A Simplified Transient Stability Analysis Method for AC Microgrids Based on Park Transform

Abstract

The present invention relates to the technical field of AC microgrids, and provides a simplified transient stability analysis method for AC microgrids based on Park transformation, constructs an equivalent circuit model of the AC microgrid, establishes Kirchhoff voltage and current equations; combines the voltage source converter droop equation, Kirchhoff voltage and current equations, obtains the voltage and current characteristic curves of each unit of the AC microgrid system and analyzes the steady-state operation point of the AC microgrid; performs Park transformation on the state equation of the system in the abc coordinate system to obtain the system state equation in the dq coordinate system; extracts the Jacobian matrix of the system operation state according to the system state equation in the dq coordinate system; and judges the transient stability of the system based on the Jacobian matrix of the system state and the asymptotic stability control theory of linear steady-state systems. The present invention realizes transient stability analysis while simplifying the analysis complexity, reducing the system calculation burden, and realizing online analysis of the transient stability of the AC microgrid.

Description

Technical Field

[0001] The present invention relates to the technical field of AC microgrids, and in particular to a simplified transient stability analysis method of an AC microgrid based on Park transformation. Background Art

[0002] AC microgrid is a small-scale power system that usually includes a variety of distributed energy resources (such as solar energy, wind energy, energy storage systems, etc.), load equipment and energy management systems. It can achieve efficient use of renewable energy, reliability and flexibility of energy supply when connected to the main grid or operating independently. However, with the increasing proportion of renewable energy access, the transient stability of microgrids after large disturbances or faults has been greatly challenged. Most of the existing transient stability analysis of AC microgrids is based on time domain simulation method or Lyapunov direct method.

[0003] Time domain simulation usually requires a lot of calculations, especially in the case of complex systems or large-scale systems, the calculation cost may be very high; although the Lyapunov direct method based on the transient energy function equates a series of devices in the AC microgrid to motor-type micro-sources and power electronic-type micro-sources, the transient energy function it constructs is still relatively complex and not conducive to solving; for large-scale systems, the time domain simulation method and the Lyapunov direct method based on the transient energy function may face problems such as high model complexity, complex energy functions, and large amount of calculations, which are difficult to deal with effectively. Time domain simulation usually requires some assumptions and initial conditions to be determined in advance, and it is impossible to consider all possible situations and uncertain factors that may occur in the system; the Lyapunov direct method based on the transient energy function can only construct a system stable operation domain, and for operating points outside the stable operation domain, it is impossible to determine whether the system can operate stably.

[0004] In addition, in addition to the above two methods, the DC microgrid can also use the equivalent circuit method and combine it with the transient stability criterion of the linear steady-state system to judge the transient stability of the microgrid. However, since this method is only applicable to linear steady-state systems, it can only be applied in DC systems. Summary of the invention

[0005] The present invention aims to solve at least one of the technical problems existing in the related art. To this end, the present invention provides a simplified transient stability analysis method for an AC microgrid based on Park transformation, by introducing Park transformation in a three-phase system, thereby using a dq two-phase rotating coordinate system to characterize the AC microgrid three-phase system, converting the three-phase phasor into a steady state for stability analysis, and then introducing an equivalent circuit method, realizing transient stability analysis while simplifying the analysis complexity, reducing the system calculation burden, and realizing online analysis of the transient stability of the AC microgrid.

[0006] The present invention provides a simplified transient stability analysis method for an AC microgrid based on Park transformation, comprising:

[0007] S1: Construct an equivalent circuit model of the AC microgrid, and establish Kirchhoff voltage and current equations based on the equivalent circuit model of the AC microgrid and the voltage source converter droop equation;

[0008] S2: By combining the voltage source converter droop equation, Kirchhoff voltage and current equations, the voltage and current characteristic curves of each unit of the AC microgrid system are obtained, and the steady-state operation point of the AC microgrid is analyzed based on the voltage and current characteristic curves of each unit of the system;

[0009] S3: Obtain the state equation of the system in the abc coordinate system according to the steady-state operation point of the AC microgrid and Kirchhoff voltage and current equations;

[0010] S4: By performing Park transformation on the state equation of the system in the abc coordinate system, the state equation of the system in the dq coordinate system is obtained;

[0011] S5: Extract the Jacobian matrix of the system operation state according to the system state equation in the dq coordinate system;

[0012] S6: Determine the transient stability of the system based on the Jacobian matrix of the system operating state and the asymptotic stability control theory of linear steady-state systems.

[0013] According to the present invention, a simplified transient stability analysis method for an AC microgrid based on Park transformation is provided, which also includes a photovoltaic power generation system and an energy storage system connected to the main grid of the AC microgrid after being inverted by a voltage source converter, and together with the AC system, they are equivalent to a controlled voltage source; the AC main grid-to-ground and phase-to-phase capacitances are equivalent to capacitances; the absorbed power of the constant power load is equivalent to a controlled current source, and the AC microgrid equivalent circuit model established after the equivalent is a system in which a controlled voltage source is connected in series with a resistor and an inductor, and then connected in parallel with a controlled current source, a constant power load equivalent resistor, and an equivalent capacitor.

[0014] According to a simplified transient stability analysis method of an AC microgrid based on Park transformation provided by the present invention, a controlled current source calculation expression established based on an AC microgrid equivalent circuit model is:

[0015]

[0016] in, is the output current of the controlled current source, is the equivalent power, is the impedance angle, is the equivalent capacitor three-phase voltage;

[0017] The calculation expression of the voltage source converter droop equation is:

[0018]

[0019] in, is the three-phase voltage of the AC system, is the three-phase current of the AC system, is the three-phase voltage reference value of the busbar at the voltage source converter outlet; is the droop coefficient of the AC microgrid voltage source converter.

[0020] According to a simplified transient stability analysis method of an AC microgrid based on Park transformation provided by the present invention, the method also includes establishing Kirchhoff voltage and current equations based on an AC microgrid equivalent circuit model and a voltage source converter droop equation:

[0021]

[0022]

[0023] in, is the equivalent capacitor three-phase voltage, is the equivalent resistance connected in series with the equivalent voltage source, is the equivalent inductance in series with the equivalent voltage source, is the equivalent resistance in parallel with the controlled current source, is the equivalent capacitance in parallel with the controlled current source.

[0024] According to a simplified transient stability analysis method of an AC microgrid based on Park transformation provided by the present invention, the method further includes the step S2, in which the voltage-current characteristic curve equation of constant power load operation is obtained by combining the voltage source converter droop equation, Kirchhoff voltage and current equation:

[0025]

[0026] in, is the equivalent capacitor three-phase voltage, is the three-phase voltage of the AC system, is the output current of the controlled current source, is the droop coefficient of the AC microgrid voltage source converter, is the equivalent resistance connected in series with the equivalent voltage source, is the equivalent resistance in parallel with the controlled current source;

[0027] The power source volt-ampere characteristic curve is drawn based on the voltage source converter droop equation, and the constant power load volt-ampere characteristic curve is drawn based on the constant power load operation voltage-current characteristic curve equation. The intersection of the power source volt-ampere characteristic curve and the constant power load volt-ampere characteristic curve is the initial operating point of the system. The stable operating point of the AC microgrid system should meet the following conditions:

[0028]

[0029] in, for The slope of for The slope of is the power supply volt-ampere characteristic curve, It is the constant power load volt-ampere characteristic curve.

[0030] According to a simplified transient stability analysis method of an AC microgrid based on Park transformation provided by the present invention, the method further includes the following step S3: when the AC microgrid system is in stable operation, the first system dynamic equation is obtained according to Kirchhoff's voltage equation:

[0031]

[0032] in, is the three-phase current of the AC system, is the three-phase voltage reference value of the busbar at the voltage source converter outlet; is the droop coefficient of the AC microgrid voltage source converter, is the equivalent capacitor three-phase voltage, is the equivalent resistance connected in series with the equivalent voltage source, is the equivalent inductance connected in series with the equivalent voltage source;

[0033] The state space equation of the first system in the abc coordinate system is:

[0034]

[0035] in, is the current flowing out of the converter A phase, is the current flowing out of the converter B phase, is the current flowing out of the converter C phase, is the reference value of the converter A phase voltage, is the converter B phase voltage reference value, is the converter C phase voltage reference value, is the equivalent capacitor A phase voltage, is the equivalent capacitor B phase voltage, is the equivalent capacitor C phase voltage,

[0036] According to Kirchhoff's current equation, the dynamic equation of the second system is:

[0037]

[0038] in, is the equivalent resistance in parallel with the controlled current source, is the equivalent capacitance in parallel with the controlled current source, Output current for the controlled current source;

[0039] The state space equation of the second system in the abc coordinate system is:

[0040]

[0041] in, The output of the A phase current is the controlled current source. The controlled current source outputs the B phase current, The C-phase current is output as a controlled current source.

[0042] According to a simplified transient stability analysis method of an AC microgrid based on Park transformation provided by the present invention, the method further includes, in step S4, transforming the state space equation of the first system in the abc coordinate system into the state space equation of the first system in the dq coordinate system by Park transformation:

[0043]

[0044] in, Outgoing converter Shaft current, For outgoing converter Shaft current, is the converter d-axis voltage reference value, is the converter q-axis voltage reference value, is the equivalent capacitance d-axis voltage, is the equivalent capacitance q-axis voltage;

[0045] Through Park transformation, the state space equation of the second system in the abc coordinate system is transformed into the state space equation of the second system in the dq coordinate system:

[0046]

[0047] in, The d-axis current is output by the controlled current source, The q-axis current is output as a controlled current source.

[0048] According to a simplified transient stability analysis method of an AC microgrid based on Park transformation provided by the present invention, the method further includes, in the step S5, simultaneously solving the first system state space equation in the dq coordinate system and the second system state space equation in the dq coordinate system to obtain the AC microgrid system state space equation group:

[0049]

[0050]

[0051] in, is the state variable of the AC microgrid system, is the Jacobian matrix, For Find the partial derivative, For Find the partial derivative, For Find the partial derivative, For Find the partial derivative, is the derivative of the state space equations of the AC microgrid system;

[0052] Based on the state space equations of the AC microgrid system, the Jacobian matrix representing the operating state of the AC microgrid is obtained:

[0053]

[0054] in, is the equivalent power.

[0055] According to a simplified transient stability analysis method for an AC microgrid based on Park transformation provided by the present invention, the method also includes the step S6, in which when all characteristic roots of the Jacobian matrix of the system operation state have negative real parts, the AC microgrid has transient stability; when one or more characteristic roots among all characteristic roots of the Jacobian matrix of the system operation state are greater than or equal to zero, the AC microgrid does not have transient stability.

[0056] According to a simplified transient stability analysis method for an AC microgrid based on Park transformation provided by the present invention, the method also includes modifying the Jacobian matrix of the system operating state according to the normal operating state of the system or the fault operating state of the system, and judging the transient stability of the system under multiple operating states.

[0057] The above one or more technical solutions in the embodiments of the present invention have at least one of the following technical effects:

[0058] The present invention provides a simplified transient stability analysis method for an AC microgrid based on Park transformation. By introducing Park transformation in a three-phase system, a dq two-phase rotating coordinate system is used to characterize the three-phase system of the AC microgrid, and stability analysis is performed after the three-phase phasors are converted into steady-state phases. Then, an equivalent circuit method is introduced to realize transient stability analysis while simplifying the analysis complexity, reducing the system calculation burden, and realizing online analysis of the transient stability of the AC microgrid.

[0059] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0061] Figure 1 It is a flow chart of a simplified transient stability analysis method of an AC microgrid based on Park transformation provided by the present invention.

[0062] Figure 2 It is a schematic diagram of the equivalent circuit structure of an AC microgrid according to a simplified transient stability analysis method of an AC microgrid based on Park transformation provided by the present invention.

[0063] Figure 3 It is a schematic diagram of steady-state operating points of a simplified transient stability analysis method for an AC microgrid based on Park transformation provided by the present invention. DETAILED DESCRIPTION

[0064] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention. The following embodiments are used to illustrate the present invention, but cannot be used to limit the scope of the present invention.

[0065] In the description of the embodiments of the present invention, it should be noted that the terms “first” and “second” are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0066] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the embodiment of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0067] Combine the following Figures 1 to 3The present invention describes a simplified transient stability analysis method for an AC microgrid based on Park transformation.

[0068] A simplified transient stability analysis method for AC microgrid based on Park transformation, including:

[0069] S1: Construct an equivalent circuit model of the AC microgrid, and establish Kirchhoff voltage and current equations based on the equivalent circuit model of the AC microgrid and the voltage source converter droop equation;

[0070] In the AC microgrid, the photovoltaic power generation system and the energy storage system are connected to the AC microgrid main grid after being inverted by the voltage source converter, and the output voltage and current are controlled in real time together with the AC system based on the voltage-current droop control. Therefore, they can be equivalent to controlled voltage sources; the AC main grid-to-ground and phase-to-phase capacitance can be equivalent to capacitance; in the transient process after the AC microgrid is disturbed, the absorbed power of the constant power load can be approximately regarded as a constant value, which can be equivalent to a controlled current source, such as Figure 2 As shown in the figure, after equivalent, the AC microgrid can be regarded as a system in which a controlled voltage source is connected in series with a resistor and an inductor, and then connected in parallel with a controlled current source, a constant power load equivalent resistor and an equivalent capacitor.

[0071] The calculation expression of the controlled current source established based on the AC microgrid equivalent circuit model is:

[0072]

[0073] in, is the output current of the controlled current source, is the equivalent power, is the impedance angle, is the equivalent capacitor three-phase voltage;

[0074]

[0075] in, The output power of the photovoltaic power source controlled by maximum power tracking, is a constant power load;

[0076] The calculation expression of the voltage source converter droop equation is:

[0077]

[0078] in, is the three-phase voltage of the AC system, is the three-phase current of the AC system, is the three-phase voltage reference value of the busbar at the voltage source converter outlet; is the droop coefficient of the AC microgrid voltage source converter.

[0079] Based on the AC microgrid equivalent circuit model and the voltage source converter droop equation, Kirchhoff voltage and current equations are established as follows:

[0080]

[0081]

[0082] in, is the equivalent capacitor three-phase voltage, is the equivalent resistance connected in series with the equivalent voltage source, is the equivalent inductance in series with the equivalent voltage source, is the equivalent resistance in parallel with the controlled current source, is the equivalent capacitance in parallel with the controlled current source.

[0083] S2: By combining the voltage source converter droop equation, Kirchhoff voltage and current equations, the voltage and current characteristic curves of each unit of the AC microgrid system are obtained, and the steady-state operation point of the AC microgrid is analyzed based on the voltage and current characteristic curves of each unit of the system;

[0084] By combining the voltage source converter droop equation, Kirchhoff voltage and current equations, the constant power load operation voltage and current characteristic curve equation is obtained as follows:

[0085]

[0086] in, is the equivalent capacitor three-phase voltage, is the three-phase voltage of the AC system, is the output current of the controlled current source, is the droop coefficient of the AC microgrid voltage source converter, is the equivalent resistance connected in series with the equivalent voltage source, is the equivalent resistance in parallel with the controlled current source;

[0087] like Figure 3 As shown, the power supply volt-ampere characteristic curve is drawn based on the voltage source converter droop equation, and the constant power load volt-ampere characteristic curve is drawn based on the constant power load operation voltage-current characteristic curve equation. The intersection points A and B of the power supply volt-ampere characteristic curve and the constant power load volt-ampere characteristic curve are the initial operating points of the system. The stable operating point of the AC microgrid system should meet the following conditions:

[0088]

[0089] in, for The slope of for The slope of is the power supply volt-ampere characteristic curve, It is the constant power load volt-ampere characteristic curve.

[0090] The constant power load operating voltage-current characteristic curve is modified according to the real-time power of the constant power load. When the constant power load power increases, the system voltage cannot change suddenly. In order to meet the load power demand, the load current needs to be increased. At this time, based on the voltage source converter droop equation, the system voltage will decrease. The curve becomes Curve, if Curve and If the curve can intersect at the new operating points A- and B-, the system has the ability to recover from transient stability, otherwise the system does not have the ability to recover from transient stability; similarly, when the constant power load power decreases, since the system voltage cannot change suddenly, in order to meet the load power demand, the load current needs to be reduced. At this time, based on the voltage source converter droop equation, the system voltage will rise, The curve becomes Curve, if Curve and If the curves can intersect at the new operating points A+ and B+, the system has the ability to recover from transient stability; otherwise, the system does not have the ability to recover from transient stability.

[0091] S3: Obtain the state equation of the system in the abc coordinate system according to the steady-state operation point of the AC microgrid and Kirchhoff voltage and current equations;

[0092] When the AC microgrid system is running stably, the system dynamic equation is obtained according to Kirchhoff voltage equation:

[0093]

[0094] in, is the three-phase current of the AC system, is the three-phase voltage reference value of the busbar at the voltage source converter outlet; is the droop coefficient of the AC microgrid voltage source converter, is the equivalent capacitor three-phase voltage, is the equivalent resistance connected in series with the equivalent voltage source, is the equivalent inductance connected in series with the equivalent voltage source;

[0095] The state space equation of the first system in the abc coordinate system is:

[0096]

[0097] in, is the current flowing out of the converter A phase, is the current flowing out of the converter B phase, is the current flowing out of the converter C phase, is the reference value of the converter A phase voltage, is the converter B phase voltage reference value, is the converter C phase voltage reference value, is the equivalent capacitor A phase voltage, is the equivalent capacitor B phase voltage, is the equivalent capacitor C phase voltage,

[0098] According to Kirchhoff's current equation, the dynamic equation of the second system is:

[0099]

[0100] in, is the equivalent resistance in parallel with the controlled current source, is the equivalent capacitance in parallel with the controlled current source, Output current for the controlled current source;

[0101] The state space equation of the second system in the abc coordinate system is:

[0102]

[0103] in, The output of the A phase current is the controlled current source. The controlled current source outputs the B phase current, The C-phase current is output as a controlled current source.

[0104] S4: By performing Park transformation on the state equation of the system in the abc coordinate system, the state equation of the system in the dq coordinate system is obtained;

[0105] Through Park transformation, the state space equation of the first system in the abc coordinate system is transformed into the state space equation of the first system in the dq coordinate system:

[0106]

[0107] in, For outgoing converter Shaft current, For outgoing converter Shaft current, is the converter d-axis voltage reference value, is the converter q-axis voltage reference value, is the equivalent capacitance d-axis voltage, is the equivalent capacitance q-axis voltage;

[0108] Through Park transformation, the state space equation of the second system in the abc coordinate system is transformed into the state space equation of the second system in the dq coordinate system:

[0109]

[0110] in, The d-axis current is output by the controlled current source, The q-axis current is output as a controlled current source.

[0111] At this point, the nonlinear time-varying system of the AC microgrid is transformed into a linear system in the dq coordinate system. The equivalent circuit method has the basis for applying to the transient stability analysis of the AC microgrid, and can determine the transient stability of the system based on control theory.

[0112] S5: Extract the Jacobian matrix of the system operation state according to the system state equation in the dq coordinate system;

[0113] By combining the state space equations of the first system in the dq coordinate system and the state space equations of the second system in the dq coordinate system, the state space equations of the AC microgrid system are obtained as follows:

[0114]

[0115]

[0116] in, is the state variable of the AC microgrid system, is the Jacobian matrix, For Find the partial derivative, For Find the partial derivative, For Find the partial derivative, For Find the partial derivative, is the derivative of the state space equations of the AC microgrid system;

[0117] Based on the state space equations of the AC microgrid system, the Jacobian matrix representing the operating state of the AC microgrid is obtained:

[0118]

[0119] in, is the equivalent power.

[0120] S6: Determine the transient stability of the system based on the Jacobian matrix of the system operating state and the asymptotic stability control theory of linear steady-state systems;

[0121] According to the asymptotic stability control theory of linear steady-state systems, when all the characteristic roots of the Jacobian matrix of the system operation state have negative real parts, the AC microgrid has transient stability; when one or more of the characteristic roots of the Jacobian matrix of the system operation state is greater than or equal to zero, the AC microgrid does not have transient stability.

[0122] The Jacobian matrix of the system operating state is modified according to the normal operating state of the system or the fault operating state of the system, and the transient stability of the system under various operating states is determined.

[0123] The present invention introduces Park transformation in the three-phase system, thereby using a dq two-phase rotating coordinate system to characterize the three-phase system of the AC microgrid, converts the three-phase phasors into steady-state quantities for stability analysis, and then introduces the equivalent circuit method to achieve transient stability analysis while simplifying the analysis complexity, reducing the system calculation burden, and realizing online analysis of the transient stability of the AC microgrid.

[0124] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A simplified transient stability analysis method for AC microgrid based on Park transformation, characterized in that: include: S1: Construct an equivalent circuit model of the AC microgrid, and establish Kirchhoff voltage and current equations based on the equivalent circuit model of the AC microgrid and the voltage source converter droop equation; S2: By combining the voltage source converter droop equation, Kirchhoff voltage and current equations, the voltage and current characteristic curves of each unit of the AC microgrid system are obtained, and the steady-state operation point of the AC microgrid is analyzed based on the voltage and current characteristic curves of each unit of the system; S3: Obtain the state equation of the system in the abc coordinate system according to the steady-state operation point of the AC microgrid and Kirchhoff voltage and current equations; S4: Obtain the system state equation in the dq coordinate system by performing Park transformation on the system state equation in the abc coordinate system; S5: Extract the Jacobian matrix of the system operation state according to the system state equation in the dq coordinate system; S6: Determine the transient stability of the system based on the Jacobian matrix of the system operating state and the asymptotic stability control theory of linear steady-state systems.

2. The simplified transient stability analysis method of AC microgrid based on Park transformation according to claim 1 is characterized in that: The photovoltaic power generation system and energy storage system are connected to the main grid of the AC microgrid after being inverted by a voltage source converter, and are equivalent to a controlled voltage source together with the AC system; the AC main grid-to-ground and phase-to-phase capacitance are equivalent to capacitance; the absorbed power of the constant power load is equivalent to a controlled current source. The equivalent circuit model of the AC microgrid established after the equivalent is a system in which a controlled voltage source is connected in series with a resistor and an inductor, and then in parallel with a controlled current source, a constant power load equivalent resistor, and an equivalent capacitor.

3. The simplified transient stability analysis method of AC microgrid based on Park transformation according to claim 2 is characterized in that: The calculation expression of the controlled current source established based on the AC microgrid equivalent circuit model is: in, is the output current of the controlled current source, is the equivalent power, is the impedance angle, is the equivalent capacitor three-phase voltage; The calculation expression of the voltage source converter droop equation is: in, is the three-phase voltage of the AC system, is the three-phase current of the AC system, is the three-phase voltage reference value of the busbar at the voltage source converter outlet; is the droop coefficient of the AC microgrid voltage source converter.

4. The simplified transient stability analysis method of AC microgrid based on Park transformation according to claim 3 is characterized in that: Based on the AC microgrid equivalent circuit model and the voltage source converter droop equation, Kirchhoff voltage and current equations are established as follows: in, is the equivalent capacitor three-phase voltage, is the equivalent resistance connected in series with the equivalent voltage source, is the equivalent inductance in series with the equivalent voltage source, is the equivalent resistance in parallel with the controlled current source, is the equivalent capacitance in parallel with the controlled current source.

5. The simplified transient stability analysis method of AC microgrid based on Park transformation according to claim 1 is characterized in that: In the step S2, the constant power load operation voltage-current characteristic curve equation is obtained by combining the voltage source converter droop equation, Kirchhoff voltage and current equation: in, is the equivalent capacitor three-phase voltage, is the three-phase voltage of the AC system, is the output current of the controlled current source, is the droop coefficient of the AC microgrid voltage source converter, is the equivalent resistance connected in series with the equivalent voltage source, is the equivalent resistance in parallel with the controlled current source; The power source volt-ampere characteristic curve is drawn based on the voltage source converter droop equation, and the constant power load volt-ampere characteristic curve is drawn based on the constant power load operation voltage-current characteristic curve equation. The intersection of the power source volt-ampere characteristic curve and the constant power load volt-ampere characteristic curve is the initial operating point of the system. The stable operating point of the AC microgrid system should meet the following conditions: in, for The slope of for The slope of is the power supply volt-ampere characteristic curve, It is the constant power load volt-ampere characteristic curve.

6. The simplified transient stability analysis method of AC microgrid based on Park transformation according to claim 1 is characterized in that: In step S3, when the AC microgrid system is in stable operation, the first system dynamic equation is obtained according to Kirchhoff's voltage equation: in, is the three-phase current of the AC system, is the three-phase voltage reference value of the busbar at the voltage source converter outlet; is the droop coefficient of the AC microgrid voltage source converter, is the equivalent capacitor three-phase voltage, is the equivalent resistance in series with the equivalent voltage source, is the equivalent inductance connected in series with the equivalent voltage source; The state space equation of the first system in the abc coordinate system is: in, is the current flowing out of phase A of the voltage source converter, is the current flowing out of the voltage source converter B phase, is the current flowing out of the voltage source converter phase C, is the voltage reference value of phase A of the voltage source converter, is the voltage reference value of phase B of the voltage source converter, is the voltage reference value of phase C of the voltage source converter, is the equivalent capacitor A phase voltage, is the equivalent capacitor B phase voltage, is the equivalent capacitor C phase voltage, According to Kirchhoff's current equation, the dynamic equation of the second system is: in, is the equivalent resistance in parallel with the controlled current source, is the equivalent capacitance in parallel with the controlled current source, Output current for the controlled current source; The state space equation of the second system in the abc coordinate system is: in, The controlled current source outputs the A phase current, The controlled current source outputs the B phase current, The C-phase current is output as a controlled current source.

7. The simplified transient stability analysis method of AC microgrid based on Park transformation according to claim 6 is characterized in that: In step S4, the state space equation of the first system in the abc coordinate system is transformed into the state space equation of the first system in the dq coordinate system through Park transformation: in, The output voltage source converter Shaft current, The output voltage source converter Shaft current, is the voltage source converter d-axis voltage reference value, is the q-axis voltage reference value of the voltage source converter, is the equivalent capacitance d-axis voltage, is the equivalent capacitance q-axis voltage; Through Park transformation, the state space equation of the second system in the abc coordinate system is transformed into the state space equation of the second system in the dq coordinate system: in, The d-axis current is output by the controlled current source, The q-axis current is output as a controlled current source.

8. The simplified transient stability analysis method of AC microgrid based on Park transformation according to claim 7 is characterized in that: In the step S5, the state space equation of the first system in the dq coordinate system and the state space equation of the second system in the dq coordinate system are combined to obtain the state space equation group of the AC microgrid system: in, is the state variable of the AC microgrid system, is the Jacobian matrix, For Find the partial derivative, For Find the partial derivative, For Find the partial derivative, For Find the partial derivative, is the derivative of the state space equations of the AC microgrid system; Based on the state space equations of the AC microgrid system, the Jacobian matrix representing the operating state of the AC microgrid is obtained: in, is the equivalent power.

9. The simplified transient stability analysis method of AC microgrid based on Park transformation according to claim 1, characterized in that: In the step S6, when all characteristic roots of the Jacobian matrix of the system operation state have negative real parts, the AC microgrid has transient stability; when one or more characteristic roots among all characteristic roots of the Jacobian matrix of the system operation state are greater than or equal to zero, the AC microgrid does not have transient stability.

10. The simplified transient stability analysis method of AC microgrid based on Park transformation according to claim 1, characterized in that: The Jacobian matrix of the system operating state is modified according to the normal operating state of the system or the fault operating state of the system, and the transient stability of the system under various operating states is determined.

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