Fault ride-through control method and system of network construction type converter

Abstract

The invention provides a fault ride-through control method and system for a network-forming converter, and relates to the technical field of power electronics. The fault ride-through control method of the network construction type converter comprises the following steps: determining a real-time virtual impedance value corresponding to a control loop based on a virtual admittance control module; in response to the fact that the real-time virtual impedance value is larger than or equal to the reference virtual impedance value of the control loop, transient current output by the network-building type converter is adjusted based on first-stage current limiting, and steady-state current output by the network-building type converter is adjusted based on second-stage current limiting; calculating an active power proportionality coefficient based on the real-time parameter and the reference parameter; calculating a target active power reference value according to the active power proportionality coefficient and the reference active power reference value; and executing power control based on the target active power reference value, and adjusting the output voltage of the network-forming converter. According to the method, when the power grid breaks down, the grid-forming type converter can have the functions of current limiting and transient synchronization stability improving at the same time.

Description

Technical Field

[0001] The embodiments of the present application relate to the field of power electronics technology, and in particular to a fault ride-through control method and system for a grid-connected converter. Background Art

[0002] Modern power systems present the "double high" characteristics of a high proportion of renewable energy and a high proportion of power electronic equipment, which has led to changes in the inherent characteristics of the power grid. The rotational inertia and damping effect provided by traditional synchronous generators are weakened, and the frequency regulation capability, voltage support strength and power angle stability of the power grid are affected. The current fault ride-through control methods of grid-type converters include droop control and virtual synchronous generator control. Droop control realizes autonomous power distribution of distributed power sources by simulating the active-frequency and reactive-voltage droop characteristics of synchronous generators. Virtual synchronous generator technology constructs a virtual rotor motion equation and introduces an inertia link and a damping coefficient into the converter control to simulate the dynamic response characteristics of the synchronous generator.

[0003] When a large disturbance such as a short-circuit fault occurs in the power grid, the system's transient stability decreases, leading to cascading failures and other problems. Furthermore, compared to traditional synchronous generators, the power electronic devices in grid-type converters have poor overcurrent capabilities. Therefore, grid-type converter control technology has become key to ensuring the stable operation of new power systems, and its fault ride-through capability is directly related to the reliable operation of the power grid. Current fault ride-through control methods for grid-type converters can limit overcurrent and improve transient synchronous stability by adjusting the active and reactive power reference values ​​through piecewise functions and adding virtual resistance.

[0004] However, under large disturbance conditions, the coupling effect between the control model of the grid-connected converter and the electromagnetic transient process of the power grid is strong, which can easily cause power angle instability problems. In addition, the coordination mechanism between current limiting control and system stability is imperfect, making it difficult to quickly restore grid stability after current limiting. It is impossible to take into account multiple goals such as current limiting, transient synchronization stability, maximum fault current output, and maintaining voltage source characteristics at the grid connection point. Summary of the Invention

[0005] To solve the above problems, the present application provides a fault ride-through control method and system for a grid-connected converter, which can solve the technical problem that when a grid fault occurs, the control system cannot take into account current limiting, improving stability, outputting the maximum fault current, and maintaining the voltage source characteristics at the grid connection point.

[0006] To achieve the above-mentioned objectives, in a first aspect, the present application provides a fault ride-through control method for a grid-type converter, which is applied to a power grid; the fault ride-through control method for a grid-type converter comprises: determining a real-time virtual impedance value corresponding to a control loop based on a virtual admittance control module; in response to the real-time virtual impedance value being greater than or equal to a reference virtual impedance value of the control loop, adjusting the transient current output by the grid-type converter based on a first-stage current limiting, and adjusting the steady-state current output by the grid-type converter based on a second-stage current limiting, wherein the real-time virtual impedance values ​​corresponding to the first-stage current limiting and the second-stage current limiting are different; calculating an active power proportional coefficient based on real-time parameters and reference parameters; calculating a target active power reference value based on the active power proportional coefficient and a reference active power reference value; performing power control based on the target active power reference value to adjust the output voltage of the grid-type converter.

[0007] In the above method, when a voltage drop occurs in the power grid, the real-time virtual impedance value corresponding to the control loop of the grid-type converter increases. When the real-time virtual impedance value is greater than or equal to the reference virtual impedance value, the output current of the grid-type converter is adjusted through the first-stage current limiting and the second-stage current limiting to achieve the purpose of current limiting; and by calculating the active power proportional coefficient, the active power reference value is adjusted according to the active power proportional coefficient to improve the transient synchronization stability of the power grid.

[0008] In an implementation method of the first aspect, the fault ride-through control method of the grid-type converter also includes: in response to the real-time virtual impedance value being less than the reference virtual impedance value, and the voltage amplitude per unit value of the grid connection point being less than a preset value, calculating the target reactive power reference value based on the target active power reference value; performing power control based on the target active power reference value and the target reactive power reference value, and adjusting the output voltage of the grid-type converter.

[0009] In the above method, when the real-time virtual impedance value and the voltage at the grid connection point meet the preset conditions, the target reactive power reference value is calculated based on the target active power reference value. After the grid-type transformer is current limited, the output of the maximum fault current can be achieved through power control, and the voltage source characteristics at the grid point can be maintained.

[0010] In an implementation method of the first aspect, the fault ride-through control method of the grid-type converter also includes: in response to the real-time virtual impedance value being greater than or equal to the reference virtual impedance value, or the voltage amplitude per unit value of the grid-connected point being greater than or equal to a preset value, determining the reference reactive power reference value as the target reactive power reference value; performing power control based on the target active power reference value and the target reactive power reference value, and adjusting the output voltage of the grid-type converter.

[0011] In the above method, when the real-time virtual impedance value does not meet the preset conditions during a fault, the baseline reactive power reference value is used as the target reactive power reference value to implement power control, reduce the time of the second stage current limiting, and maintain the voltage source characteristics at the grid connection point.

[0012] In an implementation method of the first aspect, the fault ride-through control method of the grid-type converter also includes: in response to the real-time virtual impedance value being less than the reference virtual impedance value, stopping execution of the first-stage current limiting and the second-stage current limiting, and setting the virtual impedance value of the control loop to the reference virtual impedance value.

[0013] In the above method, whether to execute the first-stage current limiting and the second-stage current limiting can be automatically determined according to the value of the real-time virtual impedance, thereby improving the automation performance of the method.

[0014] In an implementation method of the first aspect, the transient current output by the grid-type converter is adjusted based on the first-stage current limiting, including: when the current amplitude output by the grid-type converter is greater than or equal to the maximum current value that the grid-type converter can withstand, calculating the first current limiting impedance value based on the voltage amplitude output by the grid-type converter, the voltage amplitude of the grid connection point and the maximum current value that the grid-type converter can withstand; calculating the first current limiting resistance value based on the first current limiting impedance value and the resistance-inductance ratio of the virtual admittance; calculating the target current limiting inductance value based on the resistance-inductance ratio of the virtual admittance, the first current limiting resistance value and the nominal frequency of the grid; calculating the first current limiting resistance value based on the gain coefficient and the maximum current value that the grid-type converter can withstand; and calculating the target current limiting resistance value based on the first current limiting resistance value and the second current limiting resistance value.

[0015] In the above method, since the first-stage current limiting adjusts the transient current, that is, limits the instantaneous current when a power grid fault occurs, it is necessary to increase the current limiting resistance value, calculate the first current limiting resistance value and the second current limiting resistance value respectively, and add the first current limiting resistance value and the second current limiting resistance value to obtain the target current limiting resistance value to resist the current impact at the moment of the fault and achieve the purpose of transient current limiting.

[0016] In an implementation of the first aspect, adjusting the transient current output by the grid-type converter based on the first-stage current limiting further includes: adjusting the transient current output by the grid-type converter based on a target current limiting resistance value and a target current limiting inductance value.

[0017] In the above method, it is also necessary to calculate the target current limiting inductance value corresponding to the virtual admittance control loop, and to achieve the first stage current limiting together with the target current limiting resistance value.

[0018] In an implementation method of the first aspect, the steady-state current output by the grid-type converter is adjusted based on the second-stage current limiting, including: when the current amplitude output by the grid-type converter is less than the maximum current value that the grid-type converter can withstand, and the real-time voltage of the grid-connected point is less than the reference voltage of the grid-connected point, the steady-state current output by the grid-type converter is adjusted based on the second current limiting resistance value and the target current limiting inductance value.

[0019] In the above method, since the second-stage current limiting adjusts the steady-state current, the current output by the grid-type converter is smaller than the current at the moment of the fault. Therefore, the target current limiting resistance value can be reduced, and the second current limiting resistance value and the target current limiting inductance value are used to achieve current limiting, which is beneficial to improving stability.

[0020] In an implementation method of the first aspect, the active power proportional coefficient is calculated based on real-time parameters and reference parameters, including: calculating the active power proportional coefficient based on the real-time voltage amplitude output by the grid-type converter, the real-time voltage amplitude of the grid-connected point, the real-time inductive reactance value of the virtual admittance, and the reference voltage amplitude of the grid-type converter, the reference voltage amplitude of the grid-connected point, and the reference inductive reactance value of the virtual admittance.

[0021] In the above method, the real-time parameters when a grid fault occurs and the reference parameters when no fault occurs can reflect the proportion of the active power reference value that needs to be adjusted, thereby realizing the calculation of the target active power reference value.

[0022] In an implementation method of the first aspect, the target reactive power reference value is calculated according to the target active power reference value, including: calculating the target reactive power reference value based on the voltage amplitude per unit value of the power grid, the rated apparent power of the grid-type converter and the target active power reference value.

[0023] In a second aspect, the present application also provides a fault ride-through control system for a grid-type converter, which is applied to a power grid. The fault ride-through control system for the grid-type converter includes a controller; the controller is configured to: determine the real-time virtual impedance value corresponding to the controlled loop based on a virtual admittance control module; in response to the real-time virtual impedance value being greater than or equal to the reference virtual impedance value of the control loop, adjust the transient current output by the grid-type converter based on the first-stage current limiting, and adjust the steady-state current output by the grid-type converter based on the second-stage current limiting, and the real-time virtual impedance values ​​corresponding to the first-stage current limiting and the second-stage current limiting are different; calculate the active power proportional coefficient based on the real-time parameters and the reference parameters; calculate the target active power reference value based on the active power proportional coefficient and the reference active power reference value; perform power control based on the target active power reference value to adjust the output voltage of the grid-type converter.

[0024] It can be understood that the beneficial effects that can be achieved by the technical solution of the second aspect provided above can refer to the beneficial effects of the first aspect and any possible implementation thereof, and will not be repeated here.

[0025] It can be seen from the above technical solution that the present application provides a fault ride-through control method and system for a grid-type converter, which is applied to a power grid; the fault ride-through control method for a grid-type converter includes: determining a real-time virtual impedance value corresponding to a control loop based on a virtual admittance control module; in response to the real-time virtual impedance value being greater than or equal to a reference virtual impedance value of the control loop, adjusting the transient current output by the grid-type converter based on a first-stage current limiting, and adjusting the steady-state current output by the grid-type converter based on a second-stage current limiting, wherein the real-time virtual impedance values ​​corresponding to the first-stage current limiting and the second-stage current limiting are different; calculating an active power proportional coefficient based on real-time parameters and reference parameters; calculating a target active power reference value based on the active power proportional coefficient and a reference active power reference value; performing power control based on the target active power reference value to adjust the output voltage of the grid-type converter.

[0026] In the above method, when a voltage drop occurs in the power grid, the real-time virtual impedance value corresponding to the control loop of the grid-type converter increases. When the real-time virtual impedance value is greater than or equal to the reference virtual impedance value, the output current of the grid-type converter is adjusted through the first-stage current limiting and the second-stage current limiting to achieve the purpose of current limiting; and by calculating the active power proportional coefficient, the active power reference value is adjusted according to the active power proportional coefficient to improve the transient synchronization stability of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solution of the present application, the following is a brief introduction to the drawings required for use in the embodiments. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0028] Figure 1 This is the grid-connected topology diagram of the grid-type converter; Figure 2 This is the control block diagram of the grid-type converter; Figure 3 A schematic flow chart of a fault ride-through control method for a grid-connected converter provided in an embodiment of the present application; Figure 4 A block diagram of a fault ride-through control method for a grid-type converter provided in an embodiment of the present application; Figure 5 A schematic diagram of a grid-connected equivalent circuit for executing control of a grid-connected converter provided in an embodiment of the present application; Figure 6 Schematic diagram of the power angle characteristics of the grid-type converter during transient period; Figure 7 This is an experimental waveform diagram of a fault occurrence and clearing provided in an embodiment of the present application. DETAILED DESCRIPTION

[0029] The following embodiments are described in detail, with examples shown in the accompanying drawings. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following embodiments are not intended to represent all embodiments consistent with this application.

[0030] It should be noted that the brief descriptions of terms in this application are only for the purpose of facilitating the understanding of the embodiments described below, and are not intended to limit the embodiments of this application. Unless otherwise specified, these terms should be understood according to their ordinary and usual meanings.

[0031] In this specification and the accompanying drawings, the terms "first," "second," "third," etc. are used to distinguish similar or similar objects or entities, and are not necessarily intended to limit a particular order or precedence, unless otherwise noted. It should be understood that the terms used in this manner are interchangeable under appropriate circumstances.

[0032] To facilitate understanding of the technical solution of the application, the relevant technologies involved in this application are first explained below.

[0033] Figure 1 This is the grid-connected topology diagram of the grid-type converter.

[0034] like Figure 1 As shown, in the grid-connected circuit of the grid-type converter, It is a DC voltage source, providing DC power input for the entire system; 、 、 、 、 、 The power switching devices form a three-phase full-bridge inverter circuit, which converts DC input into three-phase AC output by controlling the on and off of these switches; and A filter circuit is formed, in which is the filter inductor, It is a filter capacitor that can filter out high-frequency harmonics in the inverter output and improve the purity of the output AC power; As the line inductance, it can achieve buffering and filtering between the grid-connected converter and the grid, as well as control the current in grid-connected control; is the grid connection point voltage; is the grid voltage; The direction of current flowing from the inverter into the grid is defined as the positive direction.

[0035] In the grid-connected circuit of the grid-type converter, the DC voltage source Connect to the DC side of the three-phase full-bridge inverter circuit, and the AC output end of the three-phase full-bridge inverter circuit is connected to the filter inductor in turn. and filter capacitors The filtered AC power is connected to the grid through the filter circuit. Line inductance Connect to AC grid voltage This connection method converts DC power into AC power that meets the grid requirements through inversion, filtering and other links, and successfully connects to the grid.

[0036] When a fault such as a voltage sag occurs in the power grid, it can easily lead to excessive current and transient power angle instability, damaging components in the circuit and affecting system stability. Therefore, a fault ride-through control method for grid-connected converters is needed to limit current and improve transient stability. It is also necessary to ensure the output of the maximum fault current and maintain the voltage source characteristics at the grid connection point.

[0037] In some embodiments, the control of the grid-type converter can be achieved based on the droop control of autonomous power distribution. By simulating the external characteristics of the synchronous generator, the linear regulation relationship between active power and frequency and reactive power and voltage can be constructed to achieve the plug-and-play function of the distributed power supply. It is also possible to establish an equivalent rotor dynamics equation including inertia simulation and damping characteristics through virtual synchronous generator control to reproduce the electromechanical transient response of the synchronous generator in the grid-type converter.

[0038] Figure 2 It is the control block diagram of the grid-type converter.

[0039] like Figure 2 As shown in the figure, the control block diagram of the grid-type converter includes an outer loop power control module and an inner loop virtual admittance and current control module. The specific control process of the grid-type converter is as follows: the active power-frequency control loop adopts a P controller, and the phase angle is obtained through the active power-frequency control loop. The reactive power-voltage control loop uses a PI controller to obtain the reference amplitude of the inverter output voltage through the reactive power-voltage control loop. , the voltage amplitude is converted to and phase angle Resultant voltage vector ; The voltage vector and grid connection point voltage The virtual admittance module outputs the current reference value ; Using PI controller as current controller, In the coordinate system, the d-axis and q-axis components of the inverter output voltage are output respectively through the current controller. ;Will Transform to the natural coordinate system and output three-phase modulated wave The SPWM modulation module outputs the modulation signal of the grid-type converter switch tube to realize the control of the on / off state of each switch tube in the grid-type converter.

[0040] In some embodiments, when controlling a grid-type converter, a virtual resistor can be added to modify the modulation voltage reference of the grid-type converter to limit transient overcurrent and avoid excessive instantaneous current when a fault such as a voltage drop occurs in the grid, thereby damaging the device.

[0041] Specifically, the method of adding a virtual resistor to achieve transient current limiting is shown in formula (1): , in, is the virtual resistance value, is the proportional gain, is the output current amplitude of the grid-type converter, is the current limiting threshold. Adjust virtual resistance value , to achieve transient current limiting.

[0042] In some embodiments, when controlling the grid-type converter, the reactive power reference value and the active power reference value may be adjusted to limit steady-state overcurrent and improve the stability of system operation.

[0043] Specifically, the calculation method of the reactive power reference value is shown in formula (2): , in, is the adjusted reactive power reference value, is the reactive power reference value of the power grid under normal operating conditions, is the rated apparent power of the grid-type converter, The reactive power reference value is adjusted based on the grid voltage unit value to limit steady-state overcurrent during a grid fault.

[0044] Specifically, the calculation method of the active power reference value is shown in formula (3): , in, is the adjusted active power reference value. After the adjusted reactive power reference value is calculated by formula (2), the adjusted active power reference value is calculated based on the adjusted reactive power reference value, which further limits the steady-state overcurrent during the power grid fault and improves the stability of the system operation.

[0045] However, in the above method, although adjusting the reactive power reference value and the active power reference value through a piecewise function and adding a virtual resistor can effectively limit the fault current and improve transient synchronization stability, allowing the converter to maintain grid-connected mode during fault ride-through, it is difficult to provide the maximum fault current during a grid fault. Therefore, the above control method for grid-connected converters cannot simultaneously take into account the functions of current limiting, improving transient synchronization stability, outputting the maximum fault current, and maintaining the voltage source characteristics at the grid connection point during a grid fault.

[0046] To solve the above technical problems, an embodiment of the present application provides a fault ride-through control method for a grid-type converter, which can be applied to a power grid including a grid-type converter.

[0047] Figure 3 A schematic flow chart of a fault ride-through control method for a grid-type converter provided in an embodiment of the present application. Figure 3 As shown, the fault ride-through control method of the grid-type converter includes steps S100-S600.

[0048] Step S100: determining a real-time virtual impedance value corresponding to a control loop based on a virtual admittance control module.

[0049] Figure 4 A block diagram of a fault ride-through control method for a grid-connected converter provided in an embodiment of the present application.

[0050] like Figure 4 As shown in the figure, during the control of the grid-connected converter, virtual admittance control is used to generate the current reference value of the current control loop. Therefore, the real-time virtual impedance value corresponding to the control loop is first determined based on the virtual admittance control module. By adjusting the virtual real-time impedance value, the current is controlled, thereby achieving the current limiting function in the event of a grid fault.

[0051] Step S200: In response to the real-time virtual impedance value being greater than or equal to the reference virtual impedance value of the control loop, adjusting the transient current output by the grid-connected converter based on the first-stage current limiting.

[0052] In some embodiments, when the grid voltage drops, a large overcurrent shock will be generated. In order to achieve transient current limiting when a fault occurs, it is necessary to use the first-stage current limiting to increase the current limiting resistance value corresponding to the virtual admittance to adjust the transient current output by the grid-type converter. At this time, the real-time virtual impedance value will be greater than or equal to the baseline virtual impedance value of the control loop.

[0053] Specifically, first, a first current limiting impedance value is calculated based on the voltage amplitude output by the grid-type converter, the voltage amplitude of the grid connection point, and the maximum current value that the grid-type converter can withstand; a first current limiting resistor value is calculated based on the first current limiting impedance value and the resistance-inductance ratio of the virtual admittance; and a target current limiting inductance value is calculated based on the resistance-inductance ratio of the virtual admittance, the first current limiting resistor value, and the nominal frequency of the grid.

[0054] In some embodiments, a method for calculating the first current limiting impedance value, the first current limiting resistance value, and the target current limiting inductance value is shown in formula (4): , in, is the first current limiting impedance value, is the first current limiting resistor value, is the target current limiting inductor value, is the voltage amplitude of the grid-type converter, is the voltage amplitude at the grid connection point, is the maximum current value that the grid-type converter can withstand, is the resistance-inductance ratio of the virtual impedance, is the nominal grid frequency.

[0055] In some embodiments, the first current-limiting impedance value, the first current-limiting resistance value, and the target current-limiting inductance value calculated according to formula (4) can achieve steady-state overcurrent limiting during a grid fault. However, due to the large overcurrent at the moment of the fault, in order to improve system safety and prevent device damage due to excessive current at the moment of the fault, a second current-limiting resistance value of virtual admittance needs to be added during the first-stage current limiting process.

[0056] Specifically, the second current limiting resistor value is calculated based on the gain coefficient and the maximum current value that the grid-type converter can withstand. The calculation method of the second current limiting resistor value is shown in formula (5): , in, is the second current limiting resistor value, is the proportional gain, when The real-time current output by the grid-type converter. When the real-time current exceeds the maximum current value that the grid-type converter can withstand, the second current limiting resistor value With real-time current and maximum current is proportional to the difference.

[0057] It should be understood that when the real-time current is less than the maximum current value that the grid-type converter can withstand, there is no need to increase the current limiting resistor value, and at this time the second current limiting resistor value is 0.

[0058] In some embodiments, after calculating the added second current limiting resistance value, the target current limiting resistance value is calculated based on the first current limiting resistance value and the second current limiting resistance value, and the target current limiting resistance value is used as the current limiting resistance value of the virtual admittance to increase the instantaneous current limiting capability of the system.

[0059] Specifically, the calculation method of the target current limiting resistor value is shown in formula (6): , in, is the target current limiting resistor value, is the first current limiting resistor value, is the second current limiting resistor value.

[0060] In some embodiments, since the change in the resistance value of the virtual admittance affects the change in the virtual admittance impedance value, when the current limiting resistance value becomes the target current limiting resistance value, the first current limiting impedance value also changes to generate the target impedance value.

[0061] Specifically, the target impedance value is calculated based on the target current limiting resistance value and the target current limiting inductance value. The method for calculating the target impedance value is shown in formula (7): , in, is the target impedance value, For inductance The inductive reactance value.

[0062] In some embodiments, the transient current output by the grid-type converter can be adjusted according to the calculated target current-limiting resistance value and target current-limiting inductance value to achieve first-stage current limiting.

[0063] Step S300: adjusting the steady-state current output by the grid-connected converter based on the second-stage current limiting.

[0064] In some embodiments, when the current amplitude output by the mesh-type converter is less than the maximum current value that the mesh-type converter can withstand, there is no need to increase the second current limiting resistance value. The purpose of current limiting can be achieved by adjusting the steady-state current output by the mesh-type converter based on the first current limiting impedance value, the first current limiting resistance value and the target current limiting inductance value.

[0065] Combining steps S200 and S300, it can be seen that since formula (5) provides a method for calculating the second current limiting resistor value under two conditions: when the current amplitude output by the grid-type converter is greater than or equal to the maximum current value that the grid-type converter can withstand, and when the current amplitude output by the grid-type converter is less than the maximum current value that the grid-type converter can withstand, and the target current limiting inductor value is the same in both the first-stage current limiting and the second-stage current limiting, a general formula (8) for the first-stage current limiting and the second-stage current limiting can be obtained by combining formulas (4)-(7): , Since formula (8) can automatically determine the value of the second current limiting resistor according to the real-time current value output by the grid-type converter Therefore, when a fault occurs in the power grid, adaptive two-stage current limiting can be achieved according to formula (8).

[0066] S400: Calculating an active power proportional coefficient based on the real-time parameters and the reference parameters.

[0067] For example Figure 4 As shown, in order to improve the transient synchronization stability of the power grid when a fault occurs, ensure the output of the maximum fault current, and maintain the voltage source characteristics at the grid connection point, the fault ride-through control method of the grid-connected converter in some embodiments of the present application can also perform power control.

[0068] It should be noted that power control adjusts the voltage angle and amplitude of the grid-connected converter output based on the active power reference value and the reactive power reference value. Therefore, it is necessary to calculate the adjusted active power reference value.

[0069] Figure 5 A schematic diagram of a grid-connected equivalent circuit for executing control of a grid-connected converter provided in an embodiment of the present application.

[0070] like Figure 5 As shown in the figure, when executing the fault ride-through control method of the grid-type converter, the resistance component in the virtual admittance is ignored, and the grid impedance and virtual admittance are regarded as purely inductive. Moreover, since the current on the filter capacitor is small, in order to further simplify the equivalent circuit, the capacitor current is also ignored. The voltage output on the inverter side is equivalent to When the first-stage current limiting and the second-stage current limiting are not performed, the active power expression of the grid-connected converter to the grid is shown in formula (9): , in, is the active power transmitted to the grid by the grid-connected converter. is the voltage amplitude of the grid-type converter With grid voltage The angle between for Figure 5 The total impedance of the equivalent circuit is, When the power grid is operating in normal conditions, .

[0071] In some embodiments, the and as well as in The exact value of is unknown. Therefore, the voltage value of the grid connection point is used. Alternative , and ignore The value of , formula (9) is approximated as formula (10): , In some embodiments, formula (10) reflects the power angle characteristic (P-δ characteristic) of the grid-connected converter during transient period. Figure 6 Schematic diagram of the power angle characteristics of the grid-type converter during transient period.

[0072] like Figure 6 As shown, curve I, curve II and curve III respectively describe the P-δ characteristic relationship of the power grid before, during and after a fault occurs. and They represent the acceleration area and the maximum deceleration area respectively. is the stable equilibrium point (SEP), is an unstable equilibrium point, is the active power reference value, The maximum active power output of the grid-connected converter. During a grid fault, the active power reference value is always greater than the active power delivered to the grid by the grid-connected converter, resulting in the power angle Continue to increase. Assume is the fault clearance angle, according to the equal area criterion, if , the power grid system can remain stable; if , the power grid system will experience a "step-out" phenomenon similar to that of a traditional synchronous generator. Therefore, in order to solve the "step-out" problem of the power grid system during a fault, the power angle should be kept constant when a fault occurs. constant.

[0073] Furthermore, from formula (10) and Figure 6 It can be seen from the power angle characteristics that the introduction of virtual admittance during the power grid fault period and the implementation of the first stage current limiting and the second stage current limiting will reduce the peak value of the power angle characteristic curve, resulting in a deceleration area Therefore, in order to improve the transient synchronization stability of the system during the fault, the active power reference value can be reduced. , that is, to make Figure 6 in In summary, by reducing the active power reference value The transient synchronization stability of the system can be improved. Therefore, it is necessary to calculate the reduced active power reference value, that is, the target active power reference value.

[0074] In some embodiments, when a fault such as a voltage drop occurs in the power grid, the impedance value of the virtual admittance is increased during the first-stage current limiting and the second-stage current limiting, causing the amplitude of the P-δ characteristic curve to decrease proportionally. Therefore, when a fault occurs in the power grid, a proportional coefficient can be calculated based on the real-time active power value and the active power value of the power grid in a normal state, and this proportional coefficient is used as the active power proportional coefficient, that is, , proportionally reduce the active power reference value to keep the power angle δ unchanged, thereby ensuring the existence of a stable equilibrium point and improving transient synchronization stability.

[0075] In some embodiments, according to formula (10), The active power proportional coefficient is calculated based on the real-time voltage amplitude output by the grid-connected converter, the real-time voltage amplitude at the grid-connected point, the real-time inductive reactance value of the virtual admittance, and the reference voltage amplitude of the grid-connected converter, the reference voltage amplitude at the grid-connected point, and the reference inductive reactance value of the virtual admittance. The method for calculating the active power proportional coefficient is shown in formula (11): , in, is the active power proportional coefficient, The real-time voltage amplitude output by the grid-type converter, is the real-time voltage amplitude of the grid connection point, is the real-time inductive reactance value of the virtual admittance, is the reference voltage amplitude of the grid-type converter, is the reference voltage amplitude of the grid connection point, is the reference inductive reactance value of the virtual admittance.

[0076] S500: Calculating a target active power reference value according to the active power proportional coefficient and the benchmark active power reference value.

[0077] In some embodiments, after the active power proportional coefficient is calculated using formula (11), the target active power reference value can be calculated based on the active power proportional coefficient.

[0078] Specifically, the method for calculating the target active power reference value is shown in formula (12): , in, is the target active power reference value, It is the reference value of active power under normal operation of the power grid.

[0079] S600: Perform power control based on the target active power reference value to adjust the output voltage amplitude of the grid-type converter.

[0080] In some embodiments, the target active power reference value calculated according to formula (12) is By performing power control, the output voltage power angle of the grid-connected converter can be adjusted, thereby improving the transient synchronization stability of the system.

[0081] In some embodiments, the fault ride-through control method of the grid-type converter can also perform power control based on the reactive power value. When a fault such as a voltage drop occurs in the power grid, according to the droop principle, the reactive power actually output by the grid-type converter will increase in a short period of time, resulting in a reactive power reference value that is much smaller than the actual value of the reactive power, thereby generating a negative error. Under the influence of the negative error, the voltage amplitude output by the grid-type converter decreases rapidly, and the current limiting impedance value ( ) decreases rapidly. When the real-time virtual impedance value falls below the baseline virtual impedance value, the first and second stages of current limiting are discontinued. At this point, the control loop's virtual impedance value becomes the baseline virtual impedance value. Consequently, the system loses its ability to limit the fault current to the maximum fault current.

[0082] In some embodiments, to safeguard the system's ability to output the maximum fault current and ensure reliable operation of the relay protection device, the reactive power reference value can be adjusted based on the active power reference value. However, to quickly reduce the current-limiting impedance, accelerate the exit from the first and second stages of current limiting, and maintain the voltage source characteristics at the grid connection point, the reactive power reference value needs to be calculated in stages.

[0083] Specifically, when it is detected that the real-time virtual impedance value is greater than or equal to the benchmark virtual impedance value, or the voltage amplitude per unit value of the grid connection point is greater than or equal to the preset value, the benchmark reactive power reference value is determined as the target reactive power reference value; when it is detected that the real-time virtual impedance value is less than the benchmark virtual impedance value, and the voltage amplitude per unit value of the grid connection point is less than the preset value, the target reactive power reference value is calculated based on the target active power reference value.

[0084] In some embodiments, the method for calculating the target reactive power reference value based on the target active power reference value is as follows: the target reactive power reference value is calculated based on the per-unit value of the voltage amplitude at the grid connection point, the rated apparent power of the grid-connected converter, and the target active power reference value. In summary, the method for calculating the reactive power reference value in stages is shown in formula (13): , in, is the target reactive power reference value, is the per-unit value of the voltage amplitude of the power grid, is the reactive power reference value of the power grid under normal operating conditions, is the rated apparent power of the grid-type converter.

[0085] In some embodiments, after the target reactive power reference value is calculated according to formula (13), power control is performed based on the target active power reference value and the target reactive power reference value, so as to achieve the purpose of not only improving the transient synchronization stability of the system when a fault occurs in the power grid, but also taking into account current limiting and ensuring the output of the maximum fault current, thereby maintaining the voltage source characteristics at the grid connection point.

[0086] Below, taking the grid voltage dropping to 0.2 pu as an example, the effectiveness of the fault ride-through control method for the grid-connected converter provided in the embodiment of the present application is verified.

[0087] Figure 7 This is an experimental waveform diagram of a fault occurrence and clearing provided in an embodiment of the present application. Figure 7 (a) is the experimental waveform when the fault occurs. Figure 7 (b) in the figure is the experimental waveform when the fault is cleared.

[0088] like Figure 7 As shown in (a) in the figure, at the moment of fault occurrence, the active power drops, and the current output by the grid-type converter is limited to 1.2pu by the first-stage current limiting and the second-stage current limiting. However, the first-stage current limiting and the second-stage current limiting are released in a short time. The reactive power reference value increases rapidly, causing the grid-type converter to respond as a voltage source and provide the maximum fault current to support the grid. Figure 7 As shown in (b), after the fault is cleared, the active power and reactive power are restored to the baseline value, and the grid-type converter returns to the pre-fault state through power control adjustment, which proves the effectiveness of the fault ride-through control method of the grid-type converter provided in the embodiment of the present application.

[0089] In some embodiments, the present application also provides a fault ride-through control system for a grid-type converter, which is applied to a power grid. The fault ride-through control system for the grid-type converter includes a controller; the controller is configured to: determine the real-time virtual impedance value corresponding to the control loop based on a virtual admittance control module; in response to the real-time virtual impedance value being greater than or equal to the reference virtual impedance value of the control loop, adjust the transient current output by the grid-type converter based on the first-stage current limiting, and adjust the steady-state current output by the grid-type converter based on the second-stage current limiting, and the real-time virtual impedance values ​​corresponding to the first-stage current limiting and the second-stage current limiting are different; calculate the active power proportional coefficient based on the real-time parameters and the reference parameters; calculate the target active power reference value according to the active power proportional coefficient and the reference active power reference value; perform power control based on the target active power reference value to adjust the output voltage amplitude of the grid-type converter.

[0090] It can be seen from the above technical solution that the present application provides a fault ride-through control method and system for a grid-type converter, which is applied to a power grid; the fault ride-through control method for a grid-type converter includes: determining a real-time virtual impedance value corresponding to a control loop based on a virtual admittance control module; in response to the real-time virtual impedance value being greater than or equal to a reference virtual impedance value of the control loop, adjusting the transient current output by the grid-type converter based on a first-stage current limiting, and adjusting the steady-state current output by the grid-type converter based on a second-stage current limiting, wherein the real-time virtual impedance values ​​corresponding to the first-stage current limiting and the second-stage current limiting are different; calculating an active power proportional coefficient based on real-time parameters and reference parameters; calculating a target active power reference value based on the active power proportional coefficient and a reference active power reference value; calculating a target reactive power reference value based on the target active power reference value; performing power control based on the target active power reference value and the target reactive power reference value to adjust the voltage power angle and amplitude output by the grid-type converter.

[0091] In the above method, when a voltage drop occurs in the power grid, the real-time virtual impedance value corresponding to the control loop of the grid-type converter increases. When the real-time virtual impedance value is greater than or equal to the reference virtual impedance value, the output current of the grid-type converter is adjusted through the first-stage current limiting and the second-stage current limiting to achieve the purpose of current limiting; and by calculating the active power proportional coefficient, the active power reference value is adjusted according to the active power proportional coefficient to improve transient synchronization stability; and the reactive power reference value is adjusted according to the active power reference value to ensure the maximum fault current output and maintain the voltage source characteristics at the grid connection point.

[0092] Similar parts between the embodiments provided in this application can be referenced to each other. The specific implementation methods provided above are only a few examples under the overall concept of this application and do not constitute a limitation on the scope of protection of this application. For those skilled in the art, any other implementation methods expanded based on the scheme of this application without expending creative work shall fall within the scope of protection of this application.

Claims

1. A fault ride-through control method for a grid-connected converter, characterized by: The fault ride-through control method of the grid-type converter is applied to a power grid; The fault ride-through control method of the grid-type converter includes: Based on the virtual admittance control module, determine the real-time virtual impedance value corresponding to the control loop; In response to the real-time virtual impedance value being greater than or equal to a reference virtual impedance value of the control loop, adjusting a transient current output by the grid-type converter based on a first-stage current limiting, and adjusting a steady-state current output by the grid-type converter based on a second-stage current limiting, wherein the real-time virtual impedance values ​​corresponding to the first-stage current limiting and the second-stage current limiting are different; Calculate active power proportional coefficient based on real-time parameters and benchmark parameters; Calculating a target active power reference value according to the active power proportional coefficient and a benchmark active power reference value; Power control is performed based on the target active power reference value to adjust the output voltage of the grid-type converter.

2. The fault ride-through control method for a grid-connected converter according to claim 1, characterized in that: The method further comprises: In response to the real-time virtual impedance value being less than the reference virtual impedance value, and the per-unit value of the voltage amplitude at the grid connection point being less than a preset value, calculating a target reactive power reference value according to the target active power reference value; Power control is performed based on the target active power reference value and the target reactive power reference value to adjust the output voltage of the grid-type converter.

3. The fault ride-through control method for a grid-connected converter according to claim 2, wherein: The method further comprises: In response to the real-time virtual impedance value being greater than or equal to the reference virtual impedance value, or the per-unit value of the voltage amplitude at the grid connection point being greater than or equal to a preset value, determining a reference reactive power reference value as the target reactive power reference value; Power control is performed based on the target active power reference value and the target reactive power reference value to adjust the output voltage of the grid-type converter.

4. The fault ride-through control method for a grid-connected converter according to claim 1, wherein: The method further comprises: In response to the real-time virtual impedance value being less than the reference virtual impedance value, the first-stage current limiting and the second-stage current limiting are stopped, and the virtual impedance value of the control loop is set to the reference virtual impedance value.

5. The fault ride-through control method for a grid-connected converter according to claim 1, characterized in that: The adjusting the transient current output by the grid-type converter based on the first-stage current limiting includes: When the current amplitude output by the grid-type converter is greater than or equal to the maximum current value that the grid-type converter can withstand, a first current-limiting impedance value is calculated based on the voltage amplitude output by the grid-type converter, the voltage amplitude of the grid connection point, and the maximum current value that the grid-type converter can withstand; Calculating a first current limiting resistance value based on the first current limiting impedance value and the resistance-inductance ratio of the virtual admittance; Calculating a target current-limiting inductance value based on the resistance-to-inductance ratio of the virtual admittance, the first current-limiting resistor value, and the nominal frequency of the power grid; Calculating a first current-limiting resistor value based on a gain coefficient and a maximum current value that the grid-type current converter can withstand; A target current limiting resistance value is calculated based on the first current limiting resistance value and the second current limiting resistance value.

6. The fault ride-through control method for a grid-connected converter according to claim 5, characterized in that: The adjusting of the transient current output by the grid-type converter based on the first-stage current limiting further includes: The transient current output by the grid-type converter is adjusted based on the target current-limiting resistance value and the target current-limiting inductance value.

7. The fault ride-through control method for a grid-connected converter according to claim 6, characterized in that: The adjusting the steady-state current output by the grid-type converter based on the second-stage current limiting includes: When the current amplitude output by the grid-type converter is less than the maximum current value that the grid-type converter can withstand, and the real-time voltage of the grid-connected point is less than the reference voltage of the grid-connected point, the steady-state current output by the grid-type converter is adjusted based on the second current-limiting resistance value and the target current-limiting inductance value.

8. The fault ride-through control method for a grid-connected converter according to claim 1, wherein: The calculating of the active power proportional coefficient based on the real-time parameter and the reference parameter includes: The active power proportional coefficient is calculated based on the real-time voltage amplitude output by the grid-type converter, the real-time voltage amplitude of the grid-connected point, the real-time inductive reactance value of the virtual admittance, and the reference voltage amplitude of the grid-type converter, the reference voltage amplitude of the grid-connected point and the reference inductive reactance value of the virtual admittance.

9. The fault ride-through control method for a grid-connected converter according to claim 2, characterized in that: The calculating the target reactive power reference value according to the target active power reference value includes: The target reactive power reference value is calculated based on the per-unit value of the voltage amplitude at the grid connection point, the rated apparent power of the grid-connected converter, and the target active power reference value.

10. A fault ride-through control system for a grid-type converter, characterized in that: Applied to a power grid, the fault ride-through control system of the grid-type converter includes a controller; The controller is configured to: Based on the virtual admittance control module, determine the real-time virtual impedance value corresponding to the control loop; In response to the real-time virtual impedance value being greater than or equal to a reference virtual impedance value of the control loop, adjusting a transient current output by the grid-type converter based on a first-stage current limiting, and adjusting a steady-state current output by the grid-type converter based on a second-stage current limiting, wherein the real-time virtual impedance values ​​corresponding to the first-stage current limiting and the second-stage current limiting are different; Calculate active power proportional coefficient based on real-time parameters and benchmark parameters; Calculating a target active power reference value according to the active power proportional coefficient and a benchmark active power reference value; Power control is performed based on the target active power reference value to adjust the output voltage of the grid-type converter.

Images