Method, device and equipment for calculating fault current of networked converter, and storage medium

Abstract

The application provides a kind of network construction converter fault current calculation method, device, equipment and storage medium, it is related to network construction converter fault characteristic analysis technical field, method includes: the equivalent circuit model suitable for network construction converter grid-connected system fault current analytical calculation is constructed;Based on the dynamic equation of reactive power-voltage control link of network construction converter, the internal potential analytical expression of network construction converter is solved;The internal potential analytical expression is substituted into the dynamic equation of positive sequence equivalent circuit, and the analytical expression of fault current positive sequence component is solved;Based on the dynamic equation of negative sequence equivalent circuit, the analytical expression of fault current negative sequence component is solved;The analytical expression of fault current positive sequence component and the analytical expression of fault current negative sequence component are superimposed, and the analytical expression of total fault current of network construction converter is obtained.The calculation result of the application is high in accuracy and wide in applicability.

Description

Technical Field

[0001] This invention relates to the field of fault characteristic analysis technology for grid-connected converters, specifically to a method, apparatus, equipment, and storage medium for calculating fault current in grid-connected converters. Background Technology

[0002] With the large-scale integration of new energy sources such as wind power and photovoltaics, the power system is shifting from a traditional synchronous generator-dominated model to one dominated by power electronic interfaces. Grid-connected converters, capable of simulating synchronous generators and autonomously constructing voltage and frequency to provide inertia and damping support for the grid, have become a key technology for addressing the stability of new energy grid integration. However, when a short-circuit fault occurs in the grid, the fault current characteristics of grid-connected converters are determined by their complex control strategies, significantly different from those of traditional synchronous generators. This makes it difficult to directly apply relay protection setting principles based on synchronous generator fault characteristics, posing a challenge to the safe and stable operation of the new power system.

[0003] To accurately characterize the fault current characteristics of grid-connected converters, existing research methods mainly include numerical integration or time-domain simulation methods, hybrid calculation methods combining numerical integration and physical mechanisms, and analytical methods. While numerical integration or time-domain simulation methods can simulate fault processes, their results heavily depend on specific parameter settings, making it difficult to reveal the universal laws and physical mechanisms of fault currents. Hybrid calculation methods combining numerical integration and physical mechanisms, to some extent, reflect the coupling relationship between the control loop and the electrical interface, but are essentially still numerical iterative processes. In contrast, analytical methods, such as deriving analytical expressions for fault currents based on power control loops and network quasi-steady-state models, do not consider the electromagnetic transient process of fault currents, making it difficult to accurately characterize the initial dynamic process of a fault. Other analytical methods, such as constructing analytical models using small-signal modeling combined with Laplace transform and singular perturbation theory, do not consider virtual impedance, a key current suppression strategy in mainstream control structures, thus limiting their engineering applicability. Another analytical method is to establish the dynamic relationship between the voltage-current loop and the power control loop, and obtain the analytical expression of the fault current through back substitution and Laplace transform. The modeling process of this method only takes into account the voltage drop scenario at the grid connection point and does not take into account the influence of network parameters between the grid-connected converter and the infinite grid on the fault current characteristics.

[0004] In summary, existing analytical calculation schemes for fault current in grid-connected converters have two shortcomings: First, in order to simplify the calculation, factors such as virtual impedance and network impedance, which have a critical impact on fault current, are ignored, resulting in low calculation accuracy. Second, they focus primarily on symmetrical fault scenarios in the power grid, lacking systematic analysis of the more common asymmetrical faults in the power grid, which greatly limits their engineering applicability. Summary of the Invention

[0005] In view of this, in order to solve the above-mentioned technical problems, the present invention provides a method, apparatus, device and storage medium for calculating fault current of grid converter.

[0006] The present invention adopts the following technical solution: In a first aspect, the present invention provides a method for calculating the fault current of a grid converter, comprising: The grid-connected system topology and control strategy of the grid converter are determined, and the controller of the grid converter is used to filter out the negative sequence component of the fault current of the grid converter. Based on the grid-connected system topology and control strategy of the grid-connected converter, an equivalent circuit model suitable for the analytical calculation of fault current in the grid-connected system of the grid-connected converter is constructed. The equivalent circuit model includes a positive-sequence equivalent circuit and a negative-sequence equivalent circuit constructed based on the symmetrical component method. In the equivalent circuit model, the grid-connected converter is equivalent to a Thevenin equivalent circuit composed of a voltage source driven by internal potential and a virtual impedance connected in series. Based on the dynamic equations of the reactive power-voltage control loop of the grid converter, the analytical expression of the internal potential of the grid converter is solved. Substituting the analytical expression of the internal potential into the dynamic equation of the positive-sequence equivalent circuit, the analytical expression of the positive-sequence component of the fault current is obtained by solving the equation. Based on the dynamic equations of the negative-sequence equivalent circuit, the analytical expression of the negative-sequence component of the fault current is obtained by solving the equations. The analytical expression for the positive-sequence component of the fault current and the analytical expression for the negative-sequence component of the fault current are superimposed to obtain the analytical expression for the total fault current of the grid converter.

[0007] Optionally, the control strategy is: The active power-frequency control loop of the grid converter adopts a virtual synchronous control strategy. The reactive power-voltage control loop adopts a reactive power-voltage droop control strategy with an integral element.

[0008] Optionally, based on the dynamic equations of the reactive power-voltage control loop of the grid-connected converter, the analytical expression of the internal electromotive force of the grid-connected converter is solved, specifically including: By linearizing the nonlinear equations in the dynamic equations of the reactive power-voltage control loop, a differential equation for the internal potential is established, and the differential equation is solved to obtain the analytical expression for the internal potential of the grid converter.

[0009] Optionally, the linearization process specifically involves linearizing the grid-connected voltage of the grid converter at its steady-state operating point.

[0010] Optionally, in the grid-connected system topology, the grid-connected converter is connected to the power grid via a step-up transformer connected in a dY configuration.

[0011] Secondly, the present invention provides a device for calculating the fault current of a grid converter, comprising: The determination module is used to determine the grid-connected system topology and control strategy of the grid converter, and the controller of the grid converter is used to filter out the negative sequence component of the fault current of the grid converter; The module is used to construct an equivalent circuit model suitable for fault current analysis calculation of grid-connected systems of grid-connected converters, combining the grid-connected system topology and control strategy of grid-connected converters. The equivalent circuit model includes a positive-sequence equivalent circuit and a negative-sequence equivalent circuit constructed based on the symmetrical component method. In the equivalent circuit model, the grid-connected converter is equivalent to a Thevenin equivalent circuit composed of a voltage source driven by internal potential and a virtual impedance connected in series. The first solution module is used to solve the analytical expression of the internal potential of the grid converter based on the dynamic equation of the reactive power-voltage control link of the grid converter; The second solution module is used to substitute the analytical expression of the internal potential into the dynamic equation of the positive sequence equivalent circuit to obtain the analytical expression of the positive sequence component of the fault current. The third solution module is used to solve for the analytical expression of the negative sequence component of the fault current based on the dynamic equation of the negative sequence equivalent circuit. The superposition module is used to superimpose the analytical expression of the positive sequence component of the fault current and the analytical expression of the negative sequence component of the fault current to obtain the analytical expression of the total fault current of the grid converter.

[0012] Optionally, the control strategy is: The active power-frequency control loop of the grid converter adopts a virtual synchronous control strategy. The reactive power-voltage control loop adopts a reactive power-voltage droop control strategy with an integral element.

[0013] Optional, the first solver module is specifically used for: By linearizing the nonlinear equations in the dynamic equations of the reactive power-voltage control loop, a differential equation for the internal potential is established, and the differential equation is solved to obtain the analytical expression for the internal potential of the grid converter.

[0014] Thirdly, the present invention provides a device for calculating the fault current of a grid converter, comprising: At least one processor; and, A memory communicatively connected to the at least one processor; wherein, The memory stores instructions that can be executed by the at least one processor, which enables the at least one processor to implement the method for calculating the fault current of the grid converter as described above.

[0015] Fourthly, the present invention provides a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed by a processor, it implements the method for calculating the fault current of a grid converter as described above.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention solves the analytical calculation problem of asymmetrical faults. By introducing the symmetrical component method and solving the positive and negative sequence current components separately and then superimposing them, this invention provides a systematic and complete solution for the analytical calculation of the current dynamics of grid converters under asymmetrical faults. Compared with existing technologies that are mostly limited to symmetrical faults, this invention greatly expands the applicability of the method.

[0017] 2. Improved model accuracy and engineering practicality. The model of this invention simultaneously considers virtual impedance and network impedance, which have a significant impact on fault current, and accurately captures the influence of control loop dynamics on internal potential through the time scale separation concept. Compared with simplified models that ignore these factors, the calculation results of this method are more accurate and closer to engineering practice, providing a reliable theoretical basis for relay protection settings in new power systems.

[0018] 3. The physical mechanism of fault current is revealed. By deriving a fully analytical expression, this invention avoids the dependence on parameter settings in traditional numerical simulation methods, and can clearly reveal the influence of key factors such as virtual impedance, network impedance, and control parameters on the transient and steady-state characteristics of fault current, which has important theoretical research value. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a flowchart illustrating a method for calculating fault current in a grid converter according to an embodiment of the present invention. Figure 2 This is a schematic diagram of a grid-connected system topology for a grid-connected converter provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of a control strategy for a grid converter provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of a positive sequence equivalent circuit provided in an embodiment of the present invention; Figure 5 This is a schematic diagram of a negative sequence equivalent circuit provided in an embodiment of the present invention; Figure 6 This is a schematic diagram of a zero-sequence equivalent circuit provided in an embodiment of the present invention; Figure 7 This is a schematic diagram of the electromagnetic transient simulation results provided in an embodiment of the present invention; Figure 8 This is a schematic diagram of the structure of a grid converter fault current calculation device provided in an embodiment of the present invention; Figure 9 This is a schematic diagram of the structure of a grid converter fault current calculation device provided in an embodiment of the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0022] This embodiment provides a complete analytical calculation method for the fault current of a grid-connected converter when an asymmetrical fault occurs in the power grid. In this method, the significant differences in the time scale between the dynamics of the control system and the electromagnetic transients of the network are decoupled, and combined with the symmetrical component method, a fully analytical expression that accurately describes the transient and steady-state characteristics of the fault current is obtained.

[0023] Figure 1 This is a flowchart illustrating a method for calculating fault current in a grid-connected converter according to an embodiment of the present invention. Figure 1 As shown, this process includes: Step 101: Determine the grid-connected system topology and control strategy of the grid converter. The controller of the grid converter is used to filter out the negative sequence component of the fault current of the grid converter (i.e., the controller of the grid converter only controls and outputs the positive sequence component).

[0024] Specifically, the control strategy can be as follows: the active power-frequency control loop of the grid-connected converter adopts a virtual synchronous control strategy, and the reactive power-voltage control loop adopts a reactive power-voltage droop control strategy with an integral element. Furthermore, in the grid-connected system topology, the grid-connected converter is connected to the grid via a step-up transformer with a dY connection. It should be noted that a dY connection refers to a winding connection method where the high-voltage side of the transformer is star (Y) and the low-voltage side is delta (d).

[0025] In a specific example Figure 2 This is a schematic diagram of a grid-connected system topology for a grid-connected converter provided in an embodiment of the present invention. (Reference) Figure 2 The output of the grid converter is connected to an LC filter, which includes a filter inductor. and filter capacitor This is used to filter out high-frequency switching harmonics from the converter output. The filtered AC power is stepped up by a dY-connected step-up transformer, and then passed through the equivalent line impedance. Connected to an infinite power grid, This represents the voltage amplitude of an infinitely large power grid. Depend on and It is connected in series, where, This represents the sum of the leakage inductance of the step-up transformer and the equivalent line resistance from the high-voltage side of the step-up transformer to the infinite power grid. It represents the sum of the leakage inductance of the step-up transformer and the equivalent line inductance from the high-voltage side of the step-up transformer to the infinite power grid.

[0026] It should be noted that an infinite grid means that its voltage amplitude and frequency are constant and unaffected by the grid connection of the converter. In this embodiment, the use of a dY-connected transformer is an important technical feature, as this connection method can effectively block the flow of zero-sequence current from the grid side to the converter side.

[0027] Figure 3 This is a schematic diagram of a control strategy for a grid-connected converter provided in an embodiment of the present invention. (Reference) Figure 3 Δ ω Indicates the angular frequency of the grid converter ω With the rated angular frequency of an infinite power grid ω A deviation of 0; and D These represent the virtual inertia and damping of the grid converter, respectively. s Represents the Laplace operator; This represents the reactive power voltage regulation coefficient in the reactive power-voltage control system. This represents the integral coefficient of the reactive power-voltage control loop; This indicates the active power reference value of the grid-connected converter;P This represents the actual active power value of the grid-connected converter; This indicates the reference value for reactive power of the grid-connected converter; This represents the actual value of reactive power of the grid-connected converter; This indicates a reference value for the voltage at the grid connection point of the grid-connected converter; This represents the actual value of the voltage at the grid connection point of the grid-connected converter; This represents the internal electromotive force of the grid converter; Indicates virtual resistance; Indicates virtual inductance; Indicates the output current of the grid converter d Axial components; Indicates the output current of the grid converter q Axial components; Indicates inner loop voltage and current control d Shaft voltage reference value, Indicates inner loop voltage and current control q The shaft voltage reference value is used, and the inner loop voltage and current control adopts classic vector control. Indicates the output of the inner loop voltage and current control. , b , c Three-phase modulated wave.

[0028] Figure 3 The control system structure of the grid-connected converter is illustrated schematically. The system is divided into outer-loop control and inner-loop control. The outer-loop control is responsible for achieving macro-level control objectives. Specifically, in this embodiment, the active power-frequency control loop employs a virtual synchronous control strategy. By simulating the swing equation of a synchronous generator, the converter can respond to system frequency changes and provide inertia support. The reactive power-voltage control loop adopts a reactive power-voltage droop control strategy with an integral element. It adjusts the voltage amplitude within the converter based on the grid connection point voltage deviation and reactive power output to maintain grid connection point voltage stability.

[0029] The inner-loop control is responsible for quickly and accurately tracking the outer-loop commands, including a fast vector current control loop and a virtual impedance loop to improve the system's dynamic performance. This virtual impedance loop introduces virtual impedance into the control algorithm, which consists of a virtual resistance. and virtual inductance It is designed to simulate the stator impedance of a synchronous generator, thereby enhancing the damping and stability of the system.

[0030] Step 102: Combining the grid-connected system topology and control strategy of the grid converter, construct an equivalent circuit model suitable for the analytical calculation of fault current in the grid-connected system of the grid converter. The equivalent circuit model includes a positive-sequence equivalent circuit and a negative-sequence equivalent circuit constructed based on the symmetrical component method. In the equivalent circuit model, the grid converter is equivalent to a Thevenin equivalent circuit composed of a voltage source driven by internal potential and a virtual impedance connected in series.

[0031] Specifically, the fault current of the grid-connected converter is subject to high-dimensional nonlinearity due to the coupling of control strategy and electromagnetic transients, and is accompanied by complexity caused by the superposition of positive and negative sequence components, making its analytical calculation quite difficult. Therefore, this invention introduces the following simplifying assumptions: 1) The introduction of negative sequence components will inject high-frequency components into the grid, so it is assumed that only positive sequence components are fed into the controller of the grid converter.

[0032] 2) The dynamic response speed of the filtering stage and the voltage and current inner loop of the grid-connected converter is much faster than that of the power control stage. Therefore, the grid-connected converter can be considered... d , q The shaft current / voltage is tracked in real time to the reference value.

[0033] 3) Considering that the electromagnetic transients of the network are much faster than those of the power control stage, it can be assumed that in the calculation... P (Actual value of active power of grid converter) (Actual value of reactive power of grid converter) and When calculating the actual value of the grid-connected voltage of the grid converter, only the quasi-steady state of the network is considered.

[0034] 4) Considering that grid-connected converters have virtual inertia, it can be assumed that during a fault... ω ≈ ω 0, the phase angle difference between the internal potential and the infinite grid voltage. ≈ ( This represents the phase angle difference between the grid converter and the infinite bus under normal operating conditions.

[0035] Based on the above simplifying assumptions, and considering the grid-connected system topology and control strategy of grid-connected converters, an equivalent circuit model suitable for analytical calculation of fault currents in grid-connected systems with grid-connected converters is constructed. In a specific example, Figure 4 This is a schematic diagram of a positive sequence equivalent circuit provided in an embodiment of the present invention. Figure 5 This is a schematic diagram of a negative sequence equivalent circuit provided in an embodiment of the present invention. Figure 6 This is a schematic diagram of a zero-sequence equivalent circuit provided in an embodiment of the present invention. (Reference) Figure 4 , Figure 5 and Figure 6 , Represents virtual impedance. ; This represents the equivalent line impedance. For the positive-sequence, negative-sequence, and zero-sequence components of the grid voltage, their amplitudes are respectively represented by... U F1 , U F2 , U F0 The phase relative to the initial phase of the power grid is represented by... θ F1 , θ F2 , θ F0 The positive-sequence, negative-sequence, and zero-sequence components of the fault current in the grid converter are represented by... i abc1 , i abc2 , i abc0 express.

[0036] Figure 4 In this circuit, the grid converter is equivalent to a Thevenin circuit, consisting of an ideal voltage source in series with an impedance. The amplitude of this ideal voltage source is the internal potential E of the grid converter, and its phase angle is the power angle difference between the internal potential of the grid converter and the infinite grid voltage. The series impedance is a virtual impedance. The Thevenin equivalent circuit and line impedance and the positive sequence component amplitude of the grid voltage U F1 When connected in series, they form a closed loop; the current flowing through this loop is the positive sequence current. .

[0037] Figure 5 In a negative-sequence network, because the controller of a grid converter is typically designed to respond only to and output the positive-sequence component and not to the negative-sequence component, there is no internal potential source on the converter side. The negative-sequence equivalent circuit consists only of virtual impedance. Line impedance And the magnitude of the negative sequence component of the grid voltage U F2 The circuit is connected in series, and the current flowing through it is a negative sequence current. .

[0038] Figure 6 As mentioned earlier, the system employs a step-up transformer with a dY connection, which provides a high-impedance path for the zero-sequence current, effectively isolating it. Therefore, the zero-sequence equivalent circuit behaves as an open circuit, and the zero-sequence current is zero. In other words, under the specific topology of this embodiment, the total fault current consists only of the positive-sequence current and the negative-sequence current.

[0039] According to the symmetrical component method, the magnitudes of the positive-sequence component, negative-sequence component, and zero-sequence component of the grid voltage are calculated using the following expressions: ......(1) in, U Fa , U Fb , U Fc These represent the power grid during the fault period. The voltage amplitudes of phases A, B, and C, when the voltage of one phase drops to... U F At that time, the voltage amplitude during the phase fault is... U F ; β a , β b , β c These represent the power grid during the fault period. Phase angle jump of the three-phase voltages (a, b, and c) β = 0 indicates that no phase angle jump occurred during the phase fault.

[0040] Step 103: Based on the dynamic equation of the reactive power-voltage control loop of the grid converter, solve the analytical expression of the internal potential of the grid converter.

[0041] Understandably, the internal potential of a grid-connected converter, as a key internal variable in its external electrical characteristics, is primarily determined by the control system. Since the response speed of the control components (especially the reactive power-voltage control involving the integrator) is typically much slower than the electromagnetic transient process on the grid side, the dynamics of the internal potential can be considered a slow process, while the dynamics of the network current can be considered a fast process. Based on this time-scale separation concept, the analytical expression for the change of internal potential over time can be solved independently first.

[0042] Specifically, based on the dynamic equations of the reactive power-voltage control loop of the grid-connected converter, the analytical expression of the internal electromotive force of the grid-connected converter is solved, which may include: By linearizing the nonlinear equations in the dynamic equations of the reactive power-voltage control loop, a differential equation regarding the internal potential is established, and the differential equation is solved to obtain the analytical expression for the internal potential of the grid-connected converter. Specifically, the linearization process can involve linearizing the grid-connected voltage of the grid-connected converter at its steady-state operating point.

[0043] In a specific example, with Phase fault current For example, based on the equivalent circuit model and the aforementioned simplification assumptions, the fault current is dynamically described as follows: ......(2) in, i a1 , i a2 and i a0 They represent Phase fault current The positive, negative, and zero-order components, θ 0 indicates the initial phase of the grid voltage at the moment the fault occurs. L eq express and The sum of L eq = + , R eq express and The sum of R eq = + , (Actual value of reactive power of grid converter) and The expression for (the actual value of the grid-connected voltage of the grid converter) is as follows: ......(3) in, X v and X g They represent and The corresponding reactance, X v = ω 0 L v , X g = ω 0 L g , X eq yes X v and X g The sum of X eq = X v + X g ;| Z g| represents the magnitude of the equivalent impedance from the grid connection point of the grid-connected converter to the infinite power grid,| Z g |= ( R g 2 + X g 2 ) 1 / 2 ;| Z v | represents the magnitude of the virtual impedance,| Z v | = ( R v 2 + X v 2 ) 1 / 2 , Z s 2 This indicates the cross-coupling term between line resistance / inductance and virtual resistance / inductance. Z s The dimension of Ω is , which has no clear physical meaning.

[0044] Since equation (3) does not explicitly contain And its sequence components, therefore, the analytical expression of the internal potential E can be solved using the reactive power-voltage control dynamic equation in equation (3) and equation (2). However, in equation (3) U The nonlinear characteristics of the internal electromotive force make it difficult to directly analyze the internal electromotive force through the dynamic equations of the reactive power-voltage link control. E Therefore, the present invention will U View as E The function, and in E = E 0 ( E 0 represents the steady-state value of the internal potential during normal operation. Linearization at this point yields an approximate expression, which is then compared with the expression in equation (3). Q Substituting the expression into the differential equation describing the reactive power-voltage control dynamics, we obtain the following differential equation regarding the internal potential E: ......(4) in, m 0、 m 1. m The expression for 2 is shown in the following formula: ......(5) Solving differential equation (5) yields the internal potential of the grid converter. E The parsing expression is as follows: ......(6) in,E 1 and E The expression for 2 is shown in the following formula: ......(7) Apply equation (6) Positive sequence component of phase fault current The first-order differential equation is still difficult to derive analytical form. Note that... E 0>0, E 2<0< E 1, E 1+ E 2<0, m 2 < 0, therefore: ......(8) Therefore, in equation (6), ( E 0– E 1) / ( E 0– E 2) exp[( E 1– E 2) m 2 t Treating it as a whole and linearizing it, we obtain the processed internal potential. E The expression is: ......(9) in, E 3 and λ The expression is shown in the following formula: ......(10) Step 104: Substitute the analytical expression of the internal potential into the dynamic equation of the positive sequence equivalent circuit to obtain the analytical expression of the positive sequence component of the fault current.

[0045] Following the example above, substitute equation (10) into equation (2) to describe... Positive sequence component of phase fault current The dynamic first-order differential equation can be obtained as follows: The parsing expression is as follows: ... (11) in, , , and The expression is shown in the following formula: ......(12) Step 105: Based on the dynamic equation of the negative sequence equivalent circuit, solve for the analytical expression of the negative sequence component of the fault current.

[0046] Following the previous example, according to the description in equation (2) Negative sequence component of phase fault current Dynamic first-order differential equations, combined with the initial fault moment Given the initial condition that the negative sequence component of the phase fault current is 0, we can obtain... The parsing expression is: ......(13) Step 106: Superimpose the analytical expressions of the positive-sequence component of the fault current and the negative-sequence component of the fault current to obtain the analytical expression of the total fault current of the grid converter.

[0047] Following the previous example, combining equations (11) and (13), the analytical expression for the fault current of phase a can be obtained as follows: ......(14) To verify the effectiveness of the analytical mathematical model of internal potential (i.e., Equation 9) and the mathematical model of fault current (e.g., Equation 14) constructed in this invention during grid converter faults, a simulation platform such as PSCAD / EMTDC can be built. Figure 2 The electromagnetic transient simulation model parameters for the grid-connected converter system shown are as shown in Table 1.

[0048] Table 1

[0049] Set the following operating conditions: Operating Condition 1: Power Grid A phase failure occurs, during the failure period, The phase grid voltage amplitude dropped to 0.20 pu.

[0050] Operating Condition 2: Power Grid A phase failure occurs, during the failure period, The phase grid voltage amplitude dropped to 0.30 pu, and The phase grid voltage jumps forward by π / 4 rad.

[0051] Operating Condition 3: Power Grid Two phases, b and c, experienced a fault. During the fault, The voltage amplitude of phases b and b dropped to 0.30 pu.

[0052] Operating Condition 4: Power Grid Two phases, b and c, experienced a fault. During the fault, The voltage amplitude of phases b and b of the power grid dropped to 0.40 pu, and The voltage of phases b and c jumps backward by π / 3 rad.

[0053] The electromagnetic transient simulation results under the above four operating conditions are compared with the calculation results of formulas (9) and (14) as follows: Figure 7 As shown. According to Figure 7 The calculation results of formulas (9) and (14) are highly consistent with the electromagnetic transient simulation results, indicating that the internal potential analytical mathematical model (i.e., formula 9) and fault current mathematical model (i.e., formula 14) constructed by the present invention during the fault period of the grid converter can be used to describe the dynamics during the fault period of the grid converter, and verify the effectiveness of formulas (9) and (14).

[0054] The method provided in this embodiment constructs a sequence network model that includes virtual impedance and line impedance, and decouples and solves the internal potential and network current based on the time-scale separation concept. Finally, the total fault current is obtained by superposition, successfully solving the analytical calculation problem of fault current of grid-connected converters under asymmetrical faults. Its calculation results are accurate and can provide a powerful theoretical tool for relay protection setting and parameter optimization design of new power systems.

[0055] Based on a general inventive concept, the present invention also provides a device for calculating the fault current of a grid converter. Figure 8 This is a schematic diagram of the structure of a grid converter fault current calculation device provided in an embodiment of the present invention. Figure 8 As shown, this device includes: The determination module 81 is used to determine the grid-connected system topology and control strategy of the grid converter. The controller of the grid converter is used to filter out the negative sequence component of the fault current of the grid converter.

[0056] Module 82 is used to construct an equivalent circuit model suitable for fault current analysis calculation of grid-connected systems of grid-connected converters, combining the grid-connected system topology and control strategy of the grid-connected converter. The equivalent circuit model includes a positive-sequence equivalent circuit and a negative-sequence equivalent circuit constructed based on the symmetrical component method. In the equivalent circuit model, the grid-connected converter is equivalent to a Thevenin equivalent circuit composed of a voltage source driven by internal potential and a virtual impedance connected in series.

[0057] The first solution module 83 is used to solve the analytical expression of the internal electromotive force of the grid converter based on the dynamic equation of the reactive power-voltage control link of the grid converter.

[0058] The second solution module 84 is used to substitute the analytical expression of the internal potential into the dynamic equation of the positive sequence equivalent circuit to obtain the analytical expression of the positive sequence component of the fault current.

[0059] The third solution module 84 is used to solve for the analytical expression of the negative sequence component of the fault current based on the dynamic equation of the negative sequence equivalent circuit.

[0060] The superposition module 86 is used to superimpose the analytical expression of the positive sequence component of the fault current and the analytical expression of the negative sequence component of the fault current to obtain the analytical expression of the total fault current of the grid converter.

[0061] Optionally, the control strategy can be: The active power-frequency control loop of the grid converter adopts a virtual synchronous control strategy. The reactive power-voltage control loop adopts a reactive power-voltage droop control strategy with an integral element.

[0062] Optional, the first solver module is specifically used for: By linearizing the nonlinear equations in the dynamic equations of the reactive power-voltage control loop, a differential equation for the internal potential is established, and the differential equation is solved to obtain the analytical expression for the internal potential of the grid converter.

[0063] Optionally, the linearization process specifically involves linearizing the grid-connected voltage of the grid converter at its steady-state operating point.

[0064] Optionally, in the grid-connected system topology, the grid-connected converter is connected to the power grid via a step-up transformer connected in a dY configuration.

[0065] Based on a general inventive concept, the present invention also provides a device for calculating the fault current of a grid converter. Figure 9 This is a schematic diagram of the structure of a grid converter fault current calculation device provided in an embodiment of the present invention. Figure 9 As shown, the grid converter fault current calculation device 900 includes: At least one processor 910; and, A memory 930 is communicatively connected to the at least one processor 910; wherein, The memory 930 stores instructions 920 that can be executed by the at least one processor. The instructions 920 are executed by the at least one processor 910 to enable the at least one processor 910 to implement the method for calculating the fault current of the grid converter as described above.

[0066] Based on a general inventive concept, the present invention also provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the method for calculating the fault current of a grid converter as described above.

[0067] It is understood that the same or similar parts in the above embodiments can be referred to each other, and the contents not described in detail in some embodiments can be referred to the same or similar contents in other embodiments.

[0068] It should be noted that in the description of this invention, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means at least two.

[0069] Any process or method described in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing a particular logical function or process, and the scope of preferred embodiments of the invention includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the functions involved, as will be understood by those skilled in the art to which embodiments of the invention pertain.

[0070] It should be understood that various parts of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented using any one or a combination of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0071] Those skilled in the art will understand that all or part of the steps of the methods in the above embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, the program includes one or a combination of the steps of the method embodiments.

[0072] Furthermore, the functional units in the various embodiments of the present invention can be integrated into a processing module, or each unit can exist physically separately, or two or more units can be integrated into a module. The integrated module can be implemented in hardware or as a software functional module. If the integrated module is implemented as a software functional module and sold or used as an independent product, it can also be stored in a computer-readable storage medium.

[0073] The storage media mentioned above can be read-only memory, disk, or optical disk, etc.

[0074] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0075] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for calculating fault current in a grid-connected converter, characterized in that, include: The grid-connected system topology and control strategy of the grid converter are determined, and the controller of the grid converter is used to filter out the negative sequence component of the fault current of the grid converter. Based on the grid-connected system topology and control strategy of the grid-connected converter, an equivalent circuit model suitable for the analytical calculation of fault current in the grid-connected system of the grid-connected converter is constructed. The equivalent circuit model includes a positive-sequence equivalent circuit and a negative-sequence equivalent circuit constructed based on the symmetrical component method. In the equivalent circuit model, the grid-connected converter is equivalent to a Thevenin equivalent circuit composed of a voltage source driven by internal potential and a virtual impedance connected in series. Based on the dynamic equations of the reactive power-voltage control loop of the grid converter, the analytical expression of the internal potential of the grid converter is solved. Substituting the analytical expression of the internal potential into the dynamic equation of the positive-sequence equivalent circuit, the analytical expression of the positive-sequence component of the fault current is obtained by solving the equation. Based on the dynamic equations of the negative-sequence equivalent circuit, the analytical expression of the negative-sequence component of the fault current is obtained by solving the equations. The analytical expression for the positive-sequence component of the fault current and the analytical expression for the negative-sequence component of the fault current are superimposed to obtain the analytical expression for the total fault current of the grid converter.

2. The method for calculating the fault current of a grid converter according to claim 1, characterized in that, The control strategy is as follows: The active power-frequency control loop of the grid converter adopts a virtual synchronous control strategy. The reactive power-voltage control loop adopts a reactive power-voltage droop control strategy with an integral element.

3. The method for calculating the fault current of a grid converter according to claim 1, characterized in that, Based on the dynamic equations of the reactive power-voltage control loop of the grid-connected converter, the analytical expression of the internal potential of the grid-connected converter is solved, specifically including: By linearizing the nonlinear equations in the dynamic equations of the reactive power-voltage control loop, a differential equation for the internal potential is established, and the differential equation is solved to obtain the analytical expression for the internal potential of the grid converter.

4. The method for calculating the fault current of a grid converter according to claim 3, characterized in that, The linearization process specifically involves linearizing the grid-connected voltage of the grid converter at its steady-state operating point.

5. The method for calculating the fault current of a grid converter according to claim 1, characterized in that, In the grid-connected system topology, the grid-connected converter is connected to the power grid via a step-up transformer connected in a dY configuration.

6. A device for calculating fault current of a grid converter, characterized in that, include: The determination module is used to determine the grid-connected system topology and control strategy of the grid converter, and the controller of the grid converter is used to filter out the negative sequence component of the fault current of the grid converter; The module is used to construct an equivalent circuit model suitable for fault current analysis calculation of grid-connected systems of grid-connected converters, combining the grid-connected system topology and control strategy of grid-connected converters. The equivalent circuit model includes a positive-sequence equivalent circuit and a negative-sequence equivalent circuit constructed based on the symmetrical component method. In the equivalent circuit model, the grid-connected converter is equivalent to a Thevenin equivalent circuit composed of a voltage source driven by internal potential and a virtual impedance connected in series. The first solution module is used to solve the analytical expression of the internal potential of the grid converter based on the dynamic equation of the reactive power-voltage control link of the grid converter; The second solution module is used to substitute the analytical expression of the internal potential into the dynamic equation of the positive sequence equivalent circuit to obtain the analytical expression of the positive sequence component of the fault current. The third solution module is used to solve for the analytical expression of the negative sequence component of the fault current based on the dynamic equation of the negative sequence equivalent circuit. The superposition module is used to superimpose the analytical expression of the positive sequence component of the fault current and the analytical expression of the negative sequence component of the fault current to obtain the analytical expression of the total fault current of the grid converter.

7. The calculation device for fault current of a grid converter according to claim 6, characterized in that, The control strategy is as follows: The active power-frequency control loop of the grid converter adopts a virtual synchronous control strategy. The reactive power-voltage control loop adopts a reactive power-voltage droop control strategy with an integral element.

8. The device for calculating the fault current of a grid converter according to claim 6, characterized in that, The first solution module is specifically used for: By linearizing the nonlinear equations in the dynamic equations of the reactive power-voltage control loop, a differential equation for the internal potential is established, and the differential equation is solved to obtain the analytical expression for the internal potential of the grid converter.

9. A device for calculating fault current of a grid converter, characterized in that, include: At least one processor; as well as, A memory communicatively connected to the at least one processor; wherein, The memory stores instructions that can be executed by the at least one processor, which, when executed by the at least one processor, enables the at least one processor to implement the method for calculating the fault current of the grid converter as described in any one of claims 1 to 5.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the method for calculating the fault current of the grid converter as described in any one of claims 1 to 5.

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