graph of the first-order derivative of the Lyapunov function of the microgrid system with respect to time under the energy storage unit charging mode can be obtained. When the energy storage unit is charging, the first-order derivative of the Lyapunov function of the microgrid system with respect to time exists in an area less than zero;

[0074] Then, according to the Lyapunov function and the first-order derivative of the Lyapunov function with respect to time, combined with the asymptotic stability domain formulas of the energy storage unit when charging and discharging as shown in equations (13) and (14), the asymptotic stability domain of the microgrid system under the condition of energy storage unit charging and discharging can be obtained respectively, as follows: Fig. 9 The figure shows a schematic diagram of the asymptotic stability region of the microgrid system in the charging mode according to the example of the present invention. Fig.10 The figure shows the asymptotic stability region of the microgrid system in the discharge mode of the present invention. Fig. 9 and 10 It can be seen that the asymptotic stability domain of the microgrid system is larger when the energy storage unit is discharged, which also shows that the system will be more stable due to the increase in power supply when the energy storage unit is discharged.

[0075] In addition, the system parameters affect the size of the stability domain. When other system parameters remain unchanged, first increase the DC side voltage stabilizing capacitor C dc From 10μF to 20μF, different asymptotic stability domains are obtained. By comparing different asymptotic stability domains, we can see that increasing C dc The value of L can increase the asymptotic stability domain of the AC / DC hybrid microgrid system; then, when other system parameters remain unchanged, the AC filter inductance is increased from 1000μH to 3000μH, and different asymptotic stability domains are obtained. By comparison, it can be seen that: increasing L dc The value of will reduce the asymptotic stability region of the AC / DC hybrid microgrid system.

[0076] It can be seen that by deriving the asymptotic stability domain of the system, the key parameters that affect the large signal stability of the system can be found, the influence trend of parameter changes on the size of the asymptotic stability domain can be obtained, and design criteria can be provided for the system parameter optimization design.

[0077] It is worth noting that the contents not described in detail in the embodiments of the present invention belong to the prior art known to professional and technical personnel in the field.

[0078] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed in the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims. The information disclosed in the background technology section of this article is only intended to deepen the understanding of the overall background technology of the present invention, and should not be regarded as an admission or in any form that the information constitutes prior art known to those skilled in the art.

Claims

1. A method for deriving a three-dimensional asymptotic stability region of an AC / DC hybrid microgrid system, characterized in that: The method comprises: Step 1: According to the topology and control strategy of the AC / DC hybrid microgrid system, an energy function model of the microgrid system under the condition of charging and discharging of the energy storage unit is obtained; Among them, in the energy storage unit discharge mode, the bidirectional Buck-Boost converter of the energy storage unit adopts voltage and current dual closed-loop control, so the battery in the discharge mode, the Buck-Boost converter and the control unit can be equivalent to a controlled current source, and its discharge power is P b , P is the power consumed by the AC constant power load; the photovoltaic module is equivalent to a current source, and the power is P G ; L s , C s are filter inductance and capacitance respectively; v s is the filter capacitor C s Voltage across the two ends; R s is the equivalent resistance of the circuit; v d is the DC bus voltage; v dc is the battery side voltage; C dc is the voltage stabilizing capacitor on the battery side; i2 is the current filtering inductor L s i3 is the output current of the DC / AC converter; Thus, the equivalent topology of the microgrid system with the energy storage unit in the discharge mode is obtained, and then the energy function model in the discharge mode of the energy storage unit is obtained as follows: When the energy storage unit is in charging mode, the battery, Buck-Boost converter and its control unit are equivalent to a resistor R, thereby obtaining the equivalent topology of the microgrid system when the energy storage unit is in charging mode, and then obtaining the energy function model of the energy storage unit in charging mode as follows: Step 2: According to the obtained energy function model, establish the corresponding Lyapunov function P*, and obtain the first-order derivative dP* / dt of the Lyapunov function P* with respect to time; When the energy storage unit is in the discharge mode, the Lyapunov function P* corresponding to the energy storage unit in the discharge mode is obtained according to the energy function model shown in formula (1) as follows: In formula (5), it satisfies: The first-order derivative dP* / dt of the Lyapunov function with respect to time shown in formula (5) is expressed as: And in formula (7), it satisfies: When the energy storage unit is in charging mode, the Lyapunov function P* corresponding to the energy storage unit charging mode is obtained according to the energy function model shown in formula (3) as follows: In formula (9), it satisfies: The first-order derivative dP* / dt of the Lyapunov function with respect to time shown in formula (9) is expressed as: In formula (11), it satisfies: Step 3: According to Lyapunov's second stability theorem, a region is obtained where both the Lyapunov function P* is greater than zero and the first-order derivative dP* / dt of the Lyapunov function P* with respect to time is less than zero. This region is the asymptotic stability domain of the AC / DC hybrid microgrid system.

2. The derivation method of the three-dimensional asymptotic stability region of the AC / DC hybrid microgrid system according to claim 1 is characterized in that: In step 1: By verifying the model shown in formula (1), we can get: It can be seen from formula (2) that when the energy storage unit is in the discharge mode, the model obtained by formula (1) is correct; By verifying the model shown in formula (3), we can get: It can be seen from formula (4) that when the energy storage unit is in charging mode, the model obtained by formula (3) is correct.

3. The derivation method of the three-dimensional asymptotic stability region of the AC / DC hybrid microgrid system according to claim 1 is characterized in that: In step 3, the asymptotic stability region of the AC / DC hybrid microgrid system in the energy storage unit discharge mode is expressed as: The asymptotic stability region of the AC / DC hybrid microgrid system in the energy storage unit charging mode is expressed as:

4. The derivation method of the three-dimensional asymptotic stability region of the AC / DC hybrid microgrid system according to claim 1 is characterized in that: The method further comprises: By comparing the obtained asymptotically stable domains, the key parameters that affect the large signal stability of the system are found, and the influence trend of parameter changes on the size of the asymptotically stable domain is obtained, providing a basis for the optimal design of system parameters.

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