Energy storage converter asymmetric fault analysis method based on virtual synchronous machine technology

Abstract

The application discloses a kind of energy storage converter asymmetric fault analysis methods based on virtual synchronous machine technology, using the vector control strategy based on internal potential orientation, grid-connected point voltage is separated into positive and negative sequence;According to asymmetric fault scene adjustment control link, including using trap filter to filter out the two-frequency fluctuation component in inverter output power, the reference voltage in reactive-voltage loop is switched from rated voltage to grid-connected point positive sequence voltage, negative sequence is inhibited by balance current strategy;Based on reactive-voltage circuit equation and reactive-voltage control equation, establish the equation about internal potential;According to virtual impedance ring control equation, calculate current reference value before fault and substitute into the virtual impedance ring control equation after Laplace transform, establish the equation about fault current dq axis component;According to the equation about internal potential and the equation about fault current dq axis component, construct the analytical expression of fault current.The application considers the influence of inverter control link on fault current transient characteristic, and analytical precision is high.

Description

Technical Field

[0001] This invention relates to the field of power system relay protection technology, and in particular to a method for analyzing asymmetrical faults in energy storage converters based on virtual synchronous machine technology. Background Technology

[0002] Under the "dual carbon" goal, renewable energy sources, represented by wind and solar power, have achieved rapid development. Most renewable energy is connected to the grid via power electronic devices. The large-scale integration of these devices reduces system inertia and damping, posing a serious threat to the stable operation of the grid. Energy storage converters, due to their fast charging and discharging characteristics, have become a crucial measure to ensure the stable operation of high-penetration grids. Therefore, virtual synchronous generator (VSG) technology and its application and control techniques in energy storage converters have attracted widespread attention from scholars both domestically and internationally. Energy storage converters based on VSG technology possess the same stability as synchronous generators, improving system inertia and facilitating the integration of distributed power sources into the grid. VSGs simulate the frequency and voltage regulation characteristics of synchronous generators, and by introducing rotor motion equations, they can effectively improve the low inertia and weak damping phenomena caused by the "dual high" characteristics of the power system. However, VSGs differ from synchronous generators in short-circuit current, energy source, and control parameters, making methods used to study synchronous generator fault characteristics difficult to apply to VSGs. Furthermore, the complexity of VSG control strategies complicates the analysis and extraction of fault currents. Fault characteristic analysis is the premise and foundation of relay protection research. Therefore, it is necessary to study the fault characteristics of VSG in order to provide theoretical reference for relay protection setting calculation of new power systems.

[0003] VSG is a grid-connected control technology. Grid-connected converters are commonly described using a voltage source series impedance model. Some literature has established simplified grid-connected inverter circuits based on this model. Studies have found that when the grid voltage drops symmetrically, the current output by the VSG includes an exponentially decaying DC component and a periodic component with decreasing amplitude. However, the voltage source series impedance model does not fully consider the influence of the control loop and cannot well describe the transient transition process of the fault current. Based on this, some literature has established an analytical expression for the fault current during a symmetrical short circuit in the grid, based on the control equations of each VSG component and the reactive power-voltage circuit equations. The transient characteristics of the VSG fault current during an asymmetrical short circuit on the grid side still need further research.

[0004] In summary, the limitations of existing research methods lie in two aspects: insufficient consideration of the influence of control links and failure to consider asymmetrical voltage drop scenarios. Further improvements are needed in the study of the transient characteristics of fault current in energy storage converters based on VSG technology. Summary of the Invention

[0005] The purpose of this invention is to provide a method for analyzing asymmetrical faults in energy storage converters based on virtual synchronous machine technology. This method fully considers the influence of inverter control loops on the transient characteristics of fault current, and can improve the analysis accuracy when performing analysis calculations of transient fault currents in inverter power supplies, thus achieving accurate analysis of fault transient currents. At the same time, the proposed method is applicable to both symmetrical and asymmetrical faults.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] The present invention discloses a method for analyzing asymmetric faults in energy storage converters based on virtual synchronous machine technology, characterized by comprising the following steps:

[0008] Step 1: Adopt a vector control strategy based on internal potential orientation to orient the internal potential to the d-axis and separate the positive and negative sequence of the grid connection point voltage;

[0009] Step 2: Adjust the control loop according to the specific scenario of asymmetrical fault, including using a notch filter to filter out the second harmonic fluctuation component in the inverter output power, switching the reference voltage in the reactive power-voltage loop from the rated voltage to the positive sequence voltage at the grid connection point, and using a balance current control strategy to suppress negative sequence.

[0010] Step 3: Based on the reactive-voltage circuit equation and the reactive-voltage control equation, establish the equation regarding the internal potential;

[0011] Step 4: Based on the virtual impedance loop control equation, calculate the reference value of the current before the fault and substitute it into the virtual impedance loop control equation after Laplace transform to establish the equation for the dq axis component of the fault current.

[0012] Step 5: Based on the equations for the internal potential and the dq-axis components of the fault current, construct analytical expressions for the internal potential and the fault current.

[0013] Preferably, step 1 further includes the following:

[0014] The active-frequency circuit outputs the phase angle. Establish a dq rotating coordinate system oriented around the d-axis, and determine the d- and q-axis components of the internal potential before the fault. , , The internal potential amplitude before the fault and after the fault , , The output internal potential amplitude of the reactive power-voltage link; the grid connection point voltage is collected. U abc The positive and negative sequence voltages at the grid connection point are obtained by separating the positive and negative sequences. U + The voltage at the grid connection point before the fault U 0 and the positive sequence voltage at the grid connection point after the faultU + Perform abc / dq coordinate transformation to determine the voltage at the grid connection point before the fault. U The dq-axis component of 0 is , After the fault, the positive sequence voltage at the grid connection point U + The dq axis components are , Meanwhile, the voltage at the grid connection point before the fault... U 0 Angle ahead of the d-axis Voltage at the grid connection point before the fault U 0 and its d-axis and q-axis components satisfy the following: The angle by which the positive sequence voltage at the grid connection point leads the d-axis after a fault. After the fault, the positive sequence voltage at the grid connection point U + It satisfies the following relationship with its d-axis and q-axis components: .

[0015] Preferably, step 2 further includes the following:

[0016] 1) Use a notch filter to remove inverter output power. P e , Q e The second harmonic frequency fluctuation component causes the reference voltage in the reactive power-voltage control equation to switch from the rated voltage to the positive sequence voltage at the grid connection point after the fault. The adjusted power loop control equation is as follows:

[0017] (1)

[0018] In the formula, , The phase and amplitude of the internal potential of the power loop output. P 0、 Q 0 represents the average components of the active and reactive power output of the inverter. , These are the reference values ​​for active and reactive power, respectively. J , D , K , w , w N These represent virtual inertia, damping coefficient, reactive voltage integral coefficient, internal potential angular frequency, and rated angular frequency, respectively. s This is the Laplace operator. Among them, the active and reactive power reference values ​​are... , The value is

[0019] (2)

[0020] In the formula, This is the reference value for active power before the fault. U N , I N These are the rated voltage and rated current, respectively. I max The maximum current allowed to pass through the inverter is 1.5, which is taken as 1.5 in this invention. I N .

[0021] 2) By adjusting the power loop, the three-phase output internal potential is balanced. To obtain the positive-sequence reference current, the positive-sequence voltage at the grid connection point needs to be connected, which is called balanced current control. The virtual impedance loop control equation after adopting balanced current control is:

[0022] (3)

[0023] In the formula, R v , L v For virtual resistance and reactance, , The d-axis and q-axis components of the positive sequence current reference value;

[0024] Preferably, step 3 further includes the following:

[0025] As shown in step 2, the average component of the inverter output power needs to be substituted into the power loop. Average component of inverter output reactive power. Q The formula for calculating 0 is:

[0026] (4)

[0027] In the formula, , These are the d-axis and q-axis components of the positive sequence current output by the inverter.

[0028] Substituting equation (4) into the reactive power-voltage control equation in equation (1) and further differentiating, we can obtain...

[0029] (5)

[0030] As shown in equation (5), and Analytical expression and internal potential E ref The expression is closely related.

[0031] Preferably, step 4 further includes the following:

[0032] 1) Substitute the dq-axis components of the pre-fault internal potential and grid connection point voltage from step 1 into equation (3) to obtain the dq-axis component of the pre-fault current reference value. , for

[0033] (6)

[0034] In the formula, The virtual impedance magnitude is expressed as follows: , The virtual impedance phase angle is expressed as follows: .

[0035] 2) Perform a Laplace transform on the virtual impedance loop control equation (3) using balanced current control, to obtain...

[0036] (7)

[0037] In the formula, e dq (s), (s) (s) represents the frequency domain expressions for the internal potential, the positive-sequence component of the grid-connected point voltage, and the dq-axis component of the current reference value. Wherein, , , , .

[0038] 3) Substitute the expressions for the internal potential and the positive-sequence voltage dq-axis component at the grid connection point in the frequency domain, along with the dq-axis component of the pre-fault current reference value, into equation (7). After separating the variables and performing an inverse Laplace transform, we obtain the dq-axis component of the positive-sequence current reference value. , The equation is

[0039] (8)

[0040] In the formula, , Since the response time of the inner current loop is neglected, we have (t)= , (t)= , , This is the expression for the dq-axis component of the fault current.

[0041] Preferably, step 5 further includes the following:

[0042] 1) Equation (5) establishes the equation for the internal potential, and equation (8) establishes the equation for the dq axis component of the fault current. Therefore, by combining the two equations, we can obtain the analytical expressions for the internal potential and the fault current.

[0043] Substituting equation (8) into equation (5), we can obtain the internal potential. E ref The first-order differential equation is:

[0044] (9)

[0045] In the formula, b 2. a 1. a The expression for 2 is: , , .in, This is the per-unit value of the positive sequence voltage at the grid connection point. S N This is the rated capacity.

[0046] Solving equation (9), we can obtain

[0047] (10)

[0048] In the formula, C is a constant, determined by the initial conditions of the internal electric potential. , The expression is: , , .

[0049] 2) Substituting equation (10) into equation (8), we can obtain i dq (t), which can be further transformed to obtain the analytical expression for the fault current in a three-phase rotating coordinate system. a For example, there are

[0050] (11)

[0051] In the formula, for a Initial phase of phase current, h The expression for 3 is .

[0052] As can be seen from the technical solution provided by the present invention, the proposed method fully considers the influence of the power loop, virtual impedance loop and current loop in the VSG control loop on the transient characteristics of the fault current. It can improve the analytical accuracy when performing analytical calculation of the fault transient current of the inverter power supply and realize the accurate analysis of the fault transient current. At the same time, the proposed method is applicable to both symmetrical and asymmetrical faults.

[0053] Beneficial effects

[0054] This invention fully considers the influence of the power loop, virtual impedance loop and current loop in the VSG control loop on the transient characteristics of fault current. It can improve the analytical accuracy when performing analytical calculation of fault transient current of inverter power supply and realize accurate analysis of fault transient current. At the same time, the proposed method is applicable to both symmetrical and asymmetrical faults. Attached Figure Description

[0055] Figure 1 This is a control block diagram of the energy storage converter based on VSG technology according to an embodiment of the present invention;

[0056] Figure 2 This is a vector diagram showing the voltage changes before and after the fault as described in an embodiment of the present invention;

[0057] Figure 3 This is a block diagram of the adjusted power loop control as described in an embodiment of the present invention;

[0058] Figure 4 This is a block diagram of the balanced current control according to an embodiment of the present invention;

[0059] Figure 5 This is a fitting diagram of the analytical waveform and simulated waveform of the three-phase current during a single-phase ground fault as described in an embodiment of the present invention.

[0060] Figure 6 This is a fitting diagram of the analytical waveform and the simulated waveform of the three-phase current during a two-phase short circuit as described in an embodiment of the present invention. Detailed Implementation

[0061] The present invention will be further described below with reference to the accompanying drawings and embodiments. The present invention provides a method for analyzing asymmetric faults in energy storage converters based on virtual synchronous machine technology, the specific implementation of which is as follows:

[0062] Step 1: Adopt a vector control strategy based on internal potential orientation to orient the internal potential to the d-axis and separate the positive and negative sequence of the grid connection point voltage;

[0063] Figure 1 The diagram below shows the control block diagram of the energy storage converter based on VSG technology according to an embodiment of the present invention. The entire control strategy consists of three parts: a power loop, a virtual impedance loop, and a current loop. The power loop outputs the internal potential amplitude and phase angle. The internal potential passes through the virtual impedance loop to obtain the current command value. The current loop tracks and controls the current command value to obtain the modulated voltage. , The modulated voltage is generated into a pulse modulation wave by a PWM modulator, which controls the switching on and off of the power electronic equipment, and outputs an equivalent three-phase AC power.

[0064] The active-frequency circuit outputs the phase angle. Establish a dq rotating coordinate system oriented around the d-axis, and determine the d- and q-axis components of the internal potential before the fault. , , The internal potential amplitude before the fault and after the fault , , The output internal potential amplitude of the reactive power-voltage link; the grid connection point voltage is collected. U abc The positive and negative sequence voltages at the grid connection point are obtained by separating the positive and negative sequences. U + The voltage at the grid connection point before the fault U 0 and the positive sequence voltage at the grid connection point after the fault U + Perform abc / dq coordinate transformation to determine the voltage at the grid connection point before the fault. U The dq-axis component of 0 is , After the fault, the positive sequence voltage at the grid connection point U + The dq axis components are , Meanwhile, the voltage at the grid connection point before the fault... U 0 Angle ahead of the d-axis Voltage at the grid connection point before the fault U 0 and its d-axis and q-axis components satisfy the following: The angle by which the positive sequence voltage at the grid connection point leads the d-axis after a fault. After the fault, the positive sequence voltage at the grid connection point U + It satisfies the following relationship with its d-axis and q-axis components: The voltage change vector diagram before and after the fault is as follows: Figure 2 As shown, due to the presence of inertial elements, the VSG output reference frequency is generally set to vary by electromechanical time constant (on the order of seconds). Since the fault duration is relatively short, the frequency can be considered to remain constant during the fault period.

[0065] Step 2: Adjust the VSG control loop according to the specific scenario of asymmetrical fault, including using a notch filter to filter out the second harmonic fluctuation component in the inverter output power, switching the reference voltage in the reactive power-voltage loop from the rated voltage to the positive sequence voltage of the grid connection point after the fault, and using a balance current control strategy to suppress negative sequence.

[0066] When an asymmetrical fault occurs, the presence of negative sequence voltage causes an imbalance in the three-phase output current. In the control of wind and solar grid-connected inverters, high quality is required for the output current; therefore, three-phase balance of the output current is the control objective. Furthermore, under the influence of unbalanced voltage and current, the inverter outputs instantaneous active and reactive power... P e , Q eThe presence of a second harmonic ripple component not only affects the performance of the converter but also reflects in the amplitude and phase of the VSG output internal potential, exacerbating the three-phase imbalance of the output current. Therefore, further adjustments to the power loop are needed to achieve the control objective of three-phase balance of the output current.

[0067] The second harmonic fluctuation component in the inverter output power is reflected in the phase and amplitude of the internal potential through the active-frequency and reactive-voltage control loops, which aggravates the three-phase imbalance of the output current. Therefore, a notch filter can be used to filter out the second harmonic fluctuation component in the active and reactive power. Only the average component of the output power is substituted into the power loop to obtain the three-phase balanced internal potential.

[0068] Traditional low-voltage ride-through (LVRT) control strategies, when adjusting the power loop, simply filter out the second harmonic component of the inverter output power to obtain a constant internal potential amplitude and phase. However, after a fault, the grid connection voltage drops, and the large voltage difference between the VSG output internal potential and the grid connection voltage will generate a large current, damaging the inverter and potentially causing the VSG to disconnect from the grid, resulting in serious hazards. Therefore, during a fault, the reference voltage in the reactive power-voltage control equation can be adjusted. From rated voltage U N Switching to the positive sequence voltage of the grid connection point after the fault U + This reduces the voltage difference between the VSG internal potential and the grid connection point voltage, effectively suppressing overcurrent. The adjusted power loop control block diagram is as follows: Figure 3 As shown, 0 represents the time before the fault, and 1 represents the time after the fault. The adjusted power loop control equation is shown in equation (1):

[0069] (1)

[0070] In the formula, , The phase and amplitude of the internal potential of the power loop output. P 0、 Q 0 represents the average components of the active and reactive power output of the inverter. , These are the reference values ​​for active and reactive power, respectively. J , D , K , w , w N These represent the virtual inertia, damping coefficient, reactive voltage integral coefficient, internal potential angular frequency, and rated angular frequency, respectively. The active and reactive power reference values ​​are also provided. , The value is

[0071] (2)

[0072] In the formula, This is the reference value for active power before the fault. U N , I N These are the rated voltage and rated current, respectively. I max The maximum current allowed to pass through the inverter is 1.5, which is taken as 1.5 in this invention. I N .

[0073] To achieve three-phase balance of output current as the control objective, the positive-sequence current must be used as the current reference value. By filtering out the second harmonic fluctuation component in the power, three-phase balance of internal potential is achieved; therefore, only the positive-sequence voltage at the grid connection point needs to be considered. U + Substituting into the virtual impedance loop control equation, the positive sequence reference current can be obtained. The current loop tracks and controls the positive sequence current command value, thus achieving three-phase balance of the output current. Its control structure diagram is shown below. Figure 4 As shown. Under balanced current control, the virtual impedance loop control equation is:

[0074] (3)

[0075] In the formula, R v , L v For virtual resistance and reactance, , The d-axis and q-axis components of the positive sequence current reference value;

[0076] Step 3: Based on the reactive-voltage circuit equation and the reactive-voltage control equation, establish the equation regarding the internal potential;

[0077] As shown in step 2, the average component of the inverter output power needs to be substituted into the power loop. Average component of inverter output reactive power. Q The formula for calculating 0 is:

[0078] (4)

[0079] In the formula, , These are the d-axis and q-axis components of the positive sequence current output by the inverter.

[0080] Substituting equation (4) into the reactive power-voltage control equation in equation (1) and further differentiating, we can obtain...

[0081] (5)

[0082] As shown in equation (5), and Analytical expression and internal potentialE ref The expression is closely related.

[0083] Step 4: Based on the virtual impedance loop control equation, calculate the reference value of the current before the fault and substitute it into the virtual impedance loop control equation after Laplace transform to establish the equation for the dq axis component of the fault current.

[0084] Since the inductor current cannot change abruptly, a reference value of the current before the fault is required. (0) and (0). Substituting the dq-axis components of the pre-fault internal potential and grid connection point voltage into equation (3), we can obtain the dq-axis component of the pre-fault current reference value. , for

[0085] (6)

[0086] In the formula, The virtual impedance magnitude is expressed as follows: , The virtual impedance phase angle is expressed as follows: .

[0087] A Laplace transform is performed on the virtual impedance loop control equation (Equation (3)) using balanced current control, to obtain...

[0088] (7)

[0089] In the formula, e dq (s) (s) (s) represents the frequency domain expression for the internal electromotive force, the positive-sequence component of the grid-connected voltage, and the dq-axis component of the inverter output current reference value. Wherein, , , , .

[0090] Substituting the expressions for the dq-axis components of the internal potential and the positive-sequence voltage at the grid connection point in the frequency domain, along with the dq-axis components of the pre-fault current reference value, into equation (7), and separating the variables and performing an inverse Laplace transform, we obtain the dq-axis components of the positive-sequence current reference value. , The equation is: (8)

[0091] In the formula, , Since the response time of the inner current loop is neglected, therefore we have (t)= , (t)= . , This is the expression for the dq-axis component of the fault current.

[0092] Step 5: Based on the equations for the internal potential and the dq-axis components of the fault current, construct analytical expressions for the internal potential and the fault current.

[0093] 1) Equation (5) establishes the equation for the internal potential, and equation (8) establishes the equation for the dq-axis component of the fault current. Therefore, by solving the two equations simultaneously, we can obtain the analytical expressions for the internal potential and the fault current. Substituting equation (8) into equation (5), we can obtain the equation for the internal potential. E ref The first-order differential equation is:

[0094] (9)

[0095] In the formula, b 2. a 1. a The expression for 2 is: , , .in, This is the per-unit value of the positive sequence voltage at the grid connection point. S N This is the rated capacity.

[0096] Solving equation (9), we can obtain

[0097] (10)

[0098] In the formula, C is a constant, determined by the initial conditions of the internal electric potential. , The expression is: , , .

[0099] As can be seen from equation (10), the expression for the amplitude of the VSG internal potential includes DC quantity, decaying DC quantity, and decaying power frequency quantity. Among them, DC quantity and power frequency quantity decay with different time constants.

[0100] Substituting equation (10) into equation (8), the dq-axis component of the fault current can be obtained. i dq (t), which can be further transformed to obtain the analytical expression for the fault current in a three-phase rotating coordinate system, so as to a For example, there are

[0101] (11)

[0102] In the formula, fora Initial phase of phase current, h The expression for 3 is .

[0103] As shown in equation (11), the analytical expression for fault current contains four components: steady-state power frequency component, time constant component, and other components. Attenuated power frequency components, and time constant The attenuated DC component and the second harmonic component.

[0104] Figure 5 , Figure 6 These are single-phase ground faults ( U A =0.5pu), two-phase short circuit ( U B = U C The figure shows a comparison between the simulated and analytical waveforms of the three-phase current at a value of 0.5 pu. It is worth noting that there is a delay in the positive and negative sequence separation; the positive and negative sequence separation method used in this invention has a delay of approximately 10 ms. When calculating the analytical expression for the fault current, the steady-state voltage value reached after the positive and negative sequence separation is used. Therefore, this invention only fits the waveform 10 ms after the fault occurs. As can be seen from the figure, the simulated waveform and the analytical waveform fit well, verifying the correctness of the analytical expression for the fault current proposed in this invention.

[0105] In summary, this invention proposes an asymmetrical fault analysis method for energy storage converters based on virtual synchronous machine technology. This method fully considers the influence of the power loop, virtual impedance loop, and current loop in the VSG control loop on the transient characteristics of the fault current. It can improve the analysis accuracy when performing fault transient current analysis calculations for inverter power supplies, and achieves accurate analysis of fault transient currents. At the same time, the proposed method is applicable to both symmetrical and asymmetrical faults.

[0106] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A method for analyzing asymmetric faults in energy storage converters based on virtual synchronous machine technology, characterized in that, Includes the following steps: Step 1: Employ a vector control strategy based on internal potential orientation to orient the internal potential along the d-axis and separate the positive and negative sequences of the grid connection point voltage; specifically: The active-frequency circuit outputs the phase angle. Establish a dq rotating coordinate system oriented around the d-axis, and define the d- and q-axis components of the internal potential before the fault. , , The internal potential amplitude before the fault and after the fault , , The output internal potential amplitude of the reactive power-voltage link; the grid connection point voltage is collected. U abc The positive and negative sequence voltages at the grid connection point are obtained by separating the positive and negative sequences. The voltage at the grid connection point before the fault and the positive sequence voltage at the grid connection point after the fault Perform an abc / dq coordinate transformation to obtain the grid connection point voltage before the fault. The dq axis components are , After the fault, the positive sequence voltage at the grid connection point The dq axis components are , Meanwhile, the voltage at the grid connection point before the fault... Angle ahead of the d-axis Voltage at the grid connection point before the fault It satisfies the following relationship with its d-axis and q-axis components: The angle by which the positive sequence voltage at the grid connection point leads the d-axis after a fault. After the fault, the positive sequence voltage at the grid connection point It satisfies the following relationship with its d-axis and q-axis components: ; Step 2: Adjust the inverter control circuit according to the specific scenario of the asymmetrical fault: This includes using a notch filter to remove the second harmonic fluctuation component in the inverter output power, switching the reference voltage in the reactive power-voltage loop from the rated voltage to the grid connection point positive sequence voltage, and using a balance current control strategy to suppress negative sequence; specifically: 1) Use a notch filter to filter out the active and reactive power output of the inverter. , The second harmonic frequency fluctuation component, in the reactive power-voltage control equation, the reference voltage is switched from the rated voltage to the positive sequence voltage at the grid connection point after the fault; the adjusted power loop control equation is: （1） In the formula, , The phase and amplitude of the internal potential of the power loop output. , The average components of active and reactive power output from the inverter. , These are the reference values ​​for active and reactive power, respectively. , , , , These represent virtual inertia, damping coefficient, reactive voltage integral coefficient, internal potential angular frequency, and rated angular frequency, respectively. For the Laplace operator; where active and reactive power reference values ​​are... , The value is: （2） In the formula, This is the reference value for active power before the fault. , These are the rated voltage and rated current, respectively. The maximum current allowed to pass through the inverter is taken as 1.

5. ; 2) By adjusting the power loop, the three-phase output internal potential is balanced. To obtain the positive-sequence reference current, the positive-sequence voltage at the grid connection point needs to be connected, which is called balanced current control. The virtual impedance loop control equation after using balanced current control is: （3） In the formula, , For virtual resistance and reactance, , The d-axis and q-axis components of the positive sequence current reference value; Step 3: Based on the reactive-voltage circuit equation and the reactive-voltage control equation, establish the equation regarding the internal potential; Step 4: Based on the virtual impedance loop control equation, calculate the reference value of the current before the fault and substitute it into the virtual impedance loop control equation after Laplace transform to establish the equation for the dq axis component of the fault current. Step 5: Based on the equations for the internal potential and the dq-axis components of the fault current, construct analytical expressions for the internal potential and the fault current.

2. The method for analyzing asymmetric faults in energy storage converters based on virtual synchronous machine technology according to claim 1, characterized in that, Step 3 further includes the following: As shown in step 2, the average component of the inverter output power needs to be substituted into the power loop; the average component of the inverter output reactive power... Q The formula for calculating 0 is: （4） In the formula, , The d-axis and q-axis components of the positive sequence current output by the inverter; Substituting equation (4) into the reactive power-voltage control equation in equation (1) and further differentiating, we can obtain... （5） As shown in equation (5), and Analytical expression and internal potential E ref The expression is closely related.

3. The method for analyzing asymmetric faults in energy storage converters based on virtual synchronous machine technology according to claim 1, characterized in that, Step 4 further includes the following: 1) Substitute the dq-axis components of the pre-fault internal potential and grid connection point voltage into equation (3) to obtain the dq-axis component of the pre-fault current reference value. , for: （6） In the formula, The virtual impedance magnitude is expressed as follows: , The virtual impedance phase angle is expressed as follows: ; 2) The Laplace transform of the virtual impedance loop control equation (3) using balanced current control is as follows: （7） In the formula, e dq (s) (s) (s) represents the frequency domain expressions for the internal potential, the positive-sequence component of the grid-connected point voltage, and the dq-axis component of the positive-sequence current reference value; where, , , , ; 3) Substitute the expressions for the internal potential and the positive-sequence voltage dq-axis component at the grid connection point in the frequency domain, along with the dq-axis component of the pre-fault current reference value, into equation (7). After separating the variables and performing an inverse Laplace transform, we obtain the dq-axis component of the positive-sequence current reference value. , The equation is: （8） In the formula, , Since the response time of the inner current loop is neglected, therefore we have (t)= , (t)= , , This is the expression for the dq-axis component of the fault current.

4. The method for analyzing asymmetric faults in energy storage converters based on virtual synchronous machine technology according to claim 3, characterized in that, Step 5 further includes the following: 1) Equation (5) establishes the equation for the internal potential, and equation (8) establishes the equation for the dq-axis component of the fault current. Therefore, by solving the two equations simultaneously, we can obtain the analytical expressions for the internal potential and the fault current. Substituting equation (8) into equation (5), we can obtain the equation for the internal potential. E ref The first-order differential equation is: （9） In the formula, b 2. a 1. a The expression for 2 is: , , ;in, This is the per-unit value of the positive sequence voltage at the grid connection point. Rated capacity; Solving equation (9) yields the internal potential. E ref The expression is: （10） In the formula, C is a constant, determined by the initial conditions of the internal electric potential. , The expression is: , , ; 2) Substituting equation (10) into equation (8), the dq-axis component of the fault current can be obtained. i dq (t), which can be further transformed to obtain the analytical expression for the fault current in a three-phase rotating coordinate system, so as to a For example, there are （11） In the formula, for a Initial phase of phase current, h The expression for 3 is .

5. A non-volatile storage medium, characterized in that, The non-volatile storage medium includes a stored program, wherein the program, when running, controls the device where the non-volatile storage medium is located to execute the method of claim 1.

6. An electronic device, characterized in that, It includes a processor and a memory; the memory stores computer-readable instructions, and the processor is used to execute the computer-readable instructions, wherein the computer-readable instructions, when executed, perform the method of claim 1.

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