Vsg overcurrent combined control method, system and readable storage medium

Abstract

This invention provides a joint control method, system, and readable storage medium for VSG overcurrent. The method includes: constructing a grid-connected model of the VSG system and its control equations, and calculating the active and reactive power injected into the grid by the VSG based on the grid-connected model; constructing a first active power constraint condition when the grid voltage experiences a slight dip; and constructing a second active power constraint condition when the grid voltage experiences a severe dip; establishing an equivalent fault circuit diagram for the VSG, and calculating the steady-state and transient components of the VSG short-circuit current using a time-domain solution method based on the circuit parameters at the fault time; calculating an adaptive active power reference, and reshaping the internal potential reference value of the VSG according to the degree of grid voltage dip; and introducing a dynamic virtual impedance, adjusting the virtual impedance parameters according to the transient characteristics of the fault current. This invention can improve the transient power angle stability margin and suppress fault current peak values.

Description

Technical Field

[0001] This invention relates to the field of VSG fault control technology, and in particular to a VSG overcurrent joint control method, system and readable storage medium. Background Technology

[0002] With the large-scale grid connection of new energy power generation equipment, the proportion of traditional power generation equipment in the power grid is gradually decreasing. The power generation method with converters as the main energy conversion method is causing the system inertia to continue to decline. Moreover, in practical applications, grid-connected converters mostly adopt grid-following control strategies. Converters under this control method rely on phase-locked loops (PLLs) to track the grid voltage, and their output changes with the grid voltage, lacking inertia. Therefore, the connection of a large number of grid-following converters will reduce the stability of the system.

[0003] To address the issue of reduced system inertia in high-proportion renewable energy power systems, grid-connected converters with equivalent inertia are often controlled using grid-based control strategies. The Virtual Synchronous Generator (VSG) control strategy is a typical example of such grid-based control. By simulating the inertia and damping characteristics of a traditional synchronous generator, it makes the converter output exhibit voltage source characteristics, actively supporting grid voltage and enhancing grid inertia to improve power system stability.

[0004] However, according to the national standard GB / T19964—2024, when a short-circuit fault in the power grid causes a voltage drop, grid-connected converters, due to their voltage source characteristics, need to provide voltage support to the power system. At this time, the converter needs to rapidly inject a large amount of reactive power into the grid. Although injecting reactive power into the grid is beneficial for voltage stability, excessive reactive current injection can lead to problems such as overcurrent and large fluctuations in instantaneous output power. Compared to traditional synchronous generators, which can withstand up to 6-7 times their rated current during transient periods, the power electronic equipment in a converter can only withstand a maximum of 1.5 times the overcurrent during a fault. Therefore, excessive short-circuit current can cause the converter to disconnect from the grid, or even damage electrical equipment, resulting in irreversible consequences. Summary of the Invention

[0005] The purpose of this invention is to provide a VSG overcurrent joint control method, system, and readable storage medium, aiming to solve the problem that the overcurrent control effect is poor due to the neglect of the dynamic characteristics of the transient response process of short-circuit current and the influence of short-circuit current on the stability of system power angle in traditional technologies.

[0006] In a first aspect, the present invention provides a VSG overcurrent joint control method, the method comprising:

[0007] Construct a grid-connected model and its control equations for a grid-connected system VSG, and calculate the active and reactive power injected by the VSG into the grid based on the grid-connected model;

[0008] When the grid voltage experiences a slight drop, a first active power constraint condition is established; when the grid voltage experiences a severe drop, a second active power constraint condition is established.

[0009] Establish the equivalent fault circuit diagram of VSG, and calculate the steady-state and transient components of VSG short-circuit current using the time-domain solution method based on the circuit parameters at the fault time.

[0010] Calculate the adaptive active power reference and reshape the internal potential reference value of the VSG according to the degree of grid voltage drop;

[0011] A dynamic virtual impedance is introduced, and the virtual impedance parameters are adjusted according to the transient characteristics of the fault current.

[0012] Secondly, the present invention provides a VSG overcurrent joint control system, the system comprising:

[0013] The grid connection model construction module is used to construct the grid connection model and its control equations of the grid-connected system VSG, and calculate the active power and reactive power injected by VSG into the grid based on the grid connection model.

[0014] The constraint construction module is used to construct the first active power constraint when the grid voltage drops slightly, and the second active power constraint when the grid voltage drops severely.

[0015] The component calculation module is used to establish the equivalent fault circuit diagram of VSG and calculate the steady-state and transient components of VSG short-circuit current using the time-domain solution method based on the circuit parameters at the time of the fault.

[0016] The internal potential reshaping module is used to calculate the adaptive active power reference and reshape the internal potential reference value of the VSG according to the degree of grid voltage drop.

[0017] The parameter adjustment module is used to introduce dynamic virtual impedance and adjust the virtual impedance parameters according to the transient characteristics of the fault current.

[0018] Thirdly, the present invention provides a readable storage medium that stores one or more programs that, when executed by a processor, implement the above-described VSG overcurrent joint control method.

[0019] Fourthly, the present invention provides an electronic device, the electronic device comprising a memory and a processor, wherein:

[0020] The memory is used to store computer programs;

[0021] When the processor executes the computer program stored in the memory, it implements the above-mentioned VSG overcurrent joint control method.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] 1. This invention introduces an adaptive adjustment strategy for the active power reference value to eliminate the power imbalance problem during system faults. By real-time correction of the dynamic balance between the reference power and the output power, it effectively suppresses the risk of transient power angle instability and improves the transient stability of the system.

[0024] 2. This invention introduces an adaptive virtual internal potential dynamic reconstruction strategy, which adjusts the reference value of the virtual internal potential in real time according to the degree of grid voltage drop, thereby achieving precise control of the steady-state periodic component of the short-circuit current and ensuring that its amplitude is within the system safety margin.

[0025] 3. This invention addresses the problem of suppressing the aperiodic component of short-circuit current at the time of fault occurrence and clearing by employing a time-varying dynamic virtual impedance control strategy to suppress the amplitude of the aperiodic component and accelerate its attenuation, thereby protecting the power electronic equipment in the system. Attached Figure Description

[0026] Figure 1 This is a timing flowchart;

[0027] Figure 2 The response of the inner loop current of the VSG under a large disturbance after a limiter is added to the current loop;

[0028] Figure 3 The equivalent circuit of the system before and after the VSG inverter failure;

[0029] Figure 4 This is a flowchart of a VSG overcurrent joint control method proposed in an embodiment of the present invention;

[0030] Figure 5 For the grid connection model of VSG network-type system;

[0031] Figure 6 Here is the control structure block diagram for VSG;

[0032] Figure 7 This is a schematic diagram of the work angle curve, where, Figure 7 (a) is the power angle curve when the system is in normal operation. Figure 7 (b) is the power angle curve when the grid voltage experiences a type I slight drop; Figure 7 (c) is the power angle curve when the grid voltage experiences a Type II severe voltage drop;

[0033] Figure 8 This is the equivalent circuit diagram after a VSG failure.

[0034] Figure 9 The system power angle curve under the adaptive active power reference regulation strategy;

[0035] Figure 10 The phasor relationship diagram of voltage and current before and after the system fault occurs;

[0036] Figure 11 This is a schematic diagram of the VSG overcurrent joint control system proposed in an embodiment of the present invention.

[0037] The following detailed description, in conjunction with the accompanying drawings, will further illustrate the present invention. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art. The terms "comprising" and similar expressions used herein mean that the element or object preceding the word covers the element or object listed after the word and its equivalents, but does not exclude other elements or objects.

[0039] Current research on overcurrent suppression methods for grid-connected converters mainly falls into three categories: ① mode switching for grid-connected control strategies of the converter; ② direct limiting of the current reference value in the voltage-current dual closed loop; ③ switching of the virtual impedance loop. Details are as follows:

[0040] ① Switching between control modes

[0041] Since the low-voltage ride-through strategy for traditional grid-connected converters is relatively well-developed, this study combines the grid-connected control strategy with the VSG control strategy. The traditional low-voltage ride-through control principle for grid-connected converters involves reshaping the active and reactive current reference values ​​of the grid-connected converter according to the national standard GB / T 19964-2012 when a low voltage is detected. Then, the current control system causes the grid-connected converter to output a specified current value. The formula for calculating the current reference value is as follows:

[0042]

[0043] In the formula, i d * i q *This is the reference value for the dq-axis current of the current loop during a low-voltage fault; I N U is the rated peak current; Ug is the grid voltage; U N This is the rated voltage of the power grid.

[0044] By reshaping the current reference value during grid-connected faults using equations (1) and (2) and incorporating feedforward decoupling technology into the current control loop, the inverter can provide sufficient reactive power support to the grid while outputting the rated current amplitude. Therefore, when a fault occurs, the VSG control is switched to low-voltage ride-through control for the grid-connected converter, and after the fault is cleared, the control mode is switched back to VSG control, thus achieving low-voltage ride-through control of the VSG. The specific timing sequence is as follows: Figure 1 As shown.

[0045] ② Current reference value limiting;

[0046] Grid-type converters incorporate current limiters in both the voltage and current loops. When a system fault causes the inner loop current to exceed the maximum allowable range, the output will be limited to pre-set upper and lower current reference values. A typical current limiting expression is as follows:

[0047]

[0048] In the formula i Lref,d * i Lref,q * I is the reference current value of the dq axis after passing through the current limiter. max This refers to the maximum peak current that a power electronic device can withstand.

[0049] After adding a limiter to the current loop, the response process of the inner loop current of the VSG under large disturbances is as follows: Figure 2 As shown, Figure 2 This indicates that the steady-state operating point of VSG is O(i L,d i L,q When the system experiences a disturbance, the operating point will change along the red curve. The function of the limiting element is shown as the boundary circle in the figure; it changes with the maximum allowable current I that the equipment can withstand. max With a decrease in voltage amplitude, the VSG may trigger a limiting circuit under slight disturbances, or even enter a continuous limiting state. In this case, the VSG will be unable to maintain voltage-controlled output and will instead enter current-limiting mode, where its output characteristics are dominated by current amplitude constraints.

[0050] ③ Virtual impedance

[0051] To reduce overcurrent issues during faults, a virtual impedance loop with negative feedback regarding the load current is added between the voltage / current dual loop and the power loop. Let the equivalent total impedance after the grid voltage dip be R.F +jωL F The equivalent impedance of the line after the fault is R0 + jωL0, and the added virtual impedance is R. v +jωL v The amplitude of the virtual internal potential before the fault is E, and the amplitude of the grid voltage after the fault is U. F Finally, the equivalent circuit of the system before and after the VSG inverter failure can be obtained as follows: Figure 3 As shown.

[0052] Therefore, the expression for the short-circuit current after a fault can be obtained as follows:

[0053]

[0054] If there is no virtual impedance input in the system, then

[0055] R F +jωL F =R0+jωL0 (5)

[0056] If a virtual impedance is involved in the system, then

[0057] R F +jωL F =(R0+R v )+jω(L0+L0) (6)

[0058] By comparing equations (4), (5) and (6), it can be seen that after introducing virtual impedance, the amplitude of the system short-circuit current is smaller than that without virtual impedance, so this method can effectively suppress overcurrent.

[0059] Regarding the short-circuit current suppression techniques discussed above, existing research still has several problems that urgently need to be solved:

[0060] ① Mode switching for the grid-connected converter's control strategy: This method switches the converter's grid-connected control to grid-connected control mode when a short-circuit current exceeds the system's maximum allowable current during a fault. While this switching mechanism effectively limits the fault current, it causes the grid-connected converter to lose its voltage source characteristics, preventing it from actively providing voltage and frequency support to the system. Furthermore, the switched grid-connected control requires a PLL to synchronize the converter with the grid, which may lead to PLL synchronization stability issues when the grid impedance is high.

[0061] ② Direct limiting of the current reference value in a voltage-current dual closed-loop system: While directly limiting the current reference value can control the short-circuit current and maintain grid characteristics during a grid fault, this limiting strategy can cause the fault current to saturate and distort the current waveform, resulting in a decrease in the power angle curve and phase shift during the fault. This phenomenon reduces the maximum fault clearance margin and the maximum duration of the fault, potentially leading to system transient instability.

[0062] ③ Switching of virtual impedance loops: By adding virtual impedance loops, the output impedance of the grid-type converter is effectively increased. Compared with limiting the current reference value, this method results in a smaller decrease in the amplitude of the power angle curve and a smaller phase shift after switching the virtual impedance. While achieving short-circuit current control, it can ensure a longer maximum duration of system operation during faults. However, virtual impedance is usually a fixed value, and the calculation of virtual impedance is relatively complex. Therefore, when the line parameters in the system change, it is not possible to quickly and accurately compensate.

[0063] Based on this, the analysis of existing research methods for short-circuit current control reveals that current control strategies primarily suppress the current amplitude, neglecting two crucial issues: the dynamic characteristics of the short-circuit current transient response and the impact of the short-circuit current on the system's power angle stability. Therefore, this invention proposes a VSG overcurrent joint control method based on parameter reshaping and dynamic virtual impedance input. When designing short-circuit current control methods for grid-connected converters, this method simultaneously considers the dynamic response characteristics of the overcurrent and the transient power angle stability of the system.

[0064] First, based on the basic structure of the grid-connected converter, the transient response characteristics of the system are analyzed. Then, based on the short-circuit current generation mechanism, an analytical expression for the short-circuit current is derived, showing that it consists of steady-state and transient components. Second, during grid faults, the active power reference value is adaptively adjusted to eliminate power imbalance during the fault and improve the system's transient power angle stability. Then, based on the voltage drop during the fault, the internal potential reference value of the converter is reshaped to limit the steady-state component of the short-circuit current. Finally, during the transient processes of fault initiation and clearance, dynamic virtual impedance is used to suppress the transient component of the short-circuit current.

[0065] Specifically, such as Figure 4 As shown, this invention proposes a VSG overcurrent joint control method, which includes steps S101 to S105, wherein:

[0066] Step S101: Construct the grid connection model and control equations of the VSG grid-connected system, and calculate the active power and reactive power injected by the VSG into the grid based on the grid connection model;

[0067] First, in this embodiment, the following settings are made during the analysis and calculation:

[0068] Setting condition ①: DC bus voltage fluctuations of VSG are not considered;

[0069] Setting condition ②: The power system is regarded as an infinite power source, and its voltage and frequency are not affected by the inverter itself;

[0070] Condition ③: Ignore inner loop control and only examine the impact of power loop and virtual impedance on VSG grid connection;

[0071] Setting condition ④: All voltages are referred to the inverter output side, the transmission line is modeled using impedance, and the transformer equivalent impedance is referred to the line impedance;

[0072] Condition ⑤: The reactance in the equivalent impedance referred to the inverter output side is much greater than the resistance, XL >> R.

[0073] Furthermore, in some embodiments, the grid connection model of the VSG network-type system is as follows: Figure 5 As shown, its structure consists of a main circuit and a control system. The main circuit uses a three-phase voltage-source inverter with an LC filter connected to the grid. Figure 5 Medium: U dc I dc These are the DC side voltage and current, E and U, respectively. PCC These are the internal potential of the VSG and the grid-connected voltage of the PCC, respectively; Ug and Ig are the grid-side voltage and current, respectively; I L I C These represent the inductor current and capacitor current, respectively; Rg and Lg represent the equivalent resistance and reactance of the power grid, respectively; L f R f C and P are the parameters of the LC filter; ref Q ref These are the active and reactive power reference values ​​for VSG, respectively; U ref ω ref These represent the reference voltage amplitude and reference angular frequency of the VSG, respectively. The control block diagram of the VSG is shown below. Figure 6 As shown, ω, ω n These represent the actual output angular frequency of the system and the rated angular frequency on the grid side, respectively; D and J are the virtual damping and inertia coefficients of the VSG, respectively; D q With K q These are the reactive droop coefficient and reactive inertia coefficient, respectively; θ is the phase of the internal potential generated by the active power loop; E ref This is the reference internal potential generated by the reactive power loop.

[0074] Furthermore, in some embodiments, VSG control technology provides the power grid with equivalent inertia and damping by simulating the rotor motion and excitation voltage regulation of a traditional synchronous generator. For example... Figure 6As shown, the power loop of the VSG consists of an active-frequency control loop and a reactive-voltage control loop, which respectively simulate the rotor sway and excitation voltage regulation characteristics of a synchronous generator. Its basic control equation is:

[0075]

[0076] When the system is in steady state, with the grid voltage as a reference, assuming the VSG output internal potential is E∠δ, the grid voltage is Ug∠0, and the transmission line impedance X>>R, the active power P injected by the VSG into the grid can be calculated. e and reactive power Q e for:

[0077]

[0078] In the formula u d u q i d i q These are the voltage and current on the dq axis after Park transformation.

[0079] Step S102: When the grid voltage experiences a slight drop, construct the first active power constraint condition; when the grid voltage experiences a severe drop, construct the second active power constraint condition.

[0080] It should be noted that the output power of the VSG is shown in equations (8) and (9). Since the grid-type converter exhibits voltage source characteristics, when the grid voltage drops, the reactive power output of the converter increases, but the active power output of the converter decreases. The imbalance of the active power in the system will lead to an increase in the virtual angular velocity of the converter and an increase in the power angle, which in turn will cause the power angle stability problem of the system.

[0081] To address the issue of system power angle stability, the power angle curve of the system is plotted according to equation (8), and the power angle curves of the system under three conditions are discussed in separate tables: normal operation, slight voltage drop of type I grid voltage, and severe voltage drop of type II grid voltage. Figure 7 (a) is the power angle curve when the system is in normal operation. Figure 7 (b) is the power angle curve when the grid voltage experiences a type I slight drop; Figure 7 (c) shows the power angle curve when the grid voltage experiences a Type II severe voltage drop. Figure 7 As shown in (b) and (c), the difference between a slight voltage dip and a severe voltage dip lies in the active power reference value P of the VSG. ref With actual output active power P e Does an intersection point exist, that is, does the system reach a new stable operating point during the fault period?

[0082] Specifically, in some embodiments, when a slight drop occurs in the grid voltage, the P value of the VSG power angle curve...ref With P e There is an intersection point, and the power angle undergoes a change involving acceleration followed by deceleration. Without clearing the fault, to ensure system power angle stability, the following must be satisfied:

[0083]

[0084] In the formula, δ0, δ h δ represents the steady-state equilibrium point power angle of the system during steady-state operation and the unstable equilibrium point power angle of the system after a fault, respectively. c The power angle represents the system's stable equilibrium point during a fault.

[0085] When the grid voltage experiences a severe drop, the P value on the VSG power angle curve... ref With P e There is no intersection between them, and the power angle continues to accelerate. If the fault is not cleared in time during this process, the power angle will eventually cross the unstable equilibrium point of the power angle curve, ultimately causing the converter to disconnect from the grid. Assume that at this time, the P of the VSG... ref With P e If the difference between them is a certain value, then we can obtain from equation (7)

[0086]

[0087] Let Δω = ω - ω0, then the first differential of the relative angular frequency can be solved as

[0088]

[0089] Since the system power angle did not change abruptly at the moment of the fault, VSG remained synchronized. Therefore, C = -1 in equation (12), which can be simplified to:

[0090]

[0091] Integrating both sides with respect to time t yields

[0092]

[0093] Similarly, when a fault occurs, VSG remains synchronized with the power grid, and the power angle remains unchanged. Therefore, C1 = -J / D, and equation (14) can be rearranged to obtain

[0094]

[0095] In summary, when the power of the VSG is unbalanced, the system power angle will exhibit a transient characteristic of continuously increasing. Therefore, to eliminate active power imbalance, the active power reference value P needs to be dynamically adjusted. ref Meanwhile, by appropriately adjusting key parameters such as virtual inertia J and damping coefficient D, the power angle deviation can also be effectively suppressed, thereby improving the transient stability of the system.

[0096] Step S103: Establish the equivalent fault circuit diagram of VSG, and calculate the steady-state and transient components of the VSG short-circuit current using the time-domain solution method based on the circuit parameters at the fault time.

[0097] It should be noted that when the grid voltage drops, because the VSG exhibits voltage source characteristics, its internal potential and power angle can be approximated as constant at the moment of the fault. Therefore, since the power system experiences a three-phase symmetrical fault, the analysis of the VSG's short-circuit current can be simplified to a single-phase calculation model. The equivalent circuit after the VSG fault is as follows: Figure 8 As shown.

[0098] Assume the power grid fault occurs at time t=0, and let t be the time interval. |0| At the instant before the fault occurs, E∠δ is the internal potential generated by the VSG power loop. F ∠δ F U is the port output voltage of the VSG. gF|0 |∠δ FgF|0| U is the voltage instantaneously before the power grid fault. gF ∠δ gF For the voltage during a power grid fault, i F (t) represents the VSG fault output current, Z f Z is the output filter impedance of the VSG. v Z is the virtual impedance of VSG. g Z is the power grid impedance. s =Z f +Z g =(R f +R g )+j(L f +L g Then, the differential equation can be obtained:

[0099]

[0100] Solving using the time-domain method yields:

[0101]

[0102] In the formula:

[0103]

[0104] As shown in equation (17), the VSG short-circuit current consists of two parts: a steady-state component and a transient component. Since the grid-type converter has good voltage support capability, the steady-state component I... F It can be seen that reducing the output voltage and power angle of the VSG can reduce the IF amplitude; and for the transient component of the short-circuit current, in addition to reducing I... FThe amplitude can also be increased by reducing the time constant L of the transient component. s / R s This is to accelerate the decay of the transient process.

[0105] Step S104: Calculate the adaptive active power reference and reshape the internal potential reference value of the VSG according to the degree of grid voltage drop;

[0106] It should be noted that, based on the analysis of transient power angle stability during VSG faults in step 2, without any protective measures, the system may experience power angle instability during both minor and severe voltage dips in the grid. Therefore, during a fault, the active power reference value P can be adjusted. ref Only by ensuring that the sum of the accelerating area and the decelerating area in equation (10) is always less than 0 can the power angle of the system be kept stable.

[0107] Therefore, during a fault, the active power reference value of the VSG is adaptively adjusted according to the degree of voltage drop in the grid. At this time, the system power angle curve under the adaptive active power reference adjustment strategy is as follows: Figure 9 As shown. Figure 9 (a) and (b) are the power angle dynamic response curves after adopting the improved control strategy when the grid voltage experiences Type I and Type II voltage drops, respectively. Figure 9 China P ref ’ This is the active power reference value after adjustment based on the degree of voltage drop.

[0108] Depend on Figure 9 It can be seen that when the system is in steady-state operation, it operates normally at point a. At the instant the grid voltage drops, assuming the system phase angle remains unchanged before and after the drop, the system operating point becomes point b on the fault power angle curve. Due to the adoption of adaptive active power reference value adjustment, P is satisfied after the fault. ref ’ =P e At this point, the acceleration / deceleration area is close to zero, and the system immediately reaches a new operating equilibrium point. Therefore, by adopting an adaptive active power reference value adjustment strategy, the P value of the system can be guaranteed at any given time. ref =P e This eliminates the problem of active power imbalance during faults and improves the power angle stability of the system.

[0109] Furthermore, in some embodiments, the adaptive active power reference value P ref ’ The calculation derivation process is as follows.

[0110] First, it is necessary to determine the power angle of the system under normal operating conditions, since P ref Since the rated power of the VSG is known, P ref =Pe Substituting into equation (8), the steady-state work angle can be obtained as follows:

[0111]

[0112] The new output power is calculated based on the degree of voltage drop in the power grid.

[0113]

[0114] Since the sudden change in power angle when the system fails is not considered, substituting equation (18) into equation (19) yields the following result.

[0115]

[0116] In summary, based on the requirements of national standards GB / T 19964—2012 and GB / T 19964—2024, the permissible range of grid voltage fluctuation is ±10%U. g Below 90% U g This is for low-voltage ride-through. Therefore, the adaptive active power reference adjustment expression is:

[0117]

[0118] Furthermore, active power adaptive control can effectively eliminate power imbalance and reduce the risk of power angle instability. When the voltage drop is too large, due to the voltage source characteristics of the VSG, the drop in its output voltage is less than the drop in the grid voltage, so the short-circuit current may still exceed the limit.

[0119] According to the current amplitude expression in equation (17), if the power angle does not change, reducing the internal potential of the VSG can reduce the short-circuit current amplitude. Therefore, reshaping the reference internal potential of the VSG during a fault can further suppress overcurrent. The principle of internal potential reshaping is as follows.

[0120] Using the grid voltage as the reference phasor, Figure 10 This illustrates the phasor relationship between voltage and current before and after a system fault, where E and U... g δ0 and δ0 represent the internal potential, grid voltage, and power angle of the VSG during normal operation, respectively. F ’ U gF Represents the internal potential and grid voltage during a VSG fault, ΔU and ΔU ’ The voltage difference between the internal potential and the grid voltage is |ΔU|, and the magnitude of the voltage difference is proportional to the output current of the VSG.

[0121] Because the above steps incorporate an active power adaptive strategy, the power angle δ remains unchanged before and after the fault. Figure 7It can be seen that when the grid voltage and the internal potential of the VSG drop, the value of |ΔU| decreases significantly, meaning that the reduction in the amplitude of the internal potential has a significant effect on suppressing overcurrent. Therefore, during the voltage drop period, the internal potential E of the reshaped VSG... F_ref * The goal is to ensure that the output current is less than the maximum allowable current of the system. At this point, the following relationship must be satisfied:

[0122]

[0123] Solving for:

[0124]

[0125] In the formula:

[0126]

[0127] Typically, the magnitude of the steady-state overcurrent during a system fault is m of the rated current. c times, then

[0128]

[0129] According to the requirements of national standards GB / T 19964—2012 and GB / T 19964—2024, substituting equation (24) into equation (23) can achieve the reshaping of the internal potential of the VSG during the fault. Simultaneously, to present the voltage support characteristics of the VSG, the larger internal potential reference value is retained.

[0130]

[0131] Step S105: Introduce dynamic virtual impedance and adjust the virtual impedance parameters according to the transient characteristics of the fault current.

[0132] It should be noted that while adjusting the adaptive active power and reshaping the VSG internal potential effectively suppresses the steady-state component of the short-circuit current, the effect on suppressing the inrush current during fault occurrence and transient clearing is not ideal. The inrush current is short in duration and highly impactful, easily triggering system protection actions. Analysis of equation (17) shows that the inrush current during the transient process is caused by a decaying aperiodic component. Therefore, to suppress the amplitude of the inrush current and accelerate the decay rate of the aperiodic component, a virtual impedance is introduced into the control system.

[0133] The introduction of virtual impedance suppresses overcurrent amplitude by changing the voltage difference between the VSG internal potential and the grid voltage.

[0134]

[0135] Among them, U dref U qrefTo introduce the internal potential of the VSG after the virtual impedance is introduced, R v With L v These are virtual resistance and virtual inductance, respectively, I d I q These are the VSG output currents after dq transformation.

[0136] The decay time constant of the aperiodic component after introducing virtual impedance is:

[0137]

[0138] Observing equation (17), it can be seen that the state of the short-circuit current is related to the time and phase of the fault occurrence, and has randomness. Considering the most severe case of short-circuit current, when I N When a short-circuit fault occurs at the peak or trough of the sinusoidal function, the short-circuit inrush current will reach its maximum value, occurring half a cycle after the fault occurs. At this time, there is...

[0139]

[0140] To ensure that the inrush current will not exceed the system's maximum allowable current value, the system's maximum allowable current is set to m of the rated current. t If the multiple is doubled, then there is

[0141]

[0142] Summarized as follows:

[0143]

[0144] Because Xg >> Rg in the transmission line, ρ X / R Since it is a constant value, considering the acceleration of the decay rate of the non-periodic component of the short-circuit current in the virtual impedance, we have Rv>Xg.

[0145] Therefore, dynamic virtual impedance is used to suppress transient overcurrent.

[0146]

[0147] X v =σ X / R R v (32)

[0148]

[0149] In the formula, n is the proportionality coefficient of the virtual impedance; I e I is the current threshold; L This is the actual output current of the converter; adjusting the value of the proportional coefficient n can limit the amplitude of the inrush current during a fault to a preset value.

[0150] The proportional gain n of the virtual impedance is determined by the most severe operating condition: when a three-phase symmetrical fault occurs in the system and the grid connection voltage drops to 0, the entire voltage drop of the VSG output is borne by the virtual impedance.

[0151]

[0152] Furthermore, based on the requirement that the short-circuit current must be limited to the maximum allowable range of the system due to the virtual impedance, the proportionality coefficient can be calculated.

[0153]

[0154] In summary, based on the aforementioned VSG overcurrent joint control method, firstly, the transient response characteristics of the grid-connected converter are analyzed based on its topology, and an analytical expression for the short-circuit current, including steady-state and transient components, is derived according to the short-circuit current generation mechanism. Secondly, during grid faults, the active power reference value is adaptively adjusted to dynamically compensate for power deficits, thereby suppressing power angle oscillations and enhancing transient power angle stability. Subsequently, the internal potential reference value of the converter is reconstructed based on the voltage drop depth during the fault, precisely constraining the amplitude of the steady-state component of the short-circuit current. Finally, a dynamic virtual impedance is introduced during the transient processes of fault initiation and clearance to effectively suppress the peak value of the transient component of the short-circuit current and accelerate its decay, achieving flexible current limiting.

[0155] like Figure 11 The image shows a VSG overcurrent joint control system according to an embodiment of the present invention. The system includes:

[0156] The grid connection model construction module 10 is used to construct the grid connection model and its control equations of the grid-connected system VSG, and calculate the active power and reactive power injected by VSG into the grid based on the grid connection model.

[0157] The constraint construction module 20 is used to construct a first active power constraint when the grid voltage experiences a slight drop, and to construct a second active power constraint when the grid voltage experiences a severe drop.

[0158] The component calculation module 30 is used to establish the equivalent fault circuit diagram of VSG and calculate the steady-state and transient components of VSG short-circuit current using the time-domain solution method based on the circuit parameters at the fault time.

[0159] The internal potential reshaping module 40 is used to calculate the adaptive active power reference and reshape the internal potential reference value of the VSG according to the degree of grid voltage drop.

[0160] The parameter adjustment module 50 is used to introduce dynamic virtual impedance and adjust the virtual impedance parameters according to the transient characteristics of the fault current.

[0161] In another aspect, the present invention also proposes a readable storage medium having stored one or more programs thereon, which, when executed by a processor, implement the above-described VSG overcurrent joint control method.

[0162] In another aspect, the present invention also proposes an electronic device, including a memory and a processor, wherein the memory is used to store a computer program, and the processor is used to execute the computer program stored in the memory to implement the above-mentioned VSG overcurrent joint control method.

[0163] Those skilled in the art will understand that the logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as a ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by, or in conjunction with, an instruction execution system, apparatus, or device (such as a computer-based system, a processor-included system, or other system that can fetch and execute instructions from, an instruction execution system, apparatus, or device). For the purposes of this specification, "computer-readable medium" can mean any means that can contain stored, communicated, propagated, or transmitted programs for use by, or in conjunction with, an instruction execution system, apparatus, or device.

[0164] More specific examples of computer-readable media (a non-exhaustive list) include: electrical connections (electronic devices) having one or more wires, portable computer disk drives (magnetic devices), random access memory (RAM), read-only memory (ROM), erasable and editable read-only memory (EPROM or flash memory), fiber optic devices, and portable optical disc read-only memory (CDROM). Furthermore, computer-readable media can even be paper or other suitable media on which the program can be printed, since the program can be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, interpreting, or otherwise processing as necessary, and then stored in computer memory.

[0165] It should be understood that various parts of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented using any one or a combination of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0166] While embodiments of the present invention have been described in detail above, it will be apparent to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations fall within the scope and spirit of the invention as set forth in the claims. Furthermore, the invention described herein may have other embodiments and can be implemented or carried out in various ways.

Claims

1. A VSG overcurrent joint control method, characterized in that, The method includes: Construct a grid-connected model and its control equations for a grid-connected system VSG, and calculate the active and reactive power injected by the VSG into the grid based on the grid-connected model; When the grid voltage experiences a slight drop, a first active power constraint condition is established; when the grid voltage experiences a severe drop, a second active power constraint condition is established. When the grid voltage experiences a slight drop, the P value of the VSG power angle curve... ref With P e Given the existence of intersection points, the expression for the first active power constraint is: ； in, The power angle at the stable equilibrium point during steady-state operation of the system. The power angle at the system's stable equilibrium point during a fault. The power angle at the unstable equilibrium point of the system after a system failure. This is the active power reference value for VSG. The active power injected into the grid by the VSG; When the grid voltage experiences a severe drop, the P value on the VSG power angle curve... ref With P e There is no intersection between them. Assume that VSG's P is at this time. ref With P e The difference between them is a certain value, so we can get: ； Let ∆ω = ω - ω N Therefore, the first derivative of the relative angular frequency can be solved as: ； Integrating both sides with respect to time t, we get: ； in, This is the difference between mechanical power and output power. This is the dynamic value of the work angle. The rated angular frequency of the power grid. This is the actual output angular frequency. This is the virtual damping coefficient of the VSG. The inertia coefficient of VSG; Establish the equivalent fault circuit diagram of VSG, and calculate the steady-state and transient components of VSG short-circuit current using the time-domain solution method based on the circuit parameters at the fault time. Calculate the adaptive active power reference and reshape the internal potential reference value of the VSG according to the degree of grid voltage drop; The internal potential of the VSG during a fault is reshaped according to the following formula: ； ； in, The internal potential of VSG, The system rated current, m c The magnitude of the steady-state overcurrent during the fault is a multiple of the rated current. The internal potential of VSG, This represents the voltage amplitude of the power grid during the fault. Voltage on the grid side before the fault The total impedance of the equivalent circuit; A dynamic virtual impedance is introduced, and the virtual impedance parameters are adjusted according to the transient characteristics of the fault current. The internal potential of the VSG after introducing virtual impedance is: ； in, This is the d-axis reference value for the output voltage. This is the q-axis reference value for the output voltage. The d-axis component of the internal potential of the VSG For Laplace variables, For virtual inductance, The output current of the VSG after d-conversion is... For virtual resistance, The rated angular frequency on the power grid side. The output current of the VSG after q transformation. The internal potential of the VSG is represented by the q-axis component. The decay time constant of the aperiodic component after introducing virtual impedance is: ； in, The decay time constant, This is the equivalent inductance value of the power grid. This is the equivalent resistance value of the power grid; Using dynamic virtual impedance to suppress transient overcurrent: ； ； in, To obtain the actual output current of the converter, For current threshold, The proportionality coefficient of the virtual impedance. This is a virtual inductance value. This is the virtual reactance-resistance ratio. Let d be the d-axis component of the inductor current. This represents the q-axis component of the inductor current. The scaling factor for the virtual impedance is calculated using the following formula: ； in, This is the maximum allowable current amplitude of the system.

2. The VSG overcurrent joint control method according to claim 1, characterized in that, The steps for constructing the grid-connected model and governing equations of the VSG network system include: Construct the governing equations based on the following formula: ； in, For virtual mechanical active power, This is the active power droop coefficient. The reference angular frequency of the VSG. The phase of the internal potential generated by the active power loop. The reactive power injected into the grid by the VSG. This is the reactive power reference value for VSG. This is the reactive power droop factor. The reference voltage amplitude of VSG. This is the actual output voltage of the VSG. The reactive inertia coefficient, This is the reference internal potential generated by the reactive power loop.

3. The VSG overcurrent joint control method according to claim 2, characterized in that, The steps of calculating the active and reactive power injected into the grid by the VSG based on the grid connection model include: The active and reactive power injected into the grid by the VSG can be calculated using the following formulas: ； in, This is the grid-side voltage. , These are the voltages on the d and q axes after Park transformation, respectively. , These represent the d-axis and q-axis currents after Park transformation, respectively. For the angle of attack.

4. The VSG overcurrent joint control method according to claim 3, characterized in that, The steps of establishing the equivalent fault circuit diagram of the VSG and calculating the steady-state and transient components of the VSG short-circuit current using the time-domain solution method based on the circuit parameters at the time of the fault include: Assuming the power grid fault occurs at time t = 0, the differential equation is: ； Solving using the time-domain method yields: ； In the formula: ； in, This represents the total inductance of the equivalent circuit. For VSG fault output current, This represents the total resistance of the equivalent circuit. This represents the output voltage value during the fault. The phase angle of the VSG output voltage during the fault period. The phase angle of the grid voltage during the fault. Let be the output current at time t during a VSG fault. The rated current amplitude before the fault occurred. This represents the phase of the current before the fault occurs. This refers to the port output voltage of the VSG. For grid impedance, Generate an internal potential for the VSG power loop. For steady-state components, This represents the initial phase of the short-circuit current after the fault.

5. The VSG overcurrent joint control method according to claim 4, characterized in that, The steps for calculating the adaptive active power reference include: The adaptive active power reference value is calculated using the following formula: ； in, This is an adaptive active power reference value.

6. A VSG overcurrent joint control system, used to implement the VSG overcurrent joint control method as described in any one of claims 1-5, characterized in that, The system includes: The grid connection model construction module is used to construct the grid connection model and its control equations of the grid-connected system VSG, and calculate the active power and reactive power injected by VSG into the grid based on the grid connection model. The constraint construction module is used to construct the first active power constraint when the grid voltage drops slightly, and the second active power constraint when the grid voltage drops severely. The component calculation module is used to establish the equivalent fault circuit diagram of VSG and calculate the steady-state and transient components of VSG short-circuit current using the time-domain solution method based on the circuit parameters at the time of the fault. The internal potential reshaping module is used to calculate the adaptive active power reference and reshape the internal potential reference value of the VSG according to the degree of grid voltage drop. The parameter adjustment module is used to introduce dynamic virtual impedance and adjust the virtual impedance parameters according to the transient characteristics of the fault current.

7. A readable storage medium, characterized in that, The readable storage medium stores one or more programs that, when executed by a processor, implement the VSG overcurrent joint control method as described in any one of claims 1-5.

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