A method and device for transient control of a grid-forming converter

Abstract

The application specifically discloses a grid-forming converter transient control method and device. After detecting that a fault occurs on a grid point system side, the application calculates a fault steady-state power reference value under an optimal transient voltage support target according to an equivalent voltage source and an equivalent line impedance of the grid point system side; and calculates a steady-state virtual impedance for correcting an internal potential of a bridge arm without exiting a power outer ring of a virtual synchronous control of the grid-forming converter, so as to quickly output the required steady-state power by skipping a slow characteristic of the outer ring. The application calculates a first time-varying virtual impedance after the fault occurs according to the steady-state virtual impedance and a transient virtual impedance for limiting an impact current. After detecting that the fault occurs, the application sets a power reference value of the grid-forming converter as the fault steady-state power reference value, and simultaneously increases the first time-varying virtual impedance. After the fault is cleared, the application restores the power reference value to a normal power reference value, and simultaneously adjusts the first time-varying virtual impedance to a second time-varying virtual impedance.

Description

Technical Field

[0001] The present application belongs to the field of new energy power generation technology, and more specifically, relates to a transient control method and device for a grid-type converter. Background Art

[0002] With the large-scale development of the new power system based on new energy, a large number of power electronic equipment have been introduced into the power grid, resulting in the grid showing the characteristics of low inertia damping and low short-circuit capacity support, and the problem of safe and stable operation of the power system has become prominent.

[0003] In order to improve the grid-connected converter's ability to support grid voltage / frequency, grid-forming (GFM) control methods, represented by virtual synchronous control, have become a research hotspot in recent years. Compared to traditional grid-connected converters, grid-connected converters under grid-forming control (hereinafter referred to as grid-forming converters) can enable power electronic converters to externally exhibit the characteristics of an AC synchronous voltage source, thereby supporting the system's inertia and voltage, while also providing greater stability when operating in weak grids. However, when the grid voltage drops significantly, the voltage source characteristics of the grid-forming converter can cause severe fault overcurrent, and power electronic devices can be easily damaged by excessive current stress. Therefore, in practical applications, grid-forming converters should adopt a current limiting strategy to prevent transient overcurrent.

[0004] Currently, there are two main methods for implementing fault current limiting in grid-connected converters. The first method involves switching from virtual synchronous control mode to traditional grid-following control mode during a significant drop in grid voltage at the point of connection due to an external system fault. However, this method causes the grid-connected converter to lose its voltage source characteristics and, consequently, its grid support capability, hindering stable operation in weak grids. The second method involves retaining or freezing the grid-connected converter's power outer loop during a fault, while simultaneously employing virtual impedance for current limiting. However, freezing the power outer loop causes the grid-connected converter to lose its regulation capability during the fault, while retaining the power outer loop hinders the ability of the grid-connected converter to quickly reach its target steady-state value during the fault, making it difficult to meet the reactive power support response speed requirements of grid standards and actual systems.

[0005] Therefore, how to solve the problem in the related art that the grid-forming converter loses its grid-forming capability or fast reactive power response capability during transient periods is a problem that needs to be solved urgently. Summary of the Invention

[0006] In response to the defects of the existing technology, the purpose of this application is to provide a transient control method and device for a grid-type converter, aiming to solve the problem in the related technology that the grid-type converter loses its grid-forming capability or fast reactive power response capability during transient periods.

[0007] To achieve the above objectives, in a first aspect, the present application provides a transient control method for a grid-type converter, comprising:

[0008] When a fault is detected on the grid connection point system side, determining a fault steady-state power reference value under an optimal transient voltage support target based on an equivalent voltage source and an equivalent line impedance on the grid connection point system side after the fault occurs;

[0009] Determine the steady-state virtual impedance without exiting the virtual synchronous control power outer loop of the grid-connected converter;

[0010] determining a first time-varying virtual impedance according to the steady-state virtual impedance and a transient virtual impedance capable of limiting a maximum transient impact current when a fault occurs;

[0011] After a fault occurs on the grid-connected point system side, the power reference value of the grid-connected converter is set to the fault steady-state power reference value, and a first time-varying virtual impedance is added to the grid-connected converter;

[0012] After the fault on the grid connection point system side is cleared, the power reference value is restored to a normal power reference value, and the first time-varying virtual impedance is adjusted to a second time-varying virtual impedance.

[0013] Further preferably, when a fault is detected on the grid connection point system side, the fault steady-state power reference value under the optimal transient voltage support target is determined using the following formula:

[0014]

[0015] Among them, U gF is the equivalent voltage source on the system side of the grid connection point after the fault occurs, Z gF is the equivalent line impedance on the system side of the grid connection point after the fault occurs, I N is the rated current of the grid-type converter, K stm is the long-term steady-state current carrying capacity of the grid-type converter, U tF is the grid-connected point voltage of the grid-connected converter after the fault occurs, j is the imaginary part, P refF is the fault steady-state active power reference value under the optimal transient voltage support target, Q refF It is a fault steady-state reactive power reference value under the optimal transient voltage support target, and the fault steady-state power reference value includes a fault steady-state active power reference value and a fault steady-state reactive power reference value.

[0016] Further preferably, without exiting the virtual synchronous control power outer loop of the grid-connected converter, the steady-state virtual impedance is determined using the following formula:

[0017]

[0018] Among them, Z vst is the steady-state virtual impedance, U gF is the equivalent voltage source on the system side of the grid connection point after the fault occurs, Z gF is the equivalent line impedance on the system side of the grid connection point after the fault occurs, K stm I N is the long-term steady-state current withstand value of the grid-type converter, E is the internal potential of the virtual synchronous control power outer loop output of the grid-type converter before the fault occurs, and Z f is the filter impedance of the grid-type converter, U gF is the equivalent voltage source, Z gF is the equivalent line impedance, I N is the rated current of the grid-type converter. Further preferably, the transient virtual impedance is determined by the following formula:

[0019]

[0020] Among them, Z vtr is the transient virtual impedance, E is the internal potential of the virtual synchronous control power outer loop output of the grid-type converter before the fault occurs, U gF is the equivalent voltage source, Z gF is the equivalent line impedance, Z f is the filter impedance of the grid-type converter, I N is the rated current of the grid-type converter, K trm is the transient impact current withstand coefficient of the converter.

[0021] Further preferably, determining the first time-varying virtual impedance according to the steady-state virtual impedance and a transient virtual impedance capable of limiting a maximum transient impact current when a fault occurs includes:

[0022] The first time-varying virtual impedance is determined using the following formula:

[0023]

[0024] Where τ1 is the time constant of virtual impedance transformation when a fault occurs, Z v1 is the first time-varying virtual impedance, Z vtr is the transient virtual impedance, Z vst is the steady-state virtual impedance, t1 is the fault occurrence time, and t is the current time.

[0025] Further preferably, the second time-varying virtual impedance is determined using the following formula:

[0026]

[0027] Among them, Z v2is the second time-varying virtual impedance, τ2 is the virtual impedance transformation time constant after the fault is cleared, t2 is the fault clearing time, Z vre To restore the virtual impedance, t is the current time;

[0028] Restore virtual impedance Z vre , the following formula must be satisfied:

[0029]

[0030] Where E is the internal potential of the virtual synchronous control power outer loop output of the grid-connected converter before the fault occurs, U g0 is the normal grid voltage, Z g is the normal line impedance, Z f is the filter impedance of the grid-type converter, K stm I N is the long-term steady-state current carrying capacity of the grid-type converter, K trm I N It is the transient impulse withstand current value of the grid-type converter.

[0031] In a second aspect, the present application provides a grid-type converter transient control device, comprising:

[0032] A first acquisition module is configured to determine, when a fault is detected on the grid connection point system side, a fault steady-state power reference value under an optimal transient voltage support target based on an equivalent voltage source and an equivalent line impedance on the grid connection point system side after the fault occurs;

[0033] A second acquisition module is used to determine the steady-state virtual impedance without exiting the virtual synchronous control power outer loop of the grid-connected converter;

[0034] a third acquisition module, configured to determine a first time-varying virtual impedance according to the steady-state virtual impedance and a transient virtual impedance capable of limiting a maximum transient impact current when a fault occurs;

[0035] A control module is used to set the power reference value of the grid-type converter to the fault steady-state power reference value after a fault occurs on the grid-connected point system side, and to increase the first time-varying virtual impedance in the grid-type converter.

[0036] In a third aspect, the present application provides an electronic device comprising: at least one memory for storing programs; and at least one processor for executing the programs stored in the memory. When the programs stored in the memory are executed, the processor is used to execute the method described in the first aspect or any possible implementation of the first aspect.

[0037] In a fourth aspect, the present application provides a computer-readable storage medium, which stores a computer program. When the computer program runs on a processor, the processor executes the method described in the first aspect or any possible implementation of the first aspect.

[0038] In a fifth aspect, the present application provides a computer program product, which, when executed on a processor, enables the processor to execute the method described in the first aspect or any possible implementation of the first aspect.

[0039] In general, the above technical solutions conceived by this application have the following beneficial effects compared with the existing technologies:

[0040] The method and device for improving the transient support capability of a grid-type converter provided in this application can retain the outer loop control of the grid-type converter during a fault, thereby retaining its grid-type converter capability and voltage source characteristics, which is beneficial to its stable operation in a weak power grid.

[0041] The method and device for improving the transient support capability of a grid-type converter provided in this application calculates the fault steady-state power reference value under the optimal transient voltage support target based on the amplitude, phase and line impedance information of the external equivalent voltage source after the fault, including P refF and Q refF , thereby achieving the optimal support effect on the grid connection point voltage under the premise that the current amplitude is limited during the fault steady state.

[0042] The method and device for improving the transient support capability of a grid-type converter provided in the present application introduce time-varying virtual impedance in the fault occurrence stage and the fault recovery stage, thereby limiting transient impact current on the one hand and correcting the phase and amplitude of the potential in the converter bridge arm on the other hand. This can skip the slow characteristic of the outer loop at the early stage of the fault, allowing the grid-type converter to quickly output the required steady-state power. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 1 is a flow chart of a transient control method for a grid-type converter provided in an embodiment of the present application;

[0044] Figure 2 This is a structural diagram of the main circuit and control system of the grid-type converter provided in an embodiment of the present application;

[0045] Figure 3 This is a phasor diagram of an equivalent circuit of a grid-type converter during steady state before and after a fault, provided by an embodiment of the present application;

[0046] Figure 4 This is a schematic diagram of the output current of the grid-type converter provided in an embodiment of the present application after a three-phase grounding fault, according to a given fault steady-state power reference value according to the grid standard;

[0047] Figure 5 Schematic diagram of the output current of the grid-type converter provided in an embodiment of the present application after a three-phase grounding fault using a fault steady-state power reference value under an optimal transient voltage support target;

[0048] Figure 6 This is a schematic diagram of the grid connection point voltage of the grid-connected converter provided in an embodiment of the present application after a three-phase grounding fault, with a given fault steady-state power reference value according to the grid standard;

[0049] Figure 7 1. A schematic diagram of the grid connection point voltage of the grid-connected converter provided in an embodiment of the present application using a fault steady-state power reference value under an optimal transient voltage support target after a three-phase grounding fault;

[0050] Figure 8 This is a schematic diagram of the output current of the grid-type converter provided in an embodiment of the present application after a three-phase grounding fault, in which the grid-type converter does not input virtual impedance during the fault period;

[0051] Figure 9 This is a schematic diagram of the output current of the grid-type converter provided by an embodiment of the present application after a three-phase grounding fault, using a fixed virtual impedance that meets the transient impulse current limiting requirements;

[0052] Figure 10 This is a schematic diagram of the output current of the grid-type converter provided by an embodiment of the present application after a three-phase grounding fault, using a fixed virtual impedance that meets the active / reactive power response requirements;

[0053] Figure 11 This is a schematic diagram of the output current of the grid-type converter provided by an embodiment of the present application after a three-phase grounding fault occurs and the grid-type converter adopts the time-varying virtual impedance of the embodiment of the present application;

[0054] Figure 12 This is a schematic diagram of the output power of the grid-type converter provided in an embodiment of the present application after a three-phase grounding fault occurs and the grid-type converter does not use virtual impedance;

[0055] Figure 13 This is a schematic diagram of the output power of the grid-type converter provided by an embodiment of the present application after a three-phase grounding fault, in which the grid-type converter adopts a fixed virtual impedance that meets the transient impulse current limiting requirements;

[0056] Figure 14 This is a schematic diagram of the output power of the grid-type converter provided by an embodiment of the present application after a three-phase grounding fault, in which the grid-type converter adopts a fixed virtual impedance that meets the active / reactive power response requirements;

[0057] Figure 15is a schematic diagram of output power of the grid-forming converter after a three-phase grounding fault, according to an embodiment of the present application;

[0058] Figure 16 is a structural schematic diagram of the transient control device of the grid-forming converter, according to an embodiment of the present application;

[0059] Figure 17 is a structural schematic diagram of the electronic device, according to an embodiment of the present application. DETAILED DESCRIPTION

[0060] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and should not be used to limit the present application.

[0061] The term "and / or" used herein is used to describe an association relationship of associated objects, which means that there can be three relationships, for example, A and / or B can mean that there are three cases of A alone, A and B together, and B alone. The symbol " / " in this paper represents the relationship of or, for example, A / B represents A or B.

[0062] In the embodiments of the present application, the words "exemplary" or "for example" are used to represent as an example, illustration or description. Any embodiment or design scheme described as "exemplary" or "for example" in the embodiments of the present application should not be interpreted as more preferred or more advantageous than other embodiments or design schemes. Rather, the words "exemplary" or "for example" are used to present the relevant concept in a specific manner.

[0063] The embodiments of the present application will be described below with reference to the accompanying drawings in the embodiments of the present application.

[0064] Referring to Figure 1 The grid-forming converter transient control method provided by the embodiments of the present application can include steps 110, 120, 130, 140 and 150.

[0065] Step 110, in the case of detecting that a fault occurs on the grid point system side, determining a fault steady-state power reference value under an optimal transient voltage support target according to an equivalent voltage source and an equivalent line impedance of the grid point system side after the fault occurs;

[0066] Step 120, determining a steady-state virtual impedance without exiting the virtual synchronous control power outer ring of the grid-forming converter;

[0067] Step 130: Determine a first time-varying virtual impedance based on the steady-state virtual impedance and a transient virtual impedance capable of limiting a maximum transient impact current when a fault occurs.

[0068] Step 140: After a fault occurs on the grid-connected point system side, the power reference value of the grid-connected converter is set to the fault steady-state power reference value, and a first time-varying virtual impedance is added to the grid-connected converter;

[0069] Step 150: After the fault on the grid connection point system side is cleared, the power reference value is restored to a normal power reference value, and the first time-varying virtual impedance is adjusted to a second time-varying virtual impedance.

[0070] In a specific implementation, in step 110, it is possible to determine whether a fault occurs on the system side of the grid connection point based on the grid connection point voltage of the grid-connected converter. For example, when the grid connection point voltage is less than or equal to 0.9 pu (per unit), it indicates that a fault occurs on the system side of the grid connection point.

[0071] When a fault is detected on the grid connection point system side, the fault steady-state power reference value under the optimal transient voltage support target can be calculated based on the equivalent voltage source and equivalent line impedance on the grid connection point system side after the fault occurs.

[0072] The fault steady-state power reference value includes a fault steady-state active power reference value and a fault steady-state reactive power reference value.

[0073] In step 120, a steady-state virtual impedance is calculated without exiting the virtual synchronous control power outer loop of the grid-connected converter. This steady-state virtual impedance can be used to achieve phase and amplitude compensation, bypassing the slow characteristics of the outer loop and quickly outputting the required steady-state power.

[0074] In step 130, a transient virtual impedance capable of limiting the inrush current may be calculated based on the expression of the maximum transient inrush current at the initial stage of the fault, and the first time-varying virtual impedance after the fault occurs may be calculated in combination with the above-mentioned steady-state virtual impedance.

[0075] In step 140, after a fault is detected on the grid-connected point system side, the power reference value of the grid-connected converter is set to the fault steady-state power reference value under the optimal transient voltage support target obtained above, and the first time-varying virtual impedance is added to the grid-connected converter.

[0076] In step 150, after the fault on the grid-connected point system side is cleared, the power reference value is restored to the normal power reference value (ie, the power reference value when the grid-connected converter operates normally), and the first time-varying virtual impedance is adjusted to the second time-varying virtual impedance.

[0077] The transient control method of the grid-type converter provided in the embodiment of the present application, after detecting that a fault occurs on the grid-connected point system side, calculates the fault steady-state power reference value under the optimal transient voltage support target based on the equivalent voltage source and equivalent line impedance on the grid-connected point system side; and without exiting the virtual synchronous control power outer loop of the grid-type converter, calculates the steady-state virtual impedance to correct the potential in the bridge arm, skips the slow characteristics of the outer loop and quickly outputs the required steady-state power; calculates the first time-varying virtual impedance after the fault occurs based on the steady-state virtual impedance and the transient virtual impedance that can limit the impact current. After detecting that a fault occurs, the power reference value of the grid-type converter is set to the fault steady-state power reference value, and at the same time, the first time-varying virtual impedance is added. The present application quickly achieves the optimal voltage support for the grid-connected point voltage without exiting the power outer loop and retaining the transient networking capability of the grid-type converter.

[0078] Furthermore, in some embodiments, in step 110, when a fault is detected on the grid connection point system side, the following formula is used to determine the fault steady-state power reference value under the optimal transient voltage support target:

[0079]

[0080] Among them, U gF is the equivalent voltage source on the system side of the grid connection point after the fault occurs, Z gF is the equivalent line impedance on the system side of the grid connection point after the fault occurs, I N is the rated current of the grid-type converter, K stm is the long-term steady-state current carrying capacity of the grid-type converter, U tF is the grid-connected point voltage of the grid-connected converter after the fault occurs, j is the imaginary part, P refF is the fault steady-state active power reference value under the optimal transient voltage support target, Q refF It is a fault steady-state reactive power reference value under the optimal transient voltage support target, and the fault steady-state power reference value includes a fault steady-state active power reference value and a fault steady-state reactive power reference value.

[0081] In a specific implementation, in step 110, the following formula is used to calculate the fault steady-state power reference value under the optimal transient voltage support target, including the fault steady-state active power reference value under the optimal transient voltage support target and the fault steady-state reactive power reference value under the optimal transient voltage support target:

[0082]

[0083] Where U gF is the equivalent voltage source on the system side of the grid connection point after the fault occurs, Z gF is the equivalent line impedance on the system side of the grid connection point after the fault occurs, I Nis the rated current of the grid-type converter, K stm is the long-term steady-state current carrying capacity of the grid-type converter, U tF is the grid-connected point voltage of the grid-connected converter after the fault occurs, j is the imaginary part, P refF is the fault steady-state active power reference value under the optimal transient voltage support target, Q refF It is a fault steady-state reactive power reference value under the optimal transient voltage support target, and the fault steady-state power reference value includes a fault steady-state active power reference value and a fault steady-state reactive power reference value.

[0084] The transient control method of the grid-connected converter provided in the embodiment of the present application calculates the fault steady-state power reference value under the optimal transient voltage support target based on the equivalent voltage source and equivalent line impedance on the grid-connected point system side after the fault occurs, thereby achieving the optimal support effect for the grid-connected point voltage under the premise that the current amplitude is limited during the fault steady-state period.

[0085] Furthermore, in step 120, without exiting the virtual synchronous control power outer loop of the grid-connected converter, the steady-state virtual impedance is determined using the following formula:

[0086]

[0087] Among them, Z vst is the steady-state virtual impedance, U gF is the equivalent voltage source on the system side of the grid connection point after the fault occurs, Z gF is the equivalent line impedance on the system side of the grid connection point after the fault occurs, K stm I N is the long-term steady-state current withstand value of the grid-type converter, E is the internal potential of the virtual synchronous control power outer loop output of the grid-type converter before the fault occurs, and Z f is the filter impedance of the grid-type converter, U gF is the equivalent voltage source, Z gF is the equivalent line impedance, I N is the rated current of the grid-type converter.

[0088] In the specific implementation, the internal potential E of the virtual synchronous control power outer loop output of the grid-connected converter before the fault occurs and the equivalent voltage source U on the grid-connected point system side after the fault occurs can be used to calculate the internal potential E of the virtual synchronous control power outer loop output of the grid-connected converter before the fault occurs and the equivalent voltage source U on the grid-connected point system side after the fault occurs. gF , equivalent line impedance Z gF , filter impedance Z of grid-type converter f The long-term steady-state current withstand value K of the grid-connected converter stm I N , calculate the steady-state virtual impedance Z by the following formula vst :

[0089]

[0090] Where, is the steady-state virtual impedance, I N is the rated current of the grid-type converter, K stm is the long-term steady-state current carrying capacity of the converter.

[0091] The transient control method of the grid-type converter provided in the embodiment of the present application can retain the outer loop control of the grid-type converter during a fault, thereby retaining its grid-type converter's grid-forming capability and voltage source characteristics, which is beneficial to its stable operation in a weak power grid.

[0092] Furthermore, in some embodiments, in step 130, the transient virtual impedance is determined using the following formula:

[0093]

[0094] Among them, Z vtr is the transient virtual impedance, E is the internal potential of the virtual synchronous control power outer loop output of the grid-type converter before the fault occurs, U gF is the equivalent voltage source, Z gF is the equivalent line impedance, Z f is the filter impedance of the grid-type converter, I N is the rated current of the grid-type converter, K trm is the transient impact current withstand coefficient of the converter.

[0095] In the specific implementation, the transient virtual impedance that can limit the surge current is calculated based on the expression of the maximum transient surge current at the initial stage of the fault. The expressions of the transient surge current and its estimated upper limit are shown in the following formula:

[0096]

[0097] Among them, t is the current time, i Fst (t) is the assumption that the internal potential E of the virtual synchronous control power outer loop remains unchanged and the transient virtual impedance Z is always input. vtr The fault steady-state current, i0 is the current value at the moment before the fault occurs, i Fst0 is the initial value of the steady-state current component immediately after the fault occurs, i Fmax The upper limit of the maximum impact current (satisfying the fault occurrence time i0 and i Fst0 are located at the peak and trough of their own sine wave and do not consider the attenuation of the free component), τ F is the decay time constant of the free component of the fault current, i F (t) is the transient impulse current.

[0098] According to the above formula, since the instantaneous value of the output current of the grid-type converter cannot change suddenly before and after the fault occurs, the transient impact current i output by the grid-type converter after the fault occurs is F (t) will induce two parts, namely the steady-state current component and the attenuated DC bias. In the most serious case, i F (t) may reach several times the magnitude of the fault steady-state current. Therefore, although the steady-state virtual impedance can limit the current of the grid-type converter after the fault enters the steady state, it is difficult to suppress the transient impact current in the early stage of the fault.

[0099] Transient virtual impedance Z that can limit the maximum inrush current vtr The following expression must be satisfied:

[0100]

[0101] Among them, K trm I N K is the instantaneous impulse withstand current value of the grid-type converter, trm is the transient impulse withstand coefficient of the grid-type converter, the transient virtual impedance Z vtr is any value in the set of vectors that satisfies the above expression.

[0102] Furthermore, in some embodiments, in step 130, determining the first time-varying virtual impedance based on the steady-state virtual impedance and the transient virtual impedance capable of limiting the maximum transient impact current when a fault occurs may include:

[0103] The first time-varying virtual impedance is determined using the following formula:

[0104]

[0105] Where τ1 is the time constant of virtual impedance transformation when a fault occurs, Z v1 is the first time-varying virtual impedance, Z vtr is the transient virtual impedance, Z vst is the steady-state virtual impedance, t1 is the fault occurrence time, and t is the current time.

[0106] In the specific implementation, the steady-state virtual impedance Z vst , transient virtual impedance Z vtr , fault occurrence time t1 and virtual impedance transformation time constant τ1 when the fault occurs, calculate the first time-varying virtual impedance, as shown in the following formula:

[0107]

[0108] Where t is the current time, and τ1 can be set according to different scenarios. Taking the wind turbine converter as an example, half of the reactive current exit time (60ms) required by the grid standard can be taken (30ms). v1 is the first time-varying virtual impedance.

[0109] The first time-varying virtual impedance Z proposed in the embodiment of the present application v1 The initial value is Z vtr , which can limit the transient surge current and transform it into Z within τ1 vtr Realize the actual internal potential U output of the bridge arm of the grid-connected converter c Rapid correction of the phase and amplitude (at this time, the potential E inside the virtual synchronous control power outer loop output can be approximately considered unchanged due to the slow response characteristics of the virtual synchronous control).

[0110] Furthermore, in some embodiments, in the above steps, the second time-varying virtual impedance is determined using the following formula:

[0111]

[0112] Among them, Z v2 is the second time-varying virtual impedance, τ2 is the virtual impedance transformation time constant after the fault is cleared, t2 is the fault clearing time, Z vre To restore the virtual impedance;

[0113] Restore virtual impedance Z vre , the following formula must be satisfied:

[0114]

[0115] Where E is the internal potential of the virtual synchronous control power outer loop output of the grid-connected converter before the fault occurs, U g0 is the normal grid voltage, Z g is the normal line impedance, Z f is the filter impedance of the grid-type converter, K stm I N is the long-term steady-state current carrying capacity of the grid-type converter, K trm I N It is the transient impulse withstand current value of the grid-type converter.

[0116] In a specific implementation, when the grid-connected point voltage of the grid-connected converter recovers to above 0.9 pu (i.e., the fault on the grid-connected point system side has been cleared), the transient control is exited, and the fault steady-state active power reference value under the optimal transient voltage support target and the fault steady-state reactive power reference value under the optimal transient voltage support target are respectively modified to the power reference value during normal operation of the grid-connected converter (i.e., the normal power reference value), and the first time-varying virtual impedance is adjusted to the second time-varying virtual impedance. The expression of the second time-varying virtual impedance is shown as follows:

[0117]

[0118] Among them, Z v2 is the second time-varying virtual impedance, Z vre To restore the virtual impedance, τ2 is the virtual impedance transformation time constant after the fault is cleared. It can be set according to different scenarios. Taking the wind turbine converter as an example, it can be set to half (20 ms) of the reactive current exit time (40 ms) required by the grid standard.

[0119] Restore virtual impedance Z vre The following expression must be satisfied:

[0120]

[0121] The transient control method for a grid-type converter provided in an embodiment of the present application introduces a first time-varying virtual impedance and a second time-varying virtual impedance in the fault occurrence stage and the fault recovery stage, thereby limiting the transient impact current on the one hand and correcting the phase and amplitude of the potential in the bridge arm of the grid-type converter on the other hand. This method can skip the slow characteristic of the outer loop at the early stage of the fault, allowing the grid-type converter to quickly output the required steady-state power and achieve rapid reactive power support.

[0122] For example, the embodiment of the present application takes a single grid-type converter connected to a weak power grid as an example. The weak power grid adopts a single-machine infinite model, which can be equivalent to a voltage source U according to the Thevenin theorem. g Series line impedance Z g In the form of, its main circuit topology and control are as follows Figure 2 As shown in the figure. Among them, the equivalent circuit phasor diagram of the grid-type converter during the steady state before and after the fault is as follows Figure 3 shown.

[0123] Figure 2 Middle,U dc is a DC voltage; the grid-type converter is connected to the common connection point through a filter circuit, U t is the grid-connected point voltage of the grid-connected converter, U c is the actual internal potential of the bridge arm output of the grid-type converter, i o-abc are the three-phase currents output by the converter; Z fis the filter impedance of the grid-type converter. The grid-type converter adopts typical virtual synchronous control, including active-frequency control link and reactive-voltage control link: p ref and p e are active power command and converter output active power respectively; Q ref and Q e are the reactive power command and the converter output reactive power respectively; ω c and ω n are the virtual synchronous control output angular velocity and the reference angular velocity respectively; D p is the damping coefficient; K is the reactive loop integral coefficient; J is the virtual inertia.

[0124] The line parameters of the grid-connected system of the grid-connected converter are shown in Table 1, and the control parameters are shown in Table 2.

[0125] Table 1

[0126]

[0127]

[0128] Table 2

[0129] parameter Numerical <![CDATA[有功-相位控制参数(J、D p )]]> 0.61pu32.3pu Reactive-amplitude control parameter K 0.22pu

[0130] Build in Matlab / Simulink simulation platform Figure 2 The simulation conditions of the model shown are: a three-phase short-circuit grounding fault occurs near the grid-connected point of the grid-connected converter at t = 3s, and Z f The size is set to 0.5+j0.314Ω, and t=7s fault recovery.

[0131] In order to further illustrate the steady-state voltage support effect of the transient control method of the grid-type converter proposed in the embodiment of the present application, a comparative explanation is given below in combination with two specific scenarios. The embodiment of the present application uses the following two scenarios to illustrate the voltage support effect of the proposed grid-type converter crossing strategy after the fault enters the steady state, which is mainly affected by the steady-state power reference value. Therefore, the following two scenarios both use the virtual impedance proposed in the embodiment of the present application for current limiting, and use different fault steady-state power reference values ​​P refF , Q refF .

[0132] Scenario I: Fault steady-state power reference value P of grid-connected converter during fault refF1 and Q refF1 Strictly follow the grid connection guidelines:

[0133]

[0134] Among them, U tFis the voltage at the point of common coupling (PCC) after the fault occurs; U N is the rated voltage of the grid-forming converter, S N is the rated capacity of the grid-forming converter, K q is taken as 2.

[0135] Scenario II: the power reference value of the grid-forming converter during the fault is the steady-state active / reactive power reference value P refF2 / Q refF2 that is,

[0136]

[0137] Referring to Figure 4 , and further referring to Figure 5 , Figure 4 and Figure 5 are schematic diagrams of the output current of Scenario I and Scenario II during the fault, respectively, and it can be seen that the current amplitudes of both during the steady state of the fault are limited to K stm I N (51.6A in peak value).

[0138] Figure 6 and Figure 7 are schematic diagrams of the voltage at the point of common coupling (PCC) of Scenario I and Scenario II under the fault, respectively, and it can be seen that under the condition of the same output current amplitude, the voltage amplitude support effect of the grid-forming converter transient control method provided in the embodiments of the present application is better, which shows that the grid-forming converter limited output current capacity is more fully utilized.

[0139] To further illustrate the voltage support response speed and impact current limiting effect of the grid-forming converter transient control method proposed in the embodiments of the present application in the initial stage of the fault, the following four specific scenarios are compared and described. The voltage support response speed and impact current limiting effect of the grid-forming converter transient control method proposed in the embodiments of the present application in the initial stage of the fault are mainly illustrated by the following four scenarios, which are mainly affected by the virtual impedance input, so the following four scenarios all input different virtual impedances with the steady-state power reference value proposed in the present application.

[0140] Scenario I: the grid-forming converter does not input virtual impedance during the fault;

[0141] Scenario II: the grid-forming converter adopts a fixed virtual impedance that meets the instantaneous impact current limiting requirement during the fault;

[0142] Scenario III: the grid-forming converter adopts a fixed virtual impedance that meets the active / reactive power response requirement during the fault;

[0143] Scenario IV: the grid-forming converter adopts the time-varying virtual impedance provided in the embodiments of the present application during the fault.

[0144] See also Figures 8-11 , and see further Figure 12-15 .in, Figures 8-11 Schematic diagram of output current under fault conditions for scenarios I to IV. Figure 12-15 The following is a schematic diagram of the output power under fault conditions for scenarios I to IV. It can be seen that the maximum surge current in scenario I exceeds 5 p.u. after the fault occurs, far exceeding the instantaneous surge current withstand value K of the grid-type converter. trm I N (The peak value is 64.5A). In scenario II, although the impact current after the fault occurs is limited to a safe range, it is affected by the slow characteristics of the power loop and the reactive support response time reaches more than several seconds, which is difficult to meet the requirements of the grid standard or the actual system. In scenario III, although the active / reactive power response time meets the requirements of the grid standard (less than 60ms) and the current does not exceed the limit during the fault steady state, at the initial stage of the fault, due to the existence of the DC bias of the fault current, the impact current exceeds the instantaneous impact withstand current value K of the grid-type converter. trm I N In scenario IV, since the proposed time-varying virtual impedance was put into use when the fault occurred, the post-fault inrush current did not exceed the limit, and the reactive power response time was less than 60ms, meeting the grid standard requirements.

[0145] In summary, the embodiments of the present application have the following advantages compared to the prior art:

[0146] The transient control method of the grid-type converter provided in the embodiment of the present application can retain the outer loop control of the grid-type converter during a fault, thereby retaining its grid-type converter's grid-forming capability and voltage source characteristics, which is beneficial to its stable operation in a weak power grid.

[0147] The transient control method of the grid-connected converter provided in the embodiment of the present application calculates the fault steady-state power reference value under the optimal transient voltage support target based on the equivalent voltage source and equivalent line impedance on the system side of the grid-connected point after the fault, thereby achieving the optimal support effect on the grid-connected point voltage under the premise that the current amplitude is limited during the fault steady-state period.

[0148] The transient control method for a grid-type converter provided in an embodiment of the present application introduces a time-varying virtual impedance during the fault occurrence stage and the fault recovery stage, thereby limiting the transient impact current on the one hand and correcting the phase and amplitude of the potential within the converter bridge arm on the other hand. This method can skip the slow characteristic of the outer loop at the early stage of the fault, allowing the grid-type converter to quickly output the required steady-state power.

[0149] The grid-type converter transient control device provided by the present invention is described below. The grid-type converter transient control device described below and the grid-type converter transient control method described above can be referenced to each other.

[0150] See also Figure 16 An embodiment of the present application provides a grid-type converter transient control device, which may specifically include: a first acquisition module 1610, a second acquisition module 1620, a third acquisition module 1630, a first control module 1640 and a second control module 1650.

[0151] The first acquisition module 1610 is configured to determine, when a fault is detected on the grid connection point system side, a fault steady-state power reference value under an optimal transient voltage support target based on an equivalent voltage source and an equivalent line impedance on the grid connection point system side after the fault occurs;

[0152] A second acquisition module 1620 is configured to determine a steady-state virtual impedance without exiting the virtual synchronous control power outer loop of the grid-connected converter;

[0153] A third acquisition module 1630 is configured to determine a first time-varying virtual impedance based on the steady-state virtual impedance and a transient virtual impedance capable of limiting a maximum transient inrush current when a fault occurs;

[0154] A first control module 1640 is configured to set the power reference value of the grid-connected converter to the fault steady-state power reference value after a fault occurs on the grid-connected point system side, and to add a first time-varying virtual impedance to the grid-connected converter;

[0155] The second control module 1650 is configured to restore the power reference value to a normal power reference value and adjust the first time-varying virtual impedance to a second time-varying virtual impedance after the fault on the grid-connected point system side is cleared.

[0156] The transient control device for the grid-type converter provided in the embodiment of the present application, after detecting a fault on the grid-connected point system side, calculates the fault steady-state power reference value under the optimal transient voltage support target based on the equivalent voltage source and equivalent line impedance on the grid-connected point system side; and without exiting the virtual synchronous control power outer loop of the grid-type converter, calculates the steady-state virtual impedance for correcting the potential in the bridge arm, skipping the slow characteristics of the outer loop to quickly output the required steady-state power; and calculates the first time-varying virtual impedance after the fault occurs based on the steady-state virtual impedance and the transient virtual impedance that can limit the impact current. After detecting the occurrence of a fault, the power reference value of the grid-type converter is set to the fault steady-state power reference value, and at the same time, the first time-varying virtual impedance is added. The present application quickly achieves the optimal voltage support for the grid-connected point voltage without exiting the power outer loop and retaining the transient networking capability of the grid-type converter.

[0157] It is understandable that the detailed functional implementation of each of the above units / modules can be found in the introduction of the aforementioned method embodiment, and will not be repeated here.

[0158] It should be understood that the above device is used to execute the method in the above embodiment, the corresponding program module in the device, the implementation principle and technical effect are similar to the description in the above method, the working process of the device can refer to the corresponding process in the above method, and details are not described here.

[0159] Based on the method in the above embodiment, the embodiment of the present application provides an electronic device, as shown in the figure, which can include: a processor (Processor) 1710, a communication interface (Communications Interface) 1720, a memory (Memory) and a communication bus 1740, wherein the processor 1710, the communication interface 1720, the memory 1730 complete mutual communication through the communication bus 1740. The processor 1710 can call the logical instructions in the memory 1730 to execute the method in the above embodiment. Figure 17

[0160] In addition, the logical instructions in the memory 1730 described above can be realized in the form of a software function unit and sold or used as an independent product, which can be stored in a computer readable storage medium. Based on such understanding, the technical solutions of the present application can be embodied in the form of a software product or part of the existing technology, and the computer software product is stored in a storage medium, including a plurality of instructions for making a computer device (which can be a personal computer, a server, or a network device, etc.) execute all or part of the steps of the method described in the embodiments of the present application.

[0161] Based on the method in the above embodiment, the embodiment of the present application provides a computer readable storage medium, and the computer readable storage medium stores a computer program, when the computer program runs on the processor, so that the processor executes the method in the above embodiment.

[0162] Based on the method in the above embodiment, the embodiment of the present application provides a computer program product, when the computer program product runs on the processor, so that the processor executes the method in the above embodiment.

[0163] ​It is understood that the processor in the embodiments of the present application may be a central processing unit (CPU), or may be other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field programmable gate arrays (FPGA), or other programmable logic devices, transistor logic devices, hardware components, or any combination thereof. The general-purpose processor may be a microprocessor or any conventional processor.

[0164] The method steps in the embodiments of the present application can be implemented by hardware or by a processor executing software instructions. The software instructions can be composed of corresponding software modules, and the software modules can be stored in random access memory (RAM), flash memory, read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), registers, hard disks, mobile hard disks, CD-ROMs or any other form of storage medium well known in the art. An exemplary storage medium is coupled to the processor so that the processor can read information from the storage medium and write information to the storage medium. Of course, the storage medium can also be a component of the processor. The processor and the storage medium can be located in an ASIC.

[0165] In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware or any combination thereof. When implemented using software, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the process or function described in the embodiment of the present application is generated in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted via the computer-readable storage medium. The computer instructions can be transmitted from one website, computer, server or data center to another website, computer, server or data center via a wired (e.g., coaxial cable, optical fiber, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) method. The computer-readable storage medium can be any available medium that a computer can access or a data storage device such as a server or data center that includes one or more available media integrations. The available medium can be a magnetic medium (e.g., a floppy disk, a hard disk, a tape), an optical medium (e.g., a DVD), or a semiconductor medium (e.g., a solid-state drive (SSD)).

[0166] It will be understood that the various numerical numbers involved in the embodiments of the present application are merely distinctions for the convenience of description and are not intended to limit the scope of the embodiments of the present application.

[0167] It is easy for those skilled in the art to understand that the above is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A transient control method for a grid-type converter, characterized in that: include: When a fault is detected on the grid connection point system side, determining a fault steady-state power reference value under an optimal transient voltage support target based on an equivalent voltage source and an equivalent line impedance on the grid connection point system side after the fault occurs; Determine the steady-state virtual impedance without exiting the virtual synchronous control power outer loop of the grid-connected converter; determining a first time-varying virtual impedance according to the steady-state virtual impedance and a transient virtual impedance capable of limiting a maximum transient impact current when a fault occurs; After a fault occurs on the grid-connected point system side, the power reference value of the grid-connected converter is set to the fault steady-state power reference value, and a first time-varying virtual impedance is added to the grid-connected converter; Determining a first time-varying virtual impedance according to the steady-state virtual impedance and a transient virtual impedance capable of limiting a maximum transient impact current when a fault occurs includes: The first time-varying virtual impedance is determined using the following formula: in, is the virtual impedance transformation time constant when a fault occurs, is the first time-varying virtual impedance, is the transient virtual impedance, is the steady-state virtual impedance, is the time when the fault occurred, is the current time.

2. The transient control method of the grid-type converter according to claim 1, characterized in that: When a fault is detected on the grid-connected point system side, the following formula is used to determine the fault steady-state power reference value under the optimal transient voltage support target: in, is the equivalent voltage source on the system side of the grid connection point after the fault occurs, is the equivalent line impedance on the system side of the grid connection point after the fault occurs, is the rated current of the grid-type converter, is the long-term steady-state current carrying capacity of the grid-type converter, is the grid-connected point voltage of the grid-connected converter after a fault occurs, is the imaginary part, is the fault steady-state active power reference value under the optimal transient voltage support target, It is a fault steady-state reactive power reference value under the optimal transient voltage support target, and the fault steady-state power reference value includes a fault steady-state active power reference value and a fault steady-state reactive power reference value.

3. The transient control method of the grid-type converter according to claim 1, characterized in that: Without exiting the virtual synchronous control power outer loop of the grid-connected converter, the steady-state virtual impedance is determined using the following formula: in, is the steady-state virtual impedance, is the equivalent voltage source on the system side of the grid connection point after the fault occurs, is the equivalent line impedance on the system side of the grid connection point after the fault occurs, is the long-term steady-state current carrying capacity of the grid-type converter, is the internal potential of the virtual synchronous control power outer loop output of the grid-type converter before the fault occurs, is the filter impedance of the grid-type converter, is the equivalent voltage source, is the equivalent line impedance, is the rated current of the grid-type converter.

4. The transient control method of the grid-type converter according to claim 1, wherein: The transient virtual impedance is determined using the following formula: in, is the transient virtual impedance, is the internal potential of the virtual synchronous control power outer loop output of the grid-type converter before the fault occurs, is the equivalent voltage source, is the equivalent line impedance, is the filter impedance of the grid-type converter, is the rated current of the grid-type converter, is the transient impact current withstand coefficient of the converter.

5. The transient control method for a grid-type converter according to any one of claims 1 to 4, characterized in that: Also includes: After the fault on the grid connection point system side is cleared, the power reference value is restored to the normal power reference value, and the first time-varying virtual impedance is adjusted to the second time-varying virtual impedance, which is determined by the following formula: in, is the second time-varying virtual impedance, is the time constant of virtual impedance transformation after fault clearing, is the fault clearing time, To restore the virtual impedance, is the current time; Restoring virtual impedance , the following formula must be satisfied: Where, is the internal potential of the virtual synchronous control power outer loop output of the grid-type converter before the fault occurs, is the normal grid voltage, is the normal line impedance, is the filter impedance of the grid-type converter, is the long-term steady-state current carrying capacity of the grid-type converter, It is the transient impulse withstand current value of the grid-type converter.

6. A transient control device for a grid-type converter, characterized in that: include: A first acquisition module is configured to determine, when a fault is detected on the grid connection point system side, a fault steady-state power reference value under an optimal transient voltage support target based on an equivalent voltage source and an equivalent line impedance on the grid connection point system side after the fault occurs; A second acquisition module is used to determine the steady-state virtual impedance without exiting the virtual synchronous control power outer loop of the grid-connected converter; a third acquisition module, configured to determine a first time-varying virtual impedance according to the steady-state virtual impedance and a transient virtual impedance capable of limiting a maximum transient impact current when a fault occurs; A first control module is configured to set the power reference value of the grid-connected converter to the fault steady-state power reference value after a fault occurs on the grid-connected point system side, and to add a first time-varying virtual impedance to the grid-connected converter; Determining a first time-varying virtual impedance according to the steady-state virtual impedance and a transient virtual impedance capable of limiting a maximum transient impact current when a fault occurs includes: The first time-varying virtual impedance is determined using the following formula: in, is the virtual impedance transformation time constant when a fault occurs, is the first time-varying virtual impedance, is the transient virtual impedance, is the steady-state virtual impedance, is the time when the fault occurred, is the current time.

7. An electronic device, characterized in that: include: at least one memory for storing a computer program; At least one processor is used to execute the program stored in the memory, and when the program stored in the memory is executed, the processor is used to execute the method according to any one of claims 1 to 5.

8. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is run on a processor, the processor is caused to perform the method according to any one of claims 1 to 5.

9. A computer program product, characterized in that When the computer program product is run on a processor, the processor is caused to perform the method according to any one of claims 1 to 5.

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