A stability determination method and device for an ac-dc hybrid microgrid system

Abstract

The present disclosure relates to a kind of AC-DC hybrid micro-grid system stability determination method and device.Wherein, the method includes: establishing the equivalent model of the AC-DC hybrid micro-grid system under synchronous rotation dq coordinate system;Based on mixed potential function theory, generate the nonlinear model of the AC-DC hybrid micro-grid system;The steady-state operating point of the AC-DC hybrid micro-grid system is generated, and the steady-state operating point is generated according to the large signal stability criterion of the AC-DC hybrid micro-grid system;The large signal stability criterion is limited to the amplitude processing based on the preset reference current limiting value, and the stability determination of the AC-DC hybrid micro-grid system is completed based on the large signal stability criterion containing current limiting amplitude.The reference current limiting value of the converter current inner loop is limited in the present disclosure, and the large signal stability criterion is optimized, so that the accuracy and practicability of stability criterion are further improved.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of new energy applications, and in particular, to a stability determination method and device for an AC / DC hybrid micro-grid system. BACKGROUND

[0002] An AC / DC hybrid micro-grid system contains a DC sub-grid and an AC sub-grid, can directly connect distributed power sources, loads and energy storage units, reduces the number of converters and power loss, and is the main architecture of a future micro-grid system. When operating in an island mode, the stability of the AC / DC hybrid micro-grid system is a prominent problem. On the one hand, one or more distributed power sources need to provide frequency and phase support for the system using droop control, but due to limited system capacity and small inertia, the system is easily disturbed and unstable. On the other hand, most loads are connected to the bus through controllable converters, and the output power of these converters is not affected by the input voltage due to closed-loop control, so the converter and the connected load can be regarded as a constant power load. When the voltage across the constant power load rises, the input current decreases, showing a negative impedance characteristic. This characteristic amplifies the disturbance signal in the system, making the stability problem of the AC / DC hybrid micro-grid in the island mode even more serious.

[0003] The premise of studying the stability problem of a hybrid micro-grid is that the system has a steady-state operating point, that is, whether the system can be in a stable state is determined. Based on the negative impedance characteristic of the constant power load and the characteristic curve of the equivalent voltage source, the steady-state operating point of the AC / DC hybrid micro-grid system is calculated, providing a prerequisite for large signal stability analysis. In the actual process, considering that a large disturbance causes a sudden increase in converter current, the converter and other switching devices will be severely overheated and even damaged, so the current needs to be limited. Therefore, when studying the large disturbance stability problem of an AC / DC hybrid micro-grid with constant power loads, it is necessary to consider the reference current limiting under the premise of meeting the steady-state operating point, and quantitatively express the influence of parameters on the stability of the system.

[0004] At present, a large number of literatures analyze the stability problem of the micro-grid system, and the analysis methods are mainly divided into small signal stability analysis and large signal stability analysis. Among them, the small signal stability analysis is widely used, but the small signal stability analysis is only suitable for the stability of the system near the steady state operating point, and is not suitable for the case where the system encounters large disturbance. Commonly used large signal stability analysis methods include BDQLF, Takagi-Sugeno fuzzy model research and hybrid potential function analysis method. Among them, BDQLF can establish Lyapunov function and is suitable for analysis of complex systems, but the results obtained are conservative; the T-S fuzzy model method can also analyze the stability of nonlinear systems and estimate the stability domain of the system, but it cannot obtain a quantitative stability criterion, and the complexity of the analysis increases with the increase of the order of the nonlinear system.

[0005] Therefore, one or more methods are needed to solve the above problems.

[0006] It should be noted that the information disclosed in the above background section is only used to strengthen the understanding of the background of the present disclosure, and therefore can include information that does not constitute prior art known to those of ordinary skill in the art. SUMMARY

[0007] The purpose of the present disclosure is to provide a stability determination method and device for an AC-DC hybrid micro-grid system, thereby at least partially overcoming one or more problems caused by the limitations and defects of the related art.

[0008] According to one aspect of the present disclosure, a stability determination method for an AC-DC hybrid micro-grid system is provided, comprising:

[0009] The DC-AC converter and the DC-DC converter in the AC-DC hybrid micro-grid system are respectively modeled, and an equivalent model of the AC-DC hybrid micro-grid system in the synchronous rotating dq coordinate system is generated;

[0010] Based on the hybrid potential function theory, the current potential function of the output voltage and the voltage potential function of the output current in the equivalent model of the AC-DC hybrid micro-grid system in the synchronous rotating dq coordinate system are calculated respectively, and a nonlinear model of the AC-DC hybrid micro-grid system is generated;

[0011] The steady state operating point of the AC-DC hybrid micro-grid system is generated based on the equivalent model of the AC-DC hybrid micro-grid system in the dq coordinate system, and the large signal stability criterion of the AC-DC hybrid micro-grid system is generated according to the steady state operating point and the nonlinear model of the AC-DC hybrid micro-grid system;

[0012] The large signal stability criterion is amplitude-limited based on a preset reference current amplitude limit value to generate a large signal stability criterion containing current amplitude limitation, and the stability of the AC / DC hybrid micro-grid system is determined based on the large signal stability criterion containing current amplitude limitation.

[0013] In an example embodiment of the present disclosure, the method further comprises:

[0014] The DC-AC converter in the AC / DC hybrid micro-grid system is modeled and converted to generate a DC-AC converter model in a synchronous rotating dq coordinate system as

[0015]

[0016] wherein e d and e q are the projections of the grid electromotive force vector E on the d-axis and the q-axis, i d , i q are the projections of the three-phase DC-AC converter current on the d-axis and the q-axis, s d and s q are the switching function components of the DC-AC converter on the d-axis and the q-axis, i dc is the DC side bus current, i L is the current passing through the equivalent resistance R L .

[0017] In an example embodiment of the present disclosure, the method further comprises:

[0018] The DC-AC converter and the DC-DC converter in the AC / DC hybrid micro-grid system are respectively modeled to generate an equivalent model of the AC / DC hybrid micro-grid system in a synchronous rotating dq coordinate system, wherein the controlled current source i0 in the equivalent model is controlled by the voltage v d , and satisfies:

[0019]

[0020] wherein i1 is a current variable in the equivalent model, and V dc is a voltage variable in the equivalent model.

[0021] In an example embodiment of the present disclosure, the method further comprises:

[0022] Based on the hybrid potential function theory, the current potential function of the output voltage and the voltage potential function of the output current in the equivalent model of the AC / DC hybrid micro-grid system in the synchronous rotating dq coordinate system are respectively calculated to generate a nonlinear model of the AC / DC hybrid micro-grid system as

[0023]

[0024] wherein, P G is an equivalent power source of photovoltaic micro source, R1 is an equivalent resistance of line, R s is an equivalent resistance of inductor, V d is an AC side output port voltage of interconnection converter, P b is an energy storage unit, P1 is a constant power load, and i0 is an output current of the converter.

[0025] In an example embodiment of the present disclosure, the method further comprises:

[0026] generating, based on an equivalent model of the AC / DC hybrid micro-grid system in a dq coordinate system, that the steady-state operating point of the AC / DC hybrid micro-grid system satisfies

[0027]

[0028] wherein, V s is a voltage across the equivalent power source, P b contains two operating states of the energy storage unit.

[0029] In an example embodiment of the present disclosure, the method further comprises:

[0030] generating, according to the steady-state operating point and a nonlinear model of the AC / DC hybrid micro-grid system, a large signal stability criterion of the AC / DC hybrid micro-grid system as

[0031]

[0032] wherein, P1 is a power of the constant power load, P b is a power of the energy storage unit, v dc is a DC bus voltage, L s is an AC filter inductance, R s is an equivalent resistance, C dc is a DC side capacitance, and k ip is a current inner loop proportional parameter of the DC-AC converter.

[0033] In an example embodiment of the present disclosure, the method further comprises:

[0034] calculating, based on a preset reference current amplitude i dmax , an instantaneous power p max of the DC-AC converter of the AC / DC hybrid micro-grid system;

[0035] performing, according to the preset reference current amplitude i dmax and the instantaneous power p max , amplitude limiting processing on the large signal stability criterion to generate a large signal stability criterion containing current amplitude limiting as

[0036]

[0037] wherein, a is the ratio of the time for the DC-AC converter d-axis current of the AC-DC hybrid micro-grid system to reach i dmax and the time of the entire transient process, and the value range of the a is 0 < a < 1.

[0038] In an aspect of the present disclosure, an AC-DC hybrid micro-grid system stability determination device is provided, comprising:

[0039] a system modeling module, configured to model the DC-AC converter and the DC-DC converter in the AC-DC hybrid micro-grid system respectively, and generate an equivalent model of the AC-DC hybrid micro-grid system in the dq coordinate system;

[0040] a nonlinear model modeling module, configured to calculate the current potential function of the output voltage and the voltage potential function of the output current in the equivalent model of the AC-DC hybrid micro-grid system in the dq coordinate system respectively based on the hybrid potential function theory, and generate a nonlinear model of the AC-DC hybrid micro-grid system;

[0041] a large signal stability criterion generation module, configured to generate a steady state working point of the AC-DC hybrid micro-grid system based on the equivalent model of the AC-DC hybrid micro-grid system in the dq coordinate system, and generate a large signal stability criterion of the AC-DC hybrid micro-grid system according to the steady state working point and the nonlinear model of the AC-DC hybrid micro-grid system;

[0042] a stability determination module, configured to perform amplitude limiting processing on the large signal stability criterion based on a preset reference current amplitude, generate a large signal stability criterion with current amplitude, and complete the stability determination of the AC-DC hybrid micro-grid system based on the large signal stability criterion with current amplitude.

[0043] In an example embodiment of the present disclosure, an AC-DC hybrid micro-grid system stability determination method is provided, wherein the method comprises: establishing an equivalent model of the AC-DC hybrid micro-grid system in the synchronous rotating dq coordinate system; generating a nonlinear model of the AC-DC hybrid micro-grid system based on the hybrid potential function theory; generating a steady state working point of the AC-DC hybrid micro-grid system, and generating a large signal stability criterion of the AC-DC hybrid micro-grid system according to the steady state working point; performing amplitude limiting processing on the large signal stability criterion based on a preset reference current amplitude, and completing the stability determination of the AC-DC hybrid micro-grid system based on the large signal stability criterion with current amplitude. The present disclosure optimizes the large signal stability criterion by limiting the reference current of the DC-AC converter current inner loop, so that the accuracy and practicability of the stability criterion are further improved.

[0044] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure. BRIEF DESCRIPTION OF DRAWINGS

[0045] The above and other features and advantages of the present disclosure will become more apparent by describing in detail exemplary embodiments thereof with reference made to the accompanying drawings.

[0046] Figure 1 A flow chart of a stability determination method of an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure is shown;

[0047] Figure 2 A main circuit topology diagram of an AC / DC hybrid microgrid system in island mode according to a stability determination method of an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure is shown;

[0048] Figure 3 A control block diagram of an AC / DC hybrid microgrid system in island mode according to a stability determination method of an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure is shown;

[0049] Figure 4 A circuit topology diagram of a DC-AC converter between AC and DC buses according to a stability determination method of an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure is shown;

[0050] Figure 5 A droop control block diagram of a DC-AC converter according to a stability determination method of an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure is shown;

[0051] Figure 6 A control block diagram of a bidirectional DC-DC converter of an energy storage unit according to a stability determination method of an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure is shown;

[0052] Figure 7 An equivalent model diagram of an energy storage system according to a stability determination method of an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure is shown;

[0053] Figure 8 A PQ control block diagram of a DC microsource DC-AC converter according to a stability determination method of an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure is shown;

[0054] Figure 9 An equivalent model diagram of a DC microsource according to a stability determination method of an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure is shown;

[0055] Figure 10Fig. 1 shows a DC equivalent model diagram of an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure;

[0056] Figure 11 Fig. 2 shows a V-I curve diagram of an equivalent voltage source Vs and a constant power load according to an exemplary embodiment of the present disclosure;

[0057] Figure 12 Fig. 3 shows a curve diagram of P1 and a when applying a limiting criterion respectively during discharging of a battery according to an exemplary embodiment of the present disclosure;

[0058] Figure 13 Fig. 4 shows a curve diagram of P1 and a when applying a limiting criterion respectively during charging of a battery according to an exemplary embodiment of the present disclosure;

[0059] Figure 14 Fig. 5 shows a schematic block diagram of a stability determination device for an AC / DC hybrid microgrid system according to an exemplary embodiment of the present disclosure. DETAILED DESCRIPTION

[0060] Example embodiments now will be described more fully hereinafter with reference to the accompanying drawings. Example embodiments, however, can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of example embodiments to those skilled in the art. Like reference numerals refer to like elements throughout the figures, and thus description of the same will be simplified or omitted.

[0061] Moreover, the described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of embodiments of the disclosure. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail in order to avoid obscuring aspects of the disclosure.

[0062] The block diagrams in the drawings show only the functionality of the embodiments and do not imply that the functions must be implemented in a particular manner. For example, the functions of one or more of the illustrated components can be implemented in hardware, software, or a combination of both hardware and software. Further, the functions of the illustrated components can be combined or divided into other components not illustrated. The embodiments can be implemented in software and / or firmware that is executable by one or more processing units of one or more computers in the system. Examples of computers include personal computers, server computers, handheld devices, programmable consumer electronics, and the like.

[0063] In the example embodiment, first, a hybrid AC-DC micro-grid system stability determination method is provided; as shown in Figure 1 The hybrid AC-DC micro-grid system stability determination method can include the following steps:

[0064] In step S110, the DC-AC converter and the DC-DC converter in the hybrid AC-DC micro-grid system are modeled respectively, and an equivalent model of the hybrid AC-DC micro-grid system in the synchronous rotating dq coordinate system is generated;

[0065] In step S120, based on the mixed potential function theory, the current potential function of the output voltage and the voltage potential function of the output current in the equivalent model of the hybrid AC-DC micro-grid system in the synchronous rotating dq coordinate system are calculated respectively, and a nonlinear model of the hybrid AC-DC micro-grid system is generated;

[0066] In step S130, the steady-state operating point of the hybrid AC-DC micro-grid system is generated based on the equivalent model of the hybrid AC-DC micro-grid system in the dq coordinate system, and the large signal stability criterion of the hybrid AC-DC micro-grid system is generated according to the steady-state operating point and the nonlinear model of the hybrid AC-DC micro-grid system;

[0067] In step S140, the large signal stability criterion is amplitude-limited based on a preset reference current amplitude value, a large signal stability criterion with current amplitude is generated, and the stability determination of the hybrid AC-DC micro-grid system is completed based on the large signal stability criterion with current amplitude.

[0068] The hybrid AC-DC micro-grid system stability determination method in the example embodiment of the disclosure, wherein the method comprises: establishing an equivalent model of the hybrid AC-DC micro-grid system in the synchronous rotating dq coordinate system; generating a nonlinear model of the hybrid AC-DC micro-grid system based on the mixed potential function theory; generating a steady-state operating point of the hybrid AC-DC micro-grid system, and generating a large signal stability criterion of the hybrid AC-DC micro-grid system according to the steady-state operating point; amplitude-limiting the large signal stability criterion based on a preset reference current amplitude value, and completing the stability determination of the hybrid AC-DC micro-grid system based on the large signal stability criterion with current amplitude. The disclosure optimizes the large signal stability criterion by limiting the reference current of the DC-AC converter current inner loop, so that the accuracy and practicability of the stability criterion are further improved.

[0069] In the following, the hybrid AC-DC micro-grid system stability determination method in the example embodiment will be further described.

[0070] Embodiment one:

[0071] In step S110, the DC-AC converter and the DC-DC converter in the AC-DC hybrid micro-grid system can be modeled respectively, and an equivalent model of the AC-DC hybrid micro-grid system in the synchronous rotating dq coordinate system is generated.

[0072] In an embodiment of the present example, the method further comprises:

[0073] The DC-AC converter in the AC-DC hybrid micro-grid system is modeled and converted to generate a DC-AC converter model in the synchronous rotating dq coordinate system as

[0074]

[0075] wherein e d and e q are the projections of the grid electromotive force vector E on the d-axis and the q-axis, i d , i q are the projections of the three-phase DC-AC converter current on the d-axis and the q-axis, s d and s q are the switching function components of the DC-AC converter on the d-axis and the q-axis, i dc is the DC side bus current, i L is the current passing through the equivalent resistance R L .

[0076] In an embodiment of the present example, the method further comprises:

[0077] The DC-AC converter and the DC-DC converter in the AC-DC hybrid micro-grid system are modeled respectively, and an equivalent model of the AC-DC hybrid micro-grid system in the synchronous rotating dq coordinate system is generated, wherein the i0 of the controlled current source in the equivalent model is controlled by the voltage v d , and satisfies:

[0078]

[0079] wherein i1 is a current variable in the equivalent model, V dc is a voltage variable in the equivalent model.

[0080] In step S120, the current potential function of the output voltage and the voltage potential function of the output current in the equivalent model of the AC-DC hybrid micro-grid system in the synchronous rotating dq coordinate system can be calculated based on the hybrid potential function theory, and a nonlinear model of the AC-DC hybrid micro-grid system is generated.

[0081] In an embodiment of the present example, the method further comprises:

[0082] The current potential function of the output voltage and the voltage potential function of the output current in the equivalent model of the direct current hybrid micro-grid system in the synchronous rotating dq coordinate system are calculated based on the hybrid potential function theory, and a nonlinear model of the direct current hybrid micro-grid system is generated

[0083]

[0084] wherein, P G is the equivalent power source of the photovoltaic micro-source, R1 is the equivalent resistance of the line, R s is the equivalent resistance of the inductor, V d is the output port voltage of the AC side of the interconnection converter, P b is the energy storage unit, P1 is the constant power load, and i0 is the output current of the converter.

[0085] In step S130, the steady-state operating point of the AC / DC hybrid micro-grid system can be generated based on the equivalent model of the AC / DC hybrid micro-grid system in the dq coordinate system, and the large signal stability criterion of the AC / DC hybrid micro-grid system can be generated according to the steady-state operating point and the nonlinear model of the AC / DC hybrid micro-grid system.

[0086] In the embodiment of the present example, the method further comprises:

[0087] The steady-state operating point of the AC / DC hybrid micro-grid system generated based on the equivalent model of the AC / DC hybrid micro-grid system in the dq coordinate system satisfies

[0088]

[0089] wherein, V s is the voltage across the equivalent power source, P b is the power of the energy storage unit in two operating states.

[0090] In the embodiment of the present example, the method further comprises:

[0091] The large signal stability criterion of the AC / DC hybrid micro-grid system generated according to the steady-state operating point and the nonlinear model of the AC / DC hybrid micro-grid system is

[0092]

[0093] wherein, P1 is the power of the constant power load, P b is the power of the energy storage unit, v dc is the DC bus voltage, L s is the AC filter inductance, R s is the equivalent resistance, C dc is the DC side capacitance, and k ip is the current inner loop proportional parameter of the DC-AC converter.

[0094] In step S140, the large signal stability criterion can be amplitude-limited based on a preset reference current limiting value to generate a large signal stability criterion containing current limiting, and the stability of the AC / DC hybrid micro-grid system is determined based on the large signal stability criterion containing current limiting.

[0095] In the embodiment of the present example, the method further comprises:

[0096] based on a preset reference current limiting value i dmax The instantaneous power of the DC-AC converter of the AC / DC hybrid micro-grid system is calculated as p max ;

[0097] According to the preset reference current limiting value i dmax , the instantaneous power is p max , the large signal stability criterion is amplitude-limited to generate a large signal stability criterion containing current limiting, which is

[0098]

[0099] Wherein, α is the ratio of the time when the d-axis current of the DC-AC converter of the AC / DC hybrid micro-grid system reaches i dmax to the time of the entire transient process, and the value of α is in the range of 0 < α < 1.

[0100] Embodiment two:

[0101] In the embodiment of the present example, the equivalent circuit model step of the AC / DC hybrid micro-grid system comprises: in order to obtain the large signal stability criterion of the AC / DC hybrid micro-grid system in island mode, first, a simplified equivalent circuit model of the entire system is established. The main circuit and control block diagram of the AC / DC hybrid micro-grid system are as shown in Figure 2 and Figure 3 In island mode, the DC-AC converter between the AC and DC bus is taken as the main control unit, and the droop control is used to provide voltage and frequency support for the system. The circuit topology of the DC-AC converter is as shown in Figure 4 Figure 5 ​The DC-AC converter droop control block diagram, the main droop controller includes power control, voltage and current double closed loop control two links, wherein the power control includes instantaneous power calculation module, droop characteristic control module and reference voltage synthesis link three parts. The AC bus voltage and current are collected, the instantaneous active power P and the instantaneous reactive power Q output by the distributed power are calculated by the instantaneous power calculation module, and formulas (1) and (2) are applied; the given active power and reactive power are compared, and the given values of the frequency f and the voltage amplitude U of the AC bus are obtained through the droop control; then the voltage amplitude and phase are synthesized to obtain the given value U of the voltage in the abc coordinate system abcref , the given value U dref , U qref of the double closed loop voltage outer ring is obtained after abc-dq coordinate transformation; finally, the driving signal of the DC-AC converter is obtained through the double closed loop controller and the SPWM module, and the voltage and frequency control are realized.

[0102] P=U d I d +U q I q (1)

[0103] Q=U q I d -U d I q (2)

[0104] Wherein, I d , I q and U d , U q are the components of the AC bus current and voltage after abc-dq coordinate transformation in d-axis and q-axis.

[0105] In order to simplify the modeling of DC-AC converter, coordinate transformation is needed, and the transformation matrix of three-phase static coordinate system to synchronous rotating dq coordinate system is:

[0106]

[0107] According to (3), the mathematical model of DC-AC converter in synchronous rotating dq coordinate system can be obtained:

[0108]

[0109] Wherein, e d and e q are the projections of grid electromotive force vector E in d-axis and q-axis. i d , i q are the projections of three-phase DC-AC converter current in d-axis and q-axis. s d and sq are the switching function components of the DC-AC converter on the d-axis and q-axis respectively. Since the DC-AC converter works under unity power factor, i sq = 0, the power delivered to the DC side by the q-axis circuit is zero, thus the DC-AC converter model can be simplified as:

[0110]

[0111] where i dc is the DC bus current, i L is the current through the equivalent resistance R L .

[0112] The energy storage unit is connected to the DC bus through a bidirectional DC-DC converter, which adopts constant voltage control to maintain the DC bus voltage constant, and adjusts the power exchange between itself and the system according to the difference between the power generated by the DC micro source and the power consumed by the load, in order to maintain the system power balance. The control principle of the bidirectional DC-DC converter is shown in Figure 6 , which adopts voltage and current double closed loop control. The DC micro source, i.e. photovoltaic converter, adopts PQ control to access the AC bus, and the constant voltage control method of the energy storage unit DC-DC converter is considered. In order to obtain a generalized equivalent model, the power exchange between the energy storage unit and the system is represented by P b . As shown in Figure 7 , P b is positive, indicating that the energy storage unit works in discharge mode; P b is negative, indicating that the energy storage unit works in charging mode, and the working mode of the energy storage unit is only distinguished by the sign of P b .

[0113] The DC-AC converter of the DC micro source adopts PQ control, and its output voltage is determined by the AC bus. Its control block diagram is shown in Figure 8 . First, the abc coordinate system is transformed into dq coordinate system to obtain the reference value of voltage on the d-axis and q-axis, and then the PI controller is used to generate the reference value of current on the d-axis and q-axis, and finally the current inner loop control is used to generate voltage vectors u sd and u sq , and the driving signal of the photovoltaic distributed power DC-AC converter is obtained. The AC side photovoltaic micro source and the PQ controlled DC-AC converter are equivalent to a current source, and its output power is P G , as shown in Figure 9 .

[0114] Based on the above analysis, the equivalent model of the AC / DC hybrid microgrid in dq coordinate system is shown in Figure 10 . Among them, the current variables i1, i2, the voltage variables V1, V dc.

[0115] In Figure 10 , R1, L1 are the line equivalent resistance and inductance respectively; L s is the filter inductance, R s is the equivalent resistance thereof, C s is the filter capacitance, P b represents the power of the energy storage unit; the DC side load under closed-loop control is equivalent to a constant power load, which is represented by power P1. In the equivalent model Figure 10 , the i0 of the controlled current source is controlled by the voltage v d , which satisfies:

[0116]

[0117] In the embodiment of the present example, the steady-state operating point of the AC / DC hybrid microgrid system includes ensuring that the power step value under large signal disturbance is within the power range provided by the system, and ensuring that the microgrid system has a steady-state operating point. According to Figure 10 , the AC side distributed micro-source and the converter are equivalent to a power source P W with an internal resistance R0, and the steady-state operating point of the system can be represented as:

[0118]

[0119] According to (7), the negative impedance characteristic curve of the constant power load and the characteristic curve of the equivalent voltage source are shown as curve 1 and curve 2 in Figure 11 , and the steady-state operating point of the system is obtained as follows:

[0120]

[0121] According to (8), the power provided by the system satisfies:

[0122]

[0123] Equation (9) obtains the maximum value of the power that the system can provide to the constant power load, V s is the voltage across the equivalent power source, and P b represents two operating states of the energy storage unit. When the energy storage unit is discharging, the AC / DC hybrid microgrid system can carry a larger power constant power load.

[0124] In the embodiment of the present example, the step of establishing a nonlinear model of the AC / DC hybrid microgrid system includes, according to the mixed potential function theory and Figure 10 topology, based on the current variable and the voltage variable, the power source P G equivalent to the photovoltaic micro-source, the line equivalent resistance R1, the inductance equivalent resistance R s , and the AC side output port Vd Write the current potential function; for energy storage unit P b Write the voltage potential function for the constant power load P1 and the output current i0 of the converter.

[0125] The current potential function is:

[0126]

[0127] The voltage potential function is:

[0128]

[0129] Combining (10) and (11), the mixed potential function of the system is:

[0130]

[0131] Verifying the accuracy of (13), we obtain:

[0132]

[0133] Obviously, equation (14) satisfies Kirchhoff's voltage and current laws and the theory of hybrid potential function, therefore the nonlinear model of the AC / DC hybrid microgrid system shown in equation (13) is correct.

[0134] In this example embodiment, the large-signal stability criterion step for the AC / DC hybrid microgrid system is as follows: based on Figure 3 The control equation for the inner current loop of the DC-AC converter is:

[0135]

[0136] Where, k ip k is the proportional parameter of the inner current loop of the DC-AC converter. ii Let be the integral parameter of the inner current loop of the DC-AC converter. Combining the current potential function (10) and the voltage potential function (11), we can obtain:

[0137]

[0138]

[0139] Among them, A ii (i) represents the second-order partial derivative of the current potential function with respect to the current variable, B vv (v) represents the second-order partial derivative of the voltage potential function with respect to the voltage variable; in order to apply the third stability theorem for mixed potential functions, matrix L needs to be calculated. -1 / 2 A ii (i)L -1 / 2 The smallest eigenvalue μ1 and matrix C -1 / 2 B vv(v)C -1 / 2 The minimum eigenvalue μ2 of the system is calculated according to Figure 10 As shown in the equivalent model of the system, the system contains two inductances L s and L1, so The system contains two capacitances C s , C dc , so Through calculation, it can be obtained that

[0140]

[0141] Through comparison, it can be seen that The value is smaller, so the final

[0142]

[0143]

[0144] In summary, according to the third stability theorem of the mixed potential function, the large signal stability criterion of the system is:

[0145]

[0146] The criterion (21) is related to the power P1 of the constant power load, the power P b of the energy storage unit, the DC bus voltage v dc , the AC filter inductance L s and the equivalent resistance R s , the DC side capacitance C dc , and the current inner loop proportional parameter k ip of the DC-AC converter. When the parameters in the system satisfy formula (21), the AC-DC hybrid microgrid system can restore stable operation under large disturbance conditions.

[0147] However, in the actual system, due to the load step, the output current of the d-axis of the DC-AC converter is very easy to reach the reference current limiting value, and lasts for a period of time, in order to protect the power electronic switching devices in the line, the reference current limiting value is set as i dmax At this time, the instantaneous power of the DC-AC converter is p max When the current value of the d-axis output of the DC-AC converter is i dmax , the criterion shown in formula (21) becomes:

[0148]

[0149] In the AC-DC hybrid microgrid, the d-axis current of the DC-AC converter is set to i dmaxthe ratio of the time of the transient process to the time of the whole transient process is α (0 < α < 1). In combination with formula (21) and formula (22), the large signal stability criterion of the AC-DC hybrid micro-grid system in island mode is:

[0150]

[0151] Comparing formula (23) and formula (21), it can be seen that formula (23) also gives a sufficient condition for the large signal stability of the system, and the criterion of formula (23) contains the parameters such as the power of the constant power load, the power of the energy storage unit, the DC bus voltage, the AC filter inductance and the equivalent resistance, the DC side capacitor, the proportional parameter, the proportional adjustment coefficient of the current controller and the maximum transient power of the DC-AC converter, which further optimizes the large signal stability criterion.

[0152] If the AC-DC hybrid micro-grid system is disturbed, the actual d-axis current i d reaches i dmax in a very short time, and α is taken as 0. At this time, formula (23) can be converted into formula (21).

[0153] Based on the criterion formula (21) and formula (23), the maximum transient power allowed by the AC-DC hybrid micro-grid in island mode can be derived. As can be seen from formula (21), the maximum power of the load is a fixed value. The maximum power of the load obtained based on (23) is closely related to α. In order to clearly show the relationship between the maximum load power derived by the two methods shown, the maximum load power P1 and the α curve under the same parameter conditions are obtained, as shown in Figure 12 and Figure 13 When the battery is discharging or discharging, the P1-α relationship of curve No. 23 is obtained according to formula (23), and the fixed maximum load power of curve No. 21 is obtained according to formula (21).

[0154] It should be noted that although the steps of the method in the present disclosure are described in a specific order in the accompanying drawings, this does not require or imply that the steps must be performed in this specific order, or that all the steps shown must be performed to achieve the desired results. In addition or alternatively, some steps can be omitted, a plurality of steps can be combined into one step, and / or one step can be divided into a plurality of steps, etc.

[0155] In addition, in the present example embodiment, an AC-DC hybrid micro-grid system stability determination device is also provided. Referring to Figure 14 , the AC-DC hybrid micro-grid system stability determination device 200 can include a system modeling module 210, a nonlinear model modeling module 220, a large signal stability criterion generation module 230, and a stability determination module 240. Among them:

[0156] The system modeling module 210 is configured to model a DC-AC converter and a DC-DC converter in a DC-AC hybrid micro-grid system respectively, and generate an equivalent model of the DC-AC hybrid micro-grid system in a dq coordinate system.

[0157] The nonlinear model modeling module 220 is configured to calculate a current potential function of an output voltage and a voltage potential function of an output current in the equivalent model of the DC-AC hybrid micro-grid system in the dq coordinate system respectively based on a hybrid potential function theory, and generate a nonlinear model of the DC-AC hybrid micro-grid system.

[0158] The large-signal stability criterion generating module 230 is configured to generate a steady-state operating point of the DC-AC hybrid micro-grid system based on the equivalent model of the DC-AC hybrid micro-grid system in the dq coordinate system, and generate a large-signal stability criterion of the DC-AC hybrid micro-grid system according to the steady-state operating point and the nonlinear model of the DC-AC hybrid micro-grid system.

[0159] The stability determining module 240 is configured to perform amplitude limiting processing on the large-signal stability criterion based on a preset reference current amplitude limiting value, generate a large-signal stability criterion containing current amplitude limiting, and complete stability determination of the DC-AC hybrid micro-grid system based on the large-signal stability criterion containing current amplitude limiting.

[0160] The specific details of the various modules of the DC-AC hybrid micro-grid system stability determining apparatus described above have been described in detail in the corresponding DC-AC hybrid micro-grid system stability determining method, and thus will not be described here again.

[0161] It should be noted that although several modules or units of the DC-AC hybrid micro-grid system stability determining apparatus 200 are mentioned in the above detailed description, such division is not mandatory. In fact, according to the embodiments of the present disclosure, the features and functions of two or more modules or units described above can be embodied in one module or unit. Conversely, the features and functions of one module or unit described above can be further divided into a plurality of modules or units.

[0162] In addition, the above-described figures are only schematic illustrations of the processes included in the method according to the exemplary embodiments of the present application, and are not for limiting purposes. It is easy to understand that the processes shown in the above-described figures do not indicate or limit the time sequence of these processes. In addition, it is also easy to understand that these processes can be executed synchronously or asynchronously, for example, in multiple modules.

[0163] Other embodiments of the disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the features disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the disclosure being indicated by the following claims.

[0164] It should be understood that the present disclosure is not limited to the precise structures herein described and illustrated in the drawings, and that various modifications and changes can be made without departing from its scope. The scope of the present disclosure is limited only by the claims that follow.

Claims

1. A method for determining stability of an AC / DC hybrid microgrid system, characterized in that, The method comprises: The DC-AC converter and the DC-DC converter in the AC-DC hybrid micro-grid system are modeled respectively, and an equivalent model of the AC-DC hybrid micro-grid system in a synchronous rotating dq coordinate system is generated; Based on the hybrid potential function theory, the current potential function of the output voltage and the voltage potential function of the output current in the equivalent model of the AC-DC hybrid micro-grid system in the synchronous rotating dq coordinate system are calculated respectively, and a nonlinear model of the AC-DC hybrid micro-grid system is generated; Based on the equivalent model of the AC-DC hybrid micro-grid system in the dq coordinate system, a steady-state operating point of the AC-DC hybrid micro-grid system is generated, and a large-signal stability criterion of the AC-DC hybrid micro-grid system is generated according to the steady-state operating point and the nonlinear model of the AC-DC hybrid micro-grid system; amplitude limit the large signal stability criterion based on a preset reference current amplitude limit value to generate a large signal stability criterion containing current amplitude limit i dmax The instantaneous power of the DC-AC converter of the AC-DC hybrid micro-grid system is calculated as p max ; according to the preset reference current amplitude limit value i dmax , instantaneous power p max , the large signal stability criterion is amplitude limited to generate a large signal stability criterion containing current amplitude limit Wherein, a is the DC-AC converter of the AC / DC hybrid micro-grid system d The shaft current reaches i dmax The ratio of the time to the entire transient process, the value range of the alpha is 0 < alpha < 1, R s The inductance equivalent resistance 、k ip The current inner loop proportional parameter of the DC-AC converter, L s The AC filter inductance, C dc The DC side capacitor, P 1 is the power of the constant power load, P b The energy storage unit power, v dc The DC bus voltage, i 0 is the output current of the DC / AC converter, the stability of the AC / DC hybrid micro-grid system is determined based on the current limiting amplitude containing the large signal stability criterion.

2. The method of claim 1, wherein, The method further comprises: The DC-AC converter in the AC-DC hybrid micro-grid system is modeled and converted to generate a DC-AC converter model in the synchronous rotating dq coordinate system as in, e d It is the electric grid electromotive force vector. E Projected on the d-axis, i d It is the projection of the three-phase DC-AC converter current onto the d-axis. v d It is the d-axis voltage of the equivalent controlled current source. v dc It is the DC bus voltage. i dc It is the DC side bus current. ,i L Through equivalent resistance R L The current.

3. The method of claim 2, wherein, The method further comprises: Modeling the DC-AC converter and the DC-DC converter in the AC-DC hybrid micro-grid system respectively, generating an equivalent model of the AC-DC hybrid micro-grid system in a synchronous rotating dq coordinate system, the current of a controlled current source in the equivalent model i 0controlled by the current source v d , satisfies: wherein, i 1 is an electric current variable in the equivalent model, V dc is a voltage variable in the equivalent model.

4. The method of claim 3, wherein, The method further comprises: Based on the hybrid potential function theory, the current potential function of the output voltage and the voltage potential function of the output current in the equivalent model of the AC-DC hybrid micro-grid system in the synchronous rotating dq coordinate system are calculated respectively, and a nonlinear model of the AC-DC hybrid micro-grid system is generated as where variables i and v are current and voltage values, respectively, P G is the power source equivalent to photovoltaic micro-source, R 1 is the line equivalent resistance, i 2 is the current flowing through the resistance R 1, R s is the inductance equivalent resistance, i 1 is the current flowing through the resistance R s V d is the AC side output port voltage of the DC / AC converter, P b is the power of the energy storage unit, P 1 is the power of the constant power load, i 0 is the output current of the DC / AC converter, V 1 is the voltage across the capacitor C s .​ 5. The method of claim 4, wherein, The method further comprises: Based on the equivalent model of the AC-DC hybrid micro-grid system in the dq coordinate system, the steady-state operating point of the AC-DC hybrid micro-grid system satisfies wherein, V s V is the voltage across the equivalent power source, R 0 is the internal resistance of the equivalent power source, P b P is the charge and discharge power of the energy storage unit.

6. The method of claim 5, wherein, The method further comprises: According to the steady-state operating point and the nonlinear model of the AC-DC hybrid micro-grid system, the large-signal stability criterion of the AC-DC hybrid micro-grid system is generated as wherein, P 1 is the power of the constant power load, P b is the power of the energy storage unit, v dc is the DC bus voltage, L s is the AC filter inductance, R s is the inductance equivalent resistance, C dc is the DC side capacitance, k ip is the current inner loop proportional parameter of the DC-AC converter.

7. A hybrid AC / DC microgrid system stability determination device based on the method of any one of claims 1-6, characterized by, The device comprises: A system modeling module is configured to model the DC-AC converter and the DC-DC converter in the AC-DC hybrid micro-grid system respectively, and generate an equivalent model of the AC-DC hybrid micro-grid system in a dq coordinate system; A nonlinear model modeling module is configured to calculate the current potential function of the output voltage and the voltage potential function of the output current in the equivalent model of the AC-DC hybrid micro-grid system in the dq coordinate system based on the hybrid potential function theory, and generate a nonlinear model of the AC-DC hybrid micro-grid system; A large-signal stability criterion generation module is configured to generate a steady-state operating point of the AC-DC hybrid micro-grid system based on the equivalent model of the AC-DC hybrid micro-grid system in the dq coordinate system, and generate a large-signal stability criterion of the AC-DC hybrid micro-grid system according to the steady-state operating point and the nonlinear model of the AC-DC hybrid micro-grid system; A stability determination module is configured to perform amplitude limiting processing on the large-signal stability criterion based on a preset reference current amplitude, generate a large-signal stability criterion containing current amplitude, and complete stability determination of the AC-DC hybrid micro-grid system based on the large-signal stability criterion containing current amplitude.

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