Voltage transformer inrush current fault analog simulation platform establishing method

Abstract

The invention discloses a method for establishing an inrush current fault analog simulation platform of a voltage transformer. The method comprises the following steps: carrying out equivalent simplification on a power distribution network and establishing a power distribution network system model; the power distribution network system model comprises a power supply module, a voltage transformer module, a line module and a protection device module, and the power distribution network system model is a circuit model of which a neutral point is not directly grounded; establishing a superposition equation of a non-fault state and a fault additional state after the single-phase earth fault disappears according to a parameter expression in the power distribution network system model; and according to the superposition equation, obtaining the suppression effect of each module in the power distribution network system model on the inrush current generated by the power distribution network system model. According to the method, the calculation precision of calculating the size of the inrush current generated by the power distribution network system according to various line factors can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of modeling and simulation of electric power systems, and in particular to a method for establishing a voltage transformer inrush fault simulation platform. Background Art

[0002] my country's 6-35kV distribution network is large in scale and complex in structure. In order to reduce the impact of shutdowns and improve the stability of power supply, the neutral point is generally not directly grounded. Single-phase grounding faults are prone to occur, which may cause the fuse of the voltage transformer to melt or be damaged, thereby further expanding the single-phase grounding problem, forming more phase connections, and then amplifying the impact of the disaster, resulting in more serious consequences. The traditional method is mainly to connect a harmonic elimination device to the neutral point grounding on the high-voltage side of the voltage transformer, in order to eliminate the resonance and reduce the damage to the voltage transformer based on the principle of making the system parameters far exceed the ferromagnetic resonance range, but the effect is not obvious, and even the harmonic elimination device and the voltage transformer are burned together. It can be proved from the actual situation that the current neutral point protection measures on the high-voltage side of the voltage transformer have certain defects, and cannot effectively prevent the high-voltage fuse of the voltage transformer from melting or even causing damage to the body.

[0003] In recent years, after re-study and discussion, it is concluded that the surge current generated after the recovery of the single-phase grounding fault in the distribution network system is the main cause of voltage transformer damage. When a single-phase grounding fault occurs, the fault phase voltage becomes zero, and the non-fault phase voltage rises to the line voltage, and its ground capacitance will be charged with the charge under the line voltage. When the grounding fault continues, under the action of the line voltage, the charge flows between the conductor and the earth through the fault grounding point as a path, forming a capacitive current. When the grounding fault disappears, the path between the charge on the conductor-to-ground capacitor and the earth is cut off. At this time, the voltage of each line to the ground needs to be restored to the level of the normal working phase voltage, and the line capacitance of the original non-fault phase is charged with the charge under the line voltage. Since the grounding point has been cut off, the free charge can only be discharged to the earth through the primary winding of the voltage transformer. If there are too many free charges, the voltage transformer core will be saturated in the process of discharging to the earth. A large surge current will appear under the power frequency power supply voltage, which will cause the high-voltage fuse of the voltage transformer to fuse. In severe cases, the voltage transformer body will be damaged, affecting the normal operation of the power grid system.

[0004] However, there is a lack of simple and practical engineering methods and tools for calculating the size of the inrush current generated after the single-phase grounding fault in the distribution network system is restored, its influencing factors, and the suppression effect of various protection devices. The inrush current size standard generated by this system is closely related to the standards of each module of the 6-35kV distribution network, but has little correlation with other levels of distribution networks. When using EMTP modeling, the difficulty lies in correctly and reliably connecting the power module, voltage transformer module, line module and protection device module, so that the parameters of each module can be modified easily and the device can be replaced conveniently. Since various factors such as lines have a great influence on the size of the inrush current generated by the distribution network system, the parameters are numerous and the transient characteristics are unclear, making the calculation process cumbersome and difficult to determine. Therefore, a surge fault simulation platform that can reflect transient characteristics, combine modules together, quickly change parameters and devices, and quickly calculate is needed. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the present invention provides a method for establishing a voltage transformer inrush current fault simulation platform, which solves the problem in the prior art that the inrush current generated by the distribution network system cannot be calculated based on a variety of line factors.

[0006] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is: a method for establishing a voltage transformer inrush current fault simulation platform, comprising the following steps:

[0007] S1. Simplify the distribution network and establish a distribution network system model;

[0008] The distribution network system model includes a power supply module, a voltage transformer module, a line module and a protection device module, and the distribution network system model is a circuit model in which the neutral point is not directly grounded;

[0009] S2. According to the parameter expressions in the distribution network system model, a superposition equation of the non-fault state and the fault additional state after the single-phase grounding fault disappears is established;

[0010] S3. According to the superposition equation, the suppression effect of each module in the distribution network system model on the inrush current generated by the distribution network system model is obtained.

[0011] The beneficial effects of the above scheme are as follows: the present invention establishes a 6-35kV distribution network model in EMTP / ATP, sets a single-phase grounding fault, and obtains the size of the surge current generated by the voltage transformer from simulation; the influence of different factors on the surge current size is obtained by setting different parameters of the model, and a suppression device is installed at the neutral point O2 on the high-voltage side of the voltage transformer of the model, and the suppression effect of each device on the surge current generated by the system is obtained from simulation. While ensuring the simulation accuracy, it has high flexibility, and the modification and adjustment of parameters and the application conversion of each module are simple and easy, which is convenient for routine engineering calculations.

[0012] Further, in S1, the power module is connected in such a way that the three-phase effective power source is connected as a neutral point;

[0013] The volt-ampere characteristics and basic parameters in the voltage transformer module are set according to preset parameters;

[0014] A grounding time-controlled switch is arranged in any one of the line modules;

[0015] The protection device module is obtained by adding a detuning device or a surge suppressor to the neutral point on the high voltage side of the voltage transformer module.

[0016] Furthermore, before S1, the method further includes:

[0017] The preset parameters are determined according to different core saturation conditions of different types of voltage transformers.

[0018] Furthermore, S2 specifically includes:

[0019] S21. According to the parameter expression in the distribution network system model, an expression for the capacitive current flowing through the fault point when a single-phase grounding fault occurs is established;

[0020] S22, converting the capacitance current expression into an instantaneous expression, and performing Laplace transform on the instantaneous expression to obtain a first Laplace expression;

[0021] S23, simplifying the equivalent transient circuit expression after the single-phase fault disappears and performing Laplace transformation to obtain a second Laplace expression;

[0022] S24, combining the first Laplace expression and the second Laplace expression, and performing an inverse Laplace transform to obtain a voltage expression at the voltage transformer terminal of the transient circuit;

[0023] S25, determining the free oscillation angular frequency of the parallel circuit, and simplifying the voltage expression at the voltage transformer terminal to obtain a simplified voltage expression at the voltage transformer terminal;

[0024] S26, introducing the damping coefficient into the simplified voltage transformer terminal voltage expression to obtain the initial superposition equation;

[0025] S27. Integrate the initial superposition equation according to the magnetic flux relationship to obtain the superposition equation.

[0026] Furthermore, the voltage transformer module is established based on the Saturable 1phase element simulation in EMTP using piecewise linearization method;

[0027] The Saturable 1phase element is a saturable voltage transformer consisting of a high-voltage side primary coil and a low-voltage side secondary coil.

[0028] Furthermore, the protection device modules are established in EMTP based on the simulation of Type-92 components and Type-97 components respectively;

[0029] Type-92 components are varistor-based detuning devices, and Type-97 components are current-dependent resistor-based inrush suppressors.

[0030] Furthermore, the current expression is:

[0031]

[0032] in, It indicates the capacitive current flowing through the fault point when a single-phase grounding fault occurs. represents the system's A-phase equivalent power supply, j represents an imaginary unit, ω represents the system current angular frequency, C0 represents the system line-to-ground capacitance, L PT Indicates the equivalent inductance of the high-voltage winding of the voltage transformer;

[0033] The first Laplace expression is:

[0034]

[0035] Where, I(s) represents the Laplace transform result of current, I m represents the rated current of the transformer, e represents the natural constant, j represents the imaginary unit, represents the voltage phase angle when the fault disappears, s represents the transformation parameter, and ω represents the system current angular frequency;

[0036] The second Laplace expression is:

[0037]

[0038] Where Z(s) represents the system impedance function, s represents the transformation parameter, and L PT represents the equivalent inductance of the high-voltage winding of the voltage transformer, and C0 represents the capacitance of the system line to ground;

[0039] The voltage expression of the voltage transformer terminal is:

[0040]

[0041] Among them, U(t) represents the instantaneous voltage function at the output of the transformer, E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, Indicates the voltage phase angle when the fault disappears, L PTrepresents the equivalent inductance of the high-voltage winding of the voltage transformer, and C0 represents the capacitance of the system line to ground;

[0042] The free oscillation angular frequency is:

[0043]

[0044] Where ω′ represents the free oscillation angular frequency, L PT represents the equivalent inductance of the high-voltage winding of the voltage transformer, and C0 represents the capacitance of the system line to ground;

[0045] The simplified voltage expression of the voltage transformer terminal is:

[0046]

[0047] Among them, U(t) represents the instantaneous voltage function at the output of the transformer, E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, It represents the voltage phase angle when the fault disappears, and ω′ represents the free oscillation angular frequency;

[0048] The initial superposition equation is:

[0049]

[0050] Among them, U(t) represents the instantaneous voltage function at the output of the transformer, E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, represents the voltage phase angle when the fault disappears, ω′ represents the free oscillation angular frequency, δ represents the damping coefficient, and e represents the natural constant;

[0051] The initial superposition equation is:

[0052]

[0053] Among them, Ψ represents the free oscillation flux, U represents the instantaneous voltage at the output end of the transformer, and E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, It represents the voltage phase angle when the fault disappears, ω′ represents the free oscillation angular frequency, δ represents the damping coefficient, and e represents the natural constant. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 The present invention is a flowchart of a method for establishing a voltage transformer inrush fault simulation platform.

[0055] Figure 2 This is a schematic diagram of the distribution network system model.

[0056] Figure 3 A schematic diagram of a power module.

[0057] Figure 4 Schematic diagram of the voltage transformer module.

[0058] Figure 5 A schematic diagram of the line module.

[0059] Figure 6 Schematic diagram of the protection device module. DETAILED DESCRIPTION

[0060] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0061] like Figure 1 As shown, a method for establishing a voltage transformer inrush current fault simulation platform includes the following steps:

[0062] S1. Perform equivalent simplification and establish a distribution network system model; the distribution network system model includes a power module, a voltage transformer module, a line module and a protection device module, and the distribution network system model is a circuit model in which the neutral point is not directly grounded.

[0063] In this embodiment, in S1, the power module is connected in a manner that the three-phase effective power source is connected as a neutral point;

[0064] The volt-ampere characteristics and basic parameters in the voltage transformer module are set according to preset parameters;

[0065] A grounding time-controlled switch is arranged in any one of the line modules;

[0066] The protection device module is obtained by adding a detuning device or a surge suppressor to the neutral point on the high voltage side of the voltage transformer module.

[0067] Optionally, before S1, the method may further include:

[0068] The preset parameters are determined according to different core saturation conditions of different types of voltage transformers.

[0069] In a specific embodiment, the distribution network may be a 6-35 kV distribution network, or a distribution network of other voltage levels, which is not specifically limited herein.

[0070] Optionally, the distribution network is simplified and a distribution network system model is established by using an electromagnetic transient simulation program. For example, the electromagnetic transient simulation program may be EMTP / ATP. And the established distribution network system model may be as follows: Figure 2 As shown, Figure 2 Figure 1 is a schematic diagram of the distribution network system model. Figure 2In the distribution network system model, the three-phase effective power E A 、E B 、E C , conductor-to-ground capacitance C0, voltage transformer high-voltage side equivalent inductance L PT and its equivalent circuit.

[0071] For example, Figure 3 As shown, Figure 3 Figure 1 is a schematic diagram of the power module. Figure 3 In the operation mode where the power module is connected to the neutral point without direct grounding, a single-phase short circuit will not be formed after a single-phase grounding fault occurs in the system, and the symmetry of the three-phase line voltage will not be affected. The three-phase equipment can work normally and the simulation experiment can continue to run.

[0072] For example, Figure 4 As shown, Figure 4 Figure 1 is a schematic diagram of the power module. Figure 4 In this paper, the volt-ampere characteristics and basic parameter settings of the voltage transformer module are improved to simulate different types of voltage transformers and simulate different core saturation conditions.

[0073] For example, Figure 5 As shown, Figure 5 Figure 1 is a schematic diagram of the line module. Figure 5 In the process, a grounding time-controlled switch is arranged on a certain phase of the line module to complete the control of the generation and elimination of single-phase grounding faults; and a ground capacitance device is added to the line module to complete the control of the conductor-to-ground capacitance.

[0074] For example, Figure 6 As shown, Figure 6 Figure 1 is a schematic diagram of the protection device module. Figure 6 In this paper, a detuning device or surge suppressor is installed at the neutral point O2 on the high-voltage side of the voltage transformer in the established model to simulate the surge suppression effect of the system with different protection devices installed, and the transient characteristics of the surge suppression of the voltage transformer are obtained.

[0075] S2. Based on the parameter expressions in the distribution network system model, a superposition equation of the non-fault state and the fault additional state after the single-phase grounding fault disappears is established.

[0076] In this embodiment, S2 specifically includes:

[0077] S21. According to the parameter expression in the distribution network system model, an expression for the capacitive current flowing through the fault point when a single-phase grounding fault occurs is established;

[0078] S22, converting the capacitance current expression into an instantaneous expression, and performing Laplace transform on the instantaneous expression to obtain a first Laplace expression;

[0079] S23, simplifying the equivalent transient circuit expression after the single-phase fault disappears and performing Laplace transformation to obtain a second Laplace expression;

[0080] S24, combining the first Laplace expression and the second Laplace expression, and performing an inverse Laplace transform to obtain a voltage expression at the voltage transformer terminal of the transient circuit;

[0081] S25, determining the free oscillation angular frequency of the parallel circuit, and simplifying the voltage expression at the voltage transformer terminal to obtain a simplified voltage expression at the voltage transformer terminal;

[0082] S26, introducing the damping coefficient into the simplified voltage transformer terminal voltage expression to obtain the initial superposition equation;

[0083] S27. Integrate the initial superposition equation according to the magnetic flux relationship to obtain the superposition equation.

[0084] In this embodiment, the voltage transformer module is established in the EMTP based on the Saturable 1phase element simulation using the piecewise linearization method; the Saturable 1phase element is a saturable voltage transformer composed of a high-voltage side primary coil and a low-voltage side secondary coil.

[0085] In this embodiment, the protection device modules are respectively established in the EMTP based on Type-92 components and Type-97 components simulation; the Type-92 components are varistor-based detuning devices, and the Type-97 components are current-sensitive resistor-based surge suppressors.

[0086] In this embodiment, the current expression is:

[0087]

[0088] in, It indicates the capacitive current flowing through the fault point when a single-phase grounding fault occurs. represents the system's A-phase equivalent power supply, j represents an imaginary unit, ω represents the system current angular frequency, C0 represents the system line-to-ground capacitance, L PT Indicates the equivalent inductance of the high-voltage winding of the voltage transformer;

[0089] The first Laplace expression is:

[0090]

[0091] Where, I(s) represents the Laplace transform result of current, I m represents the rated current of the transformer, e represents the natural constant, j represents the imaginary unit, represents the voltage phase angle when the fault disappears, s represents the transformation parameter, and ω represents the system current angular frequency;

[0092] The second Laplace expression is:

[0093]

[0094] Where Z(s) represents the system impedance function, s represents the transformation parameter, and L PT represents the equivalent inductance of the high-voltage winding of the voltage transformer, and C0 represents the capacitance of the system line to ground;

[0095] The voltage expression of the voltage transformer terminal is:

[0096]

[0097] Among them, U(t) represents the instantaneous voltage function at the output of the transformer, E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, Indicates the voltage phase angle when the fault disappears, L PT represents the equivalent inductance of the high-voltage winding of the voltage transformer, and C0 represents the capacitance of the system line to ground;

[0098] The free oscillation angular frequency is:

[0099]

[0100] Where ω′ represents the free oscillation angular frequency, L PT represents the equivalent inductance of the high-voltage winding of the voltage transformer, and C0 represents the capacitance of the system line to ground;

[0101] The simplified voltage expression of the voltage transformer terminal is:

[0102]

[0103] Among them, U(t) represents the instantaneous voltage function at the output of the transformer, E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, It represents the voltage phase angle when the fault disappears, and ω′ represents the free oscillation angular frequency;

[0104] The initial superposition equation is:

[0105]

[0106] Among them, U(t) represents the instantaneous voltage function at the output of the transformer, E m represents the rated voltage of the transformer, ω represents the system current angular frequency, and t represents, represents the voltage phase angle when the fault disappears, ω′ represents the free oscillation angular frequency, δ represents the damping coefficient, and e represents the natural constant;

[0107] The initial superposition equation is:

[0108]

[0109] Among them, Ψ represents the free oscillation flux, U represents the instantaneous voltage at the output end of the transformer, and E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, It represents the voltage phase angle when the fault disappears, ω′ represents the free oscillation angular frequency, δ represents the damping coefficient, and e represents the natural constant.

[0110] Exemplarily, S21-S27 are superposition equations of the non-fault state and the fault additional state established when a single-phase grounding fault occurs in a certain phase (taking phase A as an example) and the single-phase grounding fault disappears, and the influence of the phase-to-phase capacitance is ignored, wherein the power supply voltage of phase A can be expressed as Therefore, before S22, the capacitor current expression can be substituted into the capacitor current expression, and then S22 is executed. That is, the A phase power supply voltage is substituted into the current expression, and we can get Among them, I m =(ωC0-1 / ωL PT )3E m .

[0111] Converting the above formula into an instantaneous expression yields:

[0112]

[0113] Perform Laplace transform on the instantaneous expression to obtain a first Laplace expression. In addition, in S24, the first Laplace expression and the second Laplace expression are combined to obtain the expression:

[0114]

[0115] Among them, U(s) represents the Laplace transform result of voltage, I(s) represents the Laplace transform result of current, Z(s) represents the system impedance function, j represents the imaginary unit, I m Indicates the rated current of the transformer, represents the voltage phase angle when the fault disappears, e represents the natural constant, C0 represents the system line-to-ground capacitance, s represents the transformation parameter, ω represents the system current angular frequency, L PT Represents the equivalent inductance of the high-voltage winding of the voltage transformer.

[0116] Optionally, according to the initial superposition equation, it can be seen that after the single-phase grounding fault disappears, the terminal voltage of the primary winding of the non-fault phase voltage transformer consists of two parts. The first part is the forced component of the voltage, which is the unattenuated part, and the second part is the free component of the voltage, which is oscillating and attenuated, and its voltage amplitude is the voltage phase angle when the fault disappears. function, and usually the system's free oscillation angular frequency ω′ is equal to the system current angular frequency ω.

[0117] Since a voltage free component with an oscillation frequency of ω′ appears in the high-voltage winding of the voltage transformer, an oscillating magnetic flux with a frequency of ω′ will be generated in the core of the voltage transformer, and a free oscillating magnetic flux Ψ with a frequency of ω′ will be generated in the voltage transformer winding accordingly. And by integrating the initial superposition equation based on the magnetic flux relationship U(Ψ)=d(Ψ) / dt, the superposition equation can be obtained.

[0118] From the superposition equation, we know that the magnitude of the flux linkage Ψ is related to the voltage phase angle when the fault disappears. and the free oscillation frequency ω′. The longer the line is, the greater its capacitance to ground is, the smaller ω′ is, and ω′=ω, then the greater the amplitude of the magnetic flux. Under the action of this oscillating magnetic flux, the core of the voltage transformer will enter saturation, and each saturation will correspond to a surge current. Repeated surge currents will cause heat to accumulate in the voltage transformer windings, which can easily cause the voltage transformer to heat up and be damaged.

[0119] S3. According to the superposition equation, the suppression effect of each module in the distribution network system model on the inrush current generated by the distribution network system model is obtained.

[0120] In this embodiment, a 6-35kV distribution network system model is established in EMTP / ATP, a single-phase grounding fault is set, and the size of the inrush current generated by the voltage transformer is obtained from the simulation; the influence of different factors on the inrush current size is obtained by setting different parameters of the distribution network system model, and a suppression device is installed at the neutral point O2 on the high-voltage side of the voltage transformer in the distribution network system model, and the suppression effect of each device on the inrush current generated by the system is obtained from the simulation.

[0121] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the invention.

Claims

1. A method for establishing a voltage transformer inrush fault simulation platform, characterized in that: The method comprises: S1. Simplify the distribution network and establish a distribution network system model; The distribution network system model includes a power supply module, a voltage transformer module, a line module and a protection device module, and the distribution network system model is a circuit model in which the neutral point is not directly grounded; S2. According to the parameter expressions in the distribution network system model, a superposition equation of a non-fault state and a fault additional state after the single-phase grounding fault disappears is established; S3. According to the superposition equation, the suppression effect of each module in the distribution network system model on the inrush current generated by the distribution network system model is obtained.

2. The method according to claim 1, characterized in that In S1, the power module is connected in such a way that the three-phase effective power source is connected as a neutral point; The volt-ampere characteristics and basic parameters in the voltage transformer module are set according to preset parameters; A grounding time-controlled switch is arranged in any one of the circuit modules; The protection device module is obtained by adding a detuning device or a surge suppressor to the neutral point on the high-voltage side of the voltage transformer module.

3. The method according to claim 2, characterized in that Before S1, the method further includes: The preset parameters are determined according to different core saturation conditions of different types of voltage transformers.

4. The method according to claim 1, characterized in that: The S2 specifically includes: S21. According to the parameter expression in the distribution network system model, an expression for the capacitive current flowing through the fault point when a single-phase grounding fault occurs is established; S22, converting the capacitance current expression into an instantaneous expression, and performing Laplace transform on the instantaneous expression to obtain a first Laplace expression; S23, simplifying the equivalent transient circuit expression after the single-phase fault disappears and performing Laplace transformation to obtain a second Laplace expression; S24, combining the first Laplace expression and the second Laplace expression, and performing an inverse Laplace transform to obtain a voltage expression at the voltage transformer terminal of the transient circuit; S25, determining the free oscillation angular frequency of the parallel circuit, and simplifying the voltage transformer terminal voltage expression to obtain a simplified voltage transformer terminal voltage expression; S26, introducing the damping coefficient into the simplified voltage transformer terminal voltage expression to obtain an initial superposition equation; S27. Integrate the initial superposition equation according to the magnetic flux relationship to obtain the superposition equation.

5. The method according to claim 2, characterized in that: The voltage transformer module is established by emulating the Saturable 1phase element in EMTP using a piecewise linearization method; The Saturable 1phase element is a saturable voltage transformer consisting of a high-voltage side primary coil and a low-voltage side secondary coil.

6. The method according to claim 2, characterized in that The protection device modules are respectively established in EMTP based on Type-92 components and Type-97 components simulation; The Type-92 component is a varistor-based detuning device, and the Type-97 component is a current-dependent resistor-based surge suppressor.

7. The method according to claim 4, characterized in that The current expression is: in, It indicates the capacitive current flowing through the fault point when a single-phase grounding fault occurs. represents the system's A-phase equivalent power supply, j represents an imaginary unit, ω represents the system current angular frequency, C0 represents the system line-to-ground capacitance, L PT Indicates the equivalent inductance of the high-voltage winding of the voltage transformer; The first Laplace expression is: Where, I(s) represents the Laplace transform result of current, I m represents the rated current of the transformer, e represents the natural constant, j represents the imaginary unit, represents the voltage phase angle when the fault disappears, s represents the transformation parameter, and ω represents the system current angular frequency; The second Laplace expression is: Where Z(s) represents the system impedance function, s represents the transformation parameter, and L PT represents the equivalent inductance of the high-voltage winding of the voltage transformer, and C0 represents the capacitance of the system line to ground; The voltage transformer terminal voltage expression is: Among them, U(t) represents the instantaneous voltage function at the output of the transformer, E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, Indicates the voltage phase angle when the fault disappears, L PT represents the equivalent inductance of the high-voltage winding of the voltage transformer, and C0 represents the capacitance of the system line to ground; The free oscillation angular frequency is: Where ω′ represents the free oscillation angular frequency, L PT represents the equivalent inductance of the high-voltage winding of the voltage transformer, and C0 represents the capacitance of the system line to ground; The simplified voltage transformer terminal voltage expression is: Among them, U(t) represents the instantaneous voltage function at the output of the transformer, E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, It represents the voltage phase angle when the fault disappears, and ω′ represents the free oscillation angular frequency; The initial superposition equation is: Among them, U(t) represents the instantaneous voltage function at the output of the transformer, E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, represents the voltage phase angle when the fault disappears, ω′ represents the free oscillation angular frequency, δ represents the damping coefficient, and e represents the natural constant; The initial superposition equation is: Among them, Ψ represents the free oscillation flux, U represents the instantaneous voltage at the output end of the transformer, and E m represents the rated voltage of the transformer, ω represents the system current angular frequency, t represents time, It represents the voltage phase angle when the fault disappears, ω′ represents the free oscillation angular frequency, δ represents the damping coefficient, and e represents the natural constant.

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