A current limiting method and system for grid-connected converter based on adaptive virtual impedance

Abstract

The present invention discloses a current limiting method and system for a grid-type converter based on adaptive virtual impedance, the method comprising: when the effective value of the output current of the grid-type converter exceeds an overcurrent threshold value I th When the overcurrent fault occurs, a virtual impedance is added for current limiting. The resistance part of the virtual impedance is used to quickly suppress the surge current component at the beginning of the overcurrent fault; the reactance part of the virtual impedance is used to accurately limit the effective value of the current in the transient stage of the overcurrent fault to the current limit value. This method is applicable to various reactive power and voltage control methods, can effectively suppress the fault current, and accurately limit the current to the current limit value. Its control structure is simple, does not rely on grid-side information, does not require complex fault detection, mode switching and scenario judgment, does not require changing control parameters or control methods according to the fault type, and is applicable to a variety of working conditions in grid-connected and island modes.

Description

Technical Field

[0001] The invention belongs to the technical field of converter control, and in particular is a current limiting method for a grid-type converter based on adaptive virtual impedance. Background Art

[0002] In order to realize a stable new energy grid-connected system, the control design problem of the grid-connected converter should be fully solved. At present, there are two basic control types of grid-connected converters: grid-building type and grid-following type. Among them, the converter using grid-following type control cannot support the grid voltage and cannot operate independently off-grid, making it difficult to adapt to the "double high" characteristic development trend of the future power system. Therefore, the grid-building converter with voltage source characteristics, which can be applied to a variety of working conditions and provide voltage and frequency support for the power system, has become the focus of research.

[0003] Typical grid-type converter control structures are as follows: Figure 1 For active power-frequency control, typical control schemes include basic droop control, generalized droop control, virtual synchronous generator (VSG) control, synchronizer control, power synchronization control, virtual oscillator control, etc.

[0004] For reactive power-voltage and inner loop control, typical control schemes include control based on reactive power loop, control based on voltage / current dual loop, and control based on voltage RMS single loop. Among them, control based on voltage / current dual loop is widely used due to its better current regulation capability, higher power quality, and better compatibility with grid following control. However, the model complexity of this control scheme is high and it requires more controllers to work together. Its control structure is as follows Figure 2 As shown. Compared with the voltage / current dual-loop control, the grid-type converter based on reactive power single-loop control has better parameter robustness, lower model complexity, smaller bus voltage offset and more balanced reactive power distribution; the grid-type converter based on voltage RMS single-loop control has a larger small signal stability margin and a simpler control structure. The control structures of the two are shown as follows: Figure 3 , 4As shown. However, the above two single-loop control schemes cannot limit the output current by limiting the current command due to the lack of current loop. Therefore, overcurrent protection has become a key issue that hinders the further promotion of grid-type converters based on reactive power single-loop control and voltage RMS single-loop control. Since the grid-type converter is essentially a voltage source, it may be subject to external interference and cause overcurrent faults, such as ground faults, grid voltage sags and overloads. Therefore, it is crucial for the grid-type converter to have overcurrent protection capabilities. Existing research has proposed some overcurrent protection methods for grid-type converters. For grid-type converters based on voltage / current dual-loop control, the fault current can be suppressed by limiting the current reference value. Although this method is easy to implement, due to the saturation of the integrator during the fault, the voltage loop may lose its regulation function, causing the grid-type converter to transform into a grid-following converter during the current limiting period, thereby losing its grid-type capability. In addition, due to the loss of the external control loop during the fault and the potential saturation of the internal loop after the fault is cleared, the system may lose stability. The most important thing is that this type of current limiting method is not suitable for control schemes based on reactive power single loop or voltage RMS single loop. Another widely used current limiting method is based on virtual impedance, which has the ability to quickly limit current and is also applicable to different inner loop control schemes. However, the current limiting method based on virtual impedance is either unable to accurately limit the overcurrent amplitude during the fault process, or requires the grid-side information to perform complex control and calculation to achieve mode switching, making it difficult to apply in practice.

[0005] In addition, most of the existing studies design grid-connected converter current limiting methods for one or several specific fault scenarios, and few studies propose current limiting methods applicable to any given fault condition. Therefore, it is urgent to propose a grid-connected converter current limiting method based on virtual impedance that can be applied to multiple working conditions in grid-connected and islanded modes. Summary of the invention

[0006] In order to overcome the shortcomings of the above-mentioned prior art, a current limiting method and system for grid-connected converters based on adaptive virtual impedance is proposed. This method can effectively suppress fault current and accurately limit the current to the current limit value. Its control structure is simple, does not rely on grid-side information, does not require complex fault detection, mode switching and scenario judgment, and is applicable to various working conditions in grid-connected and island modes.

[0007] To achieve the above object, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a current limiting method for a grid-type current transformer based on adaptive virtual impedance, comprising:

[0009] When the output current of the grid-connected converter exceeds the overcurrent threshold I thWhen , virtual impedance is added to limit the current;

[0010] The added resistance portion of the virtual impedance is used to quickly suppress the surge current component at the beginning of the overcurrent fault (i.e., the sub-transient stage);

[0011] The reactance part of the added virtual impedance is used to accurately limit the effective value of the current in the transient stage of the overcurrent fault (ie, the quasi-steady-state stage after the surge current decays) to the current limit value.

[0012] As a further improvement of the present invention, the adaptive virtual impedance current limiting is applicable to various reactive power and voltage control methods, including but not limited to the following three:

[0013] Method 1 is a grid-type control based on voltage and current double closed loops, which includes QV droop control and voltage and current loops;

[0014] Method 2 is a control method based on reactive power single loop, which includes VQ droop control and reactive power loop control;

[0015] Method three is a control method based on a voltage effective value single loop, which mainly includes QV droop control and a voltage effective value single loop.

[0016] As a further improvement of the present invention, the adaptive virtual impedance current limiting method is applied to various reactive power and voltage control methods, including but not limited to:

[0017] When the adaptive virtual impedance current limiting method is applied to voltage and current dual closed-loop control, in the αβ coordinate system or dq coordinate system, the voltage drop V across the virtual impedance is calculated by the output current and the virtual impedance. Zv , then use the virtual internal potential E to subtract the voltage drop V across the virtual impedance Zv The inverter output voltage command value V ref , thereby performing voltage loop control;

[0018] When the adaptive virtual impedance current limiting method is applied to reactive power single-loop or voltage RMS single-loop control, the voltage drop V across the virtual impedance is calculated by the output current and the virtual impedance in the αβ coordinate system or the dq coordinate system. Zv , then use the virtual internal potential E to subtract the voltage drop V across the virtual impedance Zv The modulation voltage V pwm ;

[0019] As a further improvement of the present invention, the adaptive virtual impedance current limiting method, the resistance part of the added virtual impedance (i.e., the sub-transient current limiting virtual resistor R subtran ) is generated through a two-dimensional fuzzy controller (i.e., a subtransient fuzzy controller).

[0020] As a further improvement of the present invention, the adaptive virtual impedance current limiting method, the reactance part of the added virtual impedance (i.e., the transient current limiting virtual reactance X tran ) is generated through a two-dimensional fuzzy PI controller (i.e., transient fuzzy PI controller).

[0021] As a further improvement of the present invention, the fuzzy controller of the resistance part of the virtual impedance is used to generate a sub-transient current limiting virtual resistance R according to the fault current characteristics by setting fuzzy control rules in the sub-transient stage of the overcurrent fault and utilizing the rapidity of fuzzy control. subtran To suppress the surge current component, after the surge current component is suppressed, the sub-transient current limiting virtual resistor R subtran Quickly adjust to zero.

[0022] As a further improvement of the present invention, the fuzzy PI controller for generating the reactance part of the virtual impedance generates a transient current limiting virtual reactance X by setting a fuzzy control rule in the transient stage of the overcurrent fault. tran The effective value of the fault current is accurately limited to the current limit value I lim .

[0023] As a further improvement of the present invention, the fuzzy controller of the resistance part of the virtual impedance generation (i.e., the sub-transient fuzzy controller) calculates the output current I in the αβ coordinate system or the dq coordinate system. out , output current change rate dI out / dt and output current I out and the current limit value I lim The error between Iout ;

[0024] When the output current I out Does not exceed the overcurrent threshold I th When the output current I out Exceeding the overcurrent threshold I th When dI out / dt and ε Iout As the two input variables of the subtransient fuzzy controller, through the fuzzification, fuzzy reasoning and defuzzification processing of the subtransient fuzzy controller, the subtransient current limiting virtual resistor R is finally output. subtran .

[0025] As a further improvement of the present invention, the fuzzy PI controller (i.e., transient fuzzy PI controller) for generating the reactance part of the virtual impedance calculates the output current I in the αβ coordinate system or the dq coordinate system. out , output current change rate dI out / dt and output current I out and the current limit value Ilim The error between Iout ;

[0026] When the output current I out Does not exceed the overcurrent threshold I th When the input and output variables of the transient fuzzy PI controller are both zero. out Exceeding the overcurrent threshold I th When ε Iout As the input variable of the PI controller, the transient current limiting virtual reactance X tran As the output variable of the PI controller. out / dt and ε Iout As the two input variables of the transient fuzzy controller, the compensation value ΔK of the proportional coefficient of the PI controller is output after fuzzification, fuzzy reasoning and defuzzification of the transient fuzzy controller. p_X Compensation amount ΔK with integral coefficient I_X Combining it with the original proportional coefficient and integral coefficient of the PI controller, a transient fuzzy PI controller is formed.

[0027] In a second aspect, the present invention provides a current limiting method and system for a grid-type converter based on adaptive virtual impedance, comprising:

[0028] Sampling module, used to sample output voltage, output current,

[0029] The calculation module is used to perform αβ transformation or dq transformation on the output current, calculate the output current, the output current change rate, and the error between the output current and the current limit value.

[0030] The state judgment module is used to judge whether the system enters the current limiting mode according to the relationship between the sampled output current and the threshold current.

[0031] The virtual impedance module is used to generate a sub-transient current limiting virtual resistance and a transient current limiting virtual reactance according to fuzzy control and fuzzy PI control, and then convert the virtual electromotive force E and the voltage drop V across the virtual impedance into a virtual impedance in the αβ coordinate system or the dq coordinate system. Zv Subtract and get the output voltage command value V of the converter ref Or modulation voltage V pwm .

[0032] An electronic device, characterized in that it comprises a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein when the processor executes the computer program, the steps of the adaptive virtual impedance current limiting method applicable to a grid-type converter are implemented;

[0033] A computer-readable storage medium, characterized in that the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of the adaptive virtual impedance current limiting method applicable to a grid-type converter are implemented.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The present invention is guided by a grid-type converter overcurrent protection method that can be generally applied to different working conditions and can accurately limit current, and proposes a grid-type converter current limiting method based on adaptive virtual impedance. This method is applicable to various reactive power and voltage control methods, can effectively suppress fault current, and accurately limit the current to the current limit value. Its control structure is simple, does not rely on grid-side information, does not require complex fault detection, mode switching and scenario judgment, does not require changing control parameters or control methods according to fault types, and is applicable to a variety of working conditions in grid-connected and island modes. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0037] Figure 1 This is the general control structure diagram of the grid-type converter;

[0038] Figure 2 The structure diagram of the grid-type converter based on voltage and current double closed-loop control is shown;

[0039] Figure 3 This is the structure diagram of a grid-type converter based on reactive power single-loop control;

[0040] Figure 4 The structure diagram of the grid-type converter adopting single-loop control based on voltage effective value;

[0041] Figure 5 1 is a structural diagram of a P-ω control part according to an embodiment of the present invention;

[0042] Figure 6 This is a structural diagram of a voltage-current dual closed-loop control according to an embodiment of the present invention;

[0043] Figure 7 This is a structural diagram of a reactive power single-loop control based on an embodiment of the present invention;

[0044] Figure 8This is a structural diagram of a single-loop control based on voltage effective value according to an embodiment of the present invention;

[0045] Fig. 9 A basic structural diagram of fuzzy control according to an embodiment of the present invention;

[0046] Fig.10 This is a control block diagram of an adaptive virtual impedance according to an embodiment of the present invention; (a) a sub-transient fuzzy controller; (b) a transient fuzzy PI controller;

[0047] Fig.11 A control surface of a transient fuzzy controller according to an embodiment of the present invention;

[0048] Fig.12 is the control surface of the transient fuzzy PI controller according to an embodiment of the present invention, (a) is the compensation amount ΔK of the proportional coefficient p_X With dI out / dt and ε Iout The relationship between (b) and (d) is the compensation value of the integral coefficient ΔK. I_X With dI out / dt and ε Iout relationship;

[0049] Fig.13 A simulation verification scenario for an embodiment of the present invention;

[0050] Fig.14 The current limiting effect of an embodiment of the present invention is compared with that of the existing virtual impedance current limiting method. (a) is the current limiting simulation result when a single machine is connected to the grid and a voltage drop of 0.95 pu occurs; (b) is the current limiting simulation result when a ground fault occurs when a single machine is operating in an isolated island with load; (c) is the current limiting simulation result when a load suddenly increases by 1.5 pu when a single machine is operating in an isolated island with load;

[0051] Fig.15 The current limiting simulation results of an embodiment of the present invention when a voltage drop of 0.95 pu occurs when a single machine is connected to the grid under different grid strengths;

[0052] Fig.16 The current limiting simulation result of a ground fault when a single machine island is running with different power factor loads according to an embodiment of the present invention;

[0053] Fig.17 A block diagram of a grid-type converter current limiting system based on adaptive virtual impedance is provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0054] In order to make the purpose and technical solution of the present invention clearer and easier to understand, the technical solution in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is only a part of the embodiment of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0055] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0056] Terminology explanation:

[0057] E——virtual internal potential; V pwm ——modulation voltage;

[0058] V out ——output voltage; X ls ,X f ,X line ——Virtual reactance, filter reactance, line

[0059] P0——active power command; road reactance;

[0060] P in ——virtual shaft power; R subtran ,X tran ——Sub-transient virtual current limiting resistor, temporary

[0061] P out ——Output active power; State virtual current limiting reactance;

[0062] P load ——load active power; ω0——rated angular frequency;

[0063] S base ——Rated capacity; ω m ——Virtual rotor angular frequency

[0064] V bus ——bus voltage; Iout ——output current;

[0065] The present invention proposes a current limiting method for a grid-type converter based on adaptive virtual impedance, comprising: when the effective value of the output current of the grid-type converter exceeds the overcurrent threshold value I th When the overcurrent fault occurs, a virtual impedance is added to limit the current; the resistance part of the added virtual impedance is used to quickly suppress the surge current component at the beginning of the overcurrent fault (i.e., the sub-transient stage); the reactance part of the added virtual impedance is used to accurately limit the effective value of the current in the transient stage of the overcurrent fault (i.e., the quasi-steady-state stage after the surge current attenuates) to the current limit value.

[0066] Furthermore, the present invention proposes using fuzzy control to generate the resistance part of the virtual impedance, and using a combination of fuzzy control and PI control to generate the reactance part of the virtual impedance, and can be applicable to a variety of working conditions. Currently, there is no research involving the use of fuzzy control or the use of fuzzy control in combination with PI control to generate virtual impedance for current limiting.

[0067] Because the grid-type converter has voltage and frequency support capabilities, it is a key solution for achieving new power systems with high renewable energy penetration. However, since the grid-type converter is essentially a voltage source, it may be affected by external interference and cause overcurrent faults, such as ground faults, grid voltage sags, and overloads. Therefore, it is crucial for the grid-type converter to have overcurrent protection capabilities. Existing studies have proposed some overcurrent protection methods for grid-type converters, but they all have certain defects, such as: the need for additional hardware circuits, resulting in poor economic performance and increased costs; the need for complex control methods and scene monitoring and mode switching, resulting in complex control structures and poor practical applicability; the need to know the grid-side information in advance, resulting in limited scope of application; the inability to accurately limit overcurrent, resulting in the inability to output reactive power according to specific requirements, etc. At the same time, most of the existing studies design grid-type converter current limiting methods for one or several specific fault scenarios, and few studies propose current limiting methods applicable to any given fault conditions.

[0068] The present invention is guided by a grid-type converter overcurrent protection method that can be generally applied to different working conditions and can accurately limit current, and proposes a grid-type converter current limiting method based on adaptive virtual impedance. This method is applicable to various reactive power and voltage control methods, can effectively suppress fault current, and accurately limit the current to the current limit value. Its control structure is simple, does not rely on grid-side information, does not require complex fault detection, mode switching and scenario judgment, does not require changing control parameters or control methods according to fault types, and is applicable to a variety of working conditions in grid-connected and island modes.

[0069] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings and specific implementation methods. It should be understood that the preferred embodiments are only for illustrating the present invention, rather than for limiting the protection scope of the present invention.

[0070] The present invention specifically adopts the following technical solutions:

[0071] (1) Adopting grid-type control based on voltage and current double closed loop or reactive power single loop or voltage effective value single loop

[0072] Typical control schemes for grid-connected converters are as follows: Figure 1 The P-ω control used in one embodiment of the present invention is as shown in FIG. Figure 5 As shown, it is based on VSG control, including a speed regulator, a virtual inertia and a virtual damping term based on state feedback control.

[0073] The QV control part includes QV droop and voltage-current double closed-loop control (such as Figure 2 ), or including VQ droop and reactive power loop-based control (as shown in Figure 3 ), or including QV droop and voltage RMS single-loop control (as shown in Figure 4 shown).

[0074] Since the power line between the converter and the public bus of the power grid is mainly resistive and inductive, power coupling problems will occur. Therefore, it is necessary to add a virtual inductor L ls The line impedance characteristics are adjusted to make it inductive, thereby achieving decoupling control between active and reactive power.

[0075] When the present invention is applied to voltage and current dual closed-loop control, in the αβ coordinate system or the dq coordinate system, the voltage drop V across the virtual impedance is calculated by the output current and the virtual impedance. Zv , then use the virtual internal potential E to subtract the voltage drop V across the virtual impedance Zv The inverter output voltage command value V ref , so as to perform voltage loop control, and its control structure is as follows Figure 6 When the present invention is applied to reactive power single-loop control or voltage effective value single-loop control, in the αβ coordinate system or the dq coordinate system, the voltage drop V across the virtual impedance is calculated by the output current and the virtual impedance. Zv , then use the virtual internal potential E to subtract the voltage drop V across the virtual impedance Zv The modulation voltage V pwm , and its control structures are as follows Figure 7 , 8 shown.

[0076] (2) Current limiting method using adaptive virtual impedance

[0077] The basic principle of the virtual impedance current limiting method is: when the effective value of the grid-connected converter output current exceeds the overcurrent threshold I th When the virtual impedance is increased, the overall impedance of the system is increased to achieve the effect of suppressing overcurrent. According to the expression of fault current, the overcurrent is divided into surge current component and periodic current component.

[0078] Among them, the surge current component is difficult to suppress because of its large peak value and fast growth rate. In order to solve this problem, the high robustness, wide adaptability and fast response of fuzzy control and its independence from the accurate model of the controlled system can be used to combine fuzzy control in the virtual impedance current limiting process, so that it can quickly respond to the growth of overcurrent and generate different virtual impedances according to the characteristics of the fault current. Secondly, in the stage where the surge current component exists, increasing the virtual resistance without increasing the virtual inductance can greatly accelerate the attenuation rate of the surge current component, thereby more effectively suppressing the surge current. For periodic current components, the existing virtual impedance current limiting method can only suppress overcurrent, but cannot accurately limit the current to the current limit value. In order to solve this problem, fuzzy control can be combined with PI control. By setting fuzzy control rules, fuzzy control can be used to optimize the parameters of the PI controller in the transient stage of the overcurrent fault, so as to accurately limit the periodic current component to the current limit value.

[0079] Therefore, the present invention focuses on the rapidity of virtual resistance intervention in current limiting during the fault subtransient period, and focuses on the accuracy of virtual reactance intervention in current limiting during the fault transient period. The resistance part of the added virtual impedance is used to quickly suppress the surge current component at the beginning of the overcurrent fault (i.e., the subtransient stage); the reactance part of the added virtual impedance is used to accurately limit the effective value of the current in the transient stage of the overcurrent fault (i.e., the quasi-steady stage after the surge current decays) to the current limit value.

[0080] (3) Fuzzy controller structure

[0081] The basic structure of the fuzzy controller used in one embodiment of the present invention is as follows: Fig. 9 As shown, considering the control accuracy and computational complexity, a two-dimensional fuzzy controller is selected to generate the resistance part of the virtual impedance, that is, the sub-transient fuzzy controller generates the sub-transient current limiting virtual resistance R subtran ; Select another two-dimensional fuzzy PI controller to generate the reactance part of the virtual impedance, that is, the transient fuzzy PI controller generates the transient current limiting virtual reactance X tran .

[0082] In the αβ coordinate system or dq coordinate system, calculate the output current I out , output current change rate dI out / dt and output current I outand the current limit value I lim The error between Iout Since the current increases rapidly during the sub-transient phase of the fault, dI out / dt value is large and has large fluctuations, which may lead to detection errors. out / dt needs to be multiplied by the attenuation factor K dI To reduce the impact of detection errors.

[0083] The sub-transient fuzzy controller is used to suppress the surge current component at the beginning of the fault. Its control block diagram is as follows: Fig.10 (a). When the output current I out Does not exceed the overcurrent threshold I th When the output current I out Exceeding the overcurrent threshold I th When dI out / dt and ε Iout As the two input variables of the subtransient fuzzy controller, through the fuzzification, fuzzy reasoning and defuzzification processing of the subtransient fuzzy controller, the subtransient current limiting virtual resistor R is finally output. subtran ;

[0084] The transient fuzzy PI controller is used to accurately limit the periodic current component after the surge current is attenuated to the current limit value. The control block diagram is as follows: Fig.10 (b) As shown. When the output current I out Does not exceed the overcurrent threshold I th When the input and output variables of the transient fuzzy PI controller are both zero. out Exceeding the overcurrent threshold I th When ε Iout As the input variable of the PI controller, the transient current limiting virtual reactance X tran As the output variable of the PI controller. out / dt and ε Iout As the two input variables of the transient fuzzy controller, the compensation value ΔK of the proportional coefficient of the PI controller is output after fuzzification, fuzzy reasoning and defuzzification of the transient fuzzy controller. p_X Compensation amount ΔK with integral coefficient I_X Combining it with the original proportional coefficient and integral coefficient of the PI controller, a transient fuzzy PI controller is formed.

[0085] (4) Fuzzy controller design

[0086] ① For the fuzzy controller of the resistor part that generates the virtual impedance, the rapidity of the virtual resistor's intervention in current limiting should be considered. Based on this, in the sub-transient stage of the overcurrent fault, the rapidity of fuzzy control is used to generate a sub-transient current limiting virtual resistor R that changes with the output current characteristics. subtran .

[0087] In order to improve the control accuracy, five fuzzy subsets {NB, NM, ZO, PM, PB} are selected in the fuzzification process. {NB} represents dI out / dt or ε Iout is a negative value with a large absolute value, which means that I out is rapidly decaying, or I out Much lower than I lim , {NM} represents dI out / dt or ε Iout is a negative value with a small absolute value, which means that I out is slowly decaying, or I out Slightly lower than I lim . {ZO} means dI out / dt or ε Iout The change is very small, which means that the system has entered a quasi-steady state, that is, I out Close to I lim {PM} means dI out / dt or ε Iout In the slow increase, that is, at this time I out Slowly growing, or slightly larger than I lim {PB} means dI out / dt or ε Iout In rapid increase, that is, at this time I out is growing rapidly, or is much larger than I lim .

[0088] Four fuzzy subsets {ZO, S, M, P} are selected in the defuzzification process. {ZO} represents R subtran is zero, {S} means R subtran is a smaller value, {M} represents R subtran is a medium value, {P} means R subtran is a larger value. out / dt or ε Iout When it is small, only a small subtransient virtual resistance is needed to suppress the surge current, so R subtran belongs to {S}; when dI out / dt or ε Iout When it is larger, a larger subtransient virtual resistance is required to suppress the surge current, so R subtran belongs to {M}; for more severe overcurrent conditions, R subtran Belongs to {P}. When dIout / dt and ε Iout When both are negative, it indicates that the surge current has decayed to zero or the output current is much lower than the current limit value. At this time, R subtran , R subtran It should belong to {ZO}. In the quasi-steady state, dI out / dt is very small, ε Iout Close to zero, R is no longer needed at this time subtran , at this time R subtran It also belongs to {ZO}. Based on the above discussion, we can formulate R as shown in Table 1 subtran The fuzzy control rule of the subtransient fuzzy controller is as follows: Fig.11 shown.

[0089] Table 1R subtran Fuzzy control rules

[0090]

[0091]

[0092] ② For the fuzzy controller that generates the reactance part of the virtual impedance, the accuracy of the virtual impedance intervention current limiting should be considered during the transient period of the fault. Therefore, the fuzzy-PI controller is used in the transient stage of the overcurrent fault to generate the compensation amount ΔK of the proportional coefficient according to the output current. p_X Compensation amount ΔK with integral coefficient I_X , and then generate the transient current limiting virtual reactance X through the PI controller tran .

[0093] In the fuzzification process, three fuzzification subsets {N, ZO, P} are selected, and in the defuzzification process, three defuzzification subsets {N, ZO, P} are selected. Their meanings are the same as those in the above analysis. out / dt and ε Iout When it belongs to {ZO}, it means that the system has entered a quasi-steady state, that is, I out Close to I lim At this time, we only need to change ΔK p_X and ΔK I_X To optimize the response speed and control accuracy of the PI controller. In other cases, keep ΔK p_X and ΔK I_X Belongs to {ZO}. Based on this, ΔK can be formulated as shown in Table 2 p_X and ΔK I_X The fuzzy control rules of the transient fuzzy controller are as follows: Fig.12 Through such a setting, a smooth transition from the sub-transient stage to the transient stage can be achieved.

[0094] Table 2ΔK p_X and ΔK I_X Fuzzy control rules

[0095]

[0096] Fig.11 is the control surface of the sub-transient fuzzy controller, which represents the proportional coefficient R subtran With dI out / dt and ε Iout relationship.

[0097] Fig.12 is the control surface of the transient fuzzy controller, (a) represents the compensation of the proportional coefficient ΔK p_X With dI out / dt and ε Iout (i.e. I_ error ), (b) represents the compensation amount ΔK of the proportional coefficient I_X With dI out / dt and ε Iout relationship.

[0098] Fig.13 It is a simulation verification scenario. Fig.13 (a) is a simulation verification scenario of the present invention under the condition of single machine grid connection. Fig.13 (b) is a simulation verification scenario of the present invention under the condition of single-machine island load.

[0099] Fig.14 The current limiting effect of the overcurrent protection method proposed by the present invention is compared with that of the existing virtual impedance current limiting method. Fig.14 (a) is the current limiting simulation result of the present invention when a voltage drop of 0.95pu occurs under single-machine grid-connected conditions. Fig.14 (b) is the current limiting simulation result of the present invention when a ground fault occurs during single-machine island load operation. Fig.14 (c) is the current limiting simulation result of the present invention when the load suddenly increases by 1.5pu when a single machine is operating in an isolated island. (The current limiting value I in the three cases lim Both 1.5pu)

[0100] Fig.15 The current limiting simulation results of the present invention when a voltage drop of 0.95 pu occurs when a single machine is connected to the grid under different grid strengths. (Current limit value I lim =1.5pu)

[0101] Fig.16 The current limiting simulation results of the present invention when a ground fault occurs when a single machine island is running with different power factor loads. (Current limit value I lim =1.5pu).

[0102] The present invention combines fuzzy control and virtual impedance to propose a current limiting method for grid-connected converters with wide applicability. The method is proposed based on the actual VSG system operating conditions. The method can effectively suppress the fault current and accurately limit the current to the current limit value. The control structure of the present invention is simple, does not depend on the grid impedance parameters, does not require complex fault detection, mode switching and scenario judgment, and is suitable for various working conditions in grid-connected and island modes. Its current limiting effect is as follows: Fig.14 , 15 , as shown in 16. The overcurrent protection method proposed in the present invention can provide a reference for the overcurrent protection method of the grid-type converter, thereby facilitating the further development and use of the grid-type converter in the new power system with high renewable energy penetration.

[0103] The present invention also provides a grid-type converter current limiting system based on adaptive virtual impedance, comprising:

[0104] Sampling module, used to sample output voltage, output current,

[0105] The calculation module is used to perform αβ transformation or dq transformation on the output current, calculate the output current, the output current change rate, and the error between the output current and the current limit value.

[0106] The state judgment module is used to judge whether the system enters the current limiting mode according to the relationship between the sampled output current and the threshold current.

[0107] The virtual impedance module is used to generate a sub-transient current limiting virtual resistance and a transient current limiting virtual reactance according to fuzzy control and fuzzy PI control, and then convert the virtual electromotive force E and the voltage drop V across the virtual impedance into a virtual impedance in the αβ coordinate system or the dq coordinate system. Zv Subtract and get the output voltage command value V of the converter ref Or modulation voltage V pwm .

[0108] The third aspect of the present invention is to provide an electronic device, characterized in that it includes a memory, a processor, and a computer program stored in the memory and executable on the processor, and when the processor executes the computer program, it implements the steps of the adaptive virtual impedance current limiting method applicable to a grid-type converter.

[0109] The fourth aspect of the present invention is to provide a computer-readable storage medium, characterized in that the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, it implements the steps of the adaptive virtual impedance current limiting method applicable to a grid-type converter.

[0110] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0111] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0112] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0113] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A current limiting method for a grid-type converter based on adaptive virtual impedance, characterized in that: include: When the output current of the grid-connected converter exceeds the overcurrent threshold I th When , virtual impedance is added to limit the current; The resistance part of the added virtual impedance is used to quickly suppress the surge current component at the beginning of the overcurrent fault; The reactance portion of the added virtual impedance is used to accurately limit the effective value of the current in the transient stage of the overcurrent fault to the current limit value.

2. A grid-type converter current limiting method based on adaptive virtual impedance according to claim 1, characterized in that: The adaptive virtual impedance current limiting is applicable to various reactive power and voltage control methods, including: A control method based on voltage and current double closed loops includes: QV Droop control and voltage and current loops; A control method based on a reactive power single loop includes: VQ Droop control and reactive power loop control; A control method based on a single loop of voltage effective value includes: QV Droop control and voltage RMS single loop.

3. A grid-type converter current limiting method based on adaptive virtual impedance according to claim 1, characterized in that: When the adaptive virtual impedance current limiting method is used for voltage and current dual closed-loop control, in the αβ coordinate system or the dq coordinate system, the voltage drop across the virtual impedance is calculated by the output current and the virtual impedance. V Zv , and then use the virtual internal potential E Subtract the voltage drop across the virtual impedance V Zv The output voltage command value of the converter is obtained V ref , thereby performing voltage loop control; When the adaptive virtual impedance current limiting method is used for reactive power single-loop control or voltage effective value single-loop control, in the αβ coordinate system or the dq coordinate system, the voltage drop across the virtual impedance is calculated by the output current and the virtual impedance. V Zv , and then use the virtual internal potential E Subtract the voltage drop across the virtual impedance V Zv The modulation voltage V pwm .

4. A grid-type converter current limiting method based on adaptive virtual impedance according to claim 1, characterized in that: The resistance part of the added virtual impedance is generated by a two-dimensional fuzzy controller.

5. A grid-type converter current limiting method based on adaptive virtual impedance according to claim 4, characterized in that: For the fuzzy controller of the resistor part that generates virtual impedance, by setting fuzzy control rules, in the sub-transient stage of overcurrent fault, the rapidity of fuzzy control is used to generate sub-transient current limiting virtual resistance according to the fault current characteristics. R subtran To suppress the surge current component, after the surge current component is suppressed, the sub-transient current limiting virtual resistor R subtran Quickly adjust to zero.

6. A grid-type converter current limiting method based on adaptive virtual impedance according to claim 1, characterized in that: The reactive part of the added virtual impedance is generated by a two-dimensional fuzzy PI controller.

7. A grid-type converter current limiting method based on adaptive virtual impedance according to claim 6, characterized in that: For the fuzzy PI controller that generates the reactance part of the virtual impedance, a transient current limiting virtual reactance is generated in the transient stage of the overcurrent fault by setting the fuzzy control rules. X tran The effective value of the fault current is accurately limited to the current limit value I lim .

8. A grid-type converter current limiting method based on adaptive virtual impedance according to claim 4, characterized in that: In the αβ coordinate system or dq coordinate system, calculate the output current I out , Output current change rate dI out / dt And the output current I out and current limit value I lim The error between ε Iout ; When the output current of the converter I out Does not exceed the overcurrent threshold I th When the input variable and output variable of the fuzzy controller that generates the virtual impedance resistance part are both zero; when the output current of the converter is I out Exceeding the overcurrent threshold I th When dI out / dt and ε Iout As two input variables of the fuzzy controller, through the fuzzification, fuzzy reasoning and defuzzification processing of the fuzzy controller, the sub-transient current limiting virtual resistor is finally output. R subtran .

9. A grid-type converter current limiting method based on adaptive virtual impedance according to claim 6, characterized in that: In the αβ coordinate system or dq coordinate system, calculate the output current I out , Output current change rate dI out / dt And the output current I out and current limit value I lim The error between ε Iout ; When the output current of the converter I out Does not exceed the overcurrent threshold I th When the input variable and output variable of the fuzzy PI controller that generates the virtual impedance reactance part are both zero; when the output current of the converter I out Exceeding the overcurrent threshold I th When ε Iout As the input variable of the PI controller, the transient current limiting virtual reactance X tran As the output variable of the PI controller; dI out / dt and ε Iout As the two input variables of the fuzzy controller, the compensation amount of the proportional coefficient of the PI controller is output after fuzzification, fuzzy reasoning and defuzzification of the fuzzy controller. ΔK p_X Compensation amount with integral coefficient ΔK I_X ; Combine it with the original proportional coefficient and integral coefficient of the PI controller to form a fuzzy PI controller.

10. A grid-type converter current limiting system based on adaptive virtual impedance, characterized in that: include: Sampling module, used to sample output voltage, output current, The calculation module is used to perform αβ transformation or dq transformation on the output current, calculate the output current, the output current change rate, and the error between the output current and the current limit value. The state judgment module is used to judge whether the system enters the current limiting mode according to the relationship between the sampled output current and the threshold current. The virtual impedance module is used to generate the sub-transient current limiting virtual resistance and transient current limiting virtual reactance according to the fuzzy control and fuzzy PI control respectively, and then convert the virtual electromotive force into the αβ coordinate system or the dq coordinate system. E The voltage drop across the virtual impedance V Zv Subtract and get the output voltage command value of the converter V ref Or modulation voltage V pwm .

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