Method and system for realizing variable virtual impedance of network-forming converter

Abstract

The invention discloses a variable virtual impedance implementation method and system for a network-building type converter. The method comprises the following steps: acquiring a ratio of a network-side voltage amplitude of the network-building type energy storage converter to a system rated voltage amplitude; when the ratio is smaller than a set threshold value, voltage drop occurs; at the moment, the active power given value is controlled to keep the power angle stable without divergence; meanwhile, a saturation characteristic function is calculated according to the drop amplitude of the grid-side voltage, and the output value of the virtual internal potential is changed based on the saturation characteristic function, so that the virtual impedance value in the control structure is changed, and the current limiting and voltage supporting requirements of the low-voltage ride-through strategy are met. The virtual impedance value can be changed according to different voltage drop degrees, and meanwhile the saturation characteristic is achieved.

Description

Technical Field

[0001] This invention relates to the field of energy storage converter technology, and in particular to a method and system for realizing variable virtual impedance in a grid-type converter. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Currently, grid-connected technology is widely used for power storage converters (PCS), enabling them to possess voltage source characteristics and effectively improve voltage and inertia support capabilities for "high-voltage and high-efficiency" new energy power systems. Virtual synchronous machine control is the most widely applied technology in grid-connected systems. However, unlike synchronous generators, grid-connected PCS has low tolerance to fault current during voltage dips on the grid side, and is prone to disconnection from the grid due to overcurrent during faults. Therefore, current-limiting protection mechanisms need to be added to the grid control system.

[0004] The virtual impedance method is a common approach for implementing current-limiting protection in grid-based control systems. It can simultaneously meet the requirements for current limiting and reactive power support during faults. The value of the virtual impedance during a fault is particularly important: if the value is too small, it will be difficult to achieve its current-limiting function; if the value is too large, it will limit its reactive power support capability, reduce system stability, and make it difficult for the grid connection voltage to recover after the fault ends.

[0005] Existing technologies often only consider steady-state current when designing virtual impedance, without considering transient current; voltage drops on the grid side may generate excessive transient current, thereby damaging the converter.

[0006] Existing technologies disclose the setting of virtual impedance based on the maximum current that the converter can withstand, but do not consider the reactive power support capability during faults; insufficient reactive power support capability may cause excessively low voltage after a fault, exacerbating stability problems.

[0007] Existing technologies also disclose the use of an adaptive virtual impedance strategy, which requires adaptive calculation of the virtual impedance based on the degree of short-circuit current exceeding the limit. Therefore, the response speed is slow and it cannot effectively suppress transient short-circuit circuits. Summary of the Invention

[0008] To address the issues of slow response speed and large transient current in existing virtual impedance current limiting methods, this invention proposes a variable virtual impedance implementation method and system for grid-type converters. This method can change the virtual impedance value according to different degrees of voltage drop, significantly reduce overcurrent and achieve current limiting protection when a fault occurs, while maintaining the voltage source characteristics inherent in grid-type energy storage and achieving voltage support.

[0009] In some implementations, the following technical solutions are adopted: A method for implementing variable virtual impedance in a grid-type converter includes: Obtain the ratio of the grid-side voltage amplitude of the grid-type energy storage converter to the rated voltage amplitude of the system; when the ratio is less than a set threshold, it indicates that a voltage drop has occurred; at this time, the active power setpoint is controlled to maintain a stable power angle without divergence; simultaneously, the saturation characteristic function is calculated based on the voltage drop amplitude of the grid side, and the output value of the virtual internal potential is changed based on the saturation characteristic function, thereby changing the virtual impedance value in the control structure to meet the current limiting and voltage support requirements of the low voltage ride-through strategy.

[0010] As a further solution, the control of the active power setpoint specifically includes: The active power setpoint can be reduced proportionally to the voltage drop amplitude on the grid side, or the difference between the active power setpoint and the actual active power value can be forcibly set to 0 during the voltage drop period.

[0011] As a further option, the saturation characteristic function is constructed based on the hyperbolic tangent function.

[0012] As a further solution, the calculation of the saturation characteristic function based on the voltage drop amplitude of the grid side is specifically as follows: ; in, It is a saturation characteristic function. U g * The per-unit value of the grid-side voltage amplitude is obtained through dq coordinate transformation.

[0013] As a further approach, the output value of the virtual internal potential is changed based on the aforementioned saturation characteristic function, specifically as follows: The output value of the virtual internal potential is equal to the virtual internal potential and the saturation characteristic function. The product of.

[0014] As a further solution, the converter current output value during grid-side faults is specifically as follows:

[0015] Based on the characteristic that the output internal potential of a grid-type converter remains unchanged during a fault, the equivalent impedance between the converter and the grid during the fault period is changed as follows: .

[0016] In other embodiments, the following technical solutions are adopted: A system for realizing variable virtual impedance in a grid-type converter includes: The judgment module is configured to: obtain the ratio of the grid-side voltage amplitude of the grid-type energy storage converter to the rated voltage amplitude of the system; when the ratio is less than a set threshold, it indicates that a voltage drop has occurred; The control module is configured to: control the active power setpoint to maintain a stable power angle without divergence; simultaneously, calculate the saturation characteristic function based on the voltage drop amplitude of the grid side, change the output value of the virtual internal potential based on the saturation characteristic function, and thereby change the virtual impedance value in the control structure to meet the current limiting and voltage support requirements of the low voltage ride-through strategy.

[0017] In other embodiments, the following technical solutions are adopted: A terminal device includes a processor and a memory, the processor being used to implement instructions; the memory being used to store multiple instructions adapted to be loaded and executed by the processor to implement the variable virtual impedance method of the above-described grid-type converter.

[0018] In other embodiments, the following technical solutions are adopted: A computer-readable storage medium storing a plurality of instructions adapted for loading and execution by a processor of a terminal device of the above-described method for implementing variable virtual impedance in a grid-type converter.

[0019] In other embodiments, the following technical solutions are adopted: A computer program product includes a computer program / instructions that, when executed by a processor, implement the above-described method for implementing the variable virtual impedance of a grid-type converter.

[0020] Compared with the prior art, the beneficial effects of the present invention are: (1) The virtual impedance value of the method of the present invention can vary according to different voltage drop levels and has saturation characteristics. When the voltage drop level is small, the virtual impedance value changes within a small range and appears as a small impedance to the outside of the system, so that the converter can sacrifice as little reactive power output capability as possible while ensuring current limiting. When the voltage drop level is large, in order to avoid damage to the equipment by large current exceeding the converter limit and to achieve converter current limiting, the virtual impedance value needs to appear as a large impedance. At the same time, the variable virtual impedance designed in this paper changes according to the voltage drop level on the grid side and has the advantage of fast response speed.

[0021] Other features and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0022] Figure 1 This is a structural diagram of an energy storage system controlled by a virtual synchronous generator in an embodiment of the present invention; Figure 2 This is a schematic diagram of the power angle curve in an embodiment of the present invention; Figure 3 This is the equivalent model of the virtual synchronous generator converter in the embodiments of the present invention; Figure 4 This is a schematic diagram illustrating the virtual impedance value variation characteristics in an embodiment of the present invention; Figure 5 As described in the embodiments of the present invention F ( U g * Schematic diagram of the function curve; Figure 6 This is a schematic diagram showing the relationship between the per-unit value of grid-side voltage and the per-unit value of converter output current in an embodiment of the present invention; Figure 7 This is a schematic diagram of the virtual impedance construction with saturation characteristics in an embodiment of the present invention; Figure 8 This is a flowchart illustrating the method for implementing variable virtual impedance in a grid-type converter according to an embodiment of the present invention. Figures 9(a)-(d) show the waveforms of the energy storage converter with a grid-side voltage drop of 0.2 pu in the embodiment of the present invention without a low voltage ride-through strategy. Figures 10(a)-(d) show the waveforms of the energy storage converter with a grid-side voltage drop of 0.8 pu in the embodiment of the present invention without a low voltage ride-through strategy. Figures 11(a)-(f) show the waveforms of the energy storage converter under the low voltage ride-through strategy in the embodiments of the present invention with a grid-side voltage drop of 0.2 pu; Figures 12(a)-(f) show the waveforms of the energy storage converter with a grid-side voltage drop of 0.8 pu under the low voltage ride-through strategy in the embodiments of the present invention. Figures 13(a)-(d) show the waveforms of the energy storage converter when the grid-side A-phase voltage drops by 0.8 pu under the low voltage ride-through strategy in the embodiments of the present invention. Detailed Implementation

[0023] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0024] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0025] Example 1 Low voltage ride-through strategies primarily require two things: current limiting for the converter and sufficient reactive power support. When the virtual impedance is constant, a virtual impedance design value that is too small cannot meet the current limiting requirements when the voltage drop is large, while a virtual impedance design value that is too large sacrifices the converter's output current capability.

[0026] To address the shortcomings of the aforementioned strategies, a method for implementing variable virtual impedance in a grid-connected converter is disclosed in one or more embodiments. The virtual impedance value can vary according to different voltage dip levels and exhibits saturation characteristics. When the voltage dip is small, the virtual impedance value changes within a small range and presents as a low impedance to the outside of the system, thus minimizing the sacrifice of reactive power output while ensuring current limiting. When the voltage dip is large, to prevent damage to the equipment from excessive current exceeding the converter's limits and to achieve converter current limiting, the virtual impedance value needs to exhibit a large impedance. Furthermore, the variable virtual impedance designed in this paper changes according to the grid-side voltage dip level, offering the advantage of fast response speed. Finally, simulations verify the effectiveness and feasibility of the proposed low-voltage ride-through strategy under different forms and degrees of grid-side voltage faults.

[0027] Combination Figure 8 The variable virtual impedance implementation method of the grid-type converter in this embodiment specifically includes the following process: obtaining the ratio of the grid-side voltage amplitude of the grid-type energy storage converter to the rated voltage amplitude of the system; when the ratio is less than a set threshold, it indicates that a voltage drop has occurred; at this time, the active power setpoint is controlled to maintain a stable power angle without divergence; simultaneously, the saturation characteristic function is calculated based on the voltage drop amplitude of the grid side, and the output value of the virtual internal potential is changed based on the saturation characteristic function, thereby changing the virtual impedance value in the control structure, realizing the construction of a variable virtual impedance with saturation characteristics; so that the virtual impedance presents a small impedance externally when the voltage drop is low and a large impedance externally when the voltage drop is high; thus meeting the current limiting and voltage support requirements of the low voltage ride-through strategy.

[0028] When the ratio is greater than the set threshold, it is considered that the grid-side voltage has been restored. At this time, the active power command value is adjusted back and the virtual impedance value is restored, and the grid-type PCS returns to normal operation.

[0029] In this embodiment, the threshold value can be set to 0.9.

[0030] As a specific implementation method, the fault characteristics of grid-type energy storage are first analyzed.

[0031] (1) Virtual Synchronous Machine Control: Virtual synchronous generator (VSG) control enables the inertia and voltage construction of the energy storage converter, giving it voltage source characteristics. The main circuit and control structure of the energy storage system using VSG control are as follows: Figure 1 As shown. The DC side of the energy storage converter (PCS) is connected to the energy storage medium via a capacitor, and the DC voltage is... U dc The AC output voltage of the converter is obtained through an LC filter. u abc Output current i abc . L f For filtering inductors; R f This refers to the parasitic resistance of the filter inductor; C f This is a filter capacitor. (Grid side) L g For line inductance; R g The line resistance is given, and the grid voltage is given. u gabc .

[0032] The active power component controlled by VSG employs droop control to simulate the governor control of a synchronous generator, and the virtual mechanical power is obtained based on the active power-frequency characteristics. P m As in equation (1): (1) The reactive power component employs droop control to simulate the reactive-voltage characteristics of a synchronous generator excitation system, thereby obtaining a virtual electromotive force. E As shown in equation (2): (2) Frequency and voltage regulation characteristics are established through the above-mentioned droop control.

[0033] Wherein, in equation (1) P m For virtual mechanical power; P ref This is the active power command value; k p The active power-frequency droop factor; ω ref This is the system's rated angular frequency; ω This is the actual angular frequency. In equation (2) E VSG virtual internal potential; E ref The internal potential is given a value; k q The reactive power-voltage droop factor; Qref This is the reactive power command value; Q e Output reactive power to VSG.

[0034] Meanwhile, VSG control also applies virtual inertia to the converter and achieves phase self-synchronization by simulating the rotor motion equations of a synchronous generator, thus realizing autonomous network construction. As shown in equation (3): (3) In the formula, J This is a virtual moment of inertia; D The damping coefficient; P e Output active power to VSG; θ The phase of the internal potential of the VSG.

[0035] The VSG control structure employs a virtual impedance method, making the voltage loop reference value the difference between the virtual internal potential and the virtual impedance voltage drop, thereby achieving current limiting. As shown in equation (4): (4) In the formula, u dref , u qref These are the given voltage values ​​for the outer voltage loop; L v For virtual inductance; R v For virtual resistance; i d , i q Output voltage of the converter i abc Obtained via Parker transformation; , The generated internal potential E is obtained through the Parker transformation. This indicates the angular frequency at a power frequency of 50Hz.

[0036] (2) Analysis of VSG characteristics under grid-side voltage fault: (2-1) Analysis of work angle characteristics: Active power injected into the grid by a grid-type PCS P e It can be represented as: (5) In the formula, E Internal potential; X The internal potential and the total impedance of the power grid; U g This refers to the grid-side voltage amplitude. θ For the phase difference, when the grid-side voltage phase is used as a reference, θThe phase of the internal potential of the VSG.

[0037] VSG's power angle characteristics are as follows Figure 2 As shown, Figure 2 Curve I is the power angle curve during normal operation; curve ⅠⅠ is the power angle curve during a small-scale voltage drop on the grid side; curve ⅠⅠⅠ is the power angle curve during a large-scale voltage drop on the grid side. From equation (5), it can be seen that when the grid-side voltage drops instantaneously, the internal potential phase remains unchanged, and the active power delivered to the grid decreases below the active power given value P. ref Unbalanced power causes frequency acceleration and an increase in the power angle. When the grid voltage drop is not severe, as shown by curve ⅠⅠ, the active power gradually increases to the active power setpoint and remains stable. When the grid voltage drop is severe, as shown by curve ⅠⅢⅠ, there is no intersection with the active power setpoint, so the power angle continues to increase until the voltage recovers. At this time, the active power is instantaneously greater than P. ref At this point, the frequency decreases, but the power angle continues to increase until the active power decreases to P. ref If the acceleration area of ​​curve ⅠⅢⅠ is greater than the deceleration area during this process, the network-type PCS becomes unstable.

[0038] (2-2) Reactive power demand analysis: When a voltage drop occurs on the grid side, the grid-connected PCS needs to provide sufficient reactive power support to the grid. The required support capacity must meet national standards for converter output reactive current. I Q for: (6) in, U gF This represents the magnitude of the voltage drop after the grid-side voltage decreases. U N This refers to the system's rated voltage amplitude. I N This is the rated current amplitude. From this, the required reactive power can be calculated. Q ref for: (7) According to the characteristics of VSG, when the grid voltage drops, the grid-connected converter can inject reactive power into the grid. Q e Achieving reactive power support: (8) (3) PCS output current analysis: The grid-connected PCS model can be equivalent to: Figure 3 The total impedance between the internal potential of the VSG and the grid voltage is denoted as . R 0+j ωL0, which includes the line impedance between the two, as well as the actual line impedance resulting from the superposition of filter reactance and parasitic resistance. R s +j ωL s ; and virtual impedance added in network control. R v +j ωL v .

[0039] When the grid-side voltage drops due to a fault, the post-fault current is as shown in equation (9): (9) in, This indicates the converter output current.

[0040] At this time, the grid-side voltage changes from the rated amplitude. U N Descending to U gF The virtual internal potential generated by VSG E It remains unchanged during a fault. The sudden change in grid-side voltage amplitude causes an increase in converter output current. Without configuring a virtual impedance, U g When the voltage drops sufficiently low, even to zero, excessive current can damage power electronic devices with limited current-carrying capacity. Adding a virtual impedance element increases the equivalent impedance between the internal potential and the grid voltage, thus reducing the output current amplitude under the same voltage drop, lowering the risk of converter breakdown, and achieving converter current limiting. The deeper the voltage drop, the greater the impedance required for current limiting between the internal potential and the grid.

[0041] Next, the low-voltage ride-through strategy for grid-based energy storage will be explained: (1) Low Voltage Ride-Through (LVRT) Strategy Requirements: Based on the above analysis, it can be concluded that when a grid-side voltage fault occurs, in order to ensure that the grid-type PCS continues to operate without disconnecting from the grid during the fault period, the following conditions must be met: ① Maintain a stable power angle during a fault.

[0042] ② Provide sufficient reactive power support during the fault period.

[0043] ③ Avoid PCS overcurrent during faults.

[0044] The main requirements for low-voltage ride-through (LVRT) strategies are ② and ③. From a current-limiting perspective, a larger virtual impedance value is better; the larger the virtual impedance in the control loop, the smaller the current during a fault. However, LVRT requires not only current limiting by the PCS during a fault but also, while maintaining grid connection, outputting as much power as possible. An excessively large virtual impedance value for current limiting means that the current output capability of the PCS is restricted during a fault. Therefore, the value of the virtual impedance needs to be flexibly considered based on the different degrees of voltage drop.

[0045] (2) Active power loop design: Based on the above analysis, when a voltage drop occurs on the grid side, the active power output by the PCS decreases, and the active power given by the PCS drops. P ref If the active power loop remains constant, it will maintain the PCS output active power at the active power setpoint by adjusting the phase. However, due to the inertia in this loop, the adjustment process takes time. When the grid-side voltage drop is small, it can adjust in time; when the grid-side voltage drop is large, the entire process may not be completed before the grid-side fault is recovered. Furthermore, a large phase adjustment range is also detrimental to power angle stability.

[0046] To maintain a stable power angle during a fault, the active power setpoint can be reduced proportionally to the degree of voltage drop when a grid-side voltage dip fault occurs, or the setpoint active power can be forcibly increased during the fault. P ref With actual power P e The difference between them is set to 0.

[0047] (3) Virtual impedance design with saturation characteristics: Based on the above analysis, the current after the fault is as shown in equation (9). When the voltage drop is small, the current limiting pressure of the PCS is relatively small. To ensure the output capacity of the PCS reactive current during the fault, the virtual impedance should change little and continuously present a small impedance to the outside. When the voltage drop is large, the current limiting pressure of the PCS is large. To achieve the current limiting of the PCS, a large virtual impedance is required. Therefore, as the voltage drop changes, the virtual impedance value also needs to change linearly. The virtual impedance value in the low voltage ride-through strategy changes little when the voltage drop is small and remains basically unchanged; it changes much when the voltage drop is large. Its variation characteristics are as follows: Figure 4 As shown: Virtual impedances with similar characteristics can be considered to have saturation characteristics. Saturation characteristics refer to the phenomenon where, after the system input signal reaches a certain value, the output no longer changes significantly with increasing input, exhibiting a tendency to stabilize or remain constant. Virtual impedances with saturation characteristics can maintain their reactive power support capability while maintaining current limiting in a PCS system. The following section focuses on designing virtual impedances with saturation characteristics.

[0048] For the voltage drop amplitude on the grid side in equation (9), it can be ignored when the drop is large, as shown in equation (10): (10) By constructing a function with saturation characteristics, the virtual impedance can be made to have saturation characteristics, as shown in equation (11): (11) Among them, the function F ( U g * ) is a function with saturation characteristics based on the per-unit value of the grid-side voltage amplitude, making its curve as follows Figure 5 As shown: Functions with similar curves include the hyperbolic tangent function tanh(x), and constructions based on the hyperbolic tangent function are possible. F ( U g * As shown in equation (12): (12) In the formula, This is the per-unit value of the grid-side voltage amplitude. The larger the voltage, the smaller the voltage drop; for example, if the grid-side voltage drops by 20%, then... U g * The value is 0.8. The specific solution method is as follows: measure the abc phase values ​​when the grid-side voltage drops, and obtain U through Park transformation. d and U q The square root of the sum of the squares of both is taken as the named value, and then divided by the base value to obtain the per-unit value.

[0049] The constructed saturation characteristic function F ( U g * Combining ) with equation (4) yields a virtual impedance with saturation characteristics.

[0050] In this embodiment, a saturation characteristic function is added to the output terminal of the internal potential in the network-type control strategy. Multiply this by the output value of the internal potential. Based on the converter current output value during grid-side faults as shown in equation (11), the numerator is the product of the internal potential and the constructed value, and the denominator is the sum of the line impedance and the virtual impedance. According to the characteristic that the output internal potential value of the grid-type converter remains unchanged during a fault, it can be equivalently represented that the impedance between the converter and the grid is changed during the fault as follows: This indirectly constructs a variable virtual impedance, which exhibits a change characteristic opposite to that of the saturation characteristic function. Furthermore, based on the expression for the saturation characteristic function, combined with... Figure 5 The saturation characteristic function shown has the following variation characteristics: its value increases with the increase of the per-unit value of the grid-side voltage amplitude (i.e., increases with the decrease of the grid-side voltage drop). Therefore, it satisfies the characteristic that the virtual impedance has a small impedance when the grid-side voltage drop is small and a large impedance when the grid-side voltage drop is large. This allows the virtual impedance value to be indirectly changed by modifying the internal potential process quantity in the control strategy through the function value. Figure 6 The per-unit value of the grid-side voltage is given. The diagram showing the relationship between the converter output current per unit value and the converter current shows that, under any voltage drop scenario, the converter current does not exceed 1.5 times the per unit value, which is within the acceptable range of the converter.

[0051] When there is no fault on the grid side, the grid-type PCS operates under normal conditions; when a fault occurs on the grid side, the voltage drop is detected in real time, and then... F ( U g * The output changes the amplitude of the internal potential of the network-type PCS. E If the internal potential is... E If it is considered constant, the virtual impedance value of the input is changed to make it have saturation characteristics, so that the reactive power support and current limiting requirements of the low voltage ride-through strategy of the grid-type PCS can be met for different voltage drop levels.

[0052] The specific construction of virtual impedance with saturation characteristics is as follows: Figure 7 As shown, in the control loop, the Ed and Eq generated by the Parker transformation of the internal potential E are each associated with a saturation characteristic function. F ( U g * Multiplying these values ​​and combining them with the calculation process of equation (4), the final output voltage outer loop given voltage value is obtained. V dref and V qref .

[0053] The control flowchart of the above low voltage ride-through strategy is as follows: Figure 8 As shown. First, the network-type PCS operates under normal conditions, while simultaneously... dq The obtained three-phase voltage amplitude on the grid side is compared with the rated voltage amplitude. When the obtained... U g * A value greater than 0.9 indicates the system is still operating normally; when the obtained value is... U g * A value less than 0.9 indicates a voltage dip fault. At this point, based on the obtained... U g* The value changes the active power command value in the VSG control, and at the same time, it is based on the obtained function value. F ( U g * By changing the output value of the virtual internal potential, the virtual impedance value in the control structure is altered, thus satisfying the current limiting and voltage support requirements of the LVRT strategy. U g * When the value is greater than 0.9, it is considered that the grid-side voltage has been restored. At this time, the active power command value is adjusted back and the virtual impedance value is restored, and the grid-type PCS returns to normal operation.

[0054] Finally, simulation analysis and verification are performed: To verify the feasibility of the aforementioned low-voltage ride-through strategy, this embodiment built a system on the Matlab / Simulink platform. Figure 1 The grid-type energy storage system is shown, and the feasibility of the proposed strategy under different voltage dip conditions is verified through further simulation. First, the output voltage and current characteristics of the grid-type PCS system under different voltage dip levels are verified without the LVRT strategy. Second, the feasibility and effectiveness of the LVRT strategy mentioned in this paper are verified under symmetrical and asymmetrical voltage dips on the grid side. The main system parameters are shown in Table 1.

[0055] Table 1 System Parameters name numerical values Rated amplitude of grid voltage Ug / V 311 <![CDATA[Rated frequency f N / Hz]]> 50 <![CDATA[Filter inductor L f / mH]]> 2 <![CDATA[Parasitic resistance R of the filtering inductor f1 / Ω]]> 0.05 <![CDATA[Filter capacitor C f / μF]]> 35 <![CDATA[Parasitic resistance R of the filtering inductor f2 / Ω]]> 0.5 <![CDATA[Virtual inertia J / (kg·m 2 )]]> 3.36 <![CDATA[Active power frequency droop coefficient k p > 30 Damping coefficient D 103.3 <![CDATA[Reactive voltage droop coefficient k q > 10 <![CDATA[Line resistance R g / Ω]]> 0.06 <![CDATA[Line inductance L g / mH]]> 1.35 <![CDATA[DC voltage U dc / V]]> 1000 <![CDATA[Given active power P ref / kW]]> 170 <![CDATA[Given reactive power Q ref / kvar]]> 0 (1) PCS characteristics when there is a network-side fault without LVRT strategy Without the LVRT strategy, the grid-type PCS operates under normal conditions. The grid voltage amplitude is set to drop by 0.2 pu at 1 second and returns to normal at 1.6 seconds. The RMS values ​​of the output voltage, RMS value of the output current, output active power, and reactive power of the grid-type PCS are shown in Figures 9(a)-(d).

[0056] When the grid voltage drop is low, the PCS output current overcurrent is small, with a peak value not exceeding 1.2 pu, which meets the requirements of the converter. However, the time required for the output current to reach the steady-state value is relatively long.

[0057] At 1 second, the grid voltage amplitude is set to drop by 0.8 pu, and it returns to normal at 1.6 seconds. The effective values ​​of the output phase voltage and current of the grid-type PCS, as well as the output active power and reactive power, are shown in Figures 10(a)-(d). At this time, due to the deep voltage drop, the current limiting of the PCS during the fault period cannot be met, and the output power of the PCS cannot reach the given value. P refThis causes the power angle to diverge, and the voltage and current during the fault cannot reach a steady state, resulting in oscillations. After the fault disappears in 1.6 seconds, the power angle divergence will also generate a large current that impacts the PCS.

[0058] (2) LVRT strategy verification during network-side failure To verify the feasibility of the LVRT strategy proposed in this paper under different grid-side voltage drop levels, a symmetrical grid-side voltage drop fault was set in a grid-type PCS system configured with LVRT control.

[0059] The grid voltage amplitude is set to drop by 0.2 pu at 1 second and return to normal at 1.6 seconds. The degree of drop is determined by collecting the grid-side voltage amplitude and then processed using a function. F ( U g * This increases the virtual impedance value. Because the voltage drop is relatively small, the function... F ( U g * The output value of ) is close to 1, and the virtual impedance does not need to be significantly increased to achieve converter current limiting, while also meeting the reactive power support requirements. Figures 11(a)-(f) show the waveforms of the three-phase voltage, three-phase current, RMS output voltage, RMS output current, active power, and reactive power of the grid-type PCS under the LVRT strategy with a grid-side voltage drop of 0.2 pu. Among them, the RMS output current of the grid-type PCS is shown in Figure 11(d), which is within the allowable range of the converter; the reactive power is shown in Figure 11(f), which meets the reactive power support requirements of Equation (7).

[0060] The grid-side voltage is set to drop by 0.8 pu at 1 second and recover at 1.6 seconds. During a fault, a large overcurrent is likely to occur, and the low-voltage ride-through strategy needs to quickly suppress the overcurrent to prevent damage to the converter. At this time, the degree of voltage drop is determined by collecting the grid-side voltage amplitude, and a function is used to... F ( U g * This increases the virtual impedance value. Because the voltage drop is significant, the function... F ( U g * The output value is close to 0. Meanwhile, the response speed of this process depends on the acquisition speed of the grid-side voltage, which in turn depends on the system's switching frequency. A higher switching frequency corresponds to a faster response speed and stronger overcurrent suppression capability.

[0061] Figures 12(a)-(f) show the waveforms of the three-phase voltage, three-phase current, RMS output voltage, RMS output current, active power, and reactive power of the PCS in a grid-connected system with a voltage drop of 0.8 pu under the LVRT strategy. The three-phase output current during a fault is shown in Figure 12(b), and no significant overcurrent is generated during the fault. The RMS output current is shown in Figure 12(d), and the current during the fault does not exceed 1.2 pu, demonstrating a significant current limiting effect. The output reactive power is shown in Figure 12(f), which meets the reactive power support requirements of equation (7).

[0062] To verify the feasibility of the LVRT strategy in this embodiment under asymmetrical faults, a grid-side voltage symmetrical drop fault was set in a grid-type PCS system configured with LVRT control, and a negative sequence control loop was added. At 1 second, the grid-side A-phase voltage was set to drop by 0.8 pu, while other phases remained normal; it returned to normal at 1.6 seconds.

[0063] Figures 13(a)-(d) show the output voltage, output current, active power, and reactive power when the grid-side A-phase voltage drops by 0.8 puPCS under the LVRT strategy. It can be seen that under voltage imbalance faults, the combination of the LVRT strategy and the imbalance control strategy proposed in this embodiment can achieve three-phase current balance and converter current limiting. During the fault, the current value does not exceed 1.2 pu, and the output reactive power is shown in Figure 13(d), which meets the reactive power support requirements of equation (7).

[0064] In summary, the low-voltage ride-through strategy based on variable virtual impedance proposed in this embodiment is applicable to both symmetrical and asymmetrical faults on the grid side.

[0065] Example 2 In one or more embodiments, a variable virtual impedance implementation system for a grid-type converter is disclosed, comprising: The acquisition module is configured to acquire the ratio of the grid-side voltage amplitude of the grid-type energy storage converter to the rated voltage amplitude of the system; The judgment module is configured to determine that a voltage drop has occurred when the ratio is less than a set threshold. The control module is configured to control the active power setpoint to maintain a stable power angle without divergence; at the same time, it calculates the saturation characteristic function based on the voltage drop amplitude of the grid side, and changes the output value of the virtual internal potential based on the saturation characteristic function, thereby changing the virtual impedance value in the control structure to meet the current limiting and voltage support requirements of the low voltage ride-through strategy.

[0066] It should be noted that the specific implementation methods of the above modules are exactly the same as those in Example 1, and will not be described in detail again.

[0067] Example 3 In one or more embodiments, a terminal device is disclosed, comprising a processor and a memory, wherein the processor is used to implement instructions; and the memory is used to store multiple instructions adapted to be loaded by the processor and executed by the processor to implement the variable virtual impedance method for the grid-type converter described in Embodiment 1.

[0068] It should be understood that in this embodiment, the processor can be a central processing unit (CPU), or it can be other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or any conventional processor, etc.

[0069] Memory may include read-only memory and random access memory, and provides instructions and data to the processor. A portion of memory may also include non-volatile random access memory. For example, memory may also store information about the device type.

[0070] In the implementation process, each step of the above method can be completed by the integrated logic circuits in the processor hardware or by software instructions.

[0071] Example 4 In one or more embodiments, a computer-readable storage medium is disclosed, wherein a plurality of instructions are stored, the instructions being adapted to be loaded by a processor of a terminal device and executed by the variable virtual impedance implementation method of the grid-type converter described in Embodiment 1.

[0072] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for implementing variable virtual impedance of a meshed network converter, characterized in that, The method comprises the following steps: obtaining a ratio of a grid-connected energy storage converter grid-side voltage amplitude to a system rated voltage amplitude; when the ratio is less than a set threshold, it is determined that voltage sag occurs; at this time, the active power set value is controlled to maintain the power angle stability and prevent divergence; meanwhile, a saturation characteristic function is calculated according to the voltage sag amplitude of the grid-side voltage, the output value of the virtual internal electromotive force is changed based on the saturation characteristic function, and the virtual impedance value in the control structure is changed to meet the current limiting and voltage support requirements of the low voltage ride through strategy.

2. The method of claim 1, wherein the variable virtual impedance of the networked converter is implemented by, The control of the active power set value specifically comprises the following steps: the active power set value is reduced in proportion to the voltage sag amplitude of the grid-side voltage, or the difference between the active power set value and the actual active power value is forcibly set to 0 during voltage sag.

3. The method of claim 1, wherein the variable virtual impedance of the networked converter is implemented by, The saturation characteristic function is constructed based on a hyperbolic tangent function.

4. The method of claim 3, wherein the variable virtual impedance is implemented by a networked converter. The calculation of the saturation characteristic function according to the voltage sag amplitude of the grid-side voltage specifically comprises the following steps: ； wherein is a saturation characteristic function, U g * is the grid-side voltage amplitude normalized value, obtained by dq coordinate transformation.

5. The method of claim 4, wherein the variable virtual impedance of the networked converter is implemented by, the output value of the virtual internal electromotive force is changed based on the saturation characteristic function, and the virtual impedance value in the control structure is changed to meet the current limiting and voltage support requirements of the low voltage ride through strategy. The output value of the virtual inner potential is equal to the product of the virtual inner potential and the saturation characteristic function .

6. The method of claim 5, wherein the variable virtual impedance of the networked converter is implemented by, The converter current output value during grid-side fault specifically comprises the following steps: According to the characteristic that the output inner potential value of the network-constructed converter remains unchanged at the time of fault, the equivalent impedance between the converter and the power grid during the fault is changed to: .

7. A variable virtual impedance implementation system of a meshed network type converter, characterized by, The method comprises the following steps: a judgment module is configured to obtain a ratio of a grid-connected energy storage converter grid-side voltage amplitude to a system rated voltage amplitude; when the ratio is less than a set threshold, it is determined that voltage sag occurs; a control module is configured to control the active power set value to maintain the power angle stability and prevent divergence; meanwhile, a saturation characteristic function is calculated according to the voltage sag amplitude of the grid-side voltage, the output value of the virtual internal electromotive force is changed based on the saturation characteristic function, and the virtual impedance value in the control structure is changed to meet the current limiting and voltage support requirements of the low voltage ride through strategy.

8. A terminal device comprising a processor and a memory, the processor configured to implement instructions; the memory configured to store a plurality of instructions, the terminal device characterized by, The instructions are suitable for being loaded and executed by a processor to implement the variable virtual impedance implementation method of the grid-connected converter according to any one of claims 1-6.

9. A computer-readable storage medium having stored therein a plurality of instructions, wherein the instructions, when executed by a processor, cause the processor to perform operations comprising: The instructions are suitable for being loaded and executed by a processor of a terminal device to implement the variable virtual impedance implementation method of the grid-connected converter according to any one of claims 1-6.

10. A computer program product comprising computer programs / instructions, characterized in that, The computer program / instructions, when executed by a processor, implement the variable virtual impedance implementation method of the grid-connected converter according to any one of claims 1-6.

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