Method for analyzing influence of new energy access on LCC receiving end power grid voltage stability

Abstract

The invention discloses a method for analyzing the influence of new energy access on the voltage stability of an LCC receiving end power grid, and the method comprises the steps: employing a two-node system which is equivalent to a virtual induction motor, and analyzing the changes of the electromagnetic torque and frequency difference curve of the induction motor in the fault of the two-node system, so as to obtain the initial stable frequency difference of the induction motor; obtaining a stability criterion of the system voltage; according to the mechanical load power and the electromagnetic power during the fault period, obtaining the limit removal frequency difference of the line fault after the short-circuit fault of the virtual induction motor; comparing the influence of the corresponding new energy access to the power grid on the stability of the receiving end system when the short-circuit ratio and the fault limit clearing time corresponding to the power grid are used as static stability indexes; and analyzing the influence of new energy grid connection on the fault transient voltage and the fault limit clearing time of the system. The method can accurately analyze the influence of new energy access on the voltage stability of the receiving-end power grid.

Description

Technical Field

[0001] The present invention relates to the technical field of renewable energy power generation, and in particular to a method for analyzing the impact of renewable energy access on the voltage stability of a LCC receiving-end power grid. Background Art

[0002] HVDC transmission is the main form of large-scale cross-regional and long-distance power transmission in my country. Now, a hybrid AC / DC grid with multiple DC feeds has been formed. Due to the large number of DC control methods, the stability research of AC / DC systems has become more complicated. At the same time, as more and more new energy units are connected to the grid through power electronic devices, the impact characteristics between AC / DC and new energy in the receiving-end grid system have become more complicated. At the same time, based on the grid-commutated rectifier HVDC transmission technology (LCC), the proportion of induction motors in the receiving-end system is very large, usually more than 60%. Therefore, the load characteristics of the receiving-end grid are often dominated by induction motors, and the local voltage stability problems caused by induction motors are easy to spread to the overall voltage stability problems of the receiving-end system. Therefore, it is of great significance to study the load characteristics of induction motors in the receiving-end system and improve the stability of the receiving-end grid system based on the load characteristics of induction motors in the receiving-end system to ensure the safe and stable operation of the grid. Voltage stability refers to the voltage stability of each load node of the power system after being impacted by a large disturbance. At present, the research on the impact of new energy access to the receiving-end grid on transient stability is relatively in-depth, but there is no unified view on the scenarios and entry angles for the grid connection of different new energy sources. In terms of the impact of DC multi-infeed on the receiving-end power grid, existing methods often analyze the factors affecting the voltage stability of the receiving-end power grid after DC multi-infeed, and judge the voltage stability of the power grid based on the frequency difference. There is an urgent need for an analytical method that can accurately calculate the motor limit cut-off frequency difference and the limit cut-off time of the line fault, and analyze the impact of the access of new energy such as wind and solar power on the voltage stability of the receiving-end power grid. Summary of the invention

[0003] In view of this, the present invention provides a method for analyzing the impact of new energy access on the voltage stability of the LCC receiving-end power grid.

[0004] The present invention discloses a method for analyzing the impact of new energy access on the voltage stability of an LCC receiving-end power grid, which comprises:

[0005] Step 1: The system with new energy access and DC feed-in is equivalent to a simple system model of an infinite system with a virtual motor, and then the Thevenin equivalent is used to convert it into a two-node system of a virtual induction motor. The changes in the electromagnetic torque and frequency difference curve of the induction motor during the two-node system fault of the virtual induction motor are analyzed to obtain the initial stable frequency difference of the induction motor.

[0006] Step 2: According to the relationship between the unbalanced torque acting on the rotor of the rotating motor and the mechanical angular acceleration of the rotor, the stability criterion of the system voltage is obtained; according to the mechanical load power and electromagnetic power during the fault period, the limit removal frequency difference of the line fault after the virtual induction motor short circuit fault is obtained; according to the limit removal frequency difference, the limit removal time is calculated;

[0007] Step 3: Combined with the stability criterion of the system voltage, compare the short-circuit ratio and the fault limit removal time corresponding to the power grid as static stability indicators, and the corresponding impact of the access of new energy to the power grid on the stability of the receiving-end system, so as to obtain the standard for judging the strength of the receiving-end power grid; the standard for judging the strength of the receiving-end power grid is the limit removal time;

[0008] Step 4: Combined with the impact of renewable energy access to the grid on the frequency difference of the induction motor, analyze the impact of renewable energy access to the grid on the system's fault transient voltage and fault limit clearing time.

[0009] Furthermore, in step 1, the Thevenin theorem is used to convert the simple system model of the infinite system with a virtual motor into a two-node system of a virtual induction motor through the following formula:

[0010]

[0011] Among them, R and X are variable equivalent resistances including receiving-end static load, energy storage, reactive power compensation, wind turbines, and distribution network impedance. L is the line equivalent impedance, R1 and X1 are the stator winding resistance and reactance of the induction motor, R2 and X2 are the rotor winding equivalent resistance and reactance, U'∠α is the terminal electromotive force of the virtual induction motor, U is the infinite system voltage, R1' is the stator resistance of the virtual induction motor, X1' is the stator reactance of the virtual induction motor, X m is the magnetizing reactance, X m ' is the magnetizing reactance of the virtual induction motor, R2' is the rotor resistance of the virtual induction motor, and X2' is the rotor reactance of the virtual induction motor.

[0012] Further, in said step 1, under normal operation, the electromagnetic torque and the mechanical load torque provided by the infinite system to the induction motor are equal, and the frequency difference corresponding to the stable operation of the induction motor is the initial stable frequency difference;

[0013] When a single-circuit short-circuit fault occurs in the double-circuit line of the system, it is assumed that the voltage of the infinite system remains unchanged before and after the fault, and the simple system model of the infinite system with a virtual motor is equivalent to a two-node system of a virtual induction motor. The system parameters after the fault are converted into parameter changes of the virtual induction motor, and the changes in transient voltage are converted into changes in the electromotive force at the machine end of the virtual induction motor.

[0014] Furthermore, the step 2 comprises:

[0015] Step 21: According to the relationship between the unbalanced torque acting on the rotor of the rotating electrical machine and the mechanical angular acceleration of the rotor, the relationship between the inertia time constant of the rotor and the electromagnetic power and the mechanical load power is derived;

[0016] Step 22: According to the relationship between the rotor inertia time constant and the electromagnetic power mechanical load power and the initial stable frequency difference, a stability criterion of the system voltage is obtained;

[0017] Step 23: Obtain the stable frequency difference and critical frequency difference after the accident according to the mechanical load power and electromagnetic power during the fault period;

[0018] Step 24: According to the stable frequency difference and critical frequency difference after the accident, the limit removal frequency difference of the line fault after the virtual induction motor short circuit fault is obtained;

[0019] Step 25: Calculate the limit cut-off time according to the initial stable frequency difference and the limit cut-off frequency difference.

[0020] Furthermore, the step 21 comprises:

[0021] The relationship between the unbalanced torque acting on the rotor of a rotating electrical machine and the mechanical angular acceleration of the rotor is expressed as:

[0022]

[0023] Among them, β is the rotor mechanical angular acceleration, ω is the actual mechanical angular velocity of the rotor, J is the moment of inertia of the motor rotor, T is the electromagnetic torque of the motor rotor, T1 is the sum of all resistance torques, and t is time;

[0024] The relationship between the kinetic energy and angular velocity of the rotor is:

[0025]

[0026] Among them, E K is the kinetic energy of the motor rotor at rated speed, Ω is the actual electrical angular velocity, Ω0 is the synchronous electrical angular velocity, p is the number of pole pairs, ω0 is the synchronous mechanical angular velocity, S m is the capacity of the induction motor, T j is the inertia time constant of the motor rotor;

[0027]

[0028] Where, s is the frequency difference of the induction motor, T N is the rated torque, is the per-unit value of electromagnetic power, is the per unit value of mechanical load power;

[0029] Then we get:

[0030]

[0031] Further, the step 22 comprises:

[0032] In case of line failure, It means that during the fault process, the rotor speed decreases and the kinetic energy decreases, and the work done by the insufficient electromagnetic power on time is recorded as the deceleration area; after the fault is removed, the rotor speed and kinetic energy increase during the fault process, and the work done by the excess electromagnetic power on time is recorded as the acceleration area; when the energy represented by the difference between the acceleration area and the deceleration area is less than the change in the kinetic energy of the motor rotor before and after the fault, the induction motor can run stably after the fault and the system maintains voltage stability; when the difference between the acceleration area and the deceleration area is greater than the change in the kinetic energy of the motor rotor before and after the fault, the motor will decelerate and eventually stall until the system collapses.

[0033] Furthermore, the rotor kinetic energy of the induction motor is expressed by the frequency difference:

[0034]

[0035] The voltage stability criterion is obtained:

[0036] If ΔW<1 / 2T j [(1-S1) 2 -(1-S0) 2 ], the system maintains voltage stability;

[0037] If ΔW>1 / 2T j [(1-S1) 2 -(1-S0) 2 ], the system voltage becomes unstable;

[0038] Among them, ΔW is the change in kinetic energy of the induction motor rotor before and after the fault is removed, S0 is the initial stable frequency difference, and S1 is the stable frequency difference after the fault is removed.

[0039] Further, the step 23 comprises:

[0040] The mechanical load power during the fault is expressed as the frequency difference:

[0041]

[0042] in, is the mechanical power per unit value of the induction motor at the initial frequency difference;

[0043] The electromagnetic power expression is expressed as follows using the frequency difference:

[0044]

[0045] in, is the per unit value of the terminal voltage, R1 is the resistance of the induction motor stator, X1 is the reactance of the induction motor stator, R2 is the equivalent resistance of the induction motor rotor winding, X2 is the equivalent reactance of the induction motor rotor winding, c1=X1 / X2 is the correction factor;

[0046] According to the expressions of mechanical load power and electromagnetic power during the fault period, the stable frequency difference and critical frequency difference after the accident can be calculated.

[0047] Further, the step 24 comprises:

[0048] The limiting cut-off frequency difference is calculated according to the following formula:

[0049]

[0050] Among them, s c is the limit cut-off frequency difference, and s3 is the critical frequency difference.

[0051] Further, the step 25 comprises:

[0052] The limit removal time is calculated by the following formula:

[0053]

[0054] Among them, t c is the limit removal time.

[0055] Due to the adoption of the above technical solution, the present invention has the following advantages: the present invention can accurately calculate the limit frequency difference of the virtual induction motor short circuit fault and the limit time of the line fault, and by comparing the limit time of the fault with the short circuit that characterizes the strength of the power system relative to the equipment, it is shown that the limit time of the fault can be used as a standard for judging the strength of the receiving power grid. The influence of the grid connection of new energy such as wind and solar on the fault transient voltage of the AC and DC system and the influence of the limit time of the fault are analyzed based on the change of the frequency difference, so as to accurately analyze the influence of the access of new energy on the voltage stability of the receiving power grid. At the same time, it provides a new theoretical basis for the current goal of promoting the access of new energy to the receiving power grid and the consumption of new energy, and solving the problem of a large number of DC feeds into the receiving power grid, and provides guidance for the future research on the voltage stability of the receiving power grid of new energy grid connection and DC feeds. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the embodiments of the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0057] Figure 1 is a flow chart of a CCT calculation method based on frequency difference analysis according to an embodiment of the present invention;

[0058] Figure 2 It is a schematic diagram of a simple system model of an infinite system with a virtual motor according to an embodiment of the present invention;

[0059] Figure 3 is a two-node system schematic diagram of a virtual induction motor according to an embodiment of the present invention;

[0060] Figure 4 is a schematic diagram of the operating points of the induction motor during a fault process in an embodiment of the present invention;

[0061] Figure 5 is a schematic diagram of the transient energy process of an induction motor during a fault process in an embodiment of the present invention;

[0062] Figure 6 It is a schematic diagram of the change of the initial frequency difference according to an embodiment of the present invention. DETAILED DESCRIPTION

[0063] The present invention is further described in conjunction with the accompanying drawings and embodiments, and the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by ordinary technicians in this field should fall within the scope of protection of the embodiments of the present invention.

[0064] See also Figure 1 The present invention provides an embodiment of a method for analyzing the impact of new energy access on the voltage stability of the LCC receiving-end power grid, which includes:

[0065] Step 1: The system with new energy access and DC feed-in is equivalent to a simple system model of an infinite system with a virtual motor, and then the Thevenin equivalent is used to convert it into a two-node system of a virtual induction motor. The changes in the electromagnetic torque and frequency difference curve of the induction motor during the two-node system fault of the virtual induction motor are analyzed to obtain the initial stable frequency difference of the induction motor.

[0066] Step 2: According to the relationship between the unbalanced torque acting on the rotor of the rotating motor and the mechanical angular acceleration of the rotor, the stability criterion of the system voltage is obtained; according to the mechanical load power and electromagnetic power during the fault period, the limit removal frequency difference of the line fault after the virtual induction motor short circuit fault is obtained; according to the limit removal frequency difference, the limit removal time is calculated;

[0067] Step 3: Combined with the stability criterion of the system voltage, compare the short-circuit ratio and the fault limit removal time corresponding to the power grid as static stability indicators, and the corresponding impact of the access of new energy to the power grid on the stability of the receiving-end system, so as to obtain the standard for judging the strength of the receiving-end power grid; the standard for judging the strength of the receiving-end power grid is the limit removal time;

[0068] Step 4: Combined with the impact of renewable energy access to the grid on the frequency difference of the induction motor, analyze the impact of renewable energy access to the grid on the system's fault transient voltage and fault limit clearing time.

[0069] In this embodiment, in step 1, the Thevenin theorem is used to equate the simple system model of the infinite system with a virtual motor to a two-node system of a virtual induction motor through the following formula:

[0070]

[0071] Among them, R and X are the resistance and reactance of the equivalent Thevenin circuit, and X L is the resistance of the infinite system connected to the Thevenin circuit, R1 and X1 are the stator winding resistance and reactance of the induction motor, R2 and X2 are the equivalent resistance and reactance of the rotor winding, U'∠α is the terminal electromotive force of the virtual induction motor, U is the infinite system voltage, R1' is the stator resistance of the virtual induction motor, X1' is the stator reactance of the virtual induction motor, X m is the magnetizing reactance, X m ' is the magnetizing reactance of the virtual induction motor, R2' is the rotor resistance of the virtual induction motor, and X2' is the rotor reactance of the virtual induction motor. For the specific process, please refer to Figure 2 and Figure 3 . Figure 2 In the infinite system E, the resistor X L Connect the Thevenin circuit; the Thevenin circuit includes R and X connected in series, R1, X1, X connected in series m , and R2, X2 connected in series; R and R1 have a common terminal, reactance X1, X m , resistor R2 has a common terminal, X, X m , X2 has a common terminal. Figure 3 In the m 'Series connection, X m ' is connected in parallel with R2' and X2' which are connected in series.

[0072] In this embodiment, in step 1, the change of the operating state of the induction motor is as follows: Figure 4 As shown in Figure 2, under normal operation, the infinite system in the two-node system provides the electromagnetic torque T to the induction motor. M and mechanical load torque T L are equal, point A represents the stable operating point, the frequency difference corresponding to point A is the initial stable frequency difference, T1 represents the torque slip curve before the fault, after the short circuit fault occurs, the torque slip curve of the motor changes to T3, and the operating point of the induction motor changes from point A to point B. Assuming that the fault is removed at point C, the torque slip curve of the motor changes from T3 to T2.

[0073] When a single-circuit short-circuit fault occurs in the double-circuit line of the system, it is assumed that the voltage of the infinite system remains unchanged before and after the fault, and the simple system model of the infinite system with a virtual motor is equivalent to a two-node system of a virtual induction motor. The system parameters after the fault are converted into parameter changes of the virtual induction motor, and the changes in transient voltage are converted into changes in the electromotive force at the machine end of the virtual induction motor.

[0074] In this embodiment, step 2 includes:

[0075] Step 21: According to the relationship between the unbalanced torque acting on the rotor of the rotating electrical machine and the mechanical angular acceleration of the rotor, the relationship between the inertia time constant of the rotor and the electromagnetic power and the mechanical load power is derived;

[0076] Step 22: According to the relationship between the rotor inertia time constant and the electromagnetic power mechanical load power and the initial stable frequency difference, a stability criterion of the system voltage is obtained;

[0077] Step 23: Obtain the stable frequency difference and critical frequency difference after the accident according to the mechanical load power and electromagnetic power during the fault period;

[0078] Step 24: According to the stable frequency difference and critical frequency difference after the accident, the limit removal frequency difference of the line fault after the virtual induction motor short circuit fault is obtained;

[0079] Step 25: Calculate the limit cut-off time according to the initial stable frequency difference and the limit cut-off frequency difference.

[0080] In this embodiment, the step 21 includes:

[0081] The relationship between the unbalanced torque acting on the rotor of a rotating electrical machine and the mechanical angular acceleration of the rotor is expressed as:

[0082]

[0083] Among them, β is the rotor mechanical angular acceleration, ω is the actual mechanical angular velocity of the rotor, J is the moment of inertia of the motor rotor, T is the electromagnetic torque of the motor rotor, T1 is the sum of all resistance torques, and t is time;

[0084] The relationship between the kinetic energy and angular velocity of the rotor is:

[0085]

[0086] Among them, E K is the kinetic energy of the motor rotor at rated speed, Ω is the actual electrical angular velocity, Ω0 is the synchronous electrical angular velocity, p is the number of pole pairs, ω0 is the synchronous mechanical angular velocity, S m is the capacity of the induction motor, T j is the inertia time constant of the motor rotor;

[0087]

[0088] Where, s is the frequency difference of the induction motor, T N is the rated torque, is the per-unit value of electromagnetic power, is the per unit value of mechanical load power;

[0089] Then we get:

[0090]

[0091] In this embodiment, the step 22 includes:

[0092] use Figure 5 Indicates the transient energy process of the induction motor during the fault process. When the line fails, It means that during the fault process, the rotor speed decreases and the kinetic energy decreases. Figure 5 A1 represents the area corresponding to the missing electromagnetic power, which is called the deceleration area. A2 represents the amount of work done by the excess electromagnetic power on time during the increase of the rotor speed and kinetic energy during the fault process, which is called the acceleration area. When the energy represented by the difference between the acceleration area and the deceleration area is less than the change in the kinetic energy of the motor rotor before and after the fault, the induction motor can run stably after the fault and the system maintains a stable voltage. When the difference between the acceleration area and the deceleration area is greater than the change in the kinetic energy of the motor rotor before and after the fault, the motor will decelerate and eventually stall until the system collapses.

[0093] In this embodiment, the rotor kinetic energy of the induction motor is expressed by the frequency difference:

[0094]

[0095] According to the above theory, the voltage stability criterion can be obtained:

[0096] If ΔW<1 / 2T j [(1-S1) 2 -(1-S0) 2 ], the system maintains voltage stability;

[0097] If ΔW>1 / 2T j [(1-S1) 2 -(1-S0) 2 ], the system voltage becomes unstable;

[0098] Among them, ΔW is the change in kinetic energy of the induction motor rotor before and after the fault is removed, S0 is the initial stable frequency difference, and S1 is the stable frequency difference after the fault is removed.

[0099] In this embodiment, step 23 includes:

[0100] The mechanical load power during the fault is expressed as the frequency difference:

[0101]

[0102] in, is the mechanical power per unit value of the induction motor at the initial frequency difference;

[0103] The electromagnetic power expression is expressed as follows using the frequency difference:

[0104]

[0105] in, is the per unit value of the terminal voltage, R1 is the resistance of the induction motor stator, X1 is the reactance of the induction motor stator, R2 is the equivalent resistance of the induction motor rotor winding, X2 is the equivalent reactance of the induction motor rotor winding, c1=X1 / X2 is the correction coefficient; c1 is approximately 1;

[0106] According to the expressions of mechanical load power and electromagnetic power during the fault period, the stable frequency difference and critical frequency difference after the accident can be calculated.

[0107] In this embodiment, step 24 includes:

[0108] Considering factors such as the inertia of the induction motor rotor, the limit cut-off frequency difference should be less than the critical frequency difference. The limit cut-off frequency difference is calculated according to the following formula:

[0109]

[0110] Among them, s c is the limit cut-off frequency difference, and s3 is the critical frequency difference.

[0111] In this embodiment, step 25 includes:

[0112] The limit removal time is calculated by the following formula:

[0113]

[0114] Among them, t c is the limit removal time.

[0115] In this embodiment, in step 3, according to the calculation method of the short-circuit ratio of multiple new energy stations, the short-circuit ratio of multiple new energy stations after the new energy access is calculated, and compared with the limit removal time calculated in step 2, after the power of new energy such as wind and solar power increases, the limit removal time of the system is reduced, and the short-circuit ratio of multiple new energy stations is reduced. Although the short-circuit ratio can reflect the transient stability as a static stability indicator, its mechanism of action is not clear enough. In comparison, the fault limit removal time can better reflect the impact of the access of new energy to the power grid on the stability of the system, and can more clearly reflect the stability of the receiving end system.

[0116] In this embodiment, in step 4, after the wind and solar energy and other new energy sources are connected, it can be equivalent to the connection of negative resistance, so that the equivalent impedance of the receiving end becomes larger, such as Figure 6 As shown in the figure, the torque frequency difference curve of the induction motor before the accident moves up. The initial stable slip of the motor is S0. When the load torque remains unchanged, the initial stable frequency difference increases, that is, it becomes S0′. The three curves represent the electromagnetic torque and mechanical torque before and after the wind power is connected. After the fault is removed and the system is restored to stability, the connection of new energy such as wind and solar power increases the stable frequency difference after the fault is removed. After the connection of new energy units such as wind and solar power, the voltage sag becomes larger, and the transient stability of the voltage of the receiving system is reduced. If a DC fault occurs in the system, the transient voltage rise of the system becomes larger.

[0117] The present invention mainly analyzes the transient voltage of the receiving-end power grid, and adopts the Thevenin equivalent to convert the integrated receiving-end system including wind and solar energy and other new energy sources into a two-node system of a virtual induction motor, provides voltage stability criteria and an analytical method for the motor limit frequency difference after a fault, and proposes a CCT calculation method based on frequency difference analysis. Based on the frequency difference change analysis, the influence of wind and solar energy and other new energy sources connected to the grid on the motor frequency difference and transient voltage stability of AC and DC system faults can be analyzed.

[0118] 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 above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for analyzing the impact of new energy access on the voltage stability of the LCC receiving-end power grid, characterized in that: include: Step 1: The system with new energy access and DC feed-in is equivalent to a simple system model of an infinite system with a virtual motor, and then the Thevenin equivalent is used to convert it into a two-node system of a virtual induction motor. The changes in the electromagnetic torque and frequency difference curve of the induction motor during the two-node system fault of the virtual induction motor are analyzed to obtain the initial stable frequency difference of the induction motor. Step 2: According to the relationship between the unbalanced torque acting on the rotor of the rotating motor and the mechanical angular acceleration of the rotor, the stability criterion of the system voltage is obtained; according to the mechanical load power and electromagnetic power during the fault period, the limit removal frequency difference of the line fault after the virtual induction motor short circuit fault is obtained; according to the limit removal frequency difference, the limit removal time is calculated; Step 3: Combined with the stability criterion of the system voltage, compare the short-circuit ratio and the fault limit removal time corresponding to the power grid as static stability indicators, and the corresponding impact of the access of new energy to the power grid on the stability of the receiving-end system, so as to obtain the standard for judging the strength of the receiving-end power grid; the standard for judging the strength of the receiving-end power grid is the limit removal time; Step 4: Combined with the impact of renewable energy access to the grid on the frequency difference of the induction motor, analyze the impact of renewable energy access to the grid on the system's fault transient voltage and fault limit clearing time.

2. The method according to claim 1, characterized in that In step 1, the Thévenin theorem is used to convert the simple system model of the infinite system with a virtual motor into a two-node system of a virtual induction motor through the following formula: Among them, R and X are variable equivalent resistances including receiving-end static load, energy storage, reactive power compensation, wind turbines, and distribution network impedance. L is the line equivalent impedance, R1 and X1 are the stator winding resistance and reactance of the induction motor, R2 and X2 are the rotor winding equivalent resistance and reactance, U'∠α is the terminal electromotive force of the virtual induction motor, U is the infinite system voltage, R1' is the stator resistance of the virtual induction motor, X1' is the stator reactance of the virtual induction motor, X m is the magnetizing reactance, X m ' is the magnetizing reactance of the virtual induction motor, R2' is the rotor resistance of the virtual induction motor, and X2' is the rotor reactance of the virtual induction motor.

3. The method according to claim 1, characterized in that In step 1, under normal operation, the electromagnetic torque and mechanical load torque provided by the infinite system to the induction motor are equal, and the frequency difference corresponding to the stable operation of the induction motor is the initial stable frequency difference; When a single-circuit short-circuit fault occurs in the double-circuit line of the system, it is assumed that the voltage of the infinite system remains unchanged before and after the fault, and the simple system model of the infinite system with a virtual motor is equivalent to a two-node system of a virtual induction motor. The system parameters after the fault are converted into parameter changes of the virtual induction motor, and the changes in transient voltage are converted into changes in the electromotive force at the machine end of the virtual induction motor.

4. The method according to claim 1, characterized in that: The step 2 comprises: Step 21: According to the relationship between the unbalanced torque acting on the rotor of the rotating electrical machine and the mechanical angular acceleration of the rotor, the relationship between the inertia time constant of the rotor and the electromagnetic power and the mechanical load power is derived; Step 22: According to the relationship between the rotor inertia time constant and the electromagnetic power mechanical load power and the initial stable frequency difference, a stability criterion of the system voltage is obtained; Step 23: Obtain the stable frequency difference and critical frequency difference after the accident according to the mechanical load power and electromagnetic power during the fault period; Step 24: According to the stable frequency difference and critical frequency difference after the accident, the limit removal frequency difference of the line fault after the virtual induction motor short circuit fault is obtained; Step 25: Calculate the limit cut-off time according to the initial stable frequency difference and the limit cut-off frequency difference.

5. The method according to claim 4, characterized in that The step 21 comprises: The relationship between the unbalanced torque acting on the rotor of a rotating electrical machine and the mechanical angular acceleration of the rotor is expressed as: Among them, β is the rotor mechanical angular acceleration, ω is the actual mechanical angular velocity of the rotor, J is the moment of inertia of the motor rotor, T is the electromagnetic torque of the motor rotor, T1 is the sum of all resistance torques, and t is time; The relationship between the kinetic energy and angular velocity of the rotor is: Among them, E K is the kinetic energy of the motor rotor at rated speed, Ω is the actual electrical angular velocity, Ω0 is the synchronous electrical angular velocity, p is the number of pole pairs, ω0 is the synchronous mechanical angular velocity, S m is the capacity of the induction motor, T j is the inertia time constant of the motor rotor; Where, s is the frequency difference of the induction motor, T N is the rated torque, is the per-unit value of electromagnetic power, is the per unit value of mechanical load power; Then we get:

6. The method according to claim 5, characterized in that The step 22 comprises: In case of line failure, It means that during the fault process, the rotor speed decreases and the kinetic energy decreases, and the work done by the insufficient electromagnetic power on time is recorded as the deceleration area; after the fault is removed, the rotor speed and kinetic energy increase during the fault process, and the work done by the excess electromagnetic power on time is recorded as the acceleration area; when the energy represented by the difference between the acceleration area and the deceleration area is less than the change in the kinetic energy of the motor rotor before and after the fault, the induction motor can run stably after the fault and the system maintains voltage stability; when the difference between the acceleration area and the deceleration area is greater than the change in the kinetic energy of the motor rotor before and after the fault, the motor will decelerate and eventually stall until the system collapses.

7. The method according to claim 6, characterized in that The rotor kinetic energy of an induction motor is expressed as a frequency difference: The voltage stability criterion is obtained: If ΔW<1 / 2T j [(1-S1) 2 -(1-S0) 2 ], the system maintains voltage stability; If ΔW>1 / 2T j [(1-S1) 2 -(1-S0) 2 ], the system voltage becomes unstable; Among them, ΔW is the change in kinetic energy of the induction motor rotor before and after the fault is removed, S0 is the initial stable frequency difference, and S1 is the stable frequency difference after the fault is removed.

8. The method according to claim 7, characterized in that The step 23 comprises: The mechanical load power during the fault is expressed as the frequency difference: in, is the mechanical power per unit value of the induction motor at the initial frequency difference; The electromagnetic power expression is expressed as follows using the frequency difference: in, is the per unit value of the terminal voltage, R1 is the resistance of the induction motor stator, X1 is the reactance of the induction motor stator, R2 is the equivalent resistance of the induction motor rotor winding, X2 is the equivalent reactance of the induction motor rotor winding, c1=X1 / X2 is the correction factor; According to the expressions of mechanical load power and electromagnetic power during the fault period, the stable frequency difference and critical frequency difference after the accident can be calculated.

9. The method according to claim 8, characterized in that The step 24 comprises: The limiting cut-off frequency difference is calculated according to the following formula: Among them, s c is the limit cut-off frequency difference, and s3 is the critical frequency difference.

10. The method according to claim 9, characterized in that The step 25 comprises: The limit removal time is calculated by the following formula: Among them, t c is the limit removal time.

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