A method for determining three-phase short-circuit fault current of dual-loop controlled virtual synchronous machine

Abstract

The present invention relates to a method for determining the three-phase short-circuit fault current of a dual-loop controlled virtual synchronous machine, comprising the following steps: Step 1: constructing active and reactive current equations based on the virtual synchronous machine's controller and taking dual-loop control into account; Step 2: constructing active and reactive current equations when a three-phase short-circuit fault occurs, taking into account the impact of the virtual synchronous machine's fault ride-through strategy on the fault current; and Step 3: calculating the fault current in the dq coordinate system based on the obtained active and reactive current equations. This method facilitates accurate fault current acquisition.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system fault analysis, and in particular relates to a method for determining a three-phase short-circuit fault current of a dual-loop controlled virtual synchronous machine. Background Art

[0002] With the gradual increase in the penetration rate of distributed energy, the installed proportion of synchronous generators in power systems has decreased relatively. What is more serious is that the virtual synchronous machine converters of a large number of power electronic converters enable renewable energy power generation to be connected to the power grid in a manner similar to synchronous generators, thereby improving the power grid's ability to absorb renewable energy. Therefore, in power grids where a large number of renewable energy sources are connected, virtual synchronous machine technology has broad application prospects.

[0003] Short-circuit current analysis is fundamental to the design of protection principles and the verification of protection performance. Due to the complexity of analyzing converter short-circuit current, this analysis is primarily performed through electromagnetic transient simulation. However, electromagnetic transient simulation cannot accurately represent fault currents and cannot exhaustively account for all operating conditions. Therefore, it is necessary to develop analytical methods that can accurately describe fault current characteristics.

[0004] At present, the existing technology for fault current analysis of grid-connected converters focuses on traditional grid-following converters, which are significantly different from networked virtual synchronous machines. The characteristics of their fault currents are also different. It is urgent to carry out research on the three-phase short-circuit fault current analysis method of virtual synchronous machines. Summary of the Invention

[0005] The object of the present invention is to provide a method for determining a three-phase short-circuit fault current of a dual-loop controlled virtual synchronous machine, which is conducive to accurately obtaining the fault current.

[0006] To achieve the above object, the present invention adopts a technical solution: a method for determining the three-phase short-circuit fault current of a dual-loop controlled virtual synchronous machine, comprising the following steps:

[0007] Step 1: Based on the controller of the virtual synchronous machine, consider the dual-loop control and construct the active current and reactive current equations;

[0008] Step 2: Considering the impact of the fault ride-through strategy of the virtual synchronous machine on the fault current, construct the active current and reactive current equations when a three-phase short circuit fault occurs;

[0009] Step 3: Calculate the fault current in the dq coordinate system based on the obtained active current and reactive current equations.

[0010] Furthermore, the implementation method of step 1 is:

[0011] The dual-loop control of the virtual synchronous machine is the voltage-current dual closed-loop control. According to the voltage loop control of the virtual synchronous machine, we can obtain:

[0012]

[0013] Where k up and k ui They represent the proportional coefficient and integral coefficient of the voltage loop respectively, s is the Laplace operator, V d and V q They are the d-axis and q-axis components of the output voltage of the virtual synchronous machine in the rotating coordinate system, C f is the capacitance of the filter capacitor, w is the angular frequency of the virtual synchronous machine, i dref and i qref are the command values of the d-axis and q-axis components of the VSG output current in the rotating coordinate system, i.e., the command values of the active current and reactive current;

[0014] Since the current flowing through the filter is very small, it can be neglected, and we get:

[0015]

[0016] According to the KVL equation, the output voltage of the virtual synchronous machine, the grid voltage, and the line impedance satisfy the following relationship:

[0017]

[0018] R and L are the line resistance and inductance from the virtual synchronous machine to the grid connection point, V pccd 、V pccq is the projection of the grid connection point voltage of the virtual synchronous machine in the dq rotating coordinate system. Substituting (3) into (2) yields:

[0019]

[0020] Since the response speed of the current inner loop is very fast, assuming that the active current and reactive current output of the VSG can accurately follow their reference values, the current inner loop can be ignored, so i dref =i d ,i qref =i q , we can get the equations for active current and reactive current:

[0021]

[0022] Furthermore, the implementation method of step 2 is:

[0023] The fault ride-through strategy of the virtual synchronous machine includes a current limiting strategy and a transient stability control strategy. The current limiting strategy is: when a fault occurs, the output voltage of the VSG is reduced by inputting a virtual impedance to limit the fault current. The current limiting strategy affects the amplitude of the fault current but has no effect on the process of deriving the analytical expression of the fault current. Therefore, the line resistance R and inductance L from the virtual synchronous machine to the grid connection point are changed without considering the influence of the virtual impedance. The transient stability control strategy is implemented by modifying the active frequency equation of the virtual synchronous machine: when a fault occurs, the unbalanced torque is reduced to 0 by modifying the command value of the active power. Therefore, in the process of determining the fault current, it is assumed that w remains unchanged and E d,qref and V pccd,q After a fault occurs, it drops to a fixed value;

[0024] Taking the derivative of both ends of equation (5), we get:

[0025]

[0026] Simplify (6):

[0027]

[0028] in,

[0029]

[0030] Furthermore, the implementation method of step 3 is:

[0031] From Equation (7), it can be seen that the dq axis current components in the second-order differential equations of current cannot be completely decoupled. In order to solve the equation, let m×①+n×②:

[0032]

[0033] Note that to construct new variables to solve, m and n must satisfy the following equations:

[0034]

[0035] Let m = 1, solve equation (13), and get n = ±j. Construct new variables y1 and y2:

[0036]

[0037] Therefore, Equation (9) is transformed into a second-order differential equation in the complex domain:

[0038]

[0039] The general solution of this second-order differential equation is:

[0040]

[0041] Among them, C1, C2, C3 and C4 are complex coefficients to be determined, which are obtained according to the initial values of current and current differential, and then the expressions of active and reactive currents are obtained:

[0042]

[0043] Since the inductor current does not change suddenly at the time of fault, and E qref＝ 0, so according to the impulse function balance method, the initial conditions of the second-order differential equation (13) are:

[0044]

[0045] The analytical expression of the fault current in the dq coordinate system is obtained from equations (14) and (15), and the fault current in the dq coordinate system can be calculated.

[0046] Compared with existing technologies, the present invention offers the following advantages: It provides a method for determining three-phase short-circuit fault current in a dual-loop controlled virtual synchronous machine that considers a fault ride-through strategy. This method incorporates both dual-loop control and the fault ride-through strategy, resulting in more accurate fault current calculation. Furthermore, to address the problem of incomplete decoupling of the second-order differential equations for active and reactive currents, a new variable construction method is employed to transform the equations from the time domain to the complex frequency domain for solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is a schematic diagram of a typical virtual synchronous machine converter system;

[0048] Figure 2 This is a flowchart of a method implementation according to an embodiment of the present invention. DETAILED DESCRIPTION

[0049] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0050] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0051] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0052] The essential difference between virtual synchronous machine and conventional converter control is that the virtual synchronous machine simulates the operating mechanism and external characteristics of the synchronous generator through the control algorithm, but still retains the control link of the conventional inverter. Figure 1 shown.

[0053] The outer loop controller of the virtual synchronous machine is mainly composed of a power calculation module and a virtual synchronous machine control module. The power calculation value uses instantaneous power and calculates active power and reactive power in the dq synchronous rotating coordinate system. The virtual synchronous machine control mainly simulates the rotor inertia and damping characteristics of the synchronous generator and the reactive power-voltage droop characteristics of the synchronous generator by introducing the rotor motion equation to control the voltage output.

[0054]

[0055] E d,ref =K q (Q ref -Q e )+E0

[0056] J is the moment of inertia of the rotor, D p is the damping coefficient of the generator set, K q is the reactive droop coefficient. This second-order electromechanical transient model of the synchronous generator enables the microgrid inverter to participate in grid voltage and frequency regulation, as well as certain low-frequency electromechanical transient capabilities such as inertia and damping. The output of the virtual synchronous generator controller serves as the input to the voltage loop, namely, the setpoint amplitude and phase of the output voltage.

[0057] The inner loop controller of the virtual synchronous machine includes dual closed-loop control of voltage and current. The voltage loop adopts PI control to track the voltage command output by the outer loop controller, and the current loop adopts PI control with active and reactive current decoupling to improve the dynamic response of the VSG.

[0058] Virtual synchronous machine drive controller: mainly completes the switching logic and waveform modulation. According to the modulation wave output by the inner loop, the SVWM algorithm is used to generate a PWM drive signal to control the switching action of the power tube IGBT.

[0059] like Figure 2 As shown, this embodiment provides a method for determining a three-phase short-circuit fault current of a dual-loop controlled virtual synchronous machine, comprising the following steps:

[0060] Step 1: Based on the controller of the virtual synchronous machine, considering the dual-loop control, construct the active current and reactive current equations.

[0061] Step 2: Considering the impact of the fault ride-through strategy of the virtual synchronous machine on the fault current, the active current and reactive current equations when a three-phase short circuit fault occurs are constructed.

[0062] Step 3: Calculate the fault current in the dq coordinate system based on the obtained active current and reactive current equations.

[0063] In this embodiment, the implementation method of step 1 is:

[0064] The dual-loop control of the virtual synchronous machine is the voltage-current dual closed-loop control. According to the voltage loop control of the virtual synchronous machine, we can obtain:

[0065]

[0066] Where k up and k ui They represent the proportional coefficient and integral coefficient of the voltage loop respectively, s is the Laplace operator, V d and V q They are the d-axis and q-axis components of the output voltage of the virtual synchronous machine in the rotating coordinate system, C f is the capacitance of the filter capacitor, w is the angular frequency of the virtual synchronous machine, i dref and i qref They are the command values of the d-axis and q-axis components of the VSG output current in the rotating coordinate system, that is, the command values of the active current and reactive current.

[0067] Since the current flowing through the filter is very small, it can be neglected, and we get:

[0068]

[0069] According to the KVL equation, the output voltage of the virtual synchronous machine, the grid voltage, and the line impedance satisfy the following relationship:

[0070]

[0071] R and L are the line resistance and inductance from the virtual synchronous machine to the grid connection point, V pccd 、V pccq is the projection of the grid connection point voltage of the virtual synchronous machine in the dq rotating coordinate system. Substituting (3) into (2) yields:

[0072]

[0073] Since the response speed of the current inner loop is very fast, assuming that the active current and reactive current output of the VSG can accurately follow their reference values, the current inner loop can be ignored, so i dref =i d ,i qref =i q , we can get the equations for active current and reactive current:

[0074]

[0075] In this embodiment, the implementation method of step 2 is:

[0076] The fault ride-through strategy of the virtual synchronous machine includes a current limiting strategy and a transient stability control strategy. The current limiting strategy is: when a fault occurs, the output voltage of the VSG is reduced by inputting a virtual impedance to limit the fault current. The current limiting strategy affects the amplitude of the fault current but has no effect on the process of deriving the analytical expression of the fault current. Therefore, the line resistance R and inductance L from the virtual synchronous machine to the grid connection point are changed without considering the influence of the virtual impedance. The transient stability control strategy is implemented by modifying the active frequency equation of the virtual synchronous machine: when a fault occurs, the unbalanced torque is reduced to 0 by modifying the command value of the active power. Therefore, in the process of determining the fault current, it is assumed that w remains unchanged and E d,qref and V pccd,q After a fault occurs, it drops to a fixed value.

[0077] Taking the derivative of both ends of formula (5), we get:

[0078]

[0079] Simplify (6):

[0080]

[0081] in,

[0082]

[0083] In this embodiment, the implementation method of step 3 is:

[0084] From Equation (7), it can be seen that the dq axis current components in the second-order differential equations of current cannot be completely decoupled. In order to solve the equation, let m×①+n×②:

[0085]

[0086] Note that to construct new variables to solve, m and n must satisfy the following equations:

[0087]

[0088] Let m = 1, solve equation (13), and get n = ±j. Construct new variables y1 and y2:

[0089]

[0090] Therefore, Equation (9) is transformed into a second-order differential equation in the complex domain:

[0091]

[0092] The general solution of this second-order differential equation is:

[0093]

[0094] Among them, C1, C2, C3 and C4 are complex coefficients to be determined, which are obtained according to the initial values of current and current differential, and then the expressions of active and reactive currents are obtained:

[0095]

[0096] Since the inductor current does not change suddenly at the time of fault, and E qref＝ 0, so according to the impulse function balance method, the initial conditions of the second-order differential equation (13) are:

[0097]

[0098] The analytical expression of the fault current in the dq coordinate system is obtained from equations (14) and (15), and the fault current in the dq coordinate system can be calculated.

[0099] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other manner. Any person skilled in the art may utilize the above-disclosed technical content to modify or modify the present invention into equivalent embodiments. However, any simple modifications, equivalent variations, and modifications to the above embodiments that do not depart from the technical content of the present invention and are based on the technical essence of the present invention remain within the scope of protection of the present invention.

Claims

1. A method for determining three-phase short-circuit fault current of a dual-loop controlled virtual synchronous machine, characterized in that: The steps include: Step 1: Based on the controller of the virtual synchronous machine, consider the dual-loop control and construct the active current and reactive current equations; Step 2: Considering the impact of the fault ride-through strategy of the virtual synchronous machine on the fault current, construct the active current and reactive current equations when a three-phase short circuit fault occurs; Step 3: Calculate the fault current in the dq coordinate system based on the obtained active current and reactive current equations; The implementation method of step 1 is: The dual-loop control of the virtual synchronous machine is the voltage-current dual closed-loop control. According to the voltage loop control of the virtual synchronous machine, we can obtain: Where k up and k ui They represent the proportional coefficient and integral coefficient of the voltage loop respectively, s is the Laplace operator, V d and V q They are the d-axis and q-axis components of the output voltage of the virtual synchronous machine in the rotating coordinate system, C f is the capacitance of the filter capacitor, w is the angular frequency of the virtual synchronous machine, i dref and i qref are the command values of the d-axis and q-axis components of the VSG output current in the rotating coordinate system, i.e., the command values of the active current and reactive current; Since the current flowing through the filter is very small, it can be neglected, and we get: According to the KVL equation, the output voltage of the virtual synchronous machine, the grid voltage, and the line impedance satisfy the following relationship: R and L are the line resistance and inductance from the virtual synchronous machine to the grid connection point, V pccd 、V pccq is the projection of the grid connection point voltage of the virtual synchronous machine in the dq rotating coordinate system. Substituting (3) into (2) yields: Since the response speed of the current inner loop is very fast, assuming that the active current and reactive current output of the VSG can accurately follow their reference values, the current inner loop can be ignored, so i dref =i d ,i qref =i q , we can get the equations for active current and reactive current: The implementation method of step 2 is: The fault ride-through strategy of the virtual synchronous machine includes a current limiting strategy and a transient stability control strategy. The current limiting strategy is: when a fault occurs, the output voltage of the VSG is reduced by inputting a virtual impedance to limit the fault current. The current limiting strategy affects the amplitude of the fault current but has no effect on the process of deriving the analytical expression of the fault current. Therefore, the line resistance R and inductance L from the virtual synchronous machine to the grid connection point are changed without considering the influence of the virtual impedance. The transient stability control strategy is implemented by modifying the active frequency equation of the virtual synchronous machine: when a fault occurs, the unbalanced torque is reduced to 0 by modifying the command value of the active power. Therefore, in the process of determining the fault current, it is assumed that w remains unchanged and E d,qref and V pccd,q After a fault occurs, it drops to a fixed value; Taking the derivative of both ends of formula (5), we get: Simplify (6): in, 2. A method for determining three-phase short-circuit fault current of a dual-loop controlled virtual synchronous machine according to claim 1, characterized in that: The implementation method of step 3 is: From Equation (7), it can be seen that the dq axis current components in the second-order differential equations of current cannot be completely decoupled. In order to solve the equation, let m×①+n×②: Note that to construct new variables to solve, m and n must satisfy the following equations: Let m = 1, solve equation (13), and get n = ±j. Construct new variables y1 and y2: Therefore, Equation (9) is transformed into a second-order differential equation in the complex domain: The general solution of this second-order differential equation is: Among them, C1, C2, C3 and C4 are complex coefficients to be determined, which are obtained according to the initial values of current and current differential, and then the expressions of active and reactive currents are obtained: Since the inductor current does not change suddenly at the time of fault, and E qref＝ 0, so according to the impulse function balance method, the initial conditions of the second-order differential equation (13) are: The analytical expression of the fault current in the dq coordinate system is obtained from equations (14) and (15), and the fault current in the dq coordinate system can be calculated.

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